Fail Of The Week: The Follies Of A Bootstrapped CNC Mill


Anyone who’s built their own CNC machine from scratch will tell you that it’s no walk in the park. Heck, even commercially available (but hobby priced) 3D printers are no picnic to get running reliably. This offering is the tale of how [Brian Amos] failed at building a CNC mill over and over again. But hey, that ‘over and over again’ part is what makes great hackers. He not only documented what didn’t work, but shows the hacks that he tried using to work through each scrape.

We think the most interesting bits are in his second post, but start with the first one (it’s a quick read) to get the background on the project. The real issues start with a common one: a bed that is severely unlevel compared to the cutting head’s axes. The solution is to use a sacrificial bed, milling it out to match the surface to the tool. This exposed the next issue which is a misaligned Z axis. Some give in the entire support structure means problems with slop and backlash. And there’s even a very creative spiral-cut coupler to help account for alignment issues between the lead screws and motors.

The nice thing about building a mill is that you can turn around a use it to mill more accurate replacement parts. Just keep telling yourself that as you toil away at a project that just won’t seem to work!

We’re already looking for next week’s fail post topic. Help keep the fun rolling by writing about your past failures and sending us a link to the story.

34 thoughts on “Fail Of The Week: The Follies Of A Bootstrapped CNC Mill

      1. Unfortunately this project started with MDF. You are never going to get a stable machine if your foundation is unsteady. Perhaps look into stiff, humidity and expansion resistant materials, such as metals to build the base with.

          1. Probably not. Test it with proper equipment (micrometer, calipers, depth gauge, etc) and you will see how much the wood expands and contracts over the months.

            Should you not need any greater precision than several millimeters, then it will be as good as a ‘rock’. If you make things like small gearing, snap fit parts, telescoping features, or anything else that requires more than skilled hand-tool quality, any absorbent material in your frame will just not work.

        1. The ‘problems’ MDF is claimed to have are much exaggerated. Yes it’ll absorb water, but so will a sponge. Point?

          That said, those threaded inserts were never going to work.

    1. yeah, part of the problem here was materials and tools. I was trying to leverage one of the Mantis designs as a base (featured here several times), but i decided to use MDF instead of ply wood. That, coupled with the limitations of only using a jigsaw and some poor design decisions on my part lead to a pretty rocky road.

      The mill is working now, although not nearly as accurately as I had (naively) hoped for. It’s been great for engraving other small projects, but still can’t quite manage milling out a PCB with SMD components.

      1. This philosophy is so Spot On. :-) I love that show. This world desperately needs more creativity and being creative means trying stuff, some of which doesn’t always work out as expected. Among the unexpected, you sometimes get something WONDERFULL!!

  1. Awesome post, but I’m not so sure I’d call it a failure :)

    There were some pitfalls to learn from along the way, but this seems like an already successful project on its way to even more success.

  2. “The nice thing about building a mill is that you can turn around a use it to mill more accurate replacement parts. Just keep telling yourself that as you toil away at a project that just won’t seem to work!”

    Had to laugh at this, because that is EXACTLY what I was thinking the entire time I was building my router.(which, many hours of work later, is functional but not quite finished).

    While I agree that MDF isn’t exactly the ideal material for this, you don’t exactly have a choice if you’re trying to work on a budget. Not to mention even relatively soft aluminum isn’t nearly as easy to work with.

    1. Commercial cutting boards used in restaurants can be had in 3/4 inch thickness. They are relatively not more expensive than MDF, and are immune to moisture. However when people say mdf is unstable. Your right. However so is every material we use to build anything. Meaning temperature variations will cause issues as well. The most thermally stable material would be cast iron, at least for the cost. However I don’t see anyone casting parts for a cnc. And yes you can cast iron at home, and the core material can be bought for pennies a pound. You can use exotic alloys that are probably thermally stable.

      1. You don’t “need” thermally stable material as long as the machine temperature is kept constant.

        A lot of shops do warm-up runs on their machines before starting to cut anything to avoid dimensionnal drift as the temperature increase on a “cold” machine. I think it’s a moot point to talk about thermal stability for the building material of a CNC machine unless it’s the size of a football field because you can readily control that…

        However, I do agree that using something that can change its dimensions when relative humidity changes is asking for trouble…

  3. I tried to do the same thing…I went with the shapeoko, things didn’t go 100% as planned but currently I put the whole thing together and got the motors going….all that’s left is fabricating a decent electronic board to hold everything together.
    luckily for me I bought all the mechanical parts in a 300USD package…when I tried to make my own electronics…my prototype was terrible..calling it a failure would be considered a complement..

    the only problem I have now is that the stepper’s shaft is too large to fit in the pulleys..

  4. Been there, done that. Redesigning and rebuilding the machine should be considered at least half of the total DIY CNC hobby. If you aren’t prepared to fail, you may want to consider buying a ready-made set or at least use a proven set of plans. :)

  5. I don’t know if hackaday should call these “fail of the week”. Maybe instead call it “newbie of the week”. That way you encourage people to document their failures, and we all learn from them.

  6. John Kleinbauer used to have a web site with quite a few diy designs. I built several of his designs over the years. It is too bad that he is no longer selling plans. Many of them used MDF and if you paint it there is no problem with moisture. MDF is a good material for first timers because it is cheap and can be worked with standard hardware tool. What you need in a good diy design is adjustability. This will allow you to compensate for the little boo boos you make during construction.

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