Choosing the Right Fastener

An important first step in designing with T-slot extrusion is to decide which form of fastener to use: anchor fastener, end fastener, joining plate or Drill thru and counterbore. Each has certain advantages.

 

Anchor fasteners

  • Strong and moveable connection between two extrusions connected at right angles.
  • Requires a counter-bore.
  • Available as single or double fasteners.
  • Fastener does not protrude outside the T-slot.
  • Requires a Ball-End Hex Wrench to tighten. It’s somewhat possible to tighten without it, but you will regret it.
  • Consideration is needed when using these in tight spaces, clearance required for Hex wrench. (picture 1 AF.)
  • Single anchor fasteners are not good at handling torsion loads. Double anchor fasteners are fine.
  • While anchor fasteners allow you to easily reposition the extrusion, it is not suitable to modify the length of the extrusion itself in the field (assuming  anchor fasteners are used on both ends). The distance of the counterbore from the end is critical, so if you need to shorten the extrusion less than 1.25” (15 Series), the existing counterbore will interfere with a new counterbore.
  • Requires an end mill to machine counterbores or at the very least a drill press with a vise. Simply put, you need more than hand tools to do this machining.
  • With single anchor fasteners you can drop-in or remove a piece of extrusion without needing to disassemble the rest of the frame.

 

End fasteners

  • Virtually the same cost as a single anchor fastener, even when including the cost of drilling and tapping an access hole.
  • Creates a strong joint because more of the surface of the two mating profiles are under clamp load.
  • Offers more resistance to straight and torsional moment loads than a single anchor fastener.
  • Due to the tabs on the end fastener, the joint is less subject to loosening due to twisting.
  • No measuring required once the access hole is drilled because the access hole must line up with the tapped hole in the end of the extrusion to be joined.
  • Easy to assemble.
  • Available as single and double fasteners.
  • You cannot “drop-in” extrusions like you can with anchor fasteners.
  • Fabrication and field modifications can be made with hand tools (drill, hand tap).
  • Can be fastened with standard hex wrenches (ball-end not required)
  • Not able to join 3 pieces in a “ + ” Shape, with the horizontal pieces the same distance from the end of the vertical, since the horizontal extrusion on one side will block insertion of the hex wrench into the same access hole when attempting to fasten the other extrusion from the opposite side.
  • While access holes are mostly hidden in the Tslot and the fastener itself is concealed, some may find the hole unappealing. This is particularly the case on closed sided or quarter round profiles.
  • Be sure to use a non-ball-end T-Handle wrench with end fasteners (and really any button head screws). Button heads have a shallower Hex slot than socket heads (as used in anchor fasteners). Because of this, they can be more susceptible to stripping the head. Ball-end wrenches seem to wear out quicker, which in turn will increase the likelihood that the wrench will strip the bolt head.

 

Joining plates

  • No machining required, but plates require more screws so assembly time is longer.
  • Unlike anchor or end fasteners, joining plates attach to the sides of extrusions, which makes for a less attractive joint.
  • Typically more expensive than anchor or end fasteners because of the additional assembly labor.
  • In applications subject to twisting or torque loads, joining plates are stronger than end or anchor fasteners.

 

 

Drill thru and counterbore

  • Similar to an end fastener, but provides drop-in capability (in the sense that you don’t have to assemble from the inside-out).
  • Not as good as end fasteners for torsional loads.
  • While end fasteners provide a strong connecton, with enough force there is potential to cause the end fastener to slide down the tslot it is attached to. With a Drill thru and counterbore, the bolt goes through the entire extrusion and threads into the end of the other. These are good for high load applications, since the bolt would have to shear in half or tear out the threads in order to fail.