Lathe crash!

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Jason ... Chris also wrote ^^^

MichaelG.

Hole in the 80T gear is 14mm, same as on the 63T gear. No detectable difference using a manual vernier caliper ( it's too cold this morning to go to the workshop and fetch the micrometers!)

The one remaining serviceable bush is a hard push fit on both gears, so I'm ruling out the gears being that much different from each other, if there are difference it will be in the order of a thou.

The thing I' cant figure out is this ; the cracked bush has a gap approx 2mm wide (see photos above), but the internal bore (10mm) and external dia (14mm) is exactly the same as the undamaged one.

So we have either broken Einstein's law for conservation of energy or somehow the material compacted itself, sort of collapsed - which might make sense if the bush is made from some sort of powdered bearing material which would have voids for holding oil.

Edit: Michael, yes on some it is a slide fit, but with no play, so it would be more appropriate for me to say a push fit as the bush will not drop into the gear without me helping it. Sorry for the confusion!

Edited By ChrisB on 28/12/2018 07:53:42

Well in my book a sliding fit would be something like H7 Hole and g6 Shaft so at 14mm dia clearance of 6 to 35microns, not quite the 300microns your 2% gives.

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O.K. ... evidently I was wrong

I have no better ideas, so I must leave you to it.

I will though, be interested to know what you find.

MichaelG.

It's not a case of right or wrong Michael, I can understand your difficulty trying to understand someone who's 2000miles away showing a couple of photos and some misleading text!

If I manage to get behind what's going on be sure I'll share it...but I honestly don't know.

Moving on I'm going for fixed pressed in bushes on all gears and dowel pins on the faces of the gears. Should I have a repeat of the mash up I had last week, the dowel pins should shear, which will be much easier to fix.

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For my education, Chris ...

could you please post a photo of the good bush, for comparison with that one ^^^

... perhaps I misunderstood the design !?!

Thanks

MichaelG.

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... I've obviously missed something

There you go, they are both damaged, one is just cracked at the bottom end but otherwise the shape is same as the original. The other is twisted through roughly 30deg

Thanks for that, Chris

... I was beginning to think I was losing my marbles.

MichaelG.

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After a quick measure of the image ...

Yes, I'm convinced that the material must have been compacted 

Edited By Michael Gilligan on 28/12/2018 09:24:03

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So, the failure mode is not entirely like that of Jason's Oreo biscuit

... Do we have a finite element modeller in the house ?

MichaelG.

Ah but you don't stop twisting an Oreo until it is is two

Well I braved the workshop and even though it is not up to controlled lab temperatures took a few measurements anyway

Bush OD, tried in several places and got readings between 0.5503" and 0.5506" ( 13.978mm to13.985mm) suggesting slightly oval.

ID of bush I did by slipping it onto a 10mm cutter shank which needed a push and it did not drop off.

Shank of tool measured 0.4935" (9,9949mm)

To get an idea of the internal diameter of the gears I turned the end of a bit of bar down to a firm push fit

Turned end of bar measured 0.5512" ( 14.0005mm). There was a fractional taper to the bore as I could slide it on part way before it tightened but if entering from the other side it was tight, bright area is where it has rubbed the surface.

So average clearance of 0.019mm which puts it mid way between the upper and lower tolerance limits I posted earlier

Pic of an unmolested bush. The small chamfer you can see at the end of the "key" was filed by me so they are not hard and do seem to have a granular look to them

Typical fit of bush in gear which were pushed together by hand with little effort

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Ah but ... both Oreo biscuits remain intact

These bushes seem to behave more like Marshmallow-topped biscuits

Thanks for the doing the measurements though ... Dimensionally it all seems pretty good.

MichaelG.

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Edit: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.220.2146&rep=rep1&type=pdf

Edited By Michael Gilligan on 28/12/2018 11:55:45

I would also hazard a guess that those bushes are made from sintered (powder metal) steel.

One of the characters of sintered steels (and there are many) is that they have a degree of self lubrication, like Oilite, so would be ideal in this application; and another is that they have a degree of porosity, and that would allow the apparent compaction in torsion shown in the photos.

You're right David, they are indeed sintered steel. Gave it a try on the grinding wheel and it emits yellow sparks. Then tried to squeeze the key with a plier and to my surprise it did give way - some oil oozed out from the surface.

A quick dimensional check showed the width reduced 5mm to 4.5mm - so that's the mystery solved I think.

Now that I ordered the new bushes I still think I have not resolved the problem as most probably the same will happen. Maybe I'm being too aggressive with my depth of cut when turning? What do you guys consider the max for the WM280?

I doubt it happened when cutting, more the result of your jam up. I have cut 0.25" (1/2" off diameter) in steel so they can take a decent cut though generally don't go that deep say 2.5mm roughing.

So is the oil retention an accident of cost saving or a deliberate design feature. If also designed to be weak the sheering of the key did not work in a way to remove the drive so perhaps not an intentional failure point. Perhaps a better solution would be to make a bronze bush with shallow slot and a plastic key, selecting the plastic to be weak enough to fail in a crash.

I M O the damage was caused by the crash, rather than normal cutting forces. The load involved in bringing everything to a sudden stop will far exceed anything that would normally be applied.

A suddenly applied force imposes double the energy of a gradually applied one (Plot force vs time - triangle instead of rectangle)

Possibly, inadvertently, the design incorporated this sintered bush as a failsafe rather than damaging more expensive components.

Having repaired my lathe after something similar; before final reassembly, I removed two the first two teeth of the Rack, (because they had not been used), and the next two which were damaged.The pinion now runs off the Rack just before it hits the Headstock.

Maybe, a similar modification will prevent a repetition, (unless using the Leadscrew)?

Howard

It can only crash on thread cutting (half nuts on leadscrew engaged) On power feed the lathe has a clutch on the feed shaft. I'm not saying the damage is not from the crash, far from it, but I could also notice the the keyed bush which was not in use during the crash (I have, had 3 bushes) is slightly deformed as well. The bush material can be compressed without much effort, so I'm a bit concerned taking large depth passes - but then, looking at Jason's post saying he took 0.25" passes (when the maximum I ever took on steel is 0.06" I should be fine.

I don't think the bushes are meant to fail as a means to protect the lathe, for that there's a 3mm brass shear pin on the lead screw ( I replaced it with an aluminium one which should fail earlier)

I am also taking Bayzle's suggestion regarding the keyed bush. I'll machine new bushes with a thicker wall, the originals are 14mm OD, 10mm ID, the new ones I'll machine will have the same 14mm OD, but a smaller 8mm ID and cut a 2mm slot for a key. I'll make the key from aluminium so hopefully if anything has to break it will not be the bush! The Bush I'll make from either steel or maybe titanium.

Did I miss something, or is the second incident and as the result of attempting to cut another thread, using a new bush in place of the one that failed, or is it from the original crash?

Other than that: Ref the oil oozing and the sparks, I hate to say it again - but I will:

It's almost certainly an 'Oilite' type product. Some have iron in them as per the quote I posted previously (hence magnetic and sparking when ground), all are designed to absorb oil which is then released as a lubricant for a bearing surface, most are quite easily compressed and have varying degrees of strength, usually quite low.

(It's how we used to get them out of blind holes - put a punch/screwdriver/nail against one edge, give it a tap and it readily deforms inwards. Pick out with pliers, or invert item and thump on bench/lump of wood.)

Could I also suggest that some enterprising designer has used the material deliberately? It can be extruded/press formed/whatever quite cheaply (why Lucas used it) in vast quantities and a bit of effort would provide a reasonable value for its shear point, thus enabling it to act as a back-up shear pin for the drive train - but perhaps in ChrisB's case, not high enough?

Will

Edited By Will Noble on 30/12/2018 16:42:55

Are you changing the spindles, on which these run, to an even smaller size than design? Or securing the key, then boring out until there is no complete ring of original metal remaining?