It sounds like you are running hot rolled steel or A36 which is sometimes horrible stuff to get a good thread in. A sulfurized tapping fluid might be worth trying but if you are using water based coolant try taking the concentration way up; such as if your normal mix is 5% go to 25%. I have done this for large fine pitch tapping with success. You can always drain some of the rich mix out and dilute again when the job is done. Another thing is take the speed up not down. I have found often faster gives a better thread and have run 1/4"-NPT at 1000 rpm in C1018. Peck tapping if your machine can do it will also help. |