Small coaxial

[MrCrowley]

With the seam issue, what I did was...

Jam it in a vice, then with one hand hold a set square (to make sure the ruler is straight in comparison with the piston) and then using a dremel with a small cylinder tool bit, I dremeled a small groove into the piston.

It fits perfectly in the pipe's seam.

BTW I'm up in Auckland

NZ Spudfiles Members:
shud_b_rite - Auckland
Battlemonkey - Hamilton
nz_cannons - Nelson
MrCrowley(me) - Auckland

[rixth]

Yeah my current piston is a Coconut Cream can (250ml) that fits PERFECTLY in 50mm PN12 PVC, but not in the steel pipe. I've cast one out of hot glue now, just waiting for it to harden. How long should a piston be?

I've thought about getting a file and removing the first few cm of seam.

I must say though, I'm worried about the schedule 10 thing... (see my last post on page 2).

Oh, and I'm in Masterton =(

Edit: ahh! My piston is stuck inside the PVC I formed it in, I hit it as hard as I could (expecting it to just pop out) and it would not budge. Meh! Guess I'll buy more glue, and make another one, this time inside a paper sleeve or something.

[rixth]

MrCrowley,

Just flicked through your instructable, saw you're using a deoderant can piston, well, I can testify that Rexona for Men cans fit PERFECTLY in 2"/50mm pipe. Plus, the end is curved in so it cups the barrel.

So yes, plan now is file the seam out of the pipe (only for 10cm), and make a 75mm long piston out of a Rexona can.

[jackssmirkingrevenge]

How long should a piston be?

The piston doesn't need to be longer than the width of the chamber, and if you can put in a lightweight core in the middle to reduce weight - have a look at my tutorial, it's writted for epoxy casting but the same principles apply.

ahh! My piston is stuck inside the PVC I formed it in, I hit it as hard as I could (expecting it to just pop out) and it would not budge. Meh! Guess I'll buy more glue, and make another one, this time inside a paper sleeve or something.

First of all you should be casting it directly in the pipe you're using as a chamber, and secondly, did you lube the sides of the pipe well first?

[MrCrowley]

Just use some silicon spray or cooking spray when casting hot glue pistons, they'll pop right out.

JSR you should write a how-to on how to minimize pilot volume to your standards

[rixth]

Ok, so piston size:
- as long as the chamber is wide
- how far should it travel?l Is 10mm enough? The barrel is 20mm.

Am I going to weaken the pipe by grinding out the seam? (see picture)

[jackssmirkingrevenge]

Am I going to weaken the pipe by grinding out the seam?

Of course you're not making it any stronger but it shouldn't influence the amount of pressure it can take, especially as it's only for a small section.

how far should it travel?l Is 10mm enough? The barrel is 20mm.

For a co-axial, piston travel more than 1/4th of the barrel diameter - 5mm in your case - is unnecessary and wasteful.

JSR you should write a how-to on how to minimize pilot volume to your standards

Tired as I am of answering the above question, I just might :p

[rixth]

Time for one of those 'how long is a piece of string' questions.

How tight SHOULD a piston fit?

[Novacastrian]

Ok, so piston size:
- as long as the chamber is wide
- how far should it travel?l Is 10mm enough? The barrel is 20mm.

Am I going to weaken the pipe by grinding out the seam? (see picture)

To answer to the best of my knowledge,
Yes that size piston should work fine
You could probably half the piston travel (D4) to 5mm
You may weaken the pipe, but it's plenty strong- don't worry about that.

Good Luck

[rixth]

Thanks for all your help guys!

Now just gotta wait until Monday for the plumbing supply stores to reopen so I can get my barrel!

Will report back.

Tom.

[MrCrowley]

Ok, so piston size:
- as long as the chamber is wide
- how far should it travel?l Is 10mm enough? The barrel is 20mm.

Am I going to weaken the pipe by grinding out the seam? (see picture)

Don't grind the seam, adjust the piston to the pipes standards not the other way round:
With the seam issue, what I did was...

Jam the piston in a vice, then with one hand hold a set square (to make sure the ruler is straight in comparison with the piston) and then using a dremel with a small cylinder tool bit, I dremeled a small groove into the piston.

It fits perfectly in the pipe's seam.

[rixth]

I don't have a dremel though, and I'd rather not buy one, as I can't see myself using it often... a drill is pretty much all I have sadly.
Hmmm found this http://www.dse.co.nz/cgi-bin/dse.storef ... duct/T4823 might consider getting one if I can justify it.

I'm curious as to why you say modify the piston and not the pipe, is that because of weakening it?

What was the piston in your steel cannon made of? Any chance of a picture?

[MrCrowley]

It was made of a PVC fitting, the job can be done with a file in no time as well.

I was going to get one of those dremel but got a Riobi one instead, they are handy in this hobby.

It may weaken the pipe a bit as well, but it would also be difficult to file the seam down evenly and completely even if it only 5cm of it. Without a dremel it may also take awhile. It would be a lot easier just modifying the piston.

[jackssmirkingrevenge]

Time for one of those 'how long is a piece of string' questions.

42

The piston should be as tight as possible but you should still be able to push it along the chamber with 1 finger and without straining yourself. As with many other piston-in-chamber situations, the use of lube is highly recommended

[rixth]

I may have found my piston, in a material that I could file a groove into.

Was looking through Marley (NZ pipe/fittings manufacturer) documents, and found the information below.

The ID of my pipe is just over 53mm. I figure I buy one, and cut if off at the red line on the diagram. Fill it with glue, then screw a seal on the end. It might be a bit short though.... about 35mm by my calculations.

Sound OK?