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Question on Dovetail Bit Angle Caluclation

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Forum topic by James Pease posted 02-02-2011 07:57 PM 2334 views 0 times favorited 6 replies Add to Favorites Watch
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James Pease

6 posts in 2147 days


02-02-2011 07:57 PM

I have an out of date Craftsman Dovetail Jig, 171.25450. Sears has deleated all information about this jig and the bits. I am trying to use the through dovetail jig template. The dovetail bit is 9/16 in. and probably should have a 1/4 shaft because the bushing is 1/2 inch for the dovetail bit. I have no idea what the angle of the dovetail bit is. I have cut tails with the 1/4 in. straight bit and appropriate bushing. I have attempted to measure the angle that way but the best I can do with my dime store protractor is somewhere between 7 and 10 degrees. My eyes don’t help much either. The craftaman number for the bit is 25414 but like I said, all information has been deleated. That number has been reassigned to an automotive part. Would anyone, by any chance, have one of these or might know what the correct angle is. Thanks, glue

-- JR Pease


6 replies so far

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richgreer

4541 posts in 2541 days


#1 posted 02-02-2011 08:58 PM

Maybe this will help – -

If you have a good, precise calipers, make 3 measurements as follows:

A = the diameter of the dovetail cut at the thickest point.
B= the diameter of the dovetail cut at the thinnest point
C = the distance along the cutter from where A was measured to where B was measured.

Let D = (A-B)/2

Let E = D/C

The angle of the dovetail cutter is the arcsin of E. There are lots of calculators around with trigonometric functions and/or you can find trigonometric functions on most spreadsheets like Excel.

Note that most calculators and spreadsheets will tell you the angle in radians. You will need to convert that to degrees.

degrees = radians * 180/pi

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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James Pease

6 posts in 2147 days


#2 posted 02-02-2011 11:09 PM

The problem is that I do not have the dovetail bit. That’s why I was trying to measure the angle on the Pins. I have cut several sets of pins and measured as precisely as I can. I think I have it norrowed down to an 8 or 9 degree bit. I’m going with an 8 and if that isn’t correct I’ll try a 9 degree. Thanks, glue

-- JR Pease

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richgreer

4541 posts in 2541 days


#3 posted 02-03-2011 01:18 AM

The procedure I outlined can also be used on the pins. In fact, it might be easier. Measure the width of the pin on each side of the board and measure the length of the side of the pin.

Actually, with the pins you only need to take 2 measurements, the length of the side of the pin and the width of the board. I’ll give you the math if interested.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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James Pease

6 posts in 2147 days


#4 posted 02-03-2011 01:41 AM

Yes, please do. I’ll measure it later tonight. I’ll let you know what I come up with. I’m thinking it might be a 10 degree now. That’s how much in the dark I am. I would be very greatful. Thanks again, glue

-- JR Pease

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richgreer

4541 posts in 2541 days


#5 posted 02-03-2011 04:14 AM

Assume that

W = width of the board
L = Length of the side of the pin

The angle will be the arccos of (W/L). “arccos” means the inverse of cosine.

FYI – I’m a retired actuary and a math nut. Trigonometry does come in handy with woodworking.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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James Pease

6 posts in 2147 days


#6 posted 02-03-2011 04:20 AM

I used the formula on the pins. I got .115. There is no way my measurements can be considered precise. That’s close enough to 10 to satisfy me. The protractor method never indicated anything more than 10 degrees. I’m going to go with a 10 degree bit. I’ll let you know what the outcome is. Thanks again. I can’t tell you how much relieved I am to get this issue resolves. I won’t hold “being a math nut” against you. Thanks again, glue

-- JR Pease

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