basic length unit vs. least command increment

[sinha_nsit]

Please check if the following statements are 100% correct and accurate. If not, pl point out:
I read somewhere that the "basic length unit" (BLU) is the distance by which the slide moves with one electrical pulse to stepper/servo motor. It would depend on step angle, gear ratio, pitch of the lead screw etc.
How much is this distance, typically?
We can call it control resolution because this is the theoretical minimum distance which the control can differentiate. (Since the control uses a large number of bits, typically 32, to store the slide position, the bit storage capacity does not put a limitation on control resolution).
How is BLU (i.e., control resolution) related to least input increment (min possible is 0.0001 mm in IS-C)?

The operator's manual uses a term "least command increment" apart from "least input increment." What exactly is "least command increment"?

From operator's manual:
"The least input increment is the least increment for programming the travel distance. The least command increment is the least increment for moving the tool on the machine.
...
The least command increment is either metric or inch depending on the machine tool".

From parameter manual:
Parameter 1001#0 (INM) Least command increment on the linear axis
= 0 : in mm (metric system machine)
= 1 : in inch (inch system machine)

What exactly is metric/inch system machine?

Is least command increment same as what I am referring to as BLU?
How is least input increment related to least command increment when we use G21 on an inch system machine, or G20 on a metric system machine?

[sinha_nsit]

No replies yet.
Possibly I asked too many questions.

Ok. Just one question:
What is least command increment?

[Chrliev]

Sinha,

Here is how I understand it to be. In inch mode most machine tools typically work to a decimal represented as thus: 3.4 Your maximum number in the inch system is 999.9999 the minimum is 0.0001

In the metric system machine tools typically work to a decimal represented as thus: 4.3 Your maximum number in the Metric system is 9999.999 the minimum is 0.001

The inch system looks to be more accurate, but from a strictly numerical perspective the metric system is actually more accurate because 0.001mm is equal to 0.00003937inches. I say a "numerical perspective" because in reality, considering all the machanicals, ball screws and machinery inaccuracies you would do very well to machine a part within 5 times the minimum resolution of either system.

In other words .0005inch or .005mm on a typical CNC machine would be considered a very precise part in either measurement system.

I'm not sure if this answers your question, if not maybe it enhances your real world understanding of machine accuracy and phalacy...

[sinha_nsit]

Thank you for your reply.
Can you differentiate between least input increment and least command increment?
What you describe, I understand, is least input increment.

I would request more people to join the discussion, even if they do not have exact or correct answer. Let us arrive at a conclusion. Just express your intuitive feeling.

[sinha_nsit]

Those who are following this thread may look into another related thread:
http://www.cnczone.com/forums/fanuc/...id_rate-2.html

[Chrliev]

Please forgive me if I misunderstand, but it sounds to me like you might be asking something similar to, what is the difference between 12 and a dozen..?

[sinha_nsit]

Least input increment is not same as least command increment, though usually these are numerically same. There can be a factor of 10 also between the two.

Actually, I want to know what happens at the design level. What is the effect of one electrical pulse, etc.? Of course, these things are not meaningful at programmers' level.

[sinha_nsit]

Got some more information:
On a metric machine, least command increment is always 0.001 mm in IS-B, irrespective of G20/G21. Thus, in G20 mode, least input increment would be 0.0001 inch whereas the least command increment would still be 0.001 mm. Least input increment is minimum INPUT data (through program), whereas least command increment is minimum machine OUTPUT. An internal conversion at the control level must be going on, to ensure programmed movement in terms of least input increment.

On an inch machine, least command increment is always 0.0001 inch in IS-B.
This is equal to 0.0025 mm. Now, what I do not understand is, how the machine would ensure 0.001 mm movement in G21 mode, since output smaller than 0.0025 mm is not possible on inch machine. Any idea?

[sinha_nsit]

The answer is, as I understand, 0.001 mm movement is not possible on an inch machine which always moves the slides in steps of 0.0001 inch (i.e., 0.0025 mm), in IS-B. Therefore, there is going to be a mismatch (of +/-0.001 mm or +/- 0.0005, depending on specified input value) between input and output values. This is very small compared to repeatibility of the machine. Hence. we do not have to bother about it. It is only a theoretical discussion.