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Well, I would post this in the TIPS & DESCRIPTIONS section, but there isn't one (yet?).
SNAP POINTS:
1) Not a great idea to use for ITERATIVE alignment points
2) Great idea for all other VECTOR points in the program
Snap Points simulate a perfect machine measuring a vector point, the perfect machine being able to STAY exactly on the approach vector, not deviating by as much as a micron. Snap points are a short cut for something YOU could do yourself with about 6 more steps.
a) Vector point nominal IJK used to create a 3-D line going through the nominals of the vector point
b) Plane perpendicular to the 3-D line is constructed THROUGH the actual measured point
c) 3-D line and PERP. Plane are intersected giving the reported XYZ location of the point. This puts the point EXACTLY on the nominal vector line eminating from the nominal XYZ location of the point.
By using SNAP point, the formula SQRT(Xdev^2+Ydev^2+Zdev^2) will match exactly the "T" axis as reported by Pcmid
By NOT using SNAP, you must use Xdev*I+Ydev*J+Zdev*K to get the matching "T" axis value.
Both formulas are correct for the RADIUS of a SPHERE, which is really what the "T" value will represent, the distance from the center (XYZ NOMS) to a point on the surface (XYZ ACTUALS). HOWEVER, if NOT using SNAP, the ACTUALS will not match due to the 'drift' of the CMM, thus requiring you to use the VECTORS (IJK) of the point in the calculations to determine the "T" value.
GAP ONLY
Gap only acts like a SNAP POINT for edge points. When doing edge points, ESPECIALLY on sheet metal, you SHOULD be taking a surface sample hit so you can control the depth of the hit on the edge. There is shear and breakage on the edge of sheet metal, you SHOULD be touching on the shear portion.
To enable you to do this, time after time after time, WITHOUT changing the program, use a surface sample hit and the proper depth (small value if the shear is on the 'top', a value = (stock thickness - a small amount) if the shear is on the 'bottom'. Once you determine the proper depth to hit the shear everytime, you need to ensure that the CMM knows where the surface of the part is, that is what the surface sample is for.
To report the edge point, you CAN do XYZ & T and T will give you the (T)otal amount the edge point is off nominal by, along the edge vector. HOWEVER, if using surface sample hits, the XYZ values will NOT jive with the "T" value. I almost always (99.995% of the time) will report the "S" axis at the same time, this gives you the SURFACE deviation (like the "T" of a vector point) as well as the (T)otal edge deviation and eliminates the need to also take a vector point (2 for 1, eh?).
IF you want to report JUST the trim edge itself, you can use GAP ONLY (in the FEATURE LOCATION dimension window. What this will do is allow you to use the surface sample hit so you can control the depth of the edge touch, but it will then 'project' the actual point BACK INTO THE NOMINAL surface and then report the XYZT of the edge point AS IF THE SURFACE CHECKED ABSOLUTELY PERFECT, thus giving you only the GAP, as if you were doing a feeler check to a hard gage.
It sure would be NICE if there were a section set up for this kind of thing. If the TITLE described EXACTLY what the post was dealing with it would make it MUCH easier for newbies to find QUICK easy help (since the search don't work so well). This could be the first such post in a TIPS and DESCRIPTIONS sections
SNAP POINTS:
1) Not a great idea to use for ITERATIVE alignment points
2) Great idea for all other VECTOR points in the program
Snap Points simulate a perfect machine measuring a vector point, the perfect machine being able to STAY exactly on the approach vector, not deviating by as much as a micron. Snap points are a short cut for something YOU could do yourself with about 6 more steps.
a) Vector point nominal IJK used to create a 3-D line going through the nominals of the vector point
b) Plane perpendicular to the 3-D line is constructed THROUGH the actual measured point
c) 3-D line and PERP. Plane are intersected giving the reported XYZ location of the point. This puts the point EXACTLY on the nominal vector line eminating from the nominal XYZ location of the point.
By using SNAP point, the formula SQRT(Xdev^2+Ydev^2+Zdev^2) will match exactly the "T" axis as reported by Pcmid
By NOT using SNAP, you must use Xdev*I+Ydev*J+Zdev*K to get the matching "T" axis value.
Both formulas are correct for the RADIUS of a SPHERE, which is really what the "T" value will represent, the distance from the center (XYZ NOMS) to a point on the surface (XYZ ACTUALS). HOWEVER, if NOT using SNAP, the ACTUALS will not match due to the 'drift' of the CMM, thus requiring you to use the VECTORS (IJK) of the point in the calculations to determine the "T" value.
GAP ONLY
Gap only acts like a SNAP POINT for edge points. When doing edge points, ESPECIALLY on sheet metal, you SHOULD be taking a surface sample hit so you can control the depth of the hit on the edge. There is shear and breakage on the edge of sheet metal, you SHOULD be touching on the shear portion.
To enable you to do this, time after time after time, WITHOUT changing the program, use a surface sample hit and the proper depth (small value if the shear is on the 'top', a value = (stock thickness - a small amount) if the shear is on the 'bottom'. Once you determine the proper depth to hit the shear everytime, you need to ensure that the CMM knows where the surface of the part is, that is what the surface sample is for.
To report the edge point, you CAN do XYZ & T and T will give you the (T)otal amount the edge point is off nominal by, along the edge vector. HOWEVER, if using surface sample hits, the XYZ values will NOT jive with the "T" value. I almost always (99.995% of the time) will report the "S" axis at the same time, this gives you the SURFACE deviation (like the "T" of a vector point) as well as the (T)otal edge deviation and eliminates the need to also take a vector point (2 for 1, eh?).
IF you want to report JUST the trim edge itself, you can use GAP ONLY (in the FEATURE LOCATION dimension window. What this will do is allow you to use the surface sample hit so you can control the depth of the edge touch, but it will then 'project' the actual point BACK INTO THE NOMINAL surface and then report the XYZT of the edge point AS IF THE SURFACE CHECKED ABSOLUTELY PERFECT, thus giving you only the GAP, as if you were doing a feeler check to a hard gage.
It sure would be NICE if there were a section set up for this kind of thing. If the TITLE described EXACTLY what the post was dealing with it would make it MUCH easier for newbies to find QUICK easy help (since the search don't work so well). This could be the first such post in a TIPS and DESCRIPTIONS sections
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