HSS Sharpening

[jocat54]

I posted a few days ago about starting to grind my own HSS bits (going pretty well)

Now I am wondering how often you need to re-sharpen? I realize that there would be a lot variables.

It seems that I can get a really nice finish when starting a long cut but that goes away pretty fast--seems to fast. Just wondering if it's me or the bit or the material.

 

[12bolts]

John,
Could be any of, or, all three.
What brand of HSS are you using?
It could jsut be your sharpening technique. Do you stone the tip after grinding it?
What material are you machining and how heavy a cut?
Any pics?

Cheers Phil

 

[jocat54]

I really don't know the brand of HSS--just some I found in a box.

I haven't stoned any of them yet, but seems like maybe a good idea.

The material is some 3/4 inch well sucker rod (I have lots of it -100's of feet) Thought it would be good to practice on, maybe not. Some is really rusty and takes a long time to clean the rust up.

 

[Tom Griffin]

About the only time you should need to re-sharpen is to change the shape of the tool. Normally an occasional stoning is all that's required. Are you calculating the appropriate rpm for the material you are cutting? Remember, HSS only tolerates around 1/5 the speed of carbide. Any blue or even straw colored chips will quickly take an edge off. Are you using standard M2 bits or cobalt? It's also possible that you are grinding too much clearance and rake on your tool. A tool with large clearance and rake angles can't tolerate as much heat.

Tom

 

[jocat54]

About the only time you should need to re-sharpen is to change the shape of the tool. Normally an occasional stoning is all that's required. Are you calculating the appropriate rpm for the material you are cutting? Remember, HSS only tolerates around 1/5 the speed of carbide. Any blue or even straw colored chips will quickly take an edge off. Are you using standard M2 bits or cobalt? It's also possible that you are grinding too much clearance and rake on your tool. A tool with large clearance and rake angles can't tolerate as much heat.

Tom

Thanks for the reply. I guess I really don't know how to calculate the right speed. I run the SB9 on the slowest speed and the feed is ? Ijust kind of pick one on the QC, about the middle of the range seems to work better than slower. That's with .003 to .005 cut.

My base on the grinder is set at 10* is that too much?

 

[yugami]

Too low of a speed creates a buildup of pressure at the shear angle which can cause small cracks chips and excessive heat buildup on the cutting edge.

 

[Tom Griffin]

For a standard left hand turning tool I generally use 6-8º end and side clearance, zero back rake and around 5º side rake. Also, zero to 5º side cutting edge angle and the same for the end. You can and should use much more when cutting non-ferrous metals like aluminum or brass. There are several guides in the download section on grinding HSS tools. Are you using one of them for reference?

Calculate your RPM as follows:

Multiply the cutting speed of the material by 4 and divide by the diameter of the work. For mild steel use a cutting speed of 100 surface feet per minute, aluminum and brass is 250 SFPM and tool steel should be around 50-60 SFPM. For example, suppose you are turning a 2" diameter piece of mild steel bar; (4 x 100)/2 = 200 rpm. Aluminum would be 2.5 times as fast or 500 rpm.

Set your speed first and then set your feed by looking at the chips. If they start turning color, add coolant or reduce the feed rate.

Tom

 

[bvd1940]

You said it was rusty metal? rust oxides will dull a bit faster than bright metal with no rust.

 

[peacock]

Sucker rod is something in the range of 4320 or 4330, the rust is hard on cutting edges. The oxide of most metal is harder than the metal it's self. Also make sure your toolbit is not slightly above center. If it is this will wear the nose of the toolbit quickly.

 

[Redirish]

Plus 1 on what has been said regarding rust. Rusty surfaces will gobble up HSS tool bits in a hurry. I remove rust before I put the stock in the lathe, as I don't want rust anywhere near my lathe. All metal oxides are very abrasive. For hard steel I use carbide inserts mostly, but sometimes use cermet.