What determines max depth of cut I can make?

[WillAdams (William Adams (Carbide 3D))]

The community has some notes on long endmill options at:

https://wiki.shapeoko.com/index.php/Endmills#Long_endmills

• http://www.ebay.com/itm/10pcs-3-175x42mm-Carbide-CNC-Router-Bits-Single-Flute-Spiral-Cutters-End-Mills-/291866010066?hash=item43f491e1d2
• [GARR TOOL - Product Details Garr Tool Series 620MEDP#41030 1/8"x3x1"] — excellent quality, long narrow bit w/ a long cutting length which makes workholding and fixturing much easier, and making through cuts (w/ a suitable number of passes) on 1" material feasible.[Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable] [http://www.carbidedepot.com/detail.aspx?ID=73347 Carbide Depot]
• https://www.banggood.com/3_175mm-2-Flute-Spiral-Bit-Carbide-End-Mill-Router-32mm-CEL-CNC-Cutting-Tool-p-1038960.html [Cutting 3/4" material with 1/8" Endmill - #9 by RichCournoyer]
• Amazon.com [Shapeoko3 2" depth HDPE - #4 by cod65]
• https://www.precisebits.com/products/carbidebits/deepreach125f2f.asp — intended for wax
• Amazon.com [Redirecting...]
• Amana [Let's cut 3" deep holes! - #2 by CNCInspiration]
  • Extra Long CNC Router Bit, Amana Tool 46590, 4" Carving ZrN Coated
  • Extra Long CNC Carving Bit, Amana Tool 46490, Tapered Ball Nose
• Redline [Let's cut 3" deep holes! - #3 by DanStory]
  • 1/4 (.2500) Diameter 3 Flute Single End Square, Carbide Endmill
  • https://www.redlinetools.com/itemdetail/RET7534
• 1/16
  • Spiral CNC Router Bits & Whiteside Router Bits - Bits & Bits [Longer Length of Cut - #17 by nystromd]
  • Amazon.com
  • https://www.ebay.com/itm/254524910002