Is it a critical dimension?

I have been over just about all the frame builds I can find, but there is a dimension I would think is critical yet never mentioned.

What is the dimension from the top of the top rail, to which the motors are mounted to the top of the 4x8 work piece. I calculate this dimension to be 21.5 inches for the 80/60 frame build. The default frame doesn’t list this dimension anywhere, and looking at the build documentation it can not even be calculated…easily anyways.

I would have thought this to be a critical dimension. If it is different between builds then every machine will react differently. Ground control, I assume, would return different results based on the variance of this dimension. Which would mean comparative data between machines would also vary making it hard to zero in on issues dealing with tolerancing and calibration. Sure this dimension shouldn’t affect a given machine as long as the calibration is done properly, but it might dirty the data when comparing data from multiple machines.

Or am I all wet and should not worry about the distance between the top of the top beam and the top of the 4x8 project plywood.

Anxiously waiting a reply, so I can finish building my frame.

I think you nailed it. That distance isn’t critical for the calibration process, but it does matter. We have seen builds where that distance was shrunk too small which was causing too much strain on the motors so now Ground Control will give you a warning during the calibration process if you try to use too small of a value. Other than that we leave it open to experimentation which does probably dirty the data, although we don’t know what the exact optimum value is yet

1 Like

It’s not mentioned in the build instructions, because it’s determined by other

But if you look at the spreadsheet, you can tinker with this value and see what
the results look like

if it’s too large (besides having trouble getting the machine through doors),
the angles in the bottom corners get too close to vertical and accuracy there

If it’s too small, the motors don’t have the strength to pull the sled across
the top (top center and moving towards the top motors)

18-22 inches seems about right. As we were doing this frame design, Bar build a
frame where this was only ~15 inches and found that it didn’t work.

David Lang


Thank you both. I will set me dimension at 20",and see how it goes. I am building a modified version of the unistrut design with the recommended 15 degree tilt.

If anything, go the other way.

You want to minimize friction (which means as near vertical as possible), but
you need to have enough of an angle so that when the bit plunges down into the
wood, there is enough force that the sled doesn’t lift away from the workpiece.

more angle means more force to keep the sled down, but also more friction.