[vonnieda]

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

[hoss2006]

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

[jstiltner]

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!

[vonnieda]

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.

[vonnieda]

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?

[vlmarshall]

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.

[vonnieda]

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!

[vlmarshall]

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.

[Hirudin]

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.

[vlmarshall]

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

[vonnieda]

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

[Regnar]

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

[SherlineGuy]

Let's beat this to death.

We live almost exclusively in the 1/8 NPT world...everything I've ever designed has at least one. NPT is tricky. You do not want the thread so deep that your fitting is bottomed out, if it does you'll need a mile of teflon tape to seal it up. In aluminum you should not need a reamer, but you will need a quality tap for any kind of NPT threading...and ones from HF may or may not be so, but ones from a true tool vendor; McMaster-Carr, MSC, J&L, etc would be.

On the other hand, a cnc mill could make you a sweet tapered pilot hole......

[vonnieda]

Just to close this out: I picked up an Irwin tap, used a Q drill and everything went great. Thanks everyone for all the excellent advice and information. My coolant manifold works perfectly!