I have a Porter Cable dovetail jig. It works reasonably well when it’s properly aligned, but properly aligning it a hilariously clumsy process of guess and check. The alignment lines on the templates are on the top surface, so there’s a quarter inch of parallax error, and the brass adjustment nuts aren’t graduated in any meaningful way. The instructions say things like “If the joint is too loose, move the jig away from you.” How far? Depends on where you hold your head. It results in a guess-and-check, guess and check mentality. There is no try, measure how far off it is, and adjust it based on that measurement.
I solved both of these problems with a knife.
I printed out a little wagon wheel looking thing to use as a guide so I could put some graduation marks around the brass thumb screws. They run on a 16TPI threaded rod, so 1 full turn drives it 1/16th inch, 1/2 turn 1/32", 1/4 turn 1/64", and 1/8 turn 1/128". I stopped there because that’s about the limits of my ability or need to measure. It’s not on an absolute scale, but now I can move both sides of the template with some precision, if not accuracy.
I also scribed an alignment line on the back of the template, and then down each side of each template tooth. The factory alignment lines are like 1/16" wide or better, so I just scribed the location of the center. That should eliminate parallax error.
I’ll give it a test run tomorrow and see if I helped it any.
When I was young and naive, I figured I should be able to buy a tool or jig, take it out of the box, and use it. The longer I play, the more I realize almost all of those things benefit by some hand tuning. This is a great example.
Don’t be afraid to scratch alignment marks or drill holes in that expensive new tool. It’s yours, and you know better how you work than some engineer in Indianapolis.
I can now attest it does help. For doing a sliding dovetail for example, I cut the tongue intentionally thick, measured the width of the bottom of the slot, measured the width of the end of the tenon, and then I adjusted the jig half that distance plus 1/128" to give me 1/64" tolerance.
It did need some way to quantify the movement of the template.
