Why Did My Cuts Not Work?


Recently I designed and cut out a baseball card sorter out of 1/4" where I used mortise and tenon joinery with the t-bone fillet where the joints met. I had to use an offset allowance because the pieces where too tight to fit together without splitting the ply.

Today I cut out an over head dust arm thing I created but had a lot of slop in the joints. I used virtually the same technique, but this time I did not put in an allowance because I wanted a very tight fit. when I measure the tenon, I am getting .99 inches and when I measure the mortise, I am getting 1.01 inches. When measuring using the software, both should be 1". Confirming the profile toolpaths, there is no allowance offset shown. I’m using a 1/8" compression bit. Both toolpaths are outside/right. The cuts arent really straight either - kind of curved.

Any thoughts as to what I am missing?

Thank you,

Usually some allowance is needed for the pieces to fit in my experience so it’s weird that you had a loose fit without any. I’m wondering if they were cut in different orientations. If one was cut parallel to the X and the other parallel to the Y then it could be that you need to adjust the steps/mm. gSender’s calibration tool can be used to check that an axis moves how much it should. If you use the calibration tool I recommend moving the axis by a large amount and then measuring as that will give you more accuracy. Also if I need to be super accurate and I’m using a tape measure I find it more precise to start measuring at 1" and then subtract the 1" from the measurement. I just don’t trust the thing at the end of the tape measure to be in the exact right spot.

The other thing that should be checked before going the calibration route would be the anti-backlash blocks. If you have a little slop in the axis that could cause a piece to come out the wrong size. It might even account for the slight curve at least that’s my thinking because maybe the forces that the bit generates moved the axis perpendicular to the cut from one side of the play to the other.

Good luck, hope this helps, it’s what came to my mind anyhow.


@Cropduster16 Are you using the same bit that you used for the successful project?

@_Michael ideas are where I would start. You may also want to check to see if the end mill is really .125". I have nominal 1/8" bits that are .118; others .121.


Both Michael and Grant point to important factors. While I can’t say step distance has ever caught me off guard, I do know that my 1/8 inch bits run .122 - .124, never .125. Similar results with my .25 bits. It’s good to mic these if you need to have a perfect fit.