Paper Plots: You Don’t Need an NC Machine

Polycase (the guys I bought my enclosures from) advertises their epic machining capabilities on their website. They sent me a sample free of charge and I was quite impressed. So before I ordered my enclosures, I used their quote wizard to see what it would cost to machine the enclosure and the mounting panel.

The numbers I got back were astronomical, partly because I was only buying two enclosures, both with different cutouts. The cost shoots up fast for each additional side of the enclosure you need to machine. I had four sides I needed machined on one box, so the quote came back in the neighborhood of $200. For $200, I can afford to drill my own holes and make my own cutouts.

But how do you make sure you drill everything in the right place? The answer: paper plots!

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The paper plot for the power electronics enclosure mounting panel.

If you drew your parts up in a CAD program and made them dimensionally accurate, you can make a paper plot in no time flat. Even if you didn’t, you can draw up something in a 2D CAD program and make one using a similar process:

  1. Make a drawing of your part that contains the features you need to cut.
  2. Include a feature of known length. In my paper plot I put a scale in the corner, but you could use a feature on the part if you know it’s length.
  3. Scale the drawing 1:1.
  4. Print it off.
  5. Physically measure the feature mentioned in (2) and ensure it measures accurately.
  6. Trim the paper plot and tape it to your part, positioning it appropriately.
  7. Use the paper plot to establish your hole locations!

It’s really as simple as that. I held the panel piece over my paper plot and aligned the edges with the lines on the paper. I recommend lots of tape because you don’t want things moving, especially after you’ve drilled a few holes.

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Use lots of tape: you don’t want the paper plot moving after you’ve drilled half your holes!

I recommend overlaying your parts with holes on the paper plot to make sure they actually align the way you expect. The DXF file for the current sensor mount did not align with the actual part I received. Measure twice, cut once is good advice to live by.

Pro-tip: add cross hairs on every hole feature you plan on drilling. This will give you something to align your drill bit to. Another good idea is to actually put a diameter dimension on your holes, so you use the right drill bit size. I had to go back and check my CAD model to see what size I needed.

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The paper plot for one of the enclosure sides.

You can use this process for just about anything. I also made paper plots for the enclosure itself. I could have used a paper plot to drill holes into the wheel chair frame, but if you have the mating part on hand, it’s better to use that, especially for items whose fit is critical.

Machinists: I’m Sorry

I have a confession to make. I’ve been making the machinists I work with drill some large hole diameters through some pretty thick steel. I didn’t think it was a big deal at the time, but I now know better. It’s kind of painful. Literally.

For mounting the power electronics box to the wheel chair chassis, I had to drill two 0.25in diameter holes through a piece of 1.5in X 1.5in steel tube. No big deal, I’ll just start out with a 0.125in drill bit and go from there.

Generally that wouldn’t be too bad of an idea, if it was actually a steel tube. The ends were plugged with some kind of threaded stud to mount the foot rest for the wheel chair. As I started drilling through the tube I quickly realized that this was actually a solid piece of bar stock.

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The first 0.125in diameter pilot hole. It only took 30 minutes!

To make matters worse, I really didn’t have any sharp drill bits, just some old ones I bought at a garage sale years ago. It helps to have sharp bits if you’re going to drill through 1.5in thick steel.

I clamped the mounting plate to the chassis and started my first pilot hole using the plate as a template. Once I had a nice dimple in the frame I removed the plate and kept drilling.

About 5 minutes in my arms were killing me. The wheelchair is pretty heavy, but I still had to brace it with my left arm to keep it from rolling off the blocks that raised it to where I could get the drill on it. And my right arm started aching from pushing the drill into the wheel chair.

I knew I should drill slowly: too many RPMs and you will heat up the drill bit and dull the cutting edge. But even going slowly I wasn’t making much progress. I tried not to apply too much pressure to the drill either, but without it, it wouldn’t make any chips at all.

I did some reading and discovered that using some cutting fluid can allow you to drill at higher RPMs, so I borrowed some non-stick vegetable oil spray from the pantry and sprayed the bit and the hole with it. That seemed to help, but not much.

In the end it took about an hour for each of the two holes, starting with a 0.125in bit, moving to a 0.1875in bit and finishing with a 0.25in bit. I can’t tell you how satisfying it felt when the drill lurched forward as it broke through the back side of the bar stock.

So machinists everywhere: I’m sorry for all the holes I’ve made you drill in steel plate. I will at least have a very good reason the next time I ask you to do it. I understand (in a small way) your pain.