lspotts38
Aluminum
- Joined
- Mar 7, 2012
- Location
- Northcentral Pennsylvania
Bit of a rant/looking for information here. I consistently find myself/processes that I have developed at the mercy/subjectivity of the inspection processes that are applied to the produced parts. I’m pretty sure that everyone that is here (in the forum, reading this thread, gives two craps about what they make) and is reasonably sure that what they make is a good part. My current issue is with the shifting application of part verification. I’m currently split between make it to the print, and I’ll make it the same every time.
Bit of context, issue number one was a drill point calculation, I figured a drill point a for a 118, drill was 135. Drill depth dimension was to the shoulder, as a practice we have been setting drills to the tip, so the P/A becacme important. Result was scrapped parts, the drill depth at full diameter was deemed to be be important, and the drilled depth ended being too deep by .050 on a .328 diameter. Was supposed to be verified by an inspector secondarily, he misinterpreted the drawing and ended up not catching that the drill depth was too deep. I’ll share some responsibility on this one, I miscalculated the depth based on a different point angle. But, I’ll also say that given a 135 degree P/A, the calculated drill depth would have still been out of spec based on the drill point length advertised by the drill manuf (standard hi-perf drill GP drill from Sandvik), something they do also reduces the effective point length. Issue is to me that the feature was to be verified secondarily, was mis interpreted and was not evidient until part specific gaging was available.
Second example is same part, different feature. Feature is a casting location, casting locators as related to machined features. Casting roughly concentric to a round cast feature, with the secondary and tertiary locators having an offset to the implied axis of the casting. Casting location was called to the secondary and tertiary locators specifically. Near as I can tell, what ends up happening is that the CMM needs/wants a primary datum to call out to given the absence of a specified primary datum, and has to construct a “perfect form” primary datum to calculate the dimensioms to the secondary and tertiary locating points. Casting location is positional of .060, and it appears that the primary datum being more than .010 out of flat negatively affects the positional accuracy, given the projection of the secondary and tertiary datums from the “A” datum, which IMO is way tighter than the .060 allowable to the secondary and tertiary.
Rant on. All of the drill point depths were within .001, probably less than that with the accuracy of the gaging method. After fixing that, I was inconsistently getting a position of cast features to machined, and didn’t understand how the CMM was <seemingly> arbitrarily referencing a phantom measurement and application of the primary datum to other measurements, even tho they were not specified, nor were included in the specification for the features that were measured “out of spec”. So I’m of the position that I will make the part the same every time, which I have demonstrated given the drill point case (incorrectly, exactly wrong) to the application of datum specification (not wrong, but not given all contributing factors).
Rant off. I need a sanity check. Five axis parts, two operations. Fairly significant quantities per year, not automotive but more than job shop. I don’t intend on making mistakes on drill point calculations on one tool of the 40 tools per part across 30 machined that are of similar conplexity, I think it happens. I also dont think I should be needing to delve deep into the datum plane construction of PCDMIS, but they had similar escalations of non-conformance. What do some of you guys do? I don’t have a problem with admitting and fixing an issue, given some reasonable investigation as to the justification of the original decision, but dammit if I’m not getting sick of changing machining programs based on changing inspection methodologies.
Bit of context, issue number one was a drill point calculation, I figured a drill point a for a 118, drill was 135. Drill depth dimension was to the shoulder, as a practice we have been setting drills to the tip, so the P/A becacme important. Result was scrapped parts, the drill depth at full diameter was deemed to be be important, and the drilled depth ended being too deep by .050 on a .328 diameter. Was supposed to be verified by an inspector secondarily, he misinterpreted the drawing and ended up not catching that the drill depth was too deep. I’ll share some responsibility on this one, I miscalculated the depth based on a different point angle. But, I’ll also say that given a 135 degree P/A, the calculated drill depth would have still been out of spec based on the drill point length advertised by the drill manuf (standard hi-perf drill GP drill from Sandvik), something they do also reduces the effective point length. Issue is to me that the feature was to be verified secondarily, was mis interpreted and was not evidient until part specific gaging was available.
Second example is same part, different feature. Feature is a casting location, casting locators as related to machined features. Casting roughly concentric to a round cast feature, with the secondary and tertiary locators having an offset to the implied axis of the casting. Casting location was called to the secondary and tertiary locators specifically. Near as I can tell, what ends up happening is that the CMM needs/wants a primary datum to call out to given the absence of a specified primary datum, and has to construct a “perfect form” primary datum to calculate the dimensioms to the secondary and tertiary locating points. Casting location is positional of .060, and it appears that the primary datum being more than .010 out of flat negatively affects the positional accuracy, given the projection of the secondary and tertiary datums from the “A” datum, which IMO is way tighter than the .060 allowable to the secondary and tertiary.
Rant on. All of the drill point depths were within .001, probably less than that with the accuracy of the gaging method. After fixing that, I was inconsistently getting a position of cast features to machined, and didn’t understand how the CMM was <seemingly> arbitrarily referencing a phantom measurement and application of the primary datum to other measurements, even tho they were not specified, nor were included in the specification for the features that were measured “out of spec”. So I’m of the position that I will make the part the same every time, which I have demonstrated given the drill point case (incorrectly, exactly wrong) to the application of datum specification (not wrong, but not given all contributing factors).
Rant off. I need a sanity check. Five axis parts, two operations. Fairly significant quantities per year, not automotive but more than job shop. I don’t intend on making mistakes on drill point calculations on one tool of the 40 tools per part across 30 machined that are of similar conplexity, I think it happens. I also dont think I should be needing to delve deep into the datum plane construction of PCDMIS, but they had similar escalations of non-conformance. What do some of you guys do? I don’t have a problem with admitting and fixing an issue, given some reasonable investigation as to the justification of the original decision, but dammit if I’m not getting sick of changing machining programs based on changing inspection methodologies.