Models for grinding HSS Lathe Tools

Finally hit a snag when I tried to grind a general purpose aluminum bit last night.

Any tips for grinding the 40 degree back rake?

My table only extends half an inch behind the platen towards the motor. I know I need to fix that before I make another attempt.


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Clamp a piece of flat plate to your tool rest to give you support on the right side of the belt. Main thing is to be safe.
 
mikey said:
When you're done with your grinder, we will be here.

Having the three grits will greatly speed up your honing. HSS is hard and trying to remove coarse grinding marks with just a fine stone will take a very long time. Given the small cost for a pack of three stones, it is worth buying in my opinion. I get them every few years because I tend to wear them out but they are worth it.
Click to expand...

Well, I got my belt grinder done enough to use for this. I have 36, 60, and 100 grit ceramic belts and 220 grit (AO, I think) belts.

I bought a set of Prolinemax 3/8" HSS lathe blanks, but I found a right hand bit that I bought previously (made in China) and tried to sharpen years ago. I also bought the credit card size set of diamond hones you recommended.

I tried the 60 grit belt on the left side of the bit with the table set to 15 degrees. I tried to figure out the other angle from your description. I think this can be computed by trig. You said to take about half of the end off and 1.5X to 2X down the length. So, that means arctan(0.5/1.5) = 18.4 degrees and arctan(0.5/2) = 14.0 degrees. So, I settled on 15 degrees to make it simple. Is this correct? I still have to do the other 2 sides.

Should I switch to the 100 or 220 grit belt before going to the diamond hones?

As far as the belt grinder goes, WOW! This things eats HSS. I was able to grind the side in a few minutes. On my bench grinder, this would have taken at least 45 - 60 minutes. Plus, the grind is straight and not concave.
 
devils4ever said:
I tried the 60 grit belt on the left side of the bit with the table set to 15 degrees. I tried to figure out the other angle from your description. I think this can be computed by trig. You said to take about half of the end off and 1.5X to 2X down the length. So, that means arctan(0.5/1.5) = 18.4 degrees and arctan(0.5/2) = 14.0 degrees. So, I settled on 15 degrees to make it simple. Is this correct? I still have to do the other 2 sides.

Should I switch to the 100 or 220 grit belt before going to the diamond hones?

As far as the belt grinder goes, WOW! This things eats HSS. I was able to grind the side in a few minutes. On my bench grinder, this would have taken at least 45 - 60 minutes. Plus, the grind is straight and not concave.
Click to expand...

The side cutting edge angle is a non-critical angle. I just grind it so about half or a little more is ground off the end while also taking off about double the width of the bit. Let me explain.

The side cutting edge angle is also the lead angle of the tool when the tool is fixed perpendicular to the work piece. If it is fixed like this, that lead angle has a significant impact on cutting forces and radial forces in particular. The greater the lead angle, the greater the cutting forces are; the less the lead angle, the lower the forces are. HOWEVER, a QCTP allows us to position the tool quickly and alter this lead angle to whatever we want and, in fact, we do this all the time. This makes the side cutting edge angle far less critical and there is no need to be precise here.

As for grits, I grind with a 36 grit to shape and an 80 grit to take off the coarse grind marks before moving to a diamond stone. 100-120 grit belts will do it as well or better.

And yes, a belt sander is far more efficient at grinding HSS tool bits vs a bench grinder. This is especially true when we use ceramic belts. That is not to say that a bench grinder won't do a fine job of tool grinding; it is just not as efficient.
 
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Devils4ever I did all of the calcs to be able to use a combo square as a guide on my belt sander. Look at my post #961 on page 97. That might help you out.
 
Okay, I gave it a try. I must say that the 36 grit belt was eating the HSS too fast. I was removing material faster than I liked and switched to the 60 grit. This was followed by the 100 grit and then I tried honing them with the coarse and fine diamond cards. I put a small radius on the end using the coarse hone.

How's the geometry look?


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Can't see the amount of back rake but from what I can see, you did a great job. The nose radius is well formed and consistent from top to bottom, which is what you want. The only suggestion I have is to hone each face better. Use your coarse card to get rid of the coarse grind marks. When each surface has a uniform scratch pattern from the coarse stone, step up to the fine stone and do the same thing. Finish up with the extra-fine stone and be sure to use that stone to freshen up the nose radius, then give it a few strokes on the top surface. A tool that is honed like this will slice paper and produce fine finishes while still being able to rough as hard as your lathe can go.

Now see if you can beat 0.010"!
 
Here's a side shot showing the back rake.

Yeah, I'm having a hard time keeping the hones flat on the faces, but I'll keep trying. Do I need to use water or oil on the diamond cards like on traditional stones?

Maybe, I'll try the 220 grit belt to help speed up the honing process.

Thanks so much, mikey!!!

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The back rake is okay. Next one, try to maintain the 15 deg angle as you push the tool into the belt.

I use water on my diamond stones and flush it often. Don't use too much pressure; let the stone cut. Try honing on the pull stroke; it works better and you won't roll your edges. Focus on making each face flat and the edges will take care of themselves.
 
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