I am lost, please help

[Eduard Nemirovsky]

Hi, I am completely lost. I want to rip with 45 degree angle 6 feet board and the same time mitering each side to 7 degree. I need 4 boards to build a floor lamp. I think I need a support board against my fence and cutting peace on this board? Questions are how to hold a cutting peace and how to make precise cut?
Would appreciate any idea, thank you. Ed:confused:

[Joe Spackle]

I would suggest you not try to do the two cuts simultaneously, rather do them as two seperate cuts.

IOW taper the edges @ 7*x 90* first, then set up rip fence with a standoff fence and make the bevel cuts. the key is that the edge you intend to bevel is the registration for the bevel cut. the standoff makes the cut safer as the piece is not trapped at the end of the cut so the potential for kickback is nearly zero as long as you hold the stock down tight to the table.

Standoff fence amounts to clamping a straightedge board to the rip fence so only 1/8" of the edge you are trying to rip is in contact with the tapered edge ( at the top) . as you "rip" the bevel the small off cut piece has clearance to fall safely away as the cut progresses The aux fence should be about 2" wide 3/4" high and as long as your rip fence. Drill two 1 1/4" holes in the face so the clamps can securely tightened without being in the way.

http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence001.jpg?t=1218074094
overall view
http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence002.jpg?t=1218074168
standoff fence should be about 1/8" lower than the stock you intend to bevel.
http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence003.jpg?t=1218074208
end of the first bevel cut
http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence005.jpg?t=1218074251
ready to start the second bevel
http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence007.jpg?t=1218074285
http://i243.photobucket.com/albums/ff168/BCStudios/drawings/Standofffence008.jpg?t=1218074322

note how the off cut safely falls away from the standoff fence

Happy taper/beveling

HTH

Joe

[John Thompson]

Ditto Joe on two separate cuts as the margin of error attempting both at once gets complicated.

Sarge..

[John Fry]

Joe's method for cutting the bevel is sound and very safe.

Be sure to make a couple of practice cuts on material of the same thickness to set the correct blade height, and its contact point with the auxiliary fence.

These settings are important in order to complete the cut and leave a sharp beveled edge. If the blade is set to low, the entire edge is not beveled leaving a "flat" tip, if it is set too high, you remove too much material and loose your "edge to fence" register right after the blade to help guide the stock through the rest of the cut.

[Joe Spackle]

I have a question...

If you want a 7* taper on each side of a six foot piece, the base would have to be 19 9/16" wide to have a 2" top :confused::confused:

Are you sure of the taper angle?



do you have some basic measurements available you could share?

Joe

[Tom Veatch]

Ed,

I'm assuming you're using plywood since the width of each piece is going to have to be greater than (2 x 72 x tan 7) = 17 11/16 (17.681") to avoid a sharp point at the top. I've never been able to get acceptable results making cuts in plywood with a knife edge running against a fence. Maybe you can, but I'm going to suggest an alternative method.

The way I would make the cut is to build a carrier from a flat piece with one straight edge to bear on the fence and long enough and wide enough to stabilize the workpiece. I would clamp the work to the carrier at the proper taper angle (7 degrees?) relative to the straight edge. Double sided tape may be all that's needed, or you may need more elaborate holddowns. I would then set the blade tilt at 44.57 degrees (or as close to that as I could get) and run the carrier with work attached through the saw. I would then flip the work end for end keeping the same side down, re-clamp it to the carrier at the proper taper angle and cut the other edge of the workpiece. Repeat 3 more times and the task is complete except for using a guided circular saw with a blade tilt of 7 degrees to square the ends.

Quick, safe, and accurate.

[John Schreiber]

. . . I would then set the blade tilt at 44.8 degrees . . .
Tom, How did you come up with that 44.8°? I knew it would have to be a hair less than 45°, but I couldn't think of math to come up with a figure.

[Eduard Nemirovsky]

thank you,guys! I will try Tom's idea with a carrier board.
Will make a pictures and will post after I done.
Joe, thank you very much for a pictures - they explain much better then any words.

Ed.:)

[Tom Veatch]

Tom, How did you come up with that 44.8°? I knew it would have to be a hair less than 45°, but I couldn't think of math to come up with a figure.

Previously posted derivation of tilt angle was in error. Proper value is 44.57 degrees. I haven't found where the error originated, but I constructed the part in TurboCad and measured the angle on the miter cut at 44.57 degrees. Got to think about where my logic error came from.

[Tom Veatch]

OK, the derivation error came in because there are actually two tapers to take into account, I only accounted for one.

There is the 7 degree taper on each side measured in the plane of the side, but there is also the inward slope of the side itself. In this case the slope is arcsin(tan 7) = 7.053 degrees. I didn't take into account that slope. So the formula is actually:

tan(tilt) = cos(taper)*cos(arcsin(tan(taper)))
or:
tan(tilt) = sqrt(2 * (cos(taper))^2 -1)

Evaluating for this case:
tan(tilt) = SQRT( 2 * (cos 7)^2 -1)
tan(tilt) = sqrt( 2 * .99255^2 - 1)
tan(tilt) = sqrt(.970230) = .98504
tilt = arctan(.98504) = 44.57 degrees.

If anybody is still reading this, first, I wonder why. Second, sorry for all the trig. As an engineer, I happen to like math but realize that a lot of people don't.

It's often easier to draw a layout of the part(s) and simply measure the angles involved.

[Eduard Nemirovsky]

wow! this is a beautiful science.
Ed.

[Bruce Volden]

OK, the derivation error came in because there are actually two tapers to take into account, I only accounted for one.

There is the 7 degree taper on each side measured in the plane of the side, but there is also the inward slope of the side itself. In this case the slope is arcsin(tan 7) = 7.053 degrees. I didn't take into account that slope. So the formula is actually:

tan(tilt) = cos(taper)*cos(arcsin(tan(taper)))
or:
tan(tilt) = sqrt(2 * (cos(taper))^2 -1)

Evaluating for this case:
tan(tilt) = SQRT( 2 * (cos 7)^2 -1)
tan(tilt) = sqrt( 2 * .99255^2 - 1)
tan(tilt) = sqrt(.970230) = .98504
tilt = arctan(.98504) = 44.57 degrees.

If anybody is still reading this, first, I wonder why. Second, sorry for all the trig. As an engineer, I happen to like math but realize that a lot of people don't.

It's often easier to draw a layout of the part(s) and simply measure the angles involved.

???:confused:???

What him say???

Bruce

[Joe Spackle]

???:confused:???

What him say???

Bruce


It's often easier to draw a layout of the part(s) and simply measure the angles involved.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
that is the pertinent version :D

[Eduard Nemirovsky]

Thank you all for a support and advices, and after reading all info - I came to conclusion that simplest (unfortunately not cheapest:() will be using my Festool.
I rip all wood using circular saw and long rail. Cleaned all edges on jointer and on pictures you can see result. Column is not glued up, just two Domino each edge for registration.