How to Drill & Tap for a 1/4-20 Bolt

by C.L. Rease

Drilling and tapping a hole in thicker gauge metal is a fundamental part of fabricating metal. Using the optimal drill bit is imperative in producing a functional and strong final tapped hole. If the drilled hole is too small, the bolt will be hard to thread. This can result in damaged threads on the bolt. A hole that is too large will result in a loose bolt, which will not have 100 percent of its structural strength when fully tightened. For a 1/4-20 bolt, the optimal drill bit size is #7 or 13/64th.

Step 1

Measure the location needed for the 1/4-20 tapped hole. Locate the hole with the tape measure, and mark the holes location with the scribe.

Step 2

Set the point of the center punch on the location of the hole to be tapped. Hit the back of the center punch with the hammer to make a depression of the hole location. This will keep the drill bit from wandering away from the hole's center.

Step 3

Insert the drill bit into the drill. Tighten the drill's chuck to secure the drill bit. Liberally apply cutting fluid to the drill bit. Place the point of the drill bit onto the center mark.

Step 4

Apply pressure to the drill. Begin drilling the hole, stop and apply additional cutting fluid to the drill bit to keep the drill bit and metal cool.

Step 5

Insert the 1/4-20 tap into the tee handle. Thoroughly secure the tap within the tee handle. If the tap comes loose from the tee handle there is a chance that the tap will break during the threading of the metal. Apply cutting fluid to the tap. Hold the tee handle level and start the tap in the hole. Turn the tee handle clockwise to thread the tap into the hole. If the tap binds in the hole, turn the tee handle 1/4 of a turn counter-clockwise to free the tap. Continue to turn the tap into the hole. Run the tap through the hole until the tap bottoms out. Turn the tap counter-clockwise to remove the tap.

Step 6

Attach the flapper wheel to the grinder. With the grinder, remove the burr from both sides of the tapped hole.

Step 7

Thread a 1/4-20 bolt into the hole to ensure that the hole is threaded properly. If the bolt will not thread into the hole, clean the tap, apply cutting fluid to the tap and the threaded hole. Re-tap the hole to clean the threads.

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