Weekend 3D Modeling and 3D Print Tolerances

posted in: 3d printing, power | 0

Update: Banana spacing now corrected to standard.  Was ~15mil short of standard before.

Hello there,

I don’t think this will be a long post and I’ve forgotten to take a lot of photos of my work before the final revision.  So I don’t expect this to be very thorough.

I’ve recently made a female banana plug adapter so that I can mount my banana plugs onto the 20mm rail that makes my Kossel K800-style printer.  I originally bought the banana plugs from Banggood and thought about using them for a power supply for charging LiPo batteries.

I measured the hole spacing between the two plugs using dial calipers and a little bit of mathematics (the process of doing this is usually included with the calipers and involves averaging two measurements).  The actual holes are formed using DD shafting and have two diameters to be measured in order to create the corresponding part correctly.  The measurements can be accomplished by measuring the included mounting plates and doing a bit of fancy caliper work, or they can be measured from the jacks directly.

Needed Parts

  • 3D Printer or equivalent 20mm T-slot
  • M4 Socket Cap Screw & Washer & T nut
  • Allen Wrench
  • Banana Plugs
  • Printed Part

Tolerances

Clearance tolerances are placed onto the modeled part in order to ensure that all the parts either “press fit” or “loose fit” onto the plastic piece.  Since the 3D printer is side-printing the holes involved, a lower tolerance than normal is used in order to ensure fitting. Side-holes are not very accurate by default because printers often leak as they form side-holes and the leaking gets stuck onto the walls inside the printed hole. Side-holes also exhibit a bowing effect that is likely caused by the plastic wanting to stick directly to other plastic than stay where it is placed by the printer. A cross-sectional picture can be generated to illustrate these effects. Maybe I should get to that.

Cut / Hole Selected Tolerance
M4 Socket Cap Screw Thread Originally 4.0mm major thread diameter, used 4.5mm on print.
M4 Socket Cap Screw Washer Originally 9.0mm diameter, used 10.0mm on print.
Aluminum T Slot Inner Width Originally 6.0mm, used 5.5mm on print.
Female Banana Plug Major DD Shaft Diameter Originally 5.26mm, used 5.55mm
Female Banana Plug Minor DD Shaft Diameter (flat-to-flat) Originally 5.0mm, used 5.05mm
Female Banana Plug Washer Diameter Originally 10.55mm, used 10.65mm
In order for 3d printer tolerances to be correct, they must be empirically tested. Most of the estimates can be done very roughly by increasing the hole diameter according to this table below. Bear in mind that most of these are designed for non-drippy Cartesian printers. Other printer kinematics seem to present slightly more nondeterministic tolerance changes. Slightly sloppy printers (they drip everywhere) also have more nondeterministic tolerance changes. Print software may compensate for printer sloppiness and change tolerances so that they are closer. It is recommended to use many test prints.
Orientation Desired Fit Diameter Increase
Perpendicular to Print Plane Loose Fit +0.3mm
Close Fit +0.2mm
Interference Fit +0.1mm
Parallel to Print Plane Loose Fit +0.2mm
Close Fit +0.1mm
Interference Fit +0.05mm

Pictures

Banana Plug Holder and Tolerances for Parallel-Plane 3D Printing
Banana Plug Holder and Tolerances for Parallel-Plane 3D Printing (Not updated to newest model)
Smoothed Model of the Banana holder
Smoothed Model of the Banana holder
Banana Holder Side View
Banana Holder Side View
Banana Holder Front View
Banana Holder Front View
Banana Holder Back View (Note the cutouts for washers)
Banana Holder Back View (Note the cutouts for washers)
Banana Plug Holder in Action
Banana Plug Holder in Action
Banana Plug Holder in Extreme Foreground
Banana Plug Holder in Extreme Foreground

I should note that my wiring is not very neat, and I plan to clean it up in the future.

Potential Issues / Future Work?

I think I may some day have issues with uninsulated portions of the wire touching the socket cap screw.

I will also someday smooth out the corners on my holder.

Apparently, dual-banana configurations have the plugs 0.75in apart.  I had it at 0.735in.  My mistake.  All the models on Thingiverse have been corrected.  Credit goes to ArizonaClark for pointing out my mistake.

Enough Talk, Gimme the Files!

Okay, the files are on Thingiverse.  It’s a better way to manage these things than trying to put all the files on my blog hosting.

http://www.thingiverse.com/thing:1547962

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