Design Con 2015

A do-it-yourself instrument cart

-September 25, 2013

When I want to use an oscilloscope, signal generator, and other instruments, I must rearrange the "clutter" on my lab table to make space. I'd rather put my instruments on a cart and place them near the table only when I need them. eBay listed several used Tektronix scope carts, but most cost several hundred dollars, and shipping would have cost about the same. I also looked at new general-purpose lab carts, but they also cost too much. Instead of buying a cart, I built one. Even if I had bought all-new materials, the cost wouldn't have broken my lab budget.

A cart would need mobility so I visited Harbor Freight Tools and bought four inexpensive 3½-in.-diameter non-locking casters. I have an industrial-type carpet in my work area, so the casters will not roll once I position the cart with the instruments on it. The casters attach to the bottom shelf with ¼-in. flat-head machine screws, four per caster. I used flat-head hardware so nothing would protrude above the surface of the bottom shelf (Figure 1). All casters can rotate 360°.

Figure 1. Casters let me roll the cart around the lab.

I had two pieces of 11/16-in. birch-veneer plywood (30in.-by-24 in.) left from a project and used them for two shelves. I bought a 2ft.-by-4ft. piece of the same type plywood and cut it to 30in.-by-24in. so I'd have three shelves–a fixed bottom and top shelf and an intermediate adjustable shelf. The shelf dimensions would provide plenty of room for a storage scope and modular instruments I use frequently.

To support the shelves I used ½-in. iron pipe removed years ago from a natural-gas line. Four 28-in. lengths of pipe would do the job. Instead of mounting threaded pipe flanges (they would cost too much and look clunky) on the top and bottom shelves, I opted to use 4-in. lengths of 2-by-4, with four underneath the top shelf and four underneath the bottom shelf. The pipe would go into holes drilled in these 2-by-4 supports. Instead of iron pipe, you could use heavy-wall or intermediate metal conduit. Figure 2 shows my master-template block (right) used to position a center hole for a pipe and the screw holes for the supports (left) underneath the bottom shelf. Note the holes already drilled in the shelf for one caster.

Figure 2. A master template block let me align holes.

I drilled completely through the center of the four 2-by-4 top-shelf supports with a 7/8-in. speed bit that cut clean holes. For the supports under under the bottom shelf I drilled similar centered holes, but left about 3/8-in. of undrilled wood at the bottom of each hole. This wood helps support the pipes and keep them in place.

To properly align holes for the pipes to pass through in the bottom and middle shelves, I created a template and marked the drill points on each shelf and drilled the 7/8-in. holes. I placed two pipe supports at the back corners of the shelves. But at the front, I offset the pipes about seven inches behind the front edge (Figure 3). I figured this offset would make it easier to move instruments.

Figure 3. For convenience of use, the front support pipes are recessed by 7in.

I used a short piece of 7/8-in.-diameter aluminum pipe to align the supports and holes for the pipe in the bottom shelf (Figure 4). The four pilot holes accommodate screws that attach–along with glue–the bottom 2-by-4 supports underneath the bottom shelf. I used flat-head brass screws and Elmer's brand Carpenter's Wood Glue.

Figure 4.The short aluminum pipe will help to align the support pipes.

Next I took four ¾-in. metal conduit splices and used a Dremel tool to grind down the interior metal stop ring in each that would otherwise properly align two pieces of conduit. Grinding away the ring let the splices move freely up or down the metal pipe (Figure 5). I placed one conduit splice on each of the four pipe sections 12 in. from one end of the pipe. Then I put a small piece of tissue or paper towel in each pipe and tamped it down with a dowel to plug one end of the pipe. Next I inserted the pipes into the hole through the bottom shelf and into the hole in the bottom 2-by-4 support. The plugged end of each pipe should go into a bottom support. Then I placed the middle shelf onto the pipes. You might need help aligning each pipe and getting it through each hold. At this point, the cart started to take shape.

Figure 5. The support pipe attaches to the shelf.

To secure the pipes in the bottom shelf, I used two-part epoxy glue applied this way: First I loosened the conduit splice on one pipe and dropped the splice by about three inches. This placement of the splice let me lift the bottom end of the pipe out of its hole, but it still passed through the middle adjustable shelf. Then I put some epoxy into the hole in the bottom shelf, re-inserted the pipe, and moved the conduit splice upward to again support the middle shelf. I repeated this process four times, once per pipe end.

To get the iron pipes properly aligned, I used six magnetic welding holders that have a 90° angle between two sides (Figure 6). I put two holders on each of three pipes and ensured the pipe had a 90° alignment with the bottom shelf.

Figure 6. Holders assure that the pipes and the shelves are at 90º angles.

Finally, I dropped the top shelf onto the pipes. I had used my template to align the 2-by-4 supports underneath this shelf, and the top ends of the four pipes went into the 2-by-4s perfectly. These supports do not bear any weight, so I used only glue to attach them underneath the top shelf.

I left the black pipe unpainted and it looks good as is. To finish the shelves, I used ½-in.-by ¾-in. strips of clear pine–glued and nailed–along the edges. Two coats of water-based stain/polyurethane finish and filler for the nail holes in the trim completed the job.

Figure 7 shows the 2-by-4 support underneath the top shelf and Figure 8 shows the 2-by-4 support and caster mounted underneath the bottom shelf. Note the 7/8-inch hole does not go all the way through the four bottom supports. The hole in the support simply forms a "cup" for the epoxy cement.

Figure 7. A 2x4 used to support the front pipes.


Figure 8. Here's a 2x4 and a caster on the rear of the cart.

Builders could add a plug strip at the back of the cart and supports for probe wires and power cords.

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