Using a bench grinder

[LancsRick]

Grabbed myself a basic bench grinder today to regrind some plane irons from 30degrees to 12.two questions...

If I get a discolouration at the edge have I overheated and ruined the temper on that area, or is a level of colouring ok?

How long should an activity like this take? It took me over half an hour with gentle grinding to regrind this iron. Any greater pressure led to very rapid heating and I chickened out.

Tips welcomed! Thanks.

 

[Jacob]

12º? impossible with a bench grinder. Unheard of anyway with woodwork tools.
25º the usual angle but that's difficult without overheating. Needs doing very carefully and dipping in cold water often.
Bench grinder not the ideal tool for sharpening - really is only for rough metal work.
Discolouration is over-heating and softening. Ignore it it just means more frequent sharpening. If you try to avoid doing it again you will sharpen past the overheated bits eventually and anyway you need the practice!

 

[sunnybob]

since giving up metal for wood, i have found a good use for my bench grinder, it keeps all my pencils sharp.

 

[Phil Pascoe]

Your wheel is probably 1/ too fine and 2/dirty. Get yourself a diamond dresser or a devil stone. Cool your irons regularly to avoid burning them.

 

[LancsRick]

Not impossible, I've just done it! Thought it appears the 12degree blade refers to the bed not the blade angle! Laughs at my expense. Good practice on the grinder though!

 

[xy mosian]

A quick dip in water will not cool the blade enough. Find something else to grind alternately, chisel or another plane iron perhaps. As one blade begins to warm up, you can normally tell through the fingers, place it in the water and swap to the other blade. That way both blades have sufficient time cooling.
It really doesn't matter how long it takes to regrind a blade. Taking time is much better than having to grind back past a 'blue', which will take even longer.
xy

 

[DTR]

A quick dip in water will not cool the blade enough.

Quenching in water can also cause certain steels (e.g. HSS) to crack. I'm not sure about the common steels in woodworking though.

+1 to letting the tool cool naturally while getting on with something else instead.

Like many things in life, little and often (ahem ) is the key. If the tool only needs a little grinding, it won't get hot in the first place.

 

[xy mosian]

A quick dip in water will not cool the blade enough.

Quenching in water can also cause certain steels (e.g. HSS) to crack. I'm not sure about the common steels in woodworking though.

Good point DTR, although I have personally not had a crack appear. I suppose a great lump of metal would be an alternative as a heat sink. A short length of railway track perhaps, or an anvil.

xy

 

[ED65]

Thought it appears the 12degree blade refers to the bed not the blade angle!

Ouch! Obviously you're going to hone at a steeper angle but a 12° primary bevel will make the edge a little weak so it might be advisable to regrind it steeper to be on the safe side.

Well done on achieving it though, that would be a challenging grind job for anyone.

 

[ED65]

A quick dip in water will not cool the blade enough.

Uh, it should. I used quick dips just a couple of days ago when doing the first grinding I've done in ages. Must have dunked at least 100 times between the two chisels I was working on.

Obviously there are some variables such as the mass of steel that's hot and how hot, but even a small amount of water will cool a fat chisel that's too hot to touch (by asbestos fingers standards ) down to a reasonable temperature in just a few seconds. Don't know about anyone else but that meets my definition of "a quick dip".

Quenching in water can also cause certain steels (e.g. HSS) to crack. I'm not sure about the common steels in woodworking though.

Leonard Lee specifically warns against this practice in his well-regarded book on sharpening, without any specifics given as to the type of steel so it must be presumed his warning extends to all steels. It's one of the book's few poor points IMO since obviously the cracks he explains happen don't appear to occur in practice. The explanation given is the kind of thing I'd expect from an armchair theorist, not someone who clearly had tons of practical experience.

 

[Robbo3]

Quenching in water can also cause certain steels (e.g. HSS) to crack. I'm not sure about the common steels in woodworking though.

+1 to letting the tool cool naturally while getting on with something else instead.

Like many things in life, little and often (ahem ) is the key. If the tool only needs a little grinding, it won't get hot in the first place.

If you blue HSS whilst grinding, it will still hold its edge & can be used as normal.

As far as I know plane irons are not made of HSS so is not relevant to this topic.

 

[Phil Pascoe]

The heading of the thread is "Using a bench grinder" so although the Op was grinding irons that is debatable.

 

[xy mosian]

A quick dip in water will not cool the blade enough.

Uh, it should. I used quick dips just a couple of days ago when doing the first grinding I've done in ages. Must have dunked at least 100 times between the two chisels I was working on.

Obviously there are some variables such as the mass of steel that's hot and how hot, but even a small amount of water will cool a fat chisel that's too hot to touch (by asbestos fingers standards ) down to a reasonable temperature in just a few seconds. Don't know about anyone else but that meets my definition of "a quick dip".

A few seconds as a cooling dip? I agree. The danger is a dip which is quick in and out, once. If the steel still feels warm, after quenching to my fingers, then I feel safer with a longer quench.
xy

 

[DTR]

A quick dip in water will not cool the blade enough.

Quenching in water can also cause certain steels (e.g. HSS) to crack. I'm not sure about the common steels in woodworking though.

Good point DTR, although I have personally not had a crack appear. I suppose a great lump of metal would be an alternative as a heat sink. A short length of railway track perhaps, or an anvil.

xy

As I understand it, it's micro-cracking that's the problem rather than anything visible. The micro-cracking weakens the edge and can lead to chipping.

If you blue HSS whilst grinding, it will still hold its edge & can be used as normal.

As far as I know plane irons are not made of HSS so is not relevant to this topic.

But something in the back of my mind tells me it's not just HSS, but certain other modern steels. Cr-V, maybe? I've certainly had my Cr-V chisels chip.

 

[ED65]

As I understand it, it's micro-cracking that's the problem rather than anything visible. The micro-cracking weakens the edge and can lead to chipping.

That's the theory at least! This is Leonard Lee's point, and it gets embellished and built upon in posts online out of all proportion to the actual danger.

It does seem reasonable that it can occur, but the implication/flat statement that it will occur appears to be false. Either that or there's something else going on here.

If cracks leading to chipping were likely or always happened I feel certain the cause would have been recognised a long time ago (the tip predates electric grinders!) and there'd be more (universal?) handed-down experience saying not to do it. But instead there are many old-timers who quench in a mug of water, have done it since they learned how to grind from whoever came before them who learned it from their teacher in turn.

 

[DTR]

I will relent to the greater knowledge of the forum; metallurgy certainly isn't my field (can tea drinking be considered a field?). All I can add is that I never heard it from Leonard Lee (whomever that is?), but from machinists outside of the woodworking sphere.

 

[Mr T]

A dry grinder is not the best for woodworking tools but they are usually cheaper than wet grinders. Burning happens when grinding a fine edge because there is little to conduct the heat away. I find it useful when grinding a tool that has been chipped to grind a flat on the end back to the bottom of the chip. You can then grind the bevel more aggressively with less frequent dousing until you come close to a fine edge. Then you need extra caution.

Chris

 

[andrewgcook]

I have read a number of scientific papers (verhoeven, Landes) regarding sharpening. These don't contain conjecture or opinion and are really good, reads. A refreshing alternative to myth and mysticism in the discipline of creating sharp edges.

There is evidence that suggests that steel can lose temper at the micron level very quickly by dry/uncooled grinding. There is no change in colour to display loss of temper but micro-hardness testing proves it.

One must be extremely vigilant to avoid damage to tool steel! Regular dipping or misting with water every few seconds may go some way to avoid this damage.

Ultimately the only way to be sure is to use flood coolant or sharpen by hand exclusively.

Some HIgh speed steels, for example M2, have very high tempering temperatures and retain hardness even at 500-600 centigrade. That being said, it is suggested that temperatures in the thousands occur due to dry grinding.

Toolmakers and machinists would never surface grind hardened steel without flood coolant, why would we as woodworkers?

Regards,
Andrew

 

[Jacob]

.....
Toolmakers and machinists would never surface grind hardened steel without flood coolant, why would we as woodworkers?

Regards,
Andrew

At school the bench grinder was banned completely - we shared a workshop with metal workers and it was for them. We had a large water bath wheel instead, but only for extra work over and above normal sharpening - freehand in a stone. Later on a C&G course there was no wheel at all and it was all freehand on a stone.
Bench grinder is for metal work, not for sharpening (except when the teacher isn't looking).

 

[andrewgcook]

Jacob,

That was my experience at tech except we had Oil-cooled, low-rpm platen grinders or a barely used water cooled grinder.

Bench grinders are, on the whole, not very good for tools.

Regards,

Andrew