American Lutherie #77 featured an article on the uses of desktop CNC machines in small stringed instrument building shops. The article mentioned a shop-built desktop CNC machine but did not describe it in any detail. This page features a description of that machine and offers downloadable CAD files containing drawings of its major mechanical components. While in no way complete documentation for building such a machine, the information here may provide some guidance for those that wish to do so. The machine is small, potentially wobbly, but very inexpensive and easy to build. It is useful for light duty inlay and engraving, and as an introduction to the technology.
Last updated: Tuesday, May 08, 2012
The CNC (Computer Numerical Control) machine that is described here is a very light duty desktop (benchtop) three axis moving table router for light routing applications. The general configuration is similar to that of a vertical milling machine. A column supports the Z axis slide assembly, which moves the spindle up and down. The Y axis slide supports the X axis slide which in turn supports the table. The machine has a table size of 6” by 8” and can position anywhere in a space approximately 6” by 8” by 4” (X, Y, and Z) at a sluggish 6 ipm. It has a high speed spindle and is thus classified as a CNC router. Each axis slide uses a stepper motor and precision lead screw to move its table, and features a hand wheel for manual positioning as well as limit switches to inhibit travel that could damage slides and couplings. An integral vacuum pickup keeps machining debris out of the slides and increases bearing life. The machine is suitable for light routing operations in wood, plastic, MOP, and non-ferrous metals, and for engraving.
This DIY CNC machine was modeled after a small printed circuit board engraving machine designed by John Kleinbauer and described on his website at http://www.kleinbauer.com. John's machine is a marvel of economy, using a number of readily available components and following an elegantly simple design. John sells plans and assembly instructions for his machine, but I have not seen them and so cannot comment on their utility. One of the nicest features of John's machine is the use of inexpensive aluminum extrusion material and plastic for the linear slides, which greatly reduces the cost of the machine, but offers pretty good dimensional tolerances none the less. The extrusion material is unfortunately not very heavy and not all that rigid (weight and rigidity being two highly desirable characteristics in any material used to make a precision machine). This is not too much of an issue for an engraving machine, where so little material is removed that the spindle doesn’t “fight” the movement of the slides, but it could be a problem in a machine that does even light routing. Still, in short lengths and appropriately stiffened it can make very nice linear slides for light routing applications. The machine described here makes use of the same aluminum extrusion material for the slides, and uses the same basic configuration as John's machine as well. Dave Kush has built a similar machine – see very detailed pictures of his on his website.
All engineering is a matter of tradeoffs, and the engineering of a CNC machine is no different.In general, the design goals of this machine focused on ease of assembly and low cost, with ease of assembly taking priority most of the time. The machine can be built with the most basic woodworking tools. Even as a low cost and easy to assemble device, this CNC machine still maintains pretty good tolerances and was used to perform real work. The drive train is well matched to the power of the spindle, which means the machine will perform well, maintaining acceptable accuracy while moving the spindle in cutting operations at its performance limit.
This is a very light duty machine, meaning it will perform well and maintain its tolerances with occasional use. This was done to keep the cost of the machine as low as practical. A heavy duty machine would necessarily cost considerably more to build. Since it is a light duty machine, its linear slides have been designed with wear compensation adjustments, so that tight tolerances can be maintained even after considerable use. The main component of a slide is a length of 1.75” x 4.5” storefront aluminum channel of the type made by YKK Architectural Products and other companies.A block of Ultra High Molecular Weight (UMHW) polyethylene plastic slides in a wide slot in the extrusion and another block of the same material retains the block in the slot. The end caps on each slide are made of the same material. The mounting brackets for the end caps can be positioned so that small adjustments to the width of the slot can be made. There are also two width adjustment rods through the channel. These two features provide width adjustment for lateral wear of the bearing blocks.The rods also substantially stiffen the somewhat flexible aluminum extrusion.
Probably the most costly mechanical components are the precision lead screws and self-adjusting anti-backlash nuts used to position the tables. This machine uses relatively inexpensive components from Kerk Motion, but their cost still represents a major fraction of the total cost of components of the machine. Although it is possible to design a machine that uses much less expensive positioning devices, doing so may place pretty severe limitations on the uses to which the machine can be put, the frequency of adjustment necessary, or both. The use of these precision components means the machine will perform well even during light routing operations. Cheaper alternatives include using plain Acme screw and fashioning some kind of anti-backlash nuts or, even cheaper, using plain threaded rod and home brewed anti-backlash nuts. The whole issue of backlash and how to deal with it is discussed here.
The drive train contains the following components in order. A crank knob is on the back shaft of the stepper motor. The front shaft of the motor has a nylon thrust washer and is connected to the screw by a short length of rigid nylon tubing attached by hose clamps. This latter subassembly serves as a cheap but effective shaft alignment compensating coupling. The shank of the plastic shank knob bearing on one end of the motor case and the nylon washer on the other end constrain axial motion of the shaft and screw. The other end of the screw sits in a plain hole drilled in the end cap.
This machine uses a low cost Dremel hobby tool as a spindle, and this design decision appears to have little downside. The Dremel doesn't have much power, but a more powerful spindle would require a considerably more rigid and thus more expensive slide and bearing design. I've seen Dremel tools disparaged for alleged poor specifications, but I've measured both runout and end play on a number of such tools and found both specs to be excellent. The specs deteriorate pretty rapidly with use but fortunately the little grinders are easy and cheap to rebuild. On the other hand there are two potential sources of apparent bad runout specs with these tools, but fortunately both can be easily overcome. The first is the collet, which can severely distort when over tightened, an operation which is surprisingly easy to do. Replacing the collet with an optional keyless chuck solves this problem inexpensively. Although a chuck is not an ideal tool holder for routing operations in theory, in practice the Dremel chuck works quite well in this application. The second source of apparent bad runout specs with the Dremel tools is nose-only mounting of the tool, especially if the tool is mounted in an unthreaded hole. The solution here is to mount the device by screwing its nose into a threaded hole, and to support the upper part of the tool securely as well. With properly rigid mounting and a precision substitute for the collet, runout specs for the Dremel tool used in this machine are better than 0.001”.
In order to hasten assembly, this machine made use of an off-the-shelf electronics and motor package which includes 58 oz/in NEMA 23 pressed-case unipolar stepper motors, a controller board, parallel printer port cable, and computer power supply. This package represents a big portion of the overall cost of the machine, but the assembly time savings when using it are substantial. Those that want to build a machine and economize here can collect these parts and build their own controller board. Less powerful motors than those included in the kit could be used in this application. Also note that the computer power supply included with the kit only puts out 12V, which makes for less than speedy movement.
The machine also uses various readily available fittings and materials. The base is made of plastic laminate covered plywood. The support for the Z axis linear slide assembly is made from black pipe fittings.
I don't have formal plans available for this machine but I did put together a zipped package which includes CAD drawings in Autocad 2000 .dwg and .dxf file formats for the linear slides and for the whole machine, and a list of parts and suppliers. This is available for download below.
The downloadable CAD drawings are available for non-commercial use and are licensed under a Creative Commons License. Please read the license for restrictions on use of these drawings and documents.
The machine worked quite well for my purposes, but I have since replaced it with another desktop machine with a bit more precision.
There are also various sites on the Internet that feature plans or at least pictures of home built, shop built, and hobby CNC machines. A very nice discussion forum site is CNCZone.com.