Gas station without pumps

2017 November 28

More SolidWorks

Filed under: Robotics — gasstationwithoutpumps @ 22:24
Tags: , , , , , ,

I spent most of my day today with SolidWorks, fixing the problems noted in Bugs found in first assembly of robot and adding new layers to the robot. I’m getting a bit better at using SolidWorks, but I still find it to be an overly complicated interface with way too many modes.  I’m sure that there is a way to get it to start up with reasonable document parameters (like using mm instead of inches, or using the same settings as already open documents), but I’ve not taken the time to try to track that down.

Here is the model as it stands so far:

View from the front left of the robot. The octagon floating on top is a the beacon-detector board, which will be on standoffs that I didn’t bother to include in the model.

I cut out the three layers of the robot today, making two mistakes in the process. One mistake I caught right away, and just recut the layer after fixing the problem—there was an extra alignment circle that was not supposed to be cut that I had forgotten to erase. The other error was just as serious, but I didn’t notice it until I got home—the top layer did not have the slots cut in it for the spacers from layer 2 to layer 3. It is hard to notice this problem in looking at the SolidWorks model, as the 3D model looks the same whether the slots are cut or not. I should have noticed it when I created the dxf file for cutting the third layer, but by then I was getting pretty tired and careless. I’ll have to cut another copy on Thursday.  Luckily MDF is cheap—each layer costs me about $1, and I still have half a dozen 1-foot squares of MDF left.

Incidentally, I came up with what I think will be a cheap fix for the potential problem of the bumper springs not being stiff enough.  I added another switch front and center, just for the spring in the switch to push the bumper forward.  At 60¢ a switch, this is not a particularly expensive way to add a spring, and it saved me a lot of modeling and building time.  I could even wire up the switch if I can think of a use for it.

One other thing I made today was a “drill” test, to see what size holes were really made by the laser cutter from specifications.  I created the guide in SVG using a short Python program (so that I could tweak things easily. It took me quite a while to get the SVG just right, because of weird limitations of SVG, like that the path commands can’t take units for the coordinates. Also because I was using Inkscape to translate the SVG to the DXF format that the RDWorks laser-cutter software needs, and Inkscape assumes that the “pixels” are 90/inch for that conversion.  It is kind of messed up that SVG works in terms of “pixels”, since it is supposed to be Scalable Vector Graphics.  Inkscape only converts paths to DXF (not other shapes, like circles and text), so I wrote the program to generate paths and used Inkscape’s object-to-path conversion to convert the text.

Here is the piece I cut:

The circles were cut at 14mm/s and 100% (actually clipped at 67%) and the numbers were written at 140mm/s and 20%.

The holes were exactly the right size (to the 0.1mm limitations of my calipers), and the circular pieces that were cut out were 0.35–0.4mm smaller in diameter. That is, the kerf is about 0.19±0.02mm and it is on the inside of arcs.

Here is the code I used for generating the SVG file:

#!/usr/bin/env python

from __future__ import division, print_function

# all sizes are given in units of 0.1 mm

# Inkscape coverts pixels to real-world units in DXF at 90 pixels/in

pix_per_inch = 90
mm_per_inch = 25.4
pix_per_mm = pix_per_inch / mm_per_inch
pix_per_unit = 0.1*pix_per_mm

print('<?xml version="1.0" encoding="UTF-8" standalone="no"?>')
print('<svg width="150mm" height="150mm" xmlns="http://www.w3.org/2000/svg">');

y=100   # y-position of first row of circles

x_space = 60    # spacing between circles
stroke = 1       # stroke-width

xmax = None     # largest value for x

text_space =35  # space from circle to label

for diams in [range(5,50,5), range(50,80,5), range(80,105,5), range(105,130,5)]:
    x=100 # left edge of first circle
    for diam in diams:
        x += diam/2
        print ('<path stroke="red" fill="none" stroke-width="1" d="M {sx},{sy} \
a {r},{r} 0,0,0 {r},{r} \
a {r},{r} 0,0,0 {r},-{r} \
a {r},{r} 0,0,0 -{r},-{r} \
a {r},{r} 0,0,0 -{r},{r} z" />'.format(sx=(x-diam/2)*pix_per_unit, sy=y*pix_per_unit,
                r=diam/2*pix_per_unit))

        print('<text x="{}" y="{}" stroke="blue" fill="blue" text-anchor="middle" font-family="Verdana" font-size="10">'.format(
                x*pix_per_unit,(y+diam/2+text_space)*pix_per_unit))
        print(diam/10)
        print('</text>')
        x += diam/2 + x_space
        last_diam=diam
        if xmax is None or x>xmax:
                xmax = x
    y+=2*last_diam+text_space

print('<path stroke="red" fill="none" stroke-width="1" d="M 0,0 h {} v {} h {} z" />'.format(
        xmax*pix_per_unit,(y-last_diam)*pix_per_unit,
        -xmax*pix_per_unit))

print ('</svg>')
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