madswitcher
Well-known member
ruffrecords said:
Hi again Ian,
As Gene has mentioned below, there are mainly three types of taps for a given thread size.
So imagine you have a piece of metal you want to drill a hole in and thread it
To start with, you have to drill a hole that is smaller that the thread size. IIRC for 5mm, the hole has to be 4mm. There is a standard chart for the size of the tapping drill.
You then have to start the thread so you use a Taper (or No 1) tap where the thread on the tap gradually begins and engages with the metal. This allows you to engage the tap in the hole and start the threading process. The taper tap (top tap in the photo below the tap wrench) will in most cases self align to the hole. As I said above, I always use a tap wrench as it allows you to align the tap with the hole properly and give you more torque. Use plenty of 3 in 1 oil and apply the "Turn clockwise one full turn, turn back half a turn" until you reach bottom of the hole or length of the tap.
You can then use a No 2 tap (second tap down in the photo) that has the thread going further down the tap shank - to get further down the hole. In most cases I don't use a No 2.
A Plug (No 3) tap enables you to tap all the way to the bottom of the hole, it is threaded down to the bottom of the tap shank and has a flat bottom to it.
For your application of taping an extrusion, it depends how deep you need to go in - i.e. length of the screw and thickness of the sheet you are attaching to it. I would have thought that a No1 followed by No 2 would suffice.
Hope this helps
Mike
