Shaping rubber and rubber balls.

[T i m]

Hi all,

In the spirit of d-i-y I've taken the non return valve out of my Bambi

75/150 compressor and found out why it is leaking.

The actual valve is small rubber 'bullet' (10mm diameter x 10mm long) that has a fairly light coil compression spring clipped over a shoulder at the back and this 'bullet' is pushed against a conical seat, initially by the spring than then the stored air pressure (~120 psi).

From close inspection it looks like something has been trapped in the valve and formed a slight ridge in the rubber bullet face, stopping it sealing properly.

So, on the grounds I've little to lose (I'm probably in for a new valve anyway[1]), if I could gently hold said bullet in the lathe, what sort of tool (hand or machine) would be most likely to get me re-surface this without ripping it up do you think please?

The rubber is about the consistency of a pencil rubber.

An alternative solution could be a 10mm diameter rubber ball and I could always turn a small nylon cup to carry the spring etc.

All the best ..

T i m

[1] My local Bambi dealer doesn't carry a spare valve so I've got a bit of time to play. Also it seems a shame to throw away a perfectly good, nicely machined and chromed brass fitting for the sake of a 10p rubber valve. :-(

[mark]

It will grind nicely,but keep a good hold of it.

[dennis]

Some fine wet or dry paper I would think.

Just cup it in the hand and rotate the rubber slowly.

[George]

of wood> An alternative solution could be a 10mm diameter rubber ball and I

Do you have a conical drill bit? drill a conical hole 10mm wide in a bit of wood,then turn the rubber bullet in the wood hole. Doesnt take much effort to smooth off rubber.

[The Natural Philosopher]

Indeed. Or simply mount in anything that spins and hold some fine wet and dry against it.

Or even make a mould and cast one out of silicone ...

[T i m]

Ok ta and the keeping hold of it seems to be the issue!

I tried putting it in the lathe earlier but it's just too unstable to be reliable (and I've images of it flying across the workshop, never to be seen again) :-(

What might work though is if I can hold it further in the lathe chuck, counter drill a small hole from the back and then glue in a small plastic rod (knitting needle etc) that should give me something to hold it with to be able to spin it whilst dressing it on some wet-n-dry / file / whatever?

All the best ..

T i m

[T i m]

Understood.

I'm not quite sure yet how important concentricity is compare with surface smoothness. The existing item looks quite smooth till you look at it closely but if you view it in profile and slowly rotate it it also looks a bit wonkey (like it's not sat square in the seat for a fair while).

All the best ..

T i m

[T i m]

Conical burr probably George.

Ooo, that's a goodun. I still need to sort the holding bit then.

Ok, thanks.

All the best ..

T i m

[T i m]

That's the tricky bit as if you hold enough of it to be sure it's held there isn't anything left to tidy. :-(

And alternative to me fitting it on a plastic mandrel above I could glue it temporarily to a holder as there is enough rubber shoulder (that the little coil spring slips over) for me to shave it off the mount once finished.

Ah, there's an idea. At least I could turn a plug in something harder (like ally) but I think that might be going a bit too far (time wise , well depending on how much the replacement one is of course).

Alternatively I've been looking about for a plain rubber ball of about

10 mm diameter. I think a mate has found something that is .43" (~10.9mm) in paint ball gun round. It might just be too big though and ironically, being a bit smaller than 10mm would be ok / better.

I've also just had a quick look through a pound shop in case a kids game had a suitable ball but the nearest I saw was a mini snooker set and they were hard plastic. :-(

All the best ..

T i m

[meow2222]

plastic probably would work if you lined the valve seat with soft silicone.

NT

[Spamlet]

If you haven't already tried the suggestions - the fiddly part with rubber being finding a piece as hard/soft/solvent proof etc. as the original; if you can find a piece of the right stuff, I find an old set of lab technician's cork borers often comes in useful for making washers/bungs and the like - you might find that your original can actually be *revived*.

We used to sell 'blanket reviver' for rubber printing blankets. This was basically a mixture of white spirit and a solvent like dry cleaning fluid or paint thinners. This swells the rubber and takes off the glaze. The right mix can make rubber double or treble in size or more with soaking. In your case you only need to soak it long enough to take out the ridge. Note that while the rubber is swollen full of solvent it will be weak so let it dry to a 'no smell of solvent' state before you put it back.

Academic as you've probably finished by now...

S

[T i m]

Ooo, no I haven't tried anything as yet as I have been busy elsewhere and was waiting for a call re how much a replacement was. Good news, £19 for the part, bad news £50 minimum order and £20 postage!

Ok

Oooo ..

Not at all, so, where do I find said potion please or are you saying it is simply white spirit and dry cleaning fluid (or was that just a guess / example)

[Andy Wade]

The trick for machining rubber is to cool it down to about -50 deg C first (then work quickly). I've seen this done and it's very effective, but the trouble is that a small workpiece like yours will probably warm up too quickly for the technique to be useful.

Freezer spray or a Peltier cooler module from RS will get you down to somewhere near the target temperature.

[geoff]

And what's wrong with a british summer ?

[T i m]

I like it, rather than looking for rubber 'valve' .. ;-)

However I'm not sure how easy it will be to re-engineer in practice as the seat is at the bottom of a fairly deep body.

Something else to think about though if the rubber won't recover chemically (I've tried lighter fluid so far (no change), now on white spirit).

All the best ..

T i m

[T i m]

Thanks Andy but I think it will be easer to buy a new valve (as interesting an experiment it would be etc). ;-)

I haven't yet though ... still waiting to stumble on that perfect rubber item to be a replacement valve ...

All the best ..

T i m

[T i m]

Right, so far I've soaked the rubber valve for several hours in: lighter fluid, white spirit and panel wipe with no affect at all?

If that means anything to anyone (from their likely chemical content) what sort of things should I try next please?

All the best ..

T i m

[Spamlet]

Not sure what the 'panel wipe' is, but all the others are 'non-polar' paraffin solvents. So you now move on to the other kinds - your ketones like acetone, and mixtures like 'cellulose thinners'; and the chlorinated compounds. If you don't have any chlorinated solvents in your rather enviable list of what you have to hand, getting some methylene chloride/dichloromethane containing brush cleaner may be as near as you can get in 'off the peg' availability. Dichloromethane is pretty much the most aggressive of the 'off the peg' compounds when it comes to dissolving organics, which is why it is first choice in paint stripping applications, where cost and fumes are not an issue. This would have been my first suggestion if I wasn't concerned that you might simply dissolve the thing before you could get it out of the solution!

Pencil 'rubbers' seem to be silicone based these days, if the smeary mess many of them leave on the paper is anything to go by. I'm afraid, I don't know enough about these to know if they can be swelled, but, if this is what you do have, then you ought to be able to mould a new piece as someone suggested earlier. Sorry if my suggestions have delayed your 'final solution'.

Cheers, S