Hard turning, slightly interrupted cut, no CBN?

[Flynth]

I've used CBN inserts a lot on hard materials... they do not do well at all on interrupted cuts. I would almost guarantee HSS would last longer than the CBN inserts...

That is my experience, anyway... about the second time it hits the interruption, the tip will break off. They are great on a smooth cut...

I'll second the suggestion to cut deep enough to get under the chrome layer.

I once watched my father machine an Enfield .303 barrel... it had a hard (chrome?) coating... he machined the coating off with HSS by getting 'under' the coating on the first pass... it worked, but didn't leave a good finish. His second pass cleaned the finish up pretty nicely.

-Bear

I heard that they're very brittle so definitely no-go for interrupted cuts, but there are ex-hydraulic rods that are still very smooth.

Did you notice if your CBN inserts have this "chamfer"? It's not visible with naked eye, but it is very apparent with magnification.

Also, I heard one is supposed to run CBN as fast as possible with the lightest cut. I'm not sure about the feed. Did you run yours like this? Could you say more about feeds and speeds you used, depths of cut, materials, machine etc ?

Maybe I'll try to get under that hard layer with hss one day (very carefully and very slowly). I have a 20mm hss blank. But I doubt my machine is rigid enough.

 

[682bear]

Some of the CBN inserts have the chamfer, some don't... I really can't tell much difference in cutting between them.

As far as speeds and feeds, I likely won't be any help... we use them on large parts on a vertical turret lathe with a 48 inch chuck... for example, one diameter is a 26.5 inch diameter turning 20 to 25 RPM, 0.005" depth of cut, maybe 0.007" feed per revolution... fairly slow cutting, but does a good job.

This is usually machining tungsten based plasma spray hardcoat or HVOF plasma. We occasionally use them on inconel, also, where we don't have an interrupted surface.

These parts are all large diameter fairly thin walled parts... they have to be turned at slower speeds to control chatter, so I wouldn't know how that would translate to turning a solid rod... I don't have any experience with that.

-Bear

 

[Flynth]

Some of the CBN inserts have the chamfer, some don't... I really can't tell much difference in cutting between them.

As far as speeds and feeds, I likely won't be any help... we use them on large parts on a vertical turret lathe with a 48 inch chuck... for example, one diameter is a 26.5 inch diameter turning 20 to 25 RPM, 0.005" depth of cut, maybe 0.007" feed per revolution... fairly slow cutting, but does a good job.

This is usually machining tungsten based plasma spray hardcoat or HVOF plasma. We occasionally use them on inconel, also, where we don't have an interrupted surface.

These parts are all large diameter fairly thin walled parts... they have to be turned at slower speeds to control chatter, so I wouldn't know how that would translate to turning a solid rod... I don't have any experience with that.

-Bear

Interesting that's only ~50 sfpm.

Even though the materials we turn as hobbyists are different it is still interesting to hear how the industry does it.

 

[Jake M]

One thing that is a bit peculiar is that (presumably) 45 degree chamfer on the cutting edge. I heard CBN removes material by friction, but such positive rake? It seems insane. Well, I can't test them too soon as my lathe is otherwise occupied, but I will at some point.

I have a thought on that chamfer. I think (I don't know...), I think that these work not by "shearing" a chip, but by "bulldozing" a blob of metal, with the intention to generate a large amount of heat, so it (almost, sorta) forces the metal to "flow" a little bit. I know my little ceramic insert/holder has square edges, but it's presented with a significant negative rake. Maybe not "that" significant, but significant nonetheless. Depending on how I run it, my little half horsepower lathe can lay out long, long strings of insanely brittle (hardened but not tempered) chips, or it can toss literally glowing red hot strings/chips/whatever it feels like. I'm kind of glad I use the "cookie sheet chip tray" which can be easily slid out of it's place, because the first face plant into the learning curve with that ceramic insert was to set that chip tray ablaze...... Whatever magic they're doing with the negative rake, it doesn't strain that motor at all. I know if I attempt to grind a negative rake HSS tool, the magic doesn't happen. It might start to cut, but the horsepower doesn't allow me to pull off that sort of stuff.

I've got a different question, separate from the (apparently) ridiculous rake angle, the nose radius on that tool- I can't tell if it's real, or a camera thing, but I saw this while I was perusing eBay as well. It appears to have a "flat" point, that "might" be actually a very, very large nose radius, but with a hard transition to the leading and trailing edge of the insert. Is that accurate, or is that just a flat nose on that surface as well?

 

[Flynth]

I have a thought on that chamfer. I think (I don't know...), I think that these work not by "shearing" a chip, but by "bulldozing" a blob of metal, with the intention to generate a large amount of heat, so it (almost, sorta) forces the metal to "flow" a little bit. I know my little ceramic insert/holder has square edges, but it's presented with a significant negative rake. Maybe not "that" significant, but significant nonetheless. Depending on how I run it, my little half horsepower lathe can lay out long, long strings of insanely brittle (hardened but not tempered) chips, or it can toss literally glowing red hot strings/chips/whatever it feels like. I'm kind of glad I use the "cookie sheet chip tray" which can be easily slid out of it's place, because the first face plant into the learning curve with that ceramic insert was to set that chip tray ablaze...... Whatever magic they're doing with the negative rake, it doesn't strain that motor at all. I know if I attempt to grind a negative rake HSS tool, the magic doesn't happen. It might start to cut, but the horsepower doesn't allow me to pull off that sort of stuff.

I've got a different question, separate from the (apparently) ridiculous rake angle, the nose radius on that tool- I can't tell if it's real, or a camera thing, but I saw this while I was perusing eBay as well. It appears to have a "flat" point, that "might" be actually a very, very large nose radius, but with a hard transition to the leading and trailing edge of the insert. Is that accurate, or is that just a flat nose on that surface as well?

Yes, very likely regarding "bulldozing" the metal.

About the nose radius it seems the flat visible is just a result of how light reflected off the insert. It has a radius of 0.8mm (30 thou) . That's a lot. I mostly use 0.4(20 thou). I ordered them long time ago so I don't remember if I chose this one because I expected it to be stronger or if I got a random one. I can't see a flat when looking from other angles.

 

[Jake M]

Probably just another modern camera artifact then. Cool. One less thing to confuse myself with...

 

[Flynth]

Ok, I have an update

I finally tested that Chinese CBN insert (a bit) and it does work. I wonder for how long.

First, let me put a big fat warning. Don't run CBN as fast as your machine goes as I read to do before. I read a document from Sandvik and it turns out CBN tools have their max sfpm too, so one can burn them out. Different grades have different speeds, but I decided to use 150m/min(450sfpm) for my Chinesium grade CBN. This is about 670rpm for an 80mm shaft(3in)

I settled for 0.1mm(4 thou) feed and 0.1mm(4thou) depth of cut. This is an interrupted cut, because it seems I'm incapable of drilling a centre right... (with my steady being too small I had to use a hand drill and a punch mark). The shaft is hardened (appears to be 55~60 HRC, and chrome coated, chrome worn off on parts of the shaft). It was used as a pin/hinge in a big digger boom.

This shaft has no gouges or even there unevenness other than some chrome wear and rust, but unfortunately I have it running about 10 thou out. It seems much harder and tougher than the last generic hydraulic rod. Normal carbide lasts 10s, hard turning carbide cracks pretty quick too. HSS in a deep cut to get under the hard layer lasts about an inch, so in theory one could peel it regrinding repeatedly.

As my steady rest is too small I had to cut this shaft. So I cut it about 2mm deep (80thou) with angle grinder, then I tried my band saw with a hss blade. It did cut, but it took a very long time (1h - it woukd take even longer but half way through I rotated the shaft so it went much faster having less thickness to cut).

Then I tried the settings above with the CBN insert and it seems to cut very well. Almost effortlessly. The chip looks like an orange flame, but it doesn't burn up fully and when it lands it looks like steel wool. Unfortunately like steel wool it catches fire occasionally. It can be pretty dangerous if a fist sized bunch catches fire and it gets lifted into the chuck which throelws it around the shop. So if your lathe is near any gasoline/petrol you may want to hide that stuff.

The finish looks very nice... Pity that insert has only one edge. They are cheap, but when you think it is just one edge... It is not soo cheap anymore.

Why am I messing with this scrap in the first place? Well, I did a calculation today. I want to make a iso40 to custom drive arbor. I wanted to use a piece of 1045 80mm round(3in) I have. I bought that round (20in long) for $17 plus shipping and here I have exact same diameter scrap shaft which I paid $2 for (both bought years ago). Let's say the CBN insert costs $3 (I think I bought two for $5.5). That is still pretty big savings and I have that 1045 for something I will want to harden. Of course this only works if my time is free. Which it is at the moment.

Here is the photo of the finish I got(the rightmost inch was peeled with hss):

 

[Flynth]

Adding better photos
After the hard layer is cut (90mm length(about 3.5in, 1.5mm total depth-60thou). No visible wear: