shot blasting advice

[christofloffer]

hi chaps, been a while since i was last on. i have just got myself a 20 gallon blasting pot and i am getting mixed results so i could really do with some pointers. this is a bit long winded but i may as well explain the set up properly.
i am set up in a tin shed with the compressor in the garage separate. i have made up a blasting bay insde using some sheet and an old kitchen extractor hood as a funnel to collect the grit. i also riveted the canvas hood supplied to an old welding mask and made i much more comfortable hood. the compressor is a wolf sioux 50 litre 2.5hp 9.5cfm. i know its on the bottom end of viable but time is not something i am short of so accommodating it is better than replacing it right now. the grit i got is a medium grade crushed glass. i only got 25kg as i didnt know whether i would be better off with a different grade, no better aid for decision making than experience. the work is a mix of steel, iron and aluminium. there is not much ally and i have tried one piece to experiment and its not too bad, leaves it a bit glitters though so i will have to wash it i think. the iron bit came up really nicely and took some primer well.

so, i did the iron bit as my first go and it was a hell of a learning curve but it didnt take too long and i was pretty happy with the result. the ally was more of a faff and took a bit longer as i was holding the nozzle a bit higher to avoid eating into the metal. i made a mistake by only opening the valve at the nozzle halfway to try and make it easier to control. this inevitably ate through the side of the valve so i replaced that. so i after a little trial and error i am happier with my work space.
however i was in there today to carry on and the blaster is not being very effective. at first the amount of grit was fairly thin but it made it really easy to see what i was doing and i liked that. today though the grit is hauling through in a thick stream. this seems to be significantly less effective as when it is flooding through it isnt removing very much at all. i am guessing that its just effectively making a pile of grit rather than firing a smaller quantity that cuts. i tried holding the nozzle higher but that did even less cutting.
there isnt any regulators on the pot, its just four valves to open or close the air in/air out/grit out/nozzle. it also started pulsing but i am assuming that its just the grit getting more prone to clogging as it gets used. i am filtering the grit when putting it back in.

any advice would be really appreciated as its turning into a bit of a bust at the moment.

 

[rtbcomp]

If the grit is coming out as a thick stream that means there's too much air in the pot pushing the grit out and not enough coming down the side pipe into the mixer. The bypass valve should be fully open and the valve at the bottom of the pot set to get the correct mixture.

If you can't stop the grit by closing the bottom valve then it's worn out.

Another thing that can cause a rich mixture is a split in the blast hose, this causes a drop in pressure at the output of the mixer and so more grit gets pushed out the pot.

 

[Turbo]

It's good to hear that your set-up is working, what size nozzle are you using? I was going to buy one of those blasting pots but I thought I wouldn't have a good enough air supply from my compressors. I might buy a pot & have a go now. Like you I have plenty of time so I don't mind if its slow.

 

[christofloffer]

ok, i will set the bottom valve to half and experiment from there. thankfully i bought a pack of the valves and they are all the same so i can swap it if it is worn out. it does stop the grit as i always close it off and blow the pipe out when i am done to avoid clogging (made that mistake once). i hope the hose and valve are still ok, i know things will wear out but if things only last for two parts then i may as well pack it in now. i'm sure it will be ok though, i am expecting to wear stuff out quickly to begin with whilst i get my head around it. it is certainly different to what i was expecting.

i think its a 3mm, or maybe 4mm i cant remember at the moment.
i looked at loads of blasting options for my budget and spoke to some folks about it. a cabinet would have been the easiest to use, tidiest and most time effective. however it all seems to pivot on what kind and size of compressor you have. these pots came recommended from a few places for use with smaller compressors as they dont use so much air to suck the grit up. then my compressor is 9.5cfm and this seemed to be lower than the recommendation for most cabinets. my compressor isnt the fastest with this pot but if your patient it seems to have enough guts and doesnt seem too hard done by. i just try to work in short bursts (20-30mins) and then let the compressor rest a while as i prep then next bit.
one thing i did quickly come the decision with is to forget doing large parts. i am doing this as part of a new business selling and restoring classic car parts and the time, power and materials would not be worth it. its is more cost effective to build up a small load and take to someone with a bigger and better rig. one or two bits would be slow but doable but i have piles of land rover bodywork to do and i would be on it for a long time with this set up. but then this pot cost me about £80 and it should earn that back pretty fast so i cant complain.
on a side note, this crushed glass seems great, cuts really well and flows nicely too. not too dusty either, from what i have seem from folks using different stuff. its also pretty cheap in comparison, though sand is cheapest it would clump up in no time here.

 

[rtbcomp]

Avoid sand - it's illegal to use with compressed air and doesn't last as long as glass grit, so works out more expensive.

If you're doing panels keep a very shallow angle and keep moving the nozzle otherwise you'll distort the panel. It's surprising how much heat builds up if you keep the nozzle stationary.

The best nozzles are these http://www.blastitclean.co.uk/page9.html

 

[christofloffer]

oh, i didnt know sand was actually illegal. many people said it wasnt worth using, but that never came up. i assume its because of the free silica thing? that was one of the things that came up with the glass, most places said "no free silica" in the descriptions.
thankfully the amount of thin stuff i have to do is very small. i have been keeping it to one side until i have gotten the hang of using the gear.

those nozzles arent cheap are they! i guess that they last a long time though, most tungsten-carbide stuff is built to last. i have ceramic ones for now. i cant see how they would attach to the valve though, are they threaded or just a really gradual taper to clamp on?

 

[rtbcomp]

Dump the valve on the end of the hose and fit the nozzle into the end of the hose and secure with a jubilee clip

 

[armalites]

Not sure where you are in Shropshire but I'm North Hereford and I have 2 tonne of recycled glass in 25kg bags so could sort you some out if you wanted to try some. I think I have mdeium and fine.

I think you could kill that compressor pretty quick if you were doing quite a bit of blasting. You can never have too much air available when it comes to blasting.

 

[christofloffer]

a quick apology here, i missed the bit about sand on the previous screen. just had a read through it like i should have done already.

i'm not sure i would want to dump the valve on the hose myself. i cant see how i would start and stop it without it. i could use the valves on the bypass and the bottom of the pot but that would be very awkward for me. i start and stop it fairly often as i move the bits around, which saves the compressor having to charge any more than necessary. plus i have some nerve damage in my chest so bending down too often would cause a lot of pain. i could use an extra short bit of hose on the other end of the valve to fit it though.

armalites, i am north shropshire, technically staffordshire, up by stoke. i think i will stick with the glass as it seems pretty effective. i am not too sure about how much and what grade to go for yet though. i will probably try a slightly smaller grade than the current one next but i want to go through a few bits first. this is still very much trial and error, i am figuring out what parts are worth cleaning up myself and what should go in a pile to take elsewhere. so i want to keep things as small as possible until i know what i am playing at.
as for the working, i am very conscious of the compressor. i can just about hear it kick in when i am in the shed and i try to make life as easy as possible for it. i only do short amounts, i would rather it take longer to do but stretch the life of the equipment for now. i would have loved to get a bigger compressor but i only had a finite amount to spend and this one seemed best at the time. if i find myself in need of more blasting and it looks viable then i can invest in a more effective set up but i need to figure some bits out first. i am thinking if i can do an hour a day then it should be fairly tolerable.

 

[rtbcomp]

I work inside a blast room or a use a cabinet so i just switch the air supply off to the pot and I've fitted an extra valve to de-compress the pot so no media gets wasted. It gets expensive changing the nozzle valve all the time!

 

[Turbo]

I have read somewhere about people gutting old spark plugs and using the ceramic insulator as a blasting nozzle!

There is a method of fitting those tungsten nozzles to a ball valve using a compression plumbing fitting, here's a link.

http://www.mig-welding.co.uk/forum/threads/tungsten-nozzles-for-sandblaster.9749/

 

[henry Kadzielski]

Good morning gents.
I have several blast pots, compressors and cabinets. So am curious as to why you are using crushed glass and not garnet. Garnet is much harder, available in many more grades and can be recycled many more times, also it is much less dusty. Instead of a valve at the end of the blast hose (if I understood that correctly) use a foot operated pedal. These pedals are either direct acting or pilot control, which operates the air valve.
Now is the pot you are using a suction or pressure pot. This makes a difference as to the connecting up.
If your compressor is too small, then you can get an additional small unit and connect them together in parallel. You may need to adjust the pressure cut in / out so they start and stop at similar pressures. One major point to remember is the moisture that you are introducing into the blast pot setup. If you run your compressor for some time it will introduce moisture into the air lines and then work it's way to the pot and grit. Once in there it will clog or partially clog the flow of grit. In that case dryers of some sort are a must.
The valve that controls the amount of grit that gets down the hose, what is it, as you mentioned that the grit was flowing too much and not cutting. I have had many unbelivable things stuck in grit control valves and since the flow of grit is reduced, the operator opens the grit control valve to compensate. Once the blockage has been removed, the result is too much grit. Too much grit down the hose feels like the hose wants to spew (vomit) and convulses.
If you are using a valve on the end of the blast hose to stop the blasting, when you shut off the valve, grit may still be trickling dawn the grit control valve into the blast hose and when you open the valve at the end of the hose you get a slug of grit which chokes off the push air and then just convulses down the hose.
Send me a picture of what you have and I can possibly give you some pointers

 

[christofloffer]

this is the pot:

http://www.ebay.co.uk/itm/120500458414?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

it is a pressure pot, that was the recommendation from most people in order to make the most of a more limited air supply. it does have a moisture trap on the inlet and i make sure to loose any trapped water in the compressor tank regularly. so would a foot valve need to carry grit too, and then wear out? i made the mistake of shutting off the air before the pot once and the grit just filled the hose instantly and i had to remove it and empty it out. since that i shut the pot off and clear the line and decompress the pot from the bypass when i am done.
so far i have not had any issues with clumping, but the tin shed is a veritable oven at this time of year so winter may be a different story. i also make sure to never leave any shot in the catch bucket when i am finished, i filter it back into the pot and keep it in there. partly to make sure things are ready to go next time but mainly to avoid picking up any more moisture than necessary.
the choice of glass over garnet was mainly cost. the glass seems around £10 per 25kg cheaper than garnet for the quantities i am looking at for now. it was also mentioned a few times that i shouldnt be using garnet on aluminium and things, which given that i have a mixed bag of materials in the cleaning pile it seemed fairly relevant.
the valve on the bottom of the pot is just one of these:

http://www.ebay.co.uk/itm/400830626834?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

it was wide open the first time i used it and it wasnt too bad, but i did find a big chunk of glass in the valve at the end (lesson learned there and it will always be filtered in when filling now, even when new). second time i used it i had both valves (pot and hose) half open and blew a hole in the side of the hose valve. i do stop and start on the hose valve and get an initial spew but it went back to normal after that. the last time i used it it was pulsing and spewing.

 

[rtbcomp]

The correct way to set it up is
1) Air supply valve off
2) Bypass valve fully open
3) Hose (nozzle) valve fully open
4) Bottom valve fully closed
5) Switch air on
6) open bottom valve until the grit stream at the nozzle is just visible

Use the bottom valve to adjust the mixture
Use the regulator on the air line to adjust pressure
The bypass, supply and hose (nozzle) valves should always be fully open.

To shut down
1) close the bottom valve
2) close the supply valve when grit stops coming out of the nozzle
3) make sure the pressure in the pot is down to zero before removing the filler cap

 

[henry Kadzielski]

Good morning gents.
As rtbcomp stated above, that is the correct operation of the pot. You should never control the flow of abrasive from the or a valve at the end of the blast hose. These pots are designed to limit the air consumption by staying pressurised and putting a pusher line air shutoff valve will cause the grit to get pushed down the hose and choke the hose. In larger pots they use a 'Thompson II valve' to shut off the abrasive, and keeps the pot pressurised. They also have a push air shutoff valve to stop air down the blast hose.
The manufacturer has made this pot simple, however difficult to operate and control, compared to a commercial pressure blast pot.
On another note, having a water trap is good but not good enough. If you blast for a longer time, the liquid water may get trapped in the water trap, however water vapor will condense out of the warm air and drop out in the grit or down the hoses. This is the major problem and only starts to happen when you are almost finished
If you wanted a cheap blast pot you got one, if you wanted a pot easy to use and control you missed out

 

[rtbcomp]

.......
If you wanted a cheap blast pot you got one, if you wanted a pot easy to use and control you missed out

True - but the change from a £1000 will come in handy.

 

[henry Kadzielski]

Point taken
It is very much what you are doing and what you require. You can put up with some inconvenience of a much lower price

 

[christofloffer]

True - but the change from a £1000 will come in handy.

precisely my issue at the time. i got a loan of £1000+£400 of my own to get a business off the ground and i had other stuff to buy too. i still think that the blasting has a place and will be worthwhile but i need to test the water before i can warrant looking into bigger and better gear. i can stomach the extra work involved if it means keeping costs down to begin with.
what i really wanted was a self contained cabinet, but the general consensus was that i would have needed a bigger compressor for that. plus now that i have started i think having a separate room is better anyway as it has more options, and saves getting the workshop too mucky.

i will get out there today and see how i get on using it the right way. why would they put a nozzle valve on it if it will just causes problems? bit confusing and makes it a bit counter intuitive. i might see if i can extend the valve handle a little to save the bending, then it would be much easier to live with. hopefully i will get on better.

 

[rtbcomp]

The nozzle valve will last a bit longer if it's either full on or full off and if you operate quickly. It may help to turn it round every time you fill the pot to even out wear.

Working in a blast room you can do without it, just close the bottom valve and switch the air off.

You could mount the pot on a pedestal to avoid the bending, but then you have to lift the media higher to get it into the pot! I was thinking of using a bowden cable to operate the bottom valve.

One idea I had was to put a window and armholes in the side of my blast room so it would function as a cabinet for smaller items and I wouldn't need to get togged up in boiler suit and helmet.

 

[christofloffer]

got out to have a couple of shots with it today. in a shocking twist, using it properly worked!
thanks for the advice guys, i could have been doing it wrong for a long time. it was certainly more controlled, though definitely more work for the compressor. i could hear it kicking in more often and lost pressure a couple of times, but at least it was more effective when it was running. got through the grit a lot faster too.

i'm thinking about getting one of these little guns:
http://www.ebay.co.uk/itm/151436035747?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT
might be easier on the compressor when i am doing small parts. a lot of refilling i would expect but it would be good for quick jobs. any experience of using these?