Motor size calculation HELP

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?

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?

Thanks again

--Larry