Making a Tool to Measure Angles





Introduction: Making a Tool to Measure Angles

About: I am insane. I make things with my fingers. I derive quiet satisfaction from driving a good road. I love studying until I get a sleep-deprived rush. I love philosophy. I treasure my mind as my most powerful ...

 Yes, you're thinking: "doesn't he mean a protractor" I do a lot of welding and often I need to check if my cutoff machine has cut the angle I needed, usually 90 or 45 but sometimes random angles like 38 or whatever.

The standard protractor you get in mathematical instrument sets doesn't cut it for me, it's designed to measure angles on paper, not a 3D object. So to the drawing board I went and an hour later I had a lovely design (I ALWAYS make a technical drawing of my projects before I start building them) that required only five parts.

SoOoOoO, the briefing is out of the way, let's start:

1) two sheets of alumin-i-um (not the American spelling). Mine are 3mm and 4mm thick.
2) a bolt with matching nut to hold it all together. Mine is brass because it looks cool.
3) a spring washer to keep all moving parts happy. A normal washer would also work.

The most precise tools you've got, all I had at hand was:
1) angle grinder with 1mm cutting disk and thick grinding disk.
2) bench grinder.
3) sanding machine with belt and disk.
3) wood chisels to make notches every 5 degrees.
4) hammer.
5) drill press and assortment of drills.
6) spade drill-bits to make recesses for the washer.
7) steel ruler.
8) vernier calliper.
9) bench vice.
10) standard size steel file for shaping.
11) set of mini files for precision shaping.
12) scroll saw.
13) a couple of G-clamps.
14) a normal protractor.

I think that's about it . . . CONTINUE 

Step 1: Step 1

First cut your aluminium plates to the approximate shape, leave a little excess material on the bottom straight edge of the protractor-plate and do the same on the edge of the guide-plate which has the "bubble". This was all done with the angle grinder and scroll saw.

Now shape the round side of the protractor-plate and the "bubble" of the guide-plate. Also remove the excess you left on the measuring edges BUT MAKE SURE THEY'RE 100% STRAIGHT, use a steel ruler and vernier caliper for this. All shaping was done with the sanding machine and standard file.

Please note that the edges on both sides of the "bubble" must line up with each other as closely as you can manage, the center of the hole for the bolt must also be on this line.

Also mark the protractor in whatever sized increments you wish to use, I opted for 5 degrees and used the wood chisels (naughty naughty) and hammer. Obviously you need to use a normal protractor to get these exact.

Also drill the holes for your bolt, This will naturally go through both plates and must be just larger in diameter than the bolt you're using. In my case: bolt = 4.8mm and hole in guide plate = a teeny bit smaller than 5mm. The hole in my protractor-plate is 4.5mm.

Step 2: Step 2

 ok, now you should have two plates which resemble the ones in the image below.

Next is making the recesses in them to allow for the washer. I used wood spade drill bits (naughty again) because aluminium is soft enough that it does little damage to these cheap bits. The reason I used spade bits is because they have a tapered point which conveniently centers the bit, and also because they have a flat cutting face as opposed to normal drill bits.

Remove the head of your bolt and shape that end so that you can press-fit it into the hole on the protractor-plate. Then cut a groove in the same end of the bolt almost as if you're making dumdum bullets. This will help you to keep the bolt secured in the hole later on.

Step 3: Step 3

OK, next you need to atatch the bolt to the protractor-plate, I just filed mine away until he was a very tight press fit, I then used my bench vice to force the bolt in and it's very solid. Once I had forced it in far enough I used a flat screw driver and hammer to pry the bolt apart where I had cut the groove into it.

Also cut a deep groove in the bottom of your protractor-plate so that when you pry the bolt open it has something to grip onto. I used the angle grinder for this.


 ok, now assemble and check it out!!!

Pretty neat huh?

compare it to a normal protractor, if it's not precise enough I apologise. You'll have to start over.

Fortunately mine is only about half a degree out so I'm not complaining.

Side note: I used my sanding machine's belt sander to give the faces of both plates a nice brushed look, sort of visible in the picture.



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    Please be positive and constructive.




    What you actually made is a goniometer. We uses all the time in patient care to measure range of motion of the joints in the body. You can buy is in various sizes and in plastic or metal.

    looks realy usful i am going to make one, could be usful in wood work as well!!!!!

    I remember having a scale in middle school which was just the same. The arm's of the device acted as a scale when the angle was 180* (straight line) and otherwise it could be use to measure angles.

    Ok, "compass" has been changed to "protractor" - sorry for the confusion.

    Lemonie: yes, I will put up a picture of the tool being used.

    KelseyMH: thank you, I also admire people who spend weeks handcrafting a beautifull piece of equipment. I have an uncle who built a single cylinder fourstroke engine and fabricated EVERY SINGLE part himself apart from the bolts. forty years later it still runs. By the way I'm from South Africa, not the UK : ) The technical drawing will come soon, I just gotta find time (kid, college, work, etc).

    Caitlinsdad: I intend to build another one with higher precision. The idea is: use prototype for a week or two then decide what I want to add/improve. Your comment about measuring inside angles got me thinking, I will see if there's anyway to adapt the design to allow for that. Thanks.

    Hi guys!

    Thanks for all the possitive commentary : )

    Sorry about the compass-protractor thing, I often make thatmistake. I will edit and correct all that in a minute.

    Also, my internet connection at home is down until the end of the month,seems some hacker has stolen my account information so I can only usethe computers at college. that is why it took so long to get reply to y'all.

    Peace, love, empathy

    Nah, I was thinking more like "doesn't he mean a protractor".

    I have a pair of "dividers", used to measure distances onnautical charts.  Like a "compass" but with two points.

    What makes this tool unique is that only one hand is needed toopen/close the pair.

    This is accomplished by grasping the recurved upper portion to takethepoints further apart, or using  fingers to close them.  Seethe pic at: 
    (on the right).

    You could probably use these for taking off measurements on 3-D thingsas well.

    1 reply

    Stupid spell checker didn't recognize the application was slamming wordstogether.  Bad Spell Checker!  No dessert for you tonight.

    Awesome, usually its the DIY tools thats keep being reached for oncethere dialed in.

    One dumb question.  In the U.S., we call this device a"protractor."  A "compass" has two arms withpoints at the end (or a point and a pencil), and is used for drawingcircles.  Is the terminology different in the U.K.?

    2 replies

     I was going to ask the same thing. Language differences are sofascinating even within the same language! 

    According to Wikipedia this tool is called a Bevel Protractor. good idea to make one - I didn't even know they existed but could bevery handy for all sorts of things :)

    I think US and British terminology is the same - A Protractor is used for measuring angles, a compass is the navigational device, and a pair of compasses  is the instrument for drawing a circle (though it's normally justcalled a compass).

    Hey, cool build.  I do a lot of sculpting and I hadn't even THOUGHTto make a tool like that, I'm foolish.  I will definitely make one soon.

    I'm having a tough time trying to figure out how you use this for anyinside angles or for checking the blade on your cutoff saw. Forwoodworking, I normally use a bevel gauge to just match up the angles Ineed and push it right up to the blade to adjust the blade.  Theydo have some gauges or angle finders with angle markings but all pivotabout a point and allows you to put the vertex right into yourworkpiece.  That "arm or piece" as the dial indicatorwould not allow you to measure anything above the horizontal withoutturning your gauge upside down and then maybe needing to think abouttranslating the angle.  It would be helpful to show how you useyour gauge.  Thanks for sharing.


    What you need is a "Line of Chords". Find it here;
    enlarge the picture and fin the two center dots next to the 160 and the440. Open a pair of compasses to whatever angle you want along the topof the rule. Then open the rule (it's joined at one end) until thepoints of the compasses fit in those center dots and voila, you have theEXACT angle you were looking for. Conversely, fit the rule into whateverangle on a wall or whatever, open your compasses to fit the center dotsand then measure the angle from the top side of the rule. Sounds morecomplicated than it is. I've had one for years. Awesome tool if you needto work with angles.

    This is a very nice tool, although as kelseymh has already said it is aprotractor not a compass. Is there some confusion with a navigational compass?

    Also, a lot of the pics are very fuzzy. It's possible to figure outwhat's going on, but sharper close ups would help a lot.

    Making your own tools is a fine and honorable tradition in wood andmetalworking.  Thank you for posting this!

    Any chance we could get the PDF of your technical drawing? You can upload any arbitrary file (doesn't have to be an"image") and add it to the I'ble.

    Nice tool, can you add a picture showing how you use it?