Getting started in cnc
[size=12pt]In retrospect in writing The what is needed for CNC thread I believe I met my initial goal. That was to give a basic overview of what is needed for a cnc system. I gave links and resources for all the components of the system that are needed.
Where I fell short is not telling folks where to start. Also from the design perspective I listed things in somewhat of a reverse order. This is written to correct that error. This overview will also include some of the basic math needed to design a CNC system .I hope this will help clear up any confusion created from the old thread and answer some important questions not addressed earlier.
The first decision to be made is selecting a machine to be converted. This can be a milling machine a lathe a kit built router, a reprap/ rapman/ repman or other 3-d printer etc.
The seig x-2 mill and mini lathes are common in the home shop and are a common cnc retrofit project.
Sherline products can be purchased cnc ready. Taig mills and lathe also offer cnc options.
If you build a machine design it for cnc ie ball screws anti backlash devises and motor mounts
The second step in the design process is calculating the power needs of the selected machine.
THE WEIGHT OF THE HEAVIEST OBJECT BEING MOVED: This will typically be your gantry and, for this example, we will use a gantry with the weight of 40 pounds.
INCHES PER MINUTE DESIRED: This is a number that will make you deliriously happy with the performance of your machine and, for this example we will use 1000 IPM.
Now multiply IPM and pounds together and divide that number by the normalizing constant of 531. The answer will be the total number of watts needed for your machine. Below is the formula in basic form:
(Heaviest object * IPM) / 531 = Watts required
And solved for the numbers above:
(40 * 1000) / 531 = 75.329W
Because this number is below 100W, you will definitely want to stay in stepper territory.
less than 100 watts choose steppers
more than 200 wats choose servos
100-200 watt choose between servo or stepper.
From the Gecko drives web site
I would say for most hobby sized bench top machines a stepper would be the choice. A Bridgeport or Millrite etc you may want to go the servo route.
Once you know the wattage requirements then select a motor.
Read this : http://www.geckodrive.com/support/st...or-basics.html
Lots of info written by an expert.
Stepper motors come in various physical sizes NEMA 17,23,and 34 are the common ones.
The important number is torque rating like 270 oz/ in, 320 oz/in etc rated amps volts and impedance are also important considerations in design.
An efficient motor, defined as the smallest motor sufficient to meet the demands of the application, will run hot. Think of the motor as having fixed power conversion efficiency: Some percentage of the input power will be converted to heat and the rest will be converted to mechanical power. To get the maximum performance from the motor, the waste heat must be just under what the motor can tolerate. Usually this motor will be biased to operate just past the corner speed as well.
The place to start is to determine the load torque in oz/in, including the torque necessary to accelerate the load. The next step is to come up with the maximum speed the application has to operate at in full steps per second using the formula below. RPI is the revolutions per inch after the motor turns through the transmission, RPS is revolutions per second and PPS is the number of pulses per second from your step pulse source.
(DESIRED IPM * RPI) / 60 = RPS
RPS * 200 = PPS
Multiply the PPS value by the number of oz/in determined previously and divide the total by 4506.The answer will be how many watts mechanical are require motor to meet the load rom the application.
When picking a motor, choose one with 40% more than the calculated power. Below is an example of the equation completed for a load requiring 450 oz/in with a 3 TPI leadscrew and a desired IPM of 300.
(300 * 3) / 60 = 15
15 * 200 = 3000
(3000 * 450) / 4506 = 299 OZ/IN
299 * 1.4 = 419 OZ/IN
As you can see, you will want to use a motor with a rating of 419 oz/in for this application.
Info is from Gecko drives.
Torque is simply force times distance or simply affix a flat piece of stock to the lead screw on your mill if you place a force 10 inches from the center of the lead screw say 2 lbs(32 oz) that is 320 oz/in of torque. Or use a spring scale to measure force and multiply by lever length. Whatever motor you select I recommend one of known origin that you can get a data sheet from before you buy. The data on the sheet will help insure you are selecting the right motor for your application. Another important formula is calculating motor voltage this is based on impedance.
Once motors are selected you will need a plan of how to connect the motors to the machine. There are plan sets for this and info on the web how others have done it. Or you can be creative and figure it out.
Plans here:: http://www.stirlingsteele.com/Index.html
and here http://www.fignoggle.com/and
here is a guy who has done cnc conversion to the max. Lots more info here: http://www.hossmachine.info/
IIRC $50 per set for plans. On my little lathe no plans just winged it. Based on experience for the mill.
The next step is to select a driver board and breakout board to meet the power needs of your motors. You need to select a controller that allows a high enough voltage to get good performance from your machine . Many of the cheap boards will only handle 24 volts. Volts = speed in cnc , speed = time.
There are many options as far as driver and breakout boards kieing cnc offers many options mostly quality import products. .http://www.kelinginc.net/index.html
Other sources for components motors Power supplies couplers etc etc.
A power supply must be selected.. Mine was from Keling cnc. The selection here will be based on power needs and the motor and control board ratings. Select one that will allow for good performance but not push the limits of the equipment. This needs to be able to provide enough amps but not too many volts.
The voltage of your power supply is entirely dependent on the inductance rating of your motor, which we learned is translatable to the number of turns of wire in the stator. Every motor model will have a different inductance rating and will therefore have a different maximum voltage. To figure out what the maximum power supply voltage should be, use the following formula with the motorís inductance in millihenries (mH) used for the L value.
32 * VL = VMAX (I believe this is 32 * (square root of L)based on the G540 manual)
The easiest factor in choosing a power supply is its current rating, which is based on your motor ratings. A motor control will always draw less than 2/3 of the motorís rated current when it is parallel (or half-winding) connected and 1/3 of the motorís rated current when it is series (or full-winding) connected. That is to say, a 6 amp per phase motor will require a 4 amp power supply when wired in parallel and a 2 amp power supply when wired in series. If multiple motors and drives are used, add the current requirements of each to arrive at the total power supply current rating.
From Gecko drives
The next decision is how to interface the breakout board to the computer . This will be determined by the breakout board connection and the computer connections. My geckco and older used computer both have a db 25 connector so I just used a cable but you may need to add a db25 parallel port to your computer or add an external motion control device like a warp9 smooth stepper http://www.warp9td.com/
these are designed to work with mach 3 software and available in USB or Ethernet versions.
You will need a computer for cnc . Computer choice and software choices are interlinked.
Last but not least you will need software :
Software cnc software to operate the system
Mach 3 cnc http://www.machsupport.com/
windows based commercial software well supported a good value.$175 license lifetime support the lifetime of mach that is.
Turbo CNC http://www.dakeng.com/turbo.html
DOS based shareware $60 license
EMC2 I believe this is now called Linux cnc http://linuxcnc.org/
linux based public domain software.
Notice 3 choices three operating systems. All of these choices can be downloaded and tested before you commit to one but please once your machine is making parts please register and license the one you choose if applicable.
$0 to 175 for control software
You may also want CAD, Computer aided drawing software such as
this is what I have
A basic home version of Alibre CAD is about $ 200, more for the pro and advanced versions.
and Computer aided manufacturing software such as http://www.cambam.co.uk/
Cam BAM is $150 for the license and there is a try before you buy.
This info is based on my experience and choices. You will need to choose what is right for you . There are many cheap import products on the net I have not used or tested them. So I will not say yah or nay as an option.
An option that will simplify the process is simply order Geckco 540 and matching motors from gecko. $300 for the 4 drivers and a breakout board in a neat package and three or 4 G723-280-4 motors @ $59 1-3 for $55 ea for 4. These are made in USA quality products. And well suited to a small bench top retrofit.and a power supply Just a happy customer. Or you can buy a kit from keling cnc that has matched motors controller and power supply.
Also keep in mind the world of home shop cnc electronics is changing. The manufactures are constantly doing R&D and introducing improved products. The software developers are also constantly improving and developing new products. So do your homework if doing a build. Also keep in mind all of these components must connect and somewhat match each other. This understanding is important if buying stuff from multiple vendors. And one stop shopping can save money.
Digital machinist Magazine has sponsored a cnc build seminar in Michigan the last couple of years. The cost of the seminar is less than you can buy the parts @ street prices at the end you have a CNC mill. Ron ginger has led the seminar.
Link to original thead here: http://www.homemodelenginemachinist....p?topic=9787.0