Re-hardening a plane iron
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- This topic has 8 replies, 3 voices, and was last updated 5 years, 8 months ago by John Carruthers.
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12 July 2018 at 11:21 am #549317
I picked up a big old jack plane at the boot fair, (only £3, I couldn’t leave it there).
The iron stamp proudly proclaims “warranted cast steel – Earnshaw bros”I got round to sharpening it today, it was ground to a shallowish angle, 15 – 20 deg maybe, but badly out of square across the edge. I went through the grits to a 1000 water stone.
I wonder if the metal may have been ‘burnt’ during heavy grinding at some time? There is no blueing but the burr doesn’t want to come off the back, it just keeps hinging back and forth.
I have access to a small induction forge so I may try rehardening by brine quenching at ~ 810C, then tempering to mid straw.
Has anyone tried this?
A quick way to assess if an iron has lost its temper is to very lightly run a fresh and sharp metal file across the cutting edge section of the bevel. If the file leaves a mark on the steel at the cutting tip, then it is out of temper. The cutting part of the iron should be almost as hard as the file.
However, a Hearnshaw blade of any vintage, say late 19th C up to the first War, may have been laminated: this means that in manufacture a thin fillet of high-quality hardened steel (the back part, forming the cutting edge) was forged to a heavier and softer piece of iron (the front or underside of the blade), which gave it strength. Sometimes this joint is visible as two differing colours on the edge of the blade.
Many better-quality blades from those days were of this type. If this is the case, re-hardening the two types of steel may be temperature-critical and difficult to do. You may run the risk of distorting the metal or weakening the bond at the temperatures required.
A file should bite on a softer under part and glide off the top, harder section.
In any case, for three quid, it may be worth a punt! Let us know how it works out.
Good luck.12 July 2018 at 4:08 pm #549338Thanks for the tips, I tried with a new saw file which bites evenly all over. I shall try a mate’s hardness tester, see what that shows. I can see no evidence of lamination.
Once it’s really clean I might try lightly etching it, see what shows up.
As you say, little to loose and I may learn something.Re-reading your post I noticed that you are grinding down to about 1000 grit…. still, in my estimation, 1000 is a bit coarse.
It may be that the root of the burr, at that, is too strong and needs to be weakened by successive grits until it wears off.
For a standard fore-plane blade I would go to 2000 or 4000 followed by a stropping with a medium paste then use it to see how it wears. The use-test is still the best. If you don’t have a leather strop, 1″ thick MDF with paste on it is excellent.Good luck. Still a bargain for three quid.
13 July 2018 at 7:42 am #549354Thanks I will try a finer stone, I also have various optical grits and polishes for finishing down to elutriated tin oxide, but that would be overkill 😉
I’ll put some pictures up later.PS, what part of Wales Howard? we’re moving to Margam in a few weeks.(if the conveyancers get their act together).
- This reply was modified 5 years, 9 months ago by John Carruthers.
13 July 2018 at 11:47 am #549362One of many, millions like it no doubt.
A gentleman on another US based joinery forum maintains that ” they never cut well…” but I say the likes of Chippendale, Joseph Sanders Esq, and countless other 18th and 19C tradesmen managed very well with these tools.
Looking in a Roman toolbag found at Vindolanda there was a small wooden plane, no different to one I saw at a garage sale, no need to change a design that works.Attachments:
You must be logged in to view attached files.John, that’s a decent looking plane. You may need to spend some time fettling it so that he sole and iron are a good match. From the stamp on the blade, I would hazard a guess that it is laminated. The file test will tell you. There may well be previous users’ name stamps on the body and a maker’s stamp, usually on the toe of the plane.
Margam is on the sunny South Wales Riviera…………… I’m in mid wales- in the hills north of Tregaron – all sheep and trees up here.
Good luck with the move.Hi John,
Please send in a photo that shows where the cutting edge is in relation to the end of the slot in the blade. You might not have any high carbon steel cutting edge left on the blade. Which will also be why your saw file bites evenly all over. If the saw file bites into the blade, rather than skating off, it is soft. Generally, on these type of blades, if you have less that about half an inch of blade below the end of the slot, it is likely that you have a blade with only the wrought iron component left – no hardened steel. In that case you must look for a replacement blade.
Cheers
Mark H1 August 2018 at 10:33 am #549776Home again, I stripped it and lightly etched the blade in mild acetic acid which showed up the joint in the steel.
Then popped the business end in my mate’s induction forge to cherry red and held it there for a few minutes.
Then quenched in brine which left it glass hard.
I tempered it to a light straw colour.
It feels much better on the stone now with a clean cutting action. It feels ‘right’ now.I squared off the edge then put a slight crown on (less than a 1/64″).
The plane now works well and no doubt I shall learn its ways as we go on.
It won’t be used in anger until we move and I get the new shed built.I mentioned it to ‘old Arthur’ who thinks it may have been ground on an angle grinder with a heavy hand, he may be right.
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