Change in accuracy from reducing pre-hit distance?

As far as prehit and retract distance goes, it really depends on the machine and parts in question. When I find a part that isn't located that accurately, I might bump the prehit and retract distances up to .25". I might lose a few seconds a part, but the program always works and the machine doesn't stop. As JEFMAN eluded, there is a minimum distance needed to stabilize the machine before the hit. If you go under that distance you will lose repeatability and accuracy. Have they already determined that distance? Have you performed a Gage R&R showing that there is no loss? Are the parts made accurately enough that the program will work for every part every time and the machine won't sit? Is the risk of the machine stopping worth the extra ~1-2 hours of run time per day? Will the reduced run time end with the CMM sitting unused? Is there the potential for a burr on the part? If you run it and there is a burr it might keep going and not cost you time with the longer prehit, but at the lower prehit it could stop and not finish the part. Depending on the parts and industry in question, are a few seconds or minutes savings per part worth the potential risks of a quality escape in the future? On the older machines I program I have found that a drop of prehit distance from .1 to .05 is enough that parts that should not fail will fail. Running the longer distance helps more than it costs. I didn't guess on the prehit and retract distances I use, I tested them and found that they worked better in my situation.

As far as fly mode goes, it doesn't always work as well as intended. In my previous tries of using fly mode I might have saved a few seconds per part, but on some random parts it would sit there and take extra hits on ID holes and just do things that were unexpected, leading to me being called to the machine. The benefit for fly mode on the parts I run was not worth the risk that the program might not work correctly every time. Even in the training that Hexagon gave me the AE said it wasn't worth the time to try it out and that it was more as a feature for their sales dept.

I think you also have to look at the bigger picture. Are the parts being run lights out? Is there someone sitting at the machine all the time? If the machine stops will there be more fallout than if it continuously runs a bit more slowly? I'm all for saving time on the machine and reporting those savings to management to better my situation, but I also know that if a machine I program stops when I'm not there, I am the one that will be questioned for it, not the person running it.

As far as prehit and retract distance goes, it really depends on the machine and parts in question. When I find a part that isn't located that accurately, I might bump the prehit and retract distances up to .25". I might lose a few seconds a part, but the program always works and the machine doesn't stop. As JEFMAN eluded, there is a minimum distance needed to stabilize the machine before the hit. If you go under that distance you will lose repeatability and accuracy. Have they already determined that distance? Have you performed a Gage R&R showing that there is no loss? Are the parts made accurately enough that the program will work for every part every time and the machine won't sit? Is the risk of the machine stopping worth the extra ~1-2 hours of run time per day? Will the reduced run time end with the CMM sitting unused? Is there the potential for a burr on the part? If you run it and there is a burr it might keep going and not cost you time with the longer prehit, but at the lower prehit it could stop and not finish the part. Depending on the parts and industry in question, are a few seconds or minutes savings per part worth the potential risks of a quality escape in the future? On the older machines I program I have found that a drop of prehit distance from .1 to .05 is enough that parts that should not fail will fail. Running the longer distance helps more than it costs. I didn't guess on the prehit and retract distances I use, I tested them and found that they worked better in my situation.

As far as fly mode goes, it doesn't always work as well as intended. In my previous tries of using fly mode I might have saved a few seconds per part, but on some random parts it would sit there and take extra hits on ID holes and just do things that were unexpected, leading to me being called to the machine. The benefit for fly mode on the parts I run was not worth the risk that the program might not work correctly every time. Even in the training that Hexagon gave me the AE said it wasn't worth the time to try it out and that it was more as a feature for their sales dept.

I think you also have to look at the bigger picture. Are the parts being run lights out? Is there someone sitting at the machine all the time? If the machine stops will there be more fallout than if it continuously runs a bit more slowly? I'm all for saving time on the machine and reporting those savings to management to better my situation, but I also know that if a machine I program stops when I'm not there, I am the one that will be questioned for it, not the person running it.