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Carbide tool mirror finish interrupted cut

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JasonB08/06/2018 20:48:56
17869 forum posts
1954 photos
1 articles

Nige, if you want to do a few more practice parts then have a go at this one, PM me if you can't work out what it is for.

25mm or 1" OD, 14.9mm ID though 15mm would do, 7.5mm thick, break all corners, Mild steel. Shiny finish optionalwink

nige ring.jpg

Edited By JasonB on 08/06/2018 20:49:27

JasonB09/06/2018 06:53:27
17869 forum posts
1954 photos
1 articles

I meant to say that during those test cuts all the chips from the CCMT were breaking into small pieces and being thrown away from the finished surface BUT it was only the 0.005" per rev ones that saw them coming off blue or straw coloured which is said to indicate that the tip is working around its ideal speed/load.

The fine cut with the CCGT came of in longer coils.

Also I was belt the slower speed range for the lathe which put the motor revs to almost flat out where it gives its highest power. Running at half that motor speed with it on the fast belt setting would have put it closer to stalling point.

Andrew Johnston09/06/2018 10:44:11
5417 forum posts
627 photos

I use 4 thou per rev feed for finish turning. For roughing anything up to 20 thou per rev can be used. Sadly I'm power limited on my lathe. Provided one maintains a decent feedrate better insert utilisation comes from doubling the DOC rather than the feedrate.

Here's another example I did last night, this time for work, in 6082:


The OD of the recess is 200mm. Done with a boring bar and a Korloy CCGT insert with 0.4mm radius. Feeds were by hand. The swarf from roughing measures about 20 thou thick. It came off in short tight helices, and was ***** hot when it went down the shirt front. Finish feed was probably more like 4-5 thou per rev. Interestingly roughing DOC was about 30 thou, but the finishing cut was only 5 thou or so, towards the centre.

A (clean) fingernail is a pretty sensitive measure of surface roughness. In conjunction with a Rubert scale I reckon the surface finish is around 1µm Ra. Unlike the steel trial there appears to be no increase in roughness towards the centre.

One of the problems with forums is that one tends to get blanket statements along the lines of to achieve XYZ you need to do ABC. Often these statements have a sound basis, but they're by no means universal. For instance it is often stated that inserts give a poor finish with low DOC, period. For low carbon steels that is correct, but it doesn't apply to aluminium or, less intuitively, some free cutting stainless steels. Unfortunately the optimum point can vary with insert type and make, setup and material. Material from different batches can behave very differently. In general 303 stainless steel is very easy to turn and get a good finish. But I've used some that was awful, and took a lot of experimentation to get an acceptable finish.


Rod Ashton09/06/2018 11:17:08
311 forum posts
12 photos

What is 303 stainless like for lathe reaming please.

Raymond Anderson09/06/2018 12:45:38
767 forum posts
152 photos

304 it's a w***e, frown 303's the one for me. smiley 303 is free cutting Stainless and is not problematic to turning, milling or any other op.

Neil Wyatt09/06/2018 14:43:43
17740 forum posts
697 photos
77 articles

Curiously enough i just made an M5 thumbscrew in random stainless from an old inkjet.

Lovely steel to work, so probably 303.

Occurs to me that such things are a great practice piece, turning, facing, 45 degree bevelling knurling and threading all on a tiny bit of metal.


m5 thumb screw.jpg

MW09/06/2018 15:10:30
2051 forum posts
51 photos

I agree with some others that without experience of turning stainless 303, I treated it like any other piece of steel and used the wrong speed, which didn't work out too well with HSS tools =s (was fine once I adjusted it).

I think I've heard 316 being a very nice one to use. That might be only in plate/sheet form though.

I've certainly heard that .304 is the devilish one for sure devil. Might actually only be appropriate for welding or just wanting a plate that's dead hard. 

Michael W

Edited By Michael-w on 09/06/2018 15:15:40

Raymond Anderson09/06/2018 15:55:57
767 forum posts
152 photos

Hi Michael w, 316 is available in round bar in addition to plate. 316 comes in a few different compositions ie 316 Ti 316 H ect.

Ian S C16/04/2019 11:26:22
7468 forum posts
230 photos

Found an interesting way of taking the edge off a carbide tip on my latest wee job. A new Butcher has taken over the local butcher's shop, and he found that the cutter discs on the mincer were badly worn, he sent them to me at the Menzshed. I decided that the Box-Ford A was not the machine for the job, so I took them home to my 1326 BH Taiwanese machine with 8" chucks and 1.5 hp motor. The mincer discs are made of Inox steel, and fairly tough. The high parts on the discs were at the rim and near the centre, but the killer bit is the piece in between with dozens of 4 mm holes. I took it down until it just started to rub on the in between area, then left it, when I got to the butchers I suggested that to do better it would need to be ground and the 4 fingered knife would need regrinding to fit, But they were over the moon to get what I had managed. The steel might have been tough, but the tool left no marks, it came up like a mirror at 420 rpm.

One of the discs required to be bored, and I was getting on fine, taking off .5 mm DOC, as the hole opened out I thought(twitt) that some cuttng fluid might help, squert, snap, the tip of the insert fell off. Don't suddenly cool a carbide tip. The broken tip actually cut better than before the break, and finished the job.

Ian S C

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