Re: [CAD_CAM_EDM_DRO] Reduction Ratios for Steppers

In a message dated 6/2/2003 9:19:45 PM Central Standard Time,
cnc@... writes:

> In designing the drive reduction ratios, how important is it to achieve a
> "round" number like [.001" of travel per 1.8 degree motor step] ?
>
> If the system I settle on comes out to, say, .00125" per step, is it going
> to cause me all kinds of setup problems?
>
> And, Do all [three] axis need to have the exact same drive ratio?

SC:

Some "in here" will surely tell you that it doesn't matter, as long as the
computer and/or software can "do the math" so that your commands won't have
irrational numbers of 17 digits in your programming. Me, I opted for 0.1" pitch
screws on my FIRST machine, and connected the steppers DIRECT to the ends of
the screws, i.e., a 1:1 ratio, so, for a 200-step motor, each step is 0.0005".
That means for a lathe, a diameter-reduction of 0.001" per step. VERY fine!

Then, on my next two lathes, I used 4 mm pitch screws, so each step is 0.02
mm. That's less than 0.001" per step, but I have to "think metric"! This DOES
mean that 200 steps, one turn, is 4 mm, so I have to "remain aware of the
arithmetic" as I program, but it doesn't result in long, multidigit irrational
numbers!

No, certainly not, the three axes do NOT have to have the same pitch screws!
Me, I think I WOULD keep X & Y the same, but the Z axis could have a finer
pitch, I think, depending on what you were building!

The idea of using "timing belts" to connect your stepper-motor to the screw
gives me hives! You would have to "tighten up" the center-to-center distance,
motor-to-screw, so tight to preclude slop, that I imagine that would detract
significantly from your motors' useable torque! If you MUST put a "ratio"
between the motors and screws, consider the W.M. Berg "Min-e-Pitch" cable-chain
and sprockets. Their polyurethane overmold and "toothed chain" types (MANY
types and sizes!) are made so that the user can "make up" things so that there is
NO slop! It is very precision (and priced as if it were made on the East
Coast! [they are in NJ!] ) Nice thing about all the "Berg stuff" is that it is
precision, smooth, set-screws, many different diameters and shaft-bore diameters,
and can even be ordered "custom made" when needed. Work carefully!

Jan Rowland, Old Troll


[Non-text portions of this message have been removed]