Boring and tapping PVC blind flange.

Looking for advice. I work in a small fabrication shop and I’m used to working with metals. Mostly piping with stainless and carbon flanges. My supervisor, the real machinist, left for a better position and I’m kind of learning new things on my own.

Customer sent in two 4” PVC blind flanges, wants a 3” NPT tapped hole. I’ve never worked with PVC before and I’m not sure how to go about it. I’ll be using a lathe and boring bar with carbide inserts. I also have a 3” tap, as I haven’t master threading on the lathe. Any recommendations on feed/speed would be great. Any other tips are welcome!

I tried to search the forum but I’m not good with this. Sorry in advance for being a dummy. Thanks everyone!

Machine it as fast as you want...it's soft and makes funny strings

You say the customer wants a "wants a 3” NPT tapped hole." in a blind hole. First, this sounds like a glue fitting so I have to ask is that hole even deep enough to allow a proper, full length thread to be cut? A threaded flange would be engineered long enough to take care of this problem, but if this is a glue flange, then it may not be. You may only have enough length for a partial thread and, with a blind hole, the pipe may not fully seat in whatever thread you can cut. Heck, you may not even have a proper diameter hole for a 3" NPT thread as a glue fitting will have a hole with a diameter slightly larger than the OD of that size PVC pipe while a thread would be cut DOWN into that OD. If this needs to be liquid tight, they may have problems. Even if it is for some kind of handrail or support, it could make a loose joint and therefore be dangerous. If I were asked to do this, I would want to have the mating part on hand to ensure a proper fit. And may have more questions after that.

Pipe threads are tapered so I am not completely sure how you can do that on a lathe. I would assume it could be done with a taper attachment.

Personally, if I just had to thread this flange, I would try using a tap. It is a blind hole so a standard tap may be too long. A 3" tap will be expensive and cutting one down may be a losing proposition if it does not work. I have had good luck making a pipe thread tap from a length of galvanized pipe or a pipe fitting. I just filed some notches at the correct angle to provide a rake angle and I screwed it into a proper sized hole in the PVC part. That works just fine for one or a few parts and saves the cost of what could be an expensive tap. These filed "flutes" will be small so it must be backed out several times during the tapping process to clear the chips. The taper of the thread takes care of any tolerance issues involved as a pipe thread is tightened until it is liquid tight.

Frankly, to me this sounds like an nu-knowledgable person (the customer) who is trying to do something the wrong way. They do make PVC fittings with threads. It may take an order from someone like McMaster to get some in this size, but that would be the best way to do this as a proper fit would be ensured. It may take a phone call to their sales force as it may not be found in the catalog or on their web site. They do have people who can help with things like this.

Thank you!

For some reason, I have in my mind that they want to screw a metal pipe in a plastic flange. Becuase if they were using plastic pipe, they could use a glue joint.

IF so, and if this is a pressure fitting, the use of a female thread in plastic is not recommended. For a mixed joint like this, generally you want the female thread to be metal and the male to be plastic. That is, you want the plastic male thread to be subjected to compressive stress. This allows the metal (and presumably stronger) female thread to take on the tensile stress. There's a reason that its the female thread that normally breaks in overtightened NPT fittings.

From US Plastics Corp:Can you thread plastic and metal NPT fittings together? | U.S. Plastic

Other issues I'd really be worried about if the female thread are plastic are
1) Along the lines above, I'd be concerned about physical joint length. NPT threads in a metal flange probably have enough meat to grip and seal. In plastic? Not confident.
2) If a long section of metal pipe is connected to the flange, you have a big lever arm and even minor flexing in the assembly could cause leakage and or a physical break.

Of course, service should be low pressure liquid only (PVC is not recommended for compressed gas of any signficant pressure). Ok for near-atmospheric ducting.

This would be one I'd check with the customer about.

I think your going to hear a large CRACK !
When that tap get's 3/4 of the way in.