Yes, micro steps are not exact. This gets into the finer points of CNC design. You want this error to be MUCH smaller than what can be noticed. So the first step is to decide what you can notice. I picked (complete out of thin air) that a error under 0.0002 inch can't be detected on a Harbor Freight Mini Mill. This is likely way to conservative and I bet I'd never notice even a .0005 error.
Then I figure, again conservatively, that the worst case step error is 1/2 the step size. This is poor performance for a stepper motor but I'm using Chinese motors and they might be that bad. The good new is that these errors are NOT cumulative.
What is the pitch of the lead screw? The stock screw is 0.060 inch per rev. Lets say you are set up so 400 steps make one revolution. this is 0.00015 inch per step. So I can tolerate a full step error and still not notice the defect.
But what if I used a ball screw with a 5mm pitch. This means 5mm, not 0.060". 5mm is 0.197 inches so we get about 0.0005 inch per step. We'd notice a full step error and even a half step is over my tolerance. So I would set up this system for 800 or even 1,600 steps per revolution so that I could tolerate a 1/2 step error.
You can spend hours pushing calculator buttons and then decide to make a spread sheet. Let's say you have big goals and want 60 inch per minute jog speed and 1600 steps per revolution and a 5mm pitch ball screw. So you need about 5 revolutions per second to get the speed you want and 1600 steps per revolution to get the tolerance you want. this means 8,000 steps per second. You then check torque/speed graphs of available motors you find you need a BIG motor. So you say "maybe I can live with 0.0005 error and only 40 IPS speed. Now you find you can save $200. But maybe you don't mind spending more and want low error and high speed. But then you think "this is a $800 Chinese mill and I'm not running a production shop maybe I can run slower or tolerate a 0.0003 error?
It is not rocket science. High school level math is needed to workout step rates and jog speeds and errors. But again this is a low-end mill and BY FAR the limit to performance is the very large backlash and low rigidity of these mills. going for 0.0001" accuracy is wishful thinking and it's not going to happen. In real life 0.001 is what you will get. I am not going to build a race car transmission. I'm making a motor mount for a robot. You can trade speed, precision and cost if you make a spread sheet.
All that said I can reduce the motor error to 1/1000th of a revolution by using the new kind of closed loop stepper motors. But these double the cost. and such precision is TOTALLY wasted if using the stock lead screws. You can prove this to yourself. Turn the handweel a tenth of a tick-mark and see if the end mill even moves. You'd need to invest in some high-precision screw before you care about 1/800th or a revolution error in motor position. (Yes they DO sell better screws if you have the cash.)
On Thu, Mar 22, 2018 at 11:10 AM, Jerry Durand jdurand@durandinterstellar.com [7x12minilathe] <7x12minilathe@yahoogroups.com> wrote:
While it's unlikely that anyone doing CNC will run into them, there ARE motors with different numbers of coils.
There's even a one-coil version that's common in low cost things like the fan on your computer or in battery powered watches that have actual hands that move. In these the motor typically one runs in one direction.
Also the number of internal lobes on the magnets can vary. Most CNC motors will have two hundred "ticks" that you can feel when turning it with the power off. As a side note to this side note, when microstepping ONLY the base 200 ticks are accurate. Microsteps only approximate the movement between ticks. This can show up as have ripples in your work, the micro steps are close, but not exactly right and it repeats every 1/200th (1.6 degrees) rotation of the motor.
On 03/22/2018 10:52 AM, Chris Albertson albertson.chris@gmail.com [7x12minilathe] wrote:
Those 6 and 8 wire motors have 6 or 8 wires for a reason. If gives the user a choice.
All motor have two coils. If they run the ends of the coils out of the motor there are four leads (as each coil has two ends) this is the normal case.
-- Jerry Durand, Durand Interstellar, Inc. www.DurandInterstellar.com tel: +1 408 356-3886 @DurandInterstel
Chris Albertson
Redondo Beach, California
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Posted by: Chris Albertson <albertson.chris@gmail.com>
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