Drilling and tapping heatsinks went well

I thought I’d post some experiences related to drilling and tapping heatsinks and the like. I’ve never done this before and was a bit nervous but found it as satisfying as any other part of HiFi DIY. You may have additional experiences that would help others here. So…

TLDR:

Tap choice: Presto makes some good quality cheap taps, but Osborn’s ‘plug’ or ‘bottoming’ tap cuts threads closer to the end, so best to finish with that rather than Presto.

Thread Cutting: make sure to use some cutting fluid for cutting threads or drilling larger holes. Screwfix has some decent stuff in an aerosol can. For a small M3 or M4 tap, one could spray a few ml into a bottle cap and then apply using a dropper.

Video: This video worked a treat for me in using a cordless drill/driver to do the tapping.

The longer:

Modushop’s Dissipante chassis has 40mm heatsinks with 0.30” (7.62mm) thick base. The Modulus 686 board and LM3886 chips are fixed to a 0.10” (2.54mm) aluminium sink mount, making the total 10.16mm. So, the sink fixing screws could be M4 x 6mm, or 8mm.

M4 x 8mm seemed to me likely to be a bit more secure, but that leaves 2mm+ of clearance without drilling clear through the base of the heatsink. One has to remember that a 3.2mm drill used for an M4 tapped hole has 1mm of chamfer between the tip and the full width cutting diameter, so that is waste. I’ve also not found a bottoming tap that goes right to the end, so there is more waste there.

Presto makes some nice quality machine taps sold individually or in sets of three for not much money and likely a good bit better than the numerous no name taps on the Bay. However, the Presto bottoming tap (‘Plug’ tap) has 2mm of clearance before it cuts full threads. If one adds that 2mm waste to the 1mm waste of the drill bit, we now have 3mm of waste when drilling and tapping a blind hole into the 10.18mm heatsink assembly… which means 8mm screws won’t work.

Fortunately, Osborn makes a fairly good quality bottoming tap at a decent price with only 1mm of clearance. So, drilling and then using an Osborne for the final bottoming tap laves only 2mm of waste which will allow an 8mm screw if one carefully sets the drilling depth stop on a bench drill.

For drilling I borrowed a mate’s bench drill and set the depth stop to be 7mm from the heatsink surface to the final drill depth. A little testing got me right to the depth I wanted without breaking through the aluminium base.

A digital tyre tread depth measuring tool with steel pin is fairly cheap and should do accurate measuring into a hole. One could use a vernier caliper but most of the current digital ones have a depth pin too wide to fit in a 3mm hole.

As mentioned in the summary, this video gives simple instructions for using a cordless drill/driver to do hold the taps and worked well for me.

I also bought some aluminium square bar stock (10mm) to secure the bottom cover to the back cover. The 3mm covers didn’t really show needing it even for an 850VA transformer but it made me feel better.

Since I did those first and hadn’t yet learned the above, I was not getting the blind hole depth I wanted for the screws, so I found it easier to drill straight through the square stock. Lining things up for drilling and later assembly is another story that went fairly well but slightly off topic for this overly long initial post.