Missed steps and back EMF question

[wjfiles]

I am 70% to completing my router .
I am using a HobbyCNC Pro board with 200oz motors Linear bearings on hardened shaft. About 4 ozs will move the gantry and there does not seem to be any sticking spots.
I have noticed many references to missed steps when using steppers.
What are the major causes, and can they be minimised in the early stages of design and fabrication.
Will Micro stepping minimise the impact of missed steps. I had planned to use half step to start with.
My motors have double shaft so that I could operate manually, but I hear that Back EMF will be a problem if the motors are connected. what is the workaround for this.


( To Get The Right Answer, You Must First Know The Right Question)

[Torchhead]

You left out a lot of information about things like the size of you table and the mass (weight) of your gantry and the type of drive (leadscrew, rack and pinion, gearing, etc). Any stepper will lose steps if presented with a load, speed or acceleration it can't produce. The other main source of lost steps is resonance. At a specific speed (different for every system) the rotation of the motor through the detents of the poles sets up a vigration that causes the structure to vibrate. All mechanics have a natural resonanace frequency where the frequency is better supported. Kinda like feedback in a PA system if the mic gets to close to the speakers. The courser the steps (whole, half) the greater the resonance is "amplified" (more vibration). Microstepping helps...a lot!

The other parameters that cause lost steps are all functions of the load and power of the steppers. It may take 4oz to overcome the frictional forces of your axis but you have to also add in the cutting loads and the inertia if the gantry mass. While cutting you will have rapid moves in multiple directions. How much force will it take to stop the mass of the gantry moving at say 120ipm in one direction and reverse it and get back to 120IPM in the other? That is where the drive mechanics have to be known.

A 200oz-in stepper is rated that at stall speed (where it develops the most torque). It will lose torque as it increases in RPM. And at normall cutting speeds may have 1/2 to 1/3 the full rated stall torque.

You can move a dump truck with a 12hp motor of you gear it right and apply the acceleration slowly enough (:-)

So to recap. 200 oz-in motors will not lose steps as long as the load and mass are light and the gearing is correct. If the machine is larger than a table top engraver or small router (Dremel type spindle) you may find you have to tune the velocity and acceleration down a lot to prevent lost steps.

[NC Cams]

Don't forget noise which can ADD steps too.

Most "lost step" issues can and have been engineered OUT of the system via proper parts application, elimination of binding, proper jog spees, and/or good wiring procedures.

[wjfiles]

Thanks guys, your replies filled in some answers and the subject of inertia and resonance did not ocurr to me but I fully understand the relevance.
I plan on using up to 6 mm end/slot mills in alluminium but mostly engraving .
As for high speed movement the size of the table would hardly justify high velocities. Like all machines it will have its capabilities dependant on the design and I am sure it will not be big enough ?
Basically I am building a router/Engraver. the work area will be 14 x 10 x 4 inches.
The construction is Aluminium frame very ridgid and all parts machined to precise measurments. All parts are dowelled and screwed to ensure a flex free gantry.
I am using linear bearings on induction ground shaft for the ways.
Ultimately I will use ball screws but may settle for Acme screws and bronze nut with backlash compensation initially. The reason being that this machine is not the result of design but an evolution of a lot of Ideas and experience and will provide me with an education in CNC and lead to a second machine. I am somewhat ashamed to admit that I used computers long before the PC. and worked ind the Aero Space and Machine tool industry as a Manufacturing consultant but did not have any interest in CNC.
I now know that it is a virus of some sort and I did not have a lifetime immunity.
I will post some pictures in due course.
Thank again.