Machining a Ring or Collar from Black ABS Plastic

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At first glance, the black band at the front of the robot where the dome attaches to the robot’s body would appear to be a simple ring.

A clear acrylic dome bolted onto a black plastic collar on a robot’s body.

A clear acrylic dome bolted onto a black plastic collar on a robot’s body.

That’s not to say that machining a large ring doesn’t take some work -- it does. But, the collar piece is more complex than that it seems at first glance.

The inside of the ring is slanted to fit flush against the front of the robot’s canister body.

The inside of the ring is slanted to fit flush against the front of the robot’s canister body.

The inner diameter of the collar is angled to match the profile of the Lego Roboriders canister. Although this may seem to be a luxury, I couldn’t find any other way to have it mate securely. Let this be a lesson to all robot builders that base their robot on a tapered molded body part because it “looks cool”.

The material selected for the collar is 1/2-inch thick, black ABS plastic. ABS is tough, inexpensive, easy-to-machine, and can be glued or epoxied.

Purchase ABS plates from McMaster-Carr, MSC Direct, US Plastic, or other material supply houses. (FYI: Flat raw material that is subjectively thick is called “plate”, as opposed to a thin piece that is called “sheet”.)

Bolt holes are drilled on a plate in a vise while referencing a paper template.

Bolt holes are drilled on a plate in a vise while referencing a paper template.

As with most machining projects, it is helpful to begin with a drawing. A computer can precisely indicate the coordinates of holes, edges, and other key points.

Print the design at scale and tape the template to the workpiece. Or, if your machinery has a digital readout, you can simple move the table to the specified coordinates.

The holes are drilled and tapped for 10-32 screws/bolts. Technically, only two bolts are needed to attach the dome to the collar. However, a ring of eight bolts all the way around the dome makes it look high-tech -- like a pressure hatch on a submarine.

A milling machine cuts out a disc from a black plastic plate on a rotary table.

A milling machine cuts out a disc from a black plastic plate on a rotary table.

Four bolt holes are used to secure the plate to a rotary table on a milling machine. An end mill is brought up to speed and slowly lowered with each pass of the rotating rotary table. Over time, the disc shape is cut all of the way through the plate, such that the corners fall off.

Left: Checking the diameter of a disc with calipers. Right: A paper template roughly aligns an end mill for machining the inner diameter.

Left: Checking the diameter of a disc with calipers. Right: A paper template roughly aligns an end mill for machining the inner diameter.

When cutting away large amounts of material, it is usually more accurate to leave a little bit of extra material. In this case, the corners of the disc are cut away leaving an oversized disc (4.1 inches). Then, the diameter is measured and a final pass or two is performed more carefully to bring the disc to the desired diameter (4 inches). This is also a good time to switch to an end mill that has more flutes, as to leave a finer finish.

The same technique applies to cutting the inner diameter. The end mill can be positioned using an poorly cut-out paper template because we’re leaving enough inner material for a margin of error.

Milling the inner diameter of a disc with straight (vertical) walls.

Milling the inner diameter of a disc with straight (vertical) walls.

A ring of material is slowly removed as the table rotates and the end mill is gently plunged down over time. The inner diameter is milled out using the same technique as the outer diameter.

However, unlike the outer diameter, material tends to build-up in the ring slot. Periodically stop machining to brush out the swarf (cut up material). If you don’t, then the end mill is going to wear more quickly because it is repeatedly cutting the same material. Also, the plastic will heat up due to friction and the scrap pieces may stick to the end mill, clogging it up.

Eventually, the inner disc is freed and it can be removed for use in another project. This leaves a secure black plastic ring with straight walls/sides not yet cut to the final diameter.

Angling a Rotary Table with Angle Blocks

The Lego Roboriders packaging chosen for this robot has a beveled bottom.

Comparing an angle to a machinist’s square in a drawing program to determine the precise angle value.

Comparing an angle to a machinist’s square in a drawing program to determine the precise angle value.

The first step is determine the exact angle at the bottom of the canister. To do so, place a 90-degree straight edge (a machinists square) against the bottom and side of the canister. Take a digital photograph and import it into the computer.

The photograph is first processed by a painting program (Paint Shop Pro, in this case) to rotate the image such that the straight edge is oriented up-and-down. That is, unless you have a steady hand and perfect eye, the raw photograph is probably slightly misaligned.

Then, magnify the picture and copy the area of interest into a drawing program (Microsoft Visio, in this case). Draw a pair of lines to match the straight edge and the canister bevel. Examine the properties of each line in the drawing program, subtracting the angle of one line from the other. This shows the canister bevel line to be 15° different than the straight-edge line.

A set of fixed metal angle blocks or angle parallels.

A set of fixed metal angle blocks or angle parallels.

For convenience, a machinist can purchase a set of precise angle blocks from most shop suppliers or eBay. The rigid angled metal blocks have clean edges, ground faces, and are marked with their exact angles.

Alternatively, a machinist can use a compass, adjustable angle, or a sine plate to tilt the workpiece to any desired angle.

Left: A rotary table in a milling vise tilted to 15 degrees by an angle block. Right: Headless screws stay out of the way of the end mill.

Left: A rotary table in a milling vise tilted to 15 degrees by an angle block. Right: Headless screws stay out of the way of the end mill.

The angle block is placed underneath the rotary table in a milling vise. The vise is tightened.

Machining blocks and parallels are almost always used to temporarily hold the position of the workpiece before locking the vise. After that, the blocks shouldn’t be supporting the workpiece. That is, it should be possible to slide the blocks out from underneath if the vise is truly clamping the workpiece firmly. There are some exceptions, such as v-blocks or blocks used a parts of fixtures, but reconsider your setup if a loose metal block is somehow involved as a supporting structure during machining. Vibrations and machining forces may move the block or workpiece with dangerous or disastrous results.

Because this ring is relatively thin, the cap screw fasteners were swapped (one at a time) for headless screws. Otherwise, at this angle, the end mill might have started shaving off the tops of the caps screws.

Left: Machining the inner diameter of a ring to a precise angle. Right: Checking the diameter for an exact fit.

Left: Machining the inner diameter of a ring to a precise angle. Right: Checking the diameter for an exact fit.

In the picture above, notice that the milling machine chuck and end mill are still perfectly vertical. The tilted rotary table is what causes an angle to be cut in the inner diameter of the ring.

The vise securely holds the rotary table on angle. You may notice the 15-degree fixed angle block at the far-right bottom of the picture. It has been removed from the setup and placed on the milling table to demonstrate that it plays no role during machining.

The yellow Lego canister body is soft, cylindrical plastic with rounded edges, which makes it difficult to measure precisely with calipers or any other common measuring instrument. So, the canister itself is repeatedly test fit in the ring to determine when the inner diameter is exactly right.

Although this method allowed me to produce a nice fitting collar, it also suggested a problem that I would soon encounter: A ring on the end of a rounded beveled cylinder slides around like a ball joint. How can the collar be aligned to sit squarely on the Lego canister?