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View Full Version : Torque required to turn a pipe tap in cast 316 stainless?



Chris165
07-22-2014, 01:52 PM
I have a little project at work re-threading some 316 stainless reducing couplings. This is NOT for pressure/water/air, only mechanical threads like NPSM. We have a NPSM tap for 2" but finding one for 2-1/2" is much harder so that's why we use an NPT tap.

It is currently 2-1/2" NPT and we ran the tap all the way through to remove the taper and make a straight thread which works fine because the male thread is lathe machined to achieve the correct diameter which is a semi-loose fit.

The hand threading is done by tacking the coupling to a heavy table which is bolted to the floor and two people turning it with a 15' long "custom" tap handle. We have done two in a lathe but the time involved with synchronizing the existing threads to the threading bar and the differences in cast material which is destroying the carbide inserts makes running the tap through a better option.

Since this might become a frequent project what kind of torque does it take to run an interrupted NPT tap completely through existing NPT threads in cast 316 stainless? We are thinking of using a hydraulic wrench or some sort of torque multiplier system (possibly a big radial drill press or boring machine) and kind of need an idea of how big to go. We currently have a Ridgid pony threader but don't know if that would do it. I did a search but came up with spec sheet links from tap mfg's that were no longer valid.

becksmachine
07-22-2014, 07:39 PM
It seems to me that you may have answered your own question, or at least supplied the necessary data to compute it. :) Or at least I think so, plus I seem to be in procrastination mode at the moment, so here goes. :p Somebody check my math.

Are these two guys really leaning on that 15' long handle? Or are they taking a walk in the park? I would assume this was on a flat floor, with no cleats for them to brace themselves against?

If they are walking easily while pushing on the handle, they are probably exerting no more than 30#-40# each. If they are struggling and jerking it could be as high as 200# each, but they would have to be doing this in unison to achieve any movement. I hope the "custom handle" allows the two men to be on opposite sides to help eliminate off axis forces?

If these numbers are anywhere near reality, 30#(2)=60# total force. This total force would be at the end of a 15' lever arm so 60#(15')=900 ft/lbs

The other end of the range would be 200#(2)=400# total force. 400#(15')=6000 ft/lbs

The Zagar website lists torque requirements up to nearly 7000 in/lbs to turn a 2"-11 1/2 pipe tap in a straight hole in cast iron, this equates to a bit less than 600 ft/lbs. With stainless having approximately double or triple the Brinell of soft cast iron, this number could easily range to 1200 ft/lbs - 1800 ft/lbs. And this is for a 2" pipe tap and we are worried about a 2 1/2" tap so this number could probably double again, making the range 2400 ft/lbs - 3600 ft/lbs.

To do this at 20 rpm would require 15-20 horsepower which is very doable in a large radial drill, the problem will be how to drive the tap. A #6 Morse taper is probably capable of transmitting this much torque, but only with a substantial amount of thrust to ensure the taper remains seated. In a boring mill with an NS 50 taper, this wouldn't be a problem, but you would need the dedicated tap driver that fit the 50 taper

I do think that this eliminates the Rigid Pony threader option. :p

Dave

KiddZimaHater
07-22-2014, 08:06 PM
Is there any way you can get that job in a CNC Mill, and Threadmill it?

kc5ezc
07-22-2014, 09:35 PM
Chris, I feel for you. I hand tapped 1/2 inch NPT in 304 and it was hard to do manually. Only needed one off
so did not get into longer levers than a normal tap wrench. I had an interupted tap or I probably wouldn't have
finished the job.

Chris165
07-23-2014, 09:10 AM
It seems to me that you may have answered your own question, or at least supplied the necessary data to compute it. :) Or at least I think so, plus I seem to be in procrastination mode at the moment, so here goes. :p Somebody check my math.

Are these two guys really leaning on that 15' long handle? Or are they taking a walk in the park? I would assume this was on a flat floor, with no cleats for them to brace themselves against?

If they are walking easily while pushing on the handle, they are probably exerting no more than 30#-40# each. If they are struggling and jerking it could be as high as 200# each, but they would have to be doing this in unison to achieve any movement. I hope the "custom handle" allows the two men to be on opposite sides to help eliminate off axis forces?

If these numbers are anywhere near reality, 30#(2)=60# total force. This total force would be at the end of a 15' lever arm so 60#(15')=900 ft/lbs

The other end of the range would be 200#(2)=400# total force. 400#(15')=6000 ft/lbs

The Zagar website lists torque requirements up to nearly 7000 in/lbs to turn a 2"-11 1/2 pipe tap in a straight hole in cast iron, this equates to a bit less than 600 ft/lbs. With stainless having approximately double or triple the Brinell of soft cast iron, this number could easily range to 1200 ft/lbs - 1800 ft/lbs. And this is for a 2" pipe tap and we are worried about a 2 1/2" tap so this number could probably double again, making the range 2400 ft/lbs - 3600 ft/lbs.

To do this at 20 rpm would require 15-20 horsepower which is very doable in a large radial drill, the problem will be how to drive the tap. A #6 Morse taper is probably capable of transmitting this much torque, but only with a substantial amount of thrust to ensure the taper remains seated. In a boring mill with an NS 50 taper, this wouldn't be a problem, but you would need the dedicated tap driver that fit the 50 taper

I do think that this eliminates the Rigid Pony threader option. :p

Dave

Thank you for figuring all of that out, I now have some baseline figures for researching machinery/tools to do this job.

We are mainly walking around until the top of the tap reaches the bottom of the tapered female threads in the coupling. Once at that point, there is probably 60-80lbs being put into the handle close to what you figured. We built the handle just like a standard two handle tap wrench to equalize the force like you mentioned. So this would make each end about 7.5' in length from the center of the tap.
I have been looking at big radial drills and boring mills because we have some other uses for them vs a hydraulic wrench so the expense could be spread out.


Is there any way you can get that job in a CNC Mill, and Threadmill it?

Unfortunately this is one of those jobs that tapping is the quickest way we have found because of the unknown composition of the castings and they are usually wanting them in a couple days from placing the order.
I did one in the lathe just fine, the second was bad and the third attempt was even worse. There are already threads in the coupling so I'm not sure how you could synchronize the thread mill with them?


Chris, I feel for you. I hand tapped 1/2 inch NPT in 304 and it was hard to do manually. Only needed one off
so did not get into longer levers than a normal tap wrench. I had an interupted tap or I probably wouldn't have
finished the job.

The interrupted tap is almost a must have for stainless. We had a standard NPT tap next-day shipped just to get this one done but an interrupted one is on the shopping list.

becksmachine
07-23-2014, 12:27 PM
Thank you for figuring all of that out, I now have some baseline figures for researching machinery/tools to do this job.

So this would make each end about 7.5' in length from the center of the tap.


Ok, in that case all my numbers are just twice what they should be as I was using a 15' lever arm for each force.

Dave