Introduction: Grinder Circle Compass/Jig

Give a man a fish feed him for the day teach a man to fish feed him for life. Man that's a very circular statement! It's doubling back everywhere on itself man and fish and time.... yawn. Now scrap trash stuff and feeding ones self that might be something I know a thing or to about being a slight trash myself; just lemme help with that circle right quick too.

Intro and personal opinions aside, might be able to help with the circle seriously. I do quite a lot of remodel projects and have stuff of things laying around. Circles are a pretty common shape and are used in alot of places. Yes they sell hole saws. They even sell variable size hole saws. Skip this bid go buy one be happier - thank you your welcome.


Caution- grinders are amazing tools, occasionally they cause serious injury! Type it into Google - I'll wait. It's nasty stuff. Don't use a grinder without gloves long sleeve or eye protection. You could get seriously hurt. This is an improvised tool, it is safe but you need to be familiar with a grinder before you go mounting it in a jig and conquering circles everywhere.

Did someone just say Hobbit door? Another article maybe.


plywood 3/8 or thicker (used a 16x24 inch piece {metric system!})

A CNC machine

8 bolts and nuts to match (if you used 3/8 ply then 1 3/4 is about perfect) - found amoungst the things!

One bolt equal to or slightly greater than the radius of your grinder disk.

If you want to cheat the scraptacular goodness of this hackadelic project go buy a suction cup and put it on previously mentioned bolt.


I personally am using my grinder jiggy goodness on tile. We've all seen some really amazing circle and arc patterns and I wanted to be able to copy them. That said most tile isn't more than 24 inches- I made my jig at 18 inches.

Locate some 3/8 ply wood mine had mastic glue all over it... pretty much trash. I ripped a section of ~24x16

Found the old junk bolts in a tool box- everyone's got one of those right? I used the grinder to make my sizes work afterwards.

Step 2: Draw Stuff on the Computer

I might have seen similar stuff, maybe even have looked for similar things accomplishing this task! They make lots of circle jigs for carpentry. I didn't see this exact design just similar concepts.

Anyway, I sat down at Blender... I mean AutoDesk Fussion... I mean Easel.... OK Vetric VcarvePro. I used Vcarve Pro *sobs heavily* it has good centering features, I have spent time with all of the aforementioned programs they are all excellent and any one will work.

I'd really like to give you the file I made but compatibility is a thing. (if anyone know how to do that I'd gladly hand it off.)

Given that this is going in a CNC machine and you'll need to generate your own g-code path a simple look at the file should suffice.

Things to keep in mind when you draw your file

Where are you going to clamp your grinder and what is that shape like? I just did a rough measurement guess of about 2.5 circle and cut it to fit with an oscillating saw.

How wide are the bolts your using/found? I had a set that was just around 1/4 thread that's how big I cut all my holes.

Step 3: Throw It in the Robot

CNC machines seem tricky. They aren't really all that bad though. It'll like driving, there are things you do before starting the car and reaching your destination. There is initially a steep learning curve and the more you know the more you can do. This isn't a 3D art piece and it doesn't take months of learning to do; 3D art does take months. This is a very beginner level file, square square circle circle rectangle in a rectangle line line done! No bonus for copy and paste- but it does help *HINT*. You can do it, I believe in you.

About those things before the destination- with any CNC project make sure you have tabs holding cutout pieces so they don't turn into projectiles, a decent hold down strategy and appropriate spindle and feed speed. Mind your home/zero position and bed depth. Make sure your path strategy stays clear of all your clamps or screws! I can't tell you exactly how cause I'm not at your machine- sorry.

I have a decent machine and a low quality bit I ran at 100 inches per min 20k rpm- fast and dirty.

Could I do all of this with conventional tools yes but, not while watching Star Trek and drinking coffee. Would I? Probably not.

Step 4: Assemble the Thing

I started with the grinder side of the compass first made sure my clamping surfaces was a good fit. Made adjustments as needed with an oscillating saw, chisel, pocket knife. Once the grinder fit snug in the holding strategy I mounted the brackets in the rail slide system and trimmed the bolts to a more appropriate length.

Then I sandwiched the pivot side of the jig on the rail system with the long bolt and its pretty much done.

As stated in part one a suction cup on the end of the longer pivot bolt would be really useful. I'm just using bolts I found- opening your wallet or finding some wing nuts would be really cool.

Step 5: Use and Test the Instrument

There are many areas that could cause slack variables but you can get a very consistent product with this compass jig. Keep in mind while using it that the grinders rotation works only one direction spin the jig with the rotation. A loose mount and shifting angle of the grinder while in action will make an inconsistent product.

As with any grinder or router curving I strongly recommend testing it on scrap material before you attempt it on a final products goods. If you're not happy with what your getting throw it away and don't use it. It was scrap after all right? Don't waste or compromise expensive material unnecessarily for any build- least of all on this weird hat trick.

Thanks for reading. I hope this works and you found it entertaining to read. Love the votes. Thanks again happy circles!

Here's what I made mine for. Sorry no in progress videos don't have that many hands and I'm on a budget.

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