Turning Silver Steel

Hello.

I was wondering if anyone could help, I am trying to turn 8mm dia silversteel down to 5mm. (first time i've used it)

The problem I have is that the finish and accuracy is awful, although the closer to the chuck the more accurate the finish, its 0.4mm out the furthest point away from the chuck. (50mm hanging out of chuck)

I'm using plenty of coolant, applying a nice steady feed rate and using a sharp facing off tool.

Any suggestions would be hugely appreciated.

Andy (The Novice)

That size material sticking out that far will almost certainly be flexing. Try Facing/centre drilling the end and supporting it with a centre in the tail stock.

You should see a big improvement.

John

Edited By John Bromley on 05/11/2014 09:03:47

Ok, I'll try that now and let you know.

cheers

Andy

Hi Andy, are you supporting this with a live centre? With such a small diameter and at 2" long you will need to do this, also you will need a fairly high speed. What tool are you using, HSS OR carbide? Personally assuming you are using a small ME type lathe I would use HSS with a small corner rad. or one ground with a chip breaker and plenty of cutting oil rather than suds.

I prefer HSS to carbide for most work it's easier to handle and you can alter it to suit many applications whereas carbide disposable tip tooling is pretty well fixed to what you have, you can of course buy many variations of grades and cutting geometry but that makes it very expensive plus one must consider this tooling is largely designed for mass production on industrial machines.

John

Are you using HSS tooling, if so, I have found by using a well honed hss knife tool, almost parallel to the job, gives a very good finish. Set it up so there is a little light creeping through at the cutting edge furthest from the point.

John

Cheers gents.

im using carbide tips (for stainless material) I hadn't used a centre to support it as I didn't want to centre drill the end of the material, purely for cosmetic reasons. I'm running the machine at about 800rpm, is this too slow? I have plenty of material so will try machining with a centre support first.

really appreciate the help

Andy

Learn to use your tailstock sooner rather than later and get or make a live centre, it makes a huge difference

You can make a good steady spot drill with one of those MT1(MT2 etc) dead centres which has a carbide tip

Grind away half of the tip so it looks like a tiny half centre, it can then be used to drill a small start hole for twist drills when pushed into any workpiece

If you want, you can always face off the finished part after turning it to diameter. Just make it a tad longer to start.

As you have so much material to remove you can play with the turning speeds. Just make sure your last couple of cuts are light and fed very slowly.

Depending on the tolerances required. If you leave it a couple of thou over size you can even polish it with some emery to a improve the finish, if it needs it at all.

John

If you continue to get a poor finish ditch the carbide tooling and get a few bits of HSS. Have a play with it and see how it goes. I have never been able to get carbide tooling to give a decent finish on a small lathe.

I am too still a novice with all of this machining lark. I think hobbies should challenge you. But when the results are constantly poor it can become tediouse.

John

Turning it without centre support shouldn't be a problem if you take care.

As others have suggested use a HSS tool, not carbide. Give it good back and side rake, I use about 20° side rake for this sort of job, and take it slowly, no more than 0.5 mm cuts to start with and only 0.1 mm or less for finishing.  Make several passes at the finished size.  Make sure the tool is really sharp. After grinding give it a few strokes on an oilstone or, better still, a diamond hone.

Your speed is a bit high, I would use about 500 rpm but you could go to 1000 for the finishing cut.

I recently reduced some silver steel down to 3 mm over about 30 mm length without tailstock support with no problem but taking it carefully with a very fine hand feed.

Good luck

Russell.

P.S.  If you need a really fine finish, polish in the lathe it with some fine wet and dry paper, starting with 400 grade and going finer depending on your reqirements.  I go down to 2500 grade and then use Micromesh for clock work.

Edited By Russell Eberhardt on 05/11/2014 10:23:03

I agree with sharp HSS tools, a good cutting oil and centre support - and/or go very carefully. Even so I often find silver steel gives a poor finish. My own trick is to finish off with a very fine file and then wet n dry plus oil backed by a ruler or similar. Take care not to catch your knuckles on the chuck jaws and mind the burr on the non-reduced section. Inspector Meticulous might not approve though.

Cheers gents

You've all helped a great deal, and taught a novice a lot. The centre was very close to the tool, but just about cleared. Russell your suggested speed rate really helped. Shall polish them later on.

Thanks again

Andy

I agree with very sharp HSS tools, I finish the cutting edges with a very fine diamond stone.

You could try a HSS tool variation for your finishing cuts - a tangential tool; (this is not the same as the tangential tool holder advertised on the website). I came across the details during an internet search for something else. It's a bit like a normal knife tool but on its side. It works with a distinct slicing action. The swarf comes off like wire-wool and 1/2 thou. cuts are easy to achieve. The photo of the tool looks odd because it was difficult to photograph. The only issue is that you can't finish to a shoulder.

I used it to good effect when attempting to make a missing 7/16 Myford Collet from silver steel.

The second photo shows the difference between insert carbide and the HSS tangential tool finish on the MT2 section. You should note that I did not have the benefit of fine feed, I was manually feeding the set-over topslide to machine the MT2 taper.

Another, not so obvious benefit, came from grinding the front face to 90deg (another variation) which I think eliminates any tool height error which affects taper turning.

I've rarely had good finish with insert tooling, as I tend to use small depths of cut, I'm in no hurry! and insert tooling calls for reasonable cuts, otherwise it just rubs. However, I have found that the Super 7 is able to produce a better finish from carbide with reasonable cuts than an ML7 (in very good condition) probably because the spindle is stiffer on the Super 7.

I'll try and find the original link to the tangential tool if anyone is interested.

Regards

Phil

Crikey...a 20 thou cut on a small piece in a small lathe is a bit suicidal dont you think?

You'd need to be running about 10 times the speed (ie closer to 5000rpm) to get any where near the recommended cutting speed for carbide, assuming your motor and spindle are up to it. And don't use coolant, as it may prevent the chips from achieving the required temperature for correct cutting in the first place.

A nice sharp HSS tool with coolant / lubricant is going to give a better result but I'd be wearing my rubber pants (and safety goggles) with that amount of overhang. If you give it too much feed or the tool snags, you could experience some unplanned excitement! Minimising overhang and providing tailstock support where possible is always good practice no matter what you are turning.

Murray

Hi fizzy,

Crikey...a 20 thou cut on a small piece in a small lathe is a bit suicidal dont you think?

Well spotted - I missed that - darned metric - I don't think I've ever taken a 20 thou cut in my life on my Myfords - probable why it took so long to make the Minnie and kept the machine in good condition .

HSS most days, lot cheaper too.

Phil

If surface finish isn't critical a 20-thou cut shouldn't be excessive in a Myford or even a mini-lathe on 8mm bar?

Neil

Just had to do two similar things this morning, small dia 2 1/2" protruding to come off finished.

Lesson learnt in my toy lathe days where I used to turn stainless steel G316 S11 -If you don't know its like glass and wears hss cobalt before finishing a mild cut 5 to 10 thou. Most of these parts were 3 to 4" long x 3/16" dia from a nominal 10m dia round. No centre just take cuts thinning round down near to size within 2 thou at furthest away. Next do same again but closer to chuck working your way in. Lastly if using hss sharpen it up and send it down on power feed numerous times, even reverse power feeding away from chuck, works every time no runout but do need impeccably sharp tool.

If it still has a taper sand it and or blend the ridges in.

By thinning down furthest away you are retaining the material strength from chuck to that point. Other times it pays to go up in size on material to reduce that flex.

If use a centre it will bow in the middle, cutter pushes the round out.

Ideally could do with a roller box.

.

Andy,

You might consider using a female centre in the tailstock, which means you can have a point on the work ... very easy to trim off when the job's finished.

MichaelG.

I would start off with bar less than 10mm/3/8" out of chuck, face and centre drill. Next, pull work out to a bit more than another say 15mm/5/8" and start turning it down to near the finished size then pull it out a futher 15mm and repeat but this time bring in the tailstock support to help steady the work. When the turned down length equals desired length + allowance for trimming off the centred end, I would up the speed and take very fine cuts for the last few thou. down to finished dia. As stated above, make sure tool is very sharp, preferably honed. HTH

John