C3 Mini-Lathe Metalworking Beginners
Tapping and metalworking engineering taps
Mini-lathes are very good at cutting threads. Using the ability of the lathe spindle to drive the leadscrew we can keep cutting many different internal and external threads so long as we think of new ways to arrange the change gears. However, what happens when we want an internal thread in a job to take an M5 bolt? The threading capacity of the lathe will not be much good to us here, unless we have a very very small internal threading tool and a very large amount of time on our hands.
For small holes, up to about a diameter of 16mm, it is advisable to make an internal thread using a metalworking tap. A tap is a internal threading tool made from a peice of hardened steel specifically for making screw threads.
There are supposed to be three types of tap:
When starting with a lump of metal, we do not use the tap to make the entire hole, and the thread. No. We first use a standard drill to make a hole, and then use to tap to thread that hole. What size should the initial hole be? The table below shows the common sizes - it is often useful to keep a stock of drill bits with the taps - to avoid wasting time trying to find the correct drill bit and matching tap. It is also possible to buy sets of drills and taps of the correct sizes. A 2mm drill bit does not have a great life expectancy, so buy several drill and taps!
Here is a selection of the drill sizes that go with common tap sizes.
Tapping by hand
If you are "tapping a hole" ie using a tap to cut a thread into a drilled hole by hand, it is normal to hold the tap in a tap wrench. These are available virtually for free on places like ebay. You twist the tap into the hole, and, every couple of turns in, you make a half-turn backwards. This helps break up the chips that the tap cutting edge are making.
It is important to use a cutting fluid with the taps. When cutting aluminium, use kerosense or WD40. When cutting steel use some oil.
It is very easy, especially with thinner taps, to break the tap. If the tap snaps off in your hole, then you are in trouble. Always make sure sure keep the tap properly lined up with the hole being tapped. This is difficult to do by hand.
Tapping by micro mill
This process is not normally recommended in the text books, but I find it works well in holes up to about M6, after which, the micro mill runs out of power. It is possible to use the rotational action of the micro mill to turn the tap and tap the hole. Of course, the tap must be drawn into the hole as the thread is cut, so you must be careful.
My procedure is roughly as follows:
All images and articles copyright www.mini-lathe.org.uk
Hints and tips