How To Quickly Grind Lathe Bits

[PAturner]

Hi,
I'm in the process of grinding the various tool bits I'll need for my lathe, and I'm having trouble doing it efficiently. I have an 8" harbor freight grinder with the stock wheels and it's taking me about an hour to grind one tool! Any tips on how to speed this process up?


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[Bob Korves]

I assume you are grinding high speed steel.

Practice! Beyond that, do not start with a fine grit wheel. Remove material with a coarse wheel. Make sure the wheels are well dressed and not dull and clogged up. It is like sandpaper, it loads up and also gets dull. With a grinding wheel you can dress that off to expose a fresh, clean, sharp, and square surface. Don't be afraid to push the tool into the wheel when roughing to shape. Have a water pot to dip it in so it stays cool enough to hold on to. Do not worry about "taking the temper out" of the steel when grinding high speed steel. HSS can go to a dull red heat without losing its temper. Once it is close to size and shape, switch to a finer wheel and clean it up. You still do not need to be delicate with the tool. Finally, hone the cutting edges (I prefer to use a fine grit diamond lap stick) to make them razor sharp. It takes a while to learn to make the angles close to correct without constantly looking at it to check. Again, practice, and it will just keep getting better. Another thing is that none of the angles are critical, so do not get anal about measuring them too much. Different work requires different angles, and that is learned by study and by experience. Watch some professional machinists on YouTube to see how they go about grinding a HSS tool bit.

 

[P T Schram]

I foolishly told my machine tool tech instructor that I had signed up for the second semeter solely to learn to grind lathe bits and sharpen drill bits.

Foolish because this meant that I spent the rest of the semester sharpening the lathe bits for all the classes both adult and high school students and sharpening every drill bit that classmates brought in.

It was well worth it because I can sharpen damn near anything and I even am asked to sharpen the knives for the local meat coutner butchers.

That said, once you do get your bits shaped as you want them, surprisingly, a razor sharp point is not necessarily desirable (yeah, I know. I reacted the same way when the instructor told me this). Once you think you're finished, using yur stone or diamond lap, go over the tip and give it the slightest bit of radius. You'll be surprised how much better it cuts.

Counter-intuitive but it has worked for me.

Now that said, I have to admit that I haven't used a hand-sharpened lathe bit in YEARS and have gone almost completely to carbide inserts. This is a change I should have made YEARS ago!

 

[rgray]

It's all in the wheel. Bench grinders come with horrible wheels. That way they last a long time.
A white 46 grit and a diamond point and some kind of jig to hold it to dress the wheel face any time it gets dull or misshapen from use will speed things up x 10.

 

[Bob Korves]

(snip)
That said, once you do get your bits shaped as you want them, surprisingly, a razor sharp point is not necessarily desirable (yeah, I know. I reacted the same way when the instructor told me this). Once you think you're finished, using yur stone or diamond lap, go over the tip and give it the slightest bit of radius. You'll be surprised how much better it cuts.
(snip)

Yes, the extra effort of making razor sharp bits is only really useful for cutting certain materials, like soft or gummy metals and plastics. It is not so good for heavy cuts or harder materials, because the sharp edge can crumble and then leave a worse surface finish when you were looking for a better finish. Still, I do it most of the time, and I think it generally helps surface finish, keeps the work cooler, and uses less power for the cut, which minimizes deflection. It is how I learned, and I could be wrong...

 

[Tozguy]

The grey wheels that come with a grinder are terrible for most purposes.
A soft white wheel in the 40 to 80 grit range will make grinding HSS tools a pleasure.
Something like a 5SG80IVS or 3SG80IVS.

 

[4GSR]

Try to find some name brand wheels like Norton, Radiac, and others, and try to get the grades like Tozguy mentioned. Some of the name brands of gray wheels are very good. I agree, one's that come with a new grinder aren't worth a flip! Ken

 

[Bill Gruby]

There is only one way to speed the grinding process and that's PRACTICE. Yes, the right wheel will ease the process but it won't take the place of PRACTICE. None of us, and I mean NONE, could grind in the beginning. Don't forget that word. When you think you have it right keep practicing.

"Billy G"

 

[rgray]

There is only one way to speed the grinding process and that's PRACTICE.

Yes but I remember starting out and trying to grind on those bench grinder wheels. Ton of heat and very little stock removed. Just getting a wheel that will cut goes a long way. And save the original wheel for finish and touch up.

 

[mikey]

I can recall how inept I was when I started grinding tools on a bench grinder. I ground so many facets on my tools that I suspect I could have made a pretty fair jeweler. Burnt fingers, burnt tools, you name it, I did it. BUT I learned, and I think several things can help you beyond the excellent advice already given, namely to buy the right wheels as Tozguy and Ken pointed to and keep it dressed as Bob said.

• Using a bench grinder for HSS requires pressure, steady and firm. Its sometimes hard to consistently do that with your hands and arms. It works better when you lock your wrists and arms and use your body to apply the pressure. Move your feet to get the angle you need and then lean into it. This also helps to maintain your tool angles; as long as your feet stay planted your angles won't change. It goes without saying that the grinder must be firmly mounted so nothing moves when you apply pressure.
  
• If you use your body its hard to keep the bit angled properly under that much pressure. The bit wants to turn away from the wheel. To avoid that, use a push block. A 3/4 - 1" wide piece of wood, maybe 6 - 7" long, can be used to back the tool and apply pressure to the tool while your other hand maintains your angle. You will be surprised how much easier it is to grind with this and how much longer you can grind before you have to cool the tool. When you use a block, orient it so as to apply pressure straight into the centerline of the spindle of the grinder. You will find this greatly increases the cutting action of the wheel.
  
• Depending on the angle, it is sometimes easier to pull the tool across the grinding face rather than trying to push and pull. The more pressure you use, the harder it is to push sometimes; so pull. Use the whole face of the wheel to speed things up.
  
• HSS is already hardened but once you get it to cut, keep going because if it cools its seems to be harder to get the cutting action going again. Learn to watch the tool angle from the top; sight along the top of the tool as it meets the wheel and that tells you how the side you're grinding is aligned. The tool rest angle determines the other angle (relief, rake, etc).
• Make a good tool rest. Each face on a turning tool is angled in two planes. One plane is determined by the angle you control with your hands; the other is controlled by the tool rest. A good tool rest is made of steel, is adjustable for height and the angle you need, is ideally resettable to precise angles and is wide enough to completely support the tool bit so you can push down on it instead of hold onto it. The rest must be wide enough to provide support for the bit on both sides of the wheel.

With all due respect, I'm going to go against the common belief that your tool angles don't matter. They in fact do matter and this especially applies to the rake and relief angles. Learn about tool geometry so you know what it is you're grinding. Cutting force reduction comes from the tool grind and this results in reduced deflection, better accuracy, better finishes and longer tool life. The smaller the lathe, the more important tool geometry and cutting force reduction becomes.

One last thing (I'm cringing in anticipation of the blows I'm about to get) - consider a belt sander to grind your tools. It is so much faster and easier to grind a good tool on it vs a bench grinder.

Hope this helps,

Mike