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SECTION: 7.10 is for some "general" information about CAM, CNC, or NC type automated machine tools, see also the program files in the current distribution of my programs, the other parts of this HTML documentation, and the current On- Line version of this Web site for information more specifically about my programs. Any comparisons of my programs or methods to some others is only given as a vague generality of my opinion and is not intended as a recommendation or reference to any particular products, always make your own evaluations and comparisons before taking any action.
Of the three parts of the CAM machine tool, i.e. the computer and software, the motor drive electronics, and the mechanical components, the mechanical part is the one that will probably produce the most errors in the finished part. If the software does not have major bugs the commanded position will generally be correct most of the time. If the electronics have been set up properly the motors will generally be near their commanded position. From the end of the motor shaft to the tip of the tool is where you will probably have most of your un-resolvable problems.
In the computer software the position of the tool is represented in "perfect" numbers taken from your tool path. The motors can generally be made to correspond to the perfect numbers the software generates from the tool path file. But once you enter the real world you have to accept that nothing in the real world is going to be perfect. How close you can bring reality to what the computer "thinks" it is doing in your machine is a matter of how, and with what skill, you build your machine tool.
It is important that the machine be a stiff and rigid as possible because the computer and machine are generally "stupid" and usually do not know when the tool is being deflected off of the correct position by the tool getting dull or any other problems. Stone might be a good choice for a machine base since stone does not bend much, though it might soak up oil and other fluids and get soft. Cast iron is the normal choice for most machine tools, but you might be able to use several layers of wood glued together, e.g. about six inches thick, and sealed, or thick plastic for small machines that do not need close tolerances. Whatever you use it should be able to hold things in place under load, over time, and with changes in temperature and humidity.
Attached to the base are the machine rails used for the bottom axis. Since it is impossible to get two rails parallel, the bearings that slide on the rails should only fit tightly on one rail, the bearing for the second rail should allow side to side movement. If you use round rails, the bearing for the second rail should be rectangular, and not round. If you do not allow the second bearing to "float" side to side you will probably have problems with the carriage sticking at one end of the motion and the stepper motors losing position. Remember that the machine will not always be oiled, dirt will gum up the bearings, the weight of the carriage will bend the rails, and temperature will change the spacing between the rails. The bearings on the first round rail should be a Vee block on one side and an adjustable plate on the other to allow adjustment of the gap, split round bearings do not necessarily move in a straight line when adjusted. Although many machines use four bearings on the carriage, you should probably use only three, two adjustable Vee blocks, and one adjustable rectangle to avoid binding due to one of the rails being higher at one end or the other.
Dovetail ways, cut into metal, might be a good alternative to rails, but cannot general be adjusted since both sides are cut into one casting. The carriage for dovetail ways can be adjusted, but that does not keep the carriage from binding at one end of the ways if the ways are uneven.
To move the carriage or table along the rails you might use:
A lead screw and nut. The lead screw should be fit with a thrust bearing at one end since stepper motors generally do not have bearings that are designed for thrust, and sometimes you can move the motor shaft in and out just by pressing hard on the end of the motors shaft. Use a flexible shaft coupling between the lead screw and the motor shaft.
Angled rollers pressing on a rotating rod. This is an alternative to a fast pitch lead screw, but when used "open loop" the carriage may need to be re-homed against a mechanical stop to be sure that the rollers have not slipped on the rod since there are no grooves to keep the rollers from creeping along the rod.
A cable and bobbins. Simple and cheep, but getting the bobbin to give an exact amount of movement may be difficult since the distance moved depends on the radius of the cable, and how the cable winds up on the bobbin. The non-driven bobbin would generally have a spring or other adjustment to keep the cable taught.
A timing belt and timing belt pulleys. Similar to a cable drive, except the bumps on the timing belt and pulley keep it from creeping off position very much.
A chain and sprockets. Similar to a timing belt, except that the radius of the sprocket might change in the arc of one link's motion, due to the straight line of the link, giving an oscillation to the linear motion.
Toothed rack and pinion gear. Might be good for rapid motion, but the length of the teeth on the pinion may make the motion oscillate depending on how the shape of the teeth were cut. To take up backlash two pinions can run on one rack. As the rack wears it will become difficult to get absolute position because the teeth will be narrower at one point than another.
Linear stepper or servo motor. These are "flat" motors and directly slide under the magnetic attraction or repulsion. Might be good for rapid motion, but may need an encoder to get fine position stiffness.
Hydraulic or pneumatic pistons. Would generally require some kind of feedback encoder to make sure the carriage is where it is commanded to be.
Piezoelectric crystals. An electronic direct motion by expansion and contraction, but moves very short distances. Might be good for making very small things, i.e. micro-machining.
Voice coil drives. Like a linear stepper, but has only one pole position, rapid motion over short distances. Like pneumatic motion may need an encoder and servo to get better stiffness. A related type of application is using a galvanometer, i.e. an electro-magnetic pivoting mirror, used to deflect a laser beam for cutting by moving the beam of light around over the surface of the part, rather than moving the part to be cut under a fixed beam.
Muscle wires, or Bi-metallic strips can make slow movements by using a change in the voltage or current flowing through them according to the commanded position. Thermal expansion and contraction from electric current flowing through a rod would be very slow, but might be useful in some odd environments, and might be used for micro positioning of a diamond cutter in optical work.
If you are looking to purchase a machine tool for conversion to computer control use a dial gauge from the carriage to the tool holder and safely apply pressure on both parts of the machine in all directions to see how much you can get the needle in the dial gauge to move. If you get any movement over 0.001 inch or 0.025 mm you might want to look for a stiffer machine, because when you operate a machine manually you can compensate and constantly make adjustments, but under computer control you want to have the whole job proceed without having to make adjustments.
If you are designing a machine from scratch you may want to make your CAM machine much heavier and stiffer than you might think of making the machine for manual operation. It may be possible under some circumstances with some very careful designing of your tool path to compensate for backlash and flexing in your machine by taking light cuts at slow feed rates for the final few cuts. In a machine with backlash you will probably want to make all the roughing cuts back from the final line by twice the amount of the backlash value, e.g. if your machine has 0.015 inch of backlash you would make the roughing cuts 0.030 inch back from the final line, and work forward to the final line by several small cuts, perhaps 0.002 inch on each pass, and 0.0005 inch on the last three cuts. Some CAM software, like mine, has compensation for backlash, but this will not stop the tool from dragging the work-piece around during the roughing cuts. On fine cuts the mass and friction of the carriage and work-piece may hold the work-piece at the commanded position, so you may get better final dimensions if you do not support the work-piece on ball bearings since some friction is needed to keep the work-piece still while the final cuts are made.
For applications other than metal working you may be able to get usable parts from a flimsier machine, just keep in mind how the mechanical aspects of your machine will effect the surface finish and other cosmetic aspects of your parts in addition to the needed mechanical dimensions of the parts.
If the software your CAM machine is using lets you enter a value for the backlash you might use a dial gauge to measure the amount of backlash for each of the axis.
In my CAM programs you can enter the value of the backlash for each of the machine's axis and the software will automatically try to take up be backlash before reversing direction. In practice there needs to be some friction in the machine to hold the work-piece still while the cutter is operating, or else the work-piece will slide around within the backlash distance. Because the work-piece will be pulled around a bit during the roughing cuts, you should make the roughing cuts about twice the amount of backlash back from the finish cut line, and work up to the finish cut line in very small cuts after the roughing cuts have all been completed.
Timing belts and pulleys generally make a better way of coupling the stepper or servo motors to the machine's lead screws than using gears because of the large amount of vibration that stepper and servo motors make when operating.
The grooves in the timing belt and timing pulleys keep the two shafts from getting out of synchronization, round or vee belts are generally not adequate for motion control applications.
For applications where rapid feed is desirable the timing belt can be used directly to move the tool or work-piece without using a lead screw.
When you need to use a reduction ratio greater than about 5:1 you may be able to use two timing belts, as is shown in this photo. Using a greater reduction reduces the linear step size for the machine's motion, and increases the torque available to move the machine's parts. In the case of a machine such as one using double reduction as shown in the photo very slow feed rates of one inch per minute or less might be used much of the time, so any loss of speed from the reduction might not be a problem.
When you want to "direct drive" a lead screw from a stepper or servo motor you should use a flexible coupling. The lead screw should have its own thrust bearing generally and so the thrust load would be born on the lead screw's thrust bearing and not the bearings in the stepper motor, which are sometimes allowed to float a little in and out because the stepper motor gets hot and otherwise the bearings might bind do to thermal expansion.
The flexible coupling should not flex in rotation more than about one forth of a step, or you will get some extra backlash from the flexible coupling. Metal billows couplings might work better in some critical applications.
When you need rapid feed rates a rack and pinion can be used as an alternative to a timing belt drive or using a lead screw. The distance moved is calculated from the pitch diameter of the pinion gear. The size of the teeth used on the rack would depend on the load that would need to be moved, and you would usually use the smallest tooth size that would carry the load in order to have the motion be as linear as possible.
Using two narrow pinions on the rack might let you rotate one of the pinions relative to the other to take up some of the backlash. You could also use one wide pinion running on two slightly shifted racks to take up some of the backlash.
This photo shows a portion of a 1 tpi multi-start fast pitch lead screw, i.e. a lead screw that moves the work-piece relative to the tool one inch each time it rotates once. Such a fast pitch lead screw would be used when you want to get high feed rates, for example when used with 6:1 reduction to a servo motor turning at 1800 RPM you might get maximum feed rates of up to 5 inches per second.
This photo shows a section of a 10 tpi Acme thread lead screw, typical of those used in small machine tools. When operated by direct drive from a 200 step per revolution stepper motor you would get 2000 steps per inch, i.e. 0.0005 inches per step. With a stepper motor having a maximum useful speed of 120 RPM you would get a maximum feed rate of about 12 inches per minute, or 0.2 inches per second.
Here you see a close up view of the shape of the thread of the Acme 10 tpi lead screw. Notice that the top and the bottom of the thread are flat, and the sides are not square, but slope a little.
If you want to increase the resolution of your machine you may do better to increase the number of threads per inch on the lead screws, rather than use a greater reduction of the timing belt on the drive end on the ends of the lead screws, the reason the increase in the lead screw pitch might be better is that the timing belt will give the rotation of lead screw about 0.5 to 2 degrees of rotational slack, which will convert to less linear movement of the work-piece relative to the tool when a lead screw of higher pitch is used. A 50 tpi lead screw operated with a 5:1 reduction from a 400 half step per revolution stepper motor might give a step size close to about 0.00001 inch per step.
