What makes a good CNC router? The following is my opinion yours may vary.
I post this with some reservation as I don’t consider myself an expert but merely a user of CNC equipment and there are many folks out there that have a better understanding of CNC equipment. I am posting this for new user’s to be able to compare different machines and understand what you are getting for your money. This can also give you some talking points when talking to a manufacturer about their machines. Feel free to post any misinformation you feel I have made.
Let’s start with construction of the frame. The easiest way to make a frame is with pre-fabricated aluminum extrusions. All you need is a saw to cut the extrusions to length they make fittings to create most any angle you need. If you do not need to take heavy cuts this may make a good inexpensive solution.
Weldment frames are a step ahead of the extrusion route they create a more rigid frame with less flex. The next step up found in very expensive machines would be heavy cast iron bases thick machines in the $150K and up class way above what the average small business and hobby guy is going to have. These machines usually weigh around 12000#+. For this discussion I am not going to talk any more about the pure industrial machines.
Let’s talk about guides and rails this is the next place some manufacturers cheap out. My first CNC used a hardened rail with a guide bearing this is OK but not ideal. I have seen some cheap stuff that even use some sort of skateboard type wheel running on an aluminum structure. Linear guide rails are the best alternative in this level machine the give a higher precision fit.
Next how is the machine driven stepper motors or servos? There are many different strength steppers with different resolution you need to compare one machine to the next. Most inexpensive machines will run some level of stepper motor. What is the difference between steppers and servo motors? I will not try and explain the very construction of each but the basic difference in how they operate. In general a stepper motor is an open loop system now there may be some exceptions to this, open loop means a signal is sent to the driven motor to make a move in a direction and in so many steps. The better the stepper motor the more steps per revolution it makes giving it a better resolution. This is something you can compare the resolution in steps per revolution usually given in degrees per step. To recap the control sends out a signal for the stepper motor to take X amount of steps in one of 2 directions which will equal some distance you program needed to move.
Servo motors are usually closed loop but not necessarily. Closed loop means when a signal is sent to turn the servo a certain direction it also gets a signal back saying that it actually made that move. This does not happen in an open loop system the control assumes the stepper actually makes the move this where the term skipped steps comes from. Skipped steps or missed steps usually ruin whatever you are cutting. I have experienced this with my first machine my current machine is servo driven. Servo driven machines are going to cost much more than stepper driven machines. This is just a basic difference there is much more to this equation but I am trying to stick with basics speed is also picked up with the use of servo motors. I don’t believe you will find and entry level machines running servos. I also believe they make some closed loop stepper motors but I am not sure about this.
Now that we have the motors to move the machine and some sort of guide rails we need a drive system either gears in a rack and pinion system or acme screws or even better ball screws. This is where backlash comes into play and limits the ability of a machine to make high precision parts this is usually stated as repeatable resolution. What does that mean simply put how accurate is the machine is over a specified distance it is repeatable to some measurement usually stated in a multiple in a thousandths of an inch or MM.
A rack and pinion machine will have some level of backlash that will increase over age in fact everything starts to wear as soon as you start running it. Not all rack and pinion machines are created equal helical rack and pinion is better than a straight cut rack and pinion.
A acme screw is a heavy duty lead screw but they tend to have a fair amount of backlash sometimes they have a half nut that can be adjusted to try and remove most of the backlash as they wear especially in an area where the machines runs the most your backlash will increase because if you adjust it up close it will be too tight in areas the machine doesn’t run enough. Most manual machines use this type drive. They can make an acetal half nut that has very little backlash but it will never be as good as a ball screw and in my opinion due to wear not as good as a good helical rack and pinion. The acme screw is probably the cheapest route but in my opinion the least desirable of the 3 methods I am comparing.
Ball screw driven machines in my opinion are superior to the others but many manufacturers use this method and the level of quality of the ball screws comes into play also. A cheap machine with an extrusion frame and ball screws is not going to outperform a welded frame with very good helical cut rack and pinion and better quality stepper or servo motors.
Next let’s talk about control systems. I am not a fan of open source control systems but they are a way of keeping a machine inexpensive. I prefer a control that has a dedicated computer that is furnished with the machine and uses a separate board to control the machine rather than being a strictly windows machine running off the mother board. This is the area I have the least experience with so I am going to leave it short. The best control would be a dedicated professional control but no machine in the class being discussed would have that control would cost more than the rest of the machine.
Cutting with a hand router compared to a spindle. A spindle will have much less run out than a hand router and will have more of the rated HP throughout a larger range of RPM. The spindle is also very quiet compared to a hand router. If you live in town will your neighbors tolerate a noisy machine running for hours?
In finishing this I will say all components play a part how many of the better components that are discussed does the machine you are looking at have. Almost any CNC is better than none. The machine with no level of tech support is almost worthless in my opinion unless you are an expert you will be needing help a good user forum for the machine you are buying is great and free tech support a must. Large industrial machine manufacturers often charge for tech support by the minute so being an amateur just learning having free resources are a must.
I post this with some reservation as I don’t consider myself an expert but merely a user of CNC equipment and there are many folks out there that have a better understanding of CNC equipment. I am posting this for new user’s to be able to compare different machines and understand what you are getting for your money. This can also give you some talking points when talking to a manufacturer about their machines. Feel free to post any misinformation you feel I have made.
Let’s start with construction of the frame. The easiest way to make a frame is with pre-fabricated aluminum extrusions. All you need is a saw to cut the extrusions to length they make fittings to create most any angle you need. If you do not need to take heavy cuts this may make a good inexpensive solution.
Weldment frames are a step ahead of the extrusion route they create a more rigid frame with less flex. The next step up found in very expensive machines would be heavy cast iron bases thick machines in the $150K and up class way above what the average small business and hobby guy is going to have. These machines usually weigh around 12000#+. For this discussion I am not going to talk any more about the pure industrial machines.
Let’s talk about guides and rails this is the next place some manufacturers cheap out. My first CNC used a hardened rail with a guide bearing this is OK but not ideal. I have seen some cheap stuff that even use some sort of skateboard type wheel running on an aluminum structure. Linear guide rails are the best alternative in this level machine the give a higher precision fit.
Next how is the machine driven stepper motors or servos? There are many different strength steppers with different resolution you need to compare one machine to the next. Most inexpensive machines will run some level of stepper motor. What is the difference between steppers and servo motors? I will not try and explain the very construction of each but the basic difference in how they operate. In general a stepper motor is an open loop system now there may be some exceptions to this, open loop means a signal is sent to the driven motor to make a move in a direction and in so many steps. The better the stepper motor the more steps per revolution it makes giving it a better resolution. This is something you can compare the resolution in steps per revolution usually given in degrees per step. To recap the control sends out a signal for the stepper motor to take X amount of steps in one of 2 directions which will equal some distance you program needed to move.
Servo motors are usually closed loop but not necessarily. Closed loop means when a signal is sent to turn the servo a certain direction it also gets a signal back saying that it actually made that move. This does not happen in an open loop system the control assumes the stepper actually makes the move this where the term skipped steps comes from. Skipped steps or missed steps usually ruin whatever you are cutting. I have experienced this with my first machine my current machine is servo driven. Servo driven machines are going to cost much more than stepper driven machines. This is just a basic difference there is much more to this equation but I am trying to stick with basics speed is also picked up with the use of servo motors. I don’t believe you will find and entry level machines running servos. I also believe they make some closed loop stepper motors but I am not sure about this.
Now that we have the motors to move the machine and some sort of guide rails we need a drive system either gears in a rack and pinion system or acme screws or even better ball screws. This is where backlash comes into play and limits the ability of a machine to make high precision parts this is usually stated as repeatable resolution. What does that mean simply put how accurate is the machine is over a specified distance it is repeatable to some measurement usually stated in a multiple in a thousandths of an inch or MM.
A rack and pinion machine will have some level of backlash that will increase over age in fact everything starts to wear as soon as you start running it. Not all rack and pinion machines are created equal helical rack and pinion is better than a straight cut rack and pinion.
A acme screw is a heavy duty lead screw but they tend to have a fair amount of backlash sometimes they have a half nut that can be adjusted to try and remove most of the backlash as they wear especially in an area where the machines runs the most your backlash will increase because if you adjust it up close it will be too tight in areas the machine doesn’t run enough. Most manual machines use this type drive. They can make an acetal half nut that has very little backlash but it will never be as good as a ball screw and in my opinion due to wear not as good as a good helical rack and pinion. The acme screw is probably the cheapest route but in my opinion the least desirable of the 3 methods I am comparing.
Ball screw driven machines in my opinion are superior to the others but many manufacturers use this method and the level of quality of the ball screws comes into play also. A cheap machine with an extrusion frame and ball screws is not going to outperform a welded frame with very good helical cut rack and pinion and better quality stepper or servo motors.
Next let’s talk about control systems. I am not a fan of open source control systems but they are a way of keeping a machine inexpensive. I prefer a control that has a dedicated computer that is furnished with the machine and uses a separate board to control the machine rather than being a strictly windows machine running off the mother board. This is the area I have the least experience with so I am going to leave it short. The best control would be a dedicated professional control but no machine in the class being discussed would have that control would cost more than the rest of the machine.
Cutting with a hand router compared to a spindle. A spindle will have much less run out than a hand router and will have more of the rated HP throughout a larger range of RPM. The spindle is also very quiet compared to a hand router. If you live in town will your neighbors tolerate a noisy machine running for hours?
In finishing this I will say all components play a part how many of the better components that are discussed does the machine you are looking at have. Almost any CNC is better than none. The machine with no level of tech support is almost worthless in my opinion unless you are an expert you will be needing help a good user forum for the machine you are buying is great and free tech support a must. Large industrial machine manufacturers often charge for tech support by the minute so being an amateur just learning having free resources are a must.
