This is a CNC rotary axis (a.k.a rotary head, 4th axis, indexing table) I built to use with my CNC machine(s).

 

 

      Now that I have my Phoenix Redux back up and running I decided to try building a rotary axis so I can do more things than "simple" 3D CNC milling.

      A rotary axis is used to rotate the part itself while it is being milled (as opposed to the part being clamped down to the milling bed) allowing more complex parts to be made. It's kinda hard to explain what all one can do so you'll have to wait until I start cutting things with it to see it's capabilities.

      One of the main reasons I'm building this (aside from the sheer challenge) is because I want to attempt to cut gears from foam with a hot-wire in the hopes that, when cast, the gears will be of useful precision. Once I get the rotary axis running I'll make a new project page for gear-making.

      The heart of the rotary axis is a worm and worm gear set from Boston Gear (note to self: add link). The worm gear has 80 teeth wich means, when the worm gear is hooked up to a 1.8 degree/step stepper motor, I get a resolution of 0.0225 degrees per step (I can move the axis in 0.0225 degree increments).

      The bearings I will use are a few roller-skate bearings left over from building the Phoenix and Phoenix Redux. The shafts will be made from some 1/2" steel rod I have laying around turned down on my Harbor Freight mini-lathe. Everything else will be cast out of aluminum.

      I first designed the whole thing in AutoCad (I'll add a screenshot later). I then used those drawings to make toolpaths that were converted to G-code (the instructions that the CNC machine uses) and them milled the parts out of blue 1/2" insulation foam (I really like this material, it's available at most home-improvement stores in 4'X8' sheets).

The parts (bearings are temporarily set in their seatings to check the fit):


      The piece with the big hole surrounded by the four smaller ones is where the motor will mount. Also, the seatings for the bearings are milled a few thousandths of an inch oversized to allow for contraction of the aluminum when cast. All the parts are then hot-glued together (the hot-glue burns out just like the foam when the metal is poured).

Foam parts assembled:


      In preperation for casting spues are added to the foam pattern. Sprues are the path that the metal uses to get to the actual part. For this part I just hot-glued two scrap pieces of foam to the top of the pattern that meet at a common point. I also glued four pieces of drinking straws to the top to provide some additional passages for the burning foam-gasses to escape during the pour. Finally the whole thing is coated several time in drywall compound (the coating is allowed to completely dry between coats). The coating helps improve the surface finish of the casting and to prevent the sand from caving-in as the metal is poured.

Pattern prepped for pouring (the bearings were removed before coating):


      The pattern was buried in dry, loose sand (no special molding sand required) with just the top of the sprues and the vents above the surface of the sand. I placed a special tool, called a KHPT, wich is essentially a split piece of 4" steel pipe with a hinge and handles over the sprue, to act as a funnel and to provide plenty of hydrostatic pressure to the aluminum to make sure the mold filled completely.

      Unfortunately, I don't have any pictures of the pour. I generally work alone and both of my hands tend to be too busy, handling a 1400 degree F crucible of molten aluminum, to take pictures.

      In lost foam casting, when the foam is hit with liquid aluminum it melts and vaporizes, more or less, instantly. In the first split-second of the pour, the foam vaporizes into a thick white smoke. However, this white vapor is extemely flamable and generally catches fire just from the heat of the aluminum. The white vapor burns with a very dark orange flame that puts out enough soot to make acetylene look clean. While the products of burning styrofoam are supposedly non-toxic, you want to stay out of the smoke unless you want too look like a coal-miner from the soot. Also, "smells bad" is an extreme understatement.

      After the aluminum solidifies in the mold (takes about 5-10 minutes after the pour) the part is removed from the sand and quenched with a water-hose to cool it down and to make the charred drywall compound fall off. Then the sprues and vents (both are now solid aluminum) are cut off with a hacksaw. Then I sanded the top and bottom of the part on a belt-sander to get them flat and smooth. Finally the holes for the motor mount were tapped (the holes were there as-cast so I didn't have to drill through 1/2"aluminum).

      I machined the shafts on my HarborFreight mini-lathe to fit the gears. Nothing spectacular, just basic lathe work. You can see them pretty well in the following picture.

The rotary head sans face plate:


      Since it's pretty hard to mount a workpiece on a bare 1/2" shaft I decided to make a faceplate. The faceplate was cast using the lost-foam method also. I just took two round pieces of foam, glued them together, added a sprue, coated with drywal compound, cast, and machined on the lathe. The faceplate is 2.000" inches in diameter at it's widest point and has a 1.000" diameter raised section on the from to help in centering workpieces and any adapter plates that I may make later. (I might try mounting the chuck from my mini-lathe on it at some point.)

The rotary head with face plate:


      All I have left to do now is make a cover for the top (to keep milling debris out of the gears) and actually use it to make something.