Help please: my plane's microbevel is cracking badly

[Frederick Skelly]

Hi guys,
Can someone please help me diagnose a sharpening problem? The microbevel on my #2 is experiencing significant cracking. I cant get a good pic to post so I'll try to describe it.

Blade is 1 5/8" wide. It is an old blade in a plane I estimate to be around 100 yrs old. Blade is 3/32" thick with plenty of life left in it. Primary bevel is 25*, secondary bevel is 30*. It was 27* but I increased it to 30* because it kept getting 1-2 large chips (around 1/16"). So I used my Worksharp wiith a 100grit wheel to "grind" the bevel back until the original crack disappeared. Then I increased the microbevel to 30*., polishing to 6000 grit. The cracking started after I took 4-5 light passes on pine. I now have 7 diagonal cracks in the microbevel - one of which is 1/16". Looking down on the bevel, I have 3 diagonal cracks left of the blade's center and 4 of them to the right.

Basically looks like this (bevel away from me): /// \\\\ .

Do I just regrind the bevel behind the cracks and hope for the best, or do I have to do something else? Id like to keep using this old blade if I can, but I'll buy a new one fron LV if I cant.

Thanks folks.

[Ray Gardiner]

Grind it back past the cracks and try again, if it keeps happening, it's most likely because the steel is too hard, seeing as the blade is around 100 years old, I'm pretty confidant it will be just a high carbon steel, probably O1 or similar so you could try tempering a little, bake for an hour in a kitchen oven at 400F, maybe put a few roast potatoes in as well...

If you have an infrared thermometer, it always pays to check just how accurate the oven temperature actually is.. you could probably go up to 450F or 500, but start out conservative and try for 400F.

Ray

[Jim Koepke]

I had a blade once with a long crack in the metal. It was honed a few times before noticing the flaw. It left a line on surfaces.

I am not sure if one can do a magnaflux test with just a magnet and iron filings, but I would try it to see if the crack stops or if it goes all the way through to the slot.

jtk

[ian maybury]

I don't know how it would look in practice - and it may well have nothing to do with your situation. Only speculation, but it's not impossible though that something was a bit off on the heat treatment of the steel.

As in I have a vague recollection that overheating before oil hardening and inappropriate tempering temperatures can cause micro cracking, or at least leave a high carbon tool steel in an internally weakened state. It somehow sets it up for impurities to come out of solution as brittle compounds that concentrate in locations (around grain boundaries) that lead to the effect. Impurities might well have a similar effect....

[Jim Wheeler]

Frederick,

I like to use antique tools as well. I have a really old and large lathe gouge I tuned up and tried to use once. I'd put a really nice edge on it and every time I attempted to turn with it pieces would chip out out of the edge. I was about to toss it, but figured I'd try re-tempering it on the theory that the steel had been left too hard in the first place. I used a propane torch to draw the colors near the edge to a medium bronze color. I did not directly heat the edge itself with the torch flame, but the metal back a ways from it. After doing this I resharpened the tool and tried using it again without any problems.

I think sometimes the reason some old tools survive for a hundred years or more for someone like us to discover and use is because they were defective and were thrown in a drawyer or the bottom of the tool box and never used.

Jim

He who welds steel with flaming pine cones may accomplish anything!​

[Frederick Skelly]

Thanks for your responses guys - it helps. This weekend I'll grind her again and if I still get chipping I'll re-temper the blade as you suggest.

Fred

[Frederick Skelly]

Again, just for the archives to help the next guy.....

I ground the blade back and resharpened a couple times - no luck, it kept chipping heavily after or during each sharpening.

So I had to try and temper it after all. I put it in the oven @ 400* for 45 mins. When I resharpened, I saw no improvement.

Next I did some reading online and tried one last time 425* for 60 mins. (I decided Id buy a new blade from LV if this failed.) After it cooled I resharpened and got NO chipping and a VERY sharp edge. I went to a piece of pine and made perhaps 20 passes, with the grain - lovely, thin shavings. I did notice some very small chips forming on the bevel. In hindsight, and after more reading, I think I got it a little too hot for a little too long and ended up softening the metal a bit too much. Probably should have taken it out @ 30-45 mins. My bad.

But just for grins, I touched up the 30* microbevel bevel again on a 6000 grit stone. Made 20 more passes - even less chipping and more of those very lovely shavings.

So, sorry LV. I think I can live with this blade a while longer.
Fred

PS. Thanks again to all of you who responded to my call for help!

[Steve Voigt]

Hey Fred, glad to hear it's going better. Couple thoughts:

- If you are still getting tiny chips, you did not get it too hot.If you get it too hot, the behavior will change and the edge will fold rather than chip, and it will be harder to sharpen--the burr will be harder to work off. If those really are chips, not folding, that you're still getting, you could probably go another 20° or so. But I think I would live with it for a while before I try that.

- If you find the chipping returns, it's because the cutting edge, being thinner than the rest, may have gotten a little hotter. You may have noticed that the edge got more brown or purple than the rest. No problem though, you can just temper it again if that happens.

- I think you were a little lucky. I would start at 350° or so and work up from there. Ovens can read low, or they can have temperature spikes. It helps to have other stuff in the oven, like some cast iron pans, to absorb heat from spikes. But it sounds like you were ok.

[Frederick Skelly]

Thanks Steve! I'll live with it a bit and try again if it seems a problem. I appreciate your advice - I have no metalurgy or blacksmithing experience.

Fred