Help troubleshooting a this 2-sided machining method?

@jwr Ok, making a knight right now with your “gauge block” idea… and I love it. I have a 5mm dowel pin and it indeed works awesome to just and roll it back and forth under the bit as I drop by 0.025mm. When it just touches, I just set z=5.01 (what I measured) and go!

I could already tell that I wasn’t being consistent between my main detail pass with a 0.125in ball end and the final details put in with a 0.5mm tapered ball end mill. I’d been struggling with the final pass being just a hair too deep, which is annoying to sand out. I just used the dowel trick for the first time and it’s perfect.


After somebody else pointed it out I bought a 5mm reamer to open the locating pin holes out to proper dimension, I bore them with the CNC and then finish them manually. A reamer gives you a hole which is the same diameter all the way down.

That sounds to me like the X rail might not be square with the Y rails, I’ve had that problem and now manually check the squareness on startup for jobs where it’s critical. The problem on a Shapeoko is they don’t always start up with the X at the same angle.

I’ve done a bit of flip work and find that setting my origin to be inline with the center line of the 2 pins helps me get a step free result.
Cheers Mike

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Cool idea on the reamer. I’m familiar with them, but hadn’t thought of using them to take a hole to final diameter for this purpose. Neat idea!

Indeed, I think I did have an x/y perpendicularity issue, which I [mostly] fixed with some shimming. Looks like we engaged on that thread too :slight_smile: That said, the evidence there was a stair step that changed along the y direction of the piece. The step on my pieces now is consistent along the piece, so I think it’s just about xy repeatability/locating, not angular deviation.

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Question for you. I’ve been using the steel dowel trick for zeroing z but run into tough calls for in between land. What do you do in this situation? I’ve had positions for example where bumping up 0.025mm doesn’t cause contact but lowering it won’t roll under the bit. I’ve had others where I can just feel it contact as I roll, and middle ground where if I put a bit of downward pressure on it, the bit will roll under pretty nicely.

Is your process that sensitive, or do you just use this as a go/no-go? I can tell that being off by, I’d guess, 0.02mm for my final detail pass with a tapered 0.5mm ball end bit looks much worse if I end up too low after the previous 0.125in ball end operation.

My best guess has been to start trying to learn half steps… like if I step in -z and it’s a hard stop, but +z one step completely clears, I might go +z so there’s no contact and call that z=5.025 (my true dia is 5.01). If it just kisses the pin with no down pressure, 5.01.

Thanks for any tips if you’ve run into this. If the machine can only step in 0.025mm steps, it made me wonder if this is why your bit setter was unreliable? I don’t have one, so not clear to me how it determines z=0 with respect to the machines step size limitations.

I found that I’m getting good results by being consistent and following this rule: always start in a position where the gauge block doesn’t fit (e.g. lower XYZ value) and increase X, Y or Z until I can fit the block “reasonably easily”. There is usually a position where I can force the block in, and another step or two up where it goes in with a bit of friction, but easily. That’s where I mark my zero.

I never go back a step, this makes no sense, as the backlash will eat the results alive :slight_smile: If I have to restart, I go back a significant amount, change direction, and then repeat the procedure.

I should probably try going one more step up and see if I get a difference in stepping on the finished model.

I don’t think I can get much better given the build rigidity of the Nomad. On a Shapeoko I think things will be much less precise.

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Thanks for clarifying. Ah, that’s very interesting and I hadn’t considered backlash! By this:

I never go back a step, this makes no sense, as the backlash will eat the results alive :slight_smile:

You mean you always go to a “no-go” stage, then creep up until the block goes? (Just calibrating what you mean by “back” vs. “forward”)!

I don’t think I can get much better given the build rigidity of the Nomad. On a Shapeoko I think things will be much less precise.

Hmm, good point. I forget you’re on a Nomad. By using my not-systematic-at-all method below, I’ve been pretty darn happy on my SO3 XL:

  • drop down in 1mm until the pin doesn’t fit
  • go up by 0.25mm until it easily fits
  • drop down by 0.025mm until I just feel the bit rub

That’s the ideal anyway. What I find is that I can go down, feel nothing, then go another step and it feels like too much friction. Because it’s been surprising to me how drastic one 0.025mm step can be, I’ve even gone back and forth and verified that, indeed, I feel nothing at one position, then too much resistance for my taste at the next lowest.

I’ve then usually been going back up and calling that 5.02 (add 0.01 to the diameter), or if it’s just rubbing enough for my tastes, I’ll call that 5.0. If I can barely perceive the blip when I roll under it, I go 5.01 (exact dia I measured).

This is purely Z, though. I have been inferring x/y by contouring and chasing any overhangs by manually adjusting and re-running. Once I get a more sturdy square I can try your method on x/y as well. Still, the contour is the most direct answer to “am I where I should be” so I might not get away from it.

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