keyhole leaving mark at Zero

[mmorgan]

This problem has come up enough times that I needed to see if anyone has ever had a similar experience and/or a solution. the fronts of our plaques are coming out beautifully and when the bit returns to zero it doesn't leave a mark. When we do the keyhole on the back it leaves a small imprint of the bit in the wood when it returns to zero after making the cut. We put a sticker over the blemish (we make them for school projects). It happens with the keyhole bit but also with the 60 degree (forgot to change it once). Any ideas??
thanks,
Michele Morgan
Occupational Therapist
Warren Woods Tower High School

[BillK]

I had the same problem. I was setting Z zero near where I was going to cut the slot. Now I set Z zero at XY zero, then jog it up a few thousandths and reset Z zero. You'll find no difference in how the slot cuts or functions, and you'll leave no mark.

[4DThinker]

There is a little flex in the gantry, and the momentum of the router coming back down to 0 will often overshoot to mark the wood before springing back up. As billK suggested, setting Z=0 a couple of thousands above the surface can do the trick. When my surfaces are critical I move the bit 1/16 up after zeroing at the surface, the reset 0. Then I tell it to start all my tool paths 1/16 down.

Bits that are lower after a cut than when they started the cut may be slipping out of the collet while they cut. Gravity and the design of some bits encourage this if the bit isn't very tight.

[4DThinker]

Here is another strategy I use when the keyhole is the only thing being cut on a surface: Draw it around 0,0. Then when cutting just zero the router right where you want it and cut it. Any ding would have been consumed when the keyhole was cut.

I use this same strategy for parts I cut frequently, and usually out of odd shaped scrap material. Jog the bit to the center of any area I can get the part out of, zero it, then cut.