[Q] Plane iron curve goes from convex to concave

[Fromey]

I've observed this twice so far and think it is related to using my planes on the edges of plywood (16mm thick).

I start out with a sharpened blade with a slight convex curve to it. By the end of a day using the plane on various, including a lot on the edge of plywood, I notice the edges of the plane blade keep digging in no matter how much lateral adjuster I use. When I take the iron out and look at it with a straight edge, to my surprise (shows my naivety I suppose) I find the centre of the iron is now either flat with the edges or even slightly concave. This has happened with a WWII-vintage Record No. 5 iron (original I think) and a new quangsheng V4 No. 4 iron. I'm not taking particularly thick shavings (the plywood gives largely dust as one would expect).

Is this normal wear on a plane iron (I don't think so)?
Is this a known problem with plywood (maybe all the cross fibres and the glue eat the metal)?
Do I have some cr@p irons?

 

[Jacob]

That's how a plane will wear, as most of the work is done in the middle. Accelerated by using on ply edges, which is tough stuff. You might get a bit more use out of it by tilting the blade, use left side, then middle, then right.
Could be blades softened by over heating on the grindwheel of course.
The important thing when sharpening is to hone right to the middle - get a burr right across, especially in the middle. This sounds obvious but it's possible to miss the middle (it takes longer to get there) and to be working with blades which are never fully sharp in spite of frequent re-sharpenings.

 

[woodbloke]

It's the end grain of the plywood which is causing this sort of wear. To see it best, try planing the edge of a piece of ply on a planer-thicknesser...then be prepared to take out the blades and grind away about a mm of steel to get out the dings - Rob

 

[bugbear]

Could be blades softened by over heating on the grindwheel of course.

No it couldn't - Fromey posted about choosing an abrasive for manually working on bevel angles. You even posted in the thread!

BugBear

 

[Fromey]

Thanks again for the feedback. I suspected it was accelerated normal wear and that plywood is nasty stuff.

I can't vouch for my no. 5 plane iron as it might have been ground in the past and thus lost its temper (I remember Chris Schwarz reporting that you only need to reach 300 degC before the temper is lost and that is quite easy to accomplish on a thin cutting edge using a grinder). However, I sharpen using 3M lapping film and the quangsheng blad is new, touched only by me.

OK, so my decision is to avoid at all costs using plywood in future (I've been making a shooting board and a saw bench). It's horrid stuff in any case and the batch I bought was a bowed as a sailor with scurvy.

 

[dunbarhamlin]

Harsh stuff like ply will more quickly develop a rounding on the face of the blade as well as on the bezel. This means after sharpening the bezel until you have a bur all the way across, the edge can still be concave. You will need to grind past this (more time on your course grit) and/or use the ruler trick to ensure your honing reaches the cutting edge on both sides.