Many of the drawings I work with are for molds, so I need my part to be perfectly aligned on the Z plane.

I’m wondering what command I should use to align my objects onto the Z plane.

I have attached an example of an object that I have purposely moved off center on the Z plane.

I used the Mass Properties / Area Centroid to find the center point, but not sure how to take the next steps for centering it.

Also a follow up question… when I look at the detail for that point that was created it doesn’t show the complete number ( it has a letter inserted ) Just curious why it doesn’t show the complete number?
Here is the coordinates it shows ( notice it put that “e” into the coordinate ) [ -0.312961,1.39878e-17,-0.0921533 ]

You need to be in the Top or Perspective viewport where the default Construction Plane coordinates line up with the World coordinates.

Then

start the Move command

Select the objects to be Moved

to finish selection

With the Point Osnap turned on, select the centroid point as the point to move from

type “0” to Move them to the 0,0,0 coordinate origin

This basic operation is covered in the tutorials.
Start with the getting started videos on the Learn page, then go through the progressive tutorials in the User’s Guide.

Your “Welcome to Rhino” email message included these suggestions.

1,39878e-17 means that the actual number has 17 zeros before the number 1,398 78, with the first zero being the full number before the decimal comma, and the next 16 zeros being placed after the latter. The minus after the “e” letter means that the number is smaller than 1. With other words, 1,39878e-17 is equal to 0,000 000 000 000 000 013 987 8. Note that the first digit “1” is at the 17th place after the decimal comma. This is why it ends with -17.

If the number was 1,39878e17 (without a minus after “e”), it would add 16 zeros after “8” (the last digit of the full number 1,39878) or = 1 398 780 000 000 000 000 000.

I may be wrong, but this is what I remember from the mathematics class in the high school.

As for aligning the object to the ground plane (Z=0), it’s quite simple. Just do the following:

While in Perspective view, draw a point somewhere at a random location. It should automatically be placed on Z=0 (if you use a custom CPlane, the point will align with its own Z=0).

Select the object(s) you want to align to the ground level.

Run the “Bounding box” command ("! _BoundingBox" or “! _BoundingBox pause CPlane” to orient the latter to the current custom CPlane). That will create a simple box covering the overall size of the selected object(s). It’s also good to quickly check the exact size of the 3d model.

Use “Move” with the “Vertical” option ("! _Move _Pause _Vertical"), then pick one of the bottom edges of the bounding box and snap it to the point you created earlier. Then delete the point and the bounding box.

I’m following your instructions… but in my example I don’t know what my exact X,Y, coordinates are because my Y coordinate is not showing the complete number. ( it has an “e” inserted into the number )

If you look at the detail for that centroid point it shows it as: 1.39878e-17

That’s scientific notation.
The e indicates an exponent.
In this case -17.
For all practical purposes, that number means Zero.
0.00000000000000000139878
It is not possible to machine anything to that accuracy.
It’s Zero