Replies

1. 
   UncleDunc | Jul 10, 2002 07:45am | #1

   If the backrests were vertical, the cutting angle on the ends of the boards would be 90 degrees and the miter angle would be 45 degrees. If the 'backrests' were laid all the way down, the cutting angle would be 45 degrees and the miter angle would be 90 degrees. Anywhere in between, I think the relationship is just linear, each degree of tilt subtracts half a degree from the cutting angle and adds half a degree to the miter angle. But my brain also seems to have gone on vacation and I can't prove the linear relationship to my satisfaction. My brain is vacationing in the Snowy Mountains in central Montana. Where did yours go?

2. 
   SgianDubh | Jul 10, 2002 03:06pm | #2

   Whoops, I completely misread your question and came up with useless figures because I used incorrect numbers, hence my deleted message. Figures should be,

   Dihedral = 93.84°. Tilt the sawblade to halve this to form a mitre at 46.92°. 47° is almost certainly near enough.

   The mitre angle is 75.49° which, subtracted from 90° = mitre fence setting of 14.5°, depending on how your fence is marked using either 0° or 90° as the base figure.

   As I commented before, a 15° is a bit steep for comfortable back supported eating, this being a better angle for reposing or leaning back. It depends on the overall purpose of the bench if this is the correct angle to use. Sorry about the screw up earlier. Slainte, RJ.

   RJFurniture

   
   

   ---

   Edited 7/10/2002 8:08:26 AM ET by Sgian Dubh

1. 
   sjmerrette | Jul 10, 2002 04:05pm | #3

   Hey there, how ya been? I got to around 14° and 44° from the trial and error part. when I cut it at 45° the rear points touched but I had a gap at the inside. so if I went with a 47° miter as you suggest I would think it would accentuate the inside gap rather than close up the gap. could you please try to explain to me how you arrived at those numbers? I really would like to remember how to calculate that for myself.

   As for the 15° rake being too much, I agree if it were a high back rest. The bench is positioned in a corner where there are windows and therefore is very short. not even 8" high and the 15° angle seemed to be pleasing aesthetically, more for appearance than function in this case.Steve

   S.J.MERRETTE Carpentry & Construction • Robesonia, PA

   Nothing is impossible...It just hasn't been done yet.

2. 
   LeeGrinding1 | Jul 10, 2002 04:11pm | #4

   Sgian, Your numbers sound right on, or as right on as compound tables will get you. Where did you get these numbers? I have an old compound miter chart from a text book that gets me close but I'm usually looking for more precise numbers and it would appear that more precise numbers is what you have.

   LeeLee Grindinger

   Furniture Carver

1. 
   SgianDubh | Jul 10, 2002 04:45pm | #5

   Lee, my numbers come from punching long complicated formulae into a scientific calculator. I try to find a way of visualising all these compound cut problems as polyhedra. In Steve's case he's got two sides of a four sided a hopper, or an inverted pyramid in which the sides slope in at an angle of 75° from the ground. All I need to know is how many sides, and this angle of lean in, and I'm up and running. It makes no difference to me how many sides there are, even odd numbers like five or 13 sides, the calculations work.

   To calculate the dihedral angle I do the sum, D= cos-1((2*sin(180/N)²*sin(L)²)-1)  [The first cos-1 in the formula should read cos to the power of -1 but I can't raise the number here.] in the formula, D= dihedral angle, N is the number of sides, and L is the angle of lean in of the side from the base line.

   To calculate the true angle that the edge of the side describes to the baseline, I calculate, T=tan-1(1/(tan(180/N)*cos(L)))   [tan-1 is tan to the power of -1.] T= true angle. the other letter references remain the same.

   Steve, I think you'll find my numbers pretty close. Try them and see anyway. Slainte, RJView ImageRJFurniture

1. 
   LeeGrinding1 | Jul 10, 2002 05:21pm | #6

   Sgian, you amaze me. Thanks for opening my eyes to yet another way to get there from here.

   We need to rendevous somewhere and empty a bunch of beer bottles...first couple on me.

   LeeLee Grindinger

   Furniture Carver

2. 
   Joe_Fusco | Jul 11, 2002 12:45am | #7

   
   

   If you use this crown calculator, you will see that entering 90° as the wall angle and 15° as the "crown" angle, the program returns 14.51° for the miter cut and 43.07° for the bevel cut.To find the dihedral angle for this you subtract the bevel angle setting of 43.07° from 90° which is 46.93° then multiply by 2 to get 93.86°

   View Image

3. 
   Joe_Fusco | Jul 11, 2002 01:58am | #8

   Sgian,

   I was able to resurrect "Men is Skirts" a.k.a. Rugby from my hard drive. I posted it as an attachment.To all others. . .:   It's provocative so if you think you might have a problem with it at all, don't click it! But it just to good!

   View Image

   Edited 7/10/2002 6:59:18 PM ET by J Fusco

3. 
   JamesNowa | Mar 10, 2004 02:58am | #9

   I use this method for cutting crown moldings laying flat in a compond miter saw,it also works for sloped boxes.

   I use the Pitch-Rise-Run-Diag buttons on a construction master calulator.

   First find the rise and run of the molding by holding it to the inside corner of a framing square,measure from the inside corner to the furthest part of the molding tuching the square.

   Enter the Rise length=A

   Enter the Run length=B

   Press Diag     length=C

   clear

   Enter length B in Rise

   Enter length C in Run

   Press Diag   length=D

   Press Pitch twice,this is your Angle

   clear

   Enter length A in Rise

   Enter length D in Run

   Press Pitch twice,this is your Bevel

   You can convert 15 degrees to rise and run by using a speed square

   James

1. 
   jmoonret | Mar 10, 2004 03:27am | #10

   I have an Incra Twin liner. I have had this jig for about two years. I just took it apart and am using the fence part only. For joinery I recently bought  an Adria dovetail saw. Now this is a fine piece of equipment for fine joinery. The Incra just took up too much space that I didn' really have.