Myford leadscrew

I appreciate that the metric vs inch debate will rage for a while longer, but for a 40yo who gets lost in the 1/8ths & 16ths...and mostly wants to cut mm threads, I have a question.

I have my Myford ML7 cutting metric but, it is a pain. So is there a web page that outlines the construction of a metric lead screw & I threading dial & how it works with the std gear ratios etc? I'm thinking that I could swap lead screws & dial, and metric threads would be so much easier.

Anyone done it?

Reply to
Richard
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Oooooh now you've done it:) You'll have the purist lynch mob after you shortly...

I don't know if its been done but others may know different. Myford never made a metric leadscrew for the ML7 so it's not a bolt on, and the nearest equivalent to 8TPI standard would be one with a 3mm pitch (but with an obvious 1.4mm difference). You would also have to make up the clasp nuts to fit this as the standard ones wouldn't work. Best bet if trying this route would be to get a leadscrew and clasp nuts from a chinese import that has this size. Can't help with the thread dial indicator as I've never had one apart, but see last sentence for a possible source. As for working it out from the standard geartrains the formula is simple. (Drivers / Driven) x input speed = output speed. So you just need to work out the ratios you would get. Idlers don't count.

Or fit a QC gearbox perhaps, theres one here

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another one here
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. I haven't got one so dont know how well this would work, but as well as the gearbox I think you need a seperate quadrant and some more change gears. HTH

Peter

Reply to
Peter Neill

In message , Richard writes

Just a thought...

1" = 25.4mm (i.e. 1/2" = 12.7mm) means that 127 teeth in a gear train results in the saddle moving in metric increments per lead screw revolution. Does that plus a DRO on the saddle open a few doors?

A few years ago there was a spate of correspondence in either ME or EIM where people were publishing tables of gear train sets that gave very low errors in percentage terms without the 'magic' 127.

Reply to
Mike H

To the original poster, it is well worth splashing out about eight pounds for Martin Cleeve's excellent book "Screwcutting in the lathe". This will tell you everything you never wanted to know about lathe geartrains, including exact metric conversions and very good approximations.

As Mike said, a 127 gear is needed for an exact conversion: Myford will sell you one for about £40; a well-known cynical trader will sell the same for 40% of this price. For 'approximate' conversions, a 21 gear is useful; again the c-t has good prices.

A further option that saves a bit of maths is to use the ML7 gear progeram, available here:-

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uses a list of the gears you have to give a ratio as near as you need for practical purposes, _and_ it checks that the combination will actually fit onto the machine.

HTH.

Reply to
John Montrose

As peter mentioned Myfords have never done a metric leadscrew for the

7 series even though they sold these as metric machines.

Again not wanting to fan the flames of the imperial/metric war but there is an advantage in staying imperial when threading.

For the record I'm used to working in both and have full imperial and full metric machines, including leadscrew's.

Lets look at it logically. In imperial threading on imperial machines the threads per inch are all fractions of the leadscrew thread, very simple, and the thread dial is usually 2x or 4x the leadscrew tpi. So using examples on an 8tpi screw if we gear 1:1 we get 8tpi. If we gear 1:2 reduction we get 16 tpi.

20 tpi is expressed by 8/20 = 2/5 so 20 driving a 50 gives the same.

In metric threading everything is worked out as a decimal as we don't cut full numbers. Take a 5mm pitch screw, if we gear 1:1 it will cut 5mm pitch, 1:2 reduction will give us 2.5mm pitch. However when we get onto 1.75 pitch weird numbers come into play. The result of this is that a thread dial with a single gear can't be used. Most metric thread dials usually have 2,3 or even 4 different gears depending on leadscrew pitch.

As we all know when cutting metric threads on an imperial setup you don't open the half nuts but wind back and forwards.

In practice on a metric lathe doing metric threads it's easier to follow this same method than piss about with the extra gears and leadscrew dial.

My metric lathe came without a leadscrew dial and they had to obtain one for me. I then had to modify the guards to allow this to work. After two threads it was disconnected and left hanging on it's mounting bolt where it is today. It's far easier to reverse back out and not worry about picking the thread up. Perhaps Myfords were aware of this or at least went this way to stop the uncertainty of using metric leadscrew's ?

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

Hi Guys, Many thx for replies. (sorry re duplicate post, not sure what happened to my outbox)

The issue I have is not so much re accuracy, its the reversing process. I don't have a drum switch or even a reverse switch at the moment to reverse the motor. so I'm stopping the motor while pulling the tool out, and revesing the leadscrew.

I have worked out a switch setup to do the reversing of the motor, but I'm concerned re the chuck coming loose so I havn't fitted it as yet..

Since I'm a beginner I tend to take very light cuts, which means many passes which means turning motor on/off many times.

Since it seems a metric leadscrew doesmn't sem to be an option, what about a clutch? The ML7 super has one I think, can I make one for my ML7? Basically thinking, I want to disconnect (rather than turn off) the motor, change the leadscrew direction, then re-engage the clutch. Anyone done this? Any plans on the www?

TIA for any ideas / info.

Reply to
Richard

Richard, Changing the direction of the leadscrew is a no no, To do this you need to change the position of the tumbler gear and unless you take great pains at every change over, marking gears etc you will get crossed threads.

Fitting some form of reversing switch / system will be far easier in the long run than other methods.

I no longer run a Myford but I did for many years, Initially on single phase with stop start buttons and a toggle switch [ never did like those drum switches ] and later with a three phase motor where you could just plug reverse. I did many threads off all descriptions, in later years the Myford had to run commercially and I never had an instance of the chuck unscrewing at threading speeds.

If I had to own / use one today I would fit a three phase motor and invertor control. One advantage of this is that it would allow the return feed, out of cut, to be done faster. I would also invest in a retractable threading toolpost.

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

I never knew that. Wondered why my original chester lathe came with a couple of these, it just goes to show the benefit that experience brings to this group. Thanks for that John, its yet another useful snippet to file away for future reference.

Peter

Reply to
Peter Neill

Some of the more expensive lathes have matched dials to gears. the idea is you swap the dial and the gear as a set. The dial usually has only the marks needed to prevent errors, some only having one mark. Tables tell you what gear and dial to use per pitch.

Nowhere near enough effort has been put into designing using metric screws. The reason being that as industry came to start using metric with a vengeance the CNC lathe and turning centre appeared on the scene making and further work redundant

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

In message , Richard writes

{snip}

Standard on a Holbrook 8B (and the larger Holbrooks too I suspect) is a dog-clutch cum reversing lever operated from the carriage position. I guess that both forward and reverse direction for the lead-screw and separate fine feed drive are permanently engaged and the dog clutch switches between - but they don't make 'em like that any more!

If you live anywhere near Gloucester you are welcome to have a look at mine but since it is closely matched with the gearbox I doubt the practicality of a DIY approach.

Reply to
Mike H

Two ideas, depending on the coarseness of the thread to be cut (and in some respects the length of it): leadscrew handwheel and/or mandrel handle.

If you screwcut in backgear, you can disengage this at the end of each pass (do not touch the tumbler reverse or the half-nuts), and wind the carriage back using either of the above.

This might also work even not in backgear and without a clutch, but your belts would need to be very slack...

HTH

Reply to
John Montrose

Sorry no. When in back gear the spindle runs thru the backgear train. So disengaging the back gear will cause an error when you re engage as the train is broken. In fact if you disengage the back gear and turn the chuck, only the chuck will turn.

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

John S., I don't believe you are correct; the drive (from the countershaft) comes onto the cone pulley and drives the cone pulley pinion directly, the pinion drives the backgear and that, in turn, drives the bull wheel which is permanently locked onto the spindle. The 25 tooth pinion that drives the change wheel chain is also permanently locked onto the spindle, therefore with backgear selected (the key locking the cone pulley pinion and the bullwheel is disengaged) but the backgear shaft disengaged, the spindle is free to rotate independently of the cone pulley but remains solidly coupled to the change wheel chain. Martin

Reply to
Martin Whybrow

Ummm this sounds interesting. Hope guys can agree on this, I'll wait & see. But if Martin is correct, then to cut a metric thread, I'd do the disengage of the backwheel then manualy wind the lead screw back. Sound like this might be hard as i would need to turn the leadscrew & all the gears also?

Wait & see what you guys agree on. (in the mean time could someone possibly take a look at my wiring jpg in other thread to see if it will work?)

TIA

Reply to
Richard

Durh, Sound of wet kipper smacking forehead

No Andrew, MY KIPPER keep off.

Martin of course you are correct as the bull wheel is keyed to the spindle and so always drives the gear train.

No excuses, total brain fart, my apologies to all.

Where's that kipper again, ???? bastich he's pinched my freaking kipper.

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

Richard, Drop me an email ASAP

-- Regards,

John Stevenson Nottingham, England.

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Reply to
John Stevenson

I think he is. However, this trick may be unique to the way the Myford back gear operates, hence where the confusion may have arisen. I'm sure I read the tip in one of the GHT books, which has a definite Myford leaning.

Winding back is not too hard depending on what you turn. For example, if you use the standard Myford fine feed set-up, you would have to turn the mandrel (because you are gearing the leadscrew down/mandrel up by such an extent). The leadscrew handwheel works for 16 tpi (2:1 ratio) but as the pitch gets finer, the effort increases. A further trick which I've seen mentioned is to use a cordless drill with a socket held in it to turn the appropriate part.

Reply to
John Montrose

Thx John for offer, but I got it sorted. The diag I posted worked great. It did hiccup a couple of times when going to reverse, Had to spin motor by hand a little. And I accidently hit revese once, while I was running & it switched...thought it would not do that.

But I cut my 10x1.5mm threads OK (for me). :-)

Thx again

Reply to
Richard

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