cutting triangle shaped frame

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Maureen Nolan

Grumbler in Training
Joined
May 15, 2003
Posts
8
Loc
Keego Harbor, Michigan
Hi Guys,

Hope someone can help.

We spent most of yesturday trying to figure out how to cut the frame for a pennante shaped mat. The angles are just not coming out right, and I know it's something simple that we are missing.

We called the PPFA and Larson Juhl help lines, both gave us pages out of Barton's Book of angles. The problem is none of the examples in the book were the shape we needed, and it didn't really tell us how to figure out our own cuts.

The mat is already cut and is penante shape....long skinny triangle. The mat's short side is 15" and the 2 other sides are 31". Now I don't know how to figure out what angles to cut the frame so it will fit around the mat.

Any help/insight would be greatly appreciated!

Thanks in advance!

Maureen Nolan
Orchard Lake Framing & Gallery
Keego Harbor, MI
 
I'm sure a mathematician will jump in with some numbers - But here is what I would do. If the mat is cut, put a protractor on the corners and divide the measured angles by two. You could use a kraft paper template to make sure you count the angles on your miter saw from the right direction.

Pat :D
 
I've done a lot of triangles, lacking anything more than a ninth grade education, my aproach has always been as Pat suggested. I sneak up on it. Get some junk wood and make your test cuts untill it's working right. Your probably going to have to shim your fence as well with scrap wood to get some of the angles. It's not as tough as it seem, just time consuming.

Now after all that, some clown who finished the tenth grade is going to spit out some super simple formula for cutting these darn things.

John
 
I could figure out the math but I do as John says.
My problem with pennant frames is how do you make the cut on the point. My saws don't cut that big. Plus the wood gets very thin.
 
A tool that comes in handy in setting the saw is a sliding T bevel. After spliting the angle on the back of the mat with compass and straight edge, you set the T bevel on the desired angle and use it to transfer the angle to the saw.
I did a pennant last week and decided to make a jig for my tablesaw to aid in cutting the angles. It is basically a hinged fence that attaches to my crosscut fence and has a sliding dowel and wedge to secure the jig at any angle.
I'm sure there is something similar that could be used on a power miter box.
I am also much more comfortable using direct measurement and transfer than calculations and dialing in a saw's scale. The numbers don't lie, but with the accuracy of some of these tools, the numbers won't always help.
 
Measuring off a scale drawing, it looks as if
the 2 less pointed angles would be cut at 142
degrees and the most pointed angle would be at
166 degrees, but making a mock up as the others
suggested is much the best way to ensure that
you have it right before the real molding gets
cut.

Hugh
 
See the HH for the math. :D
 
Triangle.gif
 
Except that the angles are actually 26 for the small one and 77 for the large ones.
 
Well, I DID take tenth-grade math (and went on to flunk college calculus) and I'd still use the protractor to measure the mat angles.

The only thing I remember for sure is the angles have to total 180 degrees. In that sense, Bill and Framah are both right. You could use the trig functions to figure it out theoretically, but I'd use the protractor.

Really, what I'd do is make the frame first, then cut the mat to fit. I guess it's too late for that.
 
Except that the angles are actually 26 for the small one and 77 for the large ones.
Click to expand...
Sorry, but if you drop the perpendicular down from the apex, you end up with sin(theta) = (15/2)/31 = 14.0008º - that's the angle of the mitre. The total angle of the apex is, therefore, 28.0016º.

Each of the bottom angles would have to be 180 minus 28 divided by 2 = 76º. The mitres for those = 76/2 = 38º (37.9995º).
 
Do I have to take into account the width of the moulding so I can get that long skinny point?

(....I thought I paid attention in math


Maureen Nolan
 
Do I have to take into account the width of the moulding so I can get that long skinny point?
Click to expand...
<center><font face=madrone size=4>Oh, gosh, I hope not!</font></center>

Seriously, its near the end of the day and my mind is turning to puddin', but I don't think so. It wouldn't be the width of the moulding but the rabbet width that you might have to consider. But if you maintain your usual 1/8" allowance when you cut your mitres, you should be okay.
 
.... if you drop the perpendicular down from the apex, you end up with sin(theta) = (15/2)/31 = 14.0008º - that's the angle of the mitre. The total angle of the apex is, therefore, 28.0016º.

Each of the bottom angles would have to be 180 minus 28 divided by 2 = 76º. The mitres for those = 76/2 = 38º (37.9995º).
Click to expand...
The next time a customer complains about the high price of framing, show this to them, and tell them this is what you go through for their framing.

Who said framing isn't rocket science?


-Mike.
 
Thanks to all for the advise/math lessons...but apparently generally accepted math formulas do not work here in Michigan...or least on our street! Maybe the moon and stars were just aligned wrong, but Ken worked on this frame from 7 am to 7 pm last Saturday! UGH! Talk about not charging enough for a job! Anyway our angles came out to be something like 32 and 57? What's up with that?

We were going to attempt to do a "home plate" shaped frame for this same customer, but after spending so much time on the triangle one, he's getting a square one!

To those of you out there that do odd shaped frames on a "regular" basis, what kind of saw do you use? Would it make any difference? Ken used a compound mitre and a table saw.

Maureen Nolan
Orchard Lake Framing & Gallery
Michigan
 
Sorry, Maureen, I forgot you were in the Central time zone where the laws of Euclidian geometry don't apply. :D

Seriously, cutting mitres like that are tough since the angle markings on a mitre saw are pretty tiny, but if all of your corners finally came together the sum of all of the angles would have to be 180º.

It would be possible for the angles lower two corners to be, say, 45º and 31º if they added up to 76º as the equation requires.
 
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