Thread: Love hate relationship with my Mars lathe

Gday everyone,

I bought a Mars 'great scot' lathe a few months back it had been sitting in a garage for 30 years. Nice looking machine and ive putting it back together.

Now, i got it up a running and theres a issue with it... i had a piece of bright steel in the 3 jaw and rotated the chuck by hand with a clock on the bright steel. If i rotate the chuck 180 degrees give or take its out by .4mm! Ive since then taken the chuck off and measured the spindle and its out by .1mm.

The lathe has what appears to be brass bushing that have been placed inside the head stock with a roller ball bearing inside thats over the shaft, is this a babbit bearing? Could the bearings be worn that its throwing out the spindle or is it bend? Or the brass bushings have a thead on it with a steel collar with a series of holes in the diameter. I believe a pin spanner is the tool for this. Does this need to tighten, would that help? Or have i got to strip the head stock?

Cheers

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Welcome to the forum.
Firstly, don't be too concerned with a 0.4mm error in a 3 jaw. With any chuck (and your spindle error as well). The material held in it will machine concentric once you take off that initial eccentricity. 3 Jaw chucks are the worst for their lack of concentricity but you can still do good work with them.
Really, the only reason that I would suggest stripping the spindle down is if there is play in the bearings. I'm not familar with Mars lathes (someone here has one I think) but I'm wondering whether someone did a mod to incorporate a rolling element bearing in what was a plain bearing lathe.

Michael

Hi @Woodlyo, i have a Great Scot in good working order.

From your pictures it looks like you have the stock white metal bushings. The metal rings with holes are threaded cones used to tighten the bushings, and you need to snug them up till the spindle has no play, but will keep spinning for a turn or so if you give the chuck a spin by hand. Keep the oil up to the bearings.

Here's a picture of the pin-spanners I made for mine. You can see more pics in this Flickr Album.

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Welcome to the forum.

Ross

I've got two questions for the gurus on this one, I'm sure they'll sort these out in quick order.

Number one, I would have thought the angle of the dial indicator as shown in the pictures would potentially cause an erroneous reading in any order, both in terms of smooth movement of the plunger and also reading in fact a bit low?

Secondly, would the inside of that particular spindle necessarily be machined to any particular level of accuracy? Does that machine have a taper ground inside it to accept centres? I would have thought the section just behind the thread had more to do with locating the chuck, but I'm not up on threaded chucks.

And for the original poster, was the bright bar 'as is', or did you turn a little off the diameter, and then flip it? Bright bar 'as is' is not particularly round, although its not too bad on the whole, might be giving you a reading thats a bit off.

As Michael has said, 0.4mm is not that bad for a self centring 3 jaw really anyway, you learn to work around it. The amount of runout will vary depending on the diameter of the workpiece too. You may be able to reduce it some by stoning any burrs off the inside of the jaws, and any dings on the rear of the chuck.

Originally Posted by Jekyll and Hyde

I've got two questions for the gurus on this one, I'm sure they'll sort these out in quick order.

Number one, I would have thought the angle of the dial indicator as shown in the pictures would potentially cause an erroneous reading in any order, both in terms of smooth movement of the plunger and also reading in fact a bit low?

Secondly, would the inside of that particular spindle necessarily be machined to any particular level of accuracy? Does that machine have a taper ground inside it to accept centres? I would have thought the section just behind the thread had more to do with locating the chuck, but I'm not up on threaded chucks.

And for the original poster, was the bright bar 'as is', or did you turn a little off the diameter, and then flip it? Bright bar 'as is' is not particularly round, although its not too bad on the whole, might be giving you a reading thats a bit off.

As Michael has said, 0.4mm is not that bad for a self centring 3 jaw really anyway, you learn to work around it. The amount of runout will vary depending on the diameter of the workpiece too. You may be able to reduce it some by stoning any burrs off the inside of the jaws, and any dings on the rear of the chuck.

1. Yes having the indicator on that angle will cause it to read less than the actual run out.
2. Unless there is a taper in the id of the spindle, i doubt it would be super concentric. The parallel section after the thread and shoulder locate the chuck. that's where the OP should check the runout. On a old lathe like that .1 runout doesn't surprise me.

Originally Posted by snapatap

2. Unless there is a taper in the id of the spindle, i doubt it would be super concentric. The parallel section after the thread and shoulder locate the chuck. that's where the OP should check the runout. On a old lathe like that .1 runout doesn't surprise me.

If it has plain bearings runout should be near zero. As it is just like rotating something round in V blocks. The OP does mention a roller bearing, so maybe there has been some sort of bodgy job done on it.

I am yet to see a lathe especially an older one without a taper in the spindle. Reason being older lathes manuals all show doing stuff between centres, which means a centre is placed in the spindle.

My mars had a taper in the nose, MT3 from memory. The ball bearing was only for thrust, it was in the rear bearing.
It is a really simple spindle to pull out, just take the nut off the LH end and tap the spindle out. The bearings work best if you shim the slots, otherwise they can move a little in their housings.
The main thing is to keep the oil up to it or the bearings will wear at an alarming rate.

Hi guys, thanks for tips. Ill be pulling out the spindle over this Christmas break.

I thought I'd post some ally that I tried turning up on my lathe... surely the lathe can get a better result than this.

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Also in regards to the dial indicator on the taper inside the spindle i checked else where on the spindle theres a flat spot just behind the treaded section that I tried, she was still out at .1mm.

Cheers guys

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Some of those marks look to be because you have brought the tool back along the cut. If the cut was good to start with it would not matter but I would suggest that your tool is not correct for the application (blunt?) so it is not cutting properly, giving you a poor finish and pushing the part away from the tool so it cuts again when the tool is wound back.
A worn spindle bearing may give a similar result but eliminate the simple stuff before attacking the spindle.
Use a length of a wood for a lever and put an indicator on top of the spindle. (gently) apply pressure and see how much the spindle deflects upward. If your bearings are worn that will show it. If the bearings are good you should see very little movement.

Michael

An old machine like that isn't really suited to carbide tools either. If that's what you are using, then try a High Speed Steel tool.
peter

Thanks guys for advice, ill see what happens. Ill keep you posted.

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