How to Tap a Large Hole in a Big PVC Pipe




Introduction: How to Tap a Large Hole in a Big PVC Pipe

I'm building a vacuum kiln to dry out wood quickly.  I 'm going to use a 12" diameter piece of schedule 40 pipe as a vacuum chamber. This pipe is really big and difficult to work with.

I needed to drill about 9 holes into the pipe and then seal them so that the pipe maintains a vacuum. 

I decided to use one of the vertical mills at TechShop to do the job.

The hole on the photo on this page is 1 1/2" in diameter.

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Step 1: Secure the Pipe to the "X" Axis of the Mill

I was going to drill a hole in the pipe with a hand drill, but I quickly realized that it would not work.  The pipe is so large and flexible, that the holes would not be concentric.

I decided to use the vertical mill at TechShop.

Step one was to secure the pipe to the "X" axis of the mill.

This proved more difficult that expected. Even though the pipe was secured, it still bounced around alot. 

Step 2: Use a Center Drill to Start the Hole

Use a center drill to start the hole.  Center Drills have very small points.  They are like an awl if you are used to doing wood working.  They make it easy to then follow on with a larger bit.  

In this photo, I'm using a 3/4" bit from Harbor Freight.

Make sure that you drill slowly.  I drilled at 200 RPM.

Step 3: Use a Point to Square the Tap - Then Tap the Hole

I placed a 60 degree point into the mill.  Notice on the large tap (also from Harbor Freight) that there is a small dimple on the top.

Applying pressure on the quill of the vertical mill, I then used wrench to start the tap into the PVC.  

For plastic, PVC is surprisingly strong and resistant to being tapped. 

I used a large tap wrench to continue taping the pipe.  About every 1/2 turn, you need to back off the tap wrench to break the material.  This is the same process that you would follow if you were tapping a metal pipe too.

Step 4: Finish Tapping All of the Holes

It is ironic that to I needed to put 9 holes into a pipe that will be a sealed vacuum chamber -- but you gotta do what you gotta do!

The trick to tapping a pipe is to make sure that everything is square and also drill slowly.  If you are tapping metal, you need to use a lubricant when you tap.

The drill bit set and the NTP pipe thread tap set are each less than $40.  They both proved to be a great investment.

I made my tapped 12" diameter PVC Pipe at TechShop!  I could not have done it accurately if I have not used a vertical mill.

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    3 Discussions


    4 months ago

    Has anyone else done this successfully? I'm doing a similar project and want to be sure this will work before possibly ruining the expensive 12" PVC pipe.


    5 years ago on Introduction

    I looked at your finished chamber ibble. Could you not have drilled all of your holes in the flat end pieces? It would still be a large hole to be drilled and tapped, but on a more forgiving form.


    5 years ago

    Would like to see photos of the finished chamber. Or another ibble??