Math doesn't work


I created a lid with a rabbet on it. The outer rectangular dimension was 1600 x 1080 mm, the inner was 1380 x 840. I then scaled it down by 90%. The inner rectangle came out correct at 138 x 84. The outer, though came out at 160 x 107; one dimension is a mm off. OK, but then I measured the rabbet to see which edge to adjust. They opposite sides of the rabbet needing adjustment should equal 23 mm (107 - 84). Instead, they measure 12 x 11.7 mm. The other rabbets don’t add up either. Any ideas? This is for 3d printing and my printer is awaiting a new motherboard, so I can’t test it in hard copy.
Thanks.GPS Lid thinner 2-16-17.skp (1.2 MB)


I guess I wouldn’t have used the Scale tool to resize it in the first place I’d have drawn it at the desired size from the git go. But if I had to resize it, I would have used the Move tool to move the ends.

The different widths of the rabbets would imply they weren’t the same width before scaling it down.

What outside dimensions do you really want? How deep and wide is the rabbet supposed to be? How thick overall?

Is the knob supposed to be part of the lid or is it supposed to be floating a little more than half a millimeter above the lid?


Following earlier advice for avoiding problems with short edges and stuff, I drew this at a multiple of 10 and then scaled down. It was definitely necessary to draw a scaled up version to get the knob to pass Solid Inspector. It is, indeed, supposed to be part of the lid; I thought it was. Is there a way to tell it to make contact, or do I have to fiddle with it manually?

The dimensions I want are the ones I drew as scaled. The rabbets should have been 12 mm on the short side and 11 on the long. The total thickness of 7 mm is correct but does not need to be as accurate, so I didn’t concern myself with it.

I’m pretty sure these inaccuracies will not create a problem when I print the part (although the “floating knob” will), but I’m concerned at the fact that the math doesn’t work.


OK. I went ahead and drew aversion of your lid at 10x from the dimensions you gave.

and then I scaled it down by a factor of 0.1.

It appears to me that the math worked just fine. How did you go about scaling?

You could draw the knob in contact in the first place or you could move it down onto the face now that you’ve got it drawn.

My general approach for drawing small stuff is to start out drawing it at the finished size. When I get to a point where working at that size would create issues–such as running Follow Me on the knob profile. I would create a component, copy it and scale up the copy. I would edit the large copy and do what needed to be done. When finished, I would close it and delete the large copy of the component and then return to the original with Zoom Extents.


Thanks for your replies. I drew the lid by creating one rectangle and then using the Offset tool to create the next. I made adjustments for the fact that the perpendicular rabbets were different by 1 mm each. I scaled using the Scale tool. The outside dimension minus the inside dimension should equal the sum of the rabbets. It does in the original and it doesn’t in the scaled version. When I get my printer working again, I’ll go ahead and try to print this – see what happens.

I’m a beginner and, while I await a part for my printer by slow barge from China, I’m experimenting with technique. My laptop graphics card is incompatible with SU2017, so I’m using 2016. I cannot make the knob I want in the finished size that will pass Solid Inspector – hundreds of short edges. It took several tries to get it to work even when scaled up. Interestingly, I finally got it to work when I plugged in a larger TV monitor for the video. Don’t know if that made any difference (I didn’t turn off the screen on the laptop).

Anyway – thanks for your help, Dave.