View Full Version : tapping thread

08-08-2005, 07:14 PM
I am trying to tap some 12-24 threads in a pc of 3/8 inch stainless steel which will hold a water spigot on the outside of my house. I do not know the metalurgy of it. I was able to drill, turn and bore this pc on the lathe without dificulty. But when came time to do the tapping , it proved very difficult. I started with the recommended no 16 drill. The tap would take a turn or two , then freeze so hard , I was afraid to turn anymore in fear of breaking the tap. So I tried to enlarge the hole with a bigger drill bit. The tap still bound tightly. Then I thought the tap maybe too dull, so I bought some Cleveland spiral taps. The tap still bound. I did finally tap one hole thru by advancing the tap carefully a fraction of an inch at a time and then backing off as I usually do. But I still have more holes to tap.
Any suggestions ? How big can I drill the hole and still get a good holding thread/

08-08-2005, 07:36 PM

Rich Carlstedt
08-08-2005, 07:48 PM
Stainless is normally tapped with a 50 to 55 % thread, so you are too snug for a tap if you use the 70 % drill call on charts.
You can read more about this in Machinery's Handbook

08-09-2005, 03:17 PM
<font face="Verdana, Arial" size="2">Lubrication?</font>


08-09-2005, 03:24 PM
Seal Oil!!

Lynn Standish
08-09-2005, 04:45 PM
Are you trying to tap with a bottoming tap or similar by any chance?

08-09-2005, 10:15 PM
The problem is the drill you are using, SS work hardens very easily. A thin web carbide drill with minimal edge prep. Flood coolant (thicker than normal) or you can use oil to drill & push that drill thru. Don't let the drill or the SS heat up Also a taper tap with alot of oil while tapping. Good luck.

08-11-2005, 05:47 PM
Wish to thank all of you who contributed to my inquiry. I did resharpen my drill bit to 135 degrees and went for a 50 percent thread. I bought some RapidTap and to my surprise, all went well. Thread cut nicely. Piece of cake. Just like in the movies.
Thanks again.

08-11-2005, 06:04 PM