GD&T: TP on a form vs. PROFILE?

[dfiola66]

I've got a feature control frame that is called to a form surface and they want it held to a TP of .6mm max material to A. That's it. No other datums other than -A- (which is a surface).

Question is why use true position instead of profile? What's the difference? By my way of thinking, the form should be held to a profile tolerance of .6mm (+/-.3mm) max material to ABC. This controls the form and location of the surface (which is a weld surface).

Any thoughts?

[VONDY]

A print would be nice for us to view.

[Mike N.]

By my way of thinking, the form should be held to a profile tolerance of .6mm (+/-.3mm) max material to ABC. This controls the form and location of the surface (which is a weld surface).

Any thoughts?

I agree with you completely. I would make the cmm prog as a profile (form and location). I too have seen prints with that same strange way of thinking

[dfiola66]

A print would be nice for us to view.

here it is...toyota if it matters..

[tking]

Dave,

I agree as well. I would add that your surface if specified with a basic dimension has no feature of size with a tolerance from which determine your material bonus. So, is it an open dimension with tolerance or a basic that defines the surface nominal location?

No print in your previous post!

TK

[bob mappes]

off on a bit of a tangent.........

serveral things come in to play

1) my father being a retired mechanical engineer told me how in college they were only required to take one semester of drafting.......back in the day engineers did not do their own drawings...I doubt that they get any more training in todays world

2) when I started in the machining trade (30 years back) I took two years of drafting courses in order to understand drawings better......GD&T was part of the classes. Our instructor had a direct line with an engineer at LLNL who was on the GD&T committee at that time, so we had instruction on future releases as well as current standards...it was a great learning experience.

so in conclusion...... most all the drawings you see that callouts are "weird" are from engineers who don't know how to draw and worse yet, don't know how to apply GD&T....so they muddle through thinking they can put down on paper what they think is correct and no one challenges them....we just have to decipher what it is that they really want or rather....need

[kbotta]

Phsycic's we are. I see in the very near future...a major cycle in the earths rotation.

[NikoM]

I've got a feature control frame that is called to a form surface and they want it held to a TP of .6mm max material to A. That's it. No other datums other than -A- (which is a surface).

Question is why use true position instead of profile? What's the difference? By my way of thinking, the form should be held to a profile tolerance of .6mm (+/-.3mm) max material to ABC. This controls the form and location of the surface (which is a weld surface).

Any thoughts?

I can not find Datum A on the drawing.
TP can be location (linear TP) of average surface, but surface can have profile to it self (like flatness). In that case profile to datum is combination of position and profile to it self.

[dfiola66]

Dave,

I agree as well. I would add that your surface if specified with a basic dimension has no feature of size with a tolerance from which determine your material bonus. So, is it an open dimension with tolerance or a basic that defines the surface nominal location?

No print in your previous post!

TK

at the top of the print (added), you can see (barely) a dimension called to the pierce hole (datum B for the part, 119.4 dim). It is not a basic and the part is toleranced to the Toyota ptc standard (based on dimension ranges like 6-30 =___, 30 to 120=___, and so on).

[dfiola66]

I can not find Datum A on the drawing.
TP can be location (linear TP) of average surface, but surface can have profile to it self (like flatness). In that case profile to datum is combination of position and profile to it self.

datum -A- on the drawing is the surface the pierce holes are in. The surface with the TP callout is roughly 90* from that surface.

Is there a difference between linear TP and profile of the form (form and location)? My little mind cannot come up with one and profile is so universally used, I just cannot figure out why they'd opt for TP vs. Profile.

[craiger_ny]

When I started here everything was TP even complex surfaces. I asked the same questions. I found out it was simply because they do not know better.like Bob Mappes said they get a 3 credit hour crash course on it drawing of which the emphasis is usually line weights, font size and very little if any "smart" dimensioning.

[dfiola66]

so they muddle through thinking they can put down on paper what they think is correct and no one challenges them....we just have to decipher what it is that they really want or rather....need

Ah....yes....good ol' CIPHERIN'!

[tking]

Ah....yes....good ol' CIPHERIN'!

So, here's my cipher of the designers intent:

1) the surface must meet the 119.4 +/- whatever

2) as long as it meets 1) above the surface is further constrained to a deviation of no more than .6 from lowest reading on the surface to highest

3) the twit screwed up with the "M" 'cause he doesn't understand it's proper use

.02

TK

[Jan d.]

This call-out must be some space alien trying to communicate with us earthlings...

I have never run across this one before and I can not figure out what the intend was, other than profile.

As is, it makes no sense at all to me either.



Jan.

[dfiola66]

Thanks all for the replies. I think like you in that it should be profile. Also, forgot to mention we have attribute gages in house for these. The surface is checked with a .6mm total tolerance.

Chalk another one up to experience....or lack thereof!