Proper GD&T for cylindrical part with mounting pins
Proper GD&T for cylindrical part with mounting pins
(OP)
I'm having to make changes to a cylindrical part used for calibration and have issues with the current GD&T methods used. For one, datum A was palced on a centerline, which I disapprove of. For another, the current schema doesn't address critical features for manufacturing of the gauge and its ultimate use.
The rod is 211mm long, 10 mm in diameter. Each end is machined back about 14mm in length, 5mm in diameter, and then grooved at 4mm from each end to provide a place for hanging a calibrated weight. There are two 3mm dia pins (25mm long) mounted 40mm apart (basic) and centered on the rod. These pins mount the rod to a dynomometer and IMO should be datums (they are not.
I'm rusty on my GD&T and so my head is spinning at the prospect of fixing this. I know what NOT to do (the way it's now done) but can't come up with a simple, comprehensive solution. My thinking is that the only datums I need are the two mounting pins, and that all features should be WRT them, ie, using A-B in the datum space of the toelrance block.
The critical features are that if one pin is fixed (mounted) then the other is positioned in relation to it. They should be parallel and "perpendicular" to the body of the rod (one thing I'm not sure how to describe using GD&T). The grooves are critical in that as they deviate from the prescribed location, the calibration deviates as well (we are looking for torque). Straightness of the rod doesn't seem that important.
Any ideas?
The rod is 211mm long, 10 mm in diameter. Each end is machined back about 14mm in length, 5mm in diameter, and then grooved at 4mm from each end to provide a place for hanging a calibrated weight. There are two 3mm dia pins (25mm long) mounted 40mm apart (basic) and centered on the rod. These pins mount the rod to a dynomometer and IMO should be datums (they are not.
I'm rusty on my GD&T and so my head is spinning at the prospect of fixing this. I know what NOT to do (the way it's now done) but can't come up with a simple, comprehensive solution. My thinking is that the only datums I need are the two mounting pins, and that all features should be WRT them, ie, using A-B in the datum space of the toelrance block.
The critical features are that if one pin is fixed (mounted) then the other is positioned in relation to it. They should be parallel and "perpendicular" to the body of the rod (one thing I'm not sure how to describe using GD&T). The grooves are critical in that as they deviate from the prescribed location, the calibration deviates as well (we are looking for torque). Straightness of the rod doesn't seem that important.
Any ideas?





RE: Proper GD&T for cylindrical part with mounting pins
That might serve as apart of the solution.
But, you are right about centerlines NEVER being labelled as datum features.
I fought a similar 'battle' once but the drawing had been around so long that no would acknolege it as being labelled incorrectly.
You might want to consider making the OD of the rod a datum feature and one of the pins and another datum feature and relating WRT those features.
RE: Proper GD&T for cylindrical part with mounting pins
There's no statement on the print for Y14.5M, but i assume 1994 based on the types of datum callouts used.
I'd also considered using the rod body as a datum, but I kept running into circular references and it gave me a headache trying to resolve them. My problem here is I'm not used to dealing with cylinders intersecting other cylinders, nor can I find any examples to reference.
In addition, I tend to go by the advice of my last instructor, ie, to begin applying datums that fixture the part and stop when you have enough to lock it down. In this case, the two mounting pins do the trick and I can't see what making the rod body a datum would add. I'm willing to be taught, though!
RE: Proper GD&T for cylindrical part with mounting pins
You could then make datum A one machined end and datum B the opposite machined end. This would allow you to orient one pin to the common axis (A-B). Label this pin as datum C. From there, locate the other pin and the grooves relative to A-B and C.
I'm not sure if this is exactly what you want, but it is the first approach that came to mind.
RE: Proper GD&T for cylindrical part with mounting pins
The centerline datum was the first to go. However, you're right in that a cylindrical axis will allow me to fix the first pin so I made the main body diameter (rather than the machined ends-- I'll get to why) datum A. I then positioned the left pin WRT datum A and made it datum B. The pin to the right of the centerline is part of a pattern so I used pattern positioning to ensure that pin1 and pin2 move together. Finally, since the pins together fix the calibration tool to the dynomometer, I toleranced the machined ends and grooves WRT to B-C. This helps me avoid the circular reference (involving the datum A on the body) that made me nuts earlier. Since I am more concerned about where and in what condition the machined ends wind up, I want it to be with respect to the datums that affix the part. I may also apply straightness to the overall body although that would only be to make it easier for manufacturing to keep the machined ends in spec.
I think this gives us what we want, but I'm still open to contrary input. ; )
RE: Proper GD&T for cylindrical part with mounting pins
RE: Proper GD&T for cylindrical part with mounting pins
RE: Proper GD&T for cylindrical part with mounting pins
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Proper GD&T for cylindrical part with mounting pins
The previous designer's contention was that he needed datum A on a centerline to keep the grooves symmetrical. It took me some time to finally demonstrate to him that the grooves should be symmetrical about the two positioning pins, and that the centerline was essentially immaterial. I say this because the machined ends could be at one or the other extreme end of tolerance and throw the centerline off, and my part doesn't care-- the grooves are the critical feature. The light bulb finally went on for him yesterday. ; )
As to ringman's point, I agree, but we lack true PDM at the moment. That includes control of standards as well as robust checking. I was PDM administrator in a former life and would love to be thus involved with my present employer, but they already have a guy supposedly doing it... the less said about that the better. ; )
RE: Proper GD&T for cylindrical part with mounting pins
RE: Proper GD&T for cylindrical part with mounting pins
RE: Proper GD&T for cylindrical part with mounting pins
RE: Proper GD&T for cylindrical part with mounting pins
Sorry about that.
RE: Proper GD&T for cylindrical part with mounting pins