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Milling

Milling in a Fully-Equipped Workshop

Milling, in the context of woodworking, is the process of preparing a piece of lumber for use in a project (it can also mean cutting up a felled tree into saleable pieces, but few woodworkers go in for that at home). Milling almost always involves making the faces of the piece precisely flat and parallel to each other and the edges flat, parallel and precisely at right angles to the faces. It generally includes establishing the desired thickness and often the desired width. Cutting individual pieces to the desired length may also be considered part of the milling process.

The tools generally used for milling are a band saw (to take a "slice" off of some larger chunk of wood), a jointer (to establish the first flat face and the first flat edge at right angles to this face), a thickness planer (to make the second face flat and parallel to the first face and to reduce the thickness to what is required) and a table saw (to make the second edge and reduce the width as required). A miter saw may be used to cut pieces to length, but this can also be done with the table saw. This article in Fine Woodworking Magazine describes the steps of milling lumber in a fully-equipped workshop.

Milling Without a Jointer or Thickness Planer

I can't use the normal milling process because I don't have a band saw, a jointer or a thickness planer. I don't intend to buy these machines because I don't really want to invest that kind of money in what might be a short-lived hobby and, in any case, I don't have anywhere to put them. At first, I simply limited my projects to what could be made from lumber that I could buy at the thickness I needed. For example, my coasters were made from purchased quarter-inch thick material. The bronze pear platform uses two layers of maple for the half-inch high upper level. However, this soon became limiting and I also figured out that it wasn't economical. A ¼" thick by 3" wide by 24" long piece of curly maple costs $10.49 at Rockler. A ¾" thick by 3" wide by 24" long piece costs less than 50% more, $14.99, for three times as much wood. In addition, if you start with wood that is undersized or bowed, by the time you've got all the pieces flat and even you can end up considerably below your target thickness. So, if I could figure out how to mill ¾" thick lumber down to two pieces each a little over ¼" thick it would be cheaper and better all around.

Resawing on the Table Saw

Thicknessing with my Home-Made Drum Sander

Removing material with a sander is slower than with a planer. This is fairly obvious, as the material has to be reduced to powder instead of being shaved off in pieces.

Example of Thicknessing with my Drum Sander

Below are measurements that I made while reducing the thickness of some small wood slabs to make box lids. Two of the slabs were curly maple and the third was leopardwood. They had been resawn to a little under 27/64" and my goal for their final thickness was 3/8" (24/64", 0.375") so I had to remove a little less than 3/64". I measured the thickness with my inexpensive digital caliper at the middle of both short sides and took the average (the difference was negligible). I did this at the start and after each sanding step.

For each step, the bed of the sander was adjusted until it just contacted one of the workpieces. Then it was raised in increments of half a turn until a "normal" feeling pass through the machine was achieved. This was counted as the first half turn of the step. The other two workpieces were passed through at the same adjustment. The bed was then raised by another half turn and all pieces passed through again. Raising the bed by more than half a turn for a pass was found to be a bad idea, even for the coarse grit sander sleeve. It took an uncomfortable force to push the wood slab through and I became fearful of heating. Since this limitation was the same for all grits, it seems that there is little point in using the coarser sleeves or in stepping cautiously from 50 to 80 to 120, so I may skip some grits in future. I did note, however, that the coarser grits had less tendency to load, which makes sense. The coarse grit step, then, consisted of eight passes through the machine for each piece, raising the bed each time by half a turn.

All steps ended with two additional passes without raising the bed. The purpose here was to end with a light pass and leave sanding marks that were less deep than for a normal pass. Also to this end, the workpiece was slewed from side to side, which has been found to reduce sanding marks as compared to a straight pass.

The first slab, called "Maple 1" in the table, was already finished on one side, so all sanding passes were applied to the other side. For the other two slabs, whichever side showed untouched areas was sanded until both were covered with sanding marks and then the sides were alternated. The final side to side passes were applied one to each side.

The first step (coarse) removed 0.029" of material in eight half-turn passes, which is about 0.0035" per pass. The leopardwood started of a little thinner and ended up the same as the maples.

The second step (medium) removed 0.005" in two half-turn passes, which is 0.0025" per pass.

The third step (fine) removed 0.004" in two half-turn passes and a quarter-turn pass, which is about 0.002" per pass.

The fourth step (extra fine) removed 0.0025" in two half-turn passes, which is about 0.001" per pass.

Hand sanding to a 400-grit finish removed an additional 0.003" per side. For the maple piece that was already finished on one side, hand sanding only the other side resulted in it being very close to the target thickness. The other two had to be hand sanded on both sides and ended up slightly below the target, but still well within what I consider acceptable.

I have no good explanation for why the amount of material removed per pass fell as the sanding sleeve became finer. If the bed is raised the same amount, it would seem that the same amount of material should be removed. (Years later : could it be something to do with the reduction in the thickness of the sleeve?)

Changing the sleeve took about three minutes between coarse, medium and fine. To get to extra fine it is necessary to change the entire drum and that took five minutes.