HSS Lathe Tools chipping, what might cause this?

[WobblyHand]

It seems that most of my lathe tools that I have ground end up chipping, or micro-fracturing. I've had one crack not at the tip, but at 1/16" below the tip. For the most part, I just re-hone if I can, or give it a quick touch to 100 grit ceramic belt and re-hone. This last one seemed to crack along the cutting edge of a square cut lathe tool bit. It is giving a ribbed, grooved finish, as opposed to a nice shiny one. This tool bit was used on 6061, so it's not on anything hard. The tools are kept in QCTP carriers which are in turn in Schaller boxes, so they don't hit anything. I'm not ramming the bit into the work, the cuts aren't super deep.

Anyone have a suggestion as to why this might be happening? The edges don't wear, they seem to chip. Right now I'm getting significantly better surface finish with TCGT carbide than with HSS. And the HSS isn't lasting long. It could be something I'm doing, but I'm at a loss at what that might be.

HSS on the left, TCGT carbide on the right.

Trying to photograph the tool tip. Here's one. I was using WD40. One can see a little aluminum on the tip, but behind the tip is a bright area, which is the edge breaking off.

Having trouble with getting the other view in focus. Not possible with my phone camera. I can see it in my stereo microscope.

Here is a view of the tool on the lathe.


Any clues I could pursue? Thanks!

 

[pontiac428]

I can't quite see what you've got going on for a grind on your HSS tool, but I would recommend stoning or grinding more of a radius into the tip to achieve a better finish. Also, aluminum likes a lot of back cut on the tool.

 

[benmychree]

I am wondering if sharpening on the ceramic belt may be overheating the cutting edge, causing micro cracking and subsequent tool failure.

 

[brino]

A little more info would help.
What is the work diameter, and the RPM?

The other issue that jumps out for me is lack of tailstock centre support!
With anything that is more than 3 diameters of the work long, the chuck alone is not enough support.

Even if it is not the root cause of your issue, I just worry that you could have an accident. The "lever arm" is long so a little bit of tool push at the far end could easily get the work off centre and then it could come around, catch the tool and launch the work around your shop.

Be safe!
-brino

 

[extropic]

A few issues come to mind.
Is it good USA HSS or cheap import stuff?
100 grit ceramic belt sounds pretty course and aggressive. I read that you hone but the close-up seems to show course grind grooves.
Is the chipping happening essentially parallel with the grind marks? How about reorienting the grind on the top relief?

 

[mikey]

In all the time I have been in this hobby I have never chipped a HSS tool except for the tip of a threading tool that didn't have a flat ground on it. Otherwise, never. You would have to catch the tip/edge on a protrusion at speed to do it and it certainly would not be likely to happen when turning aluminum.

It might help to see some detailed pics of the tool. Perhaps the clues lie there.

 

[benmychree]

Agreed, Mikey.

 

[WCraig]

Are you positive the tool bits are HSS? Are they attracted to a magnet?

There are other alloys out there (Stellite, Tantung G, Blackalloy, ...) that look like HSS but need different handling. In particular, they should not be quenched while sharpening as the metal will become brittle and may fracture.

Craig

 

[erikmannie]

Is there a slight radius at the cutting edge? I don’t imagine that you tried to make it as sharp as possible.

 

[WobblyHand]

Well thank you for all your observations.

@brino I'm aware of the large stickout. This is a 2" diameter piece with about a 5" stickout. I'm checking diameter of a 0.001" thin cut near the chuck vs the end, to check alignment. Ordinarily I wouldn't do this without tail stock support.

@benmychree if the chip is large compared to what I can remove with honing, I just touch the tool to 100 git and I get to a sharp edge again. It also helps restore the relief if I messed it up while honing. I find honing by hand to be quite difficult. Maintaining the flatness is a motor skill that I'm not as proficient as I'd like to be. At some point I need to make a fixture for this, but my tool making skills need more development. Under 22X magnification I see no evidence of discoloration due to over heating. But I can see a fracture where the surface is a different color. It could be the belt, hope not. Alas, I have no camera attachment for my microscope.

@extropic Import steel from a company that was recommended by users on HM. I bought 50 pieces. This tool wasn't honed as well as I would have liked. Looking at the photo now embarrasses me. Usually my honing is better. Now that you mention it, the fractures seem to be in the direction of the grind. I'll take a look to see if there is a different way to grind the tool on the belt sander.

@WCraig The tools behave like HSS when grinding. They are marked HSS and they are magnetic. They don't discolor if very hot. Not sure if I quenched them when grinding. I know I did it to some of them in the beginning. Maybe this is a cause.

@mikey I don't have any idea why I'm getting all this edge failure. To the best of my knowledge, I've followed your basic tutorial on tool grinding. You did point out to me that the threading tool failure I had was due to not breaking the edge of the V. So I fixed that. Haven't had issues with threading since. I'll try to get some better photos of the tool bit. It's rather hard with a phone camera. Darn phone wants to focus on the wrong thing. Wish I could take some pictures with my microscope, but I don't have a camera for it. These fractures are evident under high magnification. To the unaided eye, they are only now visible as a tiny difference in color on the edge. It's been frustrating that many tools I've ground have not lasted. I don't know, might be material, or more likely my technique.

@erikmannie I tried the best I can to have sharp edges everywhere. Under an 8x magnifying glass they look sharp save for the chip area. Phone cameras are not distortion free. It wouldn't surprise me if the photo looks curved. By eye, under an 8x hand held magnifying glass and using a 22.5X stereo microscope, I don't see curvature. Edit: I am wrong. There is some curvature of the top face. I find it really hard to maintain the flat. I may have rocked the hone.

@pontiac428 I'll get better photos.

Keep your ideas coming!