Make It - Cast Aluminum Stool

Using the X-Carve to CNC a mould so I can cast my own Aluminum top for a stool.

Step 1: Make a Block of Plaster

Slap together a simple wooden box out of scraps. I used cheap 3/8" plywood that had some water damage. I mixed an entire 10KG bag of plaster for my block following the instructions on the bag. I poured it into the box and rattled it against my bench to force the air bubbles to the top.

Step 2: Mill the Block

Let your block dry, under a heat source if possible, for a few days. When the surface of the mould feels dry to the touch, it should be ready for milling. Secure your block to the X-Carve waste board. Mill the pattern in two steps. With a 5/8" bit, cut the large circle to the desired depth. Change out bits to either a 1/4" of 1/8" spiral upcut bit to machine the detail work. I was cutting at 1000mm/min and taking 2.5mm passes for the whole job. Wear a respirator and be prepared for a lot of dust.

Step 3: Melt Metal, Cast Top

There are many great guides online to building your own foundry furnace. If you’re attempting this project, I assume you’ve already figured out what metal you want to cast and how you plan on melting it. Aluminum, brass, copper, pewter, zinc, gold and silver are all options. If you don’t want the headache of melting and casting metal, you could also look at plastic or epoxy options.

I went with Aluminum and here is how I figured out how much I needed… Diameter (mm) Squared X .0007854 / 1000 gives you a coefficient (i.e – .06157). Coefficient X Depth (m) = Volume (m3). Divide that by .001 to get liters Here’s my example for a 280mm diameter top that’s 20mm deep. 280mm X 280mm = 78,400 78,400 X .0007854/1000 = .061575m3/m .061575m3/m / .001 = 61.575L/m Depth – 20mm X .001 = .02m 61.575L/m X .02m = 1.2315L of Aluminum. Since at room temperature the aluminum is a solid, I used weight to determine how much scrap I needed. 1L = 61.0237ci 1.2315L of aluminum X 61.0237ci = 75.15ci of aluminum Pure Aluminum is 168.48 lbs. per cubic foot and there are 1728 cubic inches per cubic foot. 168.48 / 1728 = .0975 lbs/ci 75.15ci Aluminum X .0975 lbs/ci = 7.33lbs of Aluminum. This is the amount I would need to fill a simple cylinder. Since the detail will reduce the total amount I actually need, my safety factor is already included in this calculation. Sorry for swapping between metric and standard. It’s standard practice in Canada.

Step 4: Legs

I loaded a piece of 3/4" Baltic Birch Plywood into my X-Carve and used a 1/4" spiral downcut bit to cut out the legs and support. My standard settings for wood are around 1000mm/min at 2.5mm per pass. With the X-Carve, this is no problem. I gave the pieces a light sanding before cutting them loose and cleaning up the edges. To hide the layers of the ply, I attached veneer to the edges with titebond 3 glue. Once the veneer was cleaned up, I attached the legs to the support and gave it a few coats of Watco semi-gloss lacquer.

Step 5: Attach the Top

I cleaned up the edges of the top to remove any sharp pieces from the casting. I smoothed the back down with a belt sander. This could have been done on the X-Carve, but I don’t like aluminum shavings. I used two part epoxy putty to attach the top to the base assembly, clamped it and let it sit for a couple hours.

Step 6: Fin

The stool came out really nice. I hope it shows through in the pictures. The casting didn't come out as perfect as I would have liked, but it's not bad for my second time casting metal.

Comments

[SuperRistopaha.]

Hi,

would it be OK to give some comments on the aluminum cast? I am also just starting out on this hobby but I have some info that might make your casts come out in more detail next time. First of all the gypsum mold is one of your issues, it is not a good material for metal casting unless you go through some additional steps before the cast / and or mix in some other ingredients. Gypsum contains chemically bound / crystalline water, about 20% by weight. I work in the fire protection industry and we use this to our advantage when protecting metal structures from fires. When heated the gypsum gives off this water starting at a bit under 100 celsius. This whater turns to steam (which is again much bigger by volume than water itself (think steam engines e.g.) Because of all the steam trying to escape from under the aluminum you have all the bubbles in your cast and as the aluminum cools (also a side effect of the steam) the pockets remain in the cast. Depending on your taste of style this may be desirable but for most it is not.

Secondly you have an open mold and sections of different thickness in your cast. Different areas cool down and shrink at different speeds. Normally in sand casting one tries to offset this effect by including a large sprue into the multi part mold into which the aluminum is then cast. It should be thicker than the thickest part of your cast and ideally be able to feed more molten aluminum into the cooling cast and such prevent shrinkage.

Thirdly for any casting in which you want the cast surface to remain decoratively visible I suggest using OBB (Europe) or Petrobond (USA ....and others?) sand which is a very fine casting sand bonded by oil and making a two part mold. The surface finish can be very impressively detailed and the cooling will be more uniform. The sand does not contain water (also not chemically bound / crystalline water) so you don't have issues with the steam. The two part mold helps with the uniform cooling. Also you should "design for casting" taking into account the thin and thick section issues etc. as much as possible.

Of course I understand if you don't want to order any special casting sand for one piece of casting.... sometimes one wants to experiment with the materials at hand.

[AustinH6]

Was the mould destroyed? I dont suppose there is anyway it wasn't unless you cut it in half before you poured and held the halves together somehow. Looks great considering how large the cast was. Wish i had room for a cnc machine...

[zensuke]

I don't have much experience casting aluminum but I have done some bronze casting and it looks like some of the problems in your cast are from the metal cooling too fast. Obviously having a hot mold is more important when your metal is only liquid around 2000 degrees but it might be worth trying to heat up the mold first.

[EcoExpatMike]

Very cool. You consider buffing it to smooth it out? I kinda like the rough look tho.

HOWEVER, it looks AMAZINGLY uncomfortable to sit on. And cold on the behind? Brrr.

the legs & top holding ring look pretty exceptional in design & appearance. You have plans (hardcopy, as I don't CNC while deployed overseas)?

[livichris]

is it comfortable? looks like it might leave you with a sore ar$e!

[dpmakestuff]

It has a nice potted plant sitting on it now. I don't think it'd be that bad for an arse

[Orngrimm]

please be aware that blank aluminium gives dark stains on light clothes (rubbed off particles) which is really really hard to wash out if at all.
Best if you go for a hard clearcoat afterwards...