Tag Archives: movement

CNC movement designs

In 3 years of building CNC machines I’ve built almost all of popular CNC  movement styles; from  Delta, CoreXY, HBot and classic 3 axis Cartesian machines.  Each of mentioned movement systems has its advantages and disadvantages. But the most interesting is Delta.

Delta configuration is by far the most attractive to watch move. It’s almost, no, it is mesmerizing. When my Kossel is working I can stare at it for hours.  Several different configuration of motors exist but the movement style is the same. DeltaRamki

Advantages of Delta setup is that it can move rapidly, much more than any other configuration (except SCARA?). Its disadvantages are small operating area and as you make it bigger (except height) it gets more and more complex.

All styles listed here at the end of the day work in Cartesian coordinate system but only one is thought about when that name is used without any additional descriptions or attributes.

CNC machines are traditionally 3 axis Cartesian machines. It’s rather simple and intuitive system where each of the axis has its own motor. Such arrangement has little to non disadvantages if you keep it small and tight. As soon as you go to 1m and above in any dimension, things start to fall apart. The stiffness of machine needs to be increased which introduces additional cost, weight and often additional motors as 1m length of any material will tend to bow if driven from only one side. The opposite side will tend to just lag behind as motion vector is transferred to it. If machine is not stiff enough the lag can be such that the machine is unusable.

sb344-90s-2200g-x-sb233-90-820gl

In addition to those issues, with larger machines there is an issue that motors travel together with axis. It’s not really an issue as it increases stiffness but complicates cable routing and adds weight to axis limiting the acceleration and speed at which it can move.

There are workaround for those issues but at the end, they are workaround solutions that do nothing to solve underlying issues and can introduce a whole set of other issues. One example is removing the need for Y axis to move and instead moving the work bed. Work bed is usually a flat stiff sheet that can be moved using one motor compared to two for Y gantry. But then, you are moving your work piece and adding stress to it. If the workpiece is brittle Y axis can not move or accelerate fast. The same is true if work piece is heavy.

Still, it is the most popular and useful movement system.

With popularization of alternative uses of CNC machines for novel uses such as 3D printers and laser cutters new movement systems and arrangements are being developed. Most of them are not intended to be used with large machines or machines whose axis are under stress like when cutting material.

Take for example Hbot arrangement of belts and pulleys.

Hbot is a very elegant solution for a lot of issues. Motors are stationary and due to pulley system there is a lot of torque. But, such arrangement needs to be built under tight tolerances. If belts become loose accuracy will be decreased drastically  and immediately. With that in mind it is still the most elegant and practical solution for X and Y axis for a CNC machine that has little to no stress on gantry, like 3D printers or Laser cutters. Additional information about this can be found on Double Jump webpage and FABtotum.

hbot

Layout-XY-2

To solve issues of slack belts CoreXY was developed and is my current favorite when building smaller machines. It eliminates the issue with slack belts but to do so adds an element that is frowned upon in engineering.

corexy

And that is crossed belts that rub against each other. That is a huge point of failure and a real problem but doesn’t need to be a dealbreaker. Several options to solve this exist.

One is to just leave it as it is and put up a finger to classical engineering practices. Just replace the belt if and when it gets worn out. Depending on use of machine this could take years. My machines are now running for a year without any significant wear on the belts. The wear is minimal as not a lot of stress is put on them. They move very light gantrys with 3D printer heads or laser assemblies. If it were under big loads like big spindle then it would be another matter.

Then there is introduction of separator in form of additional roller or rollers that are placed where belts meet. Roller/s are made of non abrasive material like delrin, teflon, nylon or polished steel and create a smooth surface against which belts can move without wearing and tearing. In a pinch even ABS can be used as separator.

Third option is the most complicated one and that is to place each motor and belt on different planes. One motor and belt is just below the other for the height of the belt itself. This eliminates the crossing of belts but unnecessarily increases complexity of the build.

c0553a_3b43456e7221473db67ce4451287a50b

No separator and option with delrin separator is usually enough for normal and low speed operations.

Styles mentioned here are just the most practical, but still, just a slice of possible movement styles. There are a lot of different ones but the rest are more or less gimmicky.

Some of more alternative movement styles:

Facebooktwittergoogle_plusredditpinterestlinkedinmail