Unimat 3 carriage feed screw - material used.

Hello:

Given your knowledge and experience in these matters, I cannot but accept your point of view as the most valid one.

Of course, there is no other way. 8^)

Thank you very much for taking the time to write it up, much appreciated.

Best,

JHM

Edited By Julius Henry Marx on 01/07/2023 13:47:28

General point, emphasising what the wise Graham Meek wrote, a few posts back.

Feedscrew bearings, ideally, need to have zero axial float, should rotate smoothly, with no torque variation around the circle, and they must not bind (stick-slip phenomenon). It's all very well trying to get a low-friction bearing, but definite drag is needed, to keep the screw from rotating, as a result of vibration, and not helped by unbalanced 'balanced' handwheels. This 'unwinding' is a particular problem with coarse-pitch, small diameter screws (eg Myford cross-slide), because of the unfavourable helix angle. But it can also afflict screws with less steep helix angles, and provoked Graham M to make a lock for his Maximat's cross slide screw. It can therefore be useful to have a bearing arrangement that can be pinched up or pre-loaded to provide (consistent) drag. Apparently simple things can get complicated, when there are conflicting requirements...

Hi Kiwi Bloke,

Some very valid points as regards stick slip and free rotation. It is a misconception that zero backlash is the best. If there is no play then there is no room for the lubrication film and this leads to accelerated wear. This wear then leads to backlash and the cycle continues. Whereas a regularly lubricated bearing will last for years without any adjustment.

On the Emco FB2 the instructions for setting the feedscrew backlash at the factory. This was set at 0.05 to 0.1 mm.

Regards

Gray,

Just a little point in passing as regards the cross-slide feedscrew nut on the U3. Backlash can be greatly reduced in this nut simply by using the taper of a screw driver blade to rotate the nut slightly before locking it up with the countersunk screw. I have found the blade is best inserted on the tailstock side of the cross-slide. With the adjustment being carried out on the least used portion of the thread.

Although the backlash has been reduced, because the nut has been rotated slightly there is always a reservoir of oil on the side of the nut not in full contact with the thread. Simple but effective.

Regards

Gray,

Please excuse this swerve off the original topic...

Graham, a space for lubricant in a plain thrust bearing makes sense, even if it results in axial float, However, with rolling element bearings, this float can be eliminated with impunity. I'm surprised by Emco's instruction, given that the FB-2's screws are supported by a pair of deep-groove ball bearings. Perhaps the instruction was to prevent gorillas applying too much pre-load to bearings with limited thrust capabilities. Any ideas?

Oh Bother! I'd just shut down the computer, when I realised I'd misunderstood Graham Meek's post. My misunderstanding is frequent - according to my dear wife... Anyway, it's the morning here, and I haven't had coffee yet, so you can't expect much sense yet.

Graham's reference to Emco's backlash adjustment presumably includes backlash in the nut, which must, of course be lubricated reliably, requiring space for the lubricant. In my earlier post, I was confining comments about axial float to the leadscrew support bearings only. Here, it seems reasonable to aim for zero end-float - easily achievable with rolling element bearings, but not simple with plain bearings.

Apologies for the confusion.

Hi Kiwi Bloke,

You have redeemed yourself, the misunderstanding was not helped by me not being more specific and mentioning the feedscrew nuts in my comment.

The comment about gorillas over-tightening the bearing end-float is another good point. I have replaced more of these bearings for friends who have purchased secondhand machines than any other bearing on the FB2. Once set at the factory these will need very little attention. It could be many years before these ever need adjustment and this will depend on the amount of use the machines get per day.

Regards

Gray

Edited By Graham Meek on 03/07/2023 10:37:03

One Tip I forgot to add with regards to the U3 Cross-slide feedscrew. I have modified the slotted M4 countersunk screw. Using a Ball Nosed slot drill as close to my grease gun nozzle as I could find. A depression is made in the head of the countersunk screw to just below the depth of the screwdriver slot. A small hole about 1 to 1.2 mm is then drilled through the length of the screw.

Grease can now be applied to the feedscrew in the centre of the feedscrew nut. The action of which forces any debris from the nut as the new charge of grease is applied.

I have not found this weakens the screw in the course of usage.

Regards

Gray,