change wheel selection

[DavidA]

The new lathe (new to me that is) came without change wheels.

I want to cut an 8tpi thread.

The gear on the back of the spindle is 34 tooth.  It drives a 21 tooth reverser set up that drives a 54 tooth  'stud' gear' that will drive the required gear train.

My leadscrew is 6 tpi.

I will need to obtain the gears,  but what gears do I need ?

I can't get my head around the selection due to the odd stud gear that means I don't have a one to one connection to the spindle.

I think it may be easier to change the stud gear for a 34 tooth gear than it will turn at the same speed as the spindle;  I can deal with the maths for that assembly.

Ideas,  anyone ?

Dave

[andyf]

Hi Dave,

There's a formula for this:
 Product of all the Drivers = leadscrew tpi
Product of all the Drivens       desired tpi

Your leadscrew is 6tpi, and you require 8 tpi. Your spindle gear is 34 teeth, and that is the first of the drivers. The reverser set can be ignored; they are just idlers, and each idler acts as both a driver and a driven, so comes on the top and bottom of the equation and so can be cancelled out. The gear below the reversing set is 54T, and that is your first driven gear.

So we have:
34 x A   =   6
54 x B        8    , where A is an extra driver and B an extra driven

This can be switched around to
A  = 54 x 6
B     34 x 8

So a 324T driver rotating with  the 54T stud gear, and a 272T driven on the leadscrew, would do the trick, if they weren't such impractical sizes. Dividing each by 4 gives an 81T driver and a 68T driven, which might fit with another idler of suitable size between them to fill up the gap. That is as low as you can go; there's nothing else which can be divided into both of them. 81T and 68T gears aren't common sizes, and you will need a third gear to act as an idler between them.

If you are going to do a lot of screwcutting of different pitches, it might be a good idea to equalise the spindle and stud gears and use 44T for each. That would mean repositioning the tumbler reverse a bit lower down, but it would enable more common gears to be used to cut different threads per inch. I suggest 44T for each in case the position of your stud gear is fixed; the gap between two 44Ts would be the same as that between the 34T and the 54T. If you can move the stud gear closer to the spindle, your suggestion of using another 34T for it would be fine.

Apologies in advance if I have erred in the arithmetic; if so, I'm sure some kind soul will correct me.


Andy

[Raggle]

If a 34t gear was on the leadscrew and all intervening gears were a simple train you would cut a thread equal to the leadscrew 8tpi.

So you have a constant: desired tpi x 34/8 or 17/4 = leadscrew gear teeth

6 tpi required = 6 x 17/4 = 25.5

Can't get a gear with 25.5 teeth, so a 51t gear on the leadscrew and a 2:1 compound (say 20 and 40) in the train would do it.

However, a 34t gear on the spindle seems very constricting to me with 17 and 2 as the prime numbers. Is it possible to change this gear for, say 32t?

In that case all simple gear trains would have a leadscrew carrying a gear equal to 4x the desired tpi. In the case of your 6 tpi this would be a 24t gear with no compounding. Simples

I hope you can make sense of my rambling. I don't know how to do andyf's delightful fractions.

Ray

[andyf]

Your way is much simpler than mine, Ray - it uses the awkward 54T as an idler, so it takes no part in determining the overall ratio.

But you have transposed things: Dave's leadscrew is 6tpi, not 8 tpi. It is his desired thread which is 8tpi.

So using your method:

If 34T was on the leadscrew, as well as the spindle, and they were connected by a simple train of idlers, Dave would cut a thread equal to the leadscrew: 6 tpi.

So he has a constant: desired tpi x 34/6 (or 17/3) = leadscrew gear teeth.

8tpi required = 8x17/3 = 45.3333 teeth.

Can't get a gear with 45.3333 teeth, so one three times the size, being 136T, on the leadscrew and a 1:3 compound (say 60 and 20) in the train would do it.

To check, apply those figures to the formula I quoted, which was
Product of all the Drivens = leadscrew tpi
Product of all the Drivens       desired tpi

Then
34x60    =   6
136x20       8

Both sides of that equation work out to 0.75, so that’s OK.

The problem is that a 136T gear for the leadscrew will be hard to find, and even if one were bought or made, there might not be enough distance between Dave’s spindle and leadscrew for a train which included 54T as an idler and 136T on the leadscrew. If there is room, the position of Dave’s 54T stud may be fixed, so he can’t adjust it get the 54T into mesh with the 136T, rendering another idler necessary to bridge the gap.

Andy

[Fergus OMore]

I think that this is where 'number crunching' as per Martin Cleeve in Screwcutting in the Lathe comes in.

I'd say that a 127 (or a 63) transposing gear would be more available- and give pretty well the same pitch

Always remember that Professor Dennis Chaddock said 'If you have 25 gears, you can have a choice of 750,000 different pitches- provided it was not all tied up in a gear box'

It's a bit like my dividing head that has ONE hole.

[John Stevenson]

I think we are missing some information here not given by Dave.

He says he has a 34 driving a 21 tumbler set up and then a 54.
If that is correct then that 54 cannot be changed or it will not mesh with the tumbler gears.

What I suspect is that the 34 runs with the tumbler gears and there is another gear behind the 54 that is always in mesh, possibly another 34 to cancel out the first set of ratios ??

Without a better explanation or picture we are flying in the wind.

John S.

[DavidA]

JS,

Nope,  It's as I stated at the beginning.  34 on the spindle,  The tumbler (reverser) consists of three 21 tooth gears in a sort of tringular formation to allow for forward or reverse.  But as they are all 21 tooth they can be treated as one gear;  only the direction will be effected.
This 21 tooth gear drives the 54 tooth directly,  nothing fixed to it.

As most of you mention,  changing the 54 for a 34 will simplify things .
If it can be made to mesh with the tumpler gear, say using a banjo,then I could then use the constant trick to get


Leadscrew threads per inch X constant /threads to be cut per inch X constant.

If I use 5 as the constant I would get

6 x 5 = 30  and 8 x 5 = 40

or put another way,  40 teeth on the leadscrew and 30 teeth on the stud.  The 30 would be driven from the back of the 34 already on the stud. i.e. keyed together.

I may have to add another 34 or 30 as idlers.

I think.

Dave.

[andyf]

Yes, Dave, if you can equalise the spindle and stud gears, it would help a lot. Then, as you say,  a 30T on the stud and a 40T on the leadscrew with an idler to fill up the gap between would result in one full turn of the spindle imparting three-quarters of a turn to the leadscrew, and give you 8tpi.

Whatever gear combination you adopt, the gears are going to be expensive if they are only for a one-off job! 

As a matter of interest, what sort of lathe is it?

Andy

[Fergus OMore]

Some of us have guessed what you are using but none of us know what gears are left to use.

Perhaps a bit more information is called for.

[DavidA]

AndyF,

What kind of lathe.   Now that is a good question.

The answer is,  I don't know.

I did a piece on here about it some time ago where I covered my repair to the tailstock..  I think I entitled it 'Mystery lathe' or something similar.  I'll try to find the article and link to it.

I'm going to have a go at making the required gears.  Can't afford to buy at the moment.  I'm sure I can dig out some suitable scrap from work. may have to be made from steel and aluminium alloy though.

Fergus,

I'll have another root around my bits to see what other gears are available. Whatever shakes out in the end it will be a mixture of half 20 dp and half 18 dp gears.  Obviously not running together.

I definately will be trying to adapt to having  equal gears on the spindle and the stud.  No point in struggling.

Thanks for all the help so far guys,

Dave.

[Fergus OMore]

pre-War Colchester Bantam.

look it up in Lathes.co.uk

Cheers
F O'M

[DavidA]

Here is a link to a picture.

http://madmodder.net/index.php?topic=4024.0

Dave.

p.s.  It's in

The Shop>Tools>  Don't try this at home.

I note it is a year ago tomorrow that I took the picture of the bed.

[Fergus OMore]

Yes, indeed, David

Last night, I wrote to two of helpers  giving details of your machine. You might not know but I'd worked it out.

Regards

F O'M

[andyf]

According to http://www.lathes.co.uk/bantam/page2.html , the headstock and bed were integrated in a single casting on pre-war Bantams. But Dave's "Don't try this at home" thread shows his bed stripped down for painting, with no headstock in sight.

Andy

[Fergus OMore]

Agreed Andy but I do recall Tubal Cain writing about the origins of lathes and - as best as he was able - to identify where all the bits actually had their beginning.  Bantams were made over a long time span and I note that Myfords were also subject to 'loose heads' and cast in heads.

Yonks ago, I was wanting spares for a Zyto - sold by Sam Tyzack and Myford refused to confirm that the change gears were actually theirs.
Under my desk is a vertical slide- ostensibly from the Zyto but it has Myford casting numbers on it.

Conclusion- don't quite rely on what is written-- even from me.

[DavidA]

Fergus,

Sorry to disappoint you,  but it's not the same machine.  Mine differs in a few respects,  not the least being that the bantam carriage seems to be driven by a rack and pinion mechanism whilst on mine the handwheel connects directly to the leadscrew. A very inconvenient idea as it makes the gearing very high (about three turns sends it all the way from end to end) and it action is counter intuitive. I.e. clockwise turning sends the saddle towards the chuck.  As this is opposite to all the machines at work,  and to my Rufold,  I must not forget that.  I engage the leadscrew and turn it by hand if I need to feed carefully.

Also the bedways are completely different.

I will need to move it again soon,  taking care not to smash the tailstock, and will get some decent pictures when I get it outside. The lighting isn't really good enough to take them in the workshop

[DavidA]

Here is (I hope) a picture of the lathe as originaly obtained by me.




I'll try to find some more detailed, and hopefully in focus,  pictures.

Dave

[DavidA]

..









Sorry about the quality of these pictures.  Must have been one of my bad days.

Dave

[andyf]

Hi Dave,

From the changewheels point of view, it would be helpful to have an end-on photo of the  headstock to show the spindle gear, tumbler reverse gears, the 54T and the banjo, but with that big gear (or handwheel?) on the end of the leadscrew removed.

Quite understand if you don't want to lug the lathe out into daylight. If your camera's flash is kaput, maybe you could borrow one that works.

Andy

[DavidA]

Andy

I'll go have a try.  Hope I can find some decent batteries for the camera.

I may be gone for some time....


Dave

[steamman]

Hi If you send me the DP of the gears you are using on your lathe I may be able to help you with a complete set of gears for your lathe as I have quite a few spare,Dont worry they are not expensive  and could well help you out in the future.Ernie

[DavidA]

Ok guys,  Don't quite know how to put this, but here goes.

Andy,
Firstly,  The pictures of the gear end of the head stock will have to wait while I move the lathe.  There simply isn't room between the end and an adjacent wall to allow me to take any.

Now the embarrassing bit.
When I was looking (at a rather uncomfortable angle) at the gears I thought 'hang on,  the gear on the stud appears to be the same size as the one on the spindle'. So I pulled it of and yes,  it was (to within a few thou'). But on the face of it was stamped 54.
Then it dawned on me, it was 54 MM diameter (2.125 inch nominal),  not 54 tooth.  It was,  in fact 34 tooth. I had not counted the teeth on this gear,  just assumed that it was 54 from the stamp.
So the problem is solved;  it wasn't a problem in the first place.

The joy at this discovery was tempered by the thought of all the humble pie I will have to eat for wasting all your time.  Even now I am putting on my Armani hair shirt.

Ernie,

Thanks for the offer.  Using the formula DP=(Number of teeth + 2)/outside diameter I get (34+2)/2.125 = 16.94.  So I reckon that the DP would be 17. Allowing for machining errors.

As this seems a bit of an odd DP I would probably be better off with DP18 gears,  or even 20.
As I have a primary drive set from the spindle I can key DP 18 to the face of the stud gear and use DP 18 for everything else.

Let me know the price and where you are.

Dave.

[Fergus OMore]

16DP is a far more 'usual'  diametrical pitch as used by several older firms such as Drummond. 20 DP is Myford et seq.

Whether this is of use is unknown. Merely chucked in for information.

Are you sure about the leadscrew?

Regards

[andyf]

 

Then it dawned on me, it was 54 MM diameter (2.125 inch nominal),  not 54 tooth.  It was,  in fact 34 tooth.

 

As you say, it doesn't matter what DP you use for the rest of the gear train, unless you use the existing stud gear as an idler (though you could always compound a 34T of a different DP with it). Indeed, you could use metric Mod 1.5 gears, which are about the same overall diameter. Actually, a 34T Mod 1.5 gear has an O/D of 54mm, which makes me wonder if Mod 1.5 is what you've got.

Cheap(ish) Mod 1.5 gears here: http://www.technobotsonline.com/gears/mod-1.5-steel-spur-gears.html . Teeth might be a bit wide though, at 17mm, and you wouldn't want the boss. I got some Mod 1s from them for my lathe; they had 15mm wide teeth and bosses, so I sliced off what I needed with the bandsaw, faced up the rough side, and then spent an age getting the resulting burrs off all the teeth. At first sight, it looks like they only supply up to 46T, but if you scroll down there are a further two pages going up to 120T. 

Andy

[DavidA]

Andy,

I also wondered if I had a metric gear.  Checking the 34 tooth gear on the spindle (which looks original) it measured 2.1 inch dia which would give me 17.14 DP.  So it looks as if it is a 17 DP set up with the stud gear being a 34 tooth mod 1.5 just tacked on to do the job. It doesn't mesh very cleanly.  I've never used the screwcutting gear on this machine. I usually do that at work. Much easier to flick a few levers to Get whatever thread I want.
But as I'm thinking of retiring (again) in a couple of month I need to get my screwcutting capabilities sorted out.

Fergus,

I'm sure about the leadscrew,  rather I was sure, I now need to check it again.

Dave