Buying HSS tool bits

Chuck, the M-2 HHS tool bits shown in the link you provided will be just fine for all steel that has not been hardened. M-2 Tool Bits with 8% Cobalt are recommended if you need to turn pre-hardened tool steels such as 4140, P20 or CSM-2. However, straight M-2 will cut these steels just mentioned, but the RPM needs to be reduced to about half.

I kinda like the extra latitude the T-4 5% cobalt & 8% cobalt provides, even though it is a bit more expensive. Here is an example why:
Most gear & cone head lathes are not variable speed. If facing 6" diameter steel at a given RPM you'll probably select that RPM by SFPM for that type of steel at its mid diameter. Then select a speed from those available on your particular machine. Neither situation is likely to be "ideal" because out at the 5 to 6" diameter surface speed will be too fast, and from work center and out to a 2" diameter, too slow.
Because higher surface speeds generate more heat, the cobalt tooling is better able to maintain a lasting cutting edge running out at the 5 & 6" dia. while not having to sacrifice so much at the smaller diameter areas.

BTW, a rule of thumb for determining whether to use HSS/HSS-Cobalt or carbide tooling: If a substance can be cut with a file, use HSS. If a substance is so hard that a file won't cut it, use carbide or ceramic.
Note that the rule of thumb may not apply if you are running high production rates where time, in seconds = dollars, which is rarely the case in a home/hobby shop.
 
Amigo, do you use the same grinding wheel to sharpen the 5% T-4 as you would the M-2 HSS? I use HSS nearly exclusively.

Pat
 
Pat:
Yes, I do use the same wheel only because I'm often too lazy to change to a softer bond wheel. To compensate, I occasionally rake across the wheel with a diamond to keep the dull abrasive knocked off and exposing sharp abrasive grains. This reduces the rate at which heat is generated as the tool bit is ground. A softer bond allows the dull grains to release/break away more readily.

Tungsten alloyed tool bits will dull a wheel pretty quickly. A dull wheel generates heat fast. Excessive heat ruins a HSS cutting edge. Cobalt and/or tungsten does not necessarily make a tool bit harder, it simply allows it to stay hard at a higher operating temperature.

While on the subject of tool bit grinding, for those new to this; keep a metal container of water close at hand. Do not hold the tool bit in contact with the wheel more than a rapid 3 or 4 count. Dunk the bit in the water after each count. If the tool bit is held longer, it will likely "color". "Color" indicates the bit edge has been tempered softer and will not last. Dunk often!

Another tip using HSS. Hone the cutting edge on an India bench stone after grinding.
I would recommend a Norton MB35 Medium Bench Stone 5" x 2" x 5/8".

Explanation: On examination under a microscope, a bench ground tool bit edge looks like a variable pitch saw blade. The saw blade points are of such small cross-section that they can not dissipate heat fast enough, virtually vaporizing, while cutting. Nor are they well supported and also tend to break off. The apparent result is a tool that dulls to quickly.
Honing the cutting edge on the bench stone removes those tiny points producing a cutting edge that not only last much longer but also give a better finish to the work. Whether turning steel or aluminum, a honed edge will have less tendency to develop "build-up" of material on the cutting edge.
The principle is to get the chip to absorb the heat, not the tool bit.
 
Knudsen, your HF tool bits may not be true M2 alloy or they got too hot while grinding.
As an added note, most quality U.S. made end mills are M2 HSS unless marked "cobalt" on the shank.
Too many R's when milling will dull an end mill just as fast as too many R's when turning.
 
"Truth in Labeling" does not seem to be a phrase or philosophy found in Mandarin or Hindi. Heh heh
Armstrong & Cleveland are both producers of good tool bits. That's what most of mine are.
 
Last edited by a moderator:
I have this grinder. [attachimg=1]

Will this work for grinding HSS and carbide bits?

Thanks

Jim
 
Last edited by a moderator:
Knudsen

5" 120-Grit Aluminum Oxide Dry Wheel,

8" 1000-Grit AO Wet Wheel

Thanks,

Jim
 
Jimbo, that nice Delta Knife Grinder will sharpen HSS but if either of those wheels are used for that purpose, it will just about be ruined for the purpose it was intended.

I've got some plane irons that sure would like to see that grinder, especially that "wet" wheel. ;)

That 5" AO wheel puts a nice hollow grind to the edge of a plane iron & the 8" will hone it fine enough to shave with.
Nice outfit!
 
Last edited by a moderator:
Thanks Amigo for the heads up. I have not ground HSS with it yet

It looks like its time to head to craigslist for another hit on the tool pipe. ;D

My name is Jim and I am a tool addict
 
You must log in or register to reply here.
Back
Top