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Trigger force gauge

Gram gauge

The trigger force gauge can be used to check or set the trigger force of certain Renishaw touch probes.

Trigger force is the amount of pressure applied by the helical compression spring onto the pivotal plate and bearing points to hold the stylus mount in place. The trigger force is preset by Renishaw but can be altered for any of the following reasons:

  • To permit the use of a longer styli on the probe
  • To permit the use of heavier styli on the probe
  • If the preset trigger force has decreased due to probe use
  • If the acceleration of the CMM is causing illegal triggers

NOTE: Changing the probe's trigger force will affect it's measurement performance. It is important to requalify the probe configuration and check the measurement performance of the probe system after any adjustment has been made to the trigger force.

Setting the optimum trigger force maximises probe performance.

Renishaw part number: P-GA01-0001.

Checking trigger force with a Renishaw gram gauge

  1. Ensure that the probe is held firmly in position (preferably on a CMM) and connected to an interface to detect a probe trigger.
  2. Establish the direction of trigger which gives the maximum resistance. The most practical way of doing this is to gently deflect the stylus with a finger, trying different directions (see sketch below). There are three lobes which produce three maximum and three minimum force directions. These can be easily found with a minimum of practice.
  3. Set the gram gauge maximum force indicator to the zero point and place fl at on the CMM table. Move the stylus tip to the same height above the surface as the gram gauge lever.
  4. Slide the gram gauge slowly sideways so that the fl at point on the end of the lever touches the probe stylus ball (ensuring that the probe is defl ected in the low trigger force direction). Very slowly continue to move the gram gauge until the probe triggers, at which point stop immediately, back off the gram gauge and read the maximum force indicator.
  5. Repeat this procedure three or four times to ensure consistent results.

NOTE: Use the following formula to convert gf to Newtons:

Newtons = gf / 100

Trigger force adjustment

The trigger force of a TP1 (S) / TP2-5W / TP6 / TP6A probe is preset by Renishaw at an optimal performance setting, but can be altered if necessary as follows:

  1. Remove the probe from the quill of your CMM.
  2. Insert the correct size hexagonal key (supplied with every probe) into the centre of the shank until you locate a grub screw.
  3. Adjust this grub screw to alter the trigger force of the probe:
  • Clockwise increases the trigger force.
  • Anticlockwise decreases the trigger force.

See also