Mark & Juanita wrote:

DJ Delorie wrote:


Mark & Juanita writes:
After considering this, I think it is also necessary to rotate the
router 180 degrees when doing pins vs. tails. When you look at
where the router bushing contacts the jig, it seems that you want
the same point contacting the jig for both pins and tails. To do
that, the router needs to be rotated 180 degrees. Would you concur?

No, because you want to consistently register the center of the
bushing against the same reference edge with the same offset. For
example, if your bushing was 1/4" to the left (assuming that were
possible), you want that offset to move the pins to the left *and*
move the tails to the left. If you rotate the router, you end up with
the pins moved to the left but the tails get moved to the right.


I agree with needing to offset the same amount from the reference edge.
However, the reference edge changes when you flip the template.
Here was the thought process:

Say the offset is as shown below:
bush bit bush
--| |-- offset
----| | | |-------
| | | |
/_\


When cutting the tails:

--| |-- offset relative to jig
----| | | |-------
| | | | ---- this part of the bushing contacts the jig,
/_\ The cut will be offset by the off-center
here

| Cutting wood on this side

If I don't rotate the router 180 degrees when cutting the pins:

--| |-- This is the now offset relative to the jig
----| | | |-------
| | | |
/_\
| Cutting wood here
This is the
side that will
contact the jig

| Now cutting wood on this side of the bit

It seems like the cut will now be greater for this offset on the pin
side
than on the tail side.

However, if I rotate the router 180 degrees:
--| |-- This is the offset relative to the jig
----| | | |-------
| | | |
/_\
| Cutting wood here
This is the
side that will
contact the jig

and the same offset from the bushing to the workpiece will be obtained.

It seems like I want the second condition to get the same offset. What
am
I missing?




If the bushing has an in/out position error, you compensate for that
when you position the D4 rack after flipping it.

This assumes that the largest error in the bushing is it's position
relative to the router bit. Errors in the shape of the bushing, while
nonzero, are likely far smaller than the position error.

From what I can tell right now, I agree, the biggest offset is in the
router bit centering.




Hopefully the ASCII art turned out better for others than when I read the
posting. If not, my apologies, let me know what I can clarify
--
If you're going to be dumb, you better be tough