Milling Plaster

[ase3558]

So I was just wanting to get some feedback. A student came in today wanting to mill a plaster block to pour glass into, essentially milling a glass mold. Has any one had any experience with milling plaster, heard of anyone doing it, or know if it is even possible.

[Rob Gunn]

I thought that using dry wall would be a cheap way to pro-to-type and test out tool paths on big projects. I figured I would rather make a mistake in a $3 sheet rather than a $200 sheet. I tried it once and found that it cut OK and gave me what I needed BUT the mess that it made was crazy. I had white dry wall dust everywhere and it doesn't like to clean up. I guess I would say go for it but have a good way to capture the dust!:eek:

[scottp55]

Austin,
100% POP(Plaster of Paris-Gypsum) has a Mohs hardness of 2, so should be entirely doable, but would have really good DC like Rob said. IF any sand added I'd say a no-no.
Not sure what this guy is using.
https://www.youtube.com/watch?v=7wrmQcjT1K4

I'd make sure EXACTLY what he used;
http://en.wikipedia.org/wiki/Plaster
Be very interested if you go for it!
Post pics and comments if you do.
scott

[Brady Watson]

Yes. You can mill plaster. However, some precautions are necessary for your life & the life of the machine.

Make sure you are wearing the proper mask for fine particulate. Dust collection is a MUST because fine particles WILL get pretty much everywhere. This can be a challenge with deep/cavernous 3D models, so you may have to get creative.

After the machining is complete, clean the machine RIGHT AWAY. The fine dust will settle on the machine & wick up moisture in the air. It will then adhere itself to the machine and be very difficult to remove. Vacuuming is the preferred method, with compressed air second. Be careful not to shoot high pressure into any of the bearings.

-B

[ase3558]

Ok! Great! Thanks guys, this helps alot!

I have a dust collector on the machine but I think I will need to figure something out to get in while making deep 3d cuts.

[chiloquinruss]

"figure something out to get in while making deep 3d cuts" I also have that problem and so have been noodling an idea around for awhile. Here tis!

An adjustable air force blowing at the tip of the bit that would keep the chips moving away from the tip. A box added to gather the now moving chips into a dust collector piece that is attached to my existing Dust collector. Haven't built it yet but opinions are always welcome. Russ

http://hobby-tronics.lenzus.com/content/21775739/r/s_34218255.jpg

[scottp55]

Hadn't thought about that Brady(and should have:( )
Like my machine too much, probably go through the extra steps to make a mold and then imprint the POP.
MAY still try carving a Gypsum crystal I have with a 3D, but will be Super careful now.
Might try Russ's idea, but think I have enough suction with the dustfoot with a .125" 30degree.

[Brady Watson]

The PRTs (3G and older) had a stationary dust foot that did not move with the Z. This let you set the foot at the top of the material and the router would move independently of the foot. You may want to try making something like that. It wasn't 'awesome' - but it sure was better than nothing. If you were machining 2" HDU for instance, it worked fine for 3D signs etc that the 4G/Alpha foot would gouge.

The newer dust feet are safer than that design because it keeps the spinning bit covered. This is certainly something to consider in an educational environment.

I'm not sure how well the blow & suck setup would work in certain situations. I would imagine that the terrain of the model being cut would influence the shape of the air cone coming out of the blower, not to mention the air stream being completely blocked by cavernous models. This may result in chips being shot/ramped off of certain shapes.

I think one of the most effective setups would be to totally enclose the YZ car in a rectangular skirt of sorts - not unlike those you see on large production CNC routers. This would at the very least reduce flying debris being rooster-tailed out of the side of the machine.

-B

[Brady Watson]

FYI: Rayite100 (http://www.talkshopbot.com/forum/showthread.php?t=5264) (no longer available) Jeez...11 years ago!

I found plaster that was slung off the bit on the 3x3" gantry end years later when I moved the bot to the new shop...and it had to be scraped off with elbow grease. When I machined the Rayite years ago I made a rubber 'way cover' that I still use on the machine that keeps chips and junk from shooting on the X2 rail while machining. It just lays there and flops when it gets to the center leg...I still have to make a set for the DT....but there's no way I'm cutting plaster or corian on that machine. Just NASTY stuff that WILL get into the bearings. I've replaced a few sets of the block bearings on tools cutting resin with plaster/wood flour fillers. Completely destroyed them...Word to the wise...

-B

[Red F]

Check out the SAWS forum on yahoo. Society of Academic Woodworkers. One of the members had a teacher do this. They found a sweet spot in the curing process where you get chips that don't stick and not dust. It seems it was around 3 days for them.

[Chuck Keysor]

Austin, a logical alternative, would be to carve a positive image out of Styrofoam, then pour the plaster around that. After the plaster has set, remove the Styrofoam. Maybe you'd have to coat the Styrofoam to keep it from sticking to the plaster, but Styrofoam machines beautifully.

Whatever way you do this, I would like to see what gets created, as I have long had a notion of doing what I think is called glass "slump molding".

Good luck, Chuck!

[steve_g]

Back in the dark ages… when we cast lead with Jr. high students… We did a “lost wax” mold. Carve the wax (even with undercuts), cast in POP, melt out the wax in an oven and pour the lead.
I suspect you could do something similar using styrene foam. Melt it out with a solvent or even a torch! Whatever method you use, be 100% sure that all moisture is baked out of the POP! Bad things happen if any moisture remains…
SG

[scottp55]

Austin, Also perhaps e-mail Sallye at Shopbot as she's into glass(saw some of her dichroic at Maine Shopbot Camp). She may have some insight from practical experience using the "Bot for molds.
scott

[Rob Gunn]

wanting to mill a plaster block to pour glass into, essentially milling a glass mold.

Lets look at this project as a true mold project. Why not use the ShopBot to make a wood pattern as the negative shape, then pore the plaster around that. In the end you have the same end result and didnt have to deal with the dust issue.

[Brady Watson]

I live in a glass town (no stone throwing buddy!) - The glass museum has plenty of wood molds (I think Oak) that are/were used for shaping glass. So wood, as Rob points out, might be a possibility for your project.

-B

[MogulTx]

A lot of small pressing molds, paddles and general handling items for glass are made from Pear wood that is soaked in water so that it does not burn when exposed to the hot glasses. ( You can probably get information on this by visiting the Corning Glass Museum's web site or other glass blowing resources)

Oak may also be used, but I have no knowledge of that...

The thing is that these products were blown against or inside of a mold, and then the mold was moved away and re-wetted. I don't know how well it does with lead being poured into it, or what-have-you.....


If he/she is CASTING into the mold with molten glass, I would suggest a "lost wax" casting or other similar process, but use olivine sand that is compacted around the shape to be cast. "Pate de verre"

[Acmeaviator]

My wife and I own a pottery business and we have worked with glass as well (though mostly for fun). I make plaster molds for both slip casting ceramic and molding glass - and have used my buddy on plaster a few times. The way to go is, as others have pointed out, to glue up a bunch of wood and machine a positive form, coat it with 1/1 solution of murphys oil soap and water and cast away. Be sure to use Pottery grade plaster and NOT plaster of Paris.