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04-29-2010, 07:30 PM
I can't seem to figure out these cuts. i am making a front bumper for my atv and am kind of going off of the original design. i am using 1.5X1 square tubing. The attachment is a quick drawing of the two main tubes that attach to the bottom of the frame. If you were to look at the bumper from the side you would see that it has about a 40 degree angle, then if you look at the same piece from the front view it has about a 25 degree angle.

I set up my chop saw at 20 degrees and cut the first piece for the 40 degree angle. Then i kept the angle on the chop saw at 20 degrees but then added a 12.5 degree angle on that and cut it. The result was two angles that didn't fit. What am i doing wrong?

steve45
04-29-2010, 09:01 PM
I cut a bunch of compound angles on a project I did recently. Even with a compound miter saw, I still confused myself, and the angles didn't always work out as planned. I found the best way to do it was to cut wood pieces first and get them to fit correctly first.

Bluesman
04-29-2010, 11:24 PM
My brain hurts just trying to think about it.

Matt_Maguire
04-30-2010, 08:31 AM

The tangent for a compound angle is equal to the tangent of either given angle times the cosine of the other given angle (tanC = tanA x cosB). This will give you a correction to the primary angle if that is all that is needed by subtracting the product from the original angle, changing sign and adding back to the original angle for a new angle setting.

Confused? Try this for a simpler solution with a miter saw and 2 angle correction.

If you are using a compound miter saw use the following.

Miter setting = arctan [tan(40) x cos(25)] = 37degrees 15min or 37 1/4 degrees

Bevel setting = arcsin [sin(40) x sin(25)] = 15degrees 45min or 15 3/4 degrees

The bevel doesn't look quite right but I think the calc is the right one. If the 25 degree angle is too shallow try a piece of scrap using the miter calc with the angles switched in the equation for the bevel setting.

The mind is the last thing to go... first my hair, then my eyes... now the mind!

Anyway post back as I may have the +/- to the original angles wrong.

Matt

04-30-2010, 11:36 AM
Thanks Matt! I will work on this project here in the next hour or so and post up some pics!

04-30-2010, 01:10 PM
Well it looks like my original degree calculations were off. It looks like the angles are 48 and 30. I don't have a graphing calculator, so i am trying to figure out how to do your calculations without one.

Matt_Maguire
04-30-2010, 01:16 PM
After further review I checked my Machinery's Handbook at the shop....

I worked the solution in a fashion I understand better (like I'm positioning a plate to put a hole in it at an angle with an second angle of rotation)

The solution came out closer to what I imagined when I first posted up in my bathrobe...

This time I came up with miter (first angle) of 43.8 degrees and bevel (angle of rotation) of 29 degrees.

This could be wrong with either angle or both being corrected the other direction from original, like 36 instead of 44 and or 21 instead of 29...

I'm gonna take the slide miter out and see which is which. A good carpenter with a framing square would probably have this cut, nailed up and most of a six pack down.

Attached vision of second solution below

Matt_Maguire
04-30-2010, 01:22 PM
Well it looks like my original degree calculations were off. It looks like the angles are 48 and 30. I don't have a graphing calculator, so i am trying to figure out how to do your calculations without one.
You posted before I posted, gotta go back to the shop I'll check with miter saw...

You are trying to do this with one cut aren't you?

Matt

RancherBill
04-30-2010, 02:01 PM
It's pretty simple look at the attached drawing

I did your 40 degree as an example.

A straight line is a 180 degree arc. You want something that is 30 degrees less. Thus it is a 140 degree Joint. The two pieces that make a 140 degree joint are 70 degrees.

IF you are cutting your metal at 90 degrees to normal they are 20 degrees (90-70).

The arc and tan are valid, but not necessary in this type of work.

04-30-2010, 02:50 PM
I figured out what i was doing wrong. I was making identical cuts on both pieces of square tubing, but what i needed to do was turn one 180 degrees and then make the same cut and it fit fine. Thanks for your help!

Matt_Maguire
04-30-2010, 06:01 PM
I figured out what i was doing wrong. I was making identical cuts on both pieces of square tubing, but what i needed to do was turn one 180 degrees and then make the same cut and it fit fine. Thanks for your help!

I was worried I was puffing bad voodoo up your poo-poo, cause I didn't understand the problem (that happens a lot with me).

Matt

William McCormick Jr
05-01-2010, 12:42 PM
That is a trick angle. Just set your two angles to half the angle you want. It comes out so close I cannot tell the difference. I got 39.5 plus degrees and 25 degrees. From 20 and 12.5 degree saw settings.

At that angle it comes out just right. It does not need an adjustment.

Obviously if you turn the 40 degree angle into a 90 degree angle you are going to get, about a 12.5 degree angle out of a 12.5 degree cut. And your ninety won't be a true ninety. Because you are only using one half the angle. It is pretty wild stuff though.

I do pipe bends that go down, and to the left or right. And it can get you laughing hysterically sometimes. It is rotation and changing the angle of the single bend to do that.

I tried it with other adjusted settings, and it did not work. Way over 40 and 25 degrees. I got almost 50 and 40 from the adjusted angles.

Sincerely,

William McCormick

tinner
05-02-2010, 12:07 AM
COS of angle A multiplied by the COS of angle B, this will give you the COS of the compound angle.

Boostinjdm
05-02-2010, 12:18 AM
Square tube is your problem. If you switched to round, you would no longer have a compound angle. That's just my .02 though.

Knotbored
05-12-2010, 09:26 AM
This is probably too late to help
http://www.csgnetwork.com/sawmitercalc.html
but this online miter saw calculator sure saves a lot of fuss cutting compound angles.