How to Guide Taping a Hole

From GT RoboJackets

Taping a hole is essentially adding matching threads to a hole so that it can retain a bolt. These holes must be smaller than through holes for bolts.

Hole Setup

Decide what size threads you want in the hole. A 1/4" bolt uses either 20 or 28 threads per inch. (Most of our stuff is 1/4-20)

NOTE: The hole size you drill MUST be smaller than the bolt diameter and approximately equal to the minimum thread diameter.

Bolt sizes commonly used: (Bolt diameter - Threads per inch)

4-40

6-32

8-32

10-32

1/4-20

1/4-28

5/16-18

3/8-16

1/2-13

Go to http://www.shender4.com/thread_chart.htm (this chart is also hung on the wall by the door of the shop).

Making the Hole

Cross reference your bolt size - threads per inch to the column labeled "75% thread" for aluminum or plastic

1. The drill size listed there is the exact size hole you need to drill .
2. Get the drill index with different sized number drills.
3. Chuck the drill in the drill press.
4. DO NOT use a hand drill unless you use a TiN (Gold colored) drill bit.
5. DO NOT use the good drill bits in the hand drill.
6. Fixture your part and drill the hole.

Tapping

Now you need to actually cut the threads:

1. Get the appropriate tap from the tap drawer.
2. Tap sizes are labeled in stamping on each tap.
3. Get a tap handle (T shaped) that fits on the square shank. DO NOT use a wrench
4. Line the tap up in the hole and slowly screw it in (clockwise).
5. Once the tap has cut into the hole about 3 full turns, back it out 1/2 turn.
6. Add cutting oil to the tap and inside of hole. Cutting oil is in the small black cans found around the shop.
7. You'll feel the chip break off inside the hole. Once this happens it is safe to continue.
8. Every 3-4 full turns, back the tap out 1/2 turn to break the chip.
9. If you don't break off the chip, the cut material will gaul and bind up inside the tap cutting edges and ruin it, or even break it.
10. BLIND HOLES: Cut the threads as deep as you need to or until the tap bottoms out on the hole floor. You should be able to feel the sudden increase in resistance.
11. THROUGH HOLES: Cut the threads with the tap until the end of the tap has cleared the end of the hole by about 1/4".
12. Once the cut is completed, remove the tap and blow out the hole.
13. CLEAN THE TAP!!! clean all your shavings and chips out of the tap flutes and put it back in its proper spot.

Enjoy your newly tapped hole. Run a bolt in and out of it a few times to make sure the threads are good.