Warco VMC Leadscrew Thread Form.

[Paul BarrettParticipant @paulbarrett57424]

I have decided to renew the Screw Nuts on the X & Y axis of my Warco VMC mill as there is serious backlash now despite adjusting them. Having stripped the mill and removed the nuts and screws I am just wondering if anyone has done their's as I am a little confused by the thread size.

I have measured the 2 leadscrews and have established that they are definiteley 3mm pitch but the od is a little odd. They measure anywhere between 22.1mm to 22.25 which would suggest possibly 7/8ths inch diameter. The smallest diameter is not where the nut runs either as you would think but at the ends. Measuring the core diameter as best I can is 18.95mm which if you apply 2 x 0.5 thread pitch (Acme form) makes it 22mm od.

Anyone out there know the exact thread size/form (acme) who have done this themselves. I know I could just but some new nuts from Warco but they are not exactly well made and I have a mod in mind anyway.

Edited By Paul Barrett on 19/01/2014 14:06:24

[Paul BarrettParticipant @paulbarrett57424]

[Sverre KvaloeParticipant @sverrekvaloe]

Well, I have the same mill and mine are 24x3mm trapeziodal threads at least very close to 24. I guess they differ a bit as most chinese things does.

[Howard LewisParticipant @howardlewis46836]

Since the pitch is 3mm, the thread is certainly going to be metric.

No one in their right mind, in volume manufacturing, is going to cut a 7/8 inch by 3mm pitch ACME thread, are they?

The worrying thing is that the OD varies from 22.1 to 22.25, which would be oversize for a 22mm, and grossly undersize for a 24mm.

Suggestion

The backlash is because the nut is a loose fit on the leadscrew, so why not experiment cutting a nut to 22 x3, and see how it feels? Adjust the O.D. until you are happy with the feel in steel (lubricated – you don't want any pick ups to damage the leadscrew) When you are happy with the fit and feel, for each leadscrew, then make the final version out of your chosen material, (Cast Iron, Brass, Bronze, Phosphor Bronze etc).

It goes without saying to lubricate the screw and new nut well before final assembly.

If you want to minimise backlash, split the nut crossways, and fit a screw to allow you to compress or expand the nut longitudinally to reduce backlash. In future you could use this to compensate for wear.

Howard

[John Stevenson 1Participant @johnstevenson1]

Posted by Howard Lewis on 21/01/2014 20:58:46:

No one in their right mind, in volume manufacturing, is going to cut a 7/8 inch by 3mm pitch ACME thread, are they?

Howard

Myford's used to use 1/2" x 2mm pitch and 13mm x 10 tpi

I have the taps here.

[John HinkleyParticipant @johnhinkley26699]

I'm neither a user nor owner of one of these particular mills but, surely, if you are intending to replace both lead screw and nut, then it really doesn't matter the exact size of the thread – so long as they match, of course! A quick google search reveals a number of suppliers of ready-made lead screws as well as acme threaded rod up to 40 feet long! Get some of this and turn it down to suit.

Good luck,

John

[AndyPParticipant @andyp13730]

Posted by John Stevenson on 21/01/2014 21:21:52:

Myford's used to use 1/2" x 2mm pitch and 13mm x 10 tpi

I have the taps here.

To be fair John Howard used the phrases "in their right mind" and "volume production" so is Myford a relevant example ?

Andy

[Paul BarrettParticipant @paulbarrett57424]

Thankyou for the replies.

Arcticturning. They probably are trapezoidal form but with 1 degree difference between acme and trapezoidal it would be difficult to measure. My screws are definately not 24mm diameter.

Howard. I would not put it past the chinese to produce a weird thread although i am more inclined to think it is suppose to be 22mm. I have come to the conclusion this vmc is a rather large paperweight. The dovetails look like they were cut with an angle grinder. No finesse at all. All the leadscrew shafts bearing points are at least 0.1mm undersize and look like they were filed round rather than turned.

John H. I don't intend on replacing the screws only the nuts. My plan is to make steel nut bodies with a pushed in/bonded phosfor bronze sleeve and cut the thread in the bronze. I have looked at the available leadscrews and for the size replacement it would have a pitch of 5mm which is a bit coarse.

Thanks again.

[AeronutParticipant @aeronut]

Just a thought Paul have you tried ringing Warco and speaking to Dave in the workshop he's been helpful to me in the past it used to be option 6 off their main number.

Regards Lee

[LathejackParticipant @lathejack]

Hi Paul.

What year is your machine, my VMC dates from 1997 and was made in Taiwan and has table feed screws that measure around 22.2mm. The current slightly heavier built Chinese made versions may have something different.

Regarding the dovetails, certainly on the Taiwanese version they can be a bit of a mess, as well as other mating faces on the head and turret which I have corrected. While the ground finish of the guideways on the column, the underside of the table and the top of the knee of my machine are excellent, its the 'hand finished' surface of those on the vertical face of the knee and the underside of the saddle where the massacre has taken place.

It's clear that the tools used for this brutal butchery on mine range from rough files, angle grinders and a big scraper of some sort. They have dug out a concave surface on the saddle and knee leaving four raised areas and then concentrated on these to get the alignment. It is possible to insert a 20 thou feeler gauge between the slides in places. The poor contact area left by this has led to scuffing and early signs of wear on the nicely ground surface that they mate with.

Despite this, and using much better quality machinery over the years, I have always liked them and it does perform very well in a home workshop. Years ago I visited the Myford works and was shown into one of the back rooms. In here was a new Warco VMC they had been looking over, and they hated it, amongst other things making much of being able to stick the feeler gauges in between the slideways.

Myfords own version of the VMC was a different machine, Warco used to offer an identical type in the 1980's. I think the Myford VMC had just been discontinued, and Malcolm Townsend of Myford recalled to me one of the few problems they once had with it was the manufacturers habit of using a bit of emery tape on a stick of wood that they would ram up the spindle to 'finish' the taper. They quickly put a stop to that.

 

 

 

 

Edited By Lathejack on 22/01/2014 22:12:37

[Paul BarrettParticipant @paulbarrett57424]

Hi Lathejack.

I have sent you a p.m.

[IDPParticipant @idp]

Paul,

Have you considered replacing the screws with 'ball nuts' and screws which can be obtained at reasonable cost these days, almost zero backlash!

I noticed one of the correspondants suggesting slitting the feedscrew nut. Instead of this consider making a second much thinner nut which is placed at the opposite end of your 'nut carrier' Around the circumference of the thinner nut drill a series of small holes. The nut carrier then has taped holes to match and by careful adjustment backlash can be eliminated. It just means you have to make two feedscrew nuts per axis rather than one, both nuts effectivley working against each other.

IDP

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