We have heard from a few people recently who have been trying to polish the backs of their plane irons and chisels to a mirror shine all over. This is a common enough misunderstanding and I have had to explain why it is unnecessary to some very experienced makers indeed. Most of them start out looking at me as if I have just come out of a tap and then the penny drops and they suddenly realise how many hours of unnecessary effort they have expended over the years and end up needing a cup of tea and a sit down.
The back of a chisel only needs to be polished for the first few millimetres under the cutting edge. This is achieved by hollowing the back by a couple of thou (no more) on coarse abrasives, preferably using a single line of contact to avoid the possibility of referencing off a bump. With the slight hollow achieved, you lay the blade flat on your finer honing abrasives and work just the underside of the cutting edge to a mirror finish. If you like to use the reflection in the back of the blade to gauge the geometry of your chopping cuts, then by all means carry on and polish the first 10mm. Zero to minus 2 thou is plenty flat enough for precision joinery so your chisel is still technically 'flat'.
The back of a plane iron is not a reference surface and only needs to be polished for the last millimetre. The back of a plane iron doesn't need to be flat, so elevating the other end of the blade by slipping a thin ruler / bit of paper / whatever underneath it when honing you can guarantee contact at the cutting edge.
The same goes for the irons of bevel up planes. The last couple of mm of the back is exposed in the mouth and doesn't register against anything, so there is absolutely no harm in using the Charlesworth ruler trick on them too.
If you find you are spending any more than ten minutes maximum preparing a new blade, there is a good chance that either the abrasives are inappropriate or the technique is not quite right. If you need some help, please get in touch and we will be happy to point you in the right direction.
Aside from saving a whole load of time and effort, the other advantage of minimising unnecessary polished surfaces on your tools is that polished steel is much more prone to pitting. A ground surface will tend to develop powdery surface rust all over, which if it does occur can be easily removed. The slightest imperfection in a polished surface (at a molecular level) will act like a sacraficial anode and fester away independently of the surrounding surface.
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Hi Matthew and thanks for the interesting post.
ReplyDeleteBut I do not agree that polished steel is much more prone to pitting.
In my experience, I think as Ron Hock wrote in his book:
"Keeping tools polished can help prevent rust by eliminating surface imperfection that can trap a droplet of water that allows rust to get started.
Polished steel is not rustproof, it still needs rust avoidance and protection measures, but shiny, polished steel tends to stay that way longer then the rough stuff."
Hi Andrea,
ReplyDeleteI think we are looking at two sides of the same coin. The polished area is less prone to corrosion, any minute defects in that surface will therefore corrode in isolation - hence you get localised pitting rather than a powdery surface rust over a wide area.
If you look at old rusty plane irons the deep holes tends to be on the back, while the face side is evenly corroded.
Cheers,
Matthew
I undersand what you say, but I don't agree again. :-)
ReplyDeleteI'm not a metallurgist, but as far as I know, the theory that an imperfection act as anode and is the cause of pitting, is true only for self passivating metals, such as stainless steel.
Carbon or low alloy steel usually uniformly corrode, and pitting start because water droplets or condensation.
A rough surface exposes a greater area then a polished surface and also a rough surface traps air and water better than a polished surface where water tends to slide off better.
Anyway I prepared a blade for half grinded and for half polished.
I will put it in the garden and in some years I'll let you know the results. :-)
Fantastic, you can't beat empirical testing. I look forward to hearing your results!
ReplyDeleteCheers,
Matthew