Datum callout question 2

[SeasonLee]

I have some questions about this print, and I tried to make some changes as shown below. The reasons for making these changes are:

• The common datum(A-B) is constructed after the individual feature is designated as the datum feature.
• The existing DRF refers to the datum feature itself, normally we will consider it as a coaxial part, but here is a multiple skewed cylinder.

Do you agree with the changes on the datum callout? Please correct me if I am wrong here.
Thanks for your help.

Season

 

[Burunduk]

If you delete the datum feature qualifying feature control frames, your datum features A and B will be left uncontrolled other than for size.
If you keep those controls, the datum features A and B are controlled relative to a datum reference frame with all 6 degrees of freedom locked, so their mutual location and orientation is under control.

 

[3DDave]

Essentially the practical gauge for the original is two recesses, one for [A] and one for [B ] with each recess basically located and oriented to each other and each having a fixed diameter of 20 + 0.03 + 0.5 = diameter 20.53. The remainder of the part will fit into a saddle with the nominal section of diameter 21 + .1 + 2 = diameter 23.1.

By deleting the FCFs there is no tolerance control on the relative location and orientation of the ends to each other. They could be anywhere and in any orientation to each other.

Coaxiality is more a condition of the nominal case being coaxial and the referring geometric characteristics being runout, which requires an axis.

 

[mfgenggear]

OP
I have experience manufacturing fuel lines for the centaur and atlas programs.
A little history any how they are manufactured.
Far as fabrication fuel lines are straight forward.
I will defer on the dimensioning to Burununduk and 3D Dave.

BUT Measuring can be a pita. Because of tolerance stack up, roundness. Diameter size.
Ends are normally machined. The tubing fabricate as straight tubing , then bent or formed in CNC tube bender.
Diameter control should be in the detail level.
Same as the two components on the ends.

The positional controls then established at
This assembly. The challenge is to design a
Line the the two ends mate. And the body
Clears the obstacles, as not to have interference.
Grab this drawing and discuss with supplier or the shop.
They are the most uptodate if tolerances can be held. I to rusty to make that suggestion.
Do concurrent engineering. And bring a red pen.
There will be changes. Mostly on tolerating.
Don't over tolerance the cost will be expensive.
And parts rejected. Causing MRB.
PS: When fitting , and welding , there will be dimensional change , weld shrink has to be accounted for. Distortion from welding
How it is fitted in a weld Assembly jig will be
Very important.

 

[Burunduk]

BUT Measuring can be a pita.

As 3DDave mentioned, most of those requirements can be verified with a functional go gage.

That is because datum reference A(M)-B(M) is specified at MMB, and the positional tolerances referencing it apply at MMC.

 

[mfgenggear]

As 3DDave mentioned, most of those requirements can be verified with a functional go gage.

That is because datum reference A(M)-B(M) is specified at MMB, and the positional tolerances referencing it apply at MMC.

Burunduk
Please advise how this gage would be design.
It's a tough cookie.
As of my last interface with this It's been a while.
The assembly would be set in the assembly fixture on a large surface plate. And then would
Use mechanical inpection procedure using hiegth gage and lay out. Very intricate and
Tedious.
If there is a CMM large enough makes life much easier. Last time I worked for convair general dynamics the had a CMM large enough.
At that point in time before computers the model shop would build a mock up. As their were under
Engineering. To make sure it would fit up, and
It would clear .

 

[Burunduk]

mfgenggear,
I think there could be a go gage fixture as 3DDave described: two recesses of diameter 20.53 and a saddle of diameter 23.1, following the sections and bends of the tubing. I suppose it's not a cheap fixture, but if enough parts are to be manufactured, it should be worth the investment and a time saver for inspection on the long run. Wouldn't you agree?

 

[3DDave]

 

[mfgenggear]

I am forgetting critical thoughts.
On the assembly fixture or inspection fixture
If done correctly precision holes are machined
On or in the fixture , then removable inspection tooling balls are added for critical points used as a datum reference as they build it and for final
Inspection. This immensely helps with inspection and fabrication. Remember fellas the tubes have to be bent before they are fitted and welded.
The details have to have extra stock at the ends
For final fit up and the details have to be fitted and build from the center out to allow for weld shrink. While this is not the designers responsibility it's informative information.
As long as it meets the final drawing.
One more thought my intention is constructive
Advice. And there may be improvements since my time. But I was a sheet metal and tubing fabricator. And this has been my experience.
We could make straight perfect diameter tube.
That's not and an issue. But after bending
Physics takes it told. Diameters go out of round,
Elliptical shapes happens and more.
Then add weld distortion and slight errors in angles. Dimensions change . Thus not a perfect machined part and has to be tolerance accordingly.

 

[mfgenggear]

Nice mock up but this not precise enough.
While this works for it purpose it can be debated of course. Wood for aerospace would not be allowed. Precision ground tooling plate, precious machined saddles, and as stated before precision tooling balls. But the concept is the same . Ti my opinion this not a gage it is inspection fixture.

 

[Burunduk]

Tolerances for the fixture design should be about 1/10 of the tolerances being verified by the fixture.

 

[mfgenggear]

Tolerances for the fixture design should be about 1/10 of the tolerances being verified by the fixture.

So if my math is correct. 5 mm (.0197) x .1 = .0019 inch , however It can be tighter.
Tooling as standard practice. 0005 inch or less
Other considerations
Overview




The precision required for an inspection fixture depends on the application and the tolerances of the parts being inspected, but generally, it needs to be highly accurate and repeatable, holding parts in the exact same position and orientation every time to ensure reliable measurements.

Here's a more detailed breakdown:

• Why Precision Matters:
  • Accurate Measurements: A well-designed and precise fixture ensures that the part being measured is held in the correct orientation and position without any movement, which is essential for achieving accurate and reliable measurement results.
    
    
  • Repeatability: The fixture must hold each part in the exact same position and orientation every time, eliminating variations and errors and ensuring repeatability in the manufacturing process.
    
    
  • Quality Control: Inspection fixtures are crucial for quality control, allowing for the verification of critical features and dimensions of parts.
    
    
  • Minimizing Deflection: A good inspection fixture must have tight tolerances and minimal deflection to ensure a high level of accuracy without damaging the product.
    
• Factors Influencing Precision Requirements:
  • Part Complexity: More complex parts with several critical features may require multiple fixtures or indexing features within the setup.
    
    
  • Measurement Method: The type of measurement equipment used (CMM, vision system, etc.) will influence the required precision of the fixture.
    
    
  • Tolerances of the Part: The tighter the tolerances of the part being inspected, the higher the precision required of the fixture.
    
    
  • Production Volume: For high-production applications, fixtures may need to be designed for efficient loading and unloading, and potentially multiple stations to optimize runtime.
    
• Best Practices for Fixture Design:
  • Secure and Repeatable Holding: The fixture should hold the part securely and repeatably without distortion.
    
    
  • Clear Markings: Marking the fixture with part number, job, and station numbers ensures fail-proof loading, increasing speed and accuracy.
    
    
  • Modular Design: Modular fixtures can be a practical and cost-effective solution for various quality control requirements.
    
    
  • Reproducibility: Ensure fixture components can be accurately and easily positioned on the base plate to measure different parts.
    
    
  • Virtual Simulations: Perform virtual simulations to detect possible interferences or clashes with other components during operation and ensure functionality.
    
    
  • Digital Verification: Thoroughly verify the design digitally before moving forward with fabrication.

 

[mfgenggear]

mfgenggear,
I think there could be a go gage fixture as 3DDave described: two recesses of diameter 20.53 and a saddle of diameter 23.1, following the sections and bends of the tubing. I suppose it's not a cheap fixture, but if enough parts are to be manufactured, it should be worth the investment and a time saver for inspection on the long run. Wouldn't you agree?

Yes Bununduk
If budgets allows with the bean counters I would design and build an inspection fixture it is a must. For proper final inspection.
I forget critical information and it brought by good questions . And brain storming I am not perfect.

 

[mfgenggear]

This better it appear the saddles material used is tool cast and should be machined. Progress since my time.

 

[mfgenggear]

This picture visually depicts part are in restrained condition. Which is a good idea. If it is permitted by the designer.
In my projects I normally had only restraind at the ends which had flanges. These would be restrained.
All the rest of the assembly if I remember correctly was in the free state. Except supported by saddles. Like I said it worked on the atlas program and it was build this way before my time. And it flew. I worked on projects with new design , larger and heavier pay loads.
Ahh the old days was fun stuff.

 

[SeasonLee]

Thanks for all comments, Burunduk, 3DDave and mfgenggear.

Please ref to the figure below for details, both callouts are legal, but what are the differences between these two callouts (with and without the datum feature qualifying FCF)?


Thanks again for your help.

Season

 

[mfgenggear]

Thanks for all comments, Burnduk, 3DDave and mfgenggear.

Please ref to the figure below for details, both callouts are legal, but what are the differences between these two callouts (with and without the datum feature qualifying FCF)?

View attachment 7318
Thanks again for your help.

Season

In my opinion I don't like true position on shafts. Runout or total runout.
On shafts the two ends are bearings.
And must run to each other. To maintain alignment which is first drawing. It forces the manufacture inspect true position to each other. The second drawing does not.

 

[Burunduk]

Season,
In the second case, there is no control over how off-coaxial the two ends can be. Here is an exaggerated illustration.

 

[SeasonLee]

Thank you, Burunduk
Understood.

Season

 

[Garland23]

I don't mean to sound like a technophobe, but why not just do a pen/pencil sketch and post a scan or picture?
That looks like a lot of work with the mouse or stylus!