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· Senior Member
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First thing is to have a good set of reference squares 90 deg and 45 deg at a minimum.

I have metal ones, but the plastic ones used for engineering drawing are accurate and inexpensive, just easy to scratch.

I went through 3 90 deg squares before I found one which was true. Shame the one with the rosewood handle and brass rivets is off. It looked very nice. Now just gathers dust.

When considering tolerance in wood working, keep in mind wood moves with the seasonal moisture changes. Metal only with temperature.

Tolerance depends on the project, method of construction, etc.

I would be happy to be within 1/100in for a typical project.

A recent project where tolerances were critical was my first segmented turning. With a ring of 12 segments, any slight deviation in wood thickness, parallel etc is compounded over the 12 segments. I got the segments as close as possible, but I ended up needing to sand the two halves so they were a good fit. I did not try a feeler gauge, but just looking at the glue line so no obvious gaps.

This is an example where sometimes the sub-assemblies need tweaking to fit. Common in my projects.

You do need to calibrate your tools as best you can. It is too easy to get burned if you assume a tool is still calibrated after a period of use.

I found this out on my first end grain cutting board. I had tuned the table saw months earlier, but did not check before cutting the pieces for the end grain board. I eventually saw the slight lack of 90 deg when I was in the last stages of assembly. Crap. Lesson learned.

If I have a project where the tolerances will really matter, I will double check and re-tune before starting the project. It is amazing how "Murphy" can impact alignment of our tools over time.

When I am making a cutting board I will put two pieces together on the glue line and hold this up to the light. If I see light I need to tweak. If no light, close enough for me. Ready to glue.

I also check the fence on my table saw as part of alignment checking. One of the few times I use a feeler gauge.

I also check the fence ruler. Set to e.g., 1in, make a cut and confirm it is 1in, otherwise tweak the ruler.
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