Monday, January 28, 2013

Linear drive: Belt/Pulley or Leadscrews

There are two common low-cost ways to drive linear motion: Belt/Pulley or Leadscrews. It seems obvious that belt/pulley can run faster, while leadscrews can take a larger load. But if the machine needs to do any 3D printing, the belt and pulley is the only choice.

The reason is simple: leadscrews do not go fast enough. Take, for example, a common cheap 1/4" x 20 threaded rod on a common NEMA 17 stepper motor. The motor turns at around 300 rpm. At that rate, the drive is moving 15 inches per minute, or 6 mm/s. That's painfully slow, as a decent 3D printers now-a-day can run about ten times as fast. Of course leadscrews can go faster with a multi-start ACME rod. But that drives the cost up quite a bit, and is no longer options in the low cost world.

As a result, all of the 3D printer or 3D printer / CNC combo design I see uses belt/pulley to drive the X/Y motion.


Does it make sense to combine 3D printer and CNC?

The question is: Does it make sense to build a combined 3D printer and CNC? It seems that despite the similarities in 3D printer and CNC, there are a few important differences.

A CNC needs to withhold a lot of lateral force from the mill head, so the mechanical structure needs to be build rigid and solid. That means any CNC machines are probably a bit heavy. On the other hand, a 3D printer only needs to move a relatively light print head, and therefore does not need to have that rigidity and mass of a CNC. A 3D printer needs to move fast, so one can use a thinner slice to achieve a better finish. That means a 3D printer probably want to be lighter.

A typical precision on a hobby CNC frame is probably need to be in the order of 1/1000th of a inch, or 1/100th of a millimeter. 3D printers, on the other hand, have nozzle size of 0.35mm or 0.5mm. That means the precision of the mechanical frame need not to be better than 1/10th of a millimeter. That is an order of magnitude different from the CNC setups. The lesser requirement on the mechanical precision means that the frames can be built lighter, and therefore can be moved faster (which is what the 3D printers need).

It seems that it makes little sense to combine these two. However, most of the sub-$1000 3D printers in the market has terrible precision. A more rigid (but not overkill) frame would definitely benefit the printing quality. Also, for a lot of hobby CNC works (especially on wood), a precision of 1/10th of a millimeter would be sufficient. So if such a compromise can be achieved in design at the right spot, maybe there is a room for such a combined machine. After all, it offers the benefit of saving the space in a garage workshop and space is always at a premium there.

After some research, I found this project called EasyMaker on the Internet. It seems to be a pretty interesting project that combines CNC and 3D printer.


Thursday, January 24, 2013

3D Printer and CNC

3D printer is all the rage recently. I got interested and decided to build one for my own amusement. At the same time, I have always been fascinated by CNC machines.

There are a lot of similarities between 3D printer and CNC. For example, they both move in the X/Y/Z Cartesian space. They all result in a real object (one by cutting, and one by building). Because I do not have a lot of space in my garage, I wanted to build one machine that may be easily converted between a 3D printer and a CNC machine.

In order to build such a machine, it is helpful to look at what makes them different. There are indeed a few important differences:

  • CNC needs a lot of sideway torque, while for 3D printer there is little (keep everything close to the cutting plane is part of the reason that most CNCs have limit Z clearance and Z range) ;
  • 3D printer needs more Z movement and Z clearance, while for CNC there is little (because for CNC there are only so much cut needed in Z anyway);
  • 3D printer head in general moves faster than CNC head (cutting versus depositing).
It is an easier to approach this from some existing designed or plans. Because of the above differences, I felt it is probably easier to look at the CNC kits first. After browsing the Internet, I found a rather comprehensive list of low cost DIY CNC kits.

While many of these may be converted to 3D printer, one of them suits my need particularly well. It is the ShapeOko design. What I particularly like about it is that fact that it has a combined X/Y/Z head. A lot of other low cost designs has a X/Z head with a Y work bed (in other word, the head moves in X and Z direction, while the bed moves in the Y direction). For a plane CNC operation, both has its advantages. But for converting to 3D printer, the X/Y/Z head has a distinct advantage. One of the main issue to consider for the convertible machine is that the 3D printer needs a larger Z range. With the combined X/Y/Z head (particular with the ShapeOko design where there is no built-in work bed), the work bed can be lowered to allow adding more Z range to the working area.

The fact that ShapeOko design has a lowest kit price does not hurt also. I do feel that the design is a bit flimsy (to achieve the amazing low cost), but I can strengthen that later. I also have concerns about the belt linear driving mechanism, and I like the rods better. But in their video, it seems that it can handle most light duty cutting job fine, so I decided to give it a try.