Temperature control when grinding HSS tool-blanks?

Hi there, all,

I've noticed that, when grinding an HSS tool-blank, some folks cool the blank at intervals by plunging it into cold water. (I'm trying to avoid use of the term 'quenching'. ) On the other hand, I've heard it said that such plunging of HSS blanks leads to micro-cracks at the tool cutting edge.

Please can any knowledgeable members here give us a metallurgist's comment on this proposition? And what is the experience of the general membership?

Some 'real' workshop tool grinders or their pedestals have water-pots built-in - is this a historical legacy from pre-HSS days when tools being ground would have been carbon steel?

Best regards,

Swarf, Mostly!

I do the little & often one, ie, a litle grind followed by a quick dip. My logic is that as I'm holding the tool, my fingers won't take kindly to any excessive heat, and excessive heat is not really that hot, 50 degrees C perhaps? The water I use is likely to be at room (garage?) temperature, so perhaps not exactly cold so I'm assumig that the overall temperature reduction is going to be quite small. Plus, of course, the quick short grind doesn't really generate that much heat.

Of course, the above may be a load of codswallop, but that's what I do. And in reality, I don't have much option to do anything else.

Peter G. Shaw

I'm a non plunge fan and use gloves

+1 on Peter's "little and often" grind / plunge into water method. Used it for many years, never had a microcrack issue with HSS. I use room temperature water but am not sure whether hot water or cold water would have much effect at all on HSS.

Most grinding is touch-up stuff and only takes seconds

Do the big part with gloves then finish the detail off later with bare hands

"General Membership" speaking here (it's certainly not a metallurgists view)

Generally, I don't grind tools beyond the point where I can comfortably hold (or touch) them. For smaller tools, I tend to keep them in their holders (or grinding guide e.g. Diamond tooling) - which also helps keep the tool cool. But it's worth mentioning that I'm generally not in a hurry either

Grinding can be a 'forming/shaping' process (new tool - heavy/much grinding) but is more often just a 'touch-up' (existing tool - light grinding) where most clearance surfaces don't need touching. It helps if the same grinding set-ups can be repeated when doing this. When grinding, I find it generally easier do multiple tools in one session and at the same general settings. So I can cycle through the tools. I also often hone a tool to either get the edge I need or to touch a tool up and this is much more ad-hoc.

So to answer the question about 'dipping' - I normally do have a 'pot' (ice-cream box) with water in it and dip or drop tools into it - but I think the key thing is that if the tool is 'cool' enough to still hold or touch, then I don't believe that it will come to any harm by dipping. If the tool really sizzles (e.g. is too hot to handle) when dipped, then this could be a very different matter of course.

You asked about HSS Swarf but I also use carbon steel tooling and much more care (when grinding) is needed with these tools to prevent heat damage. HSS is pretty robust though.

Regards,

Ian T

+ 1 .. Me too, although most of my cutting uses a tangential HSS holder, so really only a light touch up when needed. Any other heavier turning I have insert type tooling as needed.

George.

I'm no metallurgist,

Is it possible that at the elevated temps we are talking that the qualities of the hss are impaired to the point of forming micro crystals if cooled by cold water immersion?

Surely, 50-60 degrees isnt hot enough to be effective but enough to cause discomfort?

Discussed here only the other week

Jason,

Oops, I'm sorry to have started a thread so close in topic to the one to which you linked. In my own defence, the title of that other thread is not exactly an intuitive search string.

In my opening post, I should have made it clear that I was concerned with grinding a new tool blank - not merely touching up a tool that was already at the right shape.

Best regards,

Swarf, Mostly!

It's how hot the tip of the tool gets that matters, not where it's being held. Think pokers - the tip can be white hot whilst the handle end is still cool enough to touch.

Oxide colours are a much better guide to grinding temperature than fingers. If the sharp end of an HSS blank goes blue it's approaching the danger zone. Blue is about 300°C.

HSS can actually go a couple of hundred degrees hotter than blue before damage, but all that's needed to ruin the edge is to overheat the tiny amount of metal at the tip. Easily done by a cack-handed grinder.

Plunging at 300°C or less is much less likely to cause microcracking too. Plunging much hotter HSS is asking for trouble.

Dave

Yup you are right, dunno what I was thinking...🤔

Obviously not paying attention at the back....

One thing that I have noticed is that when grinding using the coarse wheel, the tool doesn't seem to get as hot so longer hand held grinds are possible. But the finger temperature test still stands even for that.

One other thing, and this may be only applicable to me. The motor on my grinder, a 150W Clarke something or other with 5 inch x 1.2inch wheels, gets very hot itself, so much so that after a modicum of grinding, I have to leave it to allow the grinder to cool down. It's been like it for over 30 years and is still going strong (famous last words!) so I assume it's being used within its design parameters. Anyway, there's another reason for short bursts of grinding.

Peter G. Shaw

I find the 1/4" square HSS blanks I use rapidly get too hot to hold, so I keep a dip tank in easy reach. I use an old kids' beach bucket, but marge tubs work just as well. When I was working on the shop floor in the 70s, a dip tank was pretty well universal.

I do find that I sometimes grind too long between dips, and it's worth trying to avoid that. Recently I had an HSS knife tool that kept sort of micro-crumbling at the tip radius - I'd put that down to a cheap blank from an unknown Ebay source until Swarf's comment above. Even though I've been doing this stuff for nearly 50 years, I can still see where maybe I've got careless. I'll try the little and often bit.

I bet a fair proportion of forum members never dress their grinding wheels. A loaded or blunt wheel will heat the tool tip much more than an opened-up, dressed wheel. Fine wheels are more prone to this, hence Peter G Shaw's observation. Also, keep the tool moving across the wheel, where possible.

'Ere, some of us do that quite often!

For a long time I managed with one of those grey cuboidal dressing stones. They work, but as you lose the corners I found it harder to get a straight wheel edge. So tthen I bought the cheapest, tiniest diamond-point dresser I could find a couple of years ago and it's just about wearing out now.

One of the things to make your mind up about is which corner of the wheel edge you want sharp and which radiused. I think the proper answer ought to be left and right respectively, since if you want to form a small rad on the work it'll usually want to be on the left of the tool - but somehow I usually seem to get it ar$e-about-face.

When the wheels start to get significantly smaller, I've always preferred to buy a new cheap bench grinder than make a project out of fitting new wheels - I don't want to invest the downtime or risk, when I can avoid it at low cost. I shall probably replace my current one in the next year or so, and I think I've had it about 7 years.

I don't have a wheel dresser. Which of these is the preferred type? C and D look similar.

Type 'B' is the diamond type Gary - which I use in addition to having a dressing stone. You can use them freehand but I think they are best if you can guide them to get an even surface. I use the diamond dresser on my fine wheels but generally clean up my coarse wheels with the stone. They both work but the diamond dresser gives a more precise finish if guided.

I have no experience of the other kinds - I've always felt that the 'Star' dressers (type A) look a bit crude...but I've never used one, so others should comment on them.

Regards,

IanT

Hi there, Peter,

I have a modern two-wheel grinder that actually specifies a duty cycle on the motor rating-plate.

Best regards,

Swarf, Mostly!