【转帖】no [ - c - ] datum on engine bracke

yang686526

no [ - c - ] datum on engine bracket?
every hole or slot is toleranced to [-a-],[-b-]. [-b-] is a slot. [-a-] is a plane perp to the slot. how do i align the part for layout purposes?  the part is 8 times longer then the [-b-] slot.     
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well, you should have a teriary datum. all dimensions should come from the centre of b datum with a tertiary datum included for orientation. in your present situation, you should align the sides of the slot for orientation but there is a big chance for error here especially since the part is quite long.
it would be better if the hole was in a positional tolerances in mmc and the slot was also in mmc. now you could use a checking fixture (functional gauge) where there would be a pin the shape of the datum b slot but in a mmc size and theoretically oriented to the hole. place the part over the datum b pin and one will see a certain slop. the positional checking pin for will be placed in the hole to confirm its location.
dave d.
thanks dave,
appreciate the function gage info. i do have mmc. for capability studies of machined hole positions i plan to set it up on the cmm rfs for the variable readings. maybe if the [-b-] slot locater had 2 spheres with a spring in between forcing them apart, but i just don't think i'll pass an under 10% r&r with that.
thanks again,
gary  
love to see how you do capability studies on positional tolerances since i know it can't be done. it is all hocus-pocus and make believe. i hope to have something online from a presentation i make to an asq group on why it can't be done properly.
as far as the 10% r & r value on critical characteristis, one must re  
joeblock:
i finally figured out how to do this. i have enclosed a web site reflecting capability studies on positional tolerances and how it just doesn't work out. this is a part of a presentation that i made to an asq group in one of their dinner meetings.
see if it makes any sense to you. i hope that this will help you.