Thread: HSS vs HSS+Cobalt

My gunsmith mate is a very experienced machinist and has tried the HSS /cobalt and thinks its best left to others as it is a real bastid to grind.

Time is money to this bloke and he reckons the extra time lost grinding it is not justified by any performance improvement.
Std HSS is as hard as you need.

Grahame

The cobolt makes it more heat resistant. Basically when HSS gets hot while cutting its hardness dcecreases. When it cools down the hardness returns. Cobolt HSS retains its hardness with temperature better. Can be useful if you want to turn at slightly higher speeds, or if you want to avoid useing coolant. As Grahame says, can be tough to grind, esspecially if you don't have a suitable grinding wheel. Once you've got the initial shape, sharpening is no more difficult than ordinary HSS. Personally I think it is well worth while for a roughing tool (maybe because coolant irritates my skin and I hate the mess it makes). For finishing and very small parts, it won't make any difference.

Thank you both.

Regards.

if you machine stainless or high carbon steels, hss + cobalt will come in handy as a roughing tool for sure.
if you dont its probably overkill and not required.
as was said, hss loses its hardness when it gets hot, cobalt helps preserve hardness when hot.
once hss loses hardness it will lose shape and in turn loss of shape will lose cutting edge
once hss has been hot and it blues its pretty well stuffed, in my experience anyway. you have to delicately grind away the blue (using alox wheel) and get a normal finish back on your tool for it to truly be the same.

depends on what you do i suppose, maybe keep it for a just in case and use regular hss for your general machining jobs on "normal" mild steel and softer materials.

Hmmm, I'm going to really throw the cat in amonst the pidgeons here.
HSS is extremely versitile and comes in a few different grades. Most of the HSS being sold now is very high quality, either with or without Cobalt added to the composition. I have used both types and find the Cobalt type is excellent for hard steels and cutting thru the "skin" of cast iron.
All HSS's have what is called "red hardness", ie, they still retain their hardness even when red hot. They wont cut for long as the edge will rapidly break down but they are still hard enough to cut into the steel. All HSS can be ground to a blue colour and it will still retain all its original properties. I have often rough ground a new tool and it has been red hot on the end being shaped. After cooling, I finish grind it and the tool has always been perfect. What you dont do, is quench the red hot (or blued) tool in water. This rapid cooling is very conducive to surface cracking that will quickly extend all the way thru the steel. You will know (by experience) by the sound of the grinding when the steel is actually cracked or not.
There were some extremely cheap and nasty HSS tool bits that were not finished ground but were all "black" just like a hot rolled mild steel bar. These were the tools that were bought at college when I was an apprentice, they cost 25 cents for a 5/16" x 3" stick. They were junk by todays standard but they did work and they did the job required. You would be hard pushed to find that stuff being sold today. In todays world, HSS must keep up with the TC and ceramics in quality and it certainly does. It needs to offer long "tool edge life" and be able to take a lot of punishment. There are many jobs where HSS is the only material suitable, being vastly superior to TC in the application. For all smaller belt driven lathes, it is the only tool material to use.

Kody

To get back to Fluxcored and answer his post,

Perhaps we should be trying to answer it with his point of view in mind,ie new bloke seeking answers as what is best for him.

Surely as new starters weren't we looking for a reasonably low cost and versatility.

If he already has a suitable wheel for Cobalt grinding then perhaps he is best to stick with it . But if he has only the Alox wheel the cobalt is pretty hard on those wheels.

I raced in in a got a a carbide tool for my lathe but quickly found the versility of being able to "roll my own" in HSS for less than the cost of an carbide insert, outweghed the carbide advantage.

The point is that every one has a different situation and thereby different requirements.

Grahame