Planer knife grinding angles

[Chipwrights]

I've discovered something I didn't know.

I've changed my planer knives recently and the ones I took out have not been in very long, so they are not so dull that they are unusable. They do, however have a little nick in them from hitting something hard. It's not bad and I'm fairly certain that if I put the knives back the nicks would be offset, so they would cover for each other.

I decided I'd make a little jig to hold them for honing. I thought that they were ground to 30 degrees. Having tried to fit them, I discover that these have been ground at 40 degrees - (fortunately the jig takes all of 15 minutes to make, so I've not lost much).

So a bit of Googling as to what they should be and I discover that planer knives can vary from 30 to 45 degrees. I should not be surprised, given that we use higher angles for hardwoods with hand planes, but I'd never given it any thought for planer knives.

So my q is this - To what angle do you have your planer blades ground and why?

Cheers
Steve

Mine are honed to 35°, they are tct, I wouldn’t normally pay much attention to this but I noticed that last time I had them sharpened they only lasted for a couple of months, when they were new they lasted for 6 months, nearly all of my work is done in Sapele so it was nothing to do with the wood, further investigation showed that the blades had been honed to 45°, much sharper, too sharp in fact as they were leaving scars on the surface of the wood, they were slicing rather than cutting, it’s all to do with the cell structure of the wood…..the cynic in me asked why they had changed the angle of the blade and the only thing I could come up with is they sharpen them 4 times a year instead of twice, so twice the revenue for them, and 50% less blade life for me, times that by every blade they sharpen and I’m sure that works out a lot of money.
40-42° is fine for softwood with hss blades but I’d never go above 35 for tct!