Woodcraft Honing Guide

[durango dude]

Wow - what a difference!

Took my bench plane blade - ran it on the wet stone with this guide - and it's a totally different piece of equipment.

I have a question, though ----

The honing guide says to project the blade 38 mm for a 30 degree blade, and 50 mm for a 25 deg blade.

Any of you guys math-minded enough to figure would the projection for a 20 degree blade?

(I know - I know -- it's basic trig --- but I forget my trig)

[jdon]

My trig skills are also rusty, but at first blush I'm not so sure it's all that basic- having to consider the roller, with its center of rotation above the sharpening surface, adds complexity to the calculation. I might try to dust away some mental cobwebs as a mental exercise in the morning, when my mind is hopefully more alert- an interesting exercise, at any rate.

Perhaps the easiest thing is to adjust by eye- extend the blade until you see full contact of the honed edge on your sharpening surface, then record the measurement of extension for future reference.

[mpc]

More than just basic trig... I assume the 38mm and 50mm measurements are from the edge of the jig? Basic/simple trig can give you the distance from the blade tip to a point directly above the roller/wheel contact point (i.e. 90 degrees to the surface of the sharpening stone making a right triangle) which is some variable distance from the edge of the tool. (variable because that magic point above the wheel depends on the angle since the whole tool rotates.)

Unless you want to drastically change the angle of a blade, just install it loose and move it in/out until the bevel seems "flat" on the sharpening stone, then lock it down. Squat down so your eyes are level with the top of the sharpening stone (or sandpaper or whatever) and look for the sliver of light between the stone and blade face. Ideally it'll disappear when you get the in-out adjustment just right unless the bevel is really inconsistent. Light at the actual cutting edge means the blade is extended too far; light at the "heel" of the bevel means it isn't extended far enough. Note: if the blade has a "micro-bevel" then do initial/coarse sharpening with the heel & bulk of the blade in contact with the stone; the micro bevel will cause a slight light gap at the cutting edge. Once the existing micro bevel is ground away, the whole bevel will be on the stone. Now pull the blade into the jig a little bit, enough to change the angle about 2 degrees and make a new micro-bevel. If you're just touching up a micro-bevel then you'll have to eyeball just the micro bevel angle initially. Once you've lined up the blade in the tool, use a Sharpie pen or something to make a reference mark lined up with the edge of the tool for quick setup next time.

Generally the absolute angle of a blade isn't important - i.e. sharpening a 25 degree blade to 24 or 26 degrees isn't going to make a perceptible difference in how it performs. Just keeping a constant angle during a sharpening session is what's important.

Another option if the blade has a constant thickness from the bevel heel to the jig body: got one of those digital readout angle gauge blocks? (Wixey digital angle gauge for example at Woodcraft) Zero it on your sharpening stone/sandpaper/whatever, then plop it onto the backside of the blade. Slide the blade in/out of the jig until the angle reads correctly. I think this will work... just occurred to me while typing this reply. Most hand plane blades should work this way (I think) while most chisels vary in thickness so they won't work with this trick.

mpc

edit: jdon got his pretty-much-the-same-reply-with-much-less-words in while I was still typing. Oh well.

[LCHIEN]

I presume this is the subject of discussion.
The exact geometry depends upon three factors: the diameter of the roller, the distance from the blade to the center of the roller and the length from the contact point to the perpendicular from the center of the roller.
As they only gave two examples and you have three unknowns you have to measure the exact geometery and can't work backwards.

But as someone MPC suggested, probably the best way is to do it empiracally, e.g. use a wixey or similar angle gauge, adjust to get the desired angle and record the projection.

[cabinetman]

As its been mentioned, the exact angle isn't as important as holding the angle. I have a few different types of guides, and I use an adjustable protractor to adjust the angle (projection amount), and then maintain light contact on the stone. Not rocket science.

.

[pelligrini]

Might try 68mm to start. There's some complex geometry, most of which is unknown to draw. See below for a quick diagram.

What are you wanting to put a 20 degree bevel on? That's kind of shallow.

[durango dude]

I think the formula is: extension = 20/sin(angle)

(you need to use radians with Excel)

My Stanley 220 cutter sits at a 21 degree angle, so the blade would extend 56mm from the edge of the honing guide.

I'll check when I get back home ----

don't worry - won't hone the blade until I can verify the angle is correct!

[jdon]

Okay, so I did some quick and dirty calculations, and came up with this formula:

x = ((9.5/cos(a) + 20.9 + 2)/tan(a)) - 20

See attached crude diagram.

x is blade extension; 9.5mm is wheel radius; a is honing angle; 20.9mm is distance from wheel axis to blade support surface; 2mm is ~blade thickness; and 20 is half the depth of the surface, i.e., in front of wheel axis.

To be obsessive, the blade thickness should be a factor in determining blade extension.

For 30 degrees, I calculate a 38.7 mm extension (vs. 38 per instructions), for 25 degrees, 51.6 mm (vs. 50); for 20 degrees, I get 70.7 mm.

Caveat: I'm not a mathematician or engineer, YMMV

[durango dude]

okay - we're sitting here working at math problems on a Friday afternoon --- that's sick......

[gsmittle]

You guys make my head hurt.

How do you know all this stuff?

g.

[LCHIEN]

You guys make my head hurt.

How do you know all this stuff?

g.

somebody gotta know this stuff, or planes will crash, buildings will collapse, bridges will fall...
computers wil %*(@!#$*&@))%*#, WHOOPS!

[cabinetman]

You guys make my head hurt.

How do you know all this stuff?

g.

For what can't be remembered, or not known, there is reference available in books or the Internet.

.

[LCHIEN]

Okay, so I did some quick and dirty calculations, and came up with this formula:

x = ((9.5/cos(a) + 20.9 + 2)/tan(a)) - 20

See attached crude diagram.

x is blade extension; 9.5mm is wheel radius; a is honing angle; 20.9mm is distance from wheel axis to blade support surface; 2mm is ~blade thickness; and 20 is half the depth of the surface, i.e., in front of wheel axis.

To be obsessive, the blade thickness should be a factor in determining blade extension.

For 30 degrees, I calculate a 38.7 mm extension (vs. 38 per instructions), for 25 degrees, 51.6 mm (vs. 50); for 20 degrees, I get 70.7 mm.


Caveat: I'm not a mathematician or engineer, YMMV

I concur 100% with jDON'S ANALYSIS. i GET EXACTLY THE SAME EQUATIONS AND RESULTS.

fwiw, Looks like the guy who did the manufacturer's calculations took a shortcut and estimated the 9.5/cos(a) term and just called it 10 degrees.

[cabinetman]

Early users of edged tooling seemed to do a nice job without the honing guides and the math...just sayin'.

.

[durango dude]

you guys are amazing.

My son - a 2nd yr engineering student, confirmed the calculations.
(being a good teenager, he wasn't available last night -- you guys were)

I also sent the solution (without diagram) to Woodcraft, and made sure they knew bt3central was the source (their tech shrugged - so I said I'd send them a formula).

With the formula in-hand, my honing guide is now just as good as a Veritas guide (calculator built-in). --- well maybe not......