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Thread: 1/8" NTP Threads: Tap or mill?

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    Default 1/8" NTP Threads: Tap or mill?

    Hi all, I am trying to make a small aluminum manifold for a flood coolant system for my Taig 2019 CNC mill. I need to drill and tap 3 1/8" NPT holes into 6061 aluminum. I am pretty new to metalworking in general, so yesterday I picked up a cheap tap set at Harbor Freight that included a 1/8" tap. I drilled the hole properly and then set to tapping it. I got about 3/8" in before I felt like I would break the tap if I pushed any harder.

    The problem, of course, is that NPT is tapered and I drilled a straight hole. Reading more about the subject, it seems I need a NPT reamer and to ream out the hole before tapping it. The cheapest reamer I can find is about $20 before shipping.

    This got me thinking about the perfectly functional CNC mill I have sitting on my bench. Could I make this job easier with it?

    I thought I'd ask and see if anyone has any suggestions for me. Consider me a total newbie but willing to learn and work

    I want to drill and tap 3 1/8" NPT holes about 3/4" deep in 6061 aluminum. I have:
    Taig 2019 CNC mill
    1/6", 1/8" and 1/4" endmills
    A 1/8" NPT hand tap

    I've been thinking that I could probably at least use the mill to "ream" the hole out without spending another $20 on a specialized tool. But what about thread milling? Can I buy a small thread mill and just throw my hand taps away forever?

    I've also been thinking about making a tooling plate with a bunch of 10-32 holes in it and it would sure be easier to have my mil cut all those threads for me than to hand tap them all.

    Thanks,
    Jason

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    Gold Member hoss2006's Avatar
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    A "Q" drill bit (.332) is just fine for a 1/8-27 NPT tap.
    You made it in 3/8, did you check the hole with the mating part, that's probably deep enough.
    5-7 threads worth of engagement is enough.
    Hoss

    http://www.hossmachine.info - Gosh, you've... really got some nice toys here. - Roy Batty -- http://www.g0704.com - http://www.bf20.com - http://www.g0602.com


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    Registered jstiltner's Avatar
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    The problem may also be in the tap you are using. I have a set of HF taps that are complete junk! it almost looks like they were cast into shape!



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    Quote Originally Posted by hoss2006 View Post
    A "Q" drill bit (.332) is just fine for a 1/8-27 NPT tap.
    You made it in 3/8, did you check the hole with the mating part, that's probably deep enough.
    5-7 threads worth of engagement is enough.
    Hoss
    Thanks for the response Hoss. I used a "R" to drill the hole. The 3/8" depth is enough to get a solid seal, but I wanted to go the full 3/4" purely for aesthetics. This is for the plug side of a Loc-Line hose and I just wanted it to be flush.



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    Quote Originally Posted by jstiltner View Post
    The problem may also be in the tap you are using. I have a set of HF taps that are complete junk! it almost looks like they were cast into shape!
    That could definitely be. The tap seems pretty junky but I figured I could at least get 3 holes out of it

    Would it be reasonable to expect that a good tap could cut NPT threads without first reaming the hole to a taper?



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    Quote Originally Posted by vonnieda View Post
    Would it be reasonable to expect that a good tap could cut NPT threads without first reaming the hole to a taper?

    Yes! Especially in Aluminum. Use a Q, like Hoss mentioned, and get another tap.



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    Quote Originally Posted by vlmarshall View Post
    Yes! Especially in Aluminum. Use a Q, like Hoss mentioned, and get another tap.
    Can someone explain to me why to use a Q instead of an R? I'm not questioning the recommendation, I just want to understand it. I've been looking at tap/drill charts and the Internet seems to be split on whether to use a Q or an R for NPT 1/8". A Q is smaller then an R so it seems like I would have even more trouble getting the tap through the work.

    Thanks all for the helpful info!



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    Quote Originally Posted by vonnieda View Post
    Can someone explain to me why to use a Q instead of an R?
    Good question...
    I've almost always used a 'Q', mainly for a better seal. Taper pipe threads (in my opinion, at least) should be a slight interference fit, and not a "100% thread". I'd rather have a pipe fitting sit a little high, than have it thread all the way in and still not seal because of a drill cutting oversize or a tap too deep.



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    Registered Hirudin's Avatar
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    I'm just beginning with CNC-ing / metalworking so I might be completely wrong, but since I was reading this thread and I have a drill/tap chart sitting right in front of me I figured I'd share what it says...

    This chart says use an "R" for NPT and use a "Q" for NPTF for tapping without reaming.

    Attached Thumbnails Attached Thumbnails -03162009138-jpg  


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    Yeah, I've got a quick-ref from Morse tools here that says the same thing. Listen to them, not me.



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    Sounds good to me. Thanks for the responses all. I'll give it a try with a Q (non-reaming) and my HF tap and if that busts off I'll go shopping



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    It sounds like you have the same crappy HF tap set that I have. I tried the 1/8 NPT that came with both kits Metric and Standard. Gave up on both and bought a Irwin one from Ace Harware. Well worth every 599 pennies. Cuts a lot easier and the finish was nicer.

    Also to help you out you will want to pick up one of these. Drill a hole, Throw this in and lining up the tap is idiot proof. http://littlemachineshop.com/product...963&category=2



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