A Tool Height Setter for Small Mills
Here is an enjoyable and useful project for the hobby machinist: A tool to zero the Z-axis when changing end mills. I built it using a CNC mill, but it could also be built it using a (non-CNC) lathe and a few hand tools. The design uses electrical contact between an end mill and a spring-loaded sensor plate to light an LED, indicating a distance of 1" above the mill table or part. A clip lead connects it to the end mill, which not only simplifies the design but is necessary for my Taig mill which has the spindle insulated from the table top.
All of the parts are layed out in the picture above. The housing is machined from block of acrylic (plexiglass); next is the sensor plate, with some machining marks shining from the camera's flash; the spring; the circuit board; a retaining clip; battery; and a length of thin music wire (look closely). A clip lead from my electrical parts bin completes the project. The materials were chosen largely out of convenience. Plexiglass has decent dimensional stability and machines well with metal cutting tools, plus the friendly folks at Fort Collins Plastics sold me a chunk big enough for 3 or 4 housings for $5. The sensor plate is from a piece of Fortal (essentially 7075) from my scrap bin, though I don't think the application requires this grade of aluminum. Other parts came from local businesses and my growing collection of miscellaneous parts.
The housing is key to the tool's accuracy and functionality. Acrylic has decent dimensional stability, acts as an electrical insulator, and lets the LED light shine through. Its coefficient of thermal expansion, 4x10^-5 in/in/F, is just enough to cause measurable change with seasonal temperature variation, but this only matters for very demanding projects. The critical machining operations were face cutting a 1.5" diameter area in the acrylic stock, and then a 1" diameter 1" deep pocket, creating an accurate "negative" height gage. Next, an access hole of diameter 0.9" was cut through to what will become the top, and then a groove (1.066" dia, 0.046" width, 0.099" margin from the edge) for the 1" internal retaining ring. All that remained was turning over the part to face the top (leaving 0.15" thickness), then drilling some very small holes just below the clip groove (turning a tiny drill bit by hand using a pin vice). These holes allow a length of wire to pass through the housing, holding the battery in place and serving as an electric contact.
The sensor plate (shown upside-down in the picture) is a 0.1" thick aluminum disk of diameter 0.99" with an island (0.1" high, 0.73" dia) to keep the spring centered. Participants on the Taig discussion group may recall the problems I had milling this, due to a combination of aggressive cutting, poor clamping, and other factors. The resulting surface finish is not perfect on the side shown, but it is beautifully flat on the sensor surface where it matters.
The spring, Serv-A-Lite part number 154U (1-1/16" length, 29/32" out. dia., 080 wire gauge), and 1" retaining ring were bought at Downtown Ace Hardware. The battery is a CR1632 (3V lithium, 16mm diameter, 3.2mm thick).
The circuit board was made from a scrap of .021" thick acrylic and the electronics are truly minimal, comprising a 5mm LED soldered to a 100 ohm resistor. Most LEDs will work, except the high brightness units which require more than 3 volts. 100 ohms is a pretty safe value but if want to be sure, you can find it here (10ma is a good choice for LED current). Note that I ground off the top of the LED for clearance. The top of the board has a round island (again 0.73" dia to center the spring) with a round pocket to hold the led. A 1/32" groove has been cut to allow on the LED's leads to wrap around the island where it will make contact with the spring. The resistor is soldered to the other lead of the LED (well away from the acrylic!) and fits into a 1/16" hole through to the opposite side of the board. After soldering, the LED/resistor assembly is glued in place with a couple of drops of cyanoacrylate glue. Note that the soldered wires are bent inward for clearance to keep them away from the spring. LEDs are polarized, but in this case it makes no difference as long as the battery is inserted with compatible polarity.
The bottom of the board has a 16mm diameter pocket to hold the battery and a loop of wire (the resistor's other lead) serves as the battery contact.
The underside of the assembled tool looks like this. The wire is held in place by friction and spring forces as it bends to make contact with the battery. It is now ready to turn right-side up and put to use! You can clearly see a gouge cut by mistake, and the poor surface finish around the circumference, but I plan to leave these flaws as a reminder that there is always room for improvement.
Because there is no seal around the sensor plate, dust and chips can get in between it and the housing, lowering the device. Cleanliness is imperative to maintain accuracy. Bringing the tool in slowly and stepping down by thousandths (or even tenths) will prevent gaps from opening and helping to maintain a clean contact.
I built the project for my own use and satisfaction and I wrote this article simply for the benefit of others who might appreciate it. The local business I mentioned are among my favorites and I like to patronize and support them - I hope no one minds their inclusion here. If you have any questions or comments feel free to write me.