I use a CAD program (Vectric VCarvePro, made by some of your countrymen!) to do all the design and layout work for UpscaleCNC. Here's a couple screenshots showing two of the clusters suggested.
To make these drawings: first, two circular vectors are made one to match the ID of the airframe (this will be the outside of the centering ring) and another centered in the first that matches the OD of the central motor tube. Then the extra tubes size is selected, here its LOC 38mm, which is 1.63ish OD. Make that vector and position it at either (0,x) or (x,0) where x= (radius inner circle + (outer radius - inner radius)). This centers the small vector halfway between the edge and the center opening. Of course, you can move them back and forth, if you wish.
Then, use the vector array tool, a circular array, centered on 0,0 and with the desired number of copies... oh- you have to select the little solitary cluster vector first, as that's the one you want to copy. Bingo, they show up, you can use the inspection tool to check distances between vectors.
Why all this detail? Because you can download a functional copy of vcarvepro from the vectric website and use it to experiment. Does you no good if you want to try to make toolpaths (it won't, in trial) or save files and open them in a full version, or export...... but you can use it to design things. Trial and error... is another way to sort out cluster arrangements.
So, the two drawings- first one is reasonable, 54mm and 6 38s, the narrowest spacing dimension is given. You could go to 8 38s easily. The second drawing is fun, the center 98mm tube is just LOC airframe, not their heavy wall 4" MMT tube as I didn't have an OD dimension handy for that. Yes, this ring could be cut, probably best to do it in 3/8 or 1/2" material though.
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Nat Kinsey
UpscaleCNC