Bead blasting

Gentlemen,
I am about to take the Renault transaxle out of my GTD to change the final drive ratio and install a long fifth gear.
Before rebuilding the box, I had intended to send the bell housing and castings for bead blasting, to remove all the external black paint, then to have them painted silver and baked.
I was telling this to a friend, who until recently was a tribologist and metallurgist, working for a well-known bearing manufacturer. He went nuts. He said that he had had to provide evidence in an investigation into why a customer-rebuilder's gearboxes frequently failed after very few miles. He discovered pieces of aluminium oxide embedded in plain-bearing surfaces, which had scoured the journals, and had done very similar things to roller-bearing tracks, synchro rings and other bearing surfaces. The particles were residues left behind when the castings had been cosmetically bead blasted during the rebuild process. He went on to emphasise the importance (if such were necessary!) of getting ALL residue out of the castings before reassembly. Because residue particles are often thicker than the oil film, bead blasting, he said, was absolute anathema to clean reassembly.
This has left me in a quandary. How can I check that the residue is excluded from before, and has been properly removed after, bead blasting? Should I give up the idea and leave the gearbox halves and bell housing in their (clean) black paint? Just do the rebuild, safe in the knowledge that the box won't wear out in a very short distance, for that reason, anyway?
My question to forum members is this. If you have had experience of bead blasting, either as a service-provider or as a customer, what precautions are necessary to protect the casting internals from aluminium oxide? What should I ensure is done to be certain that the inside of the box is completely clean afterwards? Should I have the blasting done at all?
Any advice or experiences would be very welcome.
Thank you.

I think your best bet would be to specify that the blasting media not contain aluminum oxide. /ubbthreads/images/graemlins/smile.gif

I think a walnut shell media would work fine for your application. Regardless, a good cleaning out with rags or paper towels when it's done is also in order.

Why not blast it before you open the case? Stand it up on a table with the inut shaft pointing down through a hole, seal everything up, and then blast it. You can blast with glass, plastic media or even baking soda. Sounds like your friend had more of an issue with the work habits of the shop rather than the actual deisred end result.

I can't tell you how much stuff gets ruined because someone wants it to look all nice and clean. I work in a rebuild and repair dept that does a lot of hyd. and air compressors.

We have to contantly tell people not to use the bead blaster as a cleaning system. The glass beads embed themselfs into the surface of what ever they impact. The softer the material the worst the problem. If you beadbast the inside of that gearbox you might all well throw it into the garbage.

Use a good paint remover and some elbo grease.

The only material I have heard of that will work as a medium for machine parts is walnut shells. Be safe use paint remover. I wouldn't even put my gearbox in the same room with a beadblaster. That stuff is nearly impossible to completely clean out of machine surfaces.

for your own sake don't do it.

I'm with you on this Howard -

I had Roys UN1 absolutely spotless, took a few hours, about 4 pints of a strong degreaser (Brake Cleaner works well) and a lot of elbow grease but the result was ace!

Pity we shattered 5th gear the following week filling it with bits of metal! - doh! /ubbthreads/images/graemlins/frown.gif

We use a bead blaster on occasion to clean cases and as long as certain rules all followed there shouldn't be a problem.
Make sure you use as non agressive media as you can get, as the guys have said, walnut shell is good.
Don't wack the air up, keep it at a level you have control over, yeah it takes ten times as long to do a nice job because you need to work each surface from square on, but it beats wrecking cases.
Pack the inside and mask all areas you don't want beaded, the media will damage the mask so replace it as and when it does. And it will get inside (think of it as a day at the beach, it gets in everywhere) but this way will not hit surfaces under any pressure so will not damage anything and will wash out just fine.
We have found that a cycle of good scrub up in the Safety Clean tank followed by drying with compressed air (again, no need to go too mad with the pressure) and a bake in the hot wash tank if repeated twice gets all traces of media off and leaves them squeaky clean.
Probably best not to just let any old blast cleaner do the job, explain your concerns and walk away if your not totaly convinced they will do exactly as you have asked, it's your money.
If you are still in any doubt at all don't do it, you'll find those old Hamerite spray cans do a `smoothrite` silver that's actualy pretty hard to tell from real clean alumnium, the finish doesn't last forever, but hey what does?

P.S. sorry I missed your call the other evening, I'll be home all Sat' eve' so give me a call then if you can.

Tony
I would suggest if you really need to clean that case you have it degreased with a good cleaner such as is used in transmission shops, really a soap used with hot water that does a good job. Also if you feel you need the cases blasted I would suggest finding someone who can do it with plastic media. This is used on body panels and doesen't distort at all, won't embed in the surface, and leaves a nice finish. Aluminum Oxide is a very abrasive media, glass beads are less abrasive but I would not risk any of that being left behind, and walnut shells leave a nice finish, but it takes a lot of pressure and time and the dust is terrible, but it leaves a nice finish.
Hope some of this helps
Cheers
Phil /ubbthreads/images/graemlins/beerchug.gif

Tony,
Nitromors to strip the paint and grease.
Then wash it briskly with a good aluminium wheel cleaner, abrasive pads and good old soap.
Check out Chris Martino's gearbox for the finished results - brand new.

Always wear complete eye protection and good gloves when using corrosives !!

Thank you, gentlemen, the quality of advice is as usual - excellent.
So, aluminium oxide is definitely out. My gearbox rebuilder should be able to help with cosmetic cleaning, possibly with walnut shells as suggested here.
I shall try to stay with the baked paint option, to give as durable an external surface as possible.
Further reports once the casings have been stripped and cleaned.

Hi Guys, I have similar question but don't want to bead blast for some of the reasons above, does anyone have a method to clean aluminium castings to remove the oxidization that forms on alloy parts, i was thinking of some form of dipping or bath to remove it any thoughts.Someone suggested soaking in Coke a Cola ???

Regards
GraemeS

How about using a good Alum wheel polish/cleaner? www.mothers.com
Goggle "metal polish" Too much here to list including how to make metal polish, all the links to polish makers.

What about blasting non bearing type parts such as suspension arms prior to re coating? Does media type matter then? I am thinking of getting a blasting cabinet now I have a compressor big enough to drive one.

That's what we use it for. Removeing rust and old paint from frames, steel hangers, Stuff that won't need to move. A good example would be a steel lawn chair frame. A-Arms would be ok as would chassis parts in general.

Bead blasters work very well for what they are designed for. But they are not for cleaning, solvents are. Blasters are for removing hard surface stuff like several layers of paint or rust.

I firmly believe that if it needs lubication and has machine surfaces, bearing races, spins at any speed much above a few RPM,s then don't. If you blast A Arms for example MAKE SURE you clean the bushing void completely before you press in the new ones.

Graeme
I don't know if it is available in your neck of the woods, but here we can buy a cleaner thats used by commercial truck fleets that is actually a mild acid. Also if you have a welding supply store near you there is a product that comes in quart (Sorry, Liter size) for prepping aluminum before welding, and it works equally well.
Malcolm, I would second Howards comments about those suspension parts. If you can, plug the bores and any threaded components. The blasted surface really likes to absorb coatings such as powder coat or paint, and is the ideal finish but handle the parts with clean gloves as it will absorb body oils also.
Good luck
Phil

I never knew there was so much science in blasting!

It's amazing what you can learn on here!

I understand there is a 'sonic' cleaning service available in the UK... I had an aluminium engine block cleaned this way via a thrid party. Sorry I can't give any more info, but a google may bring up some results.

Alternatively, you could assemble the bare casing & mask any bearing surfaces before blasting. Mach 1 transmissions blast their casings before rebuilding & I've never heard of any related bearing failures.....

Julian, I don't disagree with you that if an experienced profession uses a bead blaster to remove old paint from the outside of gearbox cases AND he knows, how too, and the consequence's of not keeping the medium out of the moving parts then all should be well.

But and it's a BIG BUT, many people think because the bead blaster is so much less work and the part looks so good when they are done then they have stumbled apon magic.

Using the wrong medium like metalic medium on soft parts like alum. and allowing it to get into the oil (even very little) will ruin high speed parts like gears. Most people also don't know how hard it is to clean a blasted part of medium.

IF you keep it out of the inside and off of the internal parts then it will be OK but careful prep is VERY important.

I have never heard of motor builders bead blasting engine parts. I beieve gearboxes are even more fragile. Less oil, usually no cooling, nearlt the same speeds, and no filtration. Come to think of it how about having the gearbox cases hot tanked like a engine block?

I've had a couple of gearboxes cleaned up now. They were taken to Chingford Metal Finishers (CMF) in Slough (Ajax Avenue) and they used glass bead which brought the finish up to a remarkable 'gleam'. The inside of the boxes was untouched, all bearing surfaces were masked, and all oilways plugged before they went into the cabinets. Then they were thoroughly washed (truck cleaner followed by hot pressure wash ) and then sent to Nicholson McLaren (engine builders) who then washed them again and then painted and baked the outside. All of the castings that were painted black by ZF also went to CMF and were similarly bead blast but painted gloss black at CMF. These were ZF 5ds25's that Fran Hall sourced for me. One will be fitted into my GTD with all the other bits in the near future by Hugh Absolem. Hugh makes control cables for just about anything and all my GTD cables came from him.
I dont see any problems at all if everything is thouroughly washed and cleaned prior to re assembly.

The other ZF is 'gathering parts together ' for my next project, probably (hopefully) a Mono with FIA docs.