Planer-thicknesser cutting angle definition

[isaac3d]

I have a question relating to planer-thicknesser blades and cutting angles. Various text books advise a "reduced cutting angle" when cutting hardwoods and especially "difficult hardwoods". However, I understood that the cutting angle of a planer-thicknesser blade is fixed by the slot in the cutting block/cylinder, as shown in the image below.
I'm further confused by the fact that the angle of the blade relative to the wood changes as it rotates in the cutting block, going from A to B to C as the cutting block/ cylinder rotates. Doesn't that mean that the angle at which the blade impacts the wood also changes as the cutting block spins? Doesn't the cutting edge (shown as X) first impact the wood at about 90 degrees (ie perpendicular to the wood), finally going down to zero as it leaves the wood?
In this context what is meant by a "reduced cutting angle" since the cutting angle appears to change from 90 degrees to zero degrees as the cutting block rotates?

The primary (and usually "only", as far as I know) bevel angle of the blade will obviously make a difference to the cutting action. The blue blade has an exaggerated bevel with X being the cutting edge and Y being the "heel" of the blade (if that is the right terminology). I'm not clear if the blue blade would be called a shallow bevel or a steep bevel. What is the accepted terminology? The other extreme would be the green blade which has zero bevel. Whilst this is clearly the strongest shape as far as impact is concerned, the heel of the blade (Y) would rub against the wood after the cutting edge (X) had left the surface of the wood. Therefore there must be at least sufficient bevel to prevent this. What other reason is there for changing the bevel angle on a planer-thicknesser blade?
Obviously the sharpness of the cutting edge (Y) will make a difference to how cleanly the wood is cut and of course the depth of the cut will affect the likelihood of the wood chipping or tearing. A sharp cutting edge can be enhanced by a bevel but the steeper (?) the bevel (such as the blue blade) the faster the cutting edge will get damaged (blunted).
So I come back to the question: How do you achieve a "reduced cutting angle" on a planer-thicknesser? Or is that an irrelevant question for a planer-thicknesser, and the way to avoid chipping or tearing is to take very shallow passes?
Hopefully some knowledgeable person can explain this.

 

[Jacob]

I think you are over thinking it. I'd just stick with the usual angle as for your machine. They do vary apparently - 30 to 45º is mentioned.
If at some point you are going into continuous production of a lot of hardwood then a small change might make a very marginal difference worth having.
Green is steep 90º bevel, blue is very shallow about 25º I'd guess

 

[Doug71]

What people often refer to as a small back bevel is actually really a front bevel so it gives more of a scraping action.

See @Peter Sefton reply and link in this link, it explains it well.

https://www.ukworkshop.co.uk/thread...rk-without-tearout.130140/page-3#post-1469505

 

[isaac3d]

Thanks! The article (https://www.peterseftonfurnituresch...terseftonmakingthemostofyourplanerarticle.pdf) is very interesting.
Peter's suggestion of using a small "front bevel" on the planer blade effectively reduces the angle of the main bevel (makes it less pointy) with respect to the surface of the new cutting edge created by the "front bevel". I see that a front bevel also makes the effective cutting angle smaller, as indicated in the diagram above and in Peter's article. So then presumably this is how you reduce the cutting angle on a planer-thicknesser! The mist begins to clear!

 

[RobinBHM]

Thanks! The article (https://www.peterseftonfurnituresch...terseftonmakingthemostofyourplanerarticle.pdf) is very interesting.
Peter's suggestion of using a small "front bevel" on the planer blade effectively reduces the angle of the main bevel (makes it less pointy) with respect to the surface of the new cutting edge created by the "front bevel". I see that a front bevel also makes the effective cutting angle smaller, as indicated in the diagram above and in Peter's article. So then presumably this is how you reduce the cutting angle on a planer-thicknesser! The mist begins to clear!

It's easy to hone planer knives in situ, I've done it for years.

Bang a timber wedge in at the end of the block to stop the block turning.

Then use a diamond stone, resting on the edge of the rear table and hone the knife evenly with a few strokes.
Remove wedge, rotate repeat.

I do it just to remove minor chips to get more life out of the cutters, but it also changes cutting angle

 

[isaac3d]

Indeed, I have read about that technique for sharpening the blades in situ. However, that must introduce a secondary bevel similar to what you might put on a chisel blade. Then as you say, the cutting angle would change, becoming greater, if I have understood the geometry correctly.