# CNC: Linear motion without timing belt or lead screw

Is it possible to build a CNC whose Linear motion system does not contain any timing belt(pulley) or lead screw(threaded rod). I was wondering whether I could directly control the Linear motion by securing wheels of a slider onto aluminum rails & directly connecting the wheels to a stepper motor.

The main objective of this question is to find the cheapest method for controlled Linear motion.

• If you google around for "DIY timelapse slider" or similar you will find some DIY mechanisms for running on straight rails. Wheels/rollers as you suggest, also rack & pinion (which might be more expensive). It depends on how much accuracy you need... Alternatively a simple roller scheme and a linear encoder - might be salvageable from an old inkjet printer and they are fairly accurate. More complex control is needed though. – Andy Nov 17 '16 at 13:23
• This question really needs more information about the precision needed. Several of us have suggested solutions that will work, depending on the precision needed and your experience in building things. You've said in a comment "high" resolution is not needed. Do you need to drill holes in PCBs (an example of medium precision) or a machine to draw things on paper with a pen (lower precision) etc.? – Andy Nov 18 '16 at 10:25
• Are you seriously trying to avoid the few bucks that a threaded rod costs in a hardware store? – Bending Unit 22 Nov 18 '16 at 11:45
• The CNC would be 100m×50m. Yes metres. So every cent saved per metre would mean a lot. – Jonathan Pereira Nov 18 '16 at 12:28
• @Jonathan Pereira you should state your requirements clearly in your question. It's pointless to discuss any design without knowing the constraints caused by the requirements. So please edit your question to include all relevant information. For example, it would be a lot cheaper to have no CNC at all and do everything by hand. But you likely have some requirements for accuracy and degree of automation that would obviously not allow this. – Bending Unit 22 Nov 18 '16 at 20:30

I would say the answer to the first question is "Yes and No".

Yes, you could hook a stepper motor directly to drive wheels on some sort of shuttle. CNC controllers normally have a setting for inches/step or some equivalent. If you set that correctly, the machine should drive the motors "correctly".

But No, this not likely to give you the output you want. Stepper-based CNC machines work on the idea that the machine never "loses steps": that for every electrical pulse, the motor moves a fixed amount; and that N forward steps followed by N backwards steps brings you back to exactly where you started. If you are driving wheels which rely on friction for motion, those two assumptions are almost certain to fail, and your output path will be stretched or compressed in strange ways.

• here is a video which disproves your answer in the case where there is no load on the moving cart: youtube.com/watch?v=JzR6FV-sdH4 – 50k4 Nov 22 '16 at 21:02

CNC controllers, in most cases, control rotary motion and the model of how this rotary motion is tranformed, by the mechanism attached to the motor, to a translational motion is implemented in the controller.

You can use any method of transforming rotary motion to a linear motion as long as the model for it is pre-implemented in the CNC controller or you can implement it to the CNC. Moreover, it does not even has to be a decoupled translational motion. E.g. LinuxCNC can controll multiple axes to achive 1 decoupled linear motion. On a parellel mechanism, moving all axes results in a Z translation. Video of this here.

The only limit to what mechanism can you put on the end of the motor of a CNC is what are you able to model and integrate to the CNC controller. If you do not have the possibility to implement a model, then the question becomes what pre-implemented models are there and how can you adjust their parameters to make them refelct your mechanism.

Needles to say the accuracy of the model will affect the accuracy of the CNC.

• That doesn't answer my question – Jonathan Pereira Nov 17 '16 at 11:52
• Well..in a way it does. You question was "I was wondering whether I could directly control the Linear motion by securing wheels of a slider onto aluminum rails & directly connecting the wheels to a stepper motor." Well I have no idea how well can you program, what is your previous exerience with CNCs and modelling mechanisms and you did not giv any information about the controller you are plenning to use. You also do not state any precision requirements of the final product. So.. can it be done? Certailny! can you do it? I have no idea! Will it have low precision? yes! – 50k4 Nov 17 '16 at 11:58

If you want to build something that more or less resembles CNC machine, and use a pen as a tool and draw some pictures, then you will probably be fine (as long as you can do programming and have a lot of patience to do calibration).

Why is this not a common solution?

Usually you want CNC to be precise and to have enough power to move a tool at reasonable speed. Cheap rubber wheels are... well cheap. Nobody cares if their diameter is 10mm or maybe 10.3. Or 9.7. What's more you can buy several wheels and each one of them will have different diameter. This will lead to problems when calibrating the machine.

Additionally, threaded rods and belts can transfer more torque without slipping, so higher dynamics will be achievable.

• I am not looking for a very high resolution CNC. Considering I do have wheels of equal diameter, what do you suggest is the cheapest method for controlled Linear motion. – Jonathan Pereira Nov 17 '16 at 13:41
• What do you mean? You spin the motor, and the distance travelled is equal to $n\pi d$ where $n$ is number of rotations. – mactro Nov 18 '16 at 8:47

What you are looking for is called a linear potentiometer. It is the same tech that is in your car known as a fuel sending unit, volume control knobs, etc. It is most basically a voltage divider whose resistance is increased or decreased and can be measured. Aditional details can be found on the wiki at en.wikipedia.org/wiki/Potentiometer

• This is not what I am looking for – Jonathan Pereira Nov 18 '16 at 8:48
• Then I can only asume that you are asking about how to mechanically connect stepper motors to the linear motions of the gantry and other parts – cecil merrel aka bringrainfire Nov 18 '16 at 8:52
• Yes but without using a timing belt or threaded rod. Instead directly connected the shaft of the motor to the wheels on the gantry. Will it suffice/drawbacks? – Jonathan Pereira Nov 18 '16 at 8:54
• No it will not work. The reason why is that it is basically like driving a car on ice – cecil merrel aka bringrainfire Nov 18 '16 at 8:56
• If it suffices or not can only be decided if the requirements are known. – 50k4 Nov 18 '16 at 9:43

Sort of. This is in the possible but not probable category of things. All you need is a system with very little play and active position feedback and you can get around the usual requirement of stepper motors and lead screws. What is wrong with belts though? They are really not that expensive for what they do and how well they do it compared to alternatives. At time of writing AndyMark carries timing belts for \$14. Given, however, that you want the amazingly large size of 100m×50m (!) might I suggest steel cable and a roller system on 80/20. Make a free rolling system and then use steel cables to move the positions on each of the axes. Such a large system will have both hysteresis and nonlinearity so you're going to want active position feedback and PID control scheme to make sure the machine head actually ends up where it needs to be for each line of the gcode. Can you not partition that space down to smaller quadrants though? 100mx50m is enormously demanding and de facto firmly out of the budget CNC category of projects.

• Steel cable. How will this replace timing belt and gears? – Jonathan Pereira Nov 18 '16 at 17:05
• Using rollers and pulleys you can position something precisely with cable and rails. See technocraticanarchist.blogspot.com/2011/05/… The cable serves the same function as belts but it can be cheaper when the actuation length is long. The system will be non linear though so active position feedback is a must. – amoose136 Nov 18 '16 at 18:02
• FYI I have built a lego CNC machine that used this principle but it was with nylon thread instead of steel cable as it is Lego. – amoose136 Nov 18 '16 at 18:05

Interesting question. Some degree of precision is required. There are two ways to do this:

a) use an encoder associated with a wheel which reports the rotary position. That value is then related to the wheel circumference to obtain the distance travelled. Perhaps a wifi transmitter on each carriage would be more practical than wiring.

b) use an absolute positioning measurement system. Your dimensions of 100m x 50m are very large and clearly any traditional belt drive/screw drive is impractical. As an example of absolute positioning, a laser rangefinder (+/- 1-2mm) at each corner would locate the gantry. How a commercial product would be interfaced with the PID/control system is a homework exercise.