Lathe crash!

Aren't these supposed to be a close sliding fit rather than needing "pressing"? I'm struggling to understand what forces in a jam up would cause this level of destruction in a bush. I imagine teeth would fly from the periphery of the gears before this item disintegrates.

I suppose you have one of the pair of gears being driven and the other is stopped from rotating by the carrage against the headstock there will be a very high twisting force as one gear wants to turn and the other does not. If the teeth are stronger than the bush the bush will fail.

I hope I'm imagining it but has the gear cracked too? Enhanced by me and could be wrong:

Original:

I half remember a trick using petrol or other light oil for highlighting genuine cracks. Soak the suspected area in petrol, wipe dry immediately, then watch to see if anything emerges from the crack. Have I got that right or is there a better method?

Having crashed a WM280 lathe myself I can confirm that the forces involved can be considerable. First time was nothing to write home about. Second time was far worse. So violent I replaced my steel shear pin with a brass one to break faster and am neurotic about not doing it again ...

Dave

Nah, the gear is fine, the damaged parts I found so far are two bushes on the change gears and the shear pin on the lead screw. Although the shear pin failed I find it strange the bushes failed as well...I have now replaced the brass pin with an aluminium one.

The bushes fit tight on some gears and slide fit on others, so it's a bit of a mash up honestly. I'm of the opinion that the material for the bushes could be better, but then again, it's a low cost hobby lathe so I cannot expect much better.

And it would probably cost more to replace a gear if you lost a tooth than the bush going bang.

That's very much true, got a quote for a set of 6 gears...£150. Got myself an HV6 rotary table instead.

I put an aluminium shear pin on the lead screw as I believe the shear pin should fail before anything else, hopefully I will never need to find out again!

............ and I'm still going for it being an 'Oilite'-type material which has, as I think I implied/said, the torque transmitting ability of something approaching cast cheese. It's not designed to transmit torque, just soak up a lubricant and then act as a bearing surface.

If Chris is cutting a 16mm thread - and especially if 'going for it' - I'd suggest he's applying quite a lot of torque to the drive to the saddle, especially on a smaller lathe. It's revealed a design expedient (read fault/fudge/cost saving exercise).

The conversion to a suitable shear pin sounds a better bet - but chosen with care. I'd suggest a drill shank might not be appropriate - as in too high a shear rating for the job.

All the above assumes there is no other hidden damage which is causing a lot of 'drag' on the layshaft/feedscrew/saddle side of things.

Will

ps I'm avoiding a discussion of my mention of Mazak. I think that bit is for another time/part of the Forum. Don't need another red herring.

A slightly posher way, which I use on aircraft, is Ardrox dye penetrant. A low viscosity red dye is sprayed onto the surface which needs testing. After 20 minutes or so of drying any surplus dye is removed with a solvent. Then a developer white powder is sprayed on. The powder draws out any dye hiding in cracks to give a red line. The system is very eftective, and will highlight cracks which are not easily visible to the naked eye.

Andrew

No, I would say it has failed because one gear was turning as indicated by the red arrow while the other was stuck solid due to the carrage being jammed up against something solid. Think of twisting an Oreo in half

Edited By JasonB on 27/12/2018 19:35:58

So ... how does the gap appear ?

I get where Michael's coming from, it got me puzzled as well, the bush fits tight on both shaft and gear...but...when I put the bush on the gear and the shaft on the bush, there seems to be a piece missing - almost as if the material has shrunk or collapsed into itself ! There are no missing fragments and the split ends match perfectly together. Have a look at the photos below and try to decipher what happened because I cant!

Looks to me that S O D is right, and the gear is cracked, from the corner of the keyway.

Either you are in the market for a 80T gear, or a Rotary Table, a set of Division Plates and a No 2 Gear Cutter to suit the gears on your lathe, (either DP or Module).

Howard.

I'm 99.9% sure it's not cracked. For peace of mind I'll do a penetrant test when I'm back at work next week as described by Andrew above - I'm sure it will be negative.

That said, rotary table and plates are on the way, as well as a set of mod  2  1.5 gear cutters

Edit 1: what you're seeing at the key slot corner is a scratch on the surface - in the photo it may lead you to believe its a crack.

Edit 2: As Jason correctly pointed out Mod1.5 cutters are what I need and what I indeed ordered

Edited By ChrisB on 27/12/2018 20:29:04

Edited By ChrisB on 27/12/2018 20:45:04

As you say they are too close a fit to fail by expanding but if sintered as some are saying I suppose the material could compress.

I think you will find they are 1.5MOD gears.

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My guess [and that's all it can be] is that "the bush fits tight on both shaft and gear" now ... but it was originally loose in the gear.

It would probably be useful to measure the diameter of the hole in that gear.

MichaelG.

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... and my very rough estimate is that it [the hole in the 80 tooth gear] will be about 2% oversize, compared to the original diameter of the bush.

MichaelG.

Edited By Michael Gilligan on 27/12/2018 23:34:46

I see what you mean about the shearing action JB. The obvious comparison to make is with a new/surviving one to see what the original dimensions should be.

Chris has already said that they are a tight for to the point of having to knock them in, which is similar to mine which are all a firm push fit and if you pick the pair up by one gear the other stays put. You could not do that with a hole that is 0.3mm oversize plus the gears would be rattling about all over the place and the key would soon fret and wear.

Michael how did you get your 2% oversize? I would have said the gap is about the same as the wall thickness of the bush at 2mm so the circumference of the bush would need room to increase by that amount so nearer 4% oversize or a 14.6mm hole for a 14mm bush.

Edited By JasonB on 28/12/2018 07:24:52

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By estimating [*] the angle of the gap in the damaged bush at about 8°

MichaelG.

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[*] prone to error because of perspective effects in the image ... but I tried.