Author Topic: Single point threading on the lathe  (Read 45032 times)

Offline spuddevans

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Single point threading on the lathe
« on: August 02, 2009, 04:16:17 PM »
Ok, I ( foolishly ) said I would put up a post on how I do single point threading on the lathe. To start off with let me say that this is just how I do it, there are many who are more experienced / qualified than I am, and I am really hoping that they will all chime in with their insights and experience.

I have just a (relatively) small lathe, a Sieg C2, and depending on the size and rigidness of your lathe you may be able to take deeper cuts than me ( or you might have to take shallower ).

I'll start of by apologising if it comes across like I'm speaking like you are all are beginners, I dont mean to patronise, it's just that I am still learning and this is how I understand all this. This isnt "The" idiot's guide, it's just "This" idiot's guide.  :lol:

The 1st thing to do is to mount and prepare (bring to size) the material you want to thread. In this example that I am demonstrating I will be cutting a M6 thread on a part for my Paddleducks build. On this part the threaded section is quite short, only about 3.2mm long, but if you are wanting to practice single point thread cutting I'd suggest threading about 5-10mm length of whatever thread you choose.

So to start off with, here's what my material looks like with the end section turned down to the required size ( easy for the metric system as M6 is 6mm diameter, I prefer to go a little under 6mm, about 5.85-5.95mm as it makes for an easier fit. )



The next stage is to make a little groove where you want the thread to end. Unfortunately I dont think there is a way to single point a thread all the way up to a shoulder, so we need to make a groove for the tool to stop at the end of each pass. All you need to do is grind up a tool to look something like this.


Just grind it up to whatever width that you want the groove to be.


So now you can make the groove at where the thread will end.



You will also need a tool to cut the threads, there are different angles for different types of threads, the metric system uses a 60 degree angle, and you can get carbide tools already formed, or you can grind up your own tool which is not hard at all. I ground my own tool using a grinding gauge shown below.



That threading angle gauge is your very good friend, not only useful for grinding the right angles, but it is also an essential tool to set the tool at the right angle to the workpiece. The pic below shows what I mean, you hold the angle gauge on the tip of the tool and the opposite edge of the gauge rests close to the workpiece. This will show up if the tool is not at 90degrees to the workpiece and then you can adjust the toolpost to allign it.



Once the tool is angled correctly and set at center height we can get the cross-slide zeroed on the OD of the workpiece. I do this by winding in the cross slide until the tool just contacts the workpiece enough to leave a little score like this,



Now the tool is all set you can zero your cross-slide, or if you have dro's you can zero those now.

Depending on your lathe you will either have a gear selector to select the different ratios for the leadscrew, or like mine you will have a little bag of gears and a chart that tells you where to put what gears to give you the required cutting pitch.



Now you can position the carriage clear from the workpiece and engage the half nuts and advance the cross-slide by 0.1mm (for brass, ali ect. Steel might need a shallower cut depending on your lathe)

Then select the slowest speed you can do on your lathe (this is where having variable speed really helps) and start up the lathe. Watch carefully and stop the lathe when the tool tip has got to the groove we cut earlier.


Then withdraw the cross-slide until it has backed out past the zero point, then select reverse, do not release the half-nuts, start the lathe and run it back past the workpiece, advance the cross-slide past zero and to 0.2mm this time (0.1mm deeper) and repeat the above paragraph.

Then you keep doing this until you get to the desired thread depth. Now, what is the right thread depth for each thread? I cant tell you as I dont know, but I can tell you that I seem to go to a depth of 0.650-0.700mm for M6. I have some tables that explain all about thread pitches, angles and all sort of things I dont understand, yet!! But the easy way to get the thread depth right is to use a commercially made nut of the thread you are cutting and when the thread form starts to look like it should, try the nut on it after each pass until it fits.








And that's all there is to single point threading, well that's the basics of it, there is a lot more but I'll leave it up to those better experienced and qualified to tell you about that.

By the way, in the time that it takes to read this you can single point thread literally feet of thread.

Hope this helps someone, if any of my ramblings do not make sense please tell me so I can explain or correct it.


Tim
« Last Edit: August 03, 2009, 05:27:34 AM by spuddevans »
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Offline Darren

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Re: Single point threading on the lathe
« Reply #1 on: August 02, 2009, 04:35:31 PM »
Very nicely shown Tim,

That's almost how I do it, only difference I make is I don't back the tool out after each pass. I can see others throwing their arms up in horror here.... :doh:

I leave the feedscrew nuts engaged all the time and just advance the top slide with each cutting pass.

I have read that you can't remove the work between starting and finishing as you'll loose it's placement. I'm not sure why that is as I have done it several times with no apparent ill effects? Just make sure the tool tip is firmly located in a groove before re-clamping the chuck. Seems to work....well so far anyways....


 
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Offline raynerd

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Re: Single point threading on the lathe
« Reply #2 on: August 02, 2009, 04:49:44 PM »
 :thumbup: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup:

Great job, Tim. I`ve been doing quite a bit of single point threading but my problem is my Boxford and Clarke cl300 are both imperial and being in the UK, I only ever use metric - I think this has inspired me to order the metric conversion kit for the clarke in the next few days (ideally would do the boxford but don`t know how to go about it) - anyway that is all  :offtopic:  :D

Good job mate, would love to see you do an internal thread as well - what size internal thread do you think would be achievable? I presume the limitation is in the size of the internal thread cutting tool? Am I going  :offtopic: again .... lol

Good job mate.

Chris

Offline kvom

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Re: Single point threading on the lathe
« Reply #3 on: August 02, 2009, 05:14:41 PM »
I'd add a couple of minor points.

After you cut the first pass with a scratch cut, it's a good idea to use a thread gauge to ensure you've set the pitch correctly.  If not, you can fix it before continuing.

If your lathe has a thread dial (and cutting imperial threads), it's easy to back out the cross slide and back up the carriage.  You can then use the dials or DRO to set the depth for the next pass.  For very short threads like above, I think it's faster to reverse the lathe with the leadscrew engaged, assuming there is not enough backlash that the threadform will be affected.

As for thread depth: if you assume a 60-degree thread with pitch P, then the depth D is the height of an equilateral triangle.  So P/2D = tan(30) or D = P/(2*tan(30)) or .866*P.

Offline Darren

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Re: Single point threading on the lathe
« Reply #4 on: August 02, 2009, 05:18:37 PM »
I'm glad you added the last bit.......866*P........ :lol:
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Offline Mr blindbird

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Re: Single point threading on the lathe
« Reply #5 on: August 02, 2009, 06:10:57 PM »
Very nice walktrough for those starters that have no experiance in threading at all,very nice to spend your time showing how it's done  :thumbup:
Those tables you mention abouth should have at least show the inner and outer diameter of every possible tread,it's pitch and if it's an advanced table,it will even show the flankdiameters,take a litle time to study those numbers and in no time you will see it'll all make sence to you.
Have to say since you state that you dont realy understand all the tables yet,i wanne congratulate you as you do a wonderfull job finishing that thread  :thumbup:
On my turn,i also dont wanne patronise honnest,may i ad the following;
it's ok to cut the thread,feeding in a 90° angle in materials that produce short chips like brass,cast iron...and in all other materials as long as the thread dont get to heavy,meaning;you dont wanne feed an M20 at a 90° angle as that will put to much force on the threadingtool,leavin ugly marks and burrs behind and even may break the tool.
In that case,you have several choises,you can everytime you feed with the cross slide,feed 1/3 of that feed on the small slide(exuse me for not knowing the right name for that   :scratch:...)so you only get your tool cutting on the left side,creating a more propper chip which will make sure the walls of the thread are nice and even to,only the few last(and very light) passes you can do a 90° feed on the crosslide to make sure both walls of the thread are eaqualy cut.
an other option is to place that small slide :scratch: at a 60° or 55° ,acording to the needed thread,and feed only with this small slide  :scratch: mmmmzz if only i knew how the litle bugger is called,i wouldnt feel such an idiot here  ::) and again,only the few last light feeds going tru to shine up the walls.
This is all i can remember for now,hope it filled in some blanks  :wave:
If force doesnt solve the problem,you're not using enough...
If aditional force still doesnt do the trick...you should have checkt if it's a right or left thread first...

Offline Darren

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Re: Single point threading on the lathe
« Reply #6 on: August 02, 2009, 06:14:47 PM »
Small slide = compound slide....

Nice addition Danny.... :thumbup:
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Offline Mr blindbird

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Re: Single point threading on the lathe
« Reply #7 on: August 02, 2009, 06:18:06 PM »
Thx for helping me out Darren  :thumbup: ...that's the second time you save my skin  :D one more and i'll have to come over to treat you on a beer...or something stronger  :headbang:
If force doesnt solve the problem,you're not using enough...
If aditional force still doesnt do the trick...you should have checkt if it's a right or left thread first...

Offline raynerd

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Re: Single point threading on the lathe
« Reply #8 on: August 02, 2009, 07:15:21 PM »
Is this what they mean about generating or forming. With your cross slide feeding you are generating the full shape (in this case the thread) and putting pressure on the tool, yet with your compound slide set at an angle and feeding with the compound you only cut on one side and ultimately generate the shape, elevating pressure on the tool. I guess it takes more calculations to work out the depth of cut if your putting it on with the compound as your going in at an angle...?

All the threading guides I have read have shown a 29-30deg angle yet I have had good results with brass with the compound set square to the work. I guess it doesn`t work with steel, would it be best to generate and use the angle of the compound?

Very nice walktrough for those starters that have no experiance in threading at all,very nice to spend your time showing how it's done  :thumbup:
Those tables you mention abouth should have at least show the inner and outer diameter of every possible tread,it's pitch and if it's an advanced table,it will even show the flankdiameters,take a litle time to study those numbers and in no time you will see it'll all make sence to you.
Have to say since you state that you dont realy understand all the tables yet,i wanne congratulate you as you do a wonderfull job finishing that thread  :thumbup:
On my turn,i also dont wanne patronise honnest,may i ad the following;
it's ok to cut the thread,feeding in a 90° angle in materials that produce short chips like brass,cast iron...and in all other materials as long as the thread dont get to heavy,meaning;you dont wanne feed an M20 at a 90° angle as that will put to much force on the threadingtool,leavin ugly marks and burrs behind and even may break the tool.
In that case,you have several choises,you can everytime you feed with the cross slide,feed 1/3 of that feed on the small slide(exuse me for not knowing the right name for that   :scratch:...)so you only get your tool cutting on the left side,creating a more propper chip which will make sure the walls of the thread are nice and even to,only the few last(and very light) passes you can do a 90° feed on the crosslide to make sure both walls of the thread are eaqualy cut.
an other option is to place that small slide :scratch: at a 60° or 55° ,acording to the needed thread,and feed only with this small slide  :scratch: mmmmzz if only i knew how the litle bugger is called,i wouldnt feel such an idiot here  ::) and again,only the few last light feeds going tru to shine up the walls.
This is all i can remember for now,hope it filled in some blanks  :wave:
« Last Edit: August 02, 2009, 07:17:56 PM by craynerd »

Offline Darren

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Re: Single point threading on the lathe
« Reply #9 on: August 02, 2009, 07:22:46 PM »
This was made by feeding in at 90 deg, it's 7/8 x 12tpi

http://madmodder.net/index.php?topic=1390.0

I didn't have any issues and it cut really nicely on my 7x12 mini lathe.

I did consider off-setting the compound slide, but thought what the heck, push your luck and see what happens...it was fine... :med:
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Offline sbwhart

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Re: Single point threading on the lathe
« Reply #10 on: August 03, 2009, 02:00:56 AM »
Is this what they mean about generating or forming.


Yes that correct Chris

Nice write up Tim and really useful input from the rest of you guys  :thumbup: a big thanks

Stew
« Last Edit: August 03, 2009, 02:12:59 AM by sbwhart »
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Re: Single point threading on the lathe
« Reply #11 on: August 03, 2009, 04:04:05 AM »
Tim
Not patronising at all. Bloody good write up.
 Remember not all of us had an apprenticeship or 40 years
experience. (not denigrating those who have)
Just helping newcomers like me to see the light about threading
IS A GOOD THING :clap:
 I'm inspired to give it a go now
BR

Offline spuddevans

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Re: Single point threading on the lathe
« Reply #12 on: August 03, 2009, 05:36:18 AM »
Tim
......
 I'm inspired to give it a go now
BR

I'm glad you are going to give it a whirl, it really is not hard to do, just take your time and threads will appear as if by magic before your eyes. :thumbup:

The great thing is, once you have got to grips with single point threading you can make any form and pitch of thread you would like ( within your lathe's capability ) Especially if your lathe has a set of gears for driving the leadscrew, with a few calculations ( or with the help of a little program called "Change" from MKlotz over on hmem ) you can come up with all sorts of combinations of gears to get the required pitch, even cutting imperial on a metric lathe or cutting metric on an imperial.

The thread world is your oyster

Tim
« Last Edit: August 03, 2009, 11:55:23 AM by spuddevans »
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Offline DavesWimshurst

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Re: Single point threading on the lathe
« Reply #13 on: August 03, 2009, 09:48:53 AM »
Tim,
"The thread world is your oyster"
I was working on my window fan one very hot day.  I had it all apart dealing with a bearing problem.  Came time to put the blades back on I couldn't find the nut that holds the blades in place, 1/4 inch left hand thread. No such tap around of course.  It was hot but my shop is in the much cooler basement so I figured why not?  I made an internal threading tool from drill rod as this was smaller than my existing ones could handle and the clearance angles were for right hand threads anyway.  Hardened and tempered the tool.  Cut the thread.  Left hand so start inside the nut blank and cut moving out of the hole.  It worked out real easy,  I was so happy.  Two days later I heard the cat playing with something under the couch,  it was the missing nut!  Damn cat!
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Offline miken

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Re: Single point threading on the lathe
« Reply #14 on: August 03, 2009, 05:26:22 PM »
Id like to add that sometimes people are reluctant to try screw-cutting on their lathe ,especially if their machine is not fitted with a clutch or perhaps does not have a  low enough speed because they are concerned that they not be able to stop in time at the end of the cut.
  There is no need for concern if you make a mandrel handle a bit like an old fashioned car starting crank that fits up the hollow head stock mandrel and allows you to carry out the entire operation by hand and under complete control.
  It might make your arm ache on a long thread though.
Mike

Offline Bernd

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Re: Single point threading on the lathe
« Reply #15 on: August 03, 2009, 07:39:16 PM »
Id like to add that sometimes people are reluctant to try screw-cutting on their lathe ,especially if their machine is not fitted with a clutch or perhaps does not have a  low enough speed because they are concerned that they not be able to stop in time at the end of the cut.
  There is no need for concern if you make a mandrel handle a bit like an old fashioned car starting crank that fits up the hollow head stock mandrel and allows you to carry out the entire operation by hand and under complete control.
  It might make your arm ache on a long thread though.
Mike

And your knukles hurt when you turn on the machine and forgot to take out the crank.  :doh: :bang: :bang: :bang:

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Offline John Hill

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Re: Single point threading on the lathe
« Reply #16 on: August 03, 2009, 07:49:26 PM »


And your knukles hurt when you turn on the machine and forgot to take out the crank.  :doh: :bang: :bang: :bang:

Bernd

Make a spindle handwheel if you want to do something like that, I think you can buy them for the small lathes but I assume they would be useful on any size lathe especially when jogging the gears when selecting ratios in the head.
« Last Edit: August 03, 2009, 07:51:21 PM by John Hill »
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Offline DavesWimshurst

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Re: Single point threading on the lathe
« Reply #17 on: August 03, 2009, 11:11:42 PM »
For short threading jobs or up to a shoulder I made a balanced crank.  If I forget to remove it at least it isn't trying to shake the lathe apart. :doh:
This is it in use when cutting a metric approximation using change gears.



I couldn't close the gear cover because of the extra gears.
Dave

Offline NickG

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Re: Single point threading on the lathe
« Reply #18 on: August 17, 2009, 10:02:33 AM »
The hand crank is a good idea as a lot of modern lathes can't attain low enough speeds.

The lat bit of threading I tried went OK, however, my depth of thread calculations just didn't work, I just had to keep taking more off until it fit. Not sure why. Maybe my internal tool and external tool weren't close enough in angles?

I also used the thread dial indicator so I could withdraw the tool from the work piece and disengage the half nuts.
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Offline Jonny

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Re: Single point threading on the lathe
« Reply #19 on: August 20, 2009, 05:20:10 PM »
Nice write up Tim, there will be variations of how its done by others and method may differ with the lathe used.

Incidently i did it the same way when using Colchester machines. Certain Harrison lathes you can disengage half nuts, wind back carriage and engage half nuts again any where on 1, 1.25, 1.5 & 1.75mm pitches. Cant do it on imperial it must remain engaged as it was built as a metric lathe in 1965.

Always use back gear 45 rpm loads of torque with one hand on the clutch. I have a variable speed mini lathe as well, threading on that powered would scare me plus its got no torque.

The reason for winding cross slide out to clear cut thread is when spindle is reversed any backlash in gears and lead screw will effectively reposition cutter or spindle.

One thing worth mentioning is the tooling of which i have always liked the HSS Cobalt ground to shape and give a reasonable finish. Indexable types theres a lot of junk out there mainly from China that flex, no substitute lash out for known brands such as Kennametal, Sandvik etc with the correct inserts.

Offline CrewCab

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Re: Single point threading on the lathe
« Reply #20 on: August 20, 2009, 06:02:47 PM »
Certain Harrison lathes you can disengage half nuts, wind back carriage and engage half nuts again any where on 1, 1.25, 1.5 & 1.75mm pitches. Cant do it on imperial it must remain engaged as it was built as a metric lathe in 1965. 

Interesting, would that apply to my Boxford metric lathe .........  ::) ......... mmmmm probably not I suspect  :scratch:

CC

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Re: Single point threading on the lathe
« Reply #21 on: August 20, 2009, 09:18:15 PM »
Dave,

Did you eventually manage to get a metric screwcutting dial for your lathe?

If you did, you should have a chart to show which marks to drop into, dependent on which gear you are using on the screwcutting dial. But even on some metric pitches, you still have to use the continuously engaged leadscrew to cut the thread.

John

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Re: Single point threading on the lathe
« Reply #22 on: August 21, 2009, 04:24:11 AM »
Did you eventually manage to get a metric screwcutting dial for your lathe?

Not yet John, but I'm still keeping my eyes open, still looking for a faceplate too.

CC

Offline Jonny

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Re: Single point threading on the lathe
« Reply #23 on: August 21, 2009, 10:44:04 AM »
would that apply to my Boxford metric lathe

I very much doubt it but by some chance it does that'll be a bonus.

Must say on my 140 its a right pain to power in opposite direction as i have to wait up to a minute for motor to stop (been known to stall it like a car), then select direction and power up hoping it dont blow a 13 amp fuse. Same process repetitively often quicker to select fastest gears and wind back by hand, then select back gear again etc. Buy fuses by the hundred never trips the 20 amp feed that the welder and compressor do.
Might rewire it over Christmas plus wire in the 32 amp sockets.




Offline John Hill

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Re: Single point threading on the lathe
« Reply #24 on: August 21, 2009, 04:13:38 PM »
Hmmmmm..... might be a need for lateral thinking here.... :scratch:

The problem as I see it using a metric feed screw to cut imperial threads and vice versa is that it is difficult to ensure the feed screw is in the same position relative to the spindle at the start of each pass.  

Now if you know the pitch of the lead screw and the pitch of the thread you are cutting, (and how could you not know this) you could divide one into the other and determine how many spindle revolutions until they come back to the relative positions they had when the thread was started. If you are using the 120/127 ratio change wheels (or the other equivalent pair)  you might need many revolutions of the lead screw until things line up again, which requires a bit more... :coffee:

Although I began by thinking it might be necessary to fit some sort of revolution counter (did I tell you how to make a counter from an old calculator?) the answer may in fact be very simple,  maybe, perhaps someone will give this a try?


Begin by putting some obvious mark on the metric/imperial change wheel pair, that is the 120/127 or its equivalent (which I can not recall right now  ::)),  anyway, put something you can see while the lathe is turning, something like a dab of paint and if necessary rig up a mirror so you can see it from where you stand to operate the lathe. Fit some sort of pointer  (magnet and a bit of wire?) so that you can line it up with the rotating mark on the change gear.

Start the first cut by engaging the half nuts as your mark lines up with its pointer and continue as normal to make the cut, disengage the half nuts, wind the carriage back to the start, advance the tool then watch the change gear and engage the half nuts when those points align.


I await reports from anyone who tries this.   Of course I may be totally wrong and all there will be to show will be a dab of paint on your change gear. :coffee:


[There is always the possibility that I dont understand the problem  :doh:]


[Later]
OOPS! No that idea will only work if the various pinion gears have teeth counts that are integer dividers of the 120/127 change gears, which of course is impossible!   :doh:  More thinking to be done!
« Last Edit: August 21, 2009, 08:05:01 PM by John Hill »
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Offline Darren

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Re: Single point threading on the lathe
« Reply #25 on: August 21, 2009, 05:54:42 PM »
I was thinking about your calculator trick the other day while out shopping of all things....yes I was indeed waiting for the other half yet again and find this is a time the mind tends to wander....


I was wondering if a calc with some "calcked" number could count the revolutions on an imperial machine to give a metric reading. of course backlash would still be an issue unlike a proper DRO.
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Offline John Hill

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Re: Single point threading on the lathe
« Reply #26 on: August 21, 2009, 08:29:42 PM »
Darren, I do not think backlash would be relevant when trying to find the point at which to re-engage the half nuts, I have a couple of ideas on the subject but the sun is shining today so not 'shop playtime until chores are done!

I will do a short topic in the calculator thingio.
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Offline HENNEGANOL

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Re: Single point threading on the lathe
« Reply #27 on: August 25, 2009, 04:05:48 AM »
When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.  The tool is set up using a gauge as described earlier, then the  feed is put on with the cross slide only.  Which results in the left face of the tool doing all the cutting.

When I served my time all our machines were calibrated in imperial units, all the lathes were supplied with a 127 tooth gear for use when cutting metric threads!

In order to cut metric threads on my Myford lathe fitted with a quick change gearbox it was necessary to purchase an additional set of gears and quadrant.  Even then you must not disengage the nut after each cut, the saddle has to be wound back by reversing the motor.

Oops, I should have said set the top slide to half the included angle
« Last Edit: August 28, 2009, 04:17:12 PM by HENNEGANOL »

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Re: Single point threading on the lathe
« Reply #28 on: August 29, 2009, 05:54:02 AM »
When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.

A couple of mistakes there HENNEGANOL, but I understand what you mean.
 
Actually, this is a much better method of cutting threads, using the compound (topslide) set to half the included thread angle, and using that for putting the cut on when leaving the leadscrew permanently engaged, and was the way I was always taught how to cut single point threads. Only using the cross slide for the final clean up.

This is due to the fact your cross slide will always be at the same setting when retracting it and putting it back to its former position, as you should do when winding back on the leadscrew, rather than trying to remember the setting you were at on the last cut, otherwise if you don't retract, backlash will kill the threads by taking extra metal off. Just use 0 for your cross slide setting, a rather easy number to remember.

When, eventually, I get my retracting topslide made, everything will be taken care of automatically.


Bogs

Offline arnoldb

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Re: Single point threading on the lathe
« Reply #29 on: August 29, 2009, 10:03:43 AM »
HENNEGANOL, Bogs gave good advice. May I just add that the angle for the topslide for cutting a 55 degree thread using this method is 27.5 degrees and not 22.5.
Not being picky - just to make sure someone does not accidentally fall into a trap and use the wrong setting  :D

Hmm... less computer and off to cut that 12 tpi internal thread - never done that; hope it comes out OK  :scratch:....
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Offline NickG

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Re: Single point threading on the lathe
« Reply #30 on: August 29, 2009, 11:36:19 AM »
I need to read through this thread more carefully!  :bang: :scratch:

I don't understand this not disengaging the half nuts thing? Judging by my questions you can probably tell I haven't cut many threads. I thought if I engaged the half nuts to start cutting on 1 of the numbers on the thread dial indicator, I could withdraw the cross slide at the end of the thread disengage the half nuts and wind the carriage back, put the extra cut on then re-engage the half nuts on the number I started on?

Nick
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Offline Darren

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Re: Single point threading on the lathe
« Reply #31 on: August 29, 2009, 11:43:37 AM »
I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

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Offline spuddevans

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Re: Single point threading on the lathe
« Reply #32 on: August 29, 2009, 12:16:39 PM »
I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

Maybe in a really good quality lathe ( I dont have one, so I'm just guessing ) there is very little backlash on the leadscrew. But on my little C2 lathe I can (with the motor turned off) engage the half-nuts and then move the carriage about 1mm in either direction. Also when the lathe is running with the halfnuts engaged and I then reverse the motor, the chuck will rotate anything between 1/4 and a whole turn (depending on what change gears are set up) before the carriage will actually move in the reverse direction.

All of that would add up to a ruined thread. It can also shorten the edge-life of your threading tool as it is rubbing the work but in reverse, this is pretty fatal for carbide tools, they really dont like to be run backwards against the work. ( go on, just ask me how I found that out  :bang: )


Tim
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Offline No1_sonuk

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Re: Single point threading on the lathe
« Reply #33 on: August 29, 2009, 12:38:26 PM »
Leaving the half nuts engaged also means you don't have to mess about working out which number on the indicator to use for which thread, or if you can use it at all.
I'm seriously considering taking the thread indicator off my lathe.  I don't use it, and it's getting in the way.

My lathe has electronic speed control.  That means I can run it forward slowly for the threading, then retract and run it in reverse at higher speed to get the carriage back to the start.

I just need practice getting the angled compound thing right.  :scratch:

Offline HENNEGANOL

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Re: Single point threading on the lathe
« Reply #34 on: August 29, 2009, 03:40:59 PM »
My thanks to Arnold and Bogstandard for pointing out my obvious mistakes, at my age I should know better.

Referring to my comments regarding cutting metric threads on my Myford and the requirement to leave the the half nut engaged at the end of the cut and reverse the saddle.  These are the instructions given by Myford when using the metric thread cutting kit on a lathe with an imperial lead screw.

Another tip that I was taught was to put a chalk mark on the chuck and another chalk mark on the lead screw and when the two marks coincided with their respective reference points to engage the half nut.  

Bearing in mind that there is a large difference between a Turner and a person who can use lathe! I must confess that I tend to rough out the thread in the lathe and then finish it either using a die, die nut or a hand chaser if they are available.  Thereby saving time and ensuring that a reasonable thread results.

Having re-read Geo Thomas'es "The Model Engineers Workshop Manual" I would reccommend that anyone who requires further information with regard to screw cutting, reads chapter 12 which contains some very useful information.
« Last Edit: August 29, 2009, 03:52:07 PM by HENNEGANOL »

bogstandard

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Re: Single point threading on the lathe
« Reply #35 on: August 29, 2009, 03:49:57 PM »
Nick,

I think you are getting confused because of the different threads that are required to be cut.

I will try to explain the reasons behind what we are on about.


Imperial Lathe.

These are almost always fitted with an imperial leadscrew, and because the imperial system uses TPI (threads per inch), then by using the drop in dial that is matched to that leadscrew, and with the correct change gears fitted, imperial threads can be cut directly by dropping in the correct numbers of the screwcutting dial.
When metric threads are being cut on an imperial machine, a transposing gear is usually fitted into the gear train to allow the leadscrew to turn at the correct rate for metric threads. Unfortunately, because metric threads use a pitch system instead of a TPI one, you cannot use the drop in dial, but instead you have to leave the half nuts engaged, and reverse the machine backwards to wind the carriage back to the beginning, to carry out the next cut.
Usually, metric threads are not true pitch when cut on an imperial machine, they are an approximation of the pitch, but usually close enough for a normally cut metric nut or bolt to fit. The problems arise when you cut a long metric thread that has to be used with a long normally cut metric hole or bolt thread. They can very easily bind up solid because of the missmatch.

Metric Lathe.

These are normally fitted with a metric pitch system of leadscrew. But unlike the imperial system, you also have to compensate on the drop in dial for the different pitches being cut. Usually this is done by having change gears on the bottom of the dial that pick up the readings from the leadscrew. There are 3 on my machine, and these little posts I did might explain what they look like.

http://madmodder.net/index.php?topic=870.0

http://madmodder.net/index.php?topic=973.0

Unfortunately, even with the compensating gears, some metric threads still have to be cut by leaving the half nuts engaged, and setting up the gear train for the correct pitch.

For cutting imperial threads on a metric machine, it is basically the same as cutting metric threads on an imperial machine. A transposing gear is fitted, the gear ratios are changed to give what TPI you require and the thread is cut with the half nuts permanently engaged. The same problem with the fit also rears it's ugly head, they are only very close approximations of the thread required.

Some very expensive lathes have all these facilities built in, just select on the gearbox. But on the sorts of lathes we normally use, we have to compromise.

I hope this has made it a little clearer.


Bogs

Offline Darren

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Re: Single point threading on the lathe
« Reply #36 on: August 29, 2009, 03:50:16 PM »
Tim,

I have a 7x12 that I cut threads with which I believe is the same as your lathe?

I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.



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Offline spuddevans

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Re: Single point threading on the lathe
« Reply #37 on: August 29, 2009, 04:09:27 PM »
I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.

You could be right are most likely right about adjusting the leadscrew, :thumbup:  I have done very little to the lathe since getting it and de-greasing it. After I finish the Paddleducks build I intend to start on a whole stream of mods to both the lathe and mill. Thinking about it, there is a lot of play in the leadscrew on mine. That's why I have to withdraw the tool before reversing the lathe when threading.


Tim
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Offline John Hill

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Re: Single point threading on the lathe
« Reply #38 on: August 29, 2009, 05:16:08 PM »
After a long, and rather exciting, exchange on another board I have concluded (actually it was pointed out to me) that when using a metric lathe to cut imperial threads the drop in position repeats at 127 revolutions of the lead screw for all (whole number) imperial pitches.  (127 is an even mulitple of 25.4 which is the ratio of millimetres to the inch).

SO....having a turns counter on the lead screw that repeats at 127 you can always withdraw the tool, wind the carriage back and re-start without phutzing about telegraphing the engine room to run astern!

I have seen one design for a 127 tooth indicator gear but if that is to engage with a 3mm pitch lead screw it would be about 5 inches in diameter.  Other suggestions have been a sprocket on the lead screw running a loop of light chain long enough to repeat every 127 turns, the chain of course has a link or so painted to indicate the drop in point.  Neither of these is impossible but both rather cumbersom.

The  4020 (there are others) CMOS integrated circuit can drive LEDs directly and has a reset pin.  It should be easy enough to mount one of these in a little plastic box complete with a reed switch which suitably position would be triggered by a magnet on the lead screw.  You would then have a little display with a number of flashing LEDs in an easily recognised pattern and when it gets to the end of the sequence thats the time to drop in the half nuts.  I confidently expect Darren will make one of these in short order.
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Offline Darren

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Re: Single point threading on the lathe
« Reply #39 on: August 29, 2009, 05:20:23 PM »
So glad to inspire confidence John....... :lol:
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Offline NickG

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Re: Single point threading on the lathe
« Reply #40 on: August 30, 2009, 04:08:44 AM »
Thanks Bogs, yes that clears it up. That is a clever modification you carried out to your lathe.  :thumbup: And I admire anyone that can get a new machine out of a box and start drilling holes in it!   :jaw: :clap:
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