Models for grinding HSS Lathe Tools

[mikey]

I would use more speed for both. With harder steels, HSS likes to rough slow and finish fast. By fast, I mean really fast with light cuts. For example, when I turn a 3/4" piece of 1144 on my little Sherline, I am running a finish cut at 2800 rpm. For your piece, I would try a fine cut at 15-1800 rpm and see what that does.

Carbide also likes speed. Finish depth of cut depends on the nose radius - what is yours? I would also try to increase speed for finish cuts on that piece; try 950-100 rpm and go up as needed. Your tool holder probably has a 5 degree positive rake and is normally used with the tool shank perpendicular to the work but try turning the tip toward the tailstock a little bit. This can really help the finish when taking light cuts at high speed.

 

[ttabbal]

I'll try that, thanks Mike! The carbide has no discernable nose radius. HSS is about 1/32" going by my gauge set. I'll set the lathe up for high speed range and give it a go. The break at 380RPM is a bit annoying, but, first world problems.

 

[mikey]

Every insert has a nose radius and you need to know what it is. The last digit is the nose radius, usually in 64ths. For example, the nose radius on a CCMT 32.51 insert is 1/64", or 0.016".



I won't go into too much depth on using carbide inserts here but it is wise to know the minimum depth of cut your insert will take accurately and then try to optimize your speed and feed to finish well at that minimum depth of cut. That way, you come in on size AND produce a good finish.

 

[ttabbal]

So, I got up to about 850RPM. I tried a 0.002 diameter cut with my M2 HSS bit. Killed the edge nearly instantly. Perhaps I should try cobalt.

The carbide is DCMT070204, so 1/64" radius. At the same RPM and feed it does decently with a 0.020 cut, but it throws a shower of red sparks doing it. 0.010 gets blue chips and cuts alright.

 

[mikey]

That material has to be high carbon steel. HSS will cut 4140 with no problem so it has to be harder than that to kill a HSS tool that fast. Did you try slowing your speed to see if you can go deeper?

 

[ttabbal]

Above about 200 RPM, the edge on the M2 dies fast. The tool gets a flat spot at the contact point. Below that, it does seem to work decently. Though I haven't gotten much depth without damaging the tool.

The carbide seems to be working fine up to about 0.030 with lower speed around 350 RPM. Beyond that, the lathe can stall.

This is my first go at mystery metal. It seems pretty tough. I had less trouble with 304 stainless.

 

[mikey]

I don't suppose you have those files for testing hardness, do you? It would be interesting to see how hard that stuff is. I've only run into a truck axle that behaved the way your material is and it was a bear to cut. In fact, I tossed it. There is some stuff for which using carbide makes the most sense; this is one of them.

 

[ttabbal]

I don't have the hardness testing files. I was actually looking at them yesterday after I did that higher speed test as I am curious what the hardness is like. I have a bunch of it, but I'm not sure I can use it now. I didn't expect it to be this hard to work. Guess I need to think about getting some of those files if I'm going to buy mystery metal.

Is it possible I hardened it when the HSS blunted the first time? I didn't look for it and the part got pretty hot before I realized it. The HSS was against the OD which looks like it could be hot rolled, so mill scale is a possibility... Guess it doesn't hurt anything to fire up the bandsaw and cut a fresh piece off to check. Use the carbide to get under anything on the outside.. It would be nice to be able to use it. The HSS seemed to work ok on the facing cut, which was the first thing I did as I wanted to drill for a live center.

I should probably grind a few extra HSS bits to have around for when I do something like this.

 

[mikey]

HSS will cut most stuff except hardened steel, which will destroy an edge quickly (like it did here). Cobalt tools might work, at least for awhile. If light cuts are producing blue chips then cobalt might last longer.

You can try a common hand file and test it. If it skates, its hardened. The Tsubosan hardness testers actually work pretty good; I have a set and they would tell you which range of hardness you have.

Since you have a bunch of this stuff it might be worth it to buy a CBN insert and see if your lathe can go fast enough to use it. You would need to take really light cuts because your lathe is not all that rigid but it might work. As for how useful this material is, if you cannot cut it except for 0.006-0.008" DOC then it will take forever to make something out of it.

 

[ttabbal]

I agree, if I can't take a reasonable DOC, it's not going to be particularly useful material. I'll hit it with a file, and try a fresh piece to make sure I didn't do something stupid. Maybe pick up a set of those files just to see what I'm dealing with. It might end up at the recyclers if I can't use it. It's making me really appreciate how easy 12L14 is to work with.