Some 3dp stuff - mostly smaller things

The following is just a little report in venturing into smaller things. I have to admit I am pushing my envelope a bit as my printers (and I) are used to print bigger things and my printers currently have .6 and .8 nozzles. But I will get into some of the issues and maybe things you might find usefull, To simplify things all pics in this post are PETG and are printed on a Predator (sub $500 printer I paid 338 for mine 3 years ago) stock with a custom 3dp effector to allow for a volcano hotend with a .6 nozzle. The PETG is from Zyltech printed at 250 C (optimized for layer adhesion. I probably could get some better results dropping it but current ones are good enough and IDK at this point how much layer adhesion I used. Also those parts have been printed without a fan as I currently have no part cooling fan mounted and petg does not like fans (strength wise) anyway.

First - using Prusa Slicer classic (there are problems with the arachne version I set the outside perimeters to .45 mm and the inside to .95 mm both well within the “sane” settings for a .6 nozzle. Layer height depending on print are between .2 and .32 mm Here is a close up of a slice of a gt2 pulley


here is the part printed

Its a 19T bevel gear on the top of a 19-23 90 deg bevel setup 1.5mm Modulus with a GT2 40 tooth pulley attached. It will use an M5 bolt as axle with 2ea 605zz bearings

These are the bearings and one inserted into one of this years bevel setup. It pays to first print a test print with multiple holes - in the case of the 605 who are 14mm od nominal from 13.8 to 14.6 and in this case 14.2 dia “felt the best”
The other end mounts on a D shaft (6mm)

This is the “fitting test” on a 6mm D shaft - steel as if there is a mishap here we are not going to potentially damage a motor. So all I print for myself or my 4 teams (1 FTC 3 FRC) will get fit tested first. For D shafts I currently use square nut inserts M3. They are 5.5mm square and 2.5mm thick and hold nice in a 6x3mm cutout (designed in cad).

I also use them for hubs

In this case the wheel is held on with 6 self tapping sheet metal screws which hold nice in the plastic (3mm OD hole predesigned like you would drill it including tolerance adjustment for plastic and yes I printed initially a piece with different dia holes to see which works best

we are going to go with double - offset omnis (as suggested bye @wgorgen) and so we also switched to M4x30mm bolts and M4 nuts are the smallest nuts that will reliably hold in plastic. To make them hold we start a hole at usually 7.5mm hex (nominal nut size 7mm) and then taper it in at 1-3 deg (depending on how much the nut is recessed to end up at 6.8mm at the bottom. Then the nut gets forced in there

The 2 omnis have washers as spacers inbetween them so the rollers clear. There is 4mm of “meat” behind the nuts and so the bolts when properly tightend just barely by a “hair” clear the nylon part of the nylock lock nuts. which in case are almost flush (slightly recessed - .5mm) with the plastic base so nothing sticks out. Then of course we have the square m3 nut inserts for the m3 setscrews to keep it from sliding off the motors or axles “D” shaft.

The reason for using square nuts are - they are easy to just slide in, are available in different sizes (you could also use T nuts if you change the layout) and give “metal advantage” in that location. Using a material like PETG (or most plastics except maybe PLA which is too hard) the plastic will give a little if you really crank down on the setscrew which will kinda work like a jam nut and keep pressure on the thread. At leas that is what I believe based on current tests. At least so far that seems a good cheap combo to use 3dp in those cases. I buy this PETG between $9 and $12 (depending on quantity) per KG. and a hub is about 20 grams. So including square nuts and set screws and the mounting screws well under $1.

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Smaller things need different tools and techniques. On my CNC router I was routinely v-carving signs in the 2" letter category. When I wanted to cut letters in the 1/4" category my router bit was far too clumsy. So I found a source (think & tinker precisebits) for tiny engraving bits, got better at leveling the material, and it came out fine.

The point is that the machine can do small things, but the workflow and tools were a learning curve.

You’ll need a smaller nozzle and some play time to dial it in.

Thanks yet again for a thoughtful and informative writeup.

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