Tag Archives: 3D Printing

Dog Weight Scale Part 12: More 3D Printing

In my previous post, I did a little woodworking on the scale.  In this post, I start designing a 3D printed part that will keep the top of the scale centered on the bottom.

Ever since I measured the center of gravity of the top plywood circle, I’ve been puzzling through how to make sure that center of gravity stays centered on the bottom part of the scale.  Without some sort of connection between the top and bottom plywood circles, the top will inevitably slide over time, messing up all the center of gravity calculations.  On the other hand, if this connection between the top and bottom has much vertical friction, it will take some of the load of the scale, throwing off the weight calculation.

After having such success 3D printing a Load Sensor holder, I decided to have a go at a plastic part that would help keep the top and bottom plywood pieces aligned.

First draft Centering Guide
First draft Centering Guide

My plan is to drive a nail down through the top of the top plywood circle. Then whenever I need to place the top plywood circle on the bottom one, I can get close enough to center that the nail will be somewhere inside the 1.5″ diameter cone in the Centering Guide.  From there, the sides of the cone should guide the nail to the center of the bottom plywood circle. Then the top and bottom pieces will be aligned.

At least that’s the theory.  We’ll see.

Being new at designing for 3D printing, I’m not sure how to build an arch or corbel to support the cone. So I relied on the default Cura support structures, which take a lot of plastic.

Centering Guide being printed
Centering Guide being printed

Note the large amount of infill in the cylinder.  I should be able to (eventually) come up with a design that doesn’t require so much plastic.  On the other hand, since I’m only making one, I can put up with using 3.5 meters of filament this time.

While I was designing the Centering Guide, I tried to print four Load Sensor holders at once.  Little did I know that I should mess with the print parameters for such a large print.  I stopped the resulting print part way through.  It was weird: the filament wouldn’t stick to the print bed, so little hairs started sticking up.  The hairs then caught other print lines, until the whole thing started looking too frizzy to be useful.  Going back to one-at-a-time prints seems to work fine.

The result of 10% infill
The result of a big print: trying 4 load sensor holders in one print

In my next post, I finish mounting the Load Sensors and complete the final assembly of the scale.

 

Dog Weight Scale Part 10: 3D Printing Load Sensor holders

In my previous post I soldered the weight scale parts to a proto-board.  In this post, I design and 3D-print the part that keeps the Load Sensors from slipping.

The Load Sensor is an oddly-shaped thing that has a few tricky constraints: the T-shaped part in the middle must be free to bend downward (my wooden mounts take care of that), and I don’t want it to slide out of place horizontally or tilt off of its position when I’m putting the top plywood piece on the scale.

I’ve tried a couple ideas – wood holders, washers and bolts – but nothing seemed to work well. So this week I decided to learn how to design and print 3D parts.

After a few false starts I designed a basic part using FreeCAD. I like FreeCAD because it’s a powerful, Open Source CAD package capable of sophisticated work.  Some people find it difficult because, unlike SketchUp it’s Parametric – everything is done through setting parameters on lines, surfaces, and solids.

We have a Lulzbot Taz 5 3D printer at work. A friend at work helped me get started with it, and printed my first part for me.

My very first 3D printed part
My very first 3D printed part

This was a great start, and really got me fired up.  All I had to do next was fix a few details in the model, and I’d be done – what could go wrong?

First I installed the missing piece of the toolchain: the Lulzbot edition of Cura, a 3D print preparation application.  Then I edited my model in FreeCAD, exported it to .stl format, read it with Cura and… woops.

My lovely CAD model
My lovely CAD model
the "transporter malfunction" result in Cura
the “transporter malfunction” result in Cura

For some reason the model didn’t export/import properly: the bolt holes were missing, the little cutouts at the front were gone, and the tabs at the top turned into strange triangular blocks.  Not Good.

So I experimented and searched. The problem seems to be that Cura is (properly) confused by internal edges, so I should Fuse the parts of the model before exporting to Cura.  I haven’t yet found how to do that (education opportunity!), but I did manage to redesign the part so that it happily made it into Cura.  The problem seems to be aggravated by using Pockets (subtracting material), so my redesigned model has no pockets – it’s all just adding parts together.

second try: no Pockets
second try: no Pockets
Successfully imported into Cura
Successfully imported into Cura

A very short time later (ok, 45 minutes), I had a printed part!

Cutting the thin supports away with a hobby knife
Cutting the thin supports away with a hobby knife
Lucky me - it fits perfectly!
Lucky me – it fits perfectly!

In my next post, I drill the right depth of holes in the plywood.