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No rating yet Subject: Re: [Other] GKH modulus Author: Chris Masterson Date: 2008-05-06 19:10:52Certainly I'm always going to leave a bit of margin in the part to account for uncontrollable errors....actually my biggest concern has less to do with the part itself and more to do with the accuracy of the tools and methods used to confirm that we aren't cheating by being underweight.
On this specific part though I was performing a rework of existing parts to get some quick test pieces at the minimum weight. I incorrectly trusted that the density in the model was valid and ended up with the first couple test pieces being underweight due to the density being set 2% too high at 7.9g/cc. My rework was designed closer to the minimum weight than normal to accomodate tools that were available in house. Obviously, at that point I learned my lesson that I should have been checking the actual pieces before the rework to make sure my assumptions were correct if it was that critical. In the process of checking many pieces it became pretty clear that the value of 7.7g/cc that I finally got off the datasheet was the correct nominal value and fairly consistent to the parts as received. The value on matweb is 7.8g/cc. Not a big difference, but in this one specific case it would have been at least inconvenient. In general, machining tolerances are certainly a far larger consideration in final mass...this part just happens to be so simple and tightly controlled for other reasons that density became a factor.
In the past I have pressed a metallurgist for a steel manufacturer to confirm a steel density and he was willing to go to 3 significant figures suggesting that their variation was less than 1%...however, at that time density turned out to be inconsequential to the issues I was chasing so I never did get confirmation of their internal specification for density variation.
Chris Masterson Toyota Racing Development
Christopher Wright Sent by: xansys-bounces_at_xansys.org 05/06/2008 10:56 AM Please respond to ANSYS User Discussion List
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Subject Re: [Xansys] [XANSYS][Other] GKH modulus
On May 6, 2008, at 12:11 PM, Chris_Masterson_at_Toyota.com wrote:
> The difference is only about 1% but that is sufficient to cause > problems when trying to design as close as possible to minimum weight > rules. I think I can pretty much guarantee that the density and pretty much everything else varies more than 1% over a chunk of material that hasn't been very specially made and shaped. Your best bet is to make it a tad heavy and remove material after the part is built by cutting lightening holes or removing ballast.
Christopher Wright P.E. |"They couldn't hit an elephant at chrisw_at_skypoint.com | this distance" (last words of Gen. .......................................| John Sedgwick, Spotsylvania 1864) http://www.skypoint.com/~chrisw/
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