Cut Off Blade Grinding...?

[EmilioG]

How do I grind a HSS cut off blade and prep it for cutting steel? Do you just grind and hone the front part or do you also put a radius on one side?
Any diagrams or photos available? I've seen differing opinions. I need help on this. Also, can someone recommend a good cut off blade holder that uses inserts?
ThinBit?
Tool: Cleveland MoMax HSS cut off blades.

 

[Ulma Doctor]

Hi Emilio,
the cutoff is pretty simple, put an end relief of 5°-10° and slightly skew the blade 10°-15° to the left when sharpening.
5° end relief for really tough stuff, 10° end relief for easier to machine materials
it will leave the parted piece without a nub. (the piece held in the chuck will have a small cone shaped nub)

i have not had good luck with insert cut-off tools,
but then again i tried a very used unit that wouldn't hold the bit tightly anymore, and threw it out- i have not replaced it but i should consider it

 

[EmilioG]

Thanks Doc. Is the relief grind rounded or sharp, like a chamfer or radius? I also read that one should hone the end with a small diamond sharpener in a down direction only. Not sure what that's all about.

 

[Ulma Doctor]

you are very welcome,
i like mine sharp as can be for when it matters, for repetitive parting i'm not as meticulous
no chamfer, the radius from your wheel is ok, if you grind the end relief and the skew while the tool is vertical.

i don't use diamond sharpeners on HSS, but others do and claim it works fine.
i have not heard of going in the downward direction only, maybe there is something to it, but i really can't see what.
i'd suggest an oilstone as, hard as you can get to hone the edge when you need to (in whatever direction floats your boat)
i try to create a very small bur at the top of the tool, then i gently stone the burr out from the other direction
i believe the method ensures the best edge, others have their way of doing the same thing

 

[mikey]

Emilio, if you go with HSS/cobalt blades, I suggest the P-type blades. This tool is built like a girder; it is very stiff and has a 5 degree side relief on each side to reduce binding in the cut. I sharpen mine similar to UlmaDoc except that I use a 7 degree nose relief and grind it flat across the face. Either way, his or mine, works and you should try both ways and see which works best for you. I use a 7 degree relief angle because I found it to be the best compromise when parting anything for which a HSS blade is appropriate. And yes, I ground and tested multiple angles to find my preferred angle. That doesn't mean it is right; it just means its right for me.

I also have an Aloris AXA-71 that takes GTN-3 inserts. This is huge blade (1.5" tall X 0.120" wide) that actually works well but the narrowest insert it takes is 1/8" wide. I use it when I cut harder stuff and have had no issues with it but there are smaller, narrower versions of this type of insert that I think would work better. Iscar makes more modern insert geometries; worth a look.

I also have an Aloris AXA-7 that will take HSS blades and smaller/narrower inserted blades. I don't like this tool much but it works okay if you get the blade perfectly vertical.

Sharpening a HSS tool is simple. Get the blade vertical in a tool holder, lay the holder on the side and rub the front of the tool against a sharpening stone while following the front relief angle. I prefer diamond stones but that is a personal preference. I stroke the blade only in the downward direction to avoid the tip from digging into the stone (which I have done all too many times). A few passes is sufficient to remove grinding marks and a pass or two on top to remove the burr and the tool is ready for use.

HSS parting tools need to be very sharp and used precisely on the centerline of the work and precisely perpendicular to the work. When parting from the front, keep your blade extension to a minimum. When parting from the rear, blade extension is not a major concern. When the tool cuts well, it only takes a slight positive feed to keep it cutting. That is, you should feel a slight positive resistance to the feed. Try to keep the feed consistent and the tool should part with no issues if the lathe is tight. In my experience, speeds should be whatever allows you to keep up a positive feed rate. Works for me anyway.

 

[cathead]

If you sharpen off your blade slightly narrow towards the chuck, the part won't end up with the dregs. Another thing I will sometimes
do is grind a small groove in the top of the center of the parting blade to facilitate chip expulsion. You can tell if the
blade is sharp with your finger tips or look at it with a loupe. I will do that before installing a parting tool to see if it needs attention.

 

[benmychree]

I prefer the T type HSS parting tool, its parallel relief lessens friction and allows a bit more side relief on the upper portion; in grinding, I just grind on the front of the wheel, usually with an angle to make for clean cut offs, but too much can cause the tool to run off sideways causing a hollow face on the cut off part. If I'm using a plain tapered tool, I also lay the tool over on its side and touch up the top of the tool if necessary on the side of the wheel; I never use hones or stones on parting tools.

 

[4ssss]

I prefer the T type HSS parting tool, its parallel relief lessens friction and allows a bit more side relief on the upper portion; in grinding, I just grind on the front of the wheel, usually with an angle to make for clean cut offs, but too much can cause the tool to run off sideways causing a hollow face on the cut off part. If I'm using a plain tapered tool, I also lay the tool over on its side and touch up the top of the tool if necessary on the side of the wheel; I never use hones or stones on parting tools.

This is my way of doing it also except I prefer a straight blade over a T blade. Also, one thing I do that probably a lot of guys don't is I use a 1/16 wide blade, keeping it as short in the tool holder as I can. The less friction, the less chatter.

 

[benmychree]

Since I do most of my lathe work on my 19" Regal, chatter is not a problem; I use 1/8" wide, both carbide insert and HSS. It took a while to get used to carbide parting, with too many broken inserts costing too much money, then a guy came along with nearly a pickup load of tools and inserts And holders I tried the inserts again, and since they were acquired quite cheaply, I was able to give it another try and master it; I use the HSS mostly for larger diameters and HSS for smaller work.

 

[Silverbullet]

I try to use the arc of the wheel to provide the tip clearance , but the rake should have a small relief with an arc also it helps curl the chips up out and away. Some like a very small vee in the tip of the blade it works to thin the chips and brake them . Very hard to tell ya how if I could show you a time or two you'd never forget how.