The question about what motion control motor one should use is pretty common. Based on my experience, I’ll give you my take on the whole thing and you can decide….
There are two major factors to consider initially; 1) What kind of precision do I need (or might need in the future) out of my machine? and 2) What’s my budget?
What kind of precision are you looking for? I’ve always been of the opinion that you buy just above what you think you might need. This can help avoid disappointment. However, if you’ll never need high precision and repeatability, then steppers are a good choice. That being said, I’m going to shoot straight with you here; I prefer, hands down, AC servos and my next choice would be DC servos. Here’s why:
- They offer higher speeds without reducing torque as speed increases like steppers. Truth is, a lot of machine time is consumed by “air movements”. Those non-cutting moves, including moving the machine for tool changes, often waste a lot of time and slow production. For some, this might not matter. However, many prefer to get their parts done as quickly and efficiently as possible…nearly everything in life we can replace, time we can’t. Plus, if you’re in business, we all know time is money. The speed and torque benefits also come into play when you want to make faster cuts or passes. Many times, those with benchtop machines aren’t making fast cuts, but sometimes it comes in handy. When I do constant contact fast material removal tool paths, I prefer speed. For steppers this is a problem. They get weaker with speed. Servo motors are the opposite.
- Positional Accuracy. Servos always know where they’re at on a cut. They use a rotary encoder. That encoder keeps track to make sure all the steps are followed and none are skipped. I hate, hate, hate losing a step after a long machine run and ruining a part. It’s frustrating and a giant waste of time and material. I understand that one can buy steppers that are closed loop. However, overall, I’ve not had the closed loop steppers perform as well as closed loop servos. Plus, you’ll still have the low power under speed issues as mentioned above.
- Quiet, smooth and cool. The noise of steppers isn’t an issue for me, but it bothers some. Servos are typically very quiet by comparison. Their operation is quite smooth and can result in a better finish on those super delicate precision jobs. If you’ve used steppers, you’ll probably notice how hot they run (or even when they aren’t in movement). Servos run much cooler overall. This means less wasted energy.
Keep in mind, commercial grade CNC machines use servos…not steppers. If you want similar results of high-end commercial machines, use similar parts. Ok, let’s talk about budget. For some, this will ultimately dictate what you’ll choose.
Let’s face it, good servos are much more costly than steppers. A good set of DMM servos will run you 5 to 6 times the cost of cheap Chinese steppers. If you’re budget won’t accommodate that kind of investment, then like all of us at some point, we simply have to get what we can afford and work around any challenges. For example, you can get a 3 axis set of Wantai NEMA 23 motors, power supply and breakout board (the breakout board will be junk) for under $200. We offer 3 Axis 200 watt AC Servo motors with cables kits for around $1,200. As you can see, that’s a big jump in total investment for some. In my experience, buying “best in breed” is the way to go…and that means servos. But, what if you can’t afford servos? You might want to try closed loop steppers. They won’t be as good as servos, but at least they can keep track of movement steps. This can avoid some ruined projects and real heart ache. Leadshine seems to be a real popular brand with this option. Frankly, I would try to gather the extra money to get closed loop steppers instead of cheap open loop steppers. However, you want to keep your priorities straight when buying a machine. Buy mechanically sound first, then work on the electronics. In other words, you’ll want give the mechanical portions such as; ballscrew or leadscrews, spindle choice, spindle bearing choice and so on, priority when choosing you machine setup. Never make poor mechanical buying decisions with the idea of making up for it with electronics. Choose great mechanical qualities and support it with great electronics when possible.
Bottom Line For Your CNC Project
Here’s my take on the subject (of course you can do as you please);
- If you can afford it, try to go with AC servos.
- If you can’t afford AC Servos, then consider DC servos or closed loop steppers.
- Finally, if your budget won’t allow or your demands aren’t really that high, then go with steppers. They are cheap and easy to setup.
- Above all…you’re mechanical precision needs to be in place first, then electronics.
I hope this all helps your decision making process. There are other considerations such as servo setup compared to stepper and so on. The aim of this article is big picture net outcome as relates to your goals and real world situations. – James, GlockCNC.com