How close is close enough? Thickness Planing

[tim walker]

I am striving for a solid 0.75" for some dove tail work I am about to start. What kind of tolerances are acceptable? I am an engineer by degree and and 0.001" tolerances are what we strive for in machining, sometimes 0.0001". Am I over thinking for wood?

[Eric Arnsdorff]

Definitely anything under 0.001" in woodworking is too small to worry with - not to mention it will change in a short time and you'll get a different measurement from wood movement.
For thickness I can usually get under 0.005" off of my planer for the most part when measuring different portions of the wood (excluding the sniped area).
But this is more than adequate for milled board thicknesses.
Just as in engineering - there isn't a singular value. 0.005" in a dovetail joint can make a difference but not in the thickness. Cleaning up the final joints after assembly will with thickness variations under 0.010" isn't an issue. The joint itself does need tighter tolerances than that. Those are in the 0.005" or under range. But I don't measure these - I do test fits.

For inlays I find that 0.002"-0.003" offset leads to a nice tight and gapless fit.

Obviously, the length measurement of the pieces making your drawer can be more than these values and will be.

I suspect others will indicate larger values as I spend way too much time trying to set all my equipment up to achieve these perfect fits and there's no doubt others are way more efficient at getting things done than me by not being a perfectionist and being a craftsman. But definitely don't worry with anything under 0.001"-0.002" (and those values are limited to a few areas).

[Jonathan Jung]

yes, there are too many variables that .001 isn't worthy fighting for

[Bill Dufour]

0.001 or more mismatch in a flat plane is visable to the naked eye. Easily corrected by sanding.
Bill D

[Mark e Kessler]

I went to school for furniture making and did that professionally for ~15 yrs before going back to school for Engineering, I work in mold making now and some of our tolerances are below 0.0001. I have spent most all my years building not using calipers and only tape measures and rules, my advice is to put down the calipers…

You can find me on IG under Kessler_Woodworks for all the shenanigans…

[Jim Becker]

IMHO, what's more important is that the material be "consistent" in thickness for best joinery results, although with hand-cut dovetails there certainly is flexibility. Nailing a "specific" thickness to multiple decimal points just isn't going to happen with a material like wood, at least over time. But "in the moment", if you can thickness all the components at the same time, it's more likely you'll have consistent thickness which makes work easier. Other dimensions can be taken as you work to account for any small differences where stuff has to fit together square and true.

[Prashun Patel]

You have to determine your own tolerance for tolerance.

Personally, the absolute thickness is never important to me - only the relative thickness.

For drawers, if machine milling, I will plane all sides to the same thickness for ease. However there are a couple caveats:

1) (unsolicited aesthetic advice alert:) Drawers of 3/4" size tend to look chunky to me. Consider 5/8" unless these are case sides or for a heavy duty drawer.
2) For dovetails, the relative thickness of the pin and tail boards is irrelevant. The critical thing here is that the base line of the pins match the thickness of the tail board and vise versa.
3) When I dovetail, I always have the pins and tails protrude the mating board by a very small amount - just enough that my fingers can feel it. This allows me to plane it flush and peen the pins if necessary after assembly. This is accomplished by making the base lines a touch deeper.
4) The pin and tail boards require separate setup. So while it's nice to have all the tail boards the same, there is no efficiency gain by making the tail boards match the pin boards exactly in thickness.

I've never met a drawer, guitar, leg, or panel that required machining tolerances. If my eye can't see light under a straight edge, it's been flat enough; if my fingers can't feel a ridge between two pieces, the consistency in thickness is good enough.

[Brian Runau]

I use a Dewalt lunch box planer and I do what Jim says, I mill all the pieces that attach to one another at the same time, and make sure they all go through the planer on the same final dimension, so they all match, if I am a little under a little over, no effect on the visual.

[David Sochar]

When you measure a board and record the numerical description, then go to the second board and mark it according to that description, you have added a place for inaccuracy with the introduction on of the abstract concept of inches, or centimeters, or ........

Avoid the inherently inaccurate ordeal of measuring altogether and mark one board from the other. This works for many joinery areas - dovetails, mortise and tenon, edge to edge or end joining, and more.

One less number to misread, one less number to forget, one less number to mismark, and so on.....

Keep it real, avoid abstractions except when they work for you.

[Prashun Patel]

That's his story stick, and he's sticking to it.

[Alex Zeller]

When I start to think about things like this I remind myself that people have been working with wood a lot longer than I have been alive and they most likely had nothing more than a wooden rule. They produced things I wish someday to be able to make. Don't over think it because wood is not metal and will change.

[roger wiegand]

It depends on the position and use. Joints like a suction-fit box top or a dovetail need to be pretty darn close. I'm guessing that sanding, planing, or scraping will easily introduce variation on the order of 10-20 thousandths. I don't try to achieve those close fits by measurement, but rather start slightly oversize and remove material a tiny bit at a time, eg with a plane, until the fit is right.

[Richard Coers]

You have to sand after going through a thickness planer, so hitting a tight tolerance is wasted time. You just want all the material the same thickness.