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TrevFond

Drilling / Tapping PVC and Acetal

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Does anyone have any information regarding SFM & IPR for drilling and tapping PVC or acetal (I know the feed of 1/tpi for tapping)? My experience is that HSS drills give better results in these materials over carbide. I have many parts that require small threads (4-40 and 6-32) but have trouble with chips binding on the drill and the hole collapsing, resulting in a tight thread that needs a hand tap ran through after machining. I have tried redrilling the hole with the tap drill once the tap has been ran, along with tapping the hole two times while in the machine; but that doesn't seem to work either. Considering trying to tap with a size larger drill bit to account for the hole collapsing or even form tapping despite it being frowned upon in plastics.

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3 hours ago, TrevFond said:

Does anyone have any information regarding SFM & IPR for drilling and tapping PVC or acetal (I know the feed of 1/tpi for tapping)? My experience is that HSS drills give better results in these materials over carbide. I have many parts that require small threads (4-40 and 6-32) but have trouble with chips binding on the drill and the hole collapsing, resulting in a tight thread that needs a hand tap ran through after machining. I have tried redrilling the hole with the tap drill once the tap has been ran, along with tapping the hole two times while in the machine; but that doesn't seem to work either. Considering trying to tap with a size larger drill bit to account for the hole collapsing or even form tapping despite it being frowned upon in plastics.

Form Tapping isn't going to work very well in that material. The plastic will not 'cold work harden',  the way metal will. You'll deform the plastic, and it will spring back, making a threaded hole that is undersized from the print requirements.

First, PVC (polyvinyl chloride) and Acetal (POM, or polyoxymethelene), are very different materials. POM machines "on size", very easily. I've cut a lot of Delrin (Acetal or POM) parts, and rarely have problems with driling or tapping this material. If you have the option; use Delrin. It's like the difference between cutting aluminum and bubble gum. Sure, the bubble gum cuts quickly, but try holding a tight dimension!

PVC on the other hand, simply sucks to machine. The material "pushes" out of the way, and you'll have difficulty getting the tool to actually cut a chip.

Tools will make a big difference here. You'll want plastic specific cutting tools for PVC. I'd consider the Single Flute cutters from Harvey Tool.

For tapping, same story. You'll want very sharp rake angles, on the cutting teeth. I've had to order special "oversized taps" before, so that the threaded holes would gauge to size properly. What is your thread callout? 2B or 3B for the Thread Class? Sometimes you can make a 3B Tap work, if you have to hit a 2B Class. But 3B is going to be extremely difficult to Tap, without an oversized tap.

For my money, I'd Thread Mill the threads, and be done with it. With Thread Milling, you control the pitch diameter by using cutter compensation. Since you have the ability to adjust the threaded hole diameter with the Thread Mill, it will give you the ability to cut the thread on-size, in a single machine operation. Once you dial-in the first hole, all the subsequent holes will be sized perfectly, without having to fiddle with anything, or run a tap through by hand.

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