Metric Thread On Imperial Lathe

[Bogstandard]

Andy,

I know you can get lathes that cut every thread under the sun (at a price), but I have always baulked at cutting BA by single pointing.

I know it can be done, but the ease of which it can be done with taps and dies makes me shy away from it, due to it having no real basis about it. It seems that each size has it's own set of rules.


Just as an aside to this topic.

I am sure that with a little work you can get a mod 1 x 127 in there.

Next year, I will be playing about with mod hobs, and if it works out OK, I won't mind cutting you some gears up to play around with, I have a full set of hobs, mod 1.25 down to mod 0.3.
But please bear in mind, it is when I get around to doing it, not when someone wants me to.

I am still trying to find some bits that I promised another member ages ago, but because I can't get to them, I have to wait until someone comes around to do it for me, and that could take days to go thru all my stash points. Two failures up to now, but they are there somewhere.

Bogs

[RichardShute]

I’ve been puzzling over the remark which Bogs made before his data got lost, and have come up with these approaches to “going in at an angle”.


Method 3.
<....> The topslide is at 30º to the work and at 60º to the cross-slide. <....>Trigonometry shows that advancing the topslide when it is set at this angle will advance the tool into the work by half the amount indicated by the topslide dial. So in this case the cross-slide and topslide can be advanced by the same amount as shown by their dials to produce the same result as Method 2 with its parallel topslide. This avoids the bother of remembering by how much the cross-slide has just been advanced and then advancing the topslide dial by half that amount. Each dial is advanced by the same amount in Method 3.

Am I on the right track. or have I got myself completely lost?

Andy

Andy,
Your trig is correct, but the scheme is that all feed is made with the topslide, other than a very fine final cleaning cut taken as a plunge striaght in. The cross slide is withdrawn after each cut and then returned to the same place on the dial each time (conveniently zero) before the next cut is taken, the feed being made with the top slide.

It is well established practice to use a compound slide at a fine angle, often 6deg as the trig works out to a convenient 1:10, for taking very fine cuts or when using a tool post grinder.

As a very near approximation to 127 you could use a ratio of 80/63 = 1.26984 which really is very close, only 16 parts in 127000 error. That may be the near miss you were referring to, it is fairly commonly used within the standard set of Myford gears I believe, 'though your gear set may well be different.


Richard

 

[BillTodd]

You beat me to it Richard

I was going to suggest 47/37. Unless you are cutting a very long thread or require extreme precision, the error is insignificant.

Bill

[andyf]

Bogs:
That's a very kind offer. When you get round to your hobbing experiments, and should they involve making some large Mod 1 gears, and if 127T happened to be a convenient size to try, I'd be very grateful. I could soon knock up a bigger banjo to accommodate it. But I know you always have a lot on your plate.

It is indeed easier to use taps and dies for BA threads because they are all so small, and I've never actually single-pointed one. While working out my gears for metric threads, I also calculated a few BA ones (see link below). At the time, I was probably trying to distract myself from doing that year's tax return    I think BA was developed as a metric thread, the logic behind it being that, as the BA numbers increase, the pitches decrease by 10%. So, 0BA is 1mm pitch and 1BA is 10% less at 0.81mm, and so on through the range, with the pitches being rounded to the nearest 0.01mm. There doesn't seem to be a corresponding logic to the diameters, though.

Richard and Bill:
43T and 37T are a handy combination which wouldn't take up much room, but I have neither. 63T and 80T are in my standard gearset, but the lathe has no quick-change gearbox, and once 63T/80T are on the banjo, there isn't much room for anything else. In any case, 63T/80T produces an error of about 0.125%, whereas most of the combinations I worked out here
< http://andysmachines.weebly.com/better-mm-threads-from-inch-leadscrew.html >
produce much less error.

Regards,
Andy
 

[RichardShute]

<....>

It is indeed easier to use taps and dies for BA threads because they are all so small, and I've never actually single-pointed one. While working out my gears for metric threads, I also calculated a few BA ones (see link below). At the time, I was probably trying to distract myself from doing that year's tax return    I think BA was developed as a metric thread, the logic behind it being that, as the BA numbers increase, the pitches decrease by 10%. So, 0BA is 1mm pitch and 1BA is 10% less at 0.81mm, and so on through the range, with the pitches being rounded to the nearest 0.01mm. There doesn't seem to be a corresponding logic to the diameters, though.

Richard and Bill:
43T and 37T are a handy combination which wouldn't take up much room, but I have neither. 63T and 80T are in my standard gearset, but the lathe has no quick-change gearbox, and once 63T/80T are on the banjo, there isn't much room for anything else. In any case, 63T/80T produces an error of about 0.125%, whereas most of the combinations I worked out here
< http://andysmachines.weebly.com/better-mm-threads-from-inch-leadscrew.html >
produce much less error.

Regards,
Andy
 

Andy,
BA threads are indeed metric and are a rare example of a system which follows a prescribed set of rules rather than just arbitrary convenient sizes. There is a description here:
http://www.enginehistory.org/british_fasteners.htm

The pitch of threads in the BA system is the 'base' dimension from which all other aspects of any given thread follow. The pitch is 0.9mm rasied to the power of thread number, hence 2BA is 0.9*0.9=0.81mm pitch etc. There is a formula quoted for the diameters in the reference, it's derived from the pitch, not vice versa.
One useful characteristic of BA threads is that the clearance drill for each thread is the tapping drill for the next but one thread in the series, the same drill is also the screw head counterbore size for the next but one down as I recall, but that warrants checking. Doesn't matter so much now, but when drills were an expensive asset, reduced tooling was handy, and is part of the reason why it was a common tendency for assemblies using BA threads to use all even or all odd numbers.  

So for example a tapping drill for 2BA is a clearance for 4BA and a counterbore size for 6BA; the same holds for 3:5:7 etc.

Just a comment, but 80/63 is 0.0125% error, not 0.125%, but it's acedemic if you can't fit other gears on the banjo at the same time. Are you aware of the little application to calculate gear ratios available here:
http://www.lathes.co.uk/threading/nthreadp.exe

It's handy for finding the best near-miss given what gears you have available.

Richard

[andyf]

  Wow, Richard - I didn't know all that BA theory.

I did use "nthreadsp" to help calculate my setups, but because it only allows for two driver and two driven gears, whereas I was using three drivers and three drivens, things got a bit complicated.  And most theoretically very accurate combinations proved physically impossible to set up. To avoid frustration I created a spreadsheet with 40T (my spindle gear) as the first driver gear and into which I could enter another two drivers and three drivens. The spreadsheet then tried all possible permutations (using 40T as the first driver) of those driver and driven gears and advised whether any of them were possible in real life. 

Andy

[BillTodd]

whereas most of the combinations I worked out...produce much less error.

Excellent:)

Given your lathe is only 300mm between centres, and even the longest of thread will only be a few tens of microns out; Why bother with a 127T gear at all?

Bill

[Bogstandard]

Thank goodness I got rid of my old lathe when I did. I used to spend hours calculating out gear ratios to give me something close to what I was after. Then overcutiing slghtly to match internal to external (if you were only cutting one).

With the way my brain is addling nowadays, I would have given up on single point threading if my lathe didn't now do it all for me.

I was very dubious when I first swapped over, as with a gear train I could always get somewhere near where I needed to be, then fiddle it afterwards, but it is now just a flick of a lever, and sometimes one or two gear swaps, and it is exactly what is needed.

As long as it is standard imperial TPI and metric pitches.




Bogs

[doubleboost]

I think i can finally put this thread    to bed
I had a visit from Rob Wilson today naturally the subject of screw cutting came up .
Rob in less than a minute showed me a very simple way of checking the compound slide angle.
The tool was simply placed on the compuund slide at 90 degrees to the lathe bed (as it would be to make a cut)
the compound slide should be at the same angle as the trailing egje of the tool.

this picture has the compound set at 30 deg as you can see the tool trailing edge is not the same as the compound

this picture has the compound set at 60 deg the tool angle and the compound are the same
The video will be edited
I have one more chuck to make i will be using the 60 deg mark
Regards
John

[Ned Ludd]

Hi John,
When you next cut a thread, will you let us know if you think it cuts better than before, then you can put the issue to bed. If after four pages you don't find it to be better, it would have been better to have kept my mouth shut or at least let my keyboard rest.   
Ned

[andyf]

Andy: whereas most of the combinations I worked out...produce much less error.

Excellent:)

Given your lathe is only 300mm between centres, and even the longest of thread will only be a few tens of microns out; Why bother with a 127T gear at all?

Bill

Good point, Bill! It would be a bit silly of me to chase after a degree of accuracy which won't, in practice, make a blind bit of difference.

Andy

[Pete.]

John thanks for the follow-up. It's always good to get a fresh set of eyes on a job.

Your video is one of the better ones and very clear and easy to follow, lots of people will gain confidence from it. I like your bronze casting ones too.