# Fun With Blast Cabinets, Air Compressors, And Vfds



## JimDawson (May 26, 2016)

I have an old HF blast cabinet and am sandblasting a couple hundred UHMW parts to finish them.

On the far right is the part after machining, center is after sandblasting, and on the left is the finished part after treating with Trim & Shine.  (an Armor All like product, but a lot less expensive)  This process gives a very nice black matt finish on the part, and covers the machining marks and minor scratches.



It was not going real well, the sand delivery was not very consistent and it was slowing me down.




So a couple quick mods.  I had previously shortened the suction hose and ran it over the top, the original configuration enters the bottom of the gun, this seems to work better.

I added a Tee to the inlet air and stuffed the end of that run up the end of the suction tube.  It works like a suction dredge.



The other end



That helped immensely with the sand delivery, but........It uses a lot of air.  I was controlling the air with a ball valve and the gun trigger.  But when I made this mod, the air was always on to the siphon tube.  Well that didn't work well, used too much air and was draining the tank too quickly because it was always on.    So...... Time for a foot valve, but no foot valve on the shelf.  But I did find an old Parker 3/8'', 5 port solenoid valve on the shelf.  OK, that'll work.....




Look around for a foot switch, nope, no foot switches....So off to Harbor Freight.  The valve only draws 17 Watts so the HF switch foot switch will probably live.



After about a hundred valve cycles, it quit working.  I had removed this valve from a machine about 30 years ago because it wasn't reliable.  Well today it failed completely.  This is really a new valve, I replaced it during testing of a new machine.  These valves are not rocket science, and I found there was no pressure to the pilot port.   But it was working, now it's not?????????  WTH?????

What I found was that the factory assembled the valve with the wrong size check ball in the pilot pressure port, and it dropped into the port and jammed, thus blocking the port.  I punched this one out, and dropped in a 3mm ball and end of problem.  (The little ball is sitting on a dime).




So now I'm back to work   But........Still using too much air.
About a year ago my air 5 HP compressor motor finally failed after about 25 years.  So I replaced the motor with a new one, but I made a slight mistake, the original motor was a 3450 RPM motor and I ordered a 1725 RPM motor.  OOPS    So I effectively turned my 5 HP compressor into a 2.5 HP compressor, but it runs at half speed and will probably last forever this way.  It keeps up with everything but sandblasting.




I traded about 20 minutes of labor for this compressor, but it has a bad motor so I can't just hook it up and go.  It says 7 HP on it, it ain't, but it is a true 5 HP.  I think they rate these at start load, not running load.



So the next question is what do I do to fix my air problems?  I have a few options.  The motor on the other compressor would run both of these pumps at half speed and double the current airflow and get me back to a full 5 HP.

So I could build a dual pump system with parts I have.

Or, I could just buy a replacement motor for the black compressor, and switch it on when needed.

Or, I found a 5 HP, 3 phase, 3450 RPM motor on Craigslist for $125, a pretty good deal.  I have a 15 HP VFD that is currently running the 3 HP vacuum pump on my router.  So I could pull the 3 HP VFD off of my mill and run the vacuum pump with that.  Then replace the mill VFD with a new sensor-less vector VFD to facilitate my direct drive conversion later.  Then I can use the 15 HP VFD to run the black compressor.   I've always wanted to try running a compressor with a VFD and vary the speed based on air use to maintain a constant pressure.

Decisions, Decisions


----------



## f350ca (May 26, 2016)

Im missing something or you missed the obvious Jim (and find that hard to believe from your work), put a new pulley on the 1750 rpm motor thats twice the size of the old one. 
Love the air injection into the suction line, I have the same problem you did with intermittent sand delivery. Maybe the POS Chinese gun Im using will work.

Greg


----------



## JimDawson (May 26, 2016)

f350ca said:


> put a new pulley on the 1750 rpm motor thats twice the size of the old one.



You are correct Greg, that would fix the problem.  I considered but didn't mention that option (and I should have) because I want to keep that compressor in it's current configuration.  It works for everything in my shop except sand blasting, and it's relatively quiet running at half speed.  Even with the 5 HP motor on it, it only draws 1.9 KW.

After sleeping on this, I have decided to go with the VFD solution.  The problem is that I want to get the current job out the door and get paid for it before I do that setup.  So what I'm going to do is go to the local rental shop and pick up a diesel powered compressor and use that to finish the job.

I'm also going to modify the suction tube on the blast cabinet and make a real suction nozzle something like this picture I grabbed off of the Internet.


----------



## kvt (May 26, 2016)

WHat size port are you planning on putting in the new suction nozzle.   I would think it would not take to big of one to help break up the sand pack and keep it flowing.


----------



## Grumpy Gator (May 26, 2016)

_In my old shop the problem was the media was not recirculating fast enough. It was sticking in the return hopper.So I got an air powered vibrator and screwed it to the outside of the chute with a small in line ball valve to control it. Sand used a lot of air flow and glass beads ¼ or less......The way to check is to give the return chute a bump with your knee while you are blasting.If the flow picks up you found the problem....Relative humidity is the culprit down here in Florida. As you blast the media gets smaller and the dust in the cabinet picks up the moisture in the air. I also put an air drier on the compressor outlet....Hope this helps..
*G*_


----------



## MSD0 (May 26, 2016)

grumpygator said:


> _In my old shop the problem was the media was not recirculating fast enough. It was sticking in the return hopper.So I got an air powered vibrator and screwed it to the outside of the chute with a small in line ball valve to control it. Sand used a lot of air flow and glass beads ¼ or less......The way to check is to give the return chute a bump with your knee while you are blasting.If the flow picks up you found the problem....Relative humidity is the culprit down here in Florida. As you blast the media gets smaller and the dust in the cabinet picks up the moisture in the air. I also put an air drier on the compressor outlet....Hope this helps..
> *G*_


Have had lots of problems with moisture, especially with really fine media.


----------



## kd4gij (May 26, 2016)

I upgraded my older hf blast cabinet with a scat pickup tube and gun and it works 100% better
http://www.tptools.com/USA-Cabinet-Gun-and-Pickup-Tube-Upgrade-Kit,2320.html?b=d*8026


----------



## JimDawson (May 26, 2016)

Gunrunner you need to order a set of these for the Z axis hand crank. Bill did you just spit coffee on your monitor? :rofl:

View attachment 253356


----------



## kvt (May 27, 2016)

"I'm thinking maybe about 20% of the pickup tube cross section area."

I was thinking more like a jet area,   just enough to help it keep from compacting.  either that or a regulator where you could adjust the flow as needed based on the size/type media you are using at the time.


----------



## JimDawson (May 27, 2016)

kvt said:


> "I'm thinking maybe about 20% of the pickup tube cross section area."
> 
> I was thinking more like a jet area,   just enough to help it keep from compacting.  either that or a regulator where you could adjust the flow as needed based on the size/type media you are using at the time.



The real purpose of the suction nozzle is to provide motive force for the media, rather than relying only on the venturi in the gun.  It may help with breaking thing up a bit, but that's not it's real function.  I like the idea of regulating the pressure to adjust flow rate.

Unfortunately I don't have time right now to experiment and build an optimized nozzle.  Maybe you could try a few things and report back?


----------



## kvt (May 27, 2016)

I would If I had a blast cabinet.   All I have is an open blast canister system  that goes through a lot of sand and clogs from the moisture.   Now if you want to loan me one for a while I will be glad to test it out while doing a bit of rebuilding on stuff.


----------



## kd4gij (Jun 2, 2016)

Another good mod for a blast cabinet is a vibrator to keep the media at the bottom.


----------

