Deep inlays and the myth of the start depth

I feel like I have finally dialed in the settings for my inlay work, and they surprised me.

I didn’t use a start depth. It never did what it was supposed to do for me, and always left a gap somewhere on the inlay. So I did a large test run, and I found that the best fit for the bits I use (4.5 degree TBN) is .25" DOC for the pocket and .29" DOC for the plug, with a start depth of 0".

Judge for yourself the fit. Doing it this way also eliminates the hassle of doing a pre-clear in really hard wood so you’re not hogging through .29" in one pass with a fragile TBN or 1/8" end mill.


How was your tool inserted into the Tool Database? V-Bit, Engraving . . . ?

I took the angles of the TBN and created a v-bit with the same parameters. I have no idea why the results are different than using a Tbn with the exact same dimensions but they are. A cut that takes 30 mins as a v-bit would take 6 hours as a tbn. And the inlay won’t work either.

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That is because the VBN in effect has a flat tip the diameter of the ball nose. This is why your numbers work. If it had been an actual v bit with a sharp point, your parts would not have fit. In reality, you are using it as an engraving bit. I’m gonna have to give this a try. It probably wouldn’t work with a different VBN with a different ball dia.

I’m not so sure about that. I don’t think the tip affects the fit. What matters is the angle. The start depth matters less as the angle approaches vertical.

You obviously need the sharpest TBN you can get or you will not get the detail you need.


Yes, the point does matter. If there was an exact point then the two halves would have the exact same dimensions at the surface and therefore would not be able to engage. That is the reason a start depth is needed for a v-bit.