Tolerances and power tools... how close is close?

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.

When CNCs showed up in the design labs where I teach my expectations for precision moved from within 1/32 "and a little bit" to .001 increments. Of course no one can tell you what fraction to leave in a joint for glue, or how much a given species can compress, but working at .001 accuracies will make you worry about such things. You can make perfect parts one morning, but come back after lunch to find the weather has swelled the parts out of tolerance. Angles are more important, but only to 1/100 a degree. You may need Pi * a number for a perfect fit, but find even 1/1000 digital accuracy has to round off that number. What to do!

In the end I simply measure fit with the part I made that has to fit. Then you sneak up on it. Actual numbers are just a starting point, but you can find "half of that", even when you don't know the dimension of "that".

As they say in motorcycling, it's about the journey

For some it's about the proces, setting things up, making perfect cuts, planing and inspecting each joint and edge.... For other's it's less about that and more about the finished piece. They say "What ever works" and just move along. Measuring and cutting as they go.
I fall in between since I enjoy the process and the satisfaction of the completed piece. I'm a cut, trim, cut, mark, sand, plane and fit guy rather than measuring and expecting things to fit precisely.
Tape measures are notoriously sloppy and difficult to make precise measurements with. I'd sooner use a long measuring stick make a mark on it and then use the stick to transfer the mark to the piece. For shorter measurements a steel scale is best for me.
As far a the machine tolerances go, I'm the weak link in the chain since my eyes are only so accurate these days, not the machine.
I make a LOT of test pieces until I get the fit I want and often have to make the piece fit the other piece by "shortening" something, one or the other. I've tried my hand at welding and unfortunately it doesn't work on wood. If you can make a mistake look like the natural grain and it doesn't show ...much, then you're good to go.:yes:

Here....one day we may be laying brick arches,over site built plywood bucks.Then it's on to some timber framing....1/16's and 1/8's are jimm dandy.

Tomorrow sees us building cabinets and furniture.....throwing in a cpl hundred feet of millwork(mouldings).Now we better break out a little better measuring than a typical tape measure.

Next day we're grinding bow(archery) laminations....to be used in international competition.Or creating a rifle stock.There's places in a bow limb where .001" is a HUGE difference.

So,there are vary'ing degrees of "needed" accuracy.What one job or craftsman deems perfectly acceptable may be completely off-base on another.The more accurately you can focus in on what's called for,WRT the job at hand.....the easier that job becomes.

Learning to measure however,isn't a bad thing.....its just back to what the job calls for.Even though the following books are more directed twds metalworking......the premises laid down do apply in certain,"shop" areas of wood working.

http://www.mooretool.com/publications.html

It's interesting how, depending upon what WW forum one reads or contributes to, opinions on tolerances can vary. On some, the posters demand .001" on everything. On others, as some posters on this thread have suggested, it's a matter of cutting, fitting, re-cutting, re-fitting until it's "right". I guess I fall somewhere in between. I think the table saw and whatever cut-off machine one uses should be as accurate as humanly possible at 45 and 90 degrees. Frankly, I'd rather take a beating than have to dink with TS trunions to get that magical .001" parallelism between blade and miter slot but I do believe that it's important for safety if not for accurate cuts. And, over the past 30 years of putzing in my shop, I've found that spending over $100 for a dial indicator and "master plate" is probably not the best use of my funds. A brass screw on the end of a hardwood "T-bar" that rides in the miter slot works great if you can discern the difference in "ping" as the screw head lightly grazes the blade tooth.

Bottom line of this long winded opinion, get your machinery as close as you can, be careful, and learn to work with what ya got. Of course it's just my opinion but spending hours trying to squeeze that final .001" is hardly my idea of a good time in the shop. Still, I do it (occasionally) because all the other woodworkers who I respect and admire tell me it's the thing to do and I believe what they tell me. I just don't enjoy it.

I doubt I helped you much but did enjoy responding to your post. :yes:

GeorgeC said:

There are several people on this forum who will vehemently disagree with me. But I think that use of a dial indicator on woodworking machines is a waste of money and time in the vast majority of situations.

1/64th of an inch is by far and away as close of a tolerance that you could possible need/use. 1/32" is far more practicable and absolutely sufficient for most applications.

Fire away guys.

George

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+1. :yes: I agree. If need be, 1/64 can be split. I use plastic drafting templates, and an adjustable protractor for my standards. I've learned to use and trust my tape measures. For the most part, that's all I use on an entire project. I may use fixed duplication/transfer of markings with a combo square, or from a sliding "T" bevel.

For a good part of my first year in the business, my table saw was a sheet of plywood with a $10 flea market special circular saw mounted underneath. All setups were done with either a tape measure or a metal yard stick. When I bought my first Unisaw, Delta sent two tech guys to deliver and set up the saw. They showed me how to check and align the saw with simple measuring devices like a tape measure.

A friend gave me as a gift some runout and dial indicator gauges as a gift. I showed them to my good friend Werner. He had a shop across the street, and I became very friendly with him...he was my mentor. He was A German immigrant whose father and grandfather and great grandfather were lifetime woodworkers and cabinetmakers . When I showed him those precision devices, he couldn't stop laughing. Actually we both got a good laugh out of it. I can't ever remember using them.