Temperature control when grinding HSS tool-blanks?

[Mark ReaParticipant @markrea67084]

Posted by IanT on 14/10/2020 14:07:13:

"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

This is it. Totaly agree.

[Howard LewisParticipant @howardlewis46836]

80C is as hot as I can stand, but the part bin contact with the wheel is going to be at a highev temperature, so I let it cool naturally, and then dip it into water.

Had no ends fall off yet, and the tools seem to cut quite well, and are more forgiving of my carelessness than carbide tips!

Howard.

[John Doe 2Participant @johndoe2]

I hope the OP doesn't mind if I piggy-back this thread:

Not a machinist, so can I ask those with experience – if I use my angle grinder to quickly dry-grind* an approximate 25° angle on the end of my wood chisels before diamond grinding them to the correct final dimensions in a Trend diamond chisel hone: will I bugger the temper of the chisel steel with the angle grinding step?

Secondly, when I sharpen my DIY HSS drill bits in a Martek drill bit sharpener containing a small drill-powered dry grinding wheel, (below), should I be dipping the drill bit end in water for similar reasons?

Finally, how do I sharpen the type of tool shown next to the drill sharpener without a mill? (It cuts a clearance hole and counter-sink as well as the hole, in one operation). The drill bit is removable but the cutting body has a ground cut-out that forms the cutting surface, but I only have a drill press?

Or a can you please recommend a good book on tool sharpening for the amateur. Many thanks.

*for example to remove a notch in the blade.

[VicParticipant @vic]

If micro cracks are indeed a consequence of dunking your tool when it’s too hot, what are the practical after effects. Tool breakage, poor finish etc or something else?

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Doe 2 on 25/01/2022 13:34:24:

…

if I use my angle grinder to quickly dry-grind* an approximate 25° angle on the end of my wood chisels before diamond grinding them to the correct final dimensions in a Trend diamond chisel hone: will I bugger the temper of the chisel steel with the angle grinding step?

Secondly, when I sharpen my DIY HSS drill bits in a Martek drill bit sharpener containing a small drill-powered dry grinding wheel, (below), should I be dipping the drill bit end in water for similar reasons?

Finally, how do I sharpen the type of tool shown next to the drill sharpener without a mill? (It cuts a clearance hole and counter-sink as well as the hole, in one operation). The drill bit is removable but the cutting body has a ground cut-out that forms the cutting surface, but I only have a drill press?

Or a can you please recommend a good book on tool sharpening for the amateur. Many thanks.

…

Yes you can overheat HSS in an a grinder. Sparey says if the metal goes blue near the tip, that section of HSS has to be ground off. He doesn't say so, but I think if the body of an HSS cutter has gone blue, the chances are the sharp edge got a lot hotter.

The answer is to not grind metal off as fast as possible and for more than several seconds! Take it down gently in stages, don't let the metal get too hot, and dip in water.

Same principle with a twist-drill except the risk of overheating is lower because the edge just needs touching up. It's unusual to have to grind of lots of metal.

Sorry, I don't know how to sharpen the countersink tool and can't remember to title of the book that told me about sharpening.

Dave

[pgk pgkParticipant @pgkpgk17461]

Agree with taking time if sharpening chisels on a dry grinder. I wouldn't bother using it unless there are significant dings/chips in the end of the chisel and just slick it up on a stone or diamond pad. For my fancy woodcarving chisels I use a Tomek water grinder and even then avoid unnecessary grinding..just re-establish the edge to chip free and hone with paste and leather. A well honed wood chisel lasts for ages with just repeat honing. For dodgy chisel work – old timber with plaster or cement stains or nails nearby – I use old cheap chisels that i don't mind damaging or grinding chunks off
I rarely bother trying to sharpen drill bits – jobbing quality drills from the likes of UKDrills are cheap enough not to waste time.
For HSS lathe tooling I doubt one will get it hot enough to damage the temper and still be able to hold it unless using thick gloves. Tom’s Techniques did a video on hot hardness to prove that point. Like many i keep a bowl of coolant by the grinder and keep dipping.
I'd guess that all-in-one tool would be best hand-dressed with grades of round stones or diamonds.

pgk

[noel shelleyParticipant @noelshelley55608]

Rapid cooling will cause cracking and an angle grinder is the wrong tool for sharpening most things ! There is a book in the workshop practice series that is on tool grinding that may help and at about £7.50 not too bad. As to drill/countersink tooling – not easy without special tooling to get correct. Good luck, Noel

[Andy StopfordParticipant @andystopford50521]

You won't harm HSS by overheating on a grinder.

I once bought a load of second hand horizontal milling cutters, including a number of large slitting saws. The saws did not survive the journey through various countries' postal systems and had been smashed by contact with the heavier cutters.

Not wishing to waste them, whenever I needed a lathe or shaper tool, I cut pieces off with an angle grinder and brazed them onto mild steel shanks, filed to provide suitable rake, etc. Despite heating them red hot for the brazing, they worked absolutely fine. In fact they were far more useful than if they had remained as slitting saws.

n.b. I let them cool in their own time – I would imagine quenching would be undesirable.

[Mike PooleParticipant @mikepoole82104]

With a dip tank close by it is no trouble to dip before things get too hot, although HSS should not lose its temper easily I am convinced that the edge of a tool that has been overheated when grinding is not a durable as one that has been kept cool. Tools with thin sections are very easy to overheat and care should be taken to avoid overheating by keeping things cool.

Mike

[MikeKParticipant @mikek40713]

Posted by Swarf, Mostly! on 14/10/2020 13:20:15:

I've heard it said that such plunging of HSS blanks leads to micro-cracks at the tool cutting edge.

I'd want proof that a) it actually happens and 2) that it actually is a problem. As many on here I've been grinding HSS blanks for years. Don't try to fix what isn't broken. Less worrying, more chip making.

[larry phelan 1Participant @larryphelan1]

I have a cheap 6" bench grinder, bough some 30 years ago, Nu Tool make, still going strong, never knew it to overheat. I simply grind, dip then grind again. When my fingers object, I dip again I never allow the HSS to change to blue, since I dont believe it,s a good idea. I dont think I have had any issues with cracking, or if I have had, I never noticed.

All very crude, like most of my efforts, but it seems to work !

Then, of course, I dont make parts for spaceships.

[Nigel McBurney 1Participant @nigelmcburney1]

in general industrial practice hss turning tools were ground on grinders which had fine and coarse large diameter wheels, with more than adeqate motors to power the wheels,hot tools were dipped into water in a bucket,where I worked an empty can from the paint shop was used,wheels were dressed using the star wheel dresser type A in the photo,they were relatively cheap and do a good job in dressing and truing up a wheel At work diamond dressers were only used on grinding machines and tool and cutter grindersI, have followed these practices in my home workshop,I do try to avoid getting HSS tools up to "blue" The micro cracking was often mentioned in our technical college books though in practice I never saw it occurr, though we were told not to get a tool red hot and then quench.A problem with bench grinders with small wheels,is that they are usually underpowered and the small diameter wheels produce a hollow ground effect to the cutting edge which wears quickly and looses cutting clearance,large diameter industrial wheels greatly reduce this effect and tools last longer, When I started my home worshop in the 1960s tool grinders were very expensive,so it was a case of live with small grinders or do like I did and make a 6 inch grinder using a motor cycle cranckcase with a home made spindle held in the m/c mainbearing housing and a belt drive passing through the aperture for the con rod,lasted for years until cheap grinders from the far east became available and I now use two two 8 inch grinders with a variety of wheels. With theadvent of 1mm thick disks for angle grinders I now rough out HSS tools ,particularly those with lots of top rake with an angle grinder and then finish grind on the bench grinder,, in the days before health and safety there was just a notice above the works grinder "goggles must be worn" nobody ever wore goggles for tool grinding , the only time that the foreman insisted on goggles was when dressing wheels as the grit did fly. And I would advise not to dress a wheel anywhere near a machine tool, I have an inner and outer workshop and all the welding,abrasive dressing ,and tool grinding takes place in the outer workshop, and I would never use a toolpost grinder on a lathe.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mike Poole on 25/01/2022 20:17:22:

With a dip tank close by it is no trouble to dip before things get too hot, although HSS should not lose its temper easily I am convinced that the edge of a tool that has been overheated when grinding is not a durable as one that has been kept cool. Tools with thin sections are very easy to overheat and care should be taken to avoid overheating by keeping things cool.

Mike

Me having quoted Sparey as the authority for damage, Andy Stopford made me think when he said 'You won't harm HSS by overheating on a grinder.' Which makes sense because modern HSS (ie made after about 1930), is still hard enough to cut at red heat. (Not a good idea because red hot HSS goes blunt faster than cool HSS.)

Also true it's exceeding difficult to soften HSS by annealing, because accurate temperature control and long soak times are needed.

'Grinder Burn' is an easy way of destroying the hardness of ordinary tool-steel, so maybe Sparey was echoing a recent folk-memory. However, I think not because his book spends a paragraph or two debunking the alleged qualities of carbon-steel lathe tools.  In 1950, HSS was new within the lifetime of many machinists, and there were plenty of chaps who preferred the old ways, believing carbon-steel can be ground sharper than HSS, and that it produces a better finish. I think Sparey knew his stuff.

Two other possibilities remain.

• I think Mike is right to worry about the very high temperatures that can be generated in thin sections. Grinding sparks are HSS undergoing a fundamental chemical change due to being hot enough to burn in air. Not unlikely that the composition of the alloy at the edge of an overheated HSS tool is altered.
• And I think Swarf Mostly and Noel are right about micro-cracking, which is mechanical damage invisible to the naked eye.

I suggest overheating and/or rapid quenching can produce a damaged edge and the effect is rapid wear rather than catastrophic failure or poor finish. A HSS edge without micro-cracks and heat damage stays sharp longer that the same item mistreated.

I think rapid wear is impossible to detect in an ordinary workshop and it doesn't matter to us. When HSS tools stop cutting we simply resharpen them, without knowing if the bluntness is premature or not. Far more important in production where getting another 10% out of cutters doing saves big money.

As it's easy to avoid overheating while grinding, I just do it. No harm, it might be worthwhile, but it's probably daft to fret about it.

I'd worry more about grinding wood-working tools though. They might be ordinary tool-steel or a different HSS alloy. As high-temperature cutting is unlikely in wood-working, the HSS alloy used in chisels etc might be optimised for edge-holding and toughness rather than heat-hardness.

I always worry about generalising. Some folk believe there are two sorts of HSS: old British, which is wonderful, and the modern rubbish made in the Far East out of Cheese. If only it were that simple!. At least seven different High Speed Steel alloys are commonly available and their exact properties are often tweaked at the factory by heat-treating to suit particular industrial needs. I've no idea exactly how each of the seven common HSS alloys behaves on a grinding wheel. I don't even know which alloy some of my HSS is made of…

Dave

Edited By SillyOldDuffer on 26/01/2022 11:04:43

[Alan Wilkinson 1Participant @alanwilkinson1]

The Huntingdon dresser type A in Pic is the one to use, it has two lugs underneath which run along the edge of the grinder rest after pulling rest to its extream out and dress the wheel square

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