We may receive a commission when you use our affiliate links. However, this does not affect our recommendations.
As I said last week, a common way to flatten a board that’s too wide for your carpenter is to build a planing sled and then pass the board and sled through the plane. Through incremental milling, the rough top surface of the board will be flattened, so you can flip the board and slide it through the plane, this time without the sled.
This is a very successful technique, especially when temporarily gluing the stabilizing shims under the panel using hot glue or double sided tape. But gluing the shims to the board and sled takes a long time and could cause unwanted glue residue. (By the way, you can wwatch some great videos at the end of this story to learn more about the thickness + glue technique.)
I recently found a way to mount the board on the sled and pass it through the plane without resorting to gluing shims. A rubbery substance that is used to improve traction on stairs and steps, manufactured by 3M, has become the perfect replacement for hot gun adhesive and has been found to be perfect for both sled and shims.
3M Safety-Walk gray for indoors / outdoors is a slightly rubberized training material that clings to shoes and prevents slipping. It is not abrasive and if it accidentally loosened inside your diary it would not blunt the cutters. It has a PSA (Pressure Sensitive Adhesive) back that allows you to securely attach it to the sleigh bed and your custom wood shims (on my shims I used the 1 “version of the tape). The Safety-Walk is available in a variety of widths and is relatively inexpensive.
To make my sled, I chose a thick plywood board and cut it to the width so it would fit nicely between the two raised cast iron fences of my Grizzly plane’s bed. To prevent the planed board from sliding backwards, I installed a heel or rear ledge in the back of the sled. I covered the surface of the sled with some wide strips of 3M Safety-Walk.
To wedge and prevent the board from wobbling you will need a variety of thicknesses to choose from. The thicker to support your board, the better. Some should be thin and some thicker, but all should have a moderate slope. Don’t make your shims too steep and remember to cover them with 3M Safety-Walk on one or both of the shim’s slopes.
Using the sled
Place your board on the sled and push it against the rear ledge. Check out the swing. Turn it over and feel if its tendency to wobble has increased. Remember, you need to place the least rocky surface face down and shimming this surface first.
Two thicknesses are better than one
A badly twisted board should be leveled and balanced along its diagonal by two shims hidden under the opposite ends of its diagonal line. We do this as we strive to standardize the milling process. The more horizontal the panel is when milling, the less material you will need to mill. If we only used one shim, the corner (and surface) raised above the shim could be excessively milled and the board would be excessively tapered.
Support a cupped board
A cupped or vaulted board that does not wobble will give you the impression that it is okay to insert it directly into the plane, however, this is misleading. The planer feed rollers will press on the panel in front of the cutters and the result will not be a true flat panel. To eliminate the pressure of the inlet cutters, filler strips or a pair of conical wedges can be inserted under the vault to act as pressure attenuators. Sliding the long wedges on top of each other allows you to reach the right height under the vault. I have provided a exaggerated illustration to demonstrate it below.
Top side milling
After verifying that the board has been shimmed correctly, start milling. Apply some downward pressure to the back of the table (around the heel area of the sled) as it enters the plane to counterbalance the jerking action of the planer feed rollers. Receive the slide at the output table and carefully lift it and bring it back for any subsequent milling operations.
turn it over
Once most of the surface has been smoothed out, it’s time to flip the board. Remember to remove the board from the sled and insert it into the plane with the real side facing down. Rely on the plane bed for this procedure.
How about Snipe?
Since the 3M material is somewhat elastic, you may notice some snipers on the front or back of the board. But rest assured, this cut can be eliminated. After the board has been turned and most of the second face has been flattened, turn it over once more and mill the first face again. This action, which can be repeated again, will eventually remove most if not all of the cut recesses.
Next time I’ll show an improvement on this basic sled I thought I’d build.
And finally, you can watch these great videos to learn how to mill a crooked board using the glued thickness technique.
Here are some supplies and tools that we find essential in our daily shop work. We may receive a commission from sales reported by our links; however, we have carefully selected these products for their usefulness and quality.