# engineering a protractor



## lizardhead (Aug 15, 2010)

I do segmented turning, so I use angles, problem is with a small protractor the margin for error is very minuet, respectfully a very large protractor would have a larger margin for error. I want to take a 24" x 24" piece of MDF and make a large protractor to cut some angled jigs for my miter slider on my band-saw. My thinking is that if 45 degrees is 1/2 of 90 then the angle would be ticked out on my MDF at from lower left to upper right, OK that's a no brainer, now if using that 24" square and 45 degree figures I would divide 24" into 45. = 9/16" Take that answer and multiply by the number of degrees I want 15 and I get 8", so if I rise 8" from the lower right the angle from that point to the lower left should be 15 degrees. Close but not right. What am I wrong about. And if you plan on giving me vectors and X's & Y's & C2R & qfb's don't bother I do not care to go to school. If that's the only way to get there then there is no bother to explain other that "THERE IS NO OTHER WAY" They have a website angles for Dummies, but I am not as smart as a Dummy.


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## Gene01 (Jan 5, 2009)

Would this help?
Miterset


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## mtenterprises (Jan 10, 2011)

You need one of these - Dial Protractor ±5 Arc Minutes Accuracy. These are cool, a bit pricey. You can get one at Grizzly model G9900 - $53.95

MIKE


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## mtenterprises (Jan 10, 2011)

Gene that's a pretty good idea too, I like it.
MIKE


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## Visions (Sep 12, 2011)

Hey, hey, hey! I like that rig there Mike!

Lizardhead,

I too do segmented turning. Unless money is an issue, just get an Incra miter gauge and never worry again.

I have an Incra V120 that I got on sale for $50, and it's awesome! After calibration, it's dead on. I've made 36 segment rings with no gap, just dead on accurate. It has 120 angle stops, so you can hit almost any angle needed.

My next project for it is going to be a miter sled like the Incra Miter Express, or if I deem it easier to by than build, I may just buy one. They're not cheap, but man do they work good!

However, if you have the money, an Incra Miter 1000HD is just an incredible piece of equipment. Not cheap, but it will end all your angle woes for good.
Even the Miter 1000 or 1000SE are incredibly accurate.

But, if money is an issue, which I completely understand (and feel in my pocket!), I will find out from my dad how to do the math you're looking for tonight.
My father is a machinist and a math genius, especially with geometry. I used to know all of it too, but having seldom used it in the 12 years since school, I can't recall now.


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## TopamaxSurvivor (May 2, 2008)

Did you take trig in school? Those function will give you precise figures to use.


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## Visions (Sep 12, 2011)

Ok, I thought about your issue for a minute. Your problem is that you are measuring in a straight line for 8", not in an arc as would be proper.

Think of a protractor, they arc half circles. You will need to measure 8" of travel a the radius appropriate to the protractor you are making.

So, find the radius of the arc you want to make, make yourself a trammel that same length, and when you figure the measurement, move the trammel point that far. It should work perfectly. But remember, you need to measure the distance of the arc, not in a straight line.


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## Visions (Sep 12, 2011)

Or, find the circumference of the circle you want to make, and use that for base number to make your calculations for the angles.

IE: The circumference of a circle 24" in diameter is 75.39". Divide that by 360 and you get .209. Multiply that by 15 and you get 3.14"

Go to a 48" diameter circle and you get 150.79" circumference, divide by 360 is .418, now multiply by 15 and you get 6.28

Just remember, you can't simply go in a straight line, not without converting the measurement in some way.

I'm not 100% positive, but I am pretty sure this will work.


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## TopamaxSurvivor (May 2, 2008)

If there are an even number of segments, it would be easier to bisect the angles to divide them than to make precise measurements on the circumference.


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## JJohnston (May 22, 2009)

The very act of asking the question indicates you do want to go to school.

Kenny's right - the distance between the "ticks" as you call them is not constant if you are measuring them along the vertical edge of the square - they get farther apart as you go up. Now, there is a formula for figuring out how far up the left side to measure for any particular angle, and it only involves looking up one value in a table (readily available on the internet) and one multiplication (the value from the table times the 24" width of the square). If you're game, I'll talk you through it. I even found an online calculator for the number, so you aren't limited to whole degrees for your angle.


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## lizardhead (Aug 15, 2010)

Well it's a done deal I got me an Incra 1000HD I'll let you know tomorrow how it works.


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## Jim Jakosh (Nov 24, 2009)

Hi Dave,
That Miterset that Gene showed is a pretty neat tool! It has all the angles calculated if tor the number of segments being cut. I like it but personally I would rig up a sine bar attachment like I put in my table saw jig- it cuts the angles right on!..............Jim


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## lizardhead (Aug 15, 2010)

I picked up 7 sheets of "Dupont Corian" 24" to 30" square each for $50 bucks from a local installation company. I set that new 1000hd up and cut me 4 Corian Jigs each one at a different angle 15-18-22.5 &30 degree. Then I had a neighbor make me 4 mitre slider bars with two 1/4" tapped holes. Maybe it would be easier to just take a pic and enter them as a project. Not made of wood so I might not hear the end of it-(for posting here instead of a blog) Wait I can do it this way--


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