thread / screwcutting and gears

[NobbyParticipant @nobby]

HI Llyod & Guys
Yes you are right . and the end of the thread you must retract the tool if you can run into a groove . diseengage the dog clutch wind back to start postion set depth on x slide engage the dog clutch etc the gears on in constant mesh . the 2&1/2 Exe lathe company it was good selling point for them. I have screwcut 10tpi LH acme thread on mine over about 8" witthin 1/2 a thou I have since fitted a dog clutch on my flatbed drummond works a treat not on the leedscrew but on the end of the gear train. on my exe lathe the dog clutch is on the spindle see photo
Its not a dim question we all learn from each other on tis forum Regards Nobby

Edited By Nobby on 13/02/2013 22:57:32

[Lloyd BowersParticipant @lloydbowers78291]

I have that one too. But I also have the one on the leadscrew. Why would I have two?
Wish my exe looked that good…

Edited By Lloyd Bowers on 13/02/2013 23:34:47

[RaggleParticipant @raggle]

I was a little puzzled at the selection of gears you got with your lathe, few of which were multipled of the magic number 3. So I did an Ebay search for Boxford change gears today and and got 28 results where multiples of 3 are rare indeed. One was for a complete set of 11 and the same applies. That vendor stated a bore of 9/16"

I suspect that you have Boxford or SB gears. My gears look like this

**LINK**

7/16" bore and pinned instead of keyed.

I can only suggest that you (and I) make a determined search for a supplier. It won't be cheap, but you won't cut regular threads until you have them. For the small lathe the basic set is 18t to 39t as Nobby said earlier. On your lathe simple train would give 6 to 13 tpi so you would need a 2:1 (say 18/36) compound to give the range 12 to 26 tpi – I hope that's right, it's late

the 38t wheel on the leadscrew would give a 2mm pitch (to an error of 381/380 or 0.99737….) so a 200mm length of thread would have 99.7 threads instead of 100. Other metrics are compounded up or down.

Ray

[Lloyd BowersParticipant @lloydbowers78291]

Yes I agree. I think only one or two of mine like like yours. Most are keyed and 9/16th bore. I’d say most of my pegs have a steel sleeve over them. Possible to make them 9/16th?? But I think my leadscrew is a keyed gear? Is yours?
My main reason for this thread is to get to grips with how to choose the right gears and to find out what I can cut with why I have, ie what can I do. It would be nice to have a correct set.

[Lloyd BowersParticipant @lloydbowers78291]

heres a photo of the lathe when i bought it.

you can just see all the gears and the two clutches.

[Lloyd BowersParticipant @lloydbowers78291]

Just trying to ensure i got the calculations correct for if i know what TPI i want to cut. can someone confirm this? i guess its easier as i dont have many gears to chose from? we it makes sense to me! lol

TPI that will be cut = 8 x Driven1 x Driven2 / 24 x Driver2

simplified to TPI to cut = (driven1 x driven2) ÷ (3 x driver2)

So 20(TPI) = Driven ÷ driver (so driven ÷ driver = 20)

rearanged to be

If no compound 20 x driver (3) = driven (60)

Compound 52/32 20 x (3 x 32) = 1920 or 1920 ÷ 52 = 36.9

so I could use spindle (24) driving 52/37 (or 36) driving 47 to give a 20.04 TPI with a 37, or 19.5 with a 36

for a 1mm metric

Compound 52/32 25.4 x (3 x 32) = 2438.4 or 2438.4 ÷ 52 = 46.89

so I could use spindle (24) driving 52/32 driving 47 to give a 25.46 TPI

I know these may not be useful threads, but im more after getting the calculations correct.

[AndyfParticipant @andyf]

Lloyd, I haven't worked through all your reasoning, but you last calculation for a 1mm (actually 25.46 TPI) pitch is certainly correct. So is the earlier one where (without a compound gear) you arrive at 20 TPI.

It is a very longwinded business to work out what gears you need to use to get a particular thread pitch. Programs like Nthreadp are only really helpful when you have a pile of gearwheels and want to know if they can be set up to cut a particular pitch. They don't help much at all when you only have a few odd-sized gears and want to know which extra ones might usefully be bought to cut standard pitches, or close approximations to standard pitches. As you said earlier, 3 is the magic number where inch threads measured in TPI are concerned. The 3 derives from the 24 teeth on the first driver divided by by the 8 TPI of your leadscrew, and means that for inch threads you need a set of gears where the tooth count increases by three, like the one I suggested. As an aside,if you had a 40T gear on your spindle, you would need a set in increments of five (40 ÷ 8 = 5).

However, help may be at hand. If you load the seven gears I suggested (24 to 42T in increments of three), plus another 24T to represent the one on your spindle, and also a 63T, and then try to find combinations for the standard metric pitches of 0.8, 1, 1.25, 1.5, 1.75, 2, 2.5 and 3mm (those being the metric coarse pitches for M5 up to M27) you will find various combinations offered with 24T as one of the drivers, and all within 1% error.

So, I suggest eight gears: 24T to 42T in increments of three, plus a 63T. If you want to get at the finer TPI pitches, you will also need a compound pair with one having twice as many teeth as the other.

Two last points:

It doesn't matter in which order the gears are assembled, as long as the ones specified as drivers are drivers and those specified as drivens are drivens. From your point of view, this means that where a compounded pair is needed (driven by your 24T spindle gear, and driving another gear on your leadscrew), it doesn't matter which of the drivens forms part of the compounded pair and which goes on the leadscrew. Swapping them can help when it comes to the practicalities of assembling the geartrain on your banjo (aka gear quadrant).

Gaps can be filled in with idlers, but bear in mind the reversing effect of each additional idler you add. Of course, this is of no consequence if your lathe has a tumbler reverse, which you can set so the leadscrew rotates in the direction you want.

My brain cell has got tired with all these sums, so I'm going for a pint.

Andy

[Lloyd BowersParticipant @lloydbowers78291]

Once again thanks very much. Enjoy your pint

[Lloyd BowersParticipant @lloydbowers78291]

with your help ive compiled what i can cut with upto 2 compound gears. but there doesn't seem many useful pitches, a couple of metrics, so i defiantly need to look for more gears as you mentioned above andy!

Can someone tell me what they normally accept as within tolerance?

Thread's to be cut
MM		TPI
0.608735501		41.8244
0.875057283		29.09524
0.894080268		28.47619
0.98919519		25.7381
1.000065466		25.45833
1.02180602		24.91667
1.130508788		22.52083
1.257894845		20.24017
1.285240385		19.80952
1.313180393		19.38804
1.421968085		17.90476
1.437594108		17.71014
1.452880435		17.52381
1.468846154		17.33333
1.500777592		16.96454
1.607442183		15.83883
1.625106383		15.66667
1.642964695		15.49638
1.660434783		15.33333
1.678681319		15.16667
1.715174391		14.84397
1.837076781		13.85897
1.857264438		13.70833

 

Edited By Lloyd Bowers on 15/02/2013 11:48:11

[Lloyd BowersParticipant @lloydbowers78291]

MM		TPI
1.897639752		13.41667
2.099516321		12.1266
2.111466346		12.05797
2.157367788		11.80142
2.336090426		10.89855
2.386875		10.66667
2.413104396		10.55072
2.438763587		10.43972
2.465563187		10.32624
2.640797872		9.641026
2.669817629		9.536232
2.698206522		9.435897
2.727857143		9.333333
2.787158385		9.134752
3.018054711		8.435897
3.083664596		8.25641
3.544247081		7.183472
3.921294643		6.492754
4.006540179		6.35461
4.338453647		5.86845
4.432767857		5.74359
4.529132376		5.621386

 

Edited By Lloyd Bowers on 15/02/2013 11:49:38

[AndyfParticipant @andyf]

Lloyd, I reckon that for simple "hold things together / nut and bolt purposes" it's OK if you are within about 1%. For example, over 6 turns, the cumulative error on your 12.1266 TPI thread will only be be about 0.005".

It looks as though you could get pretty close to 1.0, 1.5, 2.0, 3.0 and 4.0mm pitches, though the last one is coarser than the leadscrew so the gearing would be against you (but you could always turn the tables by handcranking the leadscrew so that it drives the spindle via the gears). You have nothing near enough to 1.25mm (as on M8 coarse) or 1.75mm (M12 coarse). Among the inch threads, only 12 and 20 TPI seem to have tolerably close versions in your table.

Andy.

[Lloyd BowersParticipant @lloydbowers78291]

Ive been given a face place, but the thread diameter is too small for my head stock, do you think i can thread it out to fit? would it sound right to think my spindle thread is 12 TPI, at a quick thread diameter of 1"1/16-1"1/8, where the face place is currently 15/16"-1" doutside thread diameter. if both are 12 TPI can i keep turning the inside thread of the faceplate at 12TPI till i bore out far enough? or am i talking out of my …behind!!!

P.S. remember in still green!!!! thanks

[NobbyParticipant @nobby]

Hi.
That woud be fine , to rebore it& srewcut the thread . but make sure the locating diameter is a good fit. Double check the tpi of your lathe and the locating diameter . Bore out to the core diameter of the thread required . Then check the depth of thread from the tables. when its on the machine i use a felt tip pen and mark the Dia. so you can see when you touch the bore with the tool & set the the dial to zero so you know when you have reached the depth . I cant recall the depth for 12 tpi I will have a look
tomorrow . PS i am asuming you can hold it for machining
Nobby

[AndyfParticipant @andyf]

As Nobby says, you really need to bore it out to the minor diameter of the thread (and dead on the register diameter), rather than trying to screwcut the existing thread deeper and deeper until it fits the spindle. And you need to make sure you don't overrun the thread into your nice new register diameter. If you only have the one lathe, not only do you have the problem of holding the faceplate while doing all this machining, but you can't remove it it and try it on the spindle for fit until all the machining has been done. Apart from trying to do ot by dead reckoning, I think the only way to get it right is to make an exact replica of your spindle nose which you can use to test for fit as you go along.

Andy

[Lloyd BowersParticipant @lloydbowers78291]

Ah, it just fits past the bed for fitting or trying to fit to the spindle. not sure if it fits in the chuck and still miss the bed.

[AndyfParticipant @andyf]

Lloyd, I'm not entirely sure about this, but if your spindle nose thread is 1.125" and 12TPI, and there is a plain register diameter of 1.25" behind the thread, it will be very like a Myford spindle. In that cae, it might be easier either to acquire an old Myford faceplate or a new compatible one like the second item on this page.

Myford threads are 55 degree Whitworth form, and your EXE is probably the same, given its age.

Maybe you could measure up your spindle nose more accurately, and post the outside (major) diameter of your thread, the length of the threaded section, and the diameter and length of your register diameter. I'm sure some of the Myford owners on here would then advise whether a Myford faceplate is likely to fit.

Andy

[Lloyd BowersParticipant @lloydbowers78291]

the spindle is 1" 3/16th as a quick measurement and about 1/2 long. i think ill try and make it fit, if all else fails least ive practiced and learn from it, then if it all goes down the pan, i can buy one.

cheers all for your help.

[Lloyd BowersParticipant @lloydbowers78291]

well, it worked, talk about a steep learning curve, cut my own 55deg internal thread tool form hss steel. then turned the thread into the inside of the faceplate. (well after flattening the old thread) and now just touching up the outside. but it all worked well, even using the gears etc to get the right thread, i did turn it by hand though to cut the read, ensuring slow speed!!!

thanks all for your help..

Edited By Lloyd Bowers on 24/02/2013 00:01:42

[AndyfParticipant @andyf]

That's great, Lloyd. Turning the spindle by hand takes much of the drama out of a stopping in the right place on a fairly coarse thread where the tool would gallop along under power, particularly when it's a short thread only involving a few turns.

Clean up well; if the faceplate is cast irin, there'll be black dust everywhere, and it's abrasive.

Andy

[NobbyParticipant @nobby]

Hi Lloyd & Guys
I use one of these for turning the spindle when dieing & screwcutting a sprung loaded handle for safety it pops out when you leave go.

Nobby

[Lloyd BowersParticipant @lloydbowers78291]

good idea, i also found out that ive been a muppet, and the numbers stamped on the gear ar not the what all the gear are, so ive got 60, 52,46,32,32 and 28, which does give a bit more of a range, oh well, as they say, we learn something new every day, and it never stops.

[Author]

Posts