Cutting Holes In Polypropylene

[kieran]

Hey all,

I want to cut a hole in a polypropylene fermenter. I tried once and made a bloody huge mess of it. I started with a drill, which kind of tore a hole through.. rather than boring a hole (didn't matter though because it was a small drill bit (5mm) and I needed a 25mm hole. So then I went for a hole saw. Well bloody hell, if the thing didn't go sliding and gliding across the surface of the fermenter scratching the hell out of it. That was a treat. In the end, I managed to steady it, but it tore a hole through on one side and ended up being about 28mm (or a bit over) as a consequence.. I could never get the thing to give a good seal on a 1" bulkhead for a tap, so I used some new Loctite HDPP/PP capable glue (some fancy new thing), and glued another PP chunk over the failed hole. I then gave it a thorough going over with food-grade silicon to smooth it all over. So the old hole is patched up, and holding water.

Time for round 2.

The issue is a combo of the rounded outer surface being hard to get the teeth of a hole saw started on, and then once that's happening, having a hole nicely milled out - rather than torn out.

Does anyone have any hints or tips?

I have a dremel-clone now, and am thinking of making a template and going at it with a borer/engraver type bit.. the drill is just to hardcore i think..

cheers,
kieran

 

[manticle]

Can't advise exactly on the polypropylene. However a trick I've used in work situations drilling through perspex is to use the drill on reverse which should burn/melt through rather than cut/shred through. The cutting part of the drill bit isn't activated in reverse. When used on perspex it has a tendency to crack it so I presume the shredding of the PP is caused by similar.

Might be worth a go if you have something you can practice on.

 

[Malted]

Did your holesaw have a drill bit in it?
Most holesaws have a drill bit that holds the 'saw' from skipping all over the place, it follows a pilot hole. 5mm is probably a bit too big for the pilot hole, you want it smaller than the drill bit in the holesaw. You'd want a <2mm or so pilot hole. 5mm is a relatively large bit and because the fermenter is soft, it will tear into it. Sharp, large drill bits will tear into soft stuff.

Is this what you mean by 'holesaw'?




You should NOT use them like this without the drill bit.


You might be able to (but may not need to) drill the holesaw through half way and then drill the other half from the other side. Don't drill too fast.

Edit: if the end of the hole saw is not in the drill chuck properly it will be off centre and wobble about as you drill in and create a larger hole. It is relatively easy to not have the three sides of the whole saw aligned with the grapsing parts of the drill chuck.

 

[TidalPete]

Mark out the diameter of your hole slightly undersize with a pair of dividers.
Carefully cut along the INSIDE of the line with a jig saw or a coping saw.
File to the line with a 1\2 round 2nd cut or smooth file.
Smooth finished edges with wet & dry.

TP

 

[kieran]

Thanks for the suggestions. The issue I had was more I think due to the curvature of the surface. Due to the 25mm diameter, only two opposite sides initally make contact with the surface. If you don't hold the saw bit perfectly level against the surface, one side 'bites'... and I think that is what caused it to skid off.

Edit: thanks tidal pete. I don't really have any jig or coping saws (yet!). Might be worth a trip down the hardware saw. The dremel is starting to look like a good option now.

 

[TidalPete]

Edit: thanks tidal pete. I don't really have any jig or coping saws (yet!). Might be worth a trip down the hardware saw. The dremel is starting to look like a good option now.

kieran,

A koping saw costs only a few dollars mate & I must say that in this world where power tools are seen to be the only option there are many instances when a manual tool will do the job just as well or even better. :beer:

TP

 

[dkaos]

kieran,

A koping saw costs only a few dollars mate & I must say that in this world where power tools are seen to be the only option there are many instances when a manual tool will do the job just as well or even better. :beer:

TP

Agreed. You can never have enough hand tools for delicate jobs.

 

[amiddler]

What about a 25mm spade bit?

 

[kelbygreen]

you cannot scribe skirting boards with a jig saw and copping saw is the only way although you can cut the straight cut most the way with a compound miter saw always back cut it a little as if old building not always square. So you will find uses for it one day.