NPT Tap Advice

[Komet]

I have 3 thread cutting operations I need to perform, and I'd like some advice on what to get because them HSS taps ain't cheap. Here's what I need to do:

Operation 1: tap a hole that is 5/16 / 7.9mm / .311" so I can put a bolt in it. This is to remove a press fit plug in a fuel rail by means of placing a socket between the bolt and rail, and pulling it out with heat assistance.



I was going to get the Astro kit for this as I have no tapping equipment and it would be nice to have a few sizes around:

https://www.amazon.com/dp/B01JBK68NM/?tag=atomicindus08-20

According to: https://www.guhring.com/Support/Technical/Tap-Drill-Sizes - the 3/8-16 tap would be a good choice.

Operation 2: Tap the hole where the plug resided for 1/2 NPT. This one was harder to measure but it seems like 11/16 / 17.47mm / .687". I have been having a harder time figuring out appropriate sizes for NPT thread cutting, but it seems like I could go directly into this hole with a 1/2" NPT tap and be ok? I was going to get this tap for it: https://www.mcmaster.com/2525A175/

Operation 3: Tap the return hole for NPT. I'm not sure what size on this one:



The hole size measures 15/32 / 11.81mm / .465" which seems between 1/4 NPT and 3/8 NPT. I was also planning on a HSS tap from McMaster Carr, but should I just go for the 1/4 NPT because it's good enough, or should I drill it out to (9/16? 37/64?) for 3/8 NPT? I had a harder time understanding the NPT sizing charts without the fixed thread percentage.

I plan on getting some Tap Magic Aluminum cutting fluid as well. Do I absolutely need taps for aluminum specifically, or will these generalist HSS ones be ok?

Thanks in advance, I'm new to machining type operations and I need to do a tight job for this high pressure gasoline application.

 

[manwithtools]

1/4" NPT is 7/16" drill
3/8" NPT is 37/64" drill
1/2" NPT is 23/32" drill, 11/16" is fine for aluminum

These are recommended drill sizes for straight hole prior to taping, there are procedures for using a taper reamer prior to taping, but not worth it for Aluminum. HSS steel taps are totally useable for Aluminum, Tap Magic AL fluid is perfect.

 

[no704]

Only run the tap in half way, then check it with your fitting. Sneek up on the correct tap depth. Easy to over size a npt hole.

 

[manwithtools]

Only run the tap in half way, then check it with your fitting. Sneek up on the correct tap depth. Easy to over size a npt hole.

Great advise, I should have mentioned that detail.

 

[RoninB4]

Tap Magic is fine for aluminum, so is kerosene or mineral spirits. Any thin fluid works, I've even used WD-40 for machining operations on aluminum. Do NOT tap aluminum dry as it tends to micro-weld itself to the cutter/tap/drill. Agree with no704 sneaking up on the tap depth, very easy to over-size with NPT and fittings/plugs sometimes are a bit different.

 

[Rinspeed]

Tap Magic used to be really magic until they were forced to remove the Trichlor.

 

[Komet]

Ok, I'm back. Operation 1 was a success:



Astro kit worked very well. The hole diameter is 0.712", only 0.006" undersized for 1/2 NPT so I think I'll be ok going straight to the tap for operation 2.

I have been doing a lot of reading about off center plunge drilling. I need to enlarge the return hole to 37/64" and the hole is already offset towards the regulator diaphragm so I can't afford to lose material in that direction.

I have a drill press with a table vise and I was considering using an end mill, will this work?

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Also, my manual says to drill at 1250rpm for aluminum slightly under 5/8", that seems fast. Norseman recommends 200-300rpm, seems more reasonable. I can go down to 140rpm, how fast should I spin the end mill? Is it going to be alright in my JT33 chuck or is there some other MT#2 device I should put the end mill in?

Should I just use a twist drill and stop worrying about it?

 

[gahrajmahal]

Your twist drill will walk. I have had success hole edge drilling with an endmill using my drill press. You should make a test run to see if your work hold setup will be rigid enough.

 

[RoninB4]

Also, my manual says to drill at 1250rpm for aluminum slightly under 5/8", that seems fast. Norseman recommends 200-300rpm, seems more reasonable. I can go down to 140rpm, how fast should I spin the end mill? Is it going to be alright in my JT33 chuck or is there some other MT#2 device I should put the end mill in?

Should I just use a twist drill and stop worrying about it?

First off, congratulations on getting to this point.

Secondly, if you're needing a hole off center from the existing hole DO NOT use a drill if break-out will occur. As previously stated, it will wander. A drill is one of the best choices for metal removal rates. It's also terrible at lateral rigidity. It tends to follow a hole whether in correct location or not. Reamers do too.

An end mill is the better choice for correcting/changing location of an off center hole. By design/construction it has lateral rigidity.

Speed may seem fast to you but it's not going to burn the cutter because it's not steel. You will need cutting fluid.

A drill press is a poor choice for this operation (a milling machine would be better, even a small one) but if that's all you have then you'll need to approach this carefully. Vibration will be your biggest obstacle, vibration is movement. Everything MUST be rigidly clamped down and held so there's ZERO movement. Speed can create a harmonic vibration and rattle everything from the work to the machine itself. Try starting with a speed around 600 RPM and take a brief down-cut, try to feel how much vibration is happening. Drop the speed if you feel a "buzz". You'll want to feed the cutter into the work in short cycles (called pecking) that bring the cutter up/out of the work so you can clear all chips out of the way. Taking a plunge cut in one shot is asking for trouble. The longer the cutter is engaged (in this instance) the more time it has for vibration/trouble to develop. Set a stop on the drill press so it bottoms at a positive stop.

Holding an end mill in a drill chuck is another poor choice but if that's all you got then that's that. I would suggest you work your way up to the final hole size. Starting small and increasing cutter diameter puts less stress on the cutter/machine and helps getting a more accurate location/diameter. Going full size in one shot increases the chance of wandering off location and/or oversizing the hole, especially with a drill press. You could start with a smaller twist drill and switch to an end mill if it's not breaking out into another hole. Can't tell without a photo.

Not sure what size hole you need but 37/64 is .578, that's a lot smaller than 5/8. OBTW, not all end mills are center cutting.

 

[Komet]

Operation 2 was both a success and a failure. The Tap Magic Al and Widia GTD tap combo was excellent with a center punch in my drill press holding it straight. I had to turn it with a pair of wrenches but I went slow and the threads came out very nice:



The tap bottomed out where the hole changes to a D shape deeper in the rail, which limited the amount of usable threads. Even if I drilled it out to tap far enough to flush mount the adapter, my fitting would interfere with my IAC housing:



Since I want to feed the rails from the rear anyways, I'm going to put a 1/2" plug here and I made a trial run at tapping the rear rail plug for 3/8 NPT.

I used a step bit at 540rpm with a squirt of WD40 to bring me up to 9/16" which worked really well. All I have in 37/64" is that 2 flute end mill, so in the chuck it went. Boy was that a poor idea.

The slightest kiss of the mill caused violent vibrations. It was very pissed off at 540rpm, so I dropped to 260rpm. It was slightly more controllable, but it was obvious that end mills don't belong in a jacobs chuck on a drill press. Just not rigid enough. But, I had a hole to drill so I mangled it out. The finish of the hole wall was poor. Gouges in the sides, pretty nasty wound of a hole. It did work though, and the tap cleaned up 85% of the nastiness:



There's no way I have enough space to open up the return hole to 3/8" NPT for Operation 3:



I'm going to tap it for 1/4" NPT, and thread sealant will have to suffice for the slightly oversized hole. Back to McMaster Carr I go.

 

[RoninB4]

-As you've discovered, an end mill in a drill press chuck yields poor results most of the time. Glad you were able to get through that part. One thing I noticed was the set-up in the photos. It's important to stabilize/control the part in a vise. With tall parts it's also critical to stabilize/control the top where the actual metal cutting is going on or it will vibrate like the dickens, could actually be dangerous if the vibes become too great. Even a 2x4 clamped the to column would have allowed clamping at the top. It's after the fact now but keep this in mind for the future. The part must be secured where the cutting is happening.

As for tap depth, I've also run into situations where I had an NPT tap go shallow on depth. With no other choice I measured what the depth was and ground the tap back. Yes it ruins the tap for future use but you do what you have to do. Modifying cutting tools happens daily in machine shops whether we like it or not. Relying on thread sealant is not a good idea, as you already know, is there anything else that could be done? Describe what's limiting you.

 

[alfadan]

Being a worry-wart with gasoline, that seems like a large pipe fitting size for that wall thickness in an aluminum extrusion. Will probably be fine but I would check it often for cracks for a while. Congratulations on the work though!

 

[RaisedByWolves]

Being a worry-wart with gasoline, that seems like a large pipe fitting size for that wall thickness in an aluminum extrusion. Will probably be fine but I would check it often for cracks for a while. Congratulations on the work though!

This whole thing makes me shudder.

Not wanting to spray pressurized fuel on a hot engine while driving at highway speeds does not make you a worry wart.

 

[larry_g]

This is where your location helps. I would be glad to help you with this if you dropped by, as well as others on the board.

lg
no neat sig line

 

[LXCam]

Maybe get away from a npt thread and go with a JIC fitting. You’ll need to create lip for the oring to seal too but it’ll fix your issue with the railing fitting being too proud (least that’s how I understand your issue).

One other piece of advice that’s a bit too late. I’d use a counter bore with a insert type mandrel. You’d probably have to come up with/create a custom mandrel that can reach far enough into the bore but st the point you could easily have completed this task with a hand drill and not much chance of your bit wandering off center.

Btw, where you at. There’s a ton of helpful guys on the board who have the right equipment to make this easy on you.

 

[Komet]

I'm in Washington, I appreciate the offers but I do want the capability and experience of tapping my own threads. McMaster Carr hooked me up with some excellent Viking twist drills that I used to deepen the holes for tap clearance and they cut through the aluminum like butter at 140rpm.

Final drilled and tapped 1/2 NPT hole:



I got the front plug to snug up right before it blocks the crossover tube port:



Tons of engagement on the 3/8 NPT adapter in the back of the driver's side rail:



I also used the 7/16" drill from the Astro kit to deepen the hole in the regulator return, the threads aren't the sharpest after it was tapped for 1/4" NPT since it started a little oversize but I did get the adapter to fit in pretty deep and it does snug up so I think it'll seal:



Group photo, all hand tight, waiting for these to get bone dry before installing with sealant:



Really appreciate the excellent advice, you guys helped me get this right on the first try. (Well, technically second. The rail in the first post was actually a spare).