You will need a “long reach” tool.
Garr makes some (in quite small diameters which are really fragile, DAMHIKT).
Kodiak is one brand which has been mentioned in earlier discussions — from the Shapeoko wiki Endmills page:
- 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]
- LMT Onsrud 63-776 Solid Carbide Upcut Spiral O Flute Cutting Tool, Inch, Uncoated (Bright) Finish, 21 Degree Helix, 1 Flute, 3.0000" Overall Length, 0.2500" Cutting Diameter, 0.2500" Shank Diameter: Spiral Router Bits: Amazon.com: Industrial & Scientific [Shapeoko3 2" depth HDPE - #4 by cod65]
- Carbide 2-flute deep-reach end mills for wood and plastics — intended for wax
- Kodiak Cutting Tools KCT137388 USA Made Premium Solid Carbide End Mill, Extra Long, 2 Flute, 1/4", 1-1/2" Length of Cut, 4" Overall Length: Straight Router Bits: Amazon.com: Industrial & Scientific [Redirecting...]
- Amana [Let's cut 3" deep holes! - #2 by CNCInspiration]
- Redline [Let's cut 3" deep holes! - #3 by DanStory]
- 1/16