Would tumbling improve the finish on a casting?

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jscarmozza
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Would tumbling improve the finish on a casting?

Post by jscarmozza » Thu Mar 11, 2021 10:48 am

In general, would tumbling benefit the finish on castings, if so, what would be the proper medium to use? I assume that if tumbling is done it would be on the rough casting prior to any machining, would that be correct? John

Harold_V
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Re: Would tumbling improve the finish on a casting?

Post by Harold_V » Thu Mar 11, 2021 3:59 pm

If you have a tumbler, you might consider trying a little common gravel to see how the surface responds. You can expect external corners to degrade faster than the balance of the casting, but if the item is run long enough, you may create a uniform patina.

I've used a cement mixer with gravel for rust removal on scrap steel in anticipation of melting to make gray or ductile iron. It works quite nicely. It is noisy. A proven tumbling media would likely be better, but by using gravel I don't have to worry about recovery, and it does a good job. It was used wet. Dusting would be a serious problem if it was operated dry.

One thing you might consider is buying some fine steel shot for use in a blast cabinet. I recently bought some and discovered it picks up well enough. I operate my cabinet @ 100 psi. It may perform well at lower pressure as well. Steel shot is generally applied with a Wheelabrator, which does not rely on air to propel the shot. It peens, so the action is different from that of abrasives, but it does improve surface finish and, in general, will remove adhered sand, assuming that's a problem for you. If nothing else, it leaves the object with a uniform appearance.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jscarmozza
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Joined: Mon Mar 03, 2014 5:09 pm

Re: Would tumbling improve the finish on a casting?

Post by jscarmozza » Thu Mar 11, 2021 5:20 pm

Hi Harold, I was thinking about the Heisler wheels I just cast, but I started machining them so that ship sailed. I'm going to try the gravel trick on some extra castings I have laying around and see how they come out.
When I was a kid, we had a neighbor who was a police officer, he'd regularly cast wad cutters for target practice. He used to tumble them in a makeshift tumbler with oiled sawdust for about 8 hours, when they came out they looked perfect. I'm going to see if this could improve the finish on my smaller castings. Thanks.


jscarmozza
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Re: Would tumbling improve the finish on a casting?

Post by jscarmozza » Sat Mar 13, 2021 4:57 pm

I'm doing an about face and I'm going to tumble the Heisler wheels that I started machining. I tumbled the two wheel castings that didn't turn out and was very satisfied with the finish that resulted. I tumbled them for about 4 hours in 3/4" crushed stone and it tended to remove high spots on the cast surface and slightly rounded the sharp edges and generally cleaned up the castings. I really like the result!

Harold_V
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Re: Would tumbling improve the finish on a casting?

Post by Harold_V » Sat Mar 13, 2021 7:28 pm

Figured you would!
I own a small vibratory tumbler, one called a Rockette. It was marketed for polishing semi-precious stones, but they were commonly found in small machine shops, used for finishing small parts. The results achieved are very good, which is what inspired me to try using a cement mixer for larger things. I can't think of another way to improve surfaces that isn't hugely demanding of time and effort. Turn it on and let it run until you're satisfied. Pretty hard to beat!

If you can, how about posting a before and after of your wheels. Too late? Already tumbled?

By the way, if rusting is a concern, what I do is dip the recovered items in a solution of sodium hydroxide (common lye) when removed from the tumbler. That prevents rusting. Do be careful, though. A drop of lye in the eye is pretty much a guarantee of losing sight in that eye. Wear hand and arm protection, along with eye protection.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jscarmozza
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Joined: Mon Mar 03, 2014 5:09 pm

Re: Would tumbling improve the finish on a casting?

Post by jscarmozza » Sun Mar 14, 2021 8:23 am

Harold, the wheel on the left was tumbled, the one on the right was just wire brushed, I still have 5 wheels to tumble which I'll do today. I think my casting sand has had it, I get very sharp peaks on the finish, especially with brass, and the tumbling knocks them down and gives a much smoother finish. These wheels weren't too bad to start with because they were poured against cores (I should have applied a wash), but they still had sharp peaks. I was surprised at how much damage was done to the crushed stone medium, it really took a beating. The trick with this technique will be matching the medium to what you want to accomplish; so far I like what I see, if I continue to get good results I'm going to have to make a better tumber.
John
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Harold_V
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Re: Would tumbling improve the finish on a casting?

Post by Harold_V » Mon Mar 15, 2021 3:48 am

I'm just now starting to learn about casting sand. To that end I have purchased both 100 and 115 AFS sand, which I am bonding with both Southern and Western Bentonite. While I have yet to cast anything, a friend just cast a copper ingot using some of the AFS-115 sand with outstanding results. Bear in mind, I'm talking about greensand, not oil tempered.

I'm impressed with the results you achieved with the tumbling, and I'm not the least bit surprised at the degradation of the crushed rock. Purpose made tumbling media will last better, but it's also more expensive. It may not do any better as far as what it accomplishes. For my original purpose (rust removal) I'm more than satisfied with rock and will continue its use for that project, which will be ongoing if I finally get started casting.

How coarse is the sand you're using?

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jscarmozza
Posts: 553
Joined: Mon Mar 03, 2014 5:09 pm

Re: Would tumbling improve the finish on a casting?

Post by jscarmozza » Mon Mar 15, 2021 1:58 pm

Harold, my homemade tumbler started to come apart yesterday just as I was finishing up...I'm going to to have to make a sturdier one.

Regarding my sand, I try to make the best with what I have without getting too sophisticated. I got most of my foundry equipment, with about 200 lbs. of sand around 20 years ago when a local high school sold off all of their shop equipment. Since then I have continuously added to it up to about 1,000 lbs. The sand that came from the high school was good suff, fine with good binder that rammed up nice and gave a great finish. My additions were mostly mason sand run through a flour sifter with bentonite that I got from a local well driller. That being said, the sand grain size doesn't seem to be the problem, after many uses tiny balls of dust and clay form in the sand and instead of the sharp sand grains packing together to form a relatively smooth closed surface, the clay balls leave voids that make the surface finish of the casting very rough. A few years ago I dried all of my sand, mulled it dry to break up the clay balls then sifted it in an attempt to recondition the sand. It worked for awhile, but I think there's too much fines (dust) in my sand and the clay balls started to show up after a few uses. Clean (little to no fines) sharp sand that passes through a window screen and the right amount of clay will give you a nice finish, when the clay balls start to show up it's time for new sand. I'm not sure about this but I heard that foundry sand may contain traces of lead, so dumping it in the backyard may not be the best way to dispose of your old sand. John

AllenH59
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Re: Would tumbling improve the finish on a casting?

Post by AllenH59 » Tue Jan 04, 2022 2:12 am

I have not tumbled castings, but I was raised in a rock shop family and know a few things about tumbling. If you can get small stainless steel balls and use a fine grinding compound, they will have a good finish. If you look into the SS pins that the reloaders tumble brass with it you will see what a good finish it can provide. If you are wanting to grind out surface defects, especially in soft metals like copper alloys, tumble gently, as when your lapping compound becomes embedded in the work piece, it just becomes a grinding stone and will not change shape at all. There is no end of polishing compounds to use, and a finish will look good. I would suggest a large quantity of small squares of fabric, like heavy denim when looking for the best finish.

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