Re-tapping Flanges
Why perform a re-tapping?
Proper bolt hole operation is required to tighten bolts down on flange pieces to achieve high vacuum. Bolt holes need to re-tapped if the threads are mangled and misaligned. For example, cross-threading bolt hole will require a re-tapping. If the threads are severely misaligned, re-tapping may only make the problem worse. If this is the case, the threads should probably be drilled out and replaced with a helicoil.
Supplies and set-up
Partner
Tapping bit
Tapping fluid
IPA
Cloth kimwipes
Clamps
Spare bolt that fits the holes (used for testing the hole after completing tapping)
Any needed conflat tools to remove bolts on the item to be retapped, e.g. 5/16" wrench
Small Allen key
Clean foil
Spare copper gasket for your flange size
Perform the re-tapping in the machine shop on an open flat surface by a table edge, because lots of oil will be used that should not be used in the cleanroom. Secure the tapping bit in a hand-powered bit holder, e.g. bit spindle.
Procedure
Open up a piece of clean foil, clean side facing up. Place it nearby.
Unbolt any conflat pieces that are covering the holes to be re-tapped, e.g. a blank.
Place any removed conflat pieces on the clean foil in an appropriate orientation to keep them clean.
Identify the holes to be re-tapped, if not already, using the spare bolt
Clamp the item to be re-tapped to a flat surface. Make sure it is secure
Coat the bit with tapping fluid.
With your partner holding the item to be tapped, slowly start to use the bit to drill into a target flange hole
Make sure the bit seeds, otherwise the bit will start to cut out already good grooves from the hole!
When you encounter resistance, slowly and gently screw the bit forward and backward in the hole
Every minute or so of drilling, back out the bit and remove any debris that is collected on the bit using a cloth kimwipe
Cover the small Allen key with a cloth Kimwipe. Reach into the hole and remove any debris build up.
Repeat steps 7 - 11 until the bottom of the hole is reached
Thoroughly clean out the hole with the small Allen key + Kimwipe
Use the spare bolt to test the re-tapped hole. Make sure the bolt can bolt all the way to the bottom
Unclamp the re-tapped item from the table
Using a cloth kimwipe and IPA, clean off the bolt holes and knife edge
Add the new copper gasket and bolt the flange covering conflat piece, e.g. blank, using holes that were not re-tapped
Return the re-tapped assembly to the cleanroom.
Cleaning after re-tapping
Perform in the the cleanroom using a cloth kimwipes. First, clean the the bolt holes using acetone followed by IPA. Then, clean the knife edge with acetone followed by IPA.