Hello,
I have a Harbor Freight mini mill that I converted into a 3-axis CNC machine using a Heavy Metal ball screw conversion kit and Automated Technologies electronics. For my workflow, I design parts in Fusion 360 and run them in Mach4. I also installed the Mach4 Mill post from Autodesk’s website into my local library.

To test the setup, I created a small bottle opener. However, every time I run the G-code, the finished part comes out at about half the intended size. The scaling is consistently off—the part is always noticeably smaller than the original design.

In my machine setup, the units are set to inches, while the unit mode is set to metric. I’m wondering if this mismatch could be the issue, or if my motor settings might be incorrect.

Motor settings:

Counts per unit: 40960.0
Velocity: 35 units/minute
Acceleration: 0.49 units/sec²

Motor specs:

200 steps per revolution (1.8° per step)
128 microsteps

My calculation:
(200 × 128) ÷ 0.625 = 40960 counts per unit

I’d appreciate any insight or suggestions on what might be causing this scaling issue.

You need to incorporate the screw pitch.
I also suggest lowering the microsteps.
That is very high