Shopsmith Forums

BuckeyeDennis wrote: ↑Sun Jan 09, 2022 10:33 am ***
Real-world check

I just measured my own 48" Shopsmith double bar clamps. They are 2-1/8" tall in the center, and 2" tall at the ends of a 47" chord. (Beyond that they have a relief cut for the clamp hardware.)

Plugging those numbers into the curvature calculator, I get a 184' radius of curvature. That's a big difference, and I suspect that it's causing your problem. I dunno where the numbers came from on the sketch that you linked, but they definitely don't match my Shopsmith clamps.

KCollins wrote: ↑Sun Jan 09, 2022 11:33 am Are these still needed?

Overall - 1-1/8" X 2 X 41
At the Crank End there is a recess narrowing the end to 1-5/8" for 2-3/4" - I can provide a picture if you'd like.
The holes are cut in half and are 1/2" in diameter.
On both ends there are 2 half holes that are 3-1/4 and 4-1/4" from the end.
The remaining half holes are from the end 6-1/2", 8-3/4", 11",13-1/4",15-1/2",17-3/4", 20", 22-1/4".

• Start with a straight edge on an MDF template.

• Use trig and a spreadsheet (or a CAD system) to calculate arc offsets from the straight edge. Generating an offset calculation every few inches along the straight edge.

• Use calipers to lay out the offset points on the template.

• Bandsaw and sand the template smoothly to the arc points.

• Use the template with a router and a pattern bit to transfer the curve to the actual cauls.

BuckeyeDennis wrote: ↑Sun Jan 09, 2022 1:55 pm The radius of curvature on my clamps really does work out to approximately 184 feet, or 2,209 inches.

Given that a different online radius calculator reportedly gave different results than the one I linked, I checked the calculated radius using some simple trig. The results agreed. The calculator I linked also matched the results from Larry's CAD program.

Algale, I agree that a compass isn't a very practical approach to drawing the arc! (I do have a 150 foot landscape tape measure, but it would be darned hard to find a smooth flat surface that long to work on. ) I'd do it something like this:

• Start with a straight edge on an MDF template.

• Use trig and a spreadsheet (or a CAD system) to calculate arc offsets from the straight edge. Generating an offset calculation every few inches along the straight edge.

• Use calipers to lay out the offset points on the template.

• Bandsaw and sand the template smoothly to the arc points.

• Use the template with a router and a pattern bit to transfer the curve to the actual cauls.

Now here's another question: do we really know that the profile is in fact a circular arc? I suspect that it is, just because that makes it fairly easy. But I doubt that a circular arc is optimal for equalizing the vertical clamping pressure across the width of a panel.

But even if it's not the optimal profile, it's a lot better than a straight clamping edge. The Woodpeckers panel clamps are straight, and they don't draw the glue-up flat without some external persuasion. Starting at about 5:30 in this Woodpeckers Deep Dive Video, you can watch Jeff Farris explain why, and demonstrate how, he gets his board edges into vertical alignment by literally whacking them with a mallet.

algale wrote: ↑Sun Jan 09, 2022 2:34 pm

BuckeyeDennis wrote: ↑Sun Jan 09, 2022 1:55 pm The radius of curvature on my clamps really does work out to approximately 184 feet, or 2,209 inches.

Given that a different online radius calculator reportedly gave different results than the one I linked, I checked the calculated radius using some simple trig. The results agreed. The calculator I linked also matched the results from Larry's CAD program.

Algale, I agree that a compass isn't a very practical approach to drawing the arc! (I do have a 150 foot landscape tape measure, but it would be darned hard to find a smooth flat surface that long to work on. ) I'd do it something like this:

• Start with a straight edge on an MDF template.

• Use trig and a spreadsheet (or a CAD system) to calculate arc offsets from the straight edge. Generating an offset calculation every few inches along the straight edge.

• Use calipers to lay out the offset points on the template.

• Bandsaw and sand the template smoothly to the arc points.

• Use the template with a router and a pattern bit to transfer the curve to the actual cauls.

Now here's another question: do we really know that the profile is in fact a circular arc? I suspect that it is, just because that makes it fairly easy. But I doubt that a circular arc is optimal for equalizing the vertical clamping pressure across the width of a panel.

But even if it's not the optimal profile, it's a lot better than a straight clamping edge. The Woodpeckers panel clamps are straight, and they don't draw the glue-up flat without some external persuasion. Starting at about 5:30 in this Woodpeckers Deep Dive Video, you can watch Jeff Farris explain why, and demonstrate how, he gets his board edges into vertical alignment by literally whacking them with a mallet.

What angle are you seeing between the two radii that form the arc?

larryhrockisland wrote: ↑Sun Jan 09, 2022 1:31 pm

KCollins wrote: ↑Sun Jan 09, 2022 11:33 am Are these still needed?

Overall - 1-1/8" X 2 X 41
At the Crank End there is a recess narrowing the end to 1-5/8" for 2-3/4" - I can provide a picture if you'd like.
The holes are cut in half and are 1/2" in diameter.
On both ends there are 2 half holes that are 3-1/4 and 4-1/4" from the end.
The remaining half holes are from the end 6-1/2", 8-3/4", 11",13-1/4",15-1/2",17-3/4", 20", 22-1/4".

Thanks KCollins. Can you check the dimension on end opposite the crank end? I think it will be 1-7/8" based on Dennis and JPG's information.