5. Machining Plastic Rings for Test Equipment

(article continued from previous page)

I wanted to make a unique piece of test equipment for measuring the amount of light passing through a polarizing film at each angle. The test box needs three mating rings machined for it.

The first thing I do when building something is to create a paper template on the computer using a drawing program (Microsoft Visio). This may seem like a lot of work, but it saves time in the end by avoiding rework due to mistakes.

CAD template created in Microsoft Visio for determining the sizes of discs for a test apparatus.

CAD template created in Microsoft Visio for determining the sizes of discs for a test apparatus.

The rings are going to be cut with a rotary table on a milling machine. There needs to be a way to fasten the raw material to the rotary table during machining.

Because the outside of the material is going to be cut to make it circular, externally mounted clamps won’t work. Because the center of the material is going to be cut to provide a hole for the light to pass, a secure bolt down the center won’t work during machining. Mounting screws need to be placed somewhere in between.

By placing two pair of screw holes neatly in line with each other, they’ll match up to the slots in the rotary table, holding the material firmly in the center of rotation during machining.

Select a raw material that is opaque and easy to machine. It is preferable that the material is non-reflective, as the center hole will then act like a baffle, preventing non-polarized ambient light from other angles from tainting the results. The material should be just slightly thicker than the wall of the opaque box, so that the face and back don’t rub against the box wall after being firmly attached to the bearing ring.

With a table saw or a hack saw, cut down each raw material board to slightly larger than the actual desired diameter of each ring. Because this is a rough cut, the extra material provides room for error and allows the rough edges to be removed by a finer process.

Cut three scrap boards of thinner material to insert between the ring boards when milling. These scrap pieces will prevent damage to the machining surface and the surfaces of each ring when milling the ring above it.

Tape the paper template in the center of the smallest board. Then, stack the boards with the biggest on the bottom, going up to the smallest on the top, with scrap material in between each one. Temporarily bolt the boards together through a center hole, or use a clamp.

Drill the four mounting screw holes as accurately as possible. Besides holding the boards in place during machining, these screw holes align the boards with the rotary table. The more accurate the placement of the holes, the more circular the wheel will turn out.

Remove the center bolt or clamps. This was only necessary to keep the boards together when drilling the four screw holes.

Attach the stack of boards to the rotary table using four #4-40 screws. (Technically, these are bolts, since the threads grab at nuts in the rotary table, not into the board material. But, #4 fasteners are so narrow that is just sounds weird to call them “bolts”.)

Milling three discs at the same time by stacking them in order of diameter on a rotary table.

Milling three discs at the same time by stacking them in order of diameter on a rotary table.

The outsides of three boards are being individually milled into circular shapes by turning a rotary table on a vertical milling machine. Thin white scrap material sits in between each board to prevent damage to the lower ring or rotary table face from the bottom of the end mill.

Working from the outside of the largest piece of board, mill down the edges a little bit with each rotation.

Each ring is going to have a different outside diameter. Again, the scrap pieces between each board allow you to mill a smaller ring without marring the top of the ring below it.

Occasionally, stop and check the ring’s outer diameter using digital calipers. Continue milling until each ring is the desired diameter.

You could mill each ring separately; one at a time. However, since they are all going to share the same diameter inner hole, and since you want the holes to line up nicely and be well centered, it’s better to mill all three rings at the same time.

You can jumpstart the center hole by drilling out a large pilot hole. Begin by swapping out the end mill for a drill that is as large as possible, but is still smaller than the final desired diameter.

Determining the maximum drill depth by first lowering the drill outside of the workpiece.

Determining the maximum drill depth by first lowering the drill outside of the workpiece.

To ensure that you don’t accidentally drill into the rotary table, you can lower the drill to one side of the stack, until it matches the maximum acceptable depth. Then, set the backstop on the mill/drill.

Note: The above photograph also provides a nice view of the scrap materials inserted between layers of the workpieces.

Starting the machining of the inner hole with a drill.

Starting the machining of the inner hole with a drill.

Bring the drill up, center it over the stack, and drill a hole all the way through (you should pull out several times during the process to help remove material). Not only does a drill remove more material faster than the mill, but it is better at pulling out the waste material.

This large pilot hole created by the drill provides room for the end mill to enter, and for waste material to come out.

Expanding a hole on a rotary table with a roughing end mill.

Expanding a hole on a rotary table with a roughing end mill.

The desired inner hole diameter is much larger than my largest drill. To expand the hole beyond the initial pilot hole, you can use an end mill or a boring bar.

I found it challenging to mill out the inner hole. The waste material didn’t flow out very well. Many times I had to stop and clean out the hole with a brush or a vacuum.

Another problem was with the length of the end mill. I ended up switching to a long roughing mill to get the depth required to mill all rings at the same time. I found it was easier to mill out most of the first ring, then the second ring, then the third ring, before finally milling out the final inner diameter of all three rings at the same time.

With the rings finished, let’s add them and some other equipment to the box and see the test results...