Tapping the extrusion
The next issue I faced was tapping the extrusion. A lot of my issues stem from having a tap holder made of monkey metal and a tap that was too abrupt in the start thread.
I drilled a 5mm hole in a scrap of wood exactly 10mm from the edge and clamped this to the table this meant I could start tapping with the extrusion pretty much dead on straight. After winding it in a little way and backing it out again I could tap the rest holding the tap and extrusion in my hands.
- Spray some cutting fluid in the centre of the extrusion and onto the tap
- Start the tap - I did about 20 half turns (until you can feel the pressure start to increase)
- Wind it back out (give a little back and forth motion to release the pressure)
- Clean the tap with a brush and the internal thread (I used a compressor)
- Wind the tap back in until it starts to bite and then carry on by about 10 half turns
- Goto 3. until you get the appropriate depth - I'm using M5x16 button head screws to hold the extrusion
I was having great difficulty using the black tap in the photo, you can see the cutting thread starts very abruptly and it's not coated. Using a ratchet tap holder is excellent.
Just a block of wood and holding the extrusion at roughly 90degrees, I was clamping it but I found holding the extrusion while winding the tap was easy enough. When the extrusion started to get harder to hold as the tap was biting hard I knew it was time to wind it back out.
