Focus/Kerf issues (I'm guessing!)

Guys, me again…

I’m cutting some rocket motor mounts for a friend of mine who does rocketry competition. The design is REALLY tight, to fit all the components into the size constraints, but it works and I can happily cut it in draftboard. The problem is, when I need to cut it in 1.5 or 2mm plywood. The cuts are too large and the thing falls apart.

I’ve attached the file for reference, but what’s the best way to keep the cuts as thin as possible? I’ve set the material thickness as per the measurement, but it just doesnt want to know.

motor%20mount%20x%203%20v2amotor mount x 3 v2a.pdf (52.3 KB)

Looks like a kerf issue to me. Only way to solve it is by adjusting the design slightly, I’m afraid. In Inkscape, use the “dynamic offset” for paths to add half a kerf-width to the edge on the cutting side. You can measure your kerf width by cutting a reference square of, say, 5cm and measuring that with your calipers… the difference between your actual piece and 5cm is one whole kerf width (1/2 on each side).

Also, make sure you have set Inkscape to “geometric bounding boxes” in options -> tools, otherwise the stroke width will muck things up…

Hope that helps, let us know how you get on.


Hi Ade

The problem is the design is constrained. The body tube has an internal diameter of 55mm and the motor housing tubes have an internal diameter of 24.8mm, with a 3.8mm hole for an M4 bolt.

The design was done in Illustrator. Would Inkscape be easier in this case? Can I have all the ‘cut’ in the dead space to make the holes accurate?


Hi Ian,

That shouldn’t be a problem - if you measure your part now, you’ll probably find it’s just under 55mm - in fact, a kerf width under… :wink: You’re spot on, you want all the cut in the dead space.

I used inkscape as the example as that’s what I use - I assume Illustrator would have a similar functionality somewhere. Basically - if I understand your design right - you want to make the outer circle bigger by 1 kw (tired of typing kerf-width…) and the inner ones smaller by 1 kw (1/2 kw on both sides in both cases). Off the cuff, I’d try with 0.15-0.25mm or so.


Yeah kerf varies by material. Illustrator can adjust kerf no problem.