Working with PVC

Thread Starter

cmartinez

Joined Jan 17, 2007
7,334
I thought about posting this problem in a different website, but I figured, what the heck... here's where my true friends like to hang out... :)

Anyways, here's the problem, quite briefly:

I'm trying to drill a 38mm (about 1-1/2") dia bore through a solid piece of PVC a little under 4" long. For this purpose, I purchased a forstner bit like this one:

1620439524456.png

But the result was disastrous. The material came off in long treads instead of breaking into chips (at about 450 rpm in my lathe) and it clumped together and melted, leaving a very ugly finish in the bore's wall.

1620439508512.png

I tried pecking the drill as it went through the plastic, but it made no difference. Is there a better way to do this so as to get a smooth surface finish?

@shortbus , @MaxHeadRoom
 

MrSalts

Joined Apr 2, 2020
285
Ask an assistant to spray "dust off" into the hole as you drill. Hold the can upside down to get liquid and that will keep the strand from melting - essentially cryogenic machining. Hopefully make it crack off regularly and eject.

If you don't want to go that extreme, you can also blast an air hose into the hole to keep things cool and eject debris.

last option would be to clamp it tight, run a 36mm through it as a rough cut snd then a 38mm for a final cut snd the chip will have room to eject.

Or, depending on thickness, you can try this...
 
Last edited:

SamR

Joined Mar 19, 2019
3,411
I'd expect PVC to be harder than wood so no Forstner bit. Full on drill bit with coolant fluid feed to keep it cool. Those size bits aren't cheap, and I'd prefer a drill press vertical feed if I could.
 

wayneh

Joined Sep 9, 2010
17,152
I like the idea of doing it in steps using a drill press. To avoid heat, you need to cut quickly. You can’t do that with such a big hole in one pass. Cooling as you go is a ... cool... idea but difficult and messy. Do it if you can work it out.
 

MrSalts

Joined Apr 2, 2020
285
Thanks all for your suggestions... I forgot to add that I'd like to avoid using a liquid coolant, if possible... and while I'm at it, I'd also like a pony for Christmas ... *sigh*
The coolant from a "dust-off" spray can would be a gas as soon as it warms above -10 or so. A standard liquid coolant wouldn't help much to make the soft plastic "chippable". You need it to be really cold.
 

ericgibbs

Joined Jan 29, 2010
12,932
hola cm,
Pop the PVC block into the home freezer overnight -20C and then drill the PVC.
My take a couple of shots or so, after you can drill holes in ice.;)
E
 

strantor

Joined Oct 3, 2010
5,543
Those bad finishes on plastic I think are due to friction heat and melting. Slow/steady and Pecking are not the way to go. It needs to be actually drilled, as in cut, by the depth corresponding to the bit's cutting edge angle, every revolution. Fast and violent. If this were a normal drill bit size hole, I would suggest drilling it out with a bit a size or two smaller in your normal way, pecking to clear chips, etc. then go back with the proper size and ram her home YOLO style. But it's a pretty large hole own two bits near that size. Did you get a lathe? This could be a job for a boring bar. Or a boring head if you have a mill.
 

shortbus

Joined Sep 30, 2009
8,588
From machining many different plastics over the years, most of them do the same thing long stringy chips and bad finises when drilling or even sawing. The friction makes the roughness and the plastic it's self makes the strings. If you can use another plastic, look into Delrin or even ABS they are more forgiving to machining. Stuff like PVC is usually molded for a reason and this is it.

Coolant kind of helps but not worth the mess. For a hole that big your better of drilling like 1/16" smaller and boring it to size. But doing it that way can also be a problem. When I had to make accurate parts I found the best thing was to make a few extras, to use as "sacrificial" parts. To get both accurate sizes and good finishes, the final pass to size has to be both moderately heavy and a single pass. No sneaking up on it like with most metals. But you will still get a single stringy chip, but usually a better finish.

Another thing is don't leave a bunch of those chips in the lathe chip pan. Can't count the number of times I didn't do that. The chips will and do get wrapped around the spindle/chuck and the pan gets cleared quickly faster than you can stop the spindle. And if your in front of it, it can hurt as the chips whip you. Let alone the embarrassment if there are others in the shop.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,334
Thank you all for your responses and willingness to help.

  • Yes, Strantor, I finally got myself a lathe :) ... in fact, I bought one of these. ;) ... eat your heart out! :p
  • No, I cannot change the material I'm using. It has to be PVC or bust...

After careful consideration, my conclusion is that the problem is that the material is being cut into long strips, instead of being chipped. And those strips have no way out of the bore being drilled. The problem is worsened even further by the fact that the bore is being machined horizontally, due to my machine's natural configuration.

I've been cutting solid PVC using a CNC router for many years now, and it's never been a problem. Using an end mill at high speed produces many light chips resembling flakes that are easily pulled up and blown out of the way by the spindle itself.

But said flakes are not being produced by the lathe... rather the long strings of material are being formed... exactly as has already been mentioned by Shortbus in his generous answer ...

So here's my plan.
  1. I'll drill a pilot hole through the material using a long 1/4" dia drill bit first.
  2. I'll next drill a 1/2" dia hole with another drill bit.
  3. Then I'll drill a much larger hole using a 1" dia drill bit.
I predict that those three steps should work acceptably well. This because drill bits have spiral grooves throughout their length (called flutes) that pull the material out as it's being cut. And that should avoid the material getting trapped inside the bore and form the clump that clings to the shaft and that's been breaking havoc in the process so far. The forstner bits have no such flutes and the material is not expelled all the way out of the hole the way a normal drill bit does.

If the 1" dia bit leaves an acceptable finish, then I'll take the risk and buy a 38mm bit and hope that it works just as fine... problem solved.

If the surface finish left by the 1" bit is not of the quality I need, then I'll make a robust boring bar which I'll place on the spindle (I'd rather not rotate the part, since it's rectangular and holding it and aligning it in the chuck would be a serious PITA) and remove a 0.010" layer of material from the hole while actively blowing air through it.... see what happens.

And if that last step doesn't work either, then I'll repeat it again, but this time I'll follow Eric's advice and leave the part in my freezer overnight. Maybe that way the material will be so hard while it's being machined that it'll chip instead of go spaghetti on me...

Wish me luck
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,334
Never used a step drill for anything other than on sheet metal... Which they are designed for and do an excellent job on.
I've used them on sheet plastic (also PVC), and they work fine too. But most likely their usefulness is limited to thin types of material.
 

ElectricSpidey

Joined Dec 2, 2017
1,663
I don't know if this helps or can even be adopted to your needs, but sometimes when I need a large hole cut in plywood that wont fit in my drill press I attach a guide to the bottom of the sheet, drill a "large" pilot hole and then remove the rest with my router.
 

hrs

Joined Jun 13, 2014
321
If you have a lathe couldn't you use a boring bar? Drill a small ugly or beautiful hole first and go to final size with the boring bar.
 
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