Re: Motor size calculation HELP

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Laurence Casey <lcasey@l...>
wrote:

> I have been posting a few questions over the last few days. I am

very

> happy to see the list helping me in my conversion process, but I

am a

> little confused about motor size. Many people with the same

machine I'm

> converting are using large motors (NEMA 34 305-424 in-oz). Am I
> calculating something wrong or are my gibbs too loose? Am I not

cutting

> enough material in a pass?
>
> I did the following test.
>
> Very primitive test follows:)
>
> I clamped a piece of 1" 6061 square. Using a 1/4" end mill

set to
1/8"

> deep, I ran the mill at 2300rpm. I wrapped a belt around the

handle and

> hooked my fish scale to the end. As I pulled the scale I noted the

lbs

> needed to feed the material through the mill. Doing this test both

at

> slow IPM and fast IPM, I noticed no difference in effort needed.
>
> I then did the following math to calculate the motor size needed.
>
> Machine = Micro Mark Mill (www.micromark.com), like Grizzly
> Handle diameter = 4"
> Lead pitch = 16
> Scale reading = 2.6 lbs
>
> 2.6 lbs * 2 r * 16 = 83.2 in-oz
> 2*3.14 * 16 tpi * 83.2 / 16 = 522.496 lbs on lead screw
> The above calculation would say I need a 116 in-oz motor (40%

reserve).

> Not sure how much reserve is needed. Did I miss something?

Stepper motors are rated in terms of holding torque. The torque they
produce while spinning is quite a bit less. Obviously you'll need at
least 116 oz's of running torque. Youll need a stepper with a much
higher torque number since they are rated in terms of holding torqe.

And its very important you have more power than you need. If you do
overpower the stepper, then the position is lost. Once it stalls,
the rest of that move is gone since there is no acceleration curve
to get it back up to speed. Then it proceeds to crash the tool into
something, ruin the part and potentially cause other damage. Its
better to have too much power then not enough.

What about when taking a 3/8 wide cut at 800 RPM in steel? A 1/4"
end mill 1/8 deep in aluminium is a very light cut.

>
> In another primitive way, I connected a drill to the lead screw in

hopes

> of determining IPM possible with this machine. I was not too

thrilled

> with the results. It (approx) 60 IPM the machine performed well

with

> little to no chatter.

60 IPM is very fast. Was this taking the same 1/4 x 1/8 cut in
aluminium? When people mention speeds of 100+ IPM they are not
cutting at this speed, this is the rapid speed.



Anything more caused the machine to chatter like

> crazy. Is this to be expected with these machines?
>
> Thanks again
>
> --Larry