Motorized Plunge PCB Drill Using a USB Microscope

This Instructable is an improved version of the USB Microscope Guided PCB Drill I published HERE.

The subject of PCB drilling has come up many times on the Homebrew_PCBs Yahoo Group. Among the many interesting thoughts was Harvey White's description of the PCB drill he had constructed using a TV camera and a homemade drill that drilled holes from the bottom of the PCB rather than the top. It also featured a stepper motor driven Z axis carrying a high speed rotary tool as a spindle.

The main advantage of bottom-up drilling is that dust from the drill does not obscure the camera's view. A secondary advantage is that the camera does not have to take up a lot of space below the drill table. In the original Instructable I detailed my build of a USB Microscope Guided PCB Drill in the inverted configuration using an off the shelf drill press and having a manual Z axis.

After using the manual drill awhile, I found that it was very cumbersome to hold the pcb with one hand while simultaneously pulling up on the feed lever to drill the hole. So I decided to modify the PCB Drill to include a Motorized Plunge.

I also found that the bar cantilevering the microscope over the plate was not rigid enough to keep it from flexing while I was focusing the microscope. I decided to replace the aluminum bar with a steel bar of the same dimensions. The Spacer between the Bar and the Plate can be either steel or aluminum. The only problem this presented was in threading the 4-40 holes to attach the microscope mount. I found that unless I was really careful (and lucky), the 4-40 tap would break. So, in this version of the drill I used 6-32 screws instead. I suggest you use a good quality tap, plenty of lubrication, and take great care to be sure the tap is vertically centered in the hole.

Parts

Mechanical Parts

Steel Plate - 4 x 4 x.125 inch thick. (OnlineMetals.com)

Steel Bar - .250 thick x .625 wide x 10 inch lon. (OnlineMetals.com)

Aluminum Bar - .250 thick x .750 wide x 6 inches long. (OnlineMetals.com)

Aluminum Angle - .75x.75 x .062 x 1.75 inch long Aluminum Angle. (Local hardware)

20mm Shaft Collar with Mounting Holes - McMaster-Carr (#5633T14)

Extension Spring - 20" Extension Spring .187" OD .026" Wire Mcmaster-Carr (#9665K15)

Brass Rod -.072 inch diameter x 1 foot long - McMaster-Carr (#8859K258)

Nylon Spacers - .375 inch long #10 spacers

Rare Earth Magnet

Nuts and Bolts - M4 x 6mm, M5 x 20mm, 10-32, 6-32 (or metric equivalents).

Electrical Parts

12 Volt 50 RPM Gearmotor. I used this one from Ebay: Motor This vendor indicates he has many of these motors left, but you can also use McMaster-Carr part (#6409K18). Torque is 1.5Kg.cm (9.98 lb.in).

12 Volt DC Power Supply, 1.5 Amps is more than adequate.

Power Jack, Panel Mount - to match the power supply.

Switch - Any SPST or SP NO switch that can handle 1.5 amps at 12 VDC is fine.

Phono Jack, Panel Mount - For the Footswitch

Phono Male Plug - For the Footswitch

Relay - Digikey (PB1773-ND) or any other 12 vdc spdt relay.

Reed Switch - Digikey (59150-010-ND)

Footswitch - see text for making your own from a dead mouse!

Tools

Drill Press - Any inexpensive one will do. You might be able to get away with a hand electric drill, but drilling relatively large holes in steel might be problematical.

Ohmmeter

Various other common workshop tools.

Warning: Wear safety glasses at all times! Some of these operations can result in metal or plastic pieces flying towards your eyes!

Remove the 4 screws holding the aluminum Plate to the Rectangle and put it aside.

Unscrew the Hand Feed Lever from the Spindle Wheel and discard it.

To make the Drill Arm, drill two 7/32 (.219) holes 30 mm apart in the 10 inch x .25 x .625 steel bar. Clean and deburr the holes, then use M5 x .8 pitch x 20mm long bolts to attach the lever to the 20mm shaft collar. Drill a 3/32 (.09375) hole .75 inches from the other end of the Drill Arm.

The motor I have has an 8mm diameter shaft. To make the Motor Lever Arm use either a 5/16 or 8mm drill to make a hole .5 inches from one end of the .250 x .750 x 6 inch aluminum bar. Drill a 7/64 (.109) hole from one edge of the bar into the 8 mm hole hole and tap for a 6-32 setscrew. Drill a 3/32 (.09375) hole .5 inches from the other end.

File the edges of the 4x4x.125 Steel Plate. Assuming you have the same motor as specified in the parts list, drill all the holes as shown in the drawing appended below. If you have a different motor you will have to modify the drawing as necessary. Once you have drilled the holes, tap the two .109 holes for 6-32 screws. Be careful not to break the tap! Be sure the tap is centered vertically and use plenty of lubricant. These holes are accessible from the rear, so another option would be to drill them to clear the 6-32 screws using a #25 (.1495) drill and use nuts and lock washers when you mount the Speed Controller Box.

Since the motor’s measurements are in metric and I (mostly) use imperial measurements, the Motor Plate drawing is imperial with metric value being converted to decimal equivalents.

Remove the head stock and replace it right side up on the column.

Remove the screws holding the Speed Controller Box. Remove the knob and the large control nut and washer under it, and then gently pull the Speed Controller board out of the box. Mark the wire going to the switch on the potentiometer with a black marker, then cut both wires near the PCB. Save the plastic insulator sheet. Pull the wires back through the rectangular hole in the Headstock and leave them in the Headstock Enclosure. Put the Speed Controller with all its parts aside for now.

Place two 10-32 by 1 inch bolts through the lower .203 hole in the Motor Plate. Secure them to the Motor Plate with lock washers and nuts. Then add a .375 inch long #10 Nylon spacer, followed by another nut to hold the spacer in place. These bolts are the upper and lower stops for the Lever Arm.

The Motor Plate is mounted to the Headstock by a homemade "toggle bolt". Cut a one inch piece of .250 inch x .75 inch aluminum bar and drill a #21 (.159) hole through the center. Tap it for a 10-32 bolt.

Place a 10-32 x 1 inch long bolt through the upper .203 hole in the steel plate. Put the 1 x .75 aluminum "nut" inside the Headstock, bridging the slot in the casting where the Speed Controller Box was mounted. Holding the nut in place with your fingers, slide the 10-32 bolt through the slot and into the nut. Loosely tighten the bolt to hold the plate in place. See the photos for proper positioning.

Mount the motor using two M4 x 6mm long screws. The shaft extends through the large center hole. Adjust the plate so that motor case is flush to aluminum Headstock casting, then tighten 10-32 toggle bolt to hold motor bracket in place.

Reverse the Headstock.

Pull the Speed Controller wires with their protective sleeve through the slot and large hole in the Motor Plate. Reconnect speed controller, soldering to the rear of the PCB as shown in the photos. Note that the wire that connects to the black wire from the drill power plug connects to the input of the Speed Controller, which is the blue lead in the photo. (Both leads will be red in your drill.)

If you need extra slack, you can remove the plastic wire tie and temporarily disconnect the ground screw in the Headstock. When you are done, reconnect the ground wire and tuck the wires out of the way in the Head Stock.

The plastic Speed Controller box has two edges contoured to fit the curve in the headstock. Using a small side cutter or snip carefully cut the plastic sidewall and mounting sleeve flush with the other sides of the box. See the photos.

Place the Speed Control back into the plastic box and secure the large control nut and washer on the potentiometer. Note that the PCB can be installed in either orientation. Put the plastic shield behind the PCB, then mount the Speed Controller Box to the Motor Plate using 1.5 inch long 6-32 screws. If you chose clearance holes rather than threading the Motor Plate, use long nose pliers to put lock washers and nuts on the rear of the Motor Plate to secure the Box.

We will now make a small bracket to hold the power connector, power switch, and footswitch connector. Cut a 1.75 inch piece of .75x.75 inch angle aluminum. Drill two 9/64 (.141) holes to match the mounting holes on the Motor Plate. Drill mounting holes for the power switch, power connector and footswitch connector in the other face of the angle. The hole sizes are dependent on the exact connectors and switch you have chosen.

Mount the angle on the Motor Plate as shown using 6-32 hardware. Then mount the switch in the top hole, the footswitch connector in the center hole and the power connector in the bottom hole as shown in the photos. Following the schematic, wire these three components leaving 6 inch leads to go to the relay.

Wire the Relay following the appended Schematic. The circuit works as follows:

1 - Turn the Power Switch on.

2 - Press the Footswitch. The drill moves upwards towards and through the PCB, drilling the hole.

3 - A Magnet on the Motor Lever Arm triggers the normally open Limit Switch. This latches the relay closed, removing power from the Motor.

4 - An external spring in conjunction with the Spindle Return Spring pulls the drill back to the starting position.

5 - The Footswitch then has to be released to unlatch the relay and reset the circuit.

The Relay I used is an automotive relay with a mounting flange. It also has a built in snubbing diode (unused), so it is polarized and should be wired per the schematic. Connect the wires from the Connector Bracket. Solder the motor wires to the Relay, but leave them unconnected from the motor for now. Bolt the Relay to the Motor Plate using 6-32 hardware as shown in the photos. Do not connect the Limit Switch at this time.

Connect the power supply, and a Footswitch (or jumper across the footswitch connector). Turn the Power switch on. Briefly connect the motor wires. The motor should be going counterclockwise, facing the shaft. If it isn't, reverse the wires. Then solder the motor wires in place.

Turn the Power Switch off, disconnect the Power Supply and Footswitch.

Loosen the shaft collar on the Drill Arm and slide it over the Spindle Wheel in approximately the position shown in the photos. Tighten the shaft collar.

Place a 6-32 x .250 inch setscrew into the Motor Lever Arm and slide it over the Motor Shaft. Center the Lever Arm on the Nylon spacers but do not tighten the setscrew.

The Limit Switch is used to sense when the drill has moved far enough to completely drill through the pcb. I decided to use a magnetic reed switch because it is much easier to adjust. Reed switches are activated by the presence of a magnetic field, in this case by a rare earth magnet mounted on the Motor Lever Arm. The Limit Switch is mounted to the motor as shown using an M4 screw. Note the tab on the Limit Switch has to be drilled out slightly to clear the screw.

To adjust the Limit Switch, place an ohmmeter across its leads. You will now be swinging the Lever Arm to determine exactly where to place the Magnet to trigger the Limit Switch. Hold the Magnet on the Lever Arm and slowly rotate it till the switch closes. You want the switch to close just a little before the lever Arm hits the lower stop. Use a piece of double stick tape to secure the Magnet, then tighten the setscrew on the Lever Arm.

Use a piece of .072 inch Brass Rod to connect the Motor Lever Arm and the Drill Arm. Loosen the shaft collar on the Drill Arm. Make a right angle bend about 1.5 inches from one end of the Brass Rod and a second right angle bend in the same direction about 4.5 inches from the first. Cut the rod about 1.5 inches from the second bend.

Cut a 5 inch length of spring. Use a wirecutter to separate a few turns at each end of the spring to form an eyelet. Slide an end of the Brass Rod through the hole on the end of the Drill Arm, then slide a spring eyelet over the rod. Following the photos, use a 1/4-20 bolt and nut to secure the end of the rod. Drill a 9/64 (.141) hole in the upper left corner of the aluminum Rectangle as shown, then secure the other end of the spring to the aluminum Rectangle with a 6-32 screw and nut. See the Photos.

Slide the other end of the Brass Rod through the hole in the Motor Lever Arm and use a 1/4-20 bolt and nut to secure the end of the rod. This completes the linkage, but it still must be adjusted.

Put a drill bit in the chuck.

Important! For this drill to work properly all of your drill bits must be the same length. I routinely use carbide drills with "rings" (see photo). The rings allow me to insert each drill exactly to the same depth in the chuck.

Replace the Aluminum Plate, Bar and Microscope Mount.

Put a hex wrench in the shaft collar setscrew. With the spring pulling the Motor Lever Arm to the upper stop, use one hand to lift the Drill Press Spindle at the pulley. When the tip of the drill is just below the surface of the Aluminum Plate tighten the Shaft Collar. You should find that rotating the Motor Lever Arm to the lower stop will raise the drill so that the tip of the bit will be well above the Plate.

If necessary, you can adjust the trigger point of the Limit Switch by rotating the Limit Switch on its mounting screw or by moving the Magnet. If you use an ohmmeter with the Motor Lever Arm rotating the motor, keep in mind that the emf generated by the rotating motor can "fool" your ohmmeter. Don't use the "beeper", look for (approximately) 0 ohms as the switch closes!

You will need a foot switch to operate the Plunge. Inexpensive ones are readily available on eBay, but I decided to modify an old, defective computer Mouse instead. We will only be using the two Button Switches, so open the Mouse, remove the wheel mechanism if it has one and remove the Ball or Optical Sensor. Remove the main "chip" as well.

Using an ohmmeter, determine which mouse wire(s) goes to ground - usually the two black wires. Then find the Normally Open (NO) contacts of each switch (usually the center terminal of the switch. Solder a small piece of wire connecting both NO contacts to the three non grounded wires at the connector.

Cut the usb connector off the mouse cable. Drill a 7/64 (.109) hole at the rear of the mouse and pull the cut end of the wire through. Solder the mating plug to your footswitch connector to the end of the cable. I used a simple Phono Jack from Radio Shack.

Check with an ohmmeter to be sure the switches are a high resistance when the switches are open. Mine read 182 ohms, way too low, indicating that something was still attached across the switches. I dug around on the board and cut the small molded inductor labeled JP3. This solved the problem. See the photos.

Reposition the mouse pcb in the lower mouse chassis and replace the cover to complete your Footswitch!

The drill is designed to use the 1.5 inch long carbide drills commonly used to drill PCBs. These drills have a .125 inch diameter shank and come either with or without depth rings. I strongly suggest you use rings - it makes positioning of the drill bit MUCH easier!

Since the spindle is so close to the plate, it is necessary to completely open the chuck and "wiggle" the bit into place. Once you’ve done it a few times it’s pretty easy.

For "final" calibration of the Microscope loosen the screws holding the Microscope. Connect the Microscope, open the software and turn the LEDs on. Raise the spindle so that the tip of the drill just passes through the plane of the Plate. Focus on the tip of the drill point and center the tip in the crosshairs. Now tighten the Microscope screws. Recheck the calibration and loosen the screws and redo if it is not in perfect alignment. After using the drill a while, I noticed that the vibration of the drill had a tendency to shake the Microscope out of focus. This is easily remedied by focusing on the PCB then putting either a piece of tape or a rubber band around the Microscope and focusing knob.

Your drill is now ready for use!

The drill can handle PCBs up to about 4.5 inches wide. To drill, insert the proper drill bit, start the drill, turn on the Microscope and set the software to Full Screen. Carefully position the crosshairs on the center of the hole. Hold the PCB firmly in place with both hands, and gently press the Footswitch. You should have a perfectly centered hole! The final photo shows a .021 hole perfectly centered in a pad.

Enjoy!