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jimbo_cnc
10-03-2015, 02:05 PM
I can't find where I downloaded it from now, so I'll put this in General.

Here's what I found. Perhaps someone can check this over.

I deconstructed the spreadsheet and simplified it, getting the same results.
Then I looked at the equations.

I think I have found these errors:

1. rotor inertia conversion error. Should be 1e-7 conversion not 1e-8.

2. torque to accelerate the gantry should include the ballscrew efficiency.

3. <deleted> my error


Net result is I get different values for 2 of the 4 torque components.

JAZZCNC
10-03-2015, 02:19 PM
Net result is I get different values for 3 of the 4 torque components.

Ye and then in real world use the Net result will be another complete different reality.!! . . . Why because of the stuff like build inacuurecy and friction etc not bearing any resemblence to what the limited data in those sheets work upon.! . . . . So IME treat it as a very rough guide only before wasting too much time on it.!

jimbo_cnc
10-03-2015, 02:53 PM
Not completely different I would hope.

The acceleration torques are not much influenced by alignment.

Besides I would build accurately :)

JAZZCNC
10-03-2015, 03:11 PM
Not completely different I would hope.

The acceleration torques are not much influenced by alignment.

Besides I would build accurately :)

No I'm lying to you it will be exactly the same has the sheet says.!! . . . Crack On.

jimbo_cnc
10-03-2015, 05:08 PM
I can't reply to that, because it makes no sense. 'Not completely different' does not equal 'exactly the same'.

Jonathan
10-03-2015, 05:44 PM
This is the thread:

http://www.mycncuk.com/threads/1524-What-size-stepper-motor-do-I-need?p=42489#post42489

You have correctly identified the errors.

I made my own version, as a script posted here (http://www.mycncuk.com/threads/1524-What-size-stepper-motor-do-I-need?p=42489#post42489). I added calculations for efficiencies, friction in different parts and the pulley inertia's. The latter makes a big difference... after including those things I've found it to be a good match with reality.

jimbo_cnc
10-03-2015, 05:55 PM
Thanks Jonathan. The second link is same as the first, but if it's the octave script then I've seen that. I preferred to use the spreadsheet, but now I will pick up some of the equations from your script.

Any thoughts on nice values for acceleration for large wood panel cutting, or small plastic parts cutting?

Jonathan
10-03-2015, 06:02 PM
Sorry yes, I got the wrong link:
http://www.mycncuk.com/threads/7355-What-size-servo-motor-do-I-need?highlight=octave+servo
By all means copy the equations or whatever you want...

Acceleration will limit the minimum arc radius the machine can move round whilst maintaining the specified feeedrate, so you can work it out from that.