smooth cut in brass

Help needed please.

I am trying to turn down a small length of 22 mm diameter brass bar on my Myford super 7. As I am getting closer to the 12 mm diameter size I want, the surface finish is looking more like thread cutting . I have a new cutting tool in the quick change holder but I cannot get the cutting edge to go down to the center line. Could this be the problem. I get the impression that the cutting tool is trying to push the brass out of the way as apposed to cutting it. I have tried several different shaped cutting tools but all with the same result. Anybody else had this problem. Mike

Sparey goes into some detail regarding tool shape for working brass. They are not the same as those for steel.

Might be worth checking it out.

Cutting edge must be on centre line

Cutting edge on centre height and a flat topped tool (ie no top rake) for brass. The finish can also be affected by the grade of brass that you re trying to cut. If it is CZ121 (as used by clockamkers among others) it should cut freely and easily with the swarf coming off as thousands of tiny chips spraying all over the place! If it comes off as a string and forms a birdsnest of tangles you have the "wrong" sort of brass for machining. It will machine but you will struggle to get a decent finish.

Almost 100% sure the tool-height is the cause. (too high is worse than too low).

The problem is the curve of the work fits into the rake under the tool effectively removing it and rubbing the end of the tool on the work. At large diameters a small height error won't matter much, but the bad effects become more severe as the tool gets closer to the centre. Eventually the tool tip would be above the work and not cutting at all.

Can you post a photo of the tool and tool-holder? You need to get the tool point down to centre height, perhaps by grinding off the bottom of the tool shank, or it may be a QCTP adjustment.

Dave

Edited By SillyOldDuffer on 01/08/2019 16:27:28

Slow spindle speed and high feed rate in combination with a cutter that has very small radius point can produce a poor finish like you describe.

Edited By Hollowpoint on 01/08/2019 16:32:02

When I started cutting, the brass was coming of as a fine chips , so I assume its the right brass. I tried a flat topped tool but results where the same. It would indeed seem the trouble is the height of the tool. I am only about half a millimeter above the center line but I have no more adjustment in my holder. I am trying to find a thinner tool before I can try again.

Many thanks for help. Mike

Have you surfaced the end of the bar with the cutter in use? That will easily confirm if the tool is not exactly on centre height by leaving a nipple at the centre.

0.5 mm on dia 12 is a massive amount too high, but less significant at dia. 22 that’s the problem. And why the chips flew off correctly at the start of machining.

What type of tool is it carbide, interchangeable it Hs?

What shape is the tool?

what is the tip radius? Is it flat topped?

More information required for a proper answer. Perhaps if the shank of the tool is soft (test with the corner of a file, it should not slide over the tool) so it can be machined down so your tool post can use it otherwise it’s probably junk to you

Edited By Zan on 01/08/2019 16:53:30

Mike - are you using TC tipped or HSS tool? Please attach a photo

What speed are you running the lathe at when approaching 12mm diameter?

What depth of cut are you attempting?

@Zan - beat me to it!

Edited By Andy Carruthers on 01/08/2019 16:57:24

I managed to find a thinner tool ,with this adjusted to a center height the finish is now like glass. Thanks so much for your help, this really is a great forum for novices like myself.

Cheers Mike

I find a round RCMT tipped tool gives a very good finish on brass with 3 added benefits:

1. If you set the tool holder just so you can face, turn and profile to most angles without adjustment.

2. If the tool gets dull or chipped rotate the tip so a fresh part of the edge is bearing on the work.

3. It’s a great radiusing tool.

Might I suggest you post pictures of your tool holder so the myford guys can tell you if it is the right type for your lathe.

Also this video is very good and explains how to set tool heights **LINK**

If the tool is above the centre height, you will get problems. Once the cutting edge of the tool can be set on the centre line, then you can start experimenting with different tools.

If you want a fine finish, the feed rate needs to be small, but the depth of cut and feed must not be so small that cuts intermittently, when the pressure builds up enough to cut, and then stops cutting as the pressure subsides; until the next time. For brass, the speed needs to be higher than for steel, and the rake needs to be less.

Howard.

Search the 'net for "vertical shear tool"...

F F, you should get hold of some RCGT tips for your holders, they are sharp and shiny, for non ferrous, or fine finishing with ferrous.