Poor quality finish

Hi everyone,

I found your forum looking for answers to questions about a Colchester Master lathe that I recently bought.

I am a lathe novice so I hope you might bear with me if I ask silly questions.

I've read a bit about taking trial cuts to establish the alignment of the lathe but my first problem is that when I take a cut the surface is terrible! A series of grooves and lumps (mild steel).

I have tried all the following without much joy:

1. Different chuck speeds.

2 Different feed rates.

3.Cross slide and top slide locked.

Any pointers for what to try would be very welcome.

Main priority - correct shape tool for material - sharp cutting edge - tool set to the correct height.

Chuck speed will vary depending on diam. of work so sugest start slow speed an then increase untill good cuts are produced - try using cutting oil (spray can type)

Took me a long time to get it right

Happy swarf making

Some questions:

What tooling are you using HSS or Carbide?

Is your tool on the workpiece centreline

and are you certain its Mild steel as it sounds like it's tearing?

Paul

Edited By Hopper on 18/02/2013 12:03:14

Another book is "Using the Small Lathe" by L(eonard) C Mason. Old it may be, but it is written in simple everyday language. Borrow it from the library for an initial read, then buy it!.

Regards,

Peter G. Shaw

+1 for Mason. "Small" means up to 3 1/2". I think this is the best intro to model engineering workshop practise (with apologies to Harold since I only know "Tool and Cutter Sharpening" from him. Recently re-published by TEE.

cheers,

Rod

This may sound stupid but are you sure the lathe is not running in reverse?

Hi Myles,
If you can find someone that lives close enough to visit you to see the problem they would probably spot the cause straight away.

Les.

Hi Myles

All the above plus you say your material is "Mild Steel" - but where did you source this mild steel and what size piece are you working on? If it's a bit of what looks like MS but is in fact a bit you picked up somewhere then it could be hard and that will give you problems, Your Colchester is a far more rigid machine than many here posess and should be able to perform well.

Cheers

Norman

By far the best source of basic information on tool shapes and cutting speeds is section 5 of Model Engineers Handbook by Tubal Cain. Seven pages, seven effective tool shapes, good listing of angle variations for different materials and accessible speed'n feed information. You can go a long way before you need more. Copy, enlarge for easy reading, print and pop in a suitable transparent pocket book to keep in the workshop.

OK feed and cut data given is based on a Myford, relatively wimpy compared to your machine, but there is no shame in walking at Myford cut rates whilst you have the training wheels on. Easy enough to uprate the cuts to mach your lathe strength once you have got the hang of things. Naturally assuming the work piece is strong enough to take a heavier cut. Heavier cuts mean greater quantities of hotter swarf coming at you too! My work rate is set more by the amount, and heat, of swarf than by what the lathe can do.

Getting the cutting edge on centre height is very important. Some sort of gauge being essential. Quick and easy way is to fit a suitable piece of flat stock on a base of some description, stand it on the cross slide, and scribe a line across it using a sharp centre in (preferably) the head stock although tail stock will do. Heck if push comes to shove simply eying up against the tail stock centre will get you close for now.

I don't wish to decry the efforts of the many able contributors to the model engineering press and publications but, objectively, the tool grinding equipment, advice and techniques available to the lone hand novice is very poor. Most being cut down industrial practice which pretty much assumes the user has been taught. Way I see it a novice needs simple and reliable ways to consistently reproduce a few effective tool shapes and angles along with the requisite tip radius. Given the importance of tip radius and angle on cutting performance it really doesn't help progress if every re-grind produces a slightly different shape. I know of no published designs for easily producing different tip radii or of any method other than scales and temples for getting angles right. I have my own ideas on an effective system but, unfortunately, am completely incapable of writing them up for general publication. Too many scientific & engineering reports.

Clive

Hy Miles

When I first started I stuck a piece of black iron that I got from the local blacksmith in the chuck and had the same problem. Try with a bit of bright mild or similar and a sharp tool on centre height.

Nick

Wow! many thanks for all the replies folks. I'll try and answer a few questions.

1. It is a triangular style carbide tool that came with a 'cheap' set to get me started. I did also try another triangular carbide tool that a chap gave me at work. It had come from a real machine shop and had a much bigger radius but the results were much the same.

2. I'm pretty sure it is 1" free machining mild steel. I bought it from the steel supplier as such.

3. I have tried a spray-on oil that is actually for thread tapping. It is quite thick and actually seemed to make the cut worse.

4. I set the tool height by eye using a centre in the tale stock.

Again, thank-you all for your time

Myles

I think your main problem is the cheap carbide tool.

I got a set with my lathe and threw them down the garden.

they are generally rubbish, get a high speed steel cutter and see the difference.

Roy.

Pretty sure the lathe is not in reverse. Anti-clockwise viewed into the chuck and the work advancing down onto the tool.

A friend at work who has done a lot of turning keeps saying that something must be loose. I have taken various measurements, chuck, toolpost etc but I don't really know what a 'good' number is for any of it.

Should be no perceptible shake at the spindle/chuck.

Could be the headstock spindle bearings need adjusting - something that requires a handbook/manual to be done properly.

If perceptible shake on the toolpost or cross slide, you might be able to adjust the gib strips - small strips of metal that fit against the ways and can be adjusted with three little grub screws on many lathes.

Best to get hold of a handbook/manaul first though and have a good read until you understand how it all works. See the page on www.lathes.co.uk on your lathe. It has a link at the bottom to a manual you can purchase.

http://www.lathes.co.uk/colchester/page17.html

But first thing is to get a good tool in place, quality carbide, new, or correctly ground High Speed Steel finished off on an oilstone and set at centre height with minimum overhang. (see those two books I recommended.)

Some pics of your set up would help.

And what model/year of Master is it?

Edited By Hopper on 19/02/2013 02:16:43

Edited By Hopper on 19/02/2013 02:23:40

It's a Master 2500, 1970s. I have the manual that comes with the lathe. It's mostly exploded drawings and part numbers but there are some operating and setting-up instructions.

Not really any perceptible shake when it's running, maybe a bit at higher speeds.

Edited By Myles Douglas-Withers on 19/02/2013 03:18:28

Edited By Myles Douglas-Withers on 19/02/2013 03:20:13

Myles,

While possible I think it unlikely that the problem is due to headstock bearings or anything else that is loose. I did have an issue on my lathe (Harrison M300) where one of the bolts holding the cross slide down had sheared, but this resulted in a barely perceptible spiral pattern not a torn finish.

I would suspect that the problem is down to tooling and/or material. I'll second the advice to throw away the cheap insert tooling. If you want to use insert tooling buy quality holders and inserts. Youve got a decent lathe with some horsepower behind it, so run insert tooling fast, the chips should be coming off blue. For steel a decent depth of cut helps too, at least 1mm. It's no good pussyfooting around with insert tooling when turning steel! See here for some results on turning:

**LINK**

Try steel from a different source. I've had poor quality steel from commercial suppliers that was impossible to get a good finish on.

Where are you based?

Regards,

Andrew

I live just north of Christchurch, New Zealand. I'll try getting some better tooling. Do you think I need to change the holder or could In just get some good inserts?

The last trial cut I did started with a really nice smooth shiny finish... for the first 3mm or so and then it all went to hell for the rest.

If insert tooling is good for taking heavy cuts, what would you recommend for taking fine cuts when getting close to a dimension?

Cheers

Myles

Hi Myles,

That sudden change sounds suspiciously like a chipped tool. Take a close look under a lens.

I had a similar problem with a tiny inserted tool boring bar (3/16" shaft!) that wouldn't give a decent finish. It wasn't until I rotated the insert and got a good result that I was convinced the old corner was damaged at all!

Neil

Hi Myles,

you ask what tooling to use for fine cuts, I would say a sharp HSS tool. I have a small tangential toolholder I use. You can make your own or buy one, it is easy to sharpen the toolbit. This has been discussed on this forum before ***LINK***, another ***LINK***.

Regards

Thor

Edited By Thor on 20/02/2013 05:50:46