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The purpose of a ring light is to provide adequate lighting from all sides while permitting viewing through the center hole. For equal illumination, the optimal internal shape is circular.
But, that doesn’t mean that the outer shape of the bulb or PCB needs to be round. Having a round exterior is optional. In the case of a PCB, it may be easier to leave the board with a rectangular or square shape, even if the corners are left empty (not populated with components).
Regarding the center hole, if the hole diameter is small enough for your largest drill bit, simply drill the hole. Otherwise, use a hole saw bit. In fact, you can use a hole saw to create both an outer and inner circular shape.
Since I already knew of another way to cut a circular PCB for the Milling Machine Tachometer, I figured I might as well make the inside and outside perimeter of the LED ring light into a circle using my milling machine. Unlike with a hole saw, the technique described on this page is more precise and allows for any hole diameters, not just standard sizes.
The ring light circuit is cut from a larger PCB and then the inner and outer dimensions are milled circular.
The ring light PCB includes a number of other circuits on the same panel. This was less expensive than ordering a bunch of separate PCBs.
The first step is to cut the square shape from the PCB panel with either a hacksaw, rotary table saw, or a shear. Because the board is going to be cut into a circle, it isn’t important if the edges of the square board are uneven or rough.
Notice in the above picture that the square PCB includes mounting holes in the four corners for devices that don’t need a round board. Unfortunately, these mounting holes can’t be used to hold the PCB in place during circular machining, because screw heads in those holes would likely interfere with all but the narrowest end mills. Narrow end mills tend to be expensive and brittle.
Seemingly convoluted layers of holding devices and adapters that make setup and break down more convenient.
Machinists try to run repeated operations on a piece of machinery because equipment setup and alignment can be time consuming. In my case, this results in what appears to be an overly complex arrangement to hold the LED ring light circuit board during machining:
An adapter that allows a small PCB to be turned on a 4-inch rotary table.
The adapter board has bolts that fasten to T-slot nuts in the rotary table slots. A spacer (flat piece of scrap material) lifts the PCB slightly to prevent damage to the plastic adapter board during machining. (You can see some additional details on this technique on the Caliper Capacitor page.)
Normally, I use standard or button-head screws. This time I experimented with flat head screws.
The conical top of a flathead screw makes contact with the edges of the hole, as well as the surface. Otherwise, the screw would not touch the sides of a clearance hole.
A clearance hole is slightly wider than the diameter of the screw that passes through it. This prevents the screw threads from digging into the sides and possibly lifting the material instead of pressing it flat. Normally, lateral forces are resisted by the friction of the top and bottom of the material.
Unfortunately, unless I make a scrap spacer with matching holes (or ruin some nylon washers), the PCB will not be properly supported underneath the screws. With standard or button head screws, the amount of tightening required to prevent shifting would also bend the PCB. That’s not a huge deal, because the PCB will spring back again when the screws are removed.
However, the cone shape on the top of a flat head screw makes contact with the sides as well as the top of the PCB. Although this is an unconventional use of a flat head screw, I figured it would hold the PCB better during milling. It seemed to work fine.
I admit that this is a lazy approach and would not be appropriate for production or precision purposes. Nevertheless, perhaps the flat-head screw trick will be useful for some occasion when proper support isn’t possible?
The substrate under the copper layers on most printed circuit boards is a fire-retardant epoxy resin with fiberglass cloth. This is called FR-4. Machining FR-4 produces significant dust that is a lung and eye irritant. Therefore, it is a good idea to wear a dust mask during machining and to wash your hands and face after machining.
The inner hole of the LED ring light needs to be cut to a diameter of about 0.55-inches or 14 mm. The PCB sits up high when it is on top of a vise, adapter plate, rotary table, another adapter plate, and a spacer. Therefore, I’d need to use a short (stub length) 14 mm drill. But, I don’t have one in that size. Instead, the inner hole is cut using a smaller end mill (3/32-inch) on the rotary table.
Before cutting, the end mill is lined up over a thin circle outline etched from copper. The rotary table is rotated to ensure that the thin line remains under the end mill for the entire rotation.
If the line wanders too much, then that means that the PCB is not centered on the rotary table. In that case:
Finally, the screws are fully tightened on the PCB, and the milling machine is powered up to a fast RPM.
The end mill is lowered until it makes contact with the board. If the end mill diameter is greater than the PCB thickness, it is probably safe to plunge all the way through to cut the board in a single pass. Or, play it safe and remove a thin layer with each rotation.
Inner circle of a PCB cut using a 3/32 end mill and a rotary table instead of using a large diameter drill.
The fun thing about cutting away the inside diameter of a PCB with a small diameter end mill is that you end up with a free washer at the end of the process. Do you see it sitting in the center of the dusty mess in the above photograph?
Remove the “washer” and then vacuum up the epoxy dust with a shop-vac.
Left: A dusty mess created as a PCB is milled into a ring shape. Right: The finished ring light PCB.
The outside diameter of the PCB is cut using the same process as the inside diameter. Before cutting, make sure to rotate the entire perimeter under the end mill to ensure the pads or vias clear the cutting path completely. On the right side of the photograph, you'll notice that I was slightly too aggressive on the inside diameter and some of the electrical traces were partially narrowed.
Also on the right side of the photograph, notice that the white plastic spacer has been damaged where the tip of the end mill passed over it. This damage would have occurred to the adapter board had the spacer not been there. Instead, the adapter board remains intact and can be used for additional ring lights or can have more holes drilled into it for future projects.
The PCB is now cut to shape and is ready to be populated with surface-mount components for soldering the LED ring light. After that, you can see images through the video camera and magnifying glass.
