Jointer and Planer Blade Angles? Jig Pic

[Steve Mathews]

I just finished making a jig to grind blades for my Northfield 12" jointer and Jet 20" planer. I'm using a surface grinder for the job. I thought originally that all of the blades would be ground at an included angle of 25 degrees but after doing some checking it seems angles ranging from 25 to 45 degrees are often used. The 2 spare sets for my jointer are ground at 35 and 40 degrees. I understand that some angles are better suited depending on the type of wood involved. But is there a common angle that would work for both?

Here's a photo of the first phase of the jig. I'm working a a way to adjust the angle without propping one side up with spacers. The angle as it sits is about 32 degrees. Unfortunately my surface grinder only travels about 19", not quite enough for a 20" planer blade.

430692

[Andrew Hughes]

That’s a nice setup wish I had a way to grind knives straight. The sharpness angle of the knives should probably be around 40. If you really want to dial in the perfect angle for your head you’ll need to figure out the hook angle.
For my jointer the head has a 36 hook so my the perfect sharpness for most woods is 42 this leaves 12 degrees clearance between the bevel and the wood.
I apologize if you already know all this this all boring to most .
Good Luck

[Steve Mathews]

Thanks Andrew! Actually, I didn't know this and it's not boring at all. As a hobby machinist I'm somewhat familiar with clearance and rake angles but I didn't know it came into play that much with cutting wood. Do you know of a source I can read up on the subject further?

I tried to grind one knife today but unfortunately ran into some overheating trouble. It's now somewhat warped.

[Andrew Hughes]

Darn sorry to hear about the warped knife. I hope it wasn’t your best hss it’s getting harder to find good hss like t1.
There is some info about Woodworking knife grinding angles written by Schmidt. Most of it applies to shapers very little about jointers and planers since we really can’t change the hook angle of our heads. There are some tricks but they come with a trade off as always no free lunch.
Search( Woodweb knife grinding woodworking Manual 5.)
Good Luck

[Bill Dufour]

Looks like you easily make that jig into a sine vise setup. just need to make a baseplate with grooves. That would allow quick angle changes with some math needed for the initial setups. I do not think the angle is critical as long as it is the same for all three knives. A crude sine vise will be easy to repeat to 30 seconds or better.
Bill D