New to CNC and my AltMill 4X4 (about 1.5 years). When attempting to start a relatively tall V-Carve job in GSender, I load the file, then hit start. The Z Axis gantry is positioned near the top of the allowable limit. When I hit start the Z axis moves to the top (quickly) and get an error in GSender, also I notice the following. The Z axis gantry is higher than I can move with the job buttons, and the Z axis zero changes to 0.800 inches (wrong zero). I am afraid to play to much in the homing setting in GSender, but have tried things like moving the bit slighty higher in the collet (more travel for the gantry upon job start), and attempting to delete some of the initail Z axis program commands with the editor. Also played with the magnetic stop position,. Turned of Soft and Hard homing limits in GSender. Zero the bit with the zero block on the flattened work piece surface. Have also pushed the (grayed font to the z-axis gantry height) to see if I can fix. Can I slow down the Z axis moving up so fast when the job starts. Feel like it “slams” up then gives the error code. This cause great frustration when attempting to carve taller projects. Any guidance on how to resolve would be greatly appreciated.
@Buffy Welcome to the group, Buffy.
More info would help; 
- How thick is the “relatively tall” material?
- In VCarve, what is your rapid Z gap above material set to?
- What is your Home Z gap above material set to?
Unless you are concerned about hitting clamps, those 2 values can be set to .25".
- Can you explain more about the Z axis gantry is higher than you can move with the job buttons?
I assume that you are homing the machine before you set your projects XYZ0, yes?
The material is approximately 3.75 inches thick.
The bottom of the Z axis gantry is approximately 4.675 inches clearance between the table bed and the bottom of the gantry.
I am zeroing off the material surface using the zero probe box.
The Altmill does not have this problem on thinner material 1-2 inches thick.
The rapid Z gap above material was set to 0.8 inches per your guidance changing to 0.25 inches.
Think this may fix the issue, I notice that after the machine normally error codes out, the Z axis changes to 0.800 inches (but is no longer zeroed to the material surface). After the error always have to re zero the Z-axis back to the material surface.
The Home Z gap was at 0.2 inches, leaving at 0.2 inches.
Not sure about the homing before starting. I measure physically the center of my project. I bring the V bit to this spot (jogging). I then zero the X and Y axis in Gsender. Then using the Z-axis probe zero the Z-axis to the surface of the material. Then I start the job.
@Buffy I’m pretty sure that this issue is completely owing to the thickness of the materail vs the available travel. (Talk about being obvious, eh?)
If the clearance between the bottom of the Z gantry is 4.675", that means that the clearance between the tip of the bit and the material must be damn near 0". You only have .9" to work with. What is the bit’s projection from the spindle/router? Clearly, it 's less than .9", but what is it?
If that’s correct, the Z axis is pretty much at it’s limit when the bit is zeroed on the surface of the material. So, when it rises in Z at the start, it is hitting its limit and throwing the 0 off.
I’m very new to limit switches. I never had them on my Mk1 and I have turned them off on my new Mk3 (what blasphemy!!) There are members here much more proficient in their use than I am.
FWIW, I would be tempted to turn off the limit switches. You still need to deal with the issue that you don’t have much Z travel available to you, but that can be accommodated by reducing the safe Z values.
I am not saying that you should do this. I am saying that’s what I would do. YMMV. I simply do not see another way to make this work. With luck, someone much more knowledgeable than me will have a better solution.
I’m going to ask the silly question because we’re all human and sometimes miss things that may be obvious.
Is there room to move the spindle up in the mount?
I’m guessing not based on the clearance numbers you mentioned but it doesn’t hurt to ask.