Brass facing finish.

I was facing a brass disc this evening and got (for me) a surprisingly good finish. Well, good in parts...

The disc is about 1.75 inches in diameter and I was using a CCGT insert tool. Depth of cut was 0.1mm and the lathe was running at 1200rpm with facing feed at 0.02mm/rev.

Can anyone suggest a reason for the rough bands near the circumference and a little way in? This is an experimental piece and I have made the same cut repeatedly - there is always a rough patch at the start of the cut, then at least one rough band further in, but not always in the same place - it's not systematic apart from a consistent band near the circumference.

I have a number of these to do and they will eventually be taken to a near mirror finish, so if I can get rid of the rough bands it will save me a lot of time going through the grades of abrasive. Finishing is by far the most time consuming part of making these things.

Robin.

I've had similar bands appear on (usually wider) facing cuts and assume it's one or more of the following - a transition where surface speed or cross speed reduces cutter performance , probably due to some subtle harmonic chatter. It may be worth playing with tool stick-out and varying lathe speeds to see if the zone moves.

pgk

Probably a matter of harmonics between your particular machine, the cutting tool and the surface speed of the job which changes as the tool moves further inwards toward the centre. Starts out with high surface speed at the outer edge and a very low surface speed right in the centre (theoretically zero at dead centre).

You might try changing, one at a time, the spindle speed, the feed rate, the angle of approach of the tool, tool overhang etc and see if it makes any difference. And do lock the carriage and topslide while facing for best result.

And a HSS tool is likely to give a better finish than carbide if correctly ground and honed to a sharp edge. You could also try changing the carbide insert for a fresh one in case you have a tiny chip or weldment on this one.

Tightening headstock bearings might be worth a try too. And gib strips on cross slide.

Otherwise, emery paper is your friend.

As Hopper has said, this is caused by resonance in the workpiece reacting against the cutting tool. I have noticed that the tension in the chuck, perhaps not surprisingly, plays a large part....if you tighten a 4-jaw holding tghe disc the bands of texture change positions etc.

It's worth mentioning that brass users have a range of workholding methods not usually seen in other trades and areas.....clockmakers often use wax chucks for discs where the disc is held on a flat plate using shelac while surfacd turning. You can do the same thing with superglue and my own faveourite is to surface turn brass discs using a rectangle of clearplastic held in the 4-jaw....first turn aperture fo rdisc then press the disc into this aperture. This method seems to give much less resonance.

Also, I have found that a round-nose tool with flat top gives a much better finish on discs than other types .....keep it very sharp.

This happens a lot with round bar stock and I have often wondered if the production methods resulted in a non-homogenous material that triggered this sort of result.

Martin C

Try facing from the centre outwards

Roy

Would the continually changing surface speed when facing have any influence?

More than likely; as I understand it is to do with surface speed, the nearer to the center of the work you get you have to increase the RPM to keep the surface speed constant.

George.

edited.. typo's

Edited By mechman48 on 18/03/2021 12:58:25

I always thought that was the correct way to do it anyway?

Rob

I guess the problem with facing from the centre out might be starting the cut, as the centre isn't actually revolving.

Just a thought.

Thanks for suggestions. Tonight I made some experiments and found that locking the saddle / topslide and reducing tool overhang to the minimum possible made no difference to the appearance of the rough areas at the periphery of the work. The next most obvious thing to change was RPM, as obviously the periphery is moving fastest. I went down to 700 rpm and the peripheral patches disappeared, but rings started to appear nearer the centre. So on the next pass I stopped halfway and went back up to 1200 and all was good down to the centre, apart from a rough bit where I changed speeds. So I'm near to cracking the problem - I'm pretty sure now that it is indeed due to resonances. My lathe is a gear head so I don't have the luxury of changing speed on the fly.

My wife is asking me what I want for an imminent birthday. If you hear a dull thud emanating from the Derbyshire region in the next few days it'll be the sound of her jaw dropping to the floor when I say 'well, a 2HP 3 phase motor and inverter would hit the spot darling'. She's probably thinking more along the lines of a jumper and a pair of socks.

Bob Stevenson - thanks for the info about clockmakers' techniques. I'll certainly give your suggestions a try.

Robin.

Edited By Robin Graham on 20/03/2021 00:36:38

Edited By Robin Graham on 20/03/2021 00:40:07

I've noticed this effect on steel as well, and put it down to resonances though my machine-tools are not on tip-top condition or adjustment, and that won't help.

However I have also noticed it on lengthways turning, and that may well be due to resonance nodes, though I suspect also the saddle "walking" slightly.

'

The material fault Martin suggests is possible in some metals at least.

There are two curious, small shiny patches in the faced surface of a 6-inch diameter mild-steel flywheel I have just machined on a moderately hefty lathe with powered cross-feed. I could not see them during the cutting, but could hear the tool, a 6mm button, passing through them. As I was modifying a part I had made several years ago, I can't recall if the stock was a billet from round bar, or trepanned from plate.

When I was the materials store-keeper for a printing-machine manufacturer I once had complaints my issuing about a batch of allegedly sub-standard aluminium-alloy bar. The milling and anodising revealed a peculiar elliptical "core" right through the bar's length, as in seaside rock. They had to accept what I pointed out: I could have had no idea the interior of the metal was like that, it was from our usual supplier, and my hacksawing machine left rough ends that hid any defects.