I have a question for all the pro’s out there… I want to build a general home CNC for myself… And I am stuck on motors…
I see so many different types and strengths it is really confusing for a novice such as myself… Most CNC’s are rail or bearing based builds, so there is not a lot of drag that I can see, so what is a good base motor to use… NEMA 17, NEMA 23… And the biggest question what strength?? 576 / 425 / 290 / 272 / 270 / 170 / 121 / 68 / 48 OZ/Inch … With all of the bearings and smooth tracks/guides and acme screws… Does the torque really need to be that high for a typical hobbyist to cut wood / plexiglass / thin metal??
Any thoughts, ideas, real life feedback would be appreciated.
More your motor have torque, less chance that the motor step pulse
Me I use
-Oriental Motor, PK299-F4.5B (Not Cheap)
-The Drive,
http://www.aliexpress.com/item/3-Axis-Stepper-motor-driver-PEAK-7-8A-256micsteps-CNC-longs-motor/555225038.html
-Ball Scree 5 turn for 1in,
I go up to 150in/min,
I make test on 200in/min, but I don’t like the sound, I think is a mechanical vibration.
It will really depend on the size of the machine and how heavy the moving pieces are. Bigger motors are not alway better, they will require a larger power supply and can limit speed. What size and style machine do plan to build ? What are the current and voltage limits of the drivers you will be using? Answer those and I can give you a ballpark recommendation.
in a CNC everything is related; you use the strongest motors your frame allows; the advantage of stronger motors is the increase on feed rate; but of course depending directly on the screws you are using (TPI), higher TPI, requires less torque, but lower speed with higher resolution.
There are too many variables at play when selecting the motors. Better research how DIY a CNC before buying anything
Very few things are more annoying than running out of torque. I went with Nema23 frame 287 in-oz on my raised gantry router. unfortunately for me i went with plain UHMW bearings all around and found that on the gantry one of those motors was not enough to drive it accurately. Now keep in mind also I went direct belt drive. If you are going with Lead screws ( please don’t use “all thread”…) you get a huge torque multiplier based on the pitch you choose.
so lets say you go with 3/8-10 (if you like English dimensions) gives you something on the order of ~ 10ish x the torque applied. Unless I’m having a total brain fart its more like 11.7ish times (not accounting for things like pre-load and friction, but I digress.
You can get ~ 2amps per motor if you go the Arduino base solution ( Ramps, Grbl, ect…) the plus side to those boards is that you only need to stream, or store locally the g-code file. So from there see what motor frame and torque fits then with you chosen lead screw pitch and diameter handy you calculate you available torque for motion, remove a portion for bearing losses and the remaining is what you have to work with.
Sorry that was kind of rambling… but I’ve been at this ( slowly) since before 3d printers and hobby cnc really took a foot hold in main stream media. Lots of time to research.
First decide on the size and type of your machine. Then ask for some recommendations. But a little motor goes a long ways…
Thing to remember is that you are both moving the components and resisting the cutting forces. There are two ways to adjust the power you have available: stronger motors or mechanical advantage. There are no real cut and dried ways to make the decision. It all depends. The speed of movement can be less of a factor than you might think. You can move the cutter at ridiculous speeds when you are not engaged in cutting but you can’t cut the material that fast. If you are cutting free cutting materials like wood, you might jump to direct drive rather than screws.
It also depends on what fabrication capabilities you have to start with. You might consider starting with a pre-built machine as a boot strap to getting to something more capable. A $1K Shapeoko, XCarve, or even one of the little chinese cnc machines on ebay might be a lot cheaper than tooling up to build something more sophisticated. They appear to be nice little machines that are not a whole lot more expensive than the components.
It depends on what you are building and what your expectations are. All my machines are some skate bearing variant. My first machine used Nema 23 118oz motors with 5/16 18 tpi all thread and could run at 24ipm, my last build used 425oz motors and 2 tpi 1/2 acme screws and can rapid at 180ipm (so far, still tuning).
I have heard it said a number of times over @ http://cnczone.com that you should build the machine 1st and figure out the motors and electronics needed to drive it later. I have never done that 
But I have been disappointed more than once at the speed of the completed machine.
The weight you are moving, and the speed you want it to go are the main factors that go into determining the method you use to move it (belts, screws, ball screws, etc) and how much power you need (motors, controllers, power supply).
Thanks guys, and Bryan… your not rambling this is exactly what I wanted… All my lead screws are 8mm in diameter with 2mm pitch… ( hoping thats okay, well it will have to be ) and my unit will be made of wood ( not such ply or mdf yet ) and a raised gantry on 12mm rails for all… My bearings are LM12UU if that adds anything…
My size will be 24 by 30 inches and I have a colt router as my cutter …
Also when looking into motors the big factor over looked often is impedance of the motor. The lower the value the faster you can charge and discharge the motor coils. also the lower number will require more amperage as impedance and resistance are related. so those factors need to be balanced against what can be driven by your electronics.
I was sorely disappointed the awesome deal i got on my motors was due to a high impedance value and it has had a huge impact on my ability to drive the motors reliability especially on the gantry since it is carrying the majority of the weight in my machine.
I would look for some nema23 motors in the 200-300oz-in range. You did not say what drivers you will be using, that will help determine the amps and volts the motor. As Bryan said you will want motor that is low inductance.
Thanks Eric… I guess 2 amp so I can use the arduino based drivers I currently have which would reduce the overall cost …
I used 287 oz/in nema 23’s and have a fairly heavy wood frame, with rails; and have had no problem doing any wood, lexan or light aluminum routing with them. Here’s an older picture I had posted:
https://plus.google.com/u/0/photos/115359593149072195919/album/6111812490674413681/6111812493775393922?authkey=CObkvfWEm_XVXA&sqid=106873799080099538660&ssid=4a52cf1e-93ff-49b1-b05e-dd0f77fb8cb5
You are not limited to 2 amps with arduino if you go with discrete drivers. Just get a screw shield and wire the arduino up to regular stepper drivers. You can even run the parallel boards if you get one of the parallel breakout boards.
I have since put on a full size router, with no problems either.
@Bruce_Lunde Thanks for the pic… @David_Kirtley - and more info / pics??
With hobby driver most are limited to 2.5 - 3 amps and around 30 volts for the lower cost drivers. With that in mind I would pick a motor that runs at full rated current of the driver with the lowest inductance you can find. If inductance is not speced the motor may have rating like 3 amps at 3 volts or 3 amps at 6 volts, pick the motor with the lower voltage rating it will be lower inductance.