How I Make REALLY Round Holes on the Shapeoko

Yes, I like to make weird (odd) stuff…it’s what a retired engineer does during a stay-at-home pandemic. So I’m on a path to make some tiny V8 Engines, but I thought I’d start with a simple Briggs and Stratton 3.5hp Go-Cart engine. Ok, it’s proving to be not so simple. Tiny things are HARD to make…

Now onto this problem. The block’s cylinder bore needs to have a REALLY Round hole. Let’s first take a look at what a (my) standard Shapeoko (stock, set at 40 steps/mm) made on the first attempt. Well, it looks nice…

Let’s take a closer look…

In the Y+/- Direction it measures 0.555"

But in the X+/- direction it measures (0.550") or 0.005" smaller (0.0025" per side)

Note: the 0.550 Dim, is in the X+/- direction (when in the Shapeoko)

This actually surprised me because I normally see bores within 0.002" My machine isn’t out of adjustment, I just picked a bad (perhaps the worse) spot on my table/machine.

Meaning I have mapped my entire 16x16 table, and I have areas that cut larger, smaller, and some areas that are spot-on. It’s typical for timing belts, and since my machine has large AND small cutting areas, a belt calibration will NOT fix all the errors (understand?) But the fix to cut a really round hole is the same.

I open Carbide Create and draw an ELLIPSE (circle feature) that is 0.005" LARGER in the X-Axis, to force the Shapeoko to actually cut a round hole. Sounds odd? Perhaps, but it works. See how the Width is 0.555 (X-Axis) and the Height is still 0.550" (Y-Axis)? But THIS geometry will cut a 0.550" Round hole on MY Shapeoko, at the present X0 Y0 Location.

NOTE: Sometimes the machine will need an extra 0.001 to “get it there” so some trial and error (or cut, measure, adjust, repeat) helps.

I then measure the bore and opened it up to 0.555" (Final dimension) using the same principle. (You can also do this in Fusion and use the Leave Stock feature rather than needing to draw additional ellipses in CC, same results)

Finished bore:

I have been using this method for 5+ years on my standard Shapeoko, and never really fully explained it step by step here on the forum, and thought I’d share my method today.

Do you have a different method/tool for creating tight tolerance parts?

I hope you learned something today, and if you have any questions, feel free to ask…I’m here to help.

NOTE: While I may have a 4th-axis setup on my Shapeoko, I am still running Carbide Motion to drive my Shapeoko, I have a separate controller to operate the Rotary (4th-axis) so that I can machine 4-5 sides of my part without needing a different setup.

NOTE 2: You can use the proven method to cut accurately ANY type of feature, ID, OD, a box, letters (inlays) etc…


This reminds me of a very old joke (that’s long, so I’ll do the abridged version).

A man goes to a tailor and buys a suit. He tells the tailor that he thinks the sleeve is too long, the pant leg is too short and jacket is too small. The tailor disagrees and tells the man to reach down, pull up his pant leg, stretch out his arm and hunch over a bit. Then he tells him that the suit fits “perfectly!”

The man hobbles out of the door and starts down the street, hunched over, grabbing his leg and hunching his back.

2 other guys are walking down the street and see him. One says, “Oh…look at that poor man, he’s so deformed!” to which the other guy responds, “Yeah, but look how nice his suit fits!”


I’m just impressed that you were able to get your measuring tools to that degree of accuracy.

For me, I’m struggling to get repeatable accurate readings from my $20 Home Depot digital calipers. Was wondering if I spent more would I get a better results, or is this just a case of replacing the operator (a.k.a me)

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Measuring is a skill in its own right; and knowing which measuring tool to use and when is part of that. Quality tools tend to be more robust and hold their tolerances better, but that doesn’t mean a cheap tool can’t get the job done.

What are you measuring and how?

By the way, great work Richard!


I started the machinist life (11/76) as a Tool and Die Maker…so precision and accuracy is a way of life…and the fact that I own some very good measuring tool (and a calibrated set of Gauge (Joe) Blocks to check them my tools), helps.


I measure the bore with telescopic bore gauges, and a 0.0001 micrometer. I also check the micrometer AT the desired dimension with a set of Joe Blocks.


My junior/senior year of high school I worked nights at a machine shop (who needs sleep?) and one of the first thing they teach you over and over and over and over again is how to accurately and repeatably measure things. When the parts you are making a hundred of have to be within .0003 of spec you get rather good at it. :slight_smile:

I suppose the spindle doesn’t have enough mass or can spin slow enough to run a boring bar?

I’m more impressed you were able to find the center. You have skills my friend!


Rich, Most cylinder bores are honed before installing a piston to help with oil control and so on. Since you are making a miniature engine do you have a hone for this small bore engine or is this just for demonstration purposes and will not be a running engine when you are done.


Just wood thickness, so I don’t have to be perfect, but it frustrates me that I can get different results when I complete the same measure in a row.

Brandon, The measuring instrument and its precision is maybe what is making you get different measurements. Circles are one of the worst to get accurate because you cannot tell exactly where the two center points on opposite sides of a hole are. Machinists find the exact center of a hole and then measure across. With just a caliper you can never find the center exactly. So like horse shoes and hand grenades close enough is good enough.

Most even cheap calipers are quire accurate but repeatability is usualy the issue with the way you measure and hold your instrument. Dont get wrapped around the axxle with a measurement that is 0.001" different on different measurements attempts. I have a Wixley digital caliper and when you turn it on it zeros so I press on the caliper to make sure it is at zero. as soon as I let loose of the caliper it ready 0.001 and if I press it together it goes to zero.

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Like this?


I don’t own “true” hone. Here is what I am using. A piece of 3/8" round bar, with some Scotch-bright wrapped around the bar, and a piece of 220 grit sandpaper glued to it. There won’t be any oil in the crankcase, so it’s more for the LOOK…but it works awesome.


Very nice. When you finish it will it be a display model for on top of your Rhode Island Desk?

You make very interesting things.

Thanks…and no, that heirloom desk is reserved for royalty item only…haha. No plans actually…I’m making a few of them mainly as a learning curve to figure out how to make a tiny V8. Maybe I’ll throw a couple on my Etsy (if there are any that make it) for some crazy amount. or…I’ve donated a few things to Carbide3D in the past…so this is an option too. IDK.


To be fair, aluminum doent change its dimensions too much if it rains outside. :wink:

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