Small parts thicknessing jig

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24 May 2014
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When preparing strips for my kumiko projects I sometimes need to finesse the thickness of the strips I’ve prepared. This may because the thickness is not correct straight off the table saw or to clean up the surface of the strips (for example purple heart is prone to burn marks from the table saw).

For longer strips I use a drum sander with a plywood sled. But for pieces less than 200mm long I built a thicknessing jig for use with my low angle block plane.

I was inspired by the Thickness Jig (small stock) video on the Young Je YouTube channel (but with a lot less attention to the final finish). I adapted his design by adding the ability to ‘micro adjust’ the thickness.
I’ve not included dimensions as I worked to the constraints of my supply of off-cuts and the block plane I had chosen to use with the jig.

The jig comprises an open-topped box constructed from 24mm and 18mm birch ply off-cuts and a lump of unknown dense hardwood I obtained some time ago.

The side pieces have a small rebate routed along each inside edge for the sole of the plane to run along. The width of the rebate depends on the amount of sole either side of the blade and the width of the end pieces will be set by the width of the plane sole minus the two rebates.

Thicknessing Jig 01.JPG

The view below shows the hardwood piece cut in two at a shallow angle. Apologies for stating the obvious but the bottom surface of the rebate must be parallel to the top surface of the upper piece (the planing bed). The planing bed must fit snugly within the length of the jig but still be free to rise and fall.

The lower piece is shortened to allow horizontal movement which will control the depth of the planing bed from the rebates. The amount that it is shortened determines the total rise and fall and the planing bed. This is a compromise between a greater rise a fall and the most support offered by the bottom piece.

In my case the maximum thickness I can achieve is 14mm. In hindsight I would probably go for greater stability and a maximum thickness of 10mm which would be more than adequate for the kumiko work.

Thicknessing Jig 02.JPG

The minimum thickness is determined by the height of the rebate on the sides (A) minus the dimensions on the two hardwood pieces (B + C). I went for approximately 1mm.

Again, in hindsight I would set the height (B + C) to be a hair above the rebate. Then with the jig raised to it’s highest position, use the plane to shave the planing bed and you’ll be certain to have the planing bed parallel to the rebate.

Thicknessing Jig 03.JPG

The lower piece of hardwood has a channel routed out in the base to comfortably accept a length of M8 rod with corresponding holes in the end pieces to allow the rod free horizontal travel.

Within the channel I set an M8 nut epoxied into a rebate as shown below (base removed for purposes of the picture). Again, stating the obvious, ensure the bottom piece is always resting on the base of the box throughout the travel.

Thicknessing Jig 04.JPG

Once the rod is threaded through the assembly and cut to length I attached a penny washer and a locking nut at each end. The nuts are tightened as tightly as possible while still allowing the rod assembly to turn therefore eliminating any lateral movement. A classy jig would have a nice knob instead of just a locking nut but I find I can adjust the jig perfectly well without.

A note on the height of the end pieces of the box. These will be the stops for the planing bed but I set them approximately 0.2mm below the rebate so that they wouldn’t foul the plane blade.

Thicknessing Jig 06.JPG

Finally, there is a slot in each side to allow a couple of locking knobs that screw into the planing bed to secure it once the desired thickness is set. I skipped this step when I made the jig but I will be adding it. I thought the top piece would be not move as it sat on the bottom piece. However, as the bottom piece is shorter than the tplaning bed it is possible for the planing bed to rock slightly if pressure is applied to the unsupported end of it.

As a minimum, all moving parts and corresponding faces have a generous waxing to ensure smooth movement.

This is how my jig looks at the moment – not pretty, minimum finishing, but it does the job. But as I say, I will be adding the locking knobs!

Thicknessing Jig 05.JPG

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