Thread: Lathe finish problem

You may need to reduce your feed rate.
It would appear that your sliding feed is .018" and your surfacing if you have power feed may be 1/2 to a 1/4 of your sliding feed.
With surfacing what results do you get if you feed out rather than in.

The tool appears to be rubbing a bit.. check that the cutting edge of the tool is right on centre

This advice is from a beginner

Mike

Originally Posted by pipeclay

You may need to reduce your feed rate.
It would appear that your sliding feed is .018" and your surfacing if you have power feed may be 1/2 to a 1/4 of your sliding feed.
With surfacing what results do you get if you feed out rather than in.

There's no power feed on the compound rest. Being a simple lathe, there's just a friction clutch to engage right-to-left carriage traversal.

There are also no dials or graduations on the compound rest. I basically just eyeball the tool tip relative to the work.

Surfacing from the outside to the center starts out good, but the lathe seems to get bogged down as it nears the center. Noise increases and it almost starts to shudder before I chicken out and back off. Surfacing from the inside to the outside doesn't have this problem, but for the finish.

I don't know the type of lathe but is there a feedscrew on it or just the leadscrew. Engaging the screwcutting feed is always going to be coarse.

Phil

Originally Posted by Steamwhisperer

I don't know the type of lathe but is there a feedscrew on it or just the leadscrew. Engaging the screwcutting feed is always going to be coarse.

The lathe just has a single screw down the middle, with a handle on the end. Basically the one screw does both jobs, if that makes sense.

There is some info here: Page Title

It looks like the finish I get when I turn mild steel.Is the cut too fine? With the pic blown up it looks sort of torn in places. HSS cuts better under a load.

My results using a fine cut were pretty average until I found that a heavier cut often yields a better finish.
I think I was not confident cutting to size and was trying to "shave " my way down to finish size.


Does Grandfather's Hercus/Bagear bar-bed lathe auto feed on crosscut for the facing cut you have shown?

Given that you say you tried it with the tangential tooling,that may well rule out HSS problems,but hey!,I am no expert.

Can you lay you hands on some other types of steel. Playing with the variables and taking note of the results will improve your work generally.

Cheers
Grahame

Have a look here for grinding up your tool to suit the application.
http://www.discoverlivesteam.com/books/Sharpening.pdf
Grind on a nice 1mm radius on the cutting edge, and run your lathe at its top speed of 600 rpm.
A light facing cut of .1 or .2mm and gently feed in the tool watching it to see it cuts smoothly.
Also the cut on your material seems to show a pattern given from a loose cross slide with evenly spaced grooves across the face.
Maybe check the gib tightness?
Good luck.

Could be that the headstock bearings are worn and have some play

Mike

Originally Posted by Andrew C

The lathe just has a single screw down the middle, with a handle on the end. Basically the one screw does both jobs, if that makes sense.

There is some info here: Page Title

Hi Andrew,
Thanks for the link. With your tangential tool, have you rubbed a radius on the point using a stone. I found it made a massive difference to surface finish and also assists in making sure you don't break the tip off which may have caused part of your problem. Not all of it though. The feed rate does look very coarse.

Phil

I owned one of those briefly. It was useless. Even if it hadn't been clapped out, and even if I had a clue, it would still be the flimsiest imaginable way to make a lathe bed, and only good for very light work. You will need light cuts, sharp tools and fine feeds.

If my size guesses and counting are right, I'd also say you are feeding to fast.
Is that 40mm dia bar?

Manual feeding, if its doing 600rpm and you're feeding 0.1mm* per rev it should take 20seconds to face the bar.

55 tpi is a thread. are you sure you cant set up the gear train to give you finer feed?

Stuart

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Originally Posted by Stustoys

55 tpi is a thread. are you sure you cant set up the gear train to give you finer feed?

Stuart

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Compounding should get it finer.

Phil

Originally Posted by Andrew C

...................Is this the best I can expect from this sort of lathe? Am I doing something stupid? Am I stuck manually polishing things past this point?..................Andrew

Firstly, please rest assured that a bar-bed lathe is capable of producing exactly the same high grade surface finish as any other lathe. And as accurately as any other lathe. The bar-bed will however limit how fast you can remove metal, because it does not have the rigidity to take deep cuts. Bar-bed lathes are still made today, the German WABECO D2000, D2400 and D3000 come to mind and these are not cheap toys at $ 6,000.

Like with any other lathe, accuracy and surface finish are very much affected by wear of the lathe, alignment of the lathe, and operator skill.

Wear: I have looked up lathes.co.uk, your lathe was apparently made between 1920 and late 50's. That is a long time. It is not unlikely the poor surface finish is due to excessive play in the spindle bearings, or in the saddle to bed-bars, or in the cross slide or top slide dovetails. Have a look at the spindle bearings first, this is most often the reason for poor finish.

Operator skill: you are working on this, it will take some time. And it is harder to learn with a worn lathe, than it is with a new machine. That said, with a worn machine you are likely to learn a lot more than just the basics. It could even get you a crash course at restoring a lathe .
To cut Aluminium on a light lathe you need a razor sharp tool with plenty top clearance. The tool should be exactly at center height, use shims under the tool to achieve this. A small tip radius will help improving finish. Cut dry, or use WD40 sparingly. Make sure your toolpost is clean before clamping the tool, else the tool will rock. Try to turn your Aluminium bar, taking a very light depth of cut that takes off a fine endless chip like a hair, whilst feeding very slowly by hand. If you cannot get such a fine chip, you may not be using a "free cutting" Aluminium alloy, or your tool is not really sharp and you are rubbing instead of cutting, or you have too much play in the spindle or in the ways and your tool deflects away from the cut producing a most horrible finish. Once you get the fine chip, try a deeper cut, something like 0.2mm. If the finish gets much worse, its because of excessive spindle bearing or way play. I think your lathe should be able to handle a 0.5mm deep cut in an Ally bar of about 30mm without undue chatter. Forget the automatic feed for now: a lathe that cannot produce a fine finish manually will not magically produce a fine finish with automatic feed either, it will be just as bad and only look a little more regular...Let us know how you go. Chis

Originally Posted by Stustoys

If my size guesses and counting are right, I'd also say you are feeding to fast.
Is that 40mm dia bar?

Manual feeding, if its doing 600rpm and you're feeding 0.1mm* per rev it should take 20seconds to face the bar.

You're correct, it is 40mm bar. I've not been taking 20 seconds to face the bar, more like half that. So that's a starting point.

55 tpi is a thread. are you sure you cant set up the gear train to give you finer feed?

I'm not sure that I can. I'm not near the machine at the moment, but from memory the leadscrew has a 70 tooth gear on it. This gear has a different size center hole to the other gears in the train, meaning I'm stuck with it being the last gear in the train.

The other gears available in the set are: 20, 35, 35, 40, 40, 45, 50, 52, 55.

Using an A-B : C-D arrangement, the fastest arrangement would be 40-20:50-70 and the slowest 20-55:35-70.