I recently bought a laser cutter (running on a Pi) and started working with acrylic (Perspex). I’ve sawn, routed and drilled Perspex before. It machines beautifully as long as you’re not too ‘greedy’ with your cuts (and if you’re too greedy, it cracks). In exploring the material, I wanted to investigate thread tapping as I’d heard it was possible and it opens up new design options.
So I watched a few videos about thread tapping. Some people use a CNC machine on an industrial scale, but others use a small hand tapping set in the same way you’d tap a thread in metal. Some pearls of wisdom which most of them mention are…
- Threading works best in cast acrylic rather than extruded
- Lubrication is a good idea
- Coarse thread is preferred over fine
I looked for a set of hand taps I thought I had, but I couldn’t find them. (I vaguely remember a failed experiment some 25+ years ago on a car repair – perhaps I got rid of them? Or maybe they’re just sitting in a drawer somewhere laughing at me?)
Bit Size is Key
The key to successful thread tapping, which I’d never actually achieved to-date, is to get the right size of tapping drill. This was why my experiment had failed all those years ago. I didn’t have the right bit size.
So I had a look at Amazon and ebay and something new, shiny and interesting caught my eye. Spiral tapping bits that fit in a drill/driver…
Countersink Tap Drill Bit Set
These are advertised as “HSS Countersink Tap Drill Bit Set, Combination Hex Shank Titanium Screw Thread Metric Tap Drill for Wood Plastic Aluminum Copper, NOT for Hard Metal / Iron Casting”
So the way they’re ‘billed’ is that you can drill, thread and counter-sink all in one (variable speed) operation. That sounds very attractive.
They looked interesting to me because I could easily put them in my cordless drill driver (running very slowly) and use that instead of an agonisingly tedious hand tap. Counter-sink might be useful if using thick enough acrylic and counter-sunk bolts.
There’s also a small note at the bottom of the listing that says…
☞1.Please add some oil before operate the tap, so the operation is smoother, and also to extend the life of the tap.
☞2.Application: High Speed Steel 4241 is suitable for wood, aluminum, iron and other lower hardness material, the material thickness is 3-5mm. Caution if you used it with hard metal!”
What do the Reviews Say?
As you do, I looked at the reviews before taking the plunge. On some listings, people were saying that they break. Others were saying that they’re good on soft materials. Well Perspex is pretty soft compared to steel, so I figured “six tap sizes for a tenner – let’s see how long they last – and hit the order button.”
On the reviews stating that “they broke’ it seemed like people were using them to drill their own holes before threading. To be fair, they are marketed that way, so should work that way. But since everything I’m likely to need to thread will already have been laser cut, it seems sensible to reduce the load on the tap by laser cutting the holes.
Tapping Drill Sizes
When they arrived I decided to test them all. So I looked up the tapping drill sizes needed. I wasn’t going to drill the holes – I’ve got a laser cutter – but I needed to know the size to make the holes for my test piece. Searching Google on “tapping drill sizes” sent me to a suitable page…
…and armed with this information I knocked up a quick design for a test piece…
The vendor suggests adding oil before cutting a thread. In one of the reviews, someone mentioned a squirt of WD40 on the bit for lubrication. I tried this and it works a treat.
I played with a simple test-piece and the M3 bit first. The seven holes here all tapped perfectly well. You have to be somewhat careful about alignment, but the tapping itself is fairly easy to do.
Encouraged by this, I tried each of the different taps on my test plate. This was the outcome…