Building A CNC Router - A Few Hardware Things To Consider!

[widgitmaster]

Building A CNC Router - A Few Hardware Things To Consider!

When designing the CNC Router
Start with the weight of the spindle motor, and the Z-Axis motion motor, then build everything strong enough to support & move them! Keep the Y-Axis and Z-Axis as compact as possible, because the force of the spindle needs to be as close to the slide mechanism as possible. Also keep in mind the maximum diameter cutter your spindle motor will allow, as larger diameter cutters require more rigidity!

Y-Axis Gantry Type?
Stationary Gantry requires a base that's twice as long as the X-Axis Travel.

Where should the Lead Screw be located?
The lead screw needs to be between the slides, as close to the center line as possible. If it is located off center, the sliding force will be unequal and the slide will jam or bind.

Do both ends of the lead screw need to be supported?
Yes, an unsupported screw will vibrate, or bend as RPM is increased.

How do I remove backlash from the lead screw & nut?
Simply add two nuts, and add shims behind the 2nd nut till the backlash is eliminated.

Can a router have more than one motor per Axis?
Yes, both motors must be matched, and connected to the same controller.

Which is better Supported or unsupported round rails?
Supported rails do not vibrate or flex, increasing rigidity and increasing cutter life!
Unsupported rails will flex and bounce, resulting in poor finishes and broken cutters.

Ball screws with Ball nuts or Acme screws and Delrin plastic nuts?
Ball screws are the best choice, but are the most expensive!
Acme screws are easy to locate, and are inexpensive!

What are my options for X-Y-Z axis slides?
Round bars or pipes with multiple roller bearings.
Round case hardened linear rails & ball bushings. (supported or unsupported)
Rectangular Linear slides with multiple trucks.
Square tubing or angle iron with roller bearings.
V-Groove bearings and V-Rails.
Chain drive, using roller chain and sprockets.
Rubber Timing belts, using coorigated pullys and timing belts.
Stationary timing belt segment, with stepper directly driving a corrigated pully (rubber Rack & Pinion)

Rack & Pinion or Lead Screw?
On very large router tables, a rack & pinion can be the only option.
For MOST router tables, lead screws are best.
Rack & Pinion can produce an echo in surface finishes, as the gears fill with crud!

If the spindle motor is too heavy
Add a counter weight system for balance, this will reduce the force required to lift the spindle motor.

What is the difference between Backlash & End Play?
In a mechanical system, Backlash is the relative motion between two devices, connected by a coupler, gear, screw, etc.
End Play is the amount of axial movement between two parts.

What are the X-Y-Z axis directions?

In a 3-Dimensional Coordinate System, such as a milling machine:
X-Axis positive is to the right of you.
Y-Axis Positive is away from you.
Z-Axis Positive is upwards.

What is a Servo Motor?
A motor used for motion control in robots, hard disc drives, etc. Generally designed more like an alternator than a standard motor, most Servos need special control circuitry to make them rotate electrically. Some can be used in reverse to generate alternating current.

Acceleration
Acceleration is defined as the change in velocity over time

Force
The formula is: F = ma. (Force equals mass times acceleration)

The formula to compute torque:

T = F * d, where
T = Torque
F = Force (here, the force applied perpendicular to the axis)
d = Distance

Ball Bushings
It is the bore diameter that controls the amount of loosness or preload on the ball bushings!
If the manufacturer's specifications suggest an .8750" dia. bore, for a .5000" dia. linear shaft! And the actual diameter of the shafts is .4995" so the bores must be .0005" smaller too!

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I hope the information provided above will help in the design of your 1st CNC router!
This information is as accurate to the best of my knowledge.
If you have any corrections or additional considerations, please post them for everyone's benefit!
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WidgitMaster

[wcarrothers1]

Lets see what Mad has to say about these..

I think I voilated the first one about Z as my Z is not to compact and has quite a long travel..

On another note.. That gage holder thing you make showed up today. And boy is it a cool looking little piece of work!!!!..

b.

[Madclicker]

Originally Posted by wcarrothers1 Lets see what Mad has to say about these.. I think I voilated the first one about Z as my Z is not to compact and has quite a long travel..

I agree. I said before that I thought that was a huge drop.

If you use 2 motors per axis they should have their own drives. You can have them share a single axis' signals or you can set them up as separate axes and slave one to the other in the software.

[Madclicker]

Rack & Pinion or Lead Screw?
On very large router tables, a rack & pinion can be the only option.
For smaller router tables, lead screws are best.

What do you consider a very large router? Why are lead screws best for smaller tables?

[Hyrum R]

Thanks for the information there, widgitmaster. I'm new to this stuff, so this is just to say thanks for taking some time to share your knowledge.

[widgitmaster]

Added more info!
Hope it is accurate, as a search through Google returned much misleading information!

Eric

[widgitmaster]

Stepper Motors.
In a stepper motor, power is applied to a wound stator, causing the brushless rotor to change position to correspond with the internal magnetic field. The rotor maintains its position relative to the internal magnetic field at all times. In its most common mode of operation, the stepper motor is energized by an electronic controller whose current output to the motor windings defines the position of the internally generated magnetic field. Applying a command pulse to the controller will change the motor currents to reposition the rotor. A series of pulses, accompanied by a direction command, will cause the rotation in uniformly spaced steps in the specified direction.
If the pulses are applied at a sufficiently high frequency, the rotor will be carried along with the system’s inertia and will rotate relatively uniformly but with a modulated velocity. At the other extreme, the response to a single pulse will be a step followed by an overshoot and a decaying oscillation. Where the application cannot permit the oscillation, damping can be included in the controller.
Stepper motors are often preferred because positions of the rotor are known from the number of pulses and the step size. An initial index point is required as an output position reference, and care is required in the electronic circuits to avoid introducing random pulses that will cause false positions. As a minimum, the output index point on an appropriate shaft can verify the step count during operation.

Copied from:

Machinery’s Handbook
27th Edition
Page 2493
Motion Control section

[wcarrothers1]

The way I figured it MAD.. better to have a lot of room under the gantry for bigger stuff incase it's needed.. As I figured it's easier to raise the table from it's lowest possiable possition (or have tall jigs) then lower it..

I'm only getting less then .001 worth of movement at the router tip if I lean (hard) on it at full extention and the stage is in the center of the gantry. If it's less then half the distance from either side to the center that movement drops to less then .0005ish..

After I cut a bit I might upgrade to 1" z bars but don't think I will need to. Probably only update that part if I get board some day..hehe

I'm VERY happy with my rack on X. Wish my Y was also rack. Best feature of the rack drive is you can move it manualy (pushing the thing around) if you need to... I'd love to be able to manualy move Y or Z around as easy with the power supply off. Course if Y were rack that would put 10+ more lbs of servo weight out on the gantry rather then off to the side of the gantry. So that would kinda suck..

I'd say screws are better for small since with rack you tend to need to build in a complex method to do gear reduction and on a small table there isn't the room (and that would also add weight). The lead screws are naturally geared.

Anywho.

[dertsap]

theres been no meantion of belt or chain drive as a cheaper alternative
http://www.cnczone.com/forums/showth...ighlight=chain
this system inspired me to try chain drive which may be crossed over to a belt system if i don t agree with the performance of chain ,

[widgitmaster]

Thanks Derstap!
I had forgotten about those!
Eric

[js11110]

Maybe I'm nuts, but I've been a machinist for 20 years and X positive is to the RIGHT, and Y positive is AWAY from you. If you take your left hand and make like your firing a fake gun (your index finger pointing and your thumb up) and then stick your 2nd finger out to the side(right) each finger points to the Positive direction of each axis. Learned that in high school many years ago.
Just my 2 cents.

Jason

[widgitmaster]

Thanks for the correction, I tried to find info in Google, but its a misleading mess!
Eric