Nothing much to add to the advice above. All really good points. I would start with a (non)metalugical comment that generic aluminium, while soft, is a most tenaceous material. Over the years, I have tapped for a great number of middlin' sized screws, #8, 3/16(#10), 1/4, and the like in field service conditions. And have broken a goodly number of taps where the Al strings up and seizes the tap so it wouldn't go deeper and wouldn't back out. I'm sure there is a technical reason for such a high failure rate. But in the field at 3AM, no one is interested in technical stuff, just in finishing the job and getting back to the coffee pot.
Lubrication is vital. As is starting the tap true to the hole. Aluminium has some interesting charactertics. Some is hard, like engine blocks, some is "soft" like aircraft skins. All of it is stringey and hard to tap. There is a type of "modern" buss bars that have an aluminium core clad with copper. The relative electrical characteristics are moot here, but for reliability in industrial environments, we developed a system of drilling through and fastening with bolts and washers. Electrically, we used copper to copper, with steel fasteners (grade 5) for stability. We found that tapping aluminium, even more pitches than recommended, would loosen as temperatures varied, eventually getting very loose.
From the view of your part, I would think that you wouldn't want to tap all the thickness shown. For a 3mm screw, essentially 1/8 inch, a 3/16 long tapped hole would be more than sufficient. The metric screw is very close to a 5-40. For a non-critical part mounting, I would drill the next larger number size drill for a 50% thread and overbore the rest.
That is my opinion, which is like an a$$#o1e, everybody has one and they all most likely stink. But I'm feeling verbose this morning and wanted to add some non-machinist perspective to the situation.
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www.msdiscounttool.com
and far worse than doing it by hand. One of my most regretable purchases. Might try to fix them someday, but it would be like polishing a turd.
