Sep 2011: Circuit board Assembly Jig - John D. Collier

[VernGraner]

If you have questions and comments about the article "Build a Circuit Board Assembly Jig" by John D. Collier please post them here.

[BobCochran13]

Hi, I want to build this jig and plan to place an order with McMaster-Carr this weekend for the parts. I don't have access to a metal milling machine, or expertise with this, but I can probably find a service to do this in my area. Are there any tips for finding a good milling company? Also, are the diagrams that are in the article sufficient for a milling person to do the job right?

Many thanks!

Bob Cochran
Greenbelt, Maryland

[JohnCollier]

[What follows is part of an email response I sent to Bob.]

To answer your main question, yes the text and diagrams should be detailed enough to fabricate the entire jig. In my experience, most machine shops aren't interested in small jobs like this. However, if you have a recommendation for a particular shop then it might be worth pursuing. If you do plan on working with a shop to make the individual pieces, I'd suggest providing a list of specific instructions rather than expecting them to follow the article. For example, "Cut (2) pcs of extrusion 10" long, tap both ends for 1/4"-20, 1/2" deep." They might also be able to supply most of the materials for less than you could purchase them. If they ask about tolerances, typical values for this kind of work are +/-0.005" for decimal dimensions and +/-1/64" for fractional. Be sure to mention that everything is aluminum--compared to steel, it's much easier to cut and machine.

One component that requires some forethought is the notched board support rail. I can think of three ways this could be done:
1. Fabricate it the way I did, as shown in Figure 4. By sandwiching the two layers together, the only milling required is the 0.060"x0.075" notch on the one side. This is a 10 minute job for even a rookie machinist. The two layers do need to be screwed together and I did not provide specific dimensions for these holes in the schematic. You can just space several #6 screws across the 9-7/8" length (tapped holes in one layer and countersunk clearance holes in the other).
2. Use the alternate design shown in Figure 4. By using three layers of material and offsetting the center layer, the notch can be created without any milling. I provided this option in the article mainly for those who do not have access to machine tools or a machine shop.
3. Fabricate the entire support rail from one solid piece of 1/4" x 1/2" material. This would provide the most professional result but requires the most machining. The ends would need to be milled down to 1/8" thick, as Figure 4 indicates, and the notch would have to be cut using a slitting saw or very small endmill. Still very doable by an amateur with the right equipment but would require more time.

A few other thoughts:

Most hardware stores have a section with various sizes of aluminum and steel bar stock. McMaster also carries this material (with better selection and pricing) but you typically have to buy long pieces which increases shipping cost. Regarding fasteners, you will need #8-32 button head for the toggle clamps, #6-32 flat head for connecting the support rail layers together (if you choose to fabricate them this way), 1/4"-20 flat head for attaching the side supports, and 1/4"-20 nuts and washers for the feet. Again, McMaster is a good source but you typically have to buy boxes of 50 or 100.

If you haven't used thread taps before, don't be afraid to try it. With a reversing power drill and gun tap, it's quite easy. They key is to run the tap in slow and as straight as possible. Be sure to lubricate the tap with cutting fluid or a light machine oil. The McMaster taps I listed are pricey but they are premium quality. You could probably find cheaper ones at the hardware store.

I bought my toggle clamps from an eBay listing and it is still running (item# 350254354002). The cost is about half of what you would pay elsewhere. I did not provide dimensions for the location of the clamps, so you'll have to put them in place on the extrusion and mark the holes. If you're going to have a shop drill and tap the holes, I'd suggest punching the marks with a center punch to make sure they get drilled in the right place.

Note that the assembly drawing in Figure 2 has a slight error. The width of the jig is shown as 7". When I built mine, I made it 8" wide. If you choose to make it 8" wide, you'll also have to increase the size of the compression backer plate in Figure 6.

[BobCochran13]

Hi John,

I want to thank you for all the help you have given me over the weekend. I really appreciate it.

Bob