can you tap large diameters w/ lathe? tapping acme threads?

Hi, I was wondering if it would be possible to tap the interior of a ~4" diameter pipe? I can't imagine a tap so large...? Can I tap it on the
lathe, by somehow getting the cutterhead inside (I'll need to go back to the shop on Monday to assess if it can)...? If not, are there places that will economically tap it for me? Or, just rethink my design to not require this; although it just makes the design better if I can do it.
Secondly, is there a way to tap for acme-style threads; are such threads preferred for higher-load applications like bringing together to vise jaws?
thanks! -Bernard Arnest
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wrote:

==================================It used to be commonly done and still can be.
The only special tools you need are a large diameter "boring" bar to hold the tool and the Acme thread tool. The major problem is that you must be very careful to not bottom out the boring bar!
I note that you are wanting to do a 4 inch diameter internal Acme thread. How long/deep is the thread and is this a blind hole or can the tool/bar go completely through the pipe?
Most likely you will wind up making a special fixture for a face plate to hold the pipe and may need a steady rest if the length is more than 1 diameter.
You may also wind up making a one off boring bar/tool holder and block to mount it to the cross slide or compound. The compound is better because you can set the compount to 14_1/2 degrees so you are not taking a full width cut.
You are also going to need an acme thread gage [fishtail] to grind the tool and set it square. The fence on most grinders is not nearly accurate enough for this. See the pictures on a fixture we made as a class project on my website at http://www.mcduffee-associates.us/machining/thfnce.htm and http://www.mcduffee-associates.us/machining/tabanggg.htm
Please feel free to browse the entire site.
Unka George
Unka George (George McDuffee)
What a country calls its vital economic interests are not the things which enable its citizens to live, but the things which enable it to make war. Petrol is more likely than wheat to be a cause of international conflict. Simone Weil (1909-43), French philosopher, mystic. The Power of Words, in Nouveaux Cahiers (1 and 15 April 1937; repr. in Selected Essays, ed. by Richard Rees, 1962)
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wrote: <snip>

<snip> Even better for one-way loads like a vise are buttress threads. Even harder to grind the tool for a buttress thread than for an acme though. You almost need a surface grinder and fixtures. Can be a royal pain to set square also as the "fish tail" gages are not available.
Unka George Unka George (George McDuffee)
What a country calls its vital economic interests are not the things which enable its citizens to live, but the things which enable it to make war. Petrol is more likely than wheat to be a cause of international conflict. Simone Weil (1909-43), French philosopher, mystic. The Power of Words, in Nouveaux Cahiers (1 and 15 April 1937; repr. in Selected Essays, ed. by Richard Rees, 1962)
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I think it would be costly to buy a tap that size and you would also need a large machine . But there are shops that do nothing but cut threads. Any kind you can think of. And most machine shops can cut threads. Look in Yellow pages under threads or threading. I used to use Vescio Threading in Santa Fe Springs Ca And they had a ton of threading machines maybe 50 of them and I think they also ground threads. Jim

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wrote:

yes.you can tap just about any size pipe.
However..depending on the length..it may be easier to single point thread it in the lathe
Gunner
"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences." - Proverbs 22:3
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    I can. I've seen a web site showing the building of a 12" diameter tap.

    On a lathe, for making only one or a few, the thread should be single-point cut, not made with a *very* expensive tap.
    For making a *lot* of them, you might consider a collapsing tap, mounted on a bed turret or on a lathe made from scratch as a turret lathe. This is like a Geometric die head -- but inside out. It taps to depth, then trips a release, and the chasers move to a reduced diameter position, allowing you to withdraw the tap while the workpiece is still spinning.
    However -- for your purposes, I think that it is perhaps time to learn to single-point cut threads -- first on the OD of something, and once you have that right, then you try your hand at making them on the ID. (First turn a "runout" groove at the end of the threads, to give you someplace to stop the threading pass.
    You don't say how long the workpiece is, but I suspect that at 4", you can't pass it through the spindle, so if it is short enough, you can just hold it in the chuck, or when a bit longer, you'll need to set up a steady rest to hold it concentric against the cutting forces.

    Unless you happen to luck into a shop which already has a tap of the desired thread, I consider *economical* tapping unlikely. But economical internal threading is a different matter, and that should be possible if the pipe is short enough -- or if the lathe is big enough.

    Sure -- tap, or single-point thread on the lathe. Again, Acme taps are expensive, and large ones are even more expensive, so single-point threading makes more sense.
    Note that part of this depends on the wall thickness of the pipe, and the thread pitch desired. If the wall thickness is not sufficient, you may have to select a smaller pitch. I threaded the OD of some aluminum pipe to 20 TPI, because I did not need strength (these threads were for screwing on endpieces to a waveguide antenna for wireless internet frequencies), and because a more appropriate pitch would come closer to cutting through the wall of the pipe.

    They are used for such applications -- in part because an Acme thread can survive wear longer than a standard V thread.
    Perhaps you should give a bit more detail about what you want to do, and why (including dimensions like length) and you might get more useful advice.
    Good Luck,         DoN.
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