I plan to use two motors to drive the Y axis.

My current Y axis uses pins 4 and 5 for the Step and Dir in Mach3.

Since the second drive will be a slave drive I will assume it will be the A axis.

Do I use the same 4 and 5 pins for the A axis or I have to use different pins?

You use different pins.
Hood

One motor along with the 48” long acme has been installed on the Y axis and the backlash nut has been secured to the gantry and now I’m starting to install the second drive for the same axis on the other side.

I will assume that since Mach3 will drive both Y axes motors at the same time, the backlash nuts must be exactly in the same distance from lets say the front of the machine so the gantry will be square to the table surface. I plan to pick the flange edge on the backlash nut and measure the distance from one end of the machine and assure that this distance will be the same on the other side.

I never installed a two drives on the Y axis and wonder if there is anything that I should be aware

You really should install a home switch on each side. That way Mach3 will automatically square the gantry. You just need to make the switch mounting adjustable, as the switch position will dictate the squareness of the gantry

I don’t understand Gerry, do you mean a limit switch?

Can you please explain?

When you "reference" the machine, Mach3 travels to the home switch, and sets the axis to machine coordinate zero when it hits the switch.

With slaved motors, each motor can move to it's own switch independently. So if the gantry is out of square, each side will move until it hits it's switch, squaring the gantry back up.

A home switch can be used as a limit switch, and vise versa.

Thanks Gerry I think I understand; I will install a limit switch on each side but I will have to read the manual to see how this works.

I noticed in the manual 4.5.4 and 5.6.1 has a good description about the home / limit switches and I will have to study this carefully to understand

Is there another way to square the gantry to the table top?

If the machine is square, move it all the way to one end against the stops.

The machine is under construction Gerry and at this stage I was wondering if there is another way to assure that the gantry is square to the table top.

After my build is done, the motors are connected and the home switches are installed then I guess I can fine tune the alignment.

If I’m doing something wrong, pls let me know