Extension Tables
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-15, Brent Beach

Introduction

Contents

1. Four Jigs - Why one is not enough.
2. Extension Tables - If you don't want to use the calculator.
3. Large Jig - Big iron.
4. Medium Jig - Jack irons.
5. Small Jig - Block plane irons.
6. Tiny Jig - Irons less than 4" long.
7. Back Bevel Angle - The mathematics, part 2.
8. Slip Angle - The geometry of slips.

Four Jigs

I make four different size of jigs, to accommodate different sizes of blades. I use one jig almost all the time, but have the other three sizes for special blades.

A wider jig is needed for wide blades, like those used in a Stanley #8. You might think you could build a single jig wide enough for a #8, and use it for all your plane irons. Unfortunately, the thinner part of the jig bends when you tighten the screws, so it is not a good idea to have a jig too much wider than the blade.

A taller jig means greater extension, and thus more distance between the front of the jig and the abrasive. The larger this distance the more abrasive you can use. Unfortunately, for smaller irons a large extension is just not possible. So, I make a small jig for short irons, and a tiny jig for even shorter irons (less than 4" long).

To use these tables you need to know the thickness of your plane blade. Typical older Stanley or Sargent blades are between 0.08 and 0.09". Newer after market blades like Hock and Lie-Nielsen are usually between 0.09 and 0.10", with some up to 0.125". The Mujingfang irons are 0.125". The Lie-Nielsen #62 iron is 0.16" thick.

Extension Tables

The blade extension depends on the height of the wide jaw. In all cases, the short jaw is 1/8" tall.

This tables provide the blade extension information for a variety of jig heights. For each jig size, each row contains extensions for a particular first microbevel angle. The columns give the extension for a specific blade thickness.

If you are like me and almost always use the same first microbevel angle (29 degrees in my case), then you can copy the row corresponding to that angle from each jig size onto a sheet of paper and keep it with your jigs. I have copied the rows for angles 26 and 29 degrees into separate tables at the end.

Large Jig 1-3/4"

Medium Jig 1-1/2"

Small Jig 1-1/4"

Tiny Jig 1"

Extensions for two standard angles

Two more compact tables, assuming you use one of two standard angles: 29 degrees, my original first microbevel angle, and 26 degrees, my new trial first microbevel angle.

For 26 degree first microbevel:

For 29 degree first microbevel:

Back Bevel Angle

If the jig slips off the slips, modify the slips by putting a 1/4" strip (or two) or masking tape on top of the slip. One strip on the front 1/4" and one on the back 1/4". With the jig between the strips of masking tape, it will not slide off the slip, and the masking tape has no effect on the bevel angle.

Like the front bevel angle, the back bevel angle is determined by the dimension of the jig. Unlike the front bevel angle, the thickness of the plane iron does not matter - the bevel is being formed at the downward face of the iron. Since I use a standard 1/8" back on all jigs, the back bevel angles are the same for all jigs.

You cannot select the back bevel angle independently from the front bevel angle. It is unwise to move the blade in the jig between abrasive sheets - that would introduce errors in the microbevels. So, if you use a back bevel you are more or less stuck with the angle based on the front bevel.

This table is provided just for your information - from it you can figure out the back bevel angle and thus the total included angle at the edge.

Slip Angle

I use two slips - 0.06" and 0.10". The analysis of the resulting geometry is more complicated because the jig rides on the slip. This means that the effective increase in the size of the jig depends on the angle. For the angles of interest to us, the 0.6" slip adds 0.67" to the effective size of the jig on the main bevel side, 0.6" on the back bevel side (since the angle is nearly 0 degrees anyway).

On the main bevel side, the change depends as well on the size of the jig. For an average extension of 3-1/2" on the large jig, the 0.06" slip increases the angle by almost 1 degree; the 0.10" slip increases the angle by about 1.5 degrees. For an average extension of 2-1/2" on the small jig, the 0.06" slip increases the bevel angle by about 1.2 degrees.

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