Substitute for Projected Tolerance?

[sluzzer]

1. I was going through some very old drawings. In a drawing, three separate rectangular blocks are fastened together by floating fasteners. The instruction for position of these fastener holes were given by a separate 'drilling drawing', in which the instruction given was to drill the holes by assembling the blocks together. Can we use Projected tolerance in each block instead of issuing a separate drilling drawing ? Is both the approach control the same thing?

2. If projected tolerance is to control the axis inclination of a hole (for example), which is positioned by positional tolerance, Can we use Perpendicularity to control the axis instead of projected tolerance?

 

[CheckerHater]

Projected tolerance controls the axis of the hole OUTSIDE of the body of the part.

For all it's worth:

 

[fsincox]

Projected tolerances are usually applied to the tapped hole, only, not the clearance holes are all three holes tapped?
Frank

 

[sluzzer]

@CheckerHater

Thanks for the info

@fsincox
Only applied to tapped holes?! Then how we will control the axis of clearance holes? I think we can apply to clearance holes also, whenever we want to control the axis outside of the body.

 

[pmarc]

sluzzer,
If this really is floating fastener case, Frank (fsincox) is right - there is no need to use projected tolerance zone concept at all. In the most common scenario, that is where two clearance holes are fastened together, you have to check whether sizes of inner boundaries (virtual conditions) of both holes are greater than or equal to the maximum major diameter of a fastener. The inner boundaries (virtual conditions) are calculated using following formula:
IB (VC) = MMC size of hole - pos. tol. value at MMC

In your case, since the holes are drilled when all blocks are assembled together, the pos. tol. value factor can be omitted, because it is assumed that the holes are perfectly oriented/located relative to each other.

----
Two comments about the Genium tip:
1. Although I see a reason behind doing this, the practice of applying (P) modifier next to the dimension defining minimum height of projected tolerance zone is not supported by Y14.5 standard.

2. The middle picture showing two fasteners. Although Y14.5 standard does not provide any instructions on how to do this on the drawing, from geometrical point of view optimal positional tolerance value will be defined when projected tolerance zone starts at the bottom of the top part, not at the top of the bottom part - in other words, when it is offset from the part containing tapped hole. In ISO this is doable (see attachment), in ASME I believe only option #1 could be used without confusing anybody too much.

http://files.engineering.com/getfil...-8020-6e981126f31e&file=ISO_projected_tol.png

 

[KENAT]

Usually match drilling is used because to get the corresponding tolerances from separately machined parts would be cost prohibitive.

Sure you can probably come up with a drawing with tolerances on the holes that nominally work but will it be cost effective?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[sluzzer]

Thanks Pmarc for your reply.

&
Kenat,
I got the clarity from your simple explanation. Thanks

 

[powerhound]

While I am absolutely a fan of extensions of principle, I think the bottom example is a stretch considering there is a supported way to do the same thing. My problem with it is that people who are in the process of learning on a fundamental level (and I think that's the intent of the document) might think that this is the proper way to control coaxial holes.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

John,

Could you elaborate on "supported way" please?

 

[powerhound]

The supported way would be to make the surface into which the threaded feature is shown, a primary datum feature. In other words, the inner right vertical wall.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

Nah, scratch that thought. It's not the same.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

I've had about the same chain of thoughts about that illustration.

But that actually makes interesting exercise:
Forget about thread, to keep it simple let’s have press-fit pin with one loose end (see the picture)
Suggest the way to specify functional requirement for pin A to be tight in hole B and loose in hole C without using projected tolerance.

Any thoughts?

 

[powerhound]

Back to the Genium sheet: I'm really having trouble imagining how the lower figure is standard compliant at all. I can't seem to wrap my head around how this could possibly mean what Whitmire intends it to mean. My trouble is with the DRF of the projected feature. The primary and secondary datums are parallel to the axis of the projected hole. I can't see how reference to datum feature C somehow makes the tolerance zone perpendicular to it.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II