Use a Tap
A tap cuts threads on the inside of a hole, to accept a machine screw or bolt. Taps can also be used to restore an internal thread that has been damaged or to cut a new, larger thread in the case of severe damage or stripping.
Steps
- Determine the size and depth of the thread that you want. What size screw will go into this hole?
- Refer to a technical publication for the exact measurements for hole size. For example, if you want to use a 1/4-20 screw then you must use a #7 drill bit. Your set of taps may also state the appropriate hole size to use with each tap.
- Drill the hole to the correct diameter and depth. Remember that taps generally cannot thread all the way to the bottom of a blind hole, so a blind hole must be a bit deeper than the required thread depth.
- Secure the workpiece firmly.
- Apply an appropriate lubricant such as oil, WD-40, or a proprietary formula.
- WD-40 is not any good as a lubricant on anything other than aluminum because it is just Kerosene (with the CAS of aviation grade fuel oil).
- Never lube cast iron unless you can flood it with a water based coolant which is just for chip flushing. Otherwise use compressed air.
- If the tap set has a separate handle or rod, insert it.
- Hold the tap in line with the hole and turn it clockwise. Try to keep it very straight as you turn, this is important. You may feel the material breaking.
- Every few turns, back the tap out (counterclockwise/anticlockwise) to clear away chips of loose material. This is especially important in a blind hole, one that does not go all the way through the workpiece.
- When the tap has reached the required depth, back it out, clean out chips and any cutting fluid or lubricant, and try your screw or bolt in it.
Tips
- Tapping operations create chips. Mask or cover any parts of your workpiece or surroundings that would be damaged by chips, or plan to clean them off when you are done.
- Small taps are extremely easy to snap. It takes about 4 pounds of force to break a 6-32 tap, so be very careful using small taps.
- Larger taps, such as a 3/4-10, remove too much material to be used by hand unless you are very strong.
- For very thin material, it may be better to use a nut on the other side. Other options include press-in nuts designed for sheet metal and threaded inserts for softer materials, such as plastics. A good rule of thumb for metals is that at least three full threads should be in the material.
- Take care not to change the angle of the tap, especially at the beginning. You only want to cut one thread.
- The tool for cutting the mating, external thread is called a die.
- In ductile materials the best tap is often a thread forming tap not a cutting one.
Warnings
- Taps are made of hardened steel, which provides long life and a sharp cutting surface; however, this also makes them brittle. Taps are expensive and the workpiece may be priceless. Always use lubrication and great care when performing tapping operations.
- For load-bearing threads, always verify that the threads, fasteners, and materials are adequate for the job.
- On very tough materials such as certain grades of SST you may need to decide between a very difficult job or a thread class less than the usual 60% 2B and drill the tap hole slightly larger.
- the tap should be perpendicular to the workpiece.
Things You'll Need
- Workpiece.
- Lubricant.
- Tap drill.
- Tap.
- Vise or clamp.
- Chart of screw thread sizes.
- Machinist square or a Try square (To check the tap is cutting at right angle to the workpiece face)
