First pass cuts wider than the rest

Finally decided to give my machine a go at tramming/squaring. I never really did it after I set the machine up. I ran a test cut using a .25 endmill to see if there’s still any scalloping. It appears the first pass cuts slightly wider than the rest as shown in my photo. Does anyone have an idea what would cause this?

The entire cutout is relative smooth beyond the first pass. This is consistent all the way around the entire dog bone

The surfacing looks nice and smooth

Maybe retry with a smaller DOC and slower plunge rate, to see if this is induced by a dynamic effect when cutting the first pass, or if this is due to machine geometry somehow?
It always seemed to me that the surface of an old MDF piece is slighlty harder than the rest (but it may just be a feeling/misconception)

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Yes, the surface is usually harder / different from the interior due to having less moisture if nothing else.

@pw8 — you’ll get a better cut if rather than slotting you add geometry around the part and make the cut as a pocket rather than an outside profile — for the best result add a second offset and take a finishing pass which is larger than your chipload.


I would try slowing the feedrate down in CM to 50% for the first full pass and see if that makes a difference, Bump it back up again once the first layer is cut. Looks to me like you’re getting a deflection of the bit. Like others have said, MDF tends to be harder on the outside, so you’re pushing against the crust and the torque of the bit is pushing it sideways to the direction of travel.

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