Is 6 tpi possible using a ML7 with a gearbox?

[TryingTurnerParticipant @tryingturner]

I'd be most grateful for some help with this challenge. I have a Myford Super 7 fitted with a quick change gearbox. The latter has selector positions down to 8 tpi. I have been asked by a friend to turn a thread at 6tpi. Is this possible, perhaps by some form of work-around? I have a full set of gearing to convert the lathe to metric turning of any of those gears would help in producing a revised gear train. Any advice would be greatly appreciated.

[TryingTurnerParticipant @tryingturner]

[LambtonParticipant @lambton]

To do this you will need to use the Myford "fine feed tumbler gear cluster "which you can still buy from "new" Myford.. Make sure you specify it for the S7 as the ML7 one is different.

This cluster is a direct replacement for the exiting gear cluster which incorporates the 24 tooth gear supplied with the gearbox. The new vital gear on the fine feed cluster has 12 teeth so it will halve all the marked thread pitches that can be selected using the gear box. So you would simple select the 12 tpi setting to obtain your desired 6 tpi.

A word of warning> it is generally not recommended on small lathes to screw cut threads with a pitch below that of the lead screw. I am sure if you only do it occasionally and take things easy you should be OK.

Hope this helps.

Eric

[TryingTurnerParticipant @tryingturner]

Hi Eric. Many thanks for your response, but I regret that I need to go the other way with the pitch and make it coarser (not finer) to achieve 6 tpi at a pitch of 0.166". If I've understood the workings of the gear train correctly (and I'm genuinely open to correction) halving the tumbler cluster gear from 24T to 12T would halve the rotation speed of the gearbox train, which in turn would halve the speed of the lead screw to result in half the selected pitch (i.e. a finer pitch and double the tpi). To solve my problem, would it work if I were to use the back gear? This would halve the rotation speed of the chuck with respect to the mandrel and gear train. In this way, selecting 12 tpi on the gearbox would achieve 6 tpi at the tool post. I'd appreciate your thoughts and any corrections in my understanding.

Your advice would be much appreciated.

Mike

[Harold Hall 1Participant @haroldhall1]

As you say you have the metric changewheels, I assume therefore that you have the special quadrant on which these are fitted.

If so, then go to this page on the web **LINK** and you will see there are around 20 combinations that achieve 6TPI with an 8TPI leadscrew. You should be able to find some that use the gears that you have. Obviously, the ones that use just two gears would be the first choice.

Just set your gear box to 8TPI and use your chosen combination.

Harold

[LambtonParticipant @lambton]

Mike,

You are quite correct about the effect of putting the 12 tooth gear on instead of the 24.toothe one. Sorry if I have mislead you.

However the use of back gear or not does not have any effect at all on the screw cutting ratio. It only alters the mandrel speed in relation to the countershaft speed. You should always use back gear when screw cutting on a Myford lathe.

Eric

[Ady1Participant @ady1]

Cutting a thread smaller than the leadscrew puts quite a lot of strain on the leadscrew nut

The easiest route may be to mill the bulk of the thread with a small powered milling cutter mounted on the cross slide, then finish it off in a conventional manner with a single point tool

Edited By Ady1 on 31/07/2013 08:45:58

[Brian WoodParticipant @brianwood45127]

Hello Mike,

There is a simple alternative to those Harold Hall suggests without using the metric quadrant.

Change the 24 tooth mandrel gear for one of 32 teeth and set the gearbox to 8tpi. That will give a 6tpi lead

You may have to extend the clamp slot at the bottom of the gear quadrant to accept the larger gear if it can't be swung down low enough. The gear loading will be higher than the designer allowed for so take it gently with light cuts.

Good luck

Brian

[TryingTurnerParticipant @tryingturner]

Hello Brian,

I’m sure that Harold’s method would work, but your alternative does sound simpler. It would also be easier to repeat the arrangement should I be asked to carry out another 6 tpi task. The 32T wheel has been ordered (sadly not in my spares box) and when it arrives I’ll give the process a go. I’ve noted the comments about the gear loading, and so lighter cuts will be taken. I’ll also need to read up about the use of the thread dial indicator with a 6 tpi thread! Thanks for your help; I’ll let you know how I get on by means of a later posting on this thread.

Thanks

Mike

[Brian WoodParticipant @brianwood45127]

Mike and John,

If you have one I would be inclined to use a mandrel handle so that you are cutting by arm power alone. It does allow considerable room for and time to spot mistakes or make tool depth adjustments if you feel the load getting excessive.

Thread dial indicator. Since you are cutting an imperial thread I can't see that being a problem, after all you wouldn't think twice in using it for 12tpi. A dummy run along the job without doing more than lightly marking it will give reassurance.

Brian

[TryingTurnerParticipant @tryingturner]

John & Brian

I regret that I don’t have a mandrel handle and I don’t have a spare handle to adapt but I appreciate that one would be beneficial for this job. I see that David Haythornthwaite made one (see the following link) **LINK** and am considering doing likewise from his plans. However, if you know of a source for one already made I’d be grateful.

Mike

[Brian WoodParticipant @brianwood45127]

Hello Mike,

Go for David's, people usually make their own and I imagine his will be as good as anyone else's.

I think you will find it a useful accessory in the years to come.It will certainly be handy for this job. In fact you will probably spend a lot of time making tools and fittings to make life easier for doing the things you want to do.

The whole activity is known as T to some unspecified power where T is task and the value of the multiplier depends entirely on your ingenuity and need for special tools not available outside your head!!

Brian

Edited By Brian Wood on 31/07/2013 17:08:36

[TryingTurnerParticipant @tryingturner]

Hello Brian,

I appreciate your advice. I’ve built my own sports car from plans, and know only too well about your T^x theory! I’ve made a number of tools that have proved invaluable when confronted by the need to readjust or modify! I now see that a ball turning tool has now been added to my wish list! This 6 tpi task is growing like Topsy!

Mike

[Sub MandrelParticipant @submandrel]

> Cutting a thread smaller than the leadscrew puts quite a lot of strain on the leadscrew nut

This is often stated, but I would argue the increased strain is as much due to the increased depth of cut.

An ML7 has an 8tpi leadscrew. cutting a 6tpi thread will only move the saddle 8/6 = 1 1/3 times faster, so only a modest increase in speed and therefore load on the leadscrew.

On the other hand the depth of cut for a 6tpi thread is 0.107" compared to 0.080" for 8tpi.

The increased gearing up can be compensated for by using a mandrel handle to turn the lathe slowly, or even driving the lathe using a leadscrew handle. You can't do anything about the increased depth of cut.

Neil

.

[Sub MandrelParticipant @submandrel]

Interesting observation John – its the gear train that suffers – not the leadscrew.

Neil

[Gordon WParticipant @gordonw]

I made a mandrel handle a few years ago, a bodge up using an old pulley with a handle screwed in, 'cos I needed it in a hurry. Still using it, esp. for threading.

[Ian S CParticipant @iansc]

I'v cut 4TPI threads under power on my 1324BH Taiwanese lathe, it has a 8TPI lead screw, the first ones were wood screws about 3/8" dia, quite a bit of metal to remove. Ian S C

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