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Discussion Starter · #1 ·
Gents, I have a project that involves carving 36' x72' plaster blocks for another artist.

The plaster is specifically molding plaster, the type that is used to create molds for ceramic slip casting.

I'm looking at qty of 6, 12 hour carving runs with .25' ball nose bits. It is essentially carving stone dust....so expecting it to eat bits

Anyone have any suggestions on bit selection?

Thanks.

Scott
 

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Plaster is really gypsum (calcium sulfate dihydrate) and on the Mohs Hardness scale is pretty soft - slightly greater than talc. Dihydrate means there are two molecules of chemically bound water contained within the mineral. This means that the plaster does a pretty good job of minimizing heat since the chemically bound water tends to act like a heat sink.

United States Gypsum (USG) used to have a product called Rayite Machinable Media. Not sure if they still make the product but it was specifically made to be machined.

At a minimum use carbide cutters. It's the dust that can be annoying so hopefully you have a dust collection system.
 

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ScottArt,

Besides doing woodworking, I also carve stone. I carve alabaster, soapstone and marble...the first two are semi-soft stone, whereas Marble and granite are very hard.

Plaster, as rayk2 points out, is very soft. I have specially hardened steel chisels and gouges for my stone work.

For plaster, you can easily use a standard woodworking chisel and/or gouge, since the plaster is soft. You can also buy rotary burrs that will work well to grind away at the plaster. I have a tool from Foredom Tools
( Foredom Electric Co. ), that has a motor , flex shaft and bits. But you can buy a flex shaft that will attach to a standard drill, where you can attach burrs to it. Just Google it...

Sounds like a pretty big piece to be working on, even if it is 3 ft x 6 ft...that's allot of carving and dust !!

Good luck

John
 

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Discussion Starter · #6 ·
ScottArt,

Besides doing woodworking, I also carve stone. I carve alabaster, soapstone and marble...the first two are semi-soft stone, whereas Marble and granite are very hard.

Plaster, as rayk2 points out, is very soft. I have specially hardened steel chisels and gouges for my stone work.

For plaster, you can easily use a standard woodworking chisel and/or gouge, since the plaster is soft. You can also buy rotary burrs that will work well to grind away at the plaster. I have a tool from Foredom Tools
( Foredom Electric Co. ), that has a motor , flex shaft and bits. But you can buy a flex shaft that will attach to a standard drill, where you can attach burrs to it. Just Google it...

Sounds like a pretty big piece to be working on, even if it is 3 ft x 6 ft...that's allot of carving and dust !!

Good luck

John
Thanks John, I have also carved soap stone with a foredom and dremels with burrs. This will be done at 12000 rpm in a CNC spindle.. the carving runs for each slab will approach 12 hours... so never carved soapstone 12 hours non stop..
 

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Discussion Starter · #7 ·
Plaster is really gypsum (calcium sulfate dihydrate) and on the Mohs Hardness scale is pretty soft - slightly greater than talc. Dihydrate means there are two molecules of chemically bound water contained within the mineral. This means that the plaster does a pretty good job of minimizing heat since the chemically bound water tends to act like a heat sink.

United States Gypsum (USG) used to have a product called Rayite Machinable Media. Not sure if they still make the product but it was specifically made to be machined.

At a minimum use carbide cutters. It's the dust that can be annoying so hopefully you have a dust collection system.
Thanks Ray, Yes the dust collector and masks will be on full time. Do you think carbide will stay sharp for a 12 hour run?
 

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I'd worry about DC big time...
regular DC rules do not apply...
 
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I really don't know if the cutter will last 12 hours - it depends on all the typical machining factors - depth of cut, speed, heat generation etc. Stick486 is also correct in the a typical shop vac being used as a DC will probably not like bring on for 12 hours.

If it were me I would break the machining into segments that allow for the cooling of the machine components.
 

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I run my Ridgid vac sometimes for up to 20 hours a day, and it keeps on running. But I do have it hooked to a Dust Deputy.
 
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Discussion Starter · #12 ·
I really don't know if the cutter will last 12 hours - it depends on all the typical machining factors - depth of cut, speed, heat generation etc. Stick486 is also correct in the a typical shop vac being used as a DC will probably not like bring on for 12 hours.

If it were me I would break the machining into segments that allow for the cooling of the machine components.

I do have a large dust system attached to the CNC machine, and yes, breaking this into shorter runs so i can check the bits is a great idea.

thanks.
 

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With a long finish cut --- set it all up and go to bed.

Or if the cut will run late a nap on the sofa til it's done. And then set it right back up first thing in the morning.
 
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I left a long 3D cut running once while I went to bed and the only thing wrong when I got back in the morning is that somehow part of the cut was a visible fraction thicker than it should have been. As if during the cut the MDF bed had sagged just a bit. Probably just gradually but when the bit got to one side and then went back to finish an area it had missed, that area was visibly higher than the rest of the cut. Consistent with my decades of experience using MDF, just unexpected. Makes a good argument for additional support cross bracing under the bed on the probotix CNCs.

4D
 

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With a long finish cut --- set it all up and go to bed.

Or if the cut will run late a nap on the sofa til it's done. And then set it right back up first thing in the morning.
ha ha, just got to thinking...

Some day when I get more confidence in my CNC and my ability to design, I will be able to set up a long cut, then go outside and fire up the pellet cooker, put on a couple of pork butts, and let them smoke all night! :grin::grin::grin:
 

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Discussion Starter · #20 ·
Just wrapped up the Plaster Carve.

5 Slabs 3" thick X 36" wide x 72" long

Plus three smaller slabs that were 15" x 12"x 3" thick.

The other artist provided my with 3 D models of some local Glaciers, the models came from Government satellite shots, so very detailed and interesting.

They ran from 1,170,000 vectors to over 2,254,000 vectors. Each of the big slabs took 12 to 15 hours of carving time using a .25 round nose carbide bit. So cumulatively over 50 hours of carving at 14000 RPMS, 250IPM on a single carbide bit... that seems no worse for the ware.

Pictures to follow..
 
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