Precision and strength of CNC machines is highly dependent upon the linear guides on which elements move. There are many many different ways CNC axis can move around. Part I is about general overview of existing linear guides and some alternatives. Part II will be about experimentation with T aluminum profile for use with small CNC machines.
Highly precise machines usually use linear motion guides like the one from HiWin. Such linear motion solutions are due to their complexity rather expensive. MGN12 model usually used in making high end Delta 3D printers can cost up from $40 for 40cm and up depending on length.
It is good for medium sized machines but when going for small to tiny machines the where size and weight is an issue they are not that practical.
Makerslide offers good rigidity for the price. It was aluminum profile of choice when building Shapeoko CNC machines.
Slightly beefed up version on makerslide is c-beam and V-slot from Openbuild. Also OpenRail is a good add on to standard aluminum profiles to make them into high end linear rails.
Also standard aluminum profiles 1010 or 2020 are popular in making 3D printers, especially Kossel mini.
Somewhere in between of HiWin and Makerslide are guide rods. They can be purchased from tiny to massive diameters and are used all over the place. They are not that expensive by them self but require additional items to make them fully useful.
Such guides offer a wide array of carriages that can ride on the rail.
Now, all of these are more or less standard ways of making CNC machines move with precision. While making smaller or huge CNC machines such linear guides are sometimes impractical and other times prohibitively expensive. Thus I needed an alternative solutions for linear guides. While experimenting it was found that almost anything with little imagination can be used as rails for CNC machines. Especially pipes, square, T and V aluminum profiles.
Note: I dont have images of all experiments so Im using internet images.
Some of examples are:
V profile, good for large scale 3D printers as they have no Z axis force put on them. Easy to acquire and fast to guild.
Simple setup for use with medium CNC machines. Has no limiters so prone to wobbling.
Rather robust setup for use with square or round pipes.
One of good examples of alternative guide lines is Mostly 3D printed CNC. Files and videos can be found of thingiverse.
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.
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.
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.
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.
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.
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.