The D-Series 420, 600 and 840 all have a max Z height of 140mm (5.51"). However this does not represent the maximum height of material you can process. From this height you need to subtract the spindle collet nut stick out as well as the length of the cutting tool you are using.
The following represents the spindle and spindle nut stick out you get on a D-Series machine with the HF-500 and MM-1000 spindles
NOTE: This is the case with just about every CNC machine on the market, not just STEPCRAFT CNC. Most all CNC machines will have spindle and spindle nut/tool holder stick out that takes away from the usable Z height.
So if you take into consideration the MM-1000 with dual spacers, you would have a total Z height of 140mm (5.51") minus 26.34mm (1.04") leaving you with a new max Z height of 113.66mm (4.47").
Now depending on what you intend to do, you can use this new value to plan your project. If you want to cut all the way through material, you would need to have a tool that is long enough, which you would further subtract from the new Z height value.
Example 1: New Z Value is 113.66mm (4.47"), if you divide that number in half, that would mean you can use a piece of material that is 56.83mm (2.23") thick and the tool would have to stick out 56.83mm (2.23") as well.
NOTE: This would also mean that the cutting height of the tool would have to be a min of 56.83mm (2.23") or you run the risk of rubbing the shank of the tool on the material.
Example 2: If you intend to only engrave on top of a piece of material, then your tool stick out can be much less. If you use an engraving blank that sticks out say 12mm from the bottom of the collet nut, then that means you can have a max material height of 101.66mm (4").
NOTE: please keep in mind you should also allow for a couple 2-3mm of Z height clearance for rapid, non-cutting moves on the CNC as well. So you would subtract another 2-3mm from the above numbers in a practical application.
TWO SIDED MACHINING - For thicker material
One way that you can increase your max material thickness that you can cut through is using double sided machining.
Example: You can take your new Z value of 113.66mm (4.47") and divide that into thirds. This would mean you can have a material thickness of 75.33mm (2.96"), or two-thirds and using a cutter with a length of 38mm (1.49") you can cut one side of the project and then flip it over and cut the other side - so you are using a shorter cutter to cut through a thicker material by cutting each side.
A surfacing bit typically has a larger than normal cutting diameter (1 to 3 inches) and are used to flatten or level uneven surfaces on your project material. They are also used to take the thickness of a material down to a final required size. For example, you might have a piece of rough cut lumber that measures 1 inch in thickness but you need it to be 3/4 inch thick. If you don't have a surface planer in your shop then you can use a surfacing bit with a pocket tool path to plane the material down.
Another common use for surfacing bits is to clean up and level a spoil board. After continuous use your spoil board's surface will have a lot of cuts in it and you might want to clean it up so it is perfectly smooth again. A surfacing bit is perfect for this, especially with machines where the spoil board is physically attached to the machine - you might want to prolong the use of it for as long as you can before replacing.
Leveling your spoil board is also important. If you are looking to engrave .005" into a piece of metal and your bed is off by a couple thousandths from one side to the other or front to back, then the thickness of your engraved lines will vary. By using a surfacing bit, you will ensure that your spoil board is perfectly level in relation to your gantry which will result in a uniform engraving.
Note: Because of the larger diameter of a surfacing bit, it is important to note that you can not take as deep of passes or run as fast as you might with smaller end mills on most machines. Consult your machine's manufacturer for assistance with proper speeds and feeds for surfacing bits.