Co5 is 5% Cobalt, so harder than normal HSS

Suggest using a coarse grit wheel to rough the shape. Also get a water container and keep it next to the grinder with room temperature water in it. A large tin can 400 to 500 ml size from the recycle bin (cleaned) will work great and won't melt if contacted by a hot part. Hold the workpiece in your fingers so you can feel it as it heats up. When it is uncomfortable dip the tool in the water and hold it there until cool. If you use this fingers / cooling method, the risk of overheating an HSS tool is minimal. It takes patience to grind HSS tools but once made they will last for ages. A very light touch up sharpening in a fine grit wheel will keep them cutting nicely. Some people like a whetstoned or honed edge but I don't bother with that generally, and get excellent finishes usually.

I've never noticed much difference in hardness when grinding a wide range of HSS toolbits. I'd be surprised if the cold hardness varies by more than a few points on the Rc scale. Cobalt is added to increase hot hardness, at temperatures far above those at which one can hold the toolbit.

When grinding HSS I have a water dip simply to cool the toolbit so I can hold it. The whole point of HSS is that it retains hardness at high temperatures. So, unlike straight carbon steels, it is unlikely to be annealed when hand grinding. After grinding a toolbit I give the cutting edges a lick with a diamond hone to remove any small burrs.

If I need to remove a large amount of material I use the milling machine. A carbide cutter will mill HSS with no problem, like the involute shape on this 1/4" square HSS:

Andrew

Not to disagree with Jeff (Dayman) but making a mild steel holder to take the HSS blank makes things so much easier to maintain the angles and position the tool blank especially when roughing the tool out initially.

I finally made one after far too many scorched fingers. Just a hole big enough to take the diagonal dimension in a piece of round bar with a grub screw in the side is all that's required. Yes you do need to keep an eye on overheating but it's not a major concern with HSS as it is with home made carbon steel tools. Just a container of cold water beside the grinder is all that's required for that.

I don't think I have ever spoilt the hardness of HSS of whatever grade by grinding temperatures and grinding new tools is something I approach with a degree of 'can I get by with something else' but when needs must a holder does take the pain out of the task.

Best – Tug

If I need remove a lot of metal from the blank sometimes I clear the bulk off with a cutting-disc on an angle-grinder.

Yes, the tool does need clamping firmly! I do that outdoors to minimise hurling grit around a cramped workshop.

It is speeds up the whole operation and saves wear on the bench-grinder wheels.

Recently I needed grind a grooving and threading tool to make a tiny brass plug. I used a new 1/4" HSS bar to make a double-ended tool, and kept the top surface untouched to give both the required 0º top-rake and single height adjustment.