Thread: MARS Atlas Lathe restoration

Originally Posted by Bryan

Better to rotate the spindle backwards, by hand, with a block of wood on the ways, under a chuck jaw.

Excellent. Thanks for the suggestion.

Another half-day working on the MARS lathe today. There's a 3-step pulley as well as a 2-speed gearbox on the overhead shaft arrangement, so there's plenty of speed options. I noticed that there was a lot of play between the input and output shafts of the gearbox so thought I had better check it out to check that all the gear teeth are OK. They're fine, and the oil looked OK but there a bit of sludge in the bottom with very fine metal powder. It just looks like the results of normal wear over many years but I decided I should clean the whole thing out as part of the project.

It was a bit of a struggle removing the input shaft due to burrs caused by the grub screw fixing the 3-step pulley. Fortunately we have a hydraulic press so not an insurmountable problem.

It is now completely dismantled. All looks in good nick inside. The only issues arising were that the 3-step pulley had one grub-screw missing and the other one was loose. The grub screw fixing the clutch fork in the gear change spindle was loose. Not major problems as everything is keyed and I can ensure they are tight when reassembling.

I took a few pix of progress.

Most of today's efforts were cleaning the gearbox inside and out and re-assembling. It was previously used in an old flour mill so most of the dirt on the outside ins 'flour and oil paste', almost like pant, and no amount of cleaning solvent will move it.

The old oil gasket was looking pretty scrappy so I made a new one before re-assembling.

Putting it all together again was really nice. There seems to be very little wear and everything clicks into place perfectly.

Back next week to fit the gearbox back onto the lathe.

The last two Tuesdays I have been concentrating on dismantling the headstock spindle and cleaning all the parts. With a lot of care most of it came apart pretty easily until I got to the bull gear with the broken teeth. It is keyed to the spindle and positioned against a shoulder on the short (LH) end, so there's a long piece of shaft along which it must be extracted... and it won't budge. We put it in to 20 tonne press and took it up to 2 tonnes. Stuck solid. We were reluctant to apply too much more pressure for fear of damaging either the gear or the spindle. If left it soaking on the parts washer until next week.
I did some measuring to determine the available metal on the damaged bull gear. The overall height from the rebate to the top of the teeth is 11.5mm. The rebate to the root of the teeth is 5mm, so there is only 6.5mm of 'meat' under the teeth. Need to consider whether, for the sake of three missing teeth it might be better to build up replacement teeth rather than strip off all the good teeth and shrink on the ring for re-cutting. The fourth picture shows roughly how they should mesh.
Next I started on the feed screw drive train which also has a gear with a couple of broken teeth.
The final picture is of the lathe overall in its current state.

Your first photo in post #13 shows a repaired tooth two teeth to the left of the missing one. This is a very common and acceptable repair for an occasional missing tooth in a back gear bull wheel where only one tooth is missing. Probably would not be a good idea to do two or more successive missing teeth this way though. It is done by tapping the wheel, screwing in two small studs and filing them to the shape of the missing tooth.

Why not do the other missing teeth this way and avoid having to disturb and possibly further damage the bull wheel? A Google search for repairs to back gear teeth will turn up several examples of this technique, and some other alternative successful repair methods for dealing with single tooth replacements which don't involve removing the bull wheel from the spindle.

Some time ago I assembled descriptions of half a dozen different gear tooth repair methods from various internet forums - if you would like a copy PM me with your e-mail address.

Frank.

Originally Posted by franco

Your first photo in post #13 shows a repaired tooth two teeth to the left of the missing one.

Thanks for pointing that out. I hadn't spotted that it was a repair. I just assumed the tooth was only partly broken. I'll have a better look at it next Tuesday.

More progress today.
First priority was to take some careful measurements to start discussions on the best way to fix the back gearing layshaft.

I've drawn these up in Turbo-CAD for clarity. I haven't done gear contours in Turbo-CAD before and, sure enough, when I come to do it I need to know:
Number of teeth (easy)
Diametral Pitch
Pressure Angle.

Obviously I have counted the number of teeth and have measured the root circle diameter and the addendum circle diameter. I'm wondering how to work out the Diametral Pitch and Pressure Angle, other than by trial and error until I get a fair match.

Any suggestions appreciated.

Diametral Pitch = (N+2) / OD where N is the number of teeth on the gear and OD is the outside diameter of the gear.

Gear formulae: http://www.engineersedge.com/gear_formula.htm

The gears on a lathe that age will almost certainly be 14 1/2 degrees pressure angle. If you have a look at Post #14 in this thread:

First Time Gear & Keyslot Cutting - The Home Shop Machinist & Machinist's Workshop Magazine's BBS

There is a diagram showing the tooth shapes for various pressure angles on a 40 tooth 20DP gear which gives an idea of how the tooth shape should look. It is probably worth reading the whole thread for the general gear information in it.

Frank.

Thanks Franco,

I proceeded with the trial and error approach and got something that looks OK (below).

Now I understand the maths better and have got my head around how Turbo-CAD does gears, I'll have another go with the correct calculations.

Cheers,

If you ever wondered how to fix a gear with broken teeth, just ask pipeclay. He did a beautiful piece of work on our broken back-gearing lay-shaft.

Before and after pictures below.

Thanks for your help, mate. Just like a bought one!

Back on the job tomorrow starting to repair the odd missing teeth on a few other gears.

Great thread. I hope to do nothing but stuff like this when I'm retired. (maybe just a little travel as well)

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That's an impressive lathe!

Originally Posted by matthew_g

I used to have one of those, Probably the same as yours, Mine was ex Australian Defence Force.
Matt

nice repair job ..... 10 out of 10 MIKE

Hi pipeclay, nice repair, very neat and tidy. any details on the process?

Regards
Ray

Damaged area is machined down enough to accomadate a new piece of cast.
A suitable piece of cast is faced and bored to suit,and then shrunk onto the existing shaft.
This is then finish machined to size and the gear teeth cut (at this stage you hope that your mandrel set up dosent move ) unfortunately mine did, so that piece was machined off and another piece fitted and the mandrel set up alltered.
After cutting the teeth the new gear was drilled and selock pins used as a secondary retainer.