|Chris TickTock||30/09/2020 20:40:49|
|576 forum posts|
Holding 50mm long 1/4 inch square stock whilst grinding means fingers get very hot / blistered. This is an age old issue. But is there a remedy in terms of using either a holder / square tube / tool or heat resistant gloves which still give enough feel?
|Frances IoM||30/09/2020 20:55:12|
|827 forum posts|
|most grinders have a small trough to fill with water to regularly cool the HSS - otherwise years of soldering makes finger tips fairly impervious to heat!|
|roy entwistle||30/09/2020 21:25:36|
|1245 forum posts|
I would use mole grips
|Michael Gilligan||30/09/2020 21:49:39|
16342 forum posts
Be sure to clean the fur off ... it smells awful when it gets hot
4804 forum posts
If you are getting 1/4 square HSS toolbits hot enough to burn your pinkies then you are probably overheating the cutting edge and ruining the temper.
Dip job in a tin of water at short intervals to keep it cool. Avoid getting the edge red hot. Purists will insist dipping in water embrittles the edge but a million industrial workshops have done it this way for the past 100 years without ill effect.
Also run job back and forth across the working surface of the wheel to spread the heat around. Holding it in one place builds up heat in that area.
If it's just mild steel etc the suggested Mole grips work. (Furry or not! 😆 ).
But don't wear gloves on the grinder. They can catch between wheel and tool rest and pull your fingers in with them. That's ouch.
Edited By Hopper on 30/09/2020 23:01:54
|Clive Foster||30/09/2020 22:59:27|
|2361 forum posts|
I have a pair of holders comprising simple slots in square bar with some small hex socket grub screws to hold the HSS tool stock. Bar is about 1/2" or 5/8" square with, obviously,1/4" deep slots. Came out of one of the handy stuff boxes I've hoovered up over the years.
I suspect they were actually adapters to allow small square stock to be used on a large lathe without shimming or other tool height issues. The small grub screws are much closer together than the screws in a noraml QC hlder which would support the hypothosis. As would a couple of baccy tins worth of short ends ground to special purpose shapes.
But they work just fine for holding when grinding.
Rough shaping the tool with a thin angle grinder blade is much quicker and cooler than taking it all the way down from square on a bench grinder.
|Grindstone Cowboy||30/09/2020 23:52:35|
|342 forum posts|
Not disagreeing with Hopper, as I've always done the dipping in water thing, but I have also heard that HSS can get up to red heat without losing temper. So is that correct? Or is it that the very fine cutting edge exceeds the critical temperature and is thus softened?
(In a philosophical mood this evening )
1460 forum posts
Yes just keep dipping in cold water & everything will be fine.
|Chris TickTock||01/10/2020 09:12:29|
|576 forum posts|
Thanks to all for posting. Clives thinking with his square tube or angle grinder has crossed my mind I will see what works best. Certainly dunking in water is a good idea just in case but there seems a divided opinion as to whether HSS is compromised by the heat. I have blisters on a thumb and finger at the moment.
4804 forum posts
Instead of seeking a confusing array of varying opinions on the internet, you should read any good book on beginner-level lathe work. They will tell you straight up to frequently dip your tool bit in water while grinding it to prevent overheating of the edge and softening there of.
I would recommend LH Sparey's "The Amateur's Lathe" as a starting point. For your milling work, Harold Hall's Milling A Complete Course would answer most of your questions.
6309 forum posts
Exactly my understanding. Broadly, HSS stays hard for much longer than Carbon Steel when they get hot. In theory, a particular lump of HSS might be OK up to dull-red heat, but the thin cutting edge could be hotter than that and lose hardness just were it's most needed.
There are several different types of HSS, and they aren't all the same. Each alloy is tuned for some combination of red-hardness, wear resistance, shock resistance, and toughness. Cobalt High Speed Steels have better red-hardness and are often harder than the others, but are more expensive.
I suspect the differences don't matter much in a home workshop, but in industry - for example - high-wear resistance in a flood cooled twist drill might matter more than the drill's ability to take interrupted cuts whilst red hot. So Production engineer's don't buy the highest specification in the catalogue, they match the tool to the requirement, and buy the most economic.
Anyway, because thin edges can get much hotter than the body during grinding, it pays to keep HSS blanks cool by frequent dunking. I have a theory that chaps good at grinding apply optimum pressure so the wheel always cuts and most of the heat is taken away by the chips. Being unsure and a bit clumsy, I tend to generate a lot of friction by rubbing the blank against the wheel. The cure is more practice...
|Dave Halford||01/10/2020 10:24:46|
|909 forum posts|
Don't let the edge go blue
|1811 forum posts|
A CBN wheel will run cooler; better yet a water-cooled diamond wheel (pawn the loved one's ring).
|Jeff Dayman||01/10/2020 12:52:49|
|1884 forum posts|
It occurs to me that if a person's finger nerves do not work well enough to send the brain a signal to dip a tool being ground that is getting warm in the water pot, maybe they should not be grinding tools or doing other hot work. Burns are no joke if they happen repeatedly in the same area.
|Dave S||01/10/2020 12:52:51|
|53 forum posts|
For the original question: I hold the blank in mole grips.
As for the dip in water or you’ll ruin it: Rubbish.
The “to hot to hold” temperature is around 50C IIRC ( Had to research this for a product at work) Way less than HSS annealing temps.
If you can get a HSS blank to annealing temps and hold it there on an offhand grinder then I’ll be amazed...
I used to grind slow and dip when I started, but then I gave it some thought.
Try abusing a piece of HSS - I have and it’s still as hard with blue temper colours on it. Most of my HSS tools have some tempering colour on them.
if you want a really sharp tool then grind it hard and fast, then hone it on a diamond bench stone. You’ll be making chips faster than the fussy man (it’s usually a man) who will still be faffing to not over heat his HSS.
One problem is that Starey et al were writing before HSS was commonly used in the home workshop. Sure if you grind a silver steel or carbon steel tool to blue you have drawn the temper and it won’t be as hard.
|1197 forum posts||
|Mike Poole||01/10/2020 13:47:14|
2735 forum posts
I think the secret of the water quench is to keep dipping it before you burn your fingers and before the edge reaches too high a temperature. I expect the micro fracture problem is true in extreme circumstances but just keeping things cool for comfortable grinding is unlikely to be a problem, micro fractures are also supposed to be caused by dabbing coolant rather than flood coolant but I doubt most hobby use gets things hot enough to cause any real problems though.
|Clive Foster||01/10/2020 18:19:35|
|2361 forum posts|
Given that HSS like stainless, which is often surprisingly similar in composition, has a fairly modest rate of heat conductivity by the time your fingers get "ouch" hot the part actually being ground will be far hotter. With tooling its only the first few tens of thou at the business end that really matter.
I can easily see that vital bit getting heat soaked warmer than it likes if you take too long over grinding. Hard, fast, quick grinding with the right sort of wheel kept in good condition puts surprisingly little heat into the tool blank if you only do it for a few seconds. Most of the heat seems to go away with the ground off debris. Bit like using good quality carbine inserts "in the zone". Chips come off blue but the work stays relatively cool.
But most folk are using an affordable grinder with either the wheel(s) that came with it or an affordable alternative. Usually grey and intended for general purposes so optimised more for durability under unskilled, careless or frankly risky handling than getting the best edge on machine tools. Instructive to see the difference if you show the tool blank to a proper tool & cutter grinder with appropriate wheels. T&C grinder wheels on a bench grinder might be considered something between a tadge risky and an accident waiting to happen. Yup, I've done it and probably shouldn't have.
Keeping grinding wheels in the best condition isn't that easy for the home shop person. Especially if working without training or mentors advice. Getting to some acceptable degree of OK isn't too difficult but, as ever that last bit that really makes things sing is harder. Seems the last 20% takes more effort than the first 80%. A universal rule I think.
|mark costello 1||01/10/2020 20:31:57|
607 forum posts
I was told in My apprenticeship that a light brown.on the edge did not hurt, but "They" better not see Me turning the tool bit blue.
4804 forum posts
No they were weren't. They commonly refer to HSS.
Edited By Hopper on 01/10/2020 23:17:24
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.