Re: [CAD_CAM_EDM_DRO] Motor size calculation HELP

Laurence Casey 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?
>
>

Is this for steppers or servos? You don't say, and it makes a big
difference.
Stepper motors are rated for HOLDING torque, and their torque falls off
rapidly as the speed increases. Most stepper motors lose 50% or more of
their torque well before they hit 1000 RPM. The larger motors suffer even
worse than the smaller ones, but they start off with more torque.

Servo motors can deliver roughly constant torque from zero to full speed.
Also, they have both continuous and peak torque ratings, so they have a
substantial reserve of torque for acceleration.

>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. Anything more caused the machine to chatter like
>crazy. Is this to be expected with these machines?
>
>

60 IPM is QUITE a lot. I suspect you might have been able to do more,
but you
need to turn the cutter faster. As the feed rate increases, the cutting
tooth has
to take a thicker chip off the work, and at some point the advancing
workpiece
rams into the BACK of the cutting tooth. When this occurs, you can see
smashed
metal on the back of the cuting flute. Assuming a 2-flute end mill at
2300 RPM,
that is 4600 cutting teeth per minute. At 60 IPM, each tooth is taking
a cut of
60/4600 = 0.013", which is enormous for a 1/4" end mill. If you look, I
suspect
you'll find that smeared metal on the back of the tooth. There is a way
to grind
the flutes to allow feedrates like this on aluminum, but standard end mills
won't handle it.

Jon