Dog Weight Scale part 3: the woodworking and assembly

In my previous post I described the electronics of the Dog Bed Weight Scale. In this post, I’m doing the final woodworking and assembly – at least enough assembly to test the thing.

First I needed to design some sort of support for the load sensors.  Because of the design of the sensor – a “T” bar surrounded by a “C” shaped bar – I needed to make blocks that were 1) tall enough to keep the top piece of plywood from resting on or crushing the electronics and 2) cut out to allow the “T bar to bend below the “C” shaped part as weight was added.  You can find plenty of videos of people trying to use load sensors by mounting them on a flat surface; that won’t work.

So, to design the blocks, I first measured the dimensions of the load sensor, using a Caliper, then drew up a simple design from that.

Measuring a load sensor using a caliper
Measuring a load sensor using a caliper

Meanwhile, I drilled the mounting holes for the boards.  Because the plywood base is so large (~41″ in diameter), I couldn’t use the drill press.  So to make nicely perpendicular holes, I used a Drill Guide. I really like the one I use because it’s metal and it has a guide for each drill bit I use, creating nicely straight holes.

Because I was feeling a bit lazy, I didn’t measure and mark the holes for the electronics boards.  Instead I used the old “mark and drill” method.

Step 1: holding the board in place, drill just enough to mark the first hole – don’t drill deep.

Marking the first hole to drill
Marking the first hole to drill

Step 2: remove the board (so the drilling doesn’t damage it) and use a Drill Guide to complete the marked hole.

Drilling a vertical hole, using a drill guide
Drilling a vertical hole, using a drill guide

Step 3: Place the board back, drop the first bolt into the new hole, then mark the second hole as in step 1.  Repeat for all the board’s holes.

Marking the 3rd hole
Marking the 3rd hole

To prevent the bolts from sticking out from the bottom of the plywood, I chose 3/4″ bolts for 3/4″ plywood.  Because the bolts don’t stick out, I needed to counterbore the bottom of these holes so I can attach the nuts.  Again I couldn’t do this on the drill press, so I used a plunge router, set to bore just a little into the plywood.

Setting the router to counterbore holes
Setting the router to counterbore holes
Counterboring the mounting holes
Counterboring the mounting holes

Because the router bit I used has a space in the middle, the counterbore holes leave a little disk of wood, I used a chisel to clean out the remaining little disk of wood to make the counterbore flat.

Chiseling out the little disk of leftover wood
Chiseling out the little disk of leftover wood

Here’s what the 3 boards look like, fastened to the plywood base.

The boards mounted onto the plywood base
The boards mounted onto the plywood base
The back of the plywood base, showing the counterbored holes
The back of the plywood base, showing the counterbored holes

I then cut the blocks that will support the load sensors.  These are just temporary blocks, to let me test the circuit. The real blocks will (somehow) hold the load sensors in place and keep them from slipping from side to side.

The load sensor support blocks
The load sensor support blocks
Cutting slots with a scrollsaw
Cutting slots with a scrollsaw
The slot lets the sensor's "T" bar move
The slot lets the sensor’s “T” bar move

I chose the thickness of the blocks to make sure that the plywood top wouldn’t rest against or crush the electronics. See how the cross-section of the load sensor support block is taller than the circuitry.

The load sensor support block keeps the top plywood circle from crushing the electronics
The load sensor support block keeps the top plywood circle from crushing the electronics

I then cut out the top plywood circle. This circle will lay on top of the four load sensors.

The top circle of the scale
The top circle of the scale

For good measure, I placed Pippa’s bed on the top piece of plywood.  It’s a good fit.  Yes, that’s an Encyclopedia Britannica and a VCR in the cabinet…Pippa’s a Retro Girl (seriously, what can you do with old encyclopedias? I can’t bring myself to cut them up for papier mache).

Test-fitting the dog bed to the top plywood circle
Test-fitting the dog bed to the top plywood circle

I then assembled the whole thing and started testing.  Pippa helped.

The assembled base of the weight scale
The assembled base of the weight scale
Pippa resting on the assembled scale
Pippa resting on the assembled scale

In my next post, I get to try out the circuit and try to weigh some standard weights.