Unimat 3 carrier rigidity problem

Hello:

Significant progess has been made albeit not without some annoyances, the net result being what seems to be a suitable outcome, at least for now. Time will tell.

I will most probably have to make some adjustments on the surfaces where the lift strips are mounted once they settle in with use for (as expected) the ways in this ca. 1980 U3 have some wear and the carriage gets stiffer as it moves near the headstock and tailsock but not too much.

As mentioned previously, an oil soaked felt pad was placed under each strip:

It is an experiment to see if, apart from keeping swarf out, will also aid in keeping the carriage running smoothly. An old but proven idea, looks like it will work as expected.

Here is what the final product looks like:

The length of the strips cover the whole length of the mounting surface which is 49 mm and their width is roughly the same as that of the brass stock my purveyor had on special offer at the time, 5/8".

Once I had everything in place, I took to cleaning up a short piece of ⌀ 25mm SAE64 bronze which I will use to make a bushing, with this result:

Granted, it is only 25mm worth of clean-up but I was very happy to see that my micrometer informed me that it is perfectly cylindrical, something I was absolutely unable to achieve a week ago.

It surprised me to see how much material had to be removed to get the strips to touch the ways as needed. I intended to measure it but was too concentrated in what I was doing and totally forgot.

If I had to guess, I'd think something like 0.02/0.03mm, at least.

As a side note, I would mention that while I was looking for a solution to the problem I was having with the U3 carriage, I recalled that that the Sieg C0/Grizzly G0745 were close clones/copies of the Unimat 3 and decided to look to what was available in terms of spare parts and if their lift strips would work on the U3.

The (steel?) replacement gibs these Sieg manufactured lathes use are held in place in the same manner as the Unimat (a pair of 6mm M4x0.7 screws) but each gib has an additional pair of threaded holes to hold set screws used to regulate the distance of the lift strips to the ways and keep them from being overtightened.

An interesting solution to this problem but probably not applicable to the Unimat 3 carriage: I think (?) the carriage used in the Sieg C0 is 1. made from steel and 2. does not have any voids so the set screws have a spot to lift land on.

That is all for the time being.

Thanks to all for your input.

Best,

JHM

Edited By Julius Henry Marx on 20/06/2023 14:56:58

Hello:

@Kiwi Bloke: thank you very much for taking the time to write up your very comprehensive explanation.

If you don't object/mind, I'll post the answer here as (I think) it makes more sense given this thread's context.

Indeed ... I knew it was not linear but did not know that it was that much ie: cubed.

Yes, the Sieg set screws jogged my memory and got me thinking.

I will take your word for that, you are far more experienced/knowledgeable than I. But I think you can agree with me that, at best, it looks like one intelligent choice vs. a great many negligent ones. The list is quite long.

Yes, I had been thinking all day about the wear the brass strips would be subject to. Even with good lubrication the strips would wear but then, (I think) not to the point of generating a gap between the ways and the strip. ie: the strips will suffer wear till they are not touching enough to cause more wear. Of course, at that point there's also swarf between ways and strips to take into account and that is when things will start to get complicated.

This afternoon I vaguely recalled a web page by some US chap with a Chinese 7x14 lathe (or similar) where he explained a modification he devised to allow him to adjust the carriage strips from above and on the fly. ie: no dismantling anything.

Cannot find the page now but the idea was similar in concept to what the people at Sieg did with the C0 strips so I set to see if I could do something like that with the U3 strips.

Here it is:

Each strip has acquired two 12mm M4x0.70 set screws, but unlike the Sieg C0 strips, the screws are not aligned with the screws holding the strips in place but offset so as to do this when adjusted:

The screws holding the strips are not tightened and the set screws cause the strips rock towards the ways.

It goes without saying that the amount of tilt shown in the photograph has been greatly exaggerated in order to show what happens when the strips are being adjusted. The strips do not load on the ways, they hover just above them on a film of oil so I expect that they will last a long time.

I finished the job a while ago and can say that it works really well: the slide (tested without lead screw fitted) goes from end to end very smoothly, with no jamming whatsoever and no discernible upwards movement at any point in its travel.

Of course, it has to be tested and used for a good while to see the actual results but (for now) it is looking good.

Once again, thank you so much for the explanation on how the strips work. Learned something today.

Best,

JHM

...carrying on from my post on your other thread (confused? So am I...).

The important thing here is, I believe, preload. It's difficult to arrange and control preload when a system is too rigid. Hence my belief that, counter-intuitive though it may be, Emco got it right. Or, at least more so than Sieg.

The Sieg approach looks attractive at first sight, but I suspect that it's another example of their blindly copying design ideas rather than thinking things through thoroughly. Compared to Emco's design and manufacturing integrity, I consider the Sieg machines to be seriously inferior in many areas. (I'm probably going to cop a lot of flack for that remark, and don't wish to start a war, so let's just leave it as a statement of a personal opinion...).

...and replying to your post above.

Interesting approach. No doubt your approach will work fine. Whether it will continue to do so as it wears remains to be seen. Realistically, it isn't going to wear much is it?

However, a few things to consider:

1. Tilting the strips will tend to cause line contact, which will be associated with greater wear. Don't forget that a softer material can wear a harder one if it gets embedded with abrasive - as in a lap. These strips need to be 'sacrificial'.

2. You could use Belville washers (or other spring washers) under the retaining screw heads to achieve preload. (IIRC, Emco use wavy spring washers).

3. The strips will certainly not float on a film of oil (hydrodynamic bearing), unless you make the gap large enough (and the saddle whizzes along) - and we don't want a gap, for obvious reasons. At the low speed a saddle will travel, and the loading necessary to pull the saddle down, even boundary lubrication is optimistic. There will be metal-to-metal contact.

4. Don't lose sleep over it. "She'll be right", as they say here in NZ.

I see your point, Kiwi Bloke

1. If the saddle was sufficiently heavy, then [with a prismatic bed] there would be no need for the strips.
2. Given that we can’t scale nature, [1] is not feasible in practice.
3. The original strips act as springs … thus substituting for the missing weight.
4. Comparatively very stiff metal plates, firmly bolted, turn the system into a linear bearing which does not rely upon Gravity for its operation.
5. For [4] to work … everything would need to be made/fitted/maintained to appropriate tolerances !

MichaelG.

The way Myford and many other lathes do it is have shim strips between the lift plate and the saddle casting. You peel off layers of shim, or fit thinner shims, until the desired clearance/contact is achieved. This has the advantge of keeping the whole lift plate parallel to the bed surface it bears on, resulting in contact over the full surface rather than the line contact of the tilted lift plates.

But some of the small Chinese lathes use the above pictured system with the canted plates adjusted by two grub screws and held down by two other screws.

Hooray! Michael's got it! Far more succinct than my rambling...

Apologies to Monsieur Belleville for mis-spelling his name (the shame...).

I wonder if a single plate relieved to clear the central rib would work better? You could have the jacking grub screws without the chance of single plates tipping and/or use some form of springs to apply a constant pressure.

Hi Michael,

You have captured in a nut shell the design criteria associated with the Emco Gib arrangement.

Regards

Gray,

Generally

Like Kiwi Bloke I think the current tipped Gib arrangement will need constant adjustment. Line contacts wear rapidly. If one takes a moment to consider the set-up which is shown in the above photograph. The only part in contact with the underside face of the Bed is the outer edge of the Brass Gib. I also fail to see how this arrangement is superior to a Gib which is firmly bolted down. As opposed to something which is standing on Stilts.

Perhaps this is why there been so many designs to improve this Gib set-up over the years.

The plastic Gibs of my Unimat show a distinct wear patch adjacent the hold down bolts which is in contact with the bedway face. This patch extends across the whole width of the contact face.

From what I can see in the above photographs there appears to be a ridge on the Vee-way which indicates excessive wear. Something I highlighted in the other post on this topic when I suggested "lifting" the Carriage to check for vertical play.

Regards

Gray,

Hello:

I'm sorry about that. I should have asked the moderator for instructions ...

Yes, it looks like it works but I cannot say for sure. To me, it makes sense and it should work but just how well I could not say, never seen a Sieg C0 (now apparently discontinued) myself though I have read quite a few negative opinions about it. In contrast, have read quite a few glowing opinions about the Unimat 3. 8^)

Thanks for your input and sorry for the confusion caused.

Best,

JHM

Hello:

Yes, I thought about it for hours while attempting to imagine just how it would work, if it would work, how much of the contact surface would be lost with the tilt, how much is would wear and at what rate, if it would need more lubrication, etc.

After making a pile of small sketches I settled on the ideas I am putting down below, obviously to be proven (or not) after some sustained use. Taking into account that I am not at the U3 six hours a day, that could mean a relatively long time.

I would be surprised if the wear over (say, 12 months) were to be significant enough to matter. ie: to create a situation where the carriage will act up and affect when turning a piece. The original strips showed wear along a ~3 mm band so checking the state of the brass strips in a couple of months should give us a rough idea / approximation.

As mentioned, the photograph showing how this arrangement works is an extreme to illustrate the idea.

In reality, when the strips are held in place by the mounting screws they are practically touching the ways before the set screws are turned. As mentioned, the set screws are M4x0.7 and I achieved what I considered to be right adjustment ie: smooth movement of the carriage without discernible lifting or jamming of the strips, with just a touch of the screw.

Just a touch (whatever quantity that effectively is) in this case means a very slight turn of the set screw. Given the pitch, this translates into a very small advance and as a result, a very slight lift of the trailing edge of the strip. ie: practically no pressure between the way and the strip and I assume (here's the scale factor again) a very small angle between both.

I expect that as time goes by, the leading edge of the strip will indeed suffer some wear but as this happens, the contact surface will be progressively increased.

Will have to check and see just how much further on.

Yes, the first thing that came to mind when you mentioned pre-load was the use of some sort of spring washer to replace the existing flat ones but have not come up with anything yet. All I have found are grover type washers (split) but I am not sure if they will work properly in this case.

Please excuse my being so optimistic, ie: ignorant. 8^) Slides would probably have been a more accurate term to use.

Indeed ... But I enjoy the challenge and in turn, learn something new.

The whole thing is working much better now. A day/night difference with respect to what I was going through when I started the post on the material to use for the gibs.

Thank you very much for your input.

Best,

JHM

Hello:

Interesting idea.

The first thing that comes to mind is that, given the height of the central rib, a single plate would have to be much thicker, maybe as much as ~10 mm. and there may not be enough room.

Thanks for your input.

Best,

JHM

Hello:

While I was writing my reply to your post, I had in mind mentioning this but eventually forgot. 8^°

It could happen that the tilt angle ends up being more than acceptable and cause equaly unacceptable edge wear on the strips. After all, this is an experiment, a work in progress.

But I think it can be easily solved by simply repeating the previous operation on the surfaces that the plates sit on. ie: removing a bit more material.

The optimium situation being that the surface coincides with the surface of the way so that the needed tilt is close to nothing.

I may eventually have a go at it and see if this arrangement can be improved but first I'd like to see what wear the strips show after a bit of carriage travel.

Best,

JHM

Hello:

An update:

After a fortnight or so I had to dismantle the carrier again to take some measurements and had a look at the new brass gibs. I then realised that they needed a tad more removal from the surface they are bolted on to, so went about doing that.

It was not a case of line wearing as the width of the markings on the brass were quite consistent with the markings I had measured on the OEM lift strips but as the use of the set screws was needed, I decided to go ahead and see what I could do.

It was a very small amount and the result was quite satisfactory, to the extent on not needing any adjustment from the Sieg C0 type set screws I had added.

But then I found out that the whole arrangement had become more difficult tighten, I guess due to the smaller dimensions at play and the 0.7 pitch of the mounting screws.

The solution (again) was to follow Kiwi Bloke's advise and use Bellville washers, so I added a pair arranged in series (on top of the OEM washer) to each screw.

The carrier now moves very smoothly and, as expected, gets tighter as it reaches each end of the bed.

I expect that, as the gibs wear the carrier will loosen up and then it will become necessary to use the set screws to adjust the gibs accordingly but I think it will take many hour's use of the U3 for that to happen.

Here's a photo of the gib below the flat as seen from the tailstock end, with the Bellville washers clearly visible:

It looks like it is as Kiwi Bloke said: She'll be right.
That's about it for now.

Best,

JHM

Edited By Julius Henry Marx on 23/07/2023 16:37:43

The reason why the Carriage gets tighter as it moves towards the end of the Bed is due to the geometry of the machine.

The Leadscrew centre-line is fixed in relationship to the Profile of the Slideways on the actual Bed.

Excessive wear on the Carriage Slideway Profile, now means the Leadscrew tapped hole will be sitting lower than was originally intended. This is not so bad when the Carriage is nearer the Headstock as there is some play in the system. As the Carriage nears the fixed Leadscrew bearing this will cause binding due to the misalignment of the two items. Any attempt to reduce backlash in the system will only exacerbate the binding.

This was, apart from wear in the original thread, one of the reasons I did the above salvage scheme on my Unimat restoration. As wear in the current Carriage set-up takes place a new Phos Bronze Leadscrew Nut can be made with a slightly offset tapped hole to cater for this misalignment.

The fitting of the new Nut brought the backlash back to 0.05 mm which is what I would expect. Although this is the original leadscrew of unknown age, there is no change in the above figure through-out the length of travel.

Regards

Gray,

Hello:

Sory for the delay in my reply.

I see.

Yes, I have noticed that there is some play at the left end of the leadscrew.

Makes sense. But when I was testing/adjusting the new gibs with the Bellville washers without the leadscrew in place, I observed that the was some tightness at each end of the run.

Yes. The Sieg C0 has a leadcrew nut that is adjustable in height, apparently to deal with that problem.

Good job that one. I'm still attempting to figure out how to get that done on mine.

Thanks for your input.

Best,

JHM

Edited By Julius Henry Marx on 26/07/2023 23:05:51