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Sanding or grinding some 304 stainless to make it flat, how would you do it?

TrevorMarcos

Plastic
Joined
Jun 6, 2022
Hello, first timer here. I have an old stainless steel bucket milk machine, probably 40 to 60 years old. The way it works is that a vacuum of about 13 inches of mercury is applied to the lid, which has a rubber lid gasket. This should seal the lid from the vacuum pressure itself. When milking a cow, the lid has to stay sealed. A lot of pulling goes on during the milking process, but at 13 in. Hg, there are several dozen pounds of force that keeps the lid sealed.

Unfortunately, the rim of the 5 gallon milk tank is slightly out of flatness. I therefore need to sand or grind off some 304 stainless, about 3/8 inch thick, where the lid sits. I probably only need to sand/grind 0.050" or less.

I can only get the seal to work if I press heavily on the bucket lid and vacuum seals it tight, but not tight enough. The seal tends to pop right off at the worst times due to the un-flat lip.

I need some pointers on what process and type of hand process might solve my problem. I know which sides of the bucket lip are high and low. I have some sanders, like a belt sander. But I am unsure what grit or if even a sander will perform adequately. Maybe I need a whole table sander or polisher to do this?

See pictures. Pencil for scale. It's called a Surge milker. The lip is about 7 inches in diameter. They don't make these tanks any more.
 

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TrevorMarcos

Plastic
Joined
Jun 6, 2022
Yes, the gasket is brand new and I have a spare. I checked flatness 2 different ways. It's the bucket lip, not the lid. Apparently, this is a common issue with these milk tanks sealing after heavy use/age.
 

Conrad Hoffman

Titanium
Joined
May 10, 2009
Location
Canandaigua, NY, USA
The first thing I'd do is get a flat surface about the size of the lip area. Could be a piece of glass, a square of countertop or anything known to be pretty flat. You can do this the same way you'd scrape a machine to flatness, in fact you should read the various posts on that. I wouldn't scrape it, but use the technique of a flat and some non-drying paint to locate the high spots. Then, belt sand the inked area a bit and check again. Rinse and repeat until you get paint transfer all around the surface. Hopefully there's plenty of metal so you can correct it and still have good lip area.
 

memphisjed

Stainless
Joined
Jan 21, 2019
Location
Memphis
Files or sand sticks (sandpaper super 77 to a piece of flat steel). Marking high spots is the hardest part, glass or granite counter top makes a good reference, a fresh board of mdf makes a light weight and decent roughing reference.
Check the lid too. The seal happens when you add weight, might be a kink in the surface of either. Smooth, like slick smooth helps. After flattening work both surfaces with -guessing here- about 400 grit paper by hand in a circular pattern (scratches go circumference of lid and bucket).
 

petersen

Aluminum
Joined
Dec 13, 2008
Location
minnesota
Trevor:

Check to see if the opening is still round.

A likely way for that surface on a Surge bucket to get "out of flat" is for the
bucket to be dropped or kicked and be bent out of round.

If so, stretch or squeeze to improve the "roundness" then check against a flat surface and sand the high spots to further improve flatness.

petersen
 

TrevorMarcos

Plastic
Joined
Jun 6, 2022
Thanks all. I was getting on the right track and you guys confirmed some of my ideas. Any help on grit or sandpaper types if I go that route? I've heard stainless gets work hardened just by looking at it funny. I was thinking of a wet sand and starting a bit rough, maybe with a palm sander, once the high areas are known.

I have all sorts of sharpening stones and some rouge when I get to the polishing/finish stage. I think I have some marble slabs somewhere.

Here's an idea that's reverse of what someone recommended... Maybe I can mark the bad lip with a Sharpie, then run it on marble with a sheet of 1600 grit taped to it. The high areas will have the marker removed. I guess the taping a sheet of paper to marble will work for the final finish anyway, but seems a bit unwieldy with that heavy bucket.
 

TrevorMarcos

Plastic
Joined
Jun 6, 2022
Trevor:

Check to see if the opening is still round.

A likely way for that surface on a Surge bucket to get "out of flat" is for the
bucket to be dropped or kicked and be bent out of round.

If so, stretch or squeeze to improve the "roundness" then check against a flat surface and sand the high spots to further improve flatness.

petersen
That would be cool if I can just bar clamp it back to round. The good news is that I already know the low spots are in line, 180 degrees around the circle from one another. I will measure it, but my calipers may not go that big.

It makes sense that the thing was dropped on the lip and it made the 2 low spots.

Instead of clamping the low edges in, I should probably make a wood block that I can wedge out the suspected dropped edge back to its origin. Maybe adding a little Vaseline and smacking the wood with a dead blow hammer will persuade it back to its original shape.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I have a harbor freight 9"x12" surface plate that is great for stuff like this. Check it to the surface plate with dye and file until you have multiple contact points, and then tape sandpaper to the plate and move it back and forth. The weight of the block actually makes it easier in my opinion.
 

TrevorMarcos

Plastic
Joined
Jun 6, 2022
I have a harbor freight 9"x12" surface plate that is great for stuff like this. Check it to the surface plate with dye and file until you have multiple contact points, and then tape sandpaper to the plate and move it back and forth. The weight of the block actually makes it easier in my opinion.
I didn't know they made one, certainly not in my price range. I googled for one and came up empty. I saw a massive granite one that Grizzly makes for a bunch of money and weighs 154 pounds, lol. Nope!

Problem is solved, in any case. Here's what I did....
  1. I found a fairly flat stone. It was a synthetic fireplace liner I think, as I couldn't find any marble. My stone probably only weighed 4 pounds, as it was porous and not dense.
  2. I taped 120 grit paper to it.
  3. I marked the lip with Sharpie and scraped it.
  4. I found what was high, then filed, sanded, and Dremel Tooled the edge.
  5. (Repeated step 3 & 4 a lot! I used palm sander, flat files, sanded by hand on the stone)
  6. I had to go backwards to 100 grit, then 120, 150, 220, 320 by taping it to the flat stone and wiggling back and forth with the taped stone in my hand and the Surge bucket locked still using my feet and me sitting on a nice chair. (see picture: warning: wear sunglasses)
  7. I also polished a little with some 500 grit by hand on the lip.
  8. The bucket had some nasty scratches that I also improved by using the 220, 320, and 500 grit on a palm sander.
I did everything dry. The lowest spot still looks rounded in the photos, at the lower right portion of the circular lip. It has black marker on it in the before picture. After cleaning, the working of the seal was an obvious no-brainer. I got the lip extremely flat, so I knew it would work. The original low spot just barely touches the flat plate, but it just takes a little. Areas that needed a lot of grinding are wide, perhaps even a 1/2 inch. Bucket now works for its intended purpose.
 

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