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View Full Version : Man what a Hassle Tapping?



madman
05-07-2009, 09:58 AM
I cant believe im having so much trouble tapping a 3/8-18 NPT thread . I drilled the hole to size (actually used a suitable endmill, part needs 5 holes done in a radial pattern) Thought (thats where the trouble started i think Thoughting always gets me in trouble) i would just use the mill to tap them, well the tap only went in so far and would lock up spin in my new chuck(yech) and i tried to hand tap the rest and the part turns no matter what I try or tighten it in the chuck. its a 5 inch diameter machined out piece of stainless steel 316) Now i did make all the other tapped holes larger as 50 percent ect. It worked OK but can you make the pipe tap holes larger also? or would it create leaks?? Thanx

JCHannum
05-07-2009, 10:13 AM
Tapping stainless almost always dictates using a tapered reamer for the hole. A tapered tap and a straight hole will not work in a material such as 316SS.

A pipe tap must go in a specific depth to produce the proper thread for a leak free seal. It is not like a straight thread, where the tap goes full depth.
A pipe tap can hog out enough material in a softer metal to go to a sufficient depth, but tougher materials must be taper reamed first.

rkepler
05-07-2009, 10:19 AM
I cant believe im having so much trouble tapping a 3/8-18 NPT thread . I drilled the hole to size (actually used a suitable endmill, part needs 5 holes done in a radial pattern) Thought (thats where the trouble started i think Thoughting always gets me in trouble) i would just use the mill to tap them, well the tap only went in so far and would lock up spin in my new chuck(yech) and i tried to hand tap the rest and the part turns no matter what I try or tighten it in the chuck. its a 5 inch diameter machined out piece of stainless steel 316) Now i did make all the other tapped holes larger as 50 percent ect. It worked OK but can you make the pipe tap holes larger also? or would it create leaks?? Thanx

While you can get away with holding the tap in a drill chuck for straight thread tapping it's a lot harder in a tapered thread. For something as small as 3/8-18 you might get away with it using a decent moly lube and a taper reamed hole, short of that it'll take too much grunt for the jaws to have a grip on the hardened tap shank.

In cases like yours I will occasionally use the chuck to start the tap and then a tap driver or wrench to finish the work on the table before moving on to the next hole.

As to increasing the tap hole diameter - you're better off using a pipe taper reamer to remove material but nothing is going to change the fact that the tap will be cutting all along it's length at the 'bottom' of the hole.

Paul Alciatore
05-07-2009, 11:43 AM
In ADDITION to ALL of the above, I would back out and clean both the tap and the hole FREQUENTLY - as soon as the resistance starts to build up. It will help some and may make the job a little easier. Add more cutting fluid or grease each time.

I would not use a larger drill as it could create leaks. The reamer would be the thing.

Not the best for a commercial situation, I know. It takes a lot of extra time. But if you want fast machinning, use leaded steel, not stainless.

gregl
05-07-2009, 03:27 PM
When I had a similar problem, I discovered a variety of pipe taps with assorted configurations in the MSC catalog. I bought something with a high hook rake on the cutting teeth and spiral flutes that did the job much easier than the standard tap I had started with. The catalog lists the various uses of the several styles. You might check with that.

lane
05-07-2009, 07:15 PM
We tap a lot of SS. Use a skip tooth tap . They work a lot better in SS.

David Powell
05-07-2009, 08:21 PM
Some years ago I had a job where, occasionally, I had to tap pipe threads in SS( Usually 314,) After a few problems I came up with a plan, I would drill through with the recommended drill and then drill two further larger steps, corresponding with the taper,being careful to drill them just deep enough but not too deep so that there was no danger of loosing thread, It worked, but life was much better when my employers bought some of the reamers, Maybe this hint might help. David Powell.

madman
05-08-2009, 01:33 AM
And buy a taper reamer and a skip tooth tap. Darn there goes the profit in this job

Dawai
05-08-2009, 10:19 AM
Stainless can take the profit out of a job in a snap...

If you know what I mean? I kept a bucket of lye water & weak acid around to do anodizing, it would rust rot taps out in a bit if you dribbled it on the right spot.

Things like this is why the big shops are ugly in pricing.. make up for it on the next three jobs.

JCHannum
05-08-2009, 10:49 AM
Buy once, cry once. How many hour$$ did you spend dorking around trying to tap and end up with what might be a poor job. If you have to take it back & rework the job, the initial cost of the tap & reamer is nothing.

pcarpenter
05-08-2009, 11:23 AM
Yeah...tapered tap needs a tapered hole....seems so obvious, but I recall when that first hit me. I remember thinking...gee...I am wedging a nice hard (maybe brittle) piece of tooling into a hole that is guaranteed to be an interference fit....it's a wonder I didn't break a tap.

As a relativel novice, however, I do wish that they would *mark* tapered pipe taps with what I will call the "upper bound"-- the largest diameter for a standard fit. It's a total waste to run the tap (and the reamer before that) in too far, making a hole that will never produce the desired interference fit with a mating pipe. I don't have one in front of me, but I wonder if the maximum values are listed in Machinery's Handbook such that you could measure and wrap tape to help you know when to quit.

I have even bought some pipe fittings recently (they nearly all look like third-world production) in which I could not get thread interference befor running out of threads on the mating part. This makes you rely too much on pipe dope...and makes for leaks....not at all acceptable for something like a gas line.

Paul

madman
05-11-2009, 10:01 PM
I called sowa tool today asking if they had a 3/8 and a 1/4 pipe tap reamers available. Of course said the nice lady on the phone. relatively cheap quote was given. Then i get exited jump in the car drive down and when i get there they didnt have the 3/8 Tap Reamer??? Anyway i center drilled and then drilled a 3/8 hole then after changed to the small end dia? .402??) any how I started feeding down with the reamer cautiously, then taking it out and measured. I ended up plunging it in almost up the the large end of the reamer. THEN i used a INTERUPTED CUT TAPERED PIPE TAP. WOW did it go easy. I was amazed. After all the hassle with the other ones. Thanx Guys for the tips and hints and youre time. This nightmmare jobs starting to look better. I uses a walther stainlwess tapping fluid for all the tapping and a water based coolant for drilling.

gmatov
05-12-2009, 12:12 AM
I really DO think there is a standard for tapered pipe threads, too.
Some here are saying that they use incremental drills or tapered reamers, but a straight drill should be used for pipe tapping.

The tap, after all, is tapered, too. It starts and does little work. The main diameter of the tap is over the hole size BUT NOT digging all that much.

Actually, a 3/8 NC tap is probably more difficult to turn in that a 1/4 pipe. Bolt size is the total of the thread depth. Pipe is the taper then the max D, and that is when you have to crank on the tap handle.

Doing it by machine, BFD. why bitch about how hard it is to turn a tap handle? Or is it that it costs a couple pennies more per day in electricity if the hole is not JUST so.

Cheers,

George

dp
05-12-2009, 12:24 AM
And buy a taper reamer and a skip tooth tap. Darn there goes the profit in this job

You'll make it up on the next. Well, maybe the next after, but there's almost nothing better for business than a good lesson not forgot.