As you probably know, I decided on a ShopBot PRTalpha96 for my first CNC Router. This machine has performed well for me and paid for itself several times over. It was a good investment to go with ShopBot and their PRTalpha96 tool. You might recall I purchased this tool with a Porter Cable router and later upgraded to a 4hp HSD Spindle as well as upgrading the X,Y and Z axis to the current production PRS 7.2:1 motors. All of this increased the performance of my tool and would allow me to continue to use it well into the future. So you ask why am I selling this tool and buying another one? My shop is small at 1176 sq. ft. and I would like to make better use of my space and increase production at the same time. After giving all of this a lot of thought I have decided to make the following changes: sell the current 4’X 8’ tool and replace it with a 4’X 4’ machine leaving room for a second 4’X 4’ tool, move out some equipment that is nice to have, but not used on a regular basis, and become more focused on products that I manufacture. Look for a future article on how I found my niche and how it is working out for me.
Now that I had a plan, the next step was deciding on which CNC Router manufacturer I will buy my machine(s) from…. This proved to be a much longer process than I had expected it to be with many twists and turns along the way. I started out by looking at the ShopBot PRSalpha tool and visiting shops with this tool in use. I figured that I was already familiar with ShopBot and I would have the shortest learning curve staying with their products. But I kept thinking I can get a better machine for the same, or almost the same, money so I kept looking. I have visited machines that were within a reasonable drive from me while taking notes and watching the machines cut. Along the way I met many nice folks running these machines. I was impressed by what I saw, both software and hardware wise. Now, to get more to the points of the different machines I looked at, and why I like or didn’t like each one. I am not going to name any manufacturers or do your homework for you. You have to evaluate each of your own needs as I did. I hope this will give you some ideas as to what to look for or maybe even a jump start on buying your first tool.
My first point to consider was the control system and software. Since I know the ShopBot control system, it made comparing easy for me. I just asked to have a part file I generated in Aspire2 and toolpathed, run on each of the machines that I looked at. There are machines out there that cut faster than my current ShopBot does. Remember I have toolpathed these files all using the same speed for X, Y and Z so I am seeing how the ramping is done and how smooth the machine runs. The easy operation of every machine I looked at made them all about the same in operation. Each works different, but all do the same things. So I would be happy with any of the control systems I looked at.
Second, I looked at the build of the machine. I want to get away from a bolt-together machine and go with a more rigid welded steel frame machine. There was only one other machine I looked at that was a bolt-together, the rest were welded and built quite nice! The welded frame machines will make it harder for me to get it unloaded since I do not have a fork lift or crane on site, but I continue to feel that I needed the welded frame. I also felt I needed the mass and weight of the welded frame to get the cut quality I wanted.
While I am looking at the build quality of the machine I am also looking at the motion system of the machine. What method is used to make the gantry move as well as what is conveying the weight of the gantry. As we all know, ShopBot has a V roller and rail for all axis movement. This is basic and simplistic. It requires another V roller or some method to keep the roller and rail together. The other method I am evaluating is the linear rail. Linear rails are very precision rails with a block that moves along the rail and cannot be moved in any other direction except back and forth in the direction it was intended to move in. With precision comes expense, but I continued to feel this was needed. I have even looked at adding linear rails to my current machine. All of the other machines I looked at have linear Rails. How the gantry is attached does vary from machine to machine.
I looked at servo motors both AC and DC as well as stepper motors. Servo motors have a constant torque and can run faster than the stepper, but the DC ones need their brushes replaced from time to time. Steppers do not require any maintenance and have more torque at the lower end of their speed range. Both work good if properly matched to the application. I have always been told that the servo has more resolution than the stepper. Well, that is not exactly correct! A servo has to have an encoder for feedback. The resolution of the encoder determines the resolution of the motor. So if your servo has an encoder with less resolution than a stepper, the servo will not have the best resolution. Beyond the native resolution of either motor, you have to consider the gearing that is done after the motor. The bottom line is, how accurately can you move the axis and can you repeat the movements, all at a speed that is appropriate to the application. I found that all of the machines I looked at could do an excellent job of this.
How the servo or stepper is attached to the axis was important to me and how many motors were used to move an axis also figured in. Some machines use a single motor to move the gantry. Some machines use a single ball screw under the table and run the gantry on the ends with linear rails. I found that this is nice, but allows the ends of the gantry to pivot a little. The point holding the gantry is in the middle, on a 4’ wide gantry, you have 2’ of a lever when the spindle is on the outer side of the table. The amount of “rack” that the gantry could be put in depended on the spacing between the bearings that run along the length of the table. Some machines were quite tight while others were quite loose. I also looked at machines that put motors on each end of the gantry so there could be no movement. This configuration was the tighter of the two in my checking.
Now this article was not intended to be a guide for selecting your CNC Router, but a synopsis of my personal experience while looking at machines to replace my ShopBot PRTalpha96. So in conclusion, here is what finally won out with me.
The machine I chose had the following: Control software that was easy to learn and use, stepper motors on each end of the gantry, very strong and beefy gantry, resolution that exceeded my current needs as well as what I can envision my needs will be in the future, and table design that allows me to construct my own vacuum hold-down system. This will allow me to meet my requirements as they may change or evolve. Also, the machine has mechanical systems that I can work on if repairs are needed down the road, electronics that are modular and easy to replace if needed and support that can be reached on the weekends if needed at no additional cost. The table is bolt-together but still heavy enough to give good cuts. Price vs value was good. I chose the ShopBot PRSalpha48. This is a 4’ X 4’ PRS machine that is half the table size of the machine I had, thus allowing me to put two machines into almost the same space that the other machine required.
I hope this gives you some points to think about as you are deciding on what machine is right for you. If you have questions about something I wrote about you are always welcome to call me or email.
I hope the rest of 2009 is good for your business and 2010 will be even better. Go out and find your niche. Don’t just copy someone else’s work and method, make your own place, and success will follow