How do I set up a command to move Z-Axis -0.8500?

Not sure I follow, are you just asking how to move an axis to that position? if so the MDI line will do it for you G1 Z-0.85F10 or G0Z-0.85
Hood

I tried G0Z-0.85 but the Z-Axis kept on going. I need the command to move Z-Axis 0.85 inch down because I had to make 60 steam locomotives and 60 cabooses for 30 centerpieces at the program for mentally handicappied people. I cut out two locomotives and two cabooses from 12 inches by 16 inches pieces of 1/4" birch plywood before changing pieces of plywood because my CNC router has working area of 17 inches by 20 inches. If I had a CNC router with working area of 24 inches by 48 inches, I would cut out 24 locomotives and 24 cabooses before changing pieces of plywood. When my CNC router is finished cutting out locomotives and cabooses, the Z-Axis moves 0.85 inch up and I need to move Z-Axis 0.85 inch down after changing pieces of plywood. I had to call up the jogging control (TAB key) to move Z-Axis 0.85 inch down.

Sounds like you may have your axis moving the wrong way.
 Do you have home switches?  If so where is your home position in relation to each axis, ie is Z fully up? X fully left? Y fully towards you?

Hood

I have home switches on X-Axis, Y-Axis, and Z-Axis. X-Axis and Y-Axis home position is at 0,0 which is on the left corner of my CNC router nearest me. Z-Axis "home" position is at its upper position. Is there something that I place on workpieces and move Z-Axis until the router bit touches the workpiece and stop?

Do you have Auto Zero enabled in Homing and Limits?
 If so then when you home the machine coordinates should all be zero. You can then jog each axis to your materials zero position and press the Zero Axis buttons for X Y and Z. This sets the work offset as X0 Y0 Z0, the Z zero is normally the top of your material thus if you command a G0Z1 it should move to 1 in above your material, commanding a Z negative move would vbe cutting into your work.

Hood

Auto Zero is enabled. How do I reset machine coordinates to 0,0,0 after homing the machine if something goes wrong with my machine? For example, I let the stepper motor move Z-Axis too far and I hear the "grinding" noise.

If you home the machine the X Y and Z will be zeroed in Machine coordinates. If previously you have moved to your workpiece and pressed the zero button at the side of each DRO you have set up a work offset, you can verify this by pressing the machine coords button this will display the machine coords which will be the distance you have just moveed each axis, press the button again and you will be back to displaying your work offset coordinates (x0,y0z0)
 If for any reason you have lost position you just home again and then when you press the Go to Zero button it will move back to your Work offset X0Y0Z0.
 When you shut Mach down it will probably ask if you want to save the work offsets, if you choose Yes then the next time you switch on you Home then you can go back to the same X0Y0Z0 in work offset.

Hood

Isn't there a command operator that specifies that the position move is incremental, not absolute?
Joe

G0 G91 Z-.85 should do it
make sure to do a G90 after it it's not in the program

Mark