Milling and Drilling Head
Contents
Milling and Drilling Head
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A RepRap can be converted to a light duty CNC Mill by equipping it with a rotary tool such as a router or a dremel. With one of these toolheads, it is possible to do milling and etching on lightweight materials like wood and PCB.
Due to the lateral stresses incurred, it is probably best to think of it not as a general purpose mill, but a tool for specialized etching and engraving. It is most suited as a PCB Mill.
There are actually two common types of PCB. FR4 is most common, a copper clad heat resistant fiberglass board, and FR1, copper cladded phenolic paper. FR4 is easily available, but FR1 is considered better for this type of work because it is easier to machine. It should be noted that FR1 is not in wide use due to it's lack of heat resistance, and if you are building boards that will require it, FR4 should be used.
The major advantage of PCB Milling is that it is a dry process. Unlike PCB Chemical etching, it does not require the storage or disposal of chemical waste. That said, there can be some danger when creating fiberglass and copper dust. Caution should be taken and respirators should be worn when milling. Also, it is a good idea with Mendel to cover your stepper electronics. Copper kerf is not condusive to the long life of your electronics.
Picking a Tool
Unfortunately, the mendel carriage is just not very large. Most rotary tools that spin fast are pretty large. They're also heavy. Because we don't have a lot of room, we are limited to what we can support.
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Dremel Flexshafts
Bogdan designed a mendel toolhead Dremel_Holder that holds a flex shaft. While this meets the design challenge requirement, it is not particularly suited for PCB milling. Flex shafts are good for extending a rotary tool, but generally lack good bearings in the shaft (at least with dremel tools), so there is some slop in the head. Making a circuit board will probably be the most precise work you will do with your Mendel. If you need to make a .5mm pitch pad, you cannot deal with sloppy tolerances. c If you do use a dremel flexshaft, it is advised to use a collet rather than a chuck. You will also need to make sure that your flexshaft is lubricated and free of kinks. We found out quickly that a small bend in an un-lubricated shaft will cause the cable inside to break. Mounting the dremel to the ceiling from a hook is a good way to keep bends out of the line.
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Proxxon Micromot Rotary Tool.
The Proxxon micromot is an affordable ($40 on AMZN) rotary tool that can be fitted in the X Carriage without modification. It also runs on 12V-18V DC power, and can be fitted with Steel Collets. It spins at 20k RPM at 18V. We are confident that we can run this off of the extruder controller, although we may need to heatsink the fet.
Printing your tool holder
Each tool is going to be a little different, so you will have to modify your design to fit. Also, you will need to take into consideration the M4 bolts holding your bearings in your X-Carriage.
<flickr>4476340741|right|m</flickr> It is very important that the tool be held firmly, and you retain the ability to change the bit. On the proxxon, the shaft lock is near the tool, and required a groove to be placed in the toolhead. Surprisingly, with a thick enough part, this did not impact the strength of the holder.
The Cutting Tool
Regardless of the machine uesd to spin it, you will still need a cutting head and a way to hold it.
Chuck vs Collet
Collets are a way to get more stability in your tool head. Chucks allow tools to be mismounted. If you have the ability, go with the collet.
Milling bits
The cutting bit on a mill is called an End Mill. These are basically drillbits that dont snap when going sideways. There are a LOT of different kinds of endmills, drillbits and tools related to milling.
If your goal is to cut traces in circuit boards, it seems that a spade bit is well suited to the task. Drewtronics.net has 30, 45, and 60 degree spade bits available.
If you are looking to route or drill circuit boards, endmills and pcb drillbits are more appropriate. The main difference again between an endmill and a drillbit is the ability to move sideways. If you try to mill with a pcb drill bit, it will most likely snap. Since it is spinning at 20,000 RPM, you will want to be careful.
People do tell success stories of cutting circuit boards with broken and resharpened pcb drill bits, but it is a lot easier to get results if you just pick up a spade bit.
Software
Skeinforge has cutting options, but there is definitely more than one way to skin a cat.
Cad.py - This is a all purpose tool created by the MIT Center for Bits and Atoms. It will generate gcode from a PNG image. If you want to run a .g file on a mendel, you will need to run it through some postprocessing. Brandon Martin-Anderson has written Mendelize and it is available here. http://github.com/metrixcreate/mendelize
PCB-GCODE - If you are using Eagle, this is what you want to use. The options are straightforward, and you do not have to leave Eagle to generate g-code.
