Hard Turning Solid Carbide

[jaychris]

I'm interested in turning some solid carbide. I'm planning to use a CBN insert and then I have diamond pads for polish (if needed)

I'm trying to figure out the speed/feed/spindle rpm that will work best for me. The max RPM on my lathe is 2000. According to the manufacturer, the starting value cutting data recommendations for the insert are a cutting speed of 492 ft/min, a feed of roughly .004 in/r, and a depth of cut of roughly .004.

Using those numbers on a workpiece diameter of .375, turned to between .240" and .290", I come up with a spindle speed of roughly 5000 rpm. Obviously not hobby-ist machine speeds.

So, my question is - if I have a max spindle speed of 2000 rpm, a starting workpiece diameter of .375, a cut length of 2", and a CBN insert grade CB7015, what would be the best speed/feed rate, given what I have?

Maybe the insert I'm looking at is not the best choice, so I'm open to suggestions there as well.

 

[kd4gij]

Good luck, but I have never seen carbide turned. Either EDM or ground with wet diamond wheels. And we had a tool and die shop.

 

[4GSR]

Tend to agree with kd4gij, I don't see CBN cutting carbide at all. But if the insert manufacture says it will, go for it!

 

[G MORSCH]

Hello, Jay in Settle:

As you may know, carbide is a powder metal material crushed/milled to the proper size for compacting with a binder. That binder is typically cobalt, in the cutting tool industry. But the binder can also be nickel for corrosive applications where wear resistance is needed. All of those pretty carbide wedding bands you see advertised use a corrosion resistant binder and they are machined with diamond tooling. During the sintering process the tiny carbide particles do not form an inter-crystalline bond with each other, they are simply stuck together with cobalt, which has a really high melting point. Typical particle size for a general purpose turning insert will range from 2 to 7 microns and the cobalt content will be 5% to 10% cobalt by weight. You will not form any kind of chip with this structure, it will come off the part looking like brake dust. Which is exactly what turning pre-sintered carbide looks like. The hardness of a typical insert will range from 89 Rockwell "A" scale to 93 RA. Edge definition on the CBN will typically be lightly honed at the edge and most times it will also have a "T" land from .004 up to .025 at 10 to 30 degrees. These features reduce micro chipping at the cutting edge and major chipping in general, axially, radially, and tangentially. But they can be up sharp or just lightly honed for light finishing applications as well.
So, I was just out in my shop and chucked up a 1/2" end mill in an ER40 collet. Grabbed a CNGA433 BN250 Sumitomo insert with a "T" land and took a 1/2" length of cut at .005 depth of cut and .0015 IPR feed, 810 RPM. Axial pressure was low and radial pressure was high enough to only remove .001 to .oo2 material. Then I tried .010 per side and the radial pressure chipped the top plane of the CBN segment off the top of the insert. In hard turning, this failure mechanism is not uncommon. As soon as we accumulate some flank wear, radial pressure spikes and the energy required to hold the insert in place is great enough to shear the top plane off the insert. Impact will chip an insert from the top down the flank but radial pressure from loss of clearance will cleave it across the top of the CBN segment. This can be in the form of a small chip or even the entire top of the insert. I can turn HSS easily in my Sharp 1230, this machine is pretty stout. But carbide is another story.
The dust was glowing as it came of the end mill/CBN interface, as I expected. But the tool pressure was just too much for this mini-tip insert.

Hope you have better luck than I.

Best Regards, Gary

 

[JimDawson]

Thanks for that writeup Gary!

 

[mmcmdl]

Turning carbide ? Good luck with that . How about grinding with silicon carbide wheels ?