Datum targets beginner understanding

[sendithard]

I've poked around and seen some auto body panel drafts with datum targets. I'm in a simple part environment, but this interests me. I wanted to ask how the GDT print and possible CMM alignment works with that. If you design the hood of a car you may have points to establish a Z level and origin(3,4,5,6,7 pts lets say), but when you go to make secondary and tertiary datums can those be any angle desired as lond as they lock the part down or do they have to be the typical perpendicular angle block type surface plate manual measurement deal? Thanks. And do prints typically only have the 3-2-1 points or am I seeing the basic examples online wrong. I'm seeing a few examples with only 3 datum A targets to get the primary plane(a perfect plane due to only 3 targets) then only 2 datum B targets to get the line down and then only 1 C to get the point down. That only makes sense to me in the perfect square part world. Was hoping someone could help with my understanding.

 

[Belanger]

The business about 3-2-1 points is only true for all planar datums. For instance, check out Fig. 7-16 in the 2018 standard -- the secondary datum feature might only make contact at one point when it's butted up to its simulator. Similar for Fig. 7-20.

 

[3DDave]

The alignment is done by the CMM to the datum simulators on the fixture, then the part is put on the fixture and restrained if needed. Just as in the example where the tertiary datum feature is at an angle to the secondary datum feature, Fig. 4-7 Inclined Datum Features in the 2009 version, the datums are presumed to be mutually perpendicular even when the datum features are not.


In a flexible part like the upper sheet of a stamped hood, there might be 10 datum targets, each held firm on the fixture. They will have been qualified to some reference surfaces on that fixture that are used by the CMM to align to. If done right those points correspond to features in the fixture that will hold that part and the stiffener in place while they are glued or spot welded or crimped into a single assembly.

 

[sendithard]

Hello again...quick question on these datum targets.

I guess my last hangup with datum targets is when there is a zone listed and/or the basic repeatability. If an engineer lists a part to be inspected with datum targets and there is a diam zone...in my mind there are going to be variation in inspection dimensions from part to part just due to datum target setup. Are the tolerances in these cases large enough/or accounting for this datum target variance or am I not understanding something. I'm kindoff thinking in terms of a larger object, I'm only used to inspecting small parts 2 inch max and I've never used datum targets.

Thanks.

 

[Burunduk]

sendithard,
Could you clarify your last question?
What exactly about the datum target set-up you assume may cause increased variation from part to part, and why?
If physical datum feature simulators are used, you set up the datum target simulators once and establish the datum reference frame from them, and then measure a series of parts using the same setup by positioning each part the same way to the fixture that includes those datum target simulators.

 

[sendithard]

Burunduk,

I don't understand datum target setups b/c I've never seen them.

I understand slapping a metal object on a surface plate and measuring it.

But I don't understand a hood of a car being placed on datum target points which are static....BUT bc the part is so large it's placement on said datum target is not anywhere near exact or repeatable...the REAL hood contact points for each target could be off in space by .100" and how do the measurement tolerances compensate for this???? Is the tolerance of the datum tolerance INCLUDED in the general tolerances to make up for non perfection alignment?

 

[Burunduk]

sendithard,
A car hood will be restrained to the datum target simulators or to some of them, and there will be as many datum target areas as necessary to make sure the part is positioned relative to the datum reference frame in a stable and sufficiently repeatable manner. If you have seen the car hood examples about use of datum targets in the Y14.5 standard - this is what they show (there are two examples in the 2018 edition - figures 7-49 and 7-50. In the 2009 edition it is just 4-42). And yes, in spite of the attempt to maximize repeatability, the tolerances for a large and flexible part such as a car hood are expected to be larger than the tolerances for a small rigid part with orthogonal datum features that do not require datum targets or measurement in restrained condition. The tolerances mainly reflect what is functionally appropriate, but also what's feasible to achieve and hold.

 

[3DDave]

https://www.google.com/search?source=univ&tbm=isch&q=car+hood+inspection+fixture

Each place there is a clamp the place under the clamp is a target.

Here's a small one in use:

https://www.pinterest.com/pin/620933867351974441/

 

[axym]

sendithard,

The idea is that even thought the part is large and flexible, the placement of the part on the set of datum target simulators (fixture components) is repeatable. A detailed note should be specified on the drawing, outlining exactly how the part is to be restrained to the datum target simulators (clamping locations, force, sequence, etc.)

It is true that the contact points on a real part are not exactly the same as the theoretical contact points, but they don't need to be. Ideally, the datum target specification represents how the part will be restrained when assembled.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[sendithard]

Thanks for the help, I figured out my issue from you alls replies.