Blasting issues

[droopsnoot]

I don't have a lathe, but after searching in vain last night on eBay for anything that looks similar, I decided that a piece of bar of suitable diameter could be used, and just drill a hole to get the appropriate size. Might not be quite central, but I don't imagine that matters very much for an experiment. I will measure the outer diameter later, hopefully it will be something standard, failing that I think a mate has a lathe.

It's just frustrating that it was working up to a point yesterday (I did a reasonable job on some of the wishbone) and then just seemed to stop.

 

[Shox Dr]

I found aeration of the media, the best fix, run a small bore pipe with the outlet close to the end of the suction pipe, with a inline tap to throttle the output.

Then go buy/beg/borrow/build a blast pot, you'll never go back and wonder why you farted about for so long with suction

 

[droopsnoot]

Interesting comment about the blast pot, I'll have to have a look around for more information on that, cheers.

 

[Wallace]

What size hole is in the blast nozzle? Might be too big or worn for the compressor output. I use a 3hp compressor @ 90psi with a 3mm or 4mm nozzle and J-Blast superfine in a Clarke cabinet with vacuum which works reasonably well but a bit slow on large parts.

 

[droopsnoot]

That's a good question, I'll measure it later on, but I suspect it's too big, though there's an internal nozzle that limits the air supply which is much smaller than the outer one. Reading an article elsewhere on here about blasting it seems that on my 3hp compressor it really needs to be about 3mm.

I've done a bit of research on blast pots, including several threads on here where people have made their own. I'm not sure I fancy trusting my welding to that pressure, even if I can find a suitable tank, and I notice that eBay are showing 10 gallon blast pot "Kits" for about £60, and at that price I wonder if it's worth making one.

 

[Shox Dr]

@60quid as you say just buy one, you'll always get your money back, and you'll have more money to spend on a bigger compressor then

 

[droopsnoot]

Looking at the nozzle, the outer diameter is just over 12mm and the hole is around 8.5mm. That sounds very big, though there is a much smaller air nozzle, closer to 3mm but I didn't measure it. Would that be small enough, or do I need to reduce the output nozzle as well?

 

[droopsnoot]

I had another go with this yesterday. As an experiment, I made a new nozzle from a cut-down section of an old bolt of suitable size. I drilled a 5mm hole down the centre, quite a bit smaller than the old nozzle. I also spent a bit of time sieving some of the media into another bucket. I got some better results, and was able to blast along one edge of the fuel tank quite nicely, though after a while it stopped picking up again. I did add a moisture trap which I hadn't been using before, that resurrected it for a bit but it stopped again - by then I'd blasted a lot of the suspect area of the tank edge so I took that as a sign to stop. I then had a bit of a therapy session sieving the rest of the media, and the stuff I'd just used and recovered from the bag.

So it's a bit better with the smaller nozzle, but I suspect I might have to grab one of these cheap blast pots to see if that helps. As an aside, I decided on a 5mm hole to start with because the air comes through a 3-4mm hole in the blaster handle, and I figured that if I made it smaller than that the compressed air would take the path of least resistance and just blow back into the media bucket. It was some time before I thought about how spray guns work even though the nozzle is much, much smaller. I'd drilled it out by then, though.

 

[Wallace]

Have you copied the original nozzle shape inside? The gun works on the Bernoulli Principle, high speed air passing through the nozzle creates a vacuum behind it that lifts the media. What pressure are you running?

 

[rtbcomp]

Have you copied the original nozzle shape inside? The gun works on the Bernoulli Principle, high speed air passing through the nozzle creates a vacuum behind it that lifts the media. What pressure are you running?

It's the venturi effect. https://en.wikipedia.org/wiki/Venturi_effect

The grit nozzle has a parallel bore, it's the bit between the grit & air nozzles, where the pickup tube fits, that needs to be the correct profile for the venturi to work.

 

[droopsnoot]

That's a relief, I just drilled a 5mm hole through the bit of old bolt. I can't answer the question about pressure, because of the interesting layout of the shed at the moment both the pressure gauge on the compressor, and the one on the moisture trap, are facing the wall so I can't read them. About time I had a tidy up. As I've been playing more with fuel tanks today, it appears I have a bit more blasting to do, so I'll move some stuff around before I try next time.

 

[rtbcomp]

One thing you will notice is that the bore in the nozzle will wear unevenly. There'll be greater wear at the end where the grit exits. The Bernoulli Principle mentioned by Wallace explains this nicely, the pressure will be lower the further downstream you go, so the speed of the media will increase causing more wear. I use tungsten carbide nozzles and the difference in bore size at each end is quite marked.

 

[paceman1]

the guns tend to block up behind the nozzle dampness does this clean it out with the air supply disconnected and if the nozzle hole is to large you wont get a good result smaller hole on low cfm compressors works well

 

[droopsnoot]

That's really why I tried this out - I know an old mild steel bolt isn't the best material, but it's a quick cheap way to see whether a 5mm hole (for now) is better than the ~8.5mm one I had before.

 

[slim_boy_fat]

the pressure gauge on the compressor, and the one on the moisture trap, are facing the wall so I can't read them.

Slip a mirror down there, you should be able to see them, although reversed image.