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Compressor Pump for Pneumatic Spearguns

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
So, after years of procrastination, I finally got around to this little project: Using a refrigerator compressor as a pump for pneumatic spearguns.

The idea is not new at all. In fact, it was mentioned on this very forum years ago and there are a lot of build-threads on an Italian forum as well. So, the inspiration was there, the research was easy and Youtube helped with the rest.

This wont be a very detailed build-log as it's all pretty straight forward. Where I did do things slightly different, I'll make sure to point it out.

You can use a new or old pump. I happened to buy a new one, which is likely a Danfoss clone, from an online shop in China (where I live) and had them ship it without any gas in it.
In regards to the size of these things, I think they can all put out enough pressure. But the bigger ones move more air, so it will be faster to take a gun up to pressure. I still opted for one of the smaller ones. They take up a little less space, weigh a lot less and are cheaper, too.

This is what I bought though mine is a 75w model (pic below shows a 65w version). Apologies for the vendor's pic as I forgot to snap one myself when I unboxed it:


I have a box of misc. pipe fittings lying around from building my vacuum pump and the Scuba Tank "Pump" project, so for this one, I just needed a bit of copper pipe and elbow fittings, which I soldered to the stubs on the compressor:


I actually ended up using a quite soft solder as I didn't have any luck with the stiffer stuff with more copper in it.
As you can tell from the pic, I used a U-bolt to hold the T-fitting in place on the alu frame to add some rigidity to the piping. A side bonus of that setup is that if any of my soldering joints were to give, nothing can really come flying at me. But I test drove this compressor already and the soldering job is holding up fine. The trick is to make sure your joints are well abraded, solvent-cleaned and fluxed.

On my Scuba Fill Adapter I have a safety valve, but I don't on this compressor build. I opted to leave it out for two reasons; first, I don't have another safety valve around and secondly, the compressor pump is quite slow so should leave me enough time to shut it off safely. If I change my mind, I can easily swap out the T-fitting with a four-way one and add the valve.

You can go about this a 1000 different ways when you choose your fittings, especially because you can modify and solder fittings on easily. E.g. the fitting that goes from the outlet elbow to the T-fitting is a barbed brass hose fitting that I drilled out for the copper tube. Here's a pic showing how it looks un-modified:


As for the pressure gauge, I knew I wanted one that went up to 40 bar as my Mirage guns will normally be pumped to 28-33bar. These gauges come in different sizes named after the size of their dials. Here are three sizes: 40, 50 and 60mm (all to 40bar):


In the end, I went for the bigger 60mm one. It makes it easy to read:


One thing I did do differently than any of the build-threads I have come across was to change out the oil in the compressor. The compressor motor sits in an oil bath but if I understand correctly, a bit of that oil gets pushed around with the gas in the whole system (in the case of an AC/fridge/freezer). Since I don't know if the compressor oil is any good in guns, I changed to the same oil I use in my guns. Might be a bit risky, but so be it.
Compressor oil on the left, gun oil to the right:


To change the oil, I poured out the original oil, flushed with a solvent, let it evaporate and then poured in an equal amount of new gun oil.

I've seen other guys add an air filter on the intake line which seems like a good idea to protect both the gun and the compressor from debris getting into them:


I attached the filter with a bit of PVC tube onto a barb fitting that I soldered onto an elbow which in turn is soldered onto the air intake. If I ever wanted to use this compressor as a vacuum pump, it is as easy as unplugging the air filter and plugging the vac hose on instead:


One thing I knew I wanted on this build was a handle. The compressor weighs 6.5kg and there's no obvious place to grab it for moving it around, so I added a T-handle. It also serves to lock in the alu mounting frame on the top:




Here it is in its almost final beauty - I will add an on-off switch later on:




Forgot to say that the high pressure hose is from a vendor that sells parts for PCP guns (pre-charged pneumatic air rifles and such). Those guns run at around 200 bar, so this is more than safe for my use. It has quick-couplings on both ends. But you can easily leave the hose out, a lot of builders have done that.

The pump valve adapter on the gun side of the hose is a bit of a Frankenstein rig. Its heart is the Salvimar fitting at the front (this one is for Mares, but they make and sell them for Cressi and Salvi, too) and then some adapters to get the male quick-coupling fitting mounted. This is one thing that could be made as a single part on a lathe, which could be a latter upgrade;-)
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
OP
OP
Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
So, when I mentioned that the pump-to-gun-fitting could be made on a lathe, I was kinda teasing... On occasion, and with growing frequency, I have thought about buying a so-called mini-lathe. And I just did;-)
Pros say they are worthless, but I have spent weeks looking into this and the consensus is that with a bit of care, proper setup (do not trust the Chinese assembly work) and not taking greedy cuts they are more than fine for our needs and can hold surprisingly good tolerances.

Here are my two first tests. First one is some cuts in aluminium, just trying to get a feel for the whole thing and to see if I could part of the tube easily and also cut a nice o-ring groove. This alu is a bit 'gummy' so the surface finish is not mirror perfect but again, I am not making parts for NASA and this is my first foray into machining so I am super content:


I always end up going to a local machinist to have my shafts cut down and re-threaded and while I was not expecting too much luck with hardened 17-4ph, which all my shafts are made of, I was amazed with the ease I could cut this and the finish I obtained on this little hobby lathe. (The right-hand end of this bit of shaft is my test cut):


The first real part I will set out to make is a one-piece connector for my pump instead of the jerry-rigged one above.
As for the standalone pressure gauges there is room for improvement, too. I could make one with minimal internal air space so I don't loose 2 bar each time I take a reading. And I can make them way less chunky than the ones from Omer.
But of course, it doesn't stop there - I can cut down barrels and shafts myself now and if I get really courageous, I might attempt making a vacuum muzzle some day. So, stay tuned:)
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
My new mini lathe is a ton of fun. Of course, it is not amazing out of the box but I have spent a bit of time getting rid of the worst of the run out (misalignment) and done a few minor modifications and now it isn't too bad at all.

The first project didn't turn out to be a new adapter for the pump above, but instead shortening some barrels for a another custom length Mirage (I will post in my Mirage thread later on). But today, I got around to making the adapter.
a
First a pic of how the original used to look:


The pump adapter itself is from Salvimar and then it took two other fittings to work with the quick coupling for the air hose. On top of that, the "wrench flanges" on the Salvimar adapter were rubbing on the gun, so I filed them down a bit.

In the next pic, on the top you have the old adapter and under it is my DIY version. Actually, the first "project" I did on the lathe was to take the old pump adapter and turn down the flanges. Then I had a go with a knurling tool, though that didn't turn out too super.

My own version turned out pretty OK, though:




As mentioned in a previous post, it shouldn't be too difficult making adapters for pressure gauges, either. Just need the correct female thread for the pressure gauge where the male quick coupling stub is on this one. And then, of course, I'd need a pin to depress the ball in the pump valve. There are some good ideas on how to make that pin adjustable on this Italian website.
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
Just wanted to add the newest adapter to this thread as well. This time in titanium for the heck of it.
The original brass one is actually slightly too short for one of my handles, so making the ti one longer makes it easier to screw in and out:

(BTW, the weight of the ti adapter is 18g vs. 50g for the brass one).

But I messed up and broke the tap in the part trying to cut the thread for an adjustable pin to open the ball valve, but the adapter still works just like the brass one (I might try to fix the tap issue later on):


I also made some caps to protect the pump thread from getting dirty (and getting blown into the gun):


Finally, if any of you need a schematic of the pump thread, here is mine. It is slightly modified from Seac and Mares measurements, but nothing crazy:


The thread is M14x1.This will work for Mares and Seac and possibly others. It wont work for Cressi (M18x1.5?) nor Salvimar (M14x1.25).
As to o-rings, a 6.75 x 1.78mm (ID x W) will work splendidly.
 
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Jenno1983

New Member
Jul 29, 2019
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0
1
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Rockhampton
My new mini lathe is a ton of fun. Of course, it is not amazing out of the box but I have spent a bit of time getting rid of the worst of the run out (misalignment) and done a few minor modifications and now it isn't too bad at all.

The first project didn't turn out to be a new adapter for the pump above, but instead shortening some barrels for a another custom length Mirage (I will post in my Mirage thread later on). But today, I got around to making the adapter.
a
First a pic of how the original used to look:


The pump adapter itself is from Salvimar and then it took two other fittings to work with the quick coupling for the air hose. On top of that, the "wrench flanges" on the Salvimar adapter were rubbing on the gun, so I filed them down a bit.

In the next pic, on the top you have the old adapter and under it is my DIY version. Actually, the first "project" I did on the lathe was to take the old pump adapter and turn down the flanges. Then I had a go with a knurling tool, though that didn't turn out too super.

My own version turned out pretty OK, though:




As mentioned in a previous post, it shouldn't be too difficult making adapters for pressure gauges, either. Just need the correct female thread for the pressure gauge where the male quick coupling stub is on this one. And then, of course, I'd need a pin to depress the ball in the pump valve. There are some good ideas on how to make that pin adjustable on this Italian website.
 
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Diving Gecko

Diving Gecko

shooter & shooter
Jun 24, 2008
1,268
309
138
Shanghai
Hi where did you purchase your original adaptor from before you fabricated your own?
It's a catalogue part from Salvimar, so you might be able to get any shop who regularly orders Salvimar to get one for you.
I might have gotten mine from http://www.edosub.it/
Find Edo on Facebook or email him though his website and ask if he has them in stock.