Gas station without pumps

2017 November 17

Hardware obtained today

Filed under: Robotics — gasstationwithoutpumps @ 19:56
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In addition to working on the circuits for the tape sensors, I went to the hardware store to get some threaded rod for clamping the layers of my robot together.  I found an old pice of 8-32 threaded rod around the house (left over from the days of the robotics club 5–6 years ago), and I realized that ¼” rod was way bigger than I needed, so I’m redesigning around 8-32 threaded rod instead.  I only had 3 feet of the rod, so I went to the hardware store to get more 8-32 threaded rod and some acorn nuts to go on the ends.  While I was there I also got some M10 nuts and E clips, for keeping the ballpoint set screws in place.

It turns out that 8-32 acorn nuts are only 6.5mm high, so there should be enough clearance under the robot for the acorn nuts.  I could probably have used them (or slightly larger 10-24 acorn nuts) as skids rather than the $6.01 M10 ball-end set screws, but I’ve got the set screws, so I might as well use them.

When I got the rod home, I cut the old one and part of the new one into 11″ pieces with bolt cutters, then ground the ends smooth and slightly rounded with a wet wheel.  I checked the length of each one with acorn nuts on the end, to make sure it was a couple of millimeters short of 11″, so that when mounted on the bot, the tips of the acorn nuts on top should just be at at 11″.  I had to grind a mm or two off of some of the rods to make them the right length—cutting with bolt cutters is not very precise, as it squishes a few mm of the rod, which then gets ground off.

Here are the 8-32 rods with the nuts on them. The hex nuts will sit above the top layer of the robot, clamping the layers together (I may add washers to spread out the force a bit).

You may notice that there are only 7 acorn nuts, not 8.  I bought the right number, but one of them was not threaded all the way to the end, so I’ll have to go back to the hardware store tomorrow to replace it.  While I’m there, I’ll also buy a box of 8-32 hex nuts, because I only had 8 left of my last box of 100, and I’ll need them for the robot, as shown on the rods here.

Here are the set screws with an E clip and a jam nut. I’m thinking of putting an E clip below the MDF and one or two jam nuts above the MDF, but this plan may need adjustment, depending how well the E clip works.

In other hardware news, I got four packages in the mail today:

  • a Digikey order with 100mH inductor to make a more sensitive passive track-wire detector (and some smaller shielded inductors, if I want to increase the Q of the amplifier with an LC tank for feedback).
  • another pair of gearmotors with encoders, wheels, and motor mounts for $25.53 with shipping.  These were spares I ordered on Nov 2 from AliExpress, and I didn’t expect them to arrive for a month—two weeks is pretty fast for cheap shipping from China.
  • four spare gear motors with encoders, but without wheels or motor mounts, also ordered from Ali Express on Nov 2 for $8.89 each (2 @210 rpm and 2 @300rpm)  These motors have the same part number (JGA25-370, DC 6V 210RPM) as the ones that came with the wheels, but the label is different, so I need to check—maybe these really are 210 RPM motors. The JGA25-370 number just refers to the shape of the gearmotor, not to gear ratio.  If my robot is too slow, I could try replacing the 210 rpm motors with the 300 rpm ones.
  • 10 roller microswitches from Amazon for $6.99.  These are tiny, low-quality switches, but I think they’ll work ok for the bumper switches.  It turns out that after I ordered these, I found a bag of 5 tiny microswitches (the same form factor, but different levers) at the back of my drawer of switches.  I’m not sure when and where I got the set in my drawer.  I’ll probably try both sets of switches and see which work better with the bumpers.

I also ordered today the multiplexer that I had omitted from my previous Digi-key order and some more of the TCRT5000L reflectance sensors.  My current robot design calls for 5, and I only have 4 in the parts kit for the course.  The usual team of 3 people would have 12, which is an ample supply, but 4 is probably not enough for the robot this year.

I’ve started thinking some more about the bumper design.  Originally, I was going to use stainless-steel wire to make whiskers that turn a rotary microswitch, but the rotary microswitches are expensive and large (particularly compared to the tiny ones I now plan to use).  I could still use whiskers, but I’m now thinking of a rounded bumper sticking out about 2mm from the front of the robot, pushed forward by the two switches, and retained by a couple of screws in slots.  I’ll have to sketch out the design, then laser-cut it to see whether it works.  I’ll probably be ready to laser-cut everything for my first two layers on Tuesday.


1 Comment »

  1. […] [Update 17 Nov 2017: I’m now going to use 8-32 threaded rod, not ¼”, so there will be room on the bottom for the acorn nuts. See Hardware obtained today.] […]

    Pingback by Another day fighting SolidWorks | Gas station without pumps — 2017 November 17 @ 20:01 | Reply

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