a Steel Box Section Build with SBR20 & Ballscrews Plus a few questions

[Neale]

The next step is to start drilling my holes for the Threads. I have chosen M5 allen screws and I have ordered a cobalt 4.2mm drill for the threads.
I just need to experiment getting the hole exactly centred for the thread holes to be as accurate as possible.

Couple of tricks that you might not have come across. One is to start the hole using a clearance drill that nicely fits the hole in the mating part. Just drill lightly to give a centre mark then switch to the tapping size drill which, with a bit of luck, will follow the first mark. That doesn't guarantee that the hole is square to the work, though. I use a small steel block (turned from an odd scrap of steel) with a projection on one end that fits the clearance hole, and a hole up the centre that takes the tapping size drill. That helps a lot when you are using a hand-held drill in awkward places. Don't know if you have the facilities to make something like this (very easy lathe job) or perhaps know someone who can help? I find that I can go through in one pass with the 4.2mm drill, hand-held, but anything much bigger than that can do with a smaller pilot hole first (much easier going through steel) and then open up with the correct drill.

One other tip, if you're not doing it already - get the right taps! For through holes in steel, use a spiral-point tap. For blind holes, use a spiral-flute tap. Avoid using conventional hand taps (straight flutes) if you can. They are hard work and you find yourself going in a half-turn, then backing off to release the chips, then every so often bringing it right out to clear the flutes. The spiral-form taps are designed to push the swarf in front or bring it up the flutes, and you can tap in one pass without backing off all the time. They're cheap enough from China (I recently bought a couple each of M4/M5/M6 plus one M8, and the whole lot, including postage, was just over a tenner). Well worth while.