Sanding Inside Hollow Forms

[Pete Blair]

Sanding the inside of hollow vessels

Personally, I couldn't care much less about the interior "finish" except in very rare instances. I did, however, run into a guy with a nifty system for smoothing the inside walls. He would put several pounds of marbles and carborundum granules, say 100-120 grit, into his vessel, then clamp it into a jig similar to a rock tumbler that ran off a 1/8 hp motor, and just let the thing rotate for a day or two. Inside surface felt like a newborn's butt!

Holes or voids got "sealed" with tape and some shrink-wrap.

I'm sure the "finger-stickers" loved his stuff.

Mark, I completely agree but I am now interested in doing more piercing and sometimes the openings get quite large and the interior can be a little detracting.

I can't seem to get my head around the marbles and carborundum granules but it sounds like it worked just fine. i do have a couple of rock polishers and could probably find some ball bearings so if my plan doesn't pan out I just may experiment with this.

Thinking out loud I wonder if my glass and marbles would do the same job . . . .

 

[Bill Boehme]

I think that you will risk staining the wood if you use ball bearings.

 

[Pete Blair]

Sanding inside?

Hey Bill, how about if I use glass marbles instead of ball bearings?

 

[Michael Poorman]

alternative to carborundum

Do you think that sand would be suitable instead of carborundum? I know it probably wouldn't cut as fast, but sand can be picked up as just about any hardware store...

I'm currently working on a vase that I can not sand very far into. The tumbling option sounds like a good route to try. The walls are 3/16" thick in the area that I can't reach a sanding stick into. I fear that marbles could add a bit more weight than I'd like to have rolling inside of the piece and possibly cause damage.

Thought?

 

[Mark Hepburn]

Do you think that sand would be suitable instead of carborundum? I know it probably wouldn't cut as fast, but sand can be picked up as just about any hardware store...

I'm currently working on a vase that I can not sand very far into. The tumbling option sounds like a good route to try. The walls are 3/16" thick in the area that I can't reach a sanding stick into. I fear that marbles could add a bit more weight than I'd like to have rolling inside of the piece and possibly cause damage.

Thought?

Mike, this is sort of related but not exactly the same as what you're thinking:

Last year I made a bunch of wood toys to donate for Christmas. Wobbly toddler toys with wood wheels, etc. Faced with sanding several hundred parts, I fashioned a tumbling drum out of a large piece of PVC (don't recall diameter but I'd guess 8"). Plugged one end, filled with sand and some parts, plugged the other and mounted it on my Jet 1642 and let it run for about 2 hours, gave it a rest and then the same for a couple of days.

When was all said and done, I removed the parts and hand sanded them after all. :mad:

I think that part of the problem was definitely that bag of sand from Lowe's. Perhaps if you're in an area where you can get a supply of sandblasting medium? There are better minds out there on this topic than mine, but hope this helps.

Mark

 

[Michael Poorman]

Ugh, I'd be ill after that too! Thanks for sharing the experience.

 

[Bill Boehme]

Just a guess, but sand has been around for a long time ... and like worn out sandpaper, probably isn't sharp enough to do the job unless you tumble the sand in the turning for an eon or two.

 

[Gerald Lawrence]

Just a guess, but sand has been around for a long time ... and like worn out sandpaper, probably isn't sharp enough to do the job unless you tumble the sand in the turning for an eon or two.

I believe there is a sand available called sharp sand. I think as Mark said is used for sandblasting. Another thought is walnut shells used in blasting (sand?) is gentler than the sharp sand.

 

[Mark Hepburn]

I believe there is a sand available called sharp sand. I think as Mark said is used for sandblasting. Another thought is walnut shells used in blasting (sand?) is gentler than the sharp sand.

Gerald,

You're right about abrasive types. I'll be a machine shop with a blasting booth has some media maybe for sale or can point to a vendor. I've heard of walnut shells too as an abrasive but don't know anything about it.

Seems like that auto glass makes the most sense to me. That's what I'm going to be trying with my next project.

 

[Bill Boehme]

The type of media depends on what you are abrasive blasting. If it is heavy rust then large granule sand might be used. Thick layers of paint on steel then the media might be smaller grained sand or small glass beads. For surfaces such as thin aircraft skins which carry structural loads and would be damaged by conventional abrasive blasting media, plastic beads have been used.

Something to consider is that high pressure abrasive blasting may not necessarily have much in common with rock tumbler type polishing.

 

[Mark Hepburn]

The type of media depends on what you are abrasive blasting. If it is heavy rust then large granule sand might be used. Thick layers of paint on steel then the media might be smaller grained sand or small glass beads. For surfaces such as thin aircraft skins which carry structural loads and would be damaged by conventional abrasive blasting media, plastic beads have been used.

Something to consider is that high pressure abrasive blasting may not necessarily have much in common with rock tumbler type polishing.

That last point is probably dead on, Bill. Offshore they're blasting platform legs and topsides with some black abrasive at +100 psi.

 

[Bill Boehme]

Mark, some reloaders tumble used brass in a drum of fractured walnut shells to polish them before reloading. More finely crushed walnut shells are used as an abrasive blasting medium. From what I have seen, the term hull refers to the soft outer husk. The outer hull or husk is used for a few things like dye, possibly Mahoney's Walnut Oil Finish, and a few quack remedies.

 

[Mark Hepburn]

Mark, some reloaders tumble used brass in a drum of fractured walnut shells to polish them before reloading. More finely crushed walnut shells are used as an abrasive blasting medium. From what I have seen, the term hull refers to the soft outer husk. The outer hull or husk is used for a few things like dye, possibly Mahoney's Walnut Oil Finish, and a few quack remedies.

Really, I had no idea that vets used it medically for ducks.

So the walnut shells really are not that abrasive if they're used on brass, right?

 

[Bill Boehme]

Really, I had no idea that vets used it medically for ducks.

So the walnut shells really are not that abrasive if they're used on brass, right?

From what I saw at a gun show, the brass and walnut shells are tumbled in a drum about like a rock tumbler would work.

Regarding the much more finely crushed walnut shell abrasive used in blasting, I have no idea about how, why, or when it is used. However, I do know that it is also used for kitty litter. Piece these bits of disparate information together to draw your own conclusions.

 

[Michael Poorman]

Corn cob is what I use for polishing brass before reloading, but I would not imagine it would do anything for tumbling the inside of a hollow form. I wouldn't think that walnut hulls would do much either. Generally you mix in a polishing compound while to vibratory tumbler is running. The corn cob / walnut hulls really just act as a bazzilion little scrubbers with a bit of the polishing compound on each one.

I did try fish tank gravel this morning and it worked well. I stopped by to talk with a machinist buddy and he showed me the "proper" tumbling media for metal parts. This is in no way acceptable for my purpose so he handed me the bag of fish tank flooring and told me that he used that stuff before buying the media he uses now. I didn't leave the piece running for more than 15 minutes, but I could certainly tell it had worked to smooth out the surface finish!

 

[Bill Boehme]

I did try fish tank gravel this morning and it worked well. I stopped by to talk with a machinist buddy and he showed me the "proper" tumbling media for metal parts. This is in no way acceptable for my purpose so he handed me the bag of fish tank flooring and told me that he used that stuff before buying the media he uses now. I didn't leave the piece running for more than 15 minutes, but I could certainly tell it had worked to smooth out the surface finish!

That sounds like a great idea.

 

[Mark Hepburn]

from what i saw at a gun show, the brass and walnut shells are tumbled in a drum about like a rock tumbler would work.

Regarding the much more finely crushed walnut shell abrasive used in blasting, i have no idea about how, why, or when it is used. However, i do know that it is also used for kitty litter. Piece these bits of disparate information together to draw your own conclusions.

:d:d:d:d:d

 

[Mark Hepburn]

Corn cob is what I use for polishing brass before reloading, but I would not imagine it would do anything for tumbling the inside of a hollow form. I wouldn't think that walnut hulls would do much either. Generally you mix in a polishing compound while to vibratory tumbler is running. The corn cob / walnut hulls really just act as a bazzilion little scrubbers with a bit of the polishing compound on each one.

I did try fish tank gravel this morning and it worked well. I stopped by to talk with a machinist buddy and he showed me the "proper" tumbling media for metal parts. This is in no way acceptable for my purpose so he handed me the bag of fish tank flooring and told me that he used that stuff before buying the media he uses now. I didn't leave the piece running for more than 15 minutes, but I could certainly tell it had worked to smooth out the surface finish!

Mike,

Is this plain old gravel like you can buy anywhere?

 

[Pete Blair]

Sanding with glass

Though not all that scientific I did discover that, yes indeed, glass will sand the inside of a hollow form. The tempered glass I used was from a broken car window. I got a pail of it for free from our local Glass Shop.
Prior to my test I was using a BBQ motor that turned faster than my final test devise. I was ready to start when the old faster motor just gave up the ghost so I had to revert to a much slower one. I think if I had used the first motor and turned for the same number of days the results would have been much better.

For this test I used two sections of a Birch log. each piece is about 5" ling and 2.25 square. Using my Kobra I proceeded to do the hollowing as quickly as I could then sharpened the cutter and did a final easy pass.
Here is my sample piece split in half with no sanding.

As you can see from this photo I didn't get it a smooth finish especially on the end grain where there was lots of tear-out. In reality I would have spent a lot more time and would have gotten a much better finish but for the test I mostly wanted to see if this method would sand the inside and if it would remove ridges.
Because I could see the torn grain in the bottom and top of the form I used half the rotational time with the form leaning to the bottom and half with it leaning toward the top.

The results were not as good as I had hoped but it is very evident and I think can be seen in these photos that the glass does indeed sand the inside to quite a nice finish. Sample on the left is of the piece I did not rotate with glass inside.

After thinking about this test I am certain that the number of revolutions are critical so to enlighten any of you who are interested I rotated this piece approximately 8600 times. This took me about 14 days. I am going to re-try with a faster motor and am convinced that some marbles might add to the final finish.

I would be happy to discuss this process with anyone who is interested or would like more information.

 

[Bill Boehme]

Good work , Pete. Do you think that the marbles would help smooth the ridges? I was wondering if periodically removing the wood dust would make the sanding more efficient.

 

[Mark Mandell]

Good work , Pete. Do you think that the marbles would help smooth the ridges? I was wondering if periodically removing the wood dust would make the sanding more efficient.

I suspect the opposite. As the marbles and abrasives remove wood fibers they (the fibers) are held in the slurry and would, I think, tend to smooth out the abrasive "bite" on the ridges and valleys just as the slurry in a rock polisher reduces the surfaces on the stones being tumbled.

I could (of course) be wrong.

m

 

[Bill Boehme]

I suspect the opposite. As the marbles and abrasives remove wood fibers that are held in the slurry they (the fibers) would, I think, tend to smooth out the abrasive "bite" on the ridges and vaslleys just as the slurry in a rock polisher reduces the surfaces on the stones being tumbled.

I could (of course) be wrong.

m

I was thinking of it from an "organic" perspective -- really organic ... as in gravel in a chicken's gizzard. You can't get any more organic than that.

 

[Mark Hepburn]

I was thinking of it from an "organic" perspective -- really organic ... as in gravel in a chicken's gizzard. You can't get any more organic than that.

Especially if it's a free range chicken.

Free range gravel?

If you ever watch how It's Made, you may see tht frequently round objects are used to polish metals objects. I've seen forks, knife blades and lots of metal parts going through a vibrating rotating tub.

Clearly, the glass will smooth the wood as Pete has shown. I'm wondering if small bearings or marbles wouldn't polish the surface after sanding?

 

[Pete Blair]

Sanding

Greetings again.
I don't think removing the sawdust while sanding this way would do much. The sawdust was so fine I'm certain that it wouldn't have made any difference.
Next I am modifying an Ice Cream Maker motor to fit my system and then I will screw the test pieces back together and try some more rotating. I feel that the addition of marbles will sort of 'push' the glass a little more causing the glass to slide more instead of just tumbling, and I hope, cause it to sand more aggressively.
I of course will report back in the next week or so.

 

[Pete Blair]

Next edition

Not sure what happened but I just tried to post a follow up and for some reason the site froze on me and I lost the note but here goes again.
My ice cream motor failed in short order. I had guessed that it might not work in the orientation that I wanted to use it in and I guess I was correct.
Here's a shot of the new sander. A repurposed rock timber. It turns at 1725 RPM through a 1 1/2" pulley which drives a 9" pulley which powers the lathe chuck that is attached slightly off centre which results in a sort of wobble which I think adds to the ability to sand. Here's a shot of the 'sander'.

I added a few marbles to the glass and ran the latest test for about 300,000 revolutions and am very pleased with the results.
Here's a shot of the glass, marbles and dust from my test. As you can see there is not much dust. The marbles are quite scratched and I believe they really help make the glass pieces slide rather than just tumble.

Here's a photo of an original unsanded piece along side the latest sanded piece.

And a slightly enlarged shot of the latest piece.

All in all,l I think if used more rotations I would be completely satisfied with the latest design and process.

Please feel free to comment or question my process.

 

[Richard Kane]

I suggest this technique. Hollow to a thickness thicker than you want. For example, if you want a final thickness of 1/4inch hollow to 3/8 inches. Then use your scraper (Trent Bosch or any other) and scrape to the final thickness. The end result will be a fairly smooth interior.
Richard Kane

 

[Mike Johnson]

Add 25% volume of steel ball bearings and 25% sand to the inside of your hollow vessel and put a cap on the end and run your lathe at a slow speed and you will end up with a ball milled finish on the inside.