Carbide inserts again.

[Robin GrahamParticipant @robingraham42208]

A while back I asked for advice about screwcutting with carbide inserts. The consensus was that I was (a) going way to slow and (b) not helping myself by using crappy (Banggood) inserts. Upped speed a bit, got new inserts (JB cutting tools) and all went well.

I don't do much lathe screwcutting, but I needed to make a non-standard (41×1.5 mm) male thread recently. I thought I would have a bash at pushing carbide, so I cranked the lathe up to 550 (as fast as I dared go with a 4mm relief) and plunged 0.4, 0.4, 0.3, 0.2 mm straight in on the cross slide.

I was pleased by that, straight off the machine without cleaning the crests.

This lead me to start some experiments with carbide inserts. What I am finding (with CCMT 08 inserts) is that that I get a good finish if I take a 1.5mm cut at about twice the rpm recommended for HSS, but weirdly the finish doesn't seem to depend on surface speed – when facing (surfacing) I see no difference in finish across the diameter of the piece although the cutting speed is going from whatever to zero. Does that make sense?

I know all this stuff is documented for industrial machinery, but it ain't quite the same game at home.

Robin.

 

 

 

 

 

Edited By Robin Graham on 26/02/2019 23:34:40

Edited By Robin Graham on 26/02/2019 23:35:14

[Robin GrahamParticipant @robingraham42208]

[JasonBModerator @jasonb]

The feed will affect the finish, try a cut feeding slowly and another fast, Expect the finish to improve the faster the tool moves over the work

[Anonymous]

Looking good on the screwcutting.

Finish versus cutting parameters depends upon the material. For some materials the surface speed doesn't matter, for other it is critical. Similarly for some materials you can take a shallow cut, on others a shallow cut just tears the material. As JasonB says feedrate also has an effect. I almost never feed less than 4 thou per rev for finishing and upwards of 10-12 thou per rev for roughing.

Unfortunately low and medium carbon steels are the most sensitive to cutting parameters. For EN3B facing a large, 200mm, diameter you can see the finish go from shiny to rough as the surface speed decreases towards the centre.

Andrew

[Nick HulmeParticipant @nickhulme30114]

If you cut away from the shoulder there are no worries about speed.

[Robin GrahamParticipant @robingraham42208]

Thanks for replies. I shall make some systematic experiments. I think I've got to the point where it would pay off – I spend too much time finishing stuff with with wet 'n' dry for aesthetic reasons or for final dimension.

Andrew – did you once publish a systematic study of surface finish under various conditions on this forum? If so I'd like to reread, but have failed to track it down so far. Maybe I'm mistaken. I'll keep looking.

Robin.

[Anonymous]

Posted by Robin Graham on 28/02/2019 22:04:17:

Andrew – did you once publish a systematic study of surface finish under various conditions on this forum? If so I'd like to reread, but have failed to track it down so far. Maybe I'm mistaken. I'll keep looking.

Yes I did, along with others, here's the link:

**LINK**

Andrew

[Robin GrahamParticipant @robingraham42208]

Thanks for the link Andrew. When I first read about your experiments (2011) I had a light bench lathe so your results weren't directly relevant to what I was doing, but your  approach stuck in my my mind. Now I have a bigger machine comparable to the M300 I shall read again. You've saved me a lot of time and some metal!

Robin

Edited By Robin Graham on 28/02/2019 23:40:14

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