How to mill NPT thread using a threadmill

[koenbro]

I am making a manifold for compressed air, and need advice on how to thread mill NPT threads, specifically 1/4" - 18. I looked up the thread and the small diameter is 0.4774", the large dia is 0.4916", and the length of useful thread is 0.4018". A drill size of 7/16" is recommended.

This is what the manifold will look like:

The hole on the front left will punch through and there will be one inlet on top, and three outlets on the front, right, and left walls. These are the 4 holes that I want to tap NPT 1/4" - 18 to attach air fittings. The two tabs will be used to attach to 8020 1010 profile.

I have done thread milling for 1/4" - 20 holes and liked it a lot more than tapping. I have a Tormach #34694 threadmill (for 1/4" and M6 threads) and the Tormach #34696 (for 3/8, 7/16, 1/2, M10, M12, M14 threads), which is what I plan to use.

How do I drill the tapered starter hole? Should I use a boring bar and drill a hole exactly to teh small diamater? Does anyone have any experience to share on how to do this type of thread with a threadmill?

 

[davidpbest]

Fusion 360 has support for NPT thread milling tool paths now.

 

[mmcmdl]

Pipe tap reamer .

 

[mmcmdl]

How do I drill the tapered starter hole? Should I use a boring bar and drill a hole exactly to teh small diamater? Does anyone have any experience to share on how to do this type of thread with a threadmill?

For a 1/4" NPT I would just drill and tap it with the machine and not thread mill it . We used thread mills when the machines didn't have the torque to drive the larger taps .

 

[Jason812]

Drill, ream, tap or get a NPT thread mill. The NPT thread mills have the taper and you program them the same as a normal treadmill. 2 passes split 75%/25%.

 

[koenbro]

What size (diameter) drill should I use?


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[Jason812]

Every thread mill I ever used I would use the recommended drill off a drill/tap chart. I was fortunate to work for a large manufacturer and I didn't have to use a single pointed thread mill.

These were all on large CNC machines. The only NPT I ever pre reamed the taper for was a 1/16. It was for an orifice in the bottom of a hole and I had to use a long tap. I'm not sure how a smaller "hobby" machine would handle cutting the taper.

What material are you using? If aluminum, personally, I would just use a tap in a non production environment.

 

[mksj]

I made an air manifold for my mill coolant system out of aluminum, drilled the internal connection holes 3/8", used a straight drill and then 1/4" NPT tap. Chart says to use a 7/16" drill if not using a taper reamer. Done similar tapping in steel, brass and PVC.

Tap Chart NPT National Pipe Taper Threads

Tap Chart NPT National Pipe Taper Threads

www.carbidedepot.com

 

[mmcmdl]

I've made quite a few of these manifolds in the past for the work benches . Nothing special , but it's nice having the extra outlets when needed .

 

[koenbro]

This is what the finished item looks like. Very pleased with the dimensional accuracy. Have used the mill for various small jobs like drilling, squaring ends, and chamfering but this is my first CNC project.

The mill is, for now, trammed quite well. This photo shows (barely) the fly cutter patterns crossing. I know it will get out of tram from usage, but I lucked out for now.

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