This video is of the planing of a Southbend 10" long bed that I did this past summer. As I recall the bed was 48" long. It was quite worn. There was also an odd bend at the headstock wherein at the end of the bed the last 10" slopped upward about 0.006" above the plane of the bed, and was also bent slightly toward the back as well. I have no idea why. The entire bed also had a slight "S" shape to it in the horizontal plane as can be seen in cutting the backside of the headstock Vee and the final cutting of the back side. However this was so small that it does not raise questions for me.
By the time the video begins the base had already been planed. The bed is long and thus has support pads mid-way in its length. Since these pads are needed for support while planing, the base must be machined flat to insure that all pads are in the same plane. Planing of the base for a lathe with a center foot requires that the tops of the Vees be planed first while the bed is allowed to sag by supporting it only at its ends. Then flipping the bed over and clamping these Vee tops to the table insures that the base will be machined in the plane of the sagging bed. This intern insures that when the base is clamped to the table the bed will be in the plane of the sagging bed at rest.
To cut the Vees the slide is angled and then adjusted for height to just clear the flats between the ways. Then the slide is locked and feeding is done horizontally. The DRO is used to set the spacing between the Vees. After moving the head, a new register is selected and zeroed. This will be the finished position for the opposing way. The head is then backed away from this position and cutting commences.
Of note is that at 13:30 when planing the headstock flat, when I stopped to measure the height between it and its Vee I noticed that I had forgotten to plane the backside of the central Vee. Rats! This meant that I needed to zero the Y axis, switch to another register and re-angle the head to cut the side of the Vee. After this I cold again set the head vertical and resume cutting the flat.
Incidentally, when setting the slide to 45 degrees I placed a large Vee-Block on the table and indicted to its Vee as a reference. And since the flat-tool had already been indicated to be within a tenth in 1.5" of square to the slide travel, I could be quite sure the tool would be 90 degrees to the Vee-Block surface. This is one of the keys to planing a lathe bed using a flat-tool: carefully set up the tool once, and then swivel the head to angle the tool. And is why the "Rats!" above was not in stronger language.
The final 45 seconds is of cleaning up the backside. I like to do this so that it is later easy to determine the alignment of planing, but is unnecessary if this edge is flat and in axis.
It was after this that I noticed that I had forgotten to cut the tops of the Vees. Rats again! I think it is time that I use a checklist to help keep my wits on these jobs. I had to set up the flat tool again for that, but there is no video of cutting them. All three were cut to that same height and to match the same width as the unworn portion of the bed when it arrived - the latter determined by the distance from the flat to the top of the small Vee-Block placed on the way.
I consider machining these flats to be important, not just to maintain clearance in the carriage, but they are "witness surfaces" that tell how the bed was planed. Using them and the planed backside, it can later be determined if the bed is still of the shape as when it was planed (hopefully relaxed and straight), and they also indicate how the bed was setup.
Also noteworthy is that you can see that I wrote "32" on the backside of the front way after planing it. This is a horizontal measurement, but it is also from approximately when the cutter first begins to cut. The number is high because of the bent upward headstock end. As measured from the table, the planing lowered the Vees 0.026", while if 32 thousandths were taken from each side it would have lowered it 0.032".
I did not cut the flats between the ways even though the wear suggested that it might be necessary. The owner was going to place a filler on the carriage ways to maintain leadscrew alignment and so there would be no interference. I have never had to cut these surfaces, and I do not relish the idea of re-stamping the serial and model numbers.
10 окт 2022
