Cuts smaller than measurements

I tried to cut a rabbet around a 133mm x 213mm rectangle. I placed a white rectangle (133x213) inside a larger black rectangle 158mm x 238mm and created a pocket toolpath with 1/8 inch endmill. The resulting raised rectangle is actually 130mm x 210mm and the edge of the larger rectangle measures 162mm x 242mm.

I imported an SVG into Carbide Create and checked the sizes of the rectangles before exporting the gcode.

Any ideas for the discrepancy?

@Teeman Check the bit size. An as advertised 1/8" bit is not necessarily actually 1/8". I have some 1/8" bits that are actually .118" and others that are .117". If you don’t enter that into your CAM software, you get bad dimensioned cuts.

Schoolboy error - I set my path to an 1/8 inch endmill and left the 1/4 inch endmill in the router.

@Teeman That will do it every time, Tony. :grinning:

It is worth keeping in mind, though, that bits are not necessarily the size they purport to be. Bill Korn first put me onto this when we were playing with a drag knife. We needed precision in our milling and Bill found that he was out on some early passes because his bits were smaller than advertised. That lead me to measure and then play with mine.

I will do, especially when learning with cheap disposable ebay bits. Even I could spot the difference between the 1/8 and the 1/4 though :grimacing:

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