Questions On Tapping On The Lathe.

[sanddan]

I need to tap some 1/2-13UNC holes in steel. I have the 1.25" dia bar stock chucked in the 3-jaw and have drilled a 13/32" hole thru (1.25' total length of hole). I start my tap and 2-3 threads in the part is slipping in the 3-jaw chuck and get's to a point that I can't turn the tap handle. The steel is a mystery metal given to me and seemed hard when turning it. Should hand tapping 1/2-UNC be very hard to do? Should I open up the drill hole?

 

[RJSakowski]

I need to tap some 1/2-13UNC holes in steel. I have the 1.25" dia bar stock chucked in the 3-jaw and have drilled a 13/32" hole thru (1.25' total length of hole). I start my tap and 2-3 threads in the part is slipping in the 3-jaw chuck and get's to a point that I can't turn the tap handle. The steel is a mystery metal given to me and seemed hard when turning it. Should hand tapping 1/2-UNC be very hard to do? Should I open up the drill hole?

Opening the hole will make tapping easier but will also weaken the resulting thread. Depending upon your intended use, it may be acceptable. Tapping requires a lot of torque because of the amount of metal being removed. In my experience, it requires aggressive tightening of the chuck and even then, you may get slippage. Use a sharp tap. Use a taper tap rather than plug or bottoming. If you are tapping near to the bottom of a blind hole follow up with the bottoming tap afterwards. Take it slow and use tapping fluid. Back the tap out periodically, to clean the chips. I like to turn in about a half turn and back out enough to break the chips. If tapping is really hard I may only turn in a quarter to an eighth turn to allow lube to recoat the cutting edge.

I use a center in the tailstock to keep the tap running true. I will usually provide a stop for the tap wrench and rotate the chuck, running the center in at the same time. Tapping off axis increases the amount of torque necessary, gives you a poor quality tapped hole, and increases the likelihood of breaking a tap.

Bob

 

[Wreck™Wreck]

If there is no thread fit class requirement and the customer is not going to use a plug gauge to inspect the threaded holes, use a bigger hole by a few Thou., makes a world of difference in tapping.

Tapping a .406" hole 1/2-13 will be difficult under ideal conditions, I would make the hole around .414-.420 depending on the application.

 

[18w]

1/2-13 tap drill is 27/64ths. That will give you another .015" on hole size. Everything else Bob said applies as well.
Wreck Wreck just posted while typing so what he said.....27/64ths is.4219"
Darrell

 

[kd4gij]

If it seamed hardwhen turning it then taping isn't going to be easey. What did you turn it with HSS or carbide.

 

[randyc]

For the HSM, if there is no specified thread classification, rule of thumb for tap drill size is nominal thread diameter - 1/pitch

In your case: .500 - 1/13 = .423

Follow the advice that the guys have posted above - it's all good

 

[randyc]

For the HSM, if there is no specified thread classification, rule of thumb for tap drill size is nominal thread diameter - 1/pitch

In your case: .500 - 1/13 = .423

Follow the advice that the guys have posted above - it's all good

 

[Kernbigo]

looked at the besly tap chart 27/64" is the proper drill size for 1/2" 13 thread

 

[markknx]

One more thing to add to the above. Where did you get the tap? most of what you find in the big box stores are pretty useless for tapping a deep thread. They are not the best even at shallow threads. get some better taps when you can from online or local industrial supply Co. My tap chart also calls for 27/64. All else mentioned above is great advise. One more thing always use a sharp drill when drilling holes for tapping. with this being unknown mat. you could have work hardened it, if a dull bit was used.
Mark

 

[John Hasler]

looked at the besly tap chart 27/64" is the proper drill size for 1/2" 13 thread

For 75% thread. For 50% thread use 29/64". All of my charts say to use 50% thread for steel. Maybe use 7/16" if your drill bits tend to make oversize holes like mine do.