Paul Ruud
11-23-2020, 7:53 PM
Best wishes to everyone! I dropped one of my crosscut sleds and knocked it out of square. Now with a little break from teaching I had a chance to try an idea I had for squaring the sled. It worked nicely so I thought I would share it and see what you all think of it as an alternative to BIll Ng's 5-cut method.
This method for squaring the fence of a crosscut sled relies on an old method for checking whether a cut is square. Make a cut, flip one piece, and put the two pieces together to check for a gap. Because two of the cuts in this method are only required at the start, this method is almost a two-cut method and goes through less wood.
--Paul
Method
Cut a board so that two sides are parallel. Keep the distance between these side less than the width of your crosscut sled fence.
To make the parallel sides on a crosscut sled, cut the board on one side and then slide it over to cut the other side. Do not turn the board. Just slide it sideways.
Put one of the parallel sides against the fence and cut an edge.
Flip the board end-over-end, placing the other parallel edge against the fence, and make a second cut. The offcut will be your gauge for adjusting your fence. Mark the parallel edges to keep track of whether the (bottom) edge against the fence after the second cut is narrower or wider than the other (top) edge. Use calipers to measure which end is wider.
In theory, the angle between the last two cuts is twice the correction needed for the fence to be square. If you have a keystone shape, with the widest edge at the top away from the fence, then the fence needs to rotate counterclockwise.
From the pivot screw that holds the fence, measure along the fence twice the length of the offcut. You can use the offcut itself for this.
Place one end of the offcut at the measured point. If the fence needs to rotate clockwise then put the widest end of the offcut at the point you have just measured. If the fence needs to rotate counterclockwise, place the narrowest end there. Butt a pointed stop up against the end of the offcut and fix it there on the sled. I use two pieces of blue tape and CA glue for this.
Unscrew the nonpivot end of the fence and move it so that the opposite end of the offcut is between the stop and the fence. Clamp the fence in its new position and fix it there with a screw in a new location.
Return to step 2 and repeat until the offcut has the same width at both ends.
Comments
This method does not amplify the angle as much as Bill Ng’s 5-cut method and can be less accurate, all other things the same.
Both methods rely on approximations that cause the adjustment to be slightly less than required. This explains why at least two iterations are often needed. In my experience, a second round with an additional two cuts is sufficient.
One can remove the math from Bill Ng’s 5-cut method but probably with a sacrifice in accuracy. For example, cutting the final offcut into quarters length-wise gives the adjustment to apply at the length of the offcut. This effectively divides the difference in end widths by 4 (not accounting for the saw kerf).
In my experience, measuring wood cuts at single locations gives a misleading impression of accuracy. Along a piece of wood, there is some variation (as much as a couple of thou) in the width of the offcuts that is caused by imperfections in cuts and inconsistent use of the calipers. Measure at several locations to get a sense of the accuracy.
Here is a Sketchup image to help understand. (I do not know how to make it a reasonable size.)
445559
On the right, it shows the offcut repeated to find the point to place the stop block. On the left, is what happens after switching the end to get the adjustment.
This method for squaring the fence of a crosscut sled relies on an old method for checking whether a cut is square. Make a cut, flip one piece, and put the two pieces together to check for a gap. Because two of the cuts in this method are only required at the start, this method is almost a two-cut method and goes through less wood.
--Paul
Method
Cut a board so that two sides are parallel. Keep the distance between these side less than the width of your crosscut sled fence.
To make the parallel sides on a crosscut sled, cut the board on one side and then slide it over to cut the other side. Do not turn the board. Just slide it sideways.
Put one of the parallel sides against the fence and cut an edge.
Flip the board end-over-end, placing the other parallel edge against the fence, and make a second cut. The offcut will be your gauge for adjusting your fence. Mark the parallel edges to keep track of whether the (bottom) edge against the fence after the second cut is narrower or wider than the other (top) edge. Use calipers to measure which end is wider.
In theory, the angle between the last two cuts is twice the correction needed for the fence to be square. If you have a keystone shape, with the widest edge at the top away from the fence, then the fence needs to rotate counterclockwise.
From the pivot screw that holds the fence, measure along the fence twice the length of the offcut. You can use the offcut itself for this.
Place one end of the offcut at the measured point. If the fence needs to rotate clockwise then put the widest end of the offcut at the point you have just measured. If the fence needs to rotate counterclockwise, place the narrowest end there. Butt a pointed stop up against the end of the offcut and fix it there on the sled. I use two pieces of blue tape and CA glue for this.
Unscrew the nonpivot end of the fence and move it so that the opposite end of the offcut is between the stop and the fence. Clamp the fence in its new position and fix it there with a screw in a new location.
Return to step 2 and repeat until the offcut has the same width at both ends.
Comments
This method does not amplify the angle as much as Bill Ng’s 5-cut method and can be less accurate, all other things the same.
Both methods rely on approximations that cause the adjustment to be slightly less than required. This explains why at least two iterations are often needed. In my experience, a second round with an additional two cuts is sufficient.
One can remove the math from Bill Ng’s 5-cut method but probably with a sacrifice in accuracy. For example, cutting the final offcut into quarters length-wise gives the adjustment to apply at the length of the offcut. This effectively divides the difference in end widths by 4 (not accounting for the saw kerf).
In my experience, measuring wood cuts at single locations gives a misleading impression of accuracy. Along a piece of wood, there is some variation (as much as a couple of thou) in the width of the offcuts that is caused by imperfections in cuts and inconsistent use of the calipers. Measure at several locations to get a sense of the accuracy.
Here is a Sketchup image to help understand. (I do not know how to make it a reasonable size.)
445559
On the right, it shows the offcut repeated to find the point to place the stop block. On the left, is what happens after switching the end to get the adjustment.