All items created are .015" too small

Hi Bill,

Sorry for the delay, work account so I don’t always get notifications over the weekend.

What is happening with this is that by swapping the cut directions we are changing the climb and conventional sides of the cut. In general deflection will be on the conventional side. So if we are cutting the 2 slots in the X then we are looking to see how far off they are from each other in the Y. You need to avoid the start and end of the cuts as they won’t show the full deflection since things are getting up to speed and slowing down. Here’s a horrible exaggerated image of what I mean and what you’re trying to measure.

The lines curve as the force increases while the axis is getting up to your feed. Then you have a flat section that is at your feed. As it slows down to come to a stop the forces lower creating another curve (usually this one is smaller).

In terms of how to measure it there’s a number of ways.

The cheap digital version is using a scanner. Scan the cuts, rotate the middle of the slots to be straight, measure the pixels from one cut to the other. You can then convert the pixels into your choice of measurement by the resolution.

Some people will use a rule and caliper combo, though that’s hard to tell small numbers from and is more prone to error.

Another option is to cut another line either above or below the 2 slots that is the full length of the 2 slots and gap. Then you can measure with calipers between the edges. That slot will also have deflection but it will be true relative to the slots (where you are measuring). There is the issue of having a clean enough cut as if you have any chatter that could effect the measurement.

Hope that help. Let me know if there’s something else.

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If I’m understanding what you did correctly, you should have functionally no real difference between these as your final pass will determine the final dimensional accuracy. When you cut the aggressive and less aggressive cut those would have the biggest difference as they have the largest cutting forces (chipload, pass depth, and stepover). The final pass was only taking the 0.010" plus the potential deflection left from the original making the actual chip and forces quite small (especially when including chip thinning). It is a very valid way to get dimensionally accurate cut though.

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