I know this is a basic question, but I'm hoping I'll read some alternatives I hadn't considered.
I need to make a T, which is a simple butt joint against the 'centre' of another piece of wood. the depth of the short leg of the T should be
5.234567 (exaggerated) within one RCH. I thought I would clamp two blocks
5.234567 wide to the edge of the wood, then glue and tack the long leg of my T using the blocks as a guide. Once my gap was filled with the 5.234567" filler wood I would sand to within 1 RCH>
Any better thoughts on achieving RCH accuracy, especially squareness.
Many years ago, Edward Deming (father of quality control), made the claim that sometimes it isn't the workers' fault, the fault is in the system. To fix a recurring conformance design problem, the design and the system of manufacturing may have to be changed.
For example:
- you are working with wood, the nature of the material is not steel.
- Since you are asking this question, I presume you are using home-owner class tools.
- your power tools don't weigh thousands of pounds.
- your power tools aren't bolted to the floor
- woodworking sometimes requires skills, not only book learning, but years of experience with the tools.
- any fabrication needs an error tolerance calculation due to fabrication process. What are your + / - errors allowed? (have you allowed for wood compression?)
Nobody can measure anything beyond three digits (0.000) with any meaning as the tools for measuring become inaccurate and the results meaningless. With wood, three digit precision limit is cause for laughter. You are asking for SIX digit accuracy. (The deflection of the wood by the pressure of your hands squeezing it will negate any attempt at SIX digit accuracy.)
In short, your "plan" or design is in error if you attempt to fabricate to such accuracy. Even trying to custom fit with sanding is going to get you high blood pressure. (sanding will many times cause loss of square in 3D of parts, as hand sanding is very, very in-accurate; one place always gets sanded more than another leaving a slight dip in the surface.)
Re-think your design, remember you are working in wood.
For FOUR digit accuracy, the Newsgroup you want is rec.metalworking. Their toys are a bit more $$$$$.
Ideally, the skill of the worker includes his/her choice of tooling, fixturing, and processing - essentially the "system of manufacturing".
He probably is. This is not necessarily a problem. In 2003 I held an accuracy contest here in the wreck. The idea was prompted by a guy who claimed that he could work wood in the ten-thousandths of an inch range on his "super tuned" contractor's saw (throughly de-bunked). Here's the thread with the results:
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the official results on my web site have long since been deleted. The winner (Owen Lowe) cut acrylic samples to within 0.005" using the stock miter gauge on a Unisaw (sold to home shop woodworkers everywhere in the US). He aligned his saw using a "feel the rub" technique. The best wood samples came in at 0.011" (hard maple). Probably could have done better with lignum vitae. Not a chance with pine, oak, or walnut. The thread includes my take on why it's hard to do much better with wood. My benchmark on a Unisaw cutting aluminum using the stock miter gauge was about 0.001".
The Unisaw doesn't weigh thousands of pounds.
My Unisaw is on wheels. I can do work to about 20 millionths in steel on my surface grinder. It's not bolted to the floor either. But it does weigh about 2500 lbs.
Absolutely. Experience is required to achieve such results or even comment on their feasibility.
He probably hasn't. Sounds like he's doing emperical work.
Just call me "Nobody"!
Please feel free to stop by my shop anytime and I'll show you reliable and repeatable linear measurements to within millionths of an inch and angular measurements to less than an arc second. Yes, that's six digits. No, it has nothing to do with wood. But, depending on the species, and the direction of measurement (along the grain, not across the grain) it's possible to work wood to less than 0.005" - all day long, any day of the week. Measuring it is no problem. I wouldn't be willing to claim anything better.
You did say "nobody" can measure "anything" beyond 3 digit accuracy. ;-) I think it's safe to say that we don't need to consider Hisenberg uncertainty or the observer effect for this sort of work.
I would liken hand sanding to hand scraping bed ways. Considerable skill is required but I don't doubt the feasibility. I wouldn't try it but there are guys who can scrape machine bed ways to within ten- thousandths of an inch with a hand held scraper.
I assume you mean rec.crafts.metalworking. I think a fairly decent manual mill can be had for the cost of a 3hp unisaw. I would like to learn more about Bill's application and needs. Perhaps there are better ways for him to solve the problem.
No straight answers here then? So now have to go to bed thinking... Royal College of Hairdressers? Registered Caliper Handlers? Roughly Calculated Hairsbreadth?
I am setting the pub quiz tomorrow night. I could ask this question.
Actually, RCH reminds me of the first WKRP episode where WKRP plays "You're having OUR baby", by the Mormon Tabernacle Choir, and Johnny drags the needle across the turntable. The problem is, Johnny isn't there to save the day!
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