Need advice putting barbs on my res...

[ChemMan]

I just got THIS resevoir from FrozenCPU.com and I need to put larger hose barbs on it. The plastic is 1/4" in diameter and about 2 3/8" OD. What I would really like to do is enlarge the bottom hole in the picture and then drill a new hole directly across from it. Ideally I would then thread the holes to accept 1/2" NPT threads. If that is too difficult then I will just glue in two pieces of copper tubing that are large enough for 3/4" ID tubing.

I've got a small drill press that is currently set up for metal(ie low rpm), and a decent Vee block that I can clamp everything down into. My questions are:

If I want to thread these holes to accept 1/2" NPT, what initial size should I bore the holes?

I've never drilled into plastic this thick before, not to mention cyclindrical. Is there anything I should be doing differently? Special drill bits, techiques, etc?

Thanks.

[bigben2k]

Tapping into the side of a cylinder is pretty difficult. You've got a good lead, with the existing holes though.

If I was you, I'd stay away from tapping the holes. Your best bet, and easiest thing to do, is to pick up nylon barbs (the kind with threads at one end), then pickup some "Plastic Welder" by Devcon from Wall-Wart, and gluing that nylon barb in the enlarged hole.

The glue works extremely well, with acrylic, polycarbonate and nylon. If you pick up some of those grey barbs from Home Depot, and trim off an end, you'll find that they're impossible to glue, because they're made of polypropylene, which is a "soft" plastic. You could use cyanoacrylate (a good modeling glue), but the "Plastic Welder" makes a much better bond, which is chemical: the plastics react with the glue, to make something so permanent, you'll need a crowbar to pry them apart!

For added security, you can use Goop, to seal the connection even better.

Good luck, and post a pic, if you can!

[ChemMan]

Doh! Now I wish I had posted this before I bought my barbs from McMaster. I got the polypropylene barbs of course. I agree that taping threads in this thing is not a good idea. I think I'll try and glue them on like you said.

[Yo-DUH_87]

hey, if you don't mind a small thread hijack:

How should I go about putting barbs on this:


???

Same method as you outlined for ChemMan?

Edit: it is made out of a hard plastic type stuff, was origonaly a food storage container

[bigben2k]

Yep, same thing.

The nylon barbs are available from a good hardware store, pretty cheap too.

[bigben2k]

Originally posted by ChemMan
Doh! Now I wish I had posted this before I bought my barbs from McMaster. I got the polypropylene barbs of course. I agree that taping threads in this thing is not a good idea. I think I'll try and glue them on like you said.

If it's a threaded adapter, you can still salvage it.

Go to a hardware store, and pick up a PVC threaded fitting. Trim off the non-threaded end, by giving it a rounded shape, to match the exterior of the res. Use the same "Plastic Welder".

Then, all you have to do, is screw in your polypro barb, with Teflon tape.

To go one step further, you can glue a 3/4" NPT fitting, just in case you decide to use a different size tubing, later on. Simply screw in a 3/4 to 1/2 adapter, also commonly available at a good hardware store (look in the grey, Schedule 80, fitting section).

[ChemMan]

I ran into a slight problem today. It turns out that I don't have a drill bit big enough to do the job. Well actually I have a 3/4 wood boring bit, the kind that is flat like a putty knife, but that bit won't do the job I don't think. I do have a 1/2" NPT tap though. I'll see if I can find those adapter you were talking about or try something else. Thanks for the advice.

[Yo-DUH_87]

Yeah, what kinda bit should you use for drilling plastics? Never thought about that one before...

All I have is wood bits in the size I would need

[bigben2k]

The wood drilling bit works fine: it's just acrylic that you're trying to drill. The flat blade style bits that ChemMan referred to, won't work, because they have a center spike that guides the rest of the blade, and since the hole is already started, the blade would spin out of position. Of course if you have a drill press, it's not a problem.

Just take it real slow, and don't apply a lot of pressure, otherwise the acrylic will crack, possibly into multiple pieces! Just let the drill bit do the work, and don't let it "grab" the acrylic.

A Dremel works very well here, as it allows you to simply cut around the hole, bit by bit.

[ChemMan]

Ohhh I'm good. Two 3/4 inch holes drilled and taped with 1/2 NPT threads. I used a 3/4 hole saw to make the holes. I was about to buy a expensive drill bit when I noticed the hole saws out of the corner of my eye. Much cheaper and just as good IMHO. Starting the tap was a bit tricky but all went well. I don't know if I'll get to post pics. Thanks