Tapping 1/4" threads

[Dave_424]

I need to tap a 1/4" thread in the end of my hybrid chamber. The correct drill size is 11.8mm but am wondering which drill bit would be best to use: 11.5mm or 12mm

I could use the 11.5mm and sand out the hole a bit t make it slightly larger

What do you think is best

P.S. the chamber is steel

Dave

[jeepkahn]

If it's 1/4" npt, the standard drill size is 7/16"... Says so right on the tap...

[jrrdw]

11.5 is .5 to big. 7/16ths is the same as 11mm.

[inonickname]

Agreed. 11.5 will probably work fine if you have a sharp tap, if not you can always ream it out a bit.

[Dave_424]

Thanks guys, it is a BSP tap BTW

I will go with the 11.5, use lots of cutting fluid and go slowly

Dave

[inonickname]

1/4" 19 0.518

I think you'll be right with the 11.5 in BSP, but you could try a scrap piece of material to test?

[Dave_424]

Yeah i should really try it on some scrap, just so I don't end up breaking my new tap

Dave

[inonickname]

If you have an engineering plastic like delrin then try that. Brass is an easy one, you could also try aluminum as it is a bit soft. Remember the 1/2 forward, 1/4 turn back (or anything similar) otherwise you'll have aluminum stuff your tap. (Probably plastics also..)