I'm trying to make a wood base to mount a 25 pound bronze plaque on the wall at church. I measured the four blind holes on the back and got about 9 11/16 by 17 5/8. So I figured those were cut to machine precision and carefully marked out those dimensions on the wood. The diagonals weren't a perfect match, so I went back to the plaque and measured the diagonals on the mounting holes to see which of my layout distances was closer to right. I ain't the guy on TV that gets everything to fit perfectly.
Huh? I get distinctly different diagonals on the bronze plaque itself.
So I measured everything estimating 1/64ths of an inch and again in millimeters, converted to the same units, and ran least squares. The adjustments to my measurements were mostly under 1/64 inch (0.4 mm) and the holes on the plaque seem to be off as much as 1/16 inch from forming a rectangle. I'd never have guessed.
I'll bet most of you haven't run least squares in millimeters or 1/64" increments. I'll bet most of you think I'm crazy, too.
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Related question - I was going to cut square-bottomed holes to recess the screws that mount the wood to the wall, and put plugs over the screws. I grabbed a Sears spade bit and tried it in a piece of scrap. The plug was floppy loose in that hole. So maybe I don't hold steady enough when drilling? I measured the spade bit and found it to be 0.015" wider than the nominal dimension. The others in the set are similarly oversized. I dug out a couple antique auger bits and they are 0.017" to 0.019" over nominal. Huh? All the twist drill bits for metal that I measured were within the accuracy of my caliper from nominal. Is there some reason, that I don't understand, why wood bits are larger than nominal?
Shrinkage?
I should have noted that in the end I asked myself WWTD, "What would Tommy do on TV?" I laid a paper over the back of the plaque and marked the pattern to transfer the hole spacings. I put it on the wood (right side up, even!) and center punched the holes before drilling. The screws fit okay.
Forestner bits are what you would want to use if you are looking for a clean hole for a wooden plug.
Spade bits are good for framing and holes that will be hidden from view.
I see the problem you are using inches, and not tenths.
Bill93, post: 354993, member: 87 wrote: the holes on the plaque seem to be off as much as 1/16 inch from forming a rectangle
Well of course they were off - you used least squares.
You should have used MOST squares...
Bill93, post: 354993, member: 87 wrote:
I'll bet most of you haven't run least squares in millimeters or 1/64" increments. I'll bet most of you think I'm crazy, too.
The two premises aren't mutually exclusive, right? :>
Bill93, post: 354993, member: 87 wrote: I'll bet most of you haven't run least squares in millimeters or 1/64" increments.
You would win that bet.
Bill93, post: 354993, member: 87 wrote: I'll bet most of you think I'm crazy, too.
Well, yes, but who am I to judge. I have a touch of that same crazy. My wife might argue I am a bit more than touched.
Bill93, post: 354993, member: 87 wrote: Related question - I was going to cut square-bottomed holes to recess the screws that mount the wood to the wall, and put plugs over the screws. I grabbed a Sears spade bit and tried it in a piece of scrap. The plug was floppy loose in that hole. So maybe I don't hold steady enough when drilling? I measured the spade bit and found it to be 0.015" wider than the nominal dimension. The others in the set are similarly oversized. I dug out a couple antique auger bits and they are 0.017" to 0.019" over nominal. Huh? All the twist drill bits for metal that I measured were within the accuracy of my caliper from nominal. Is there some reason, that I don't understand, why wood bits are larger than nominal?
I was working on a similar project, 5/8 hole into which I was going to glue 5/8 dowel in place. When I fit the pieces together, I found that I would have had to use about a shot glass full of glue (that generates some odd thoughts) to fill up the gap between the edges of the hole and the edges of the dowel. Like you, I thought I allowed the bit to wobble too much. I have no idea why those bits are larger than nominal. I'll have to pick up that project again and try it with a Forstner bit.
I'm glad you have the kind of crazy that drives you to inspect your drill bits so closely. You just saved me a little on my own sanity budget.
If I was going to drill a hole for a large dowel, I would probably use a drill bit like this....
I'd probably start it with a forestner bit to keep the surface from splintering. This style drill bit will drill a very uniform and straight hole. You DO have to be very careful getting the angle right when you start drilling, because it will pull it's self in what ever direction it is started and there is no making corrections.
