Gage to inspect position of countersink relative to thru hole? 3

[illini8181]

I am reviewing a design of a plate which bolts to a larger structure using countersunk flatheads into tapped holes. I recognize that this is a fixed/fixed scenario, and so if the tapped hole and countersink are not perfectly aligned, the stress on the fastener head will be uneven (See

http://www.eng-tips.com/viewthread.cfm?qid=255441).

This is an existing design that has been in use for some time.

It has just come to light that the vendor who currently makes this part sometimes “opens up” the countersink during fit up of the plate to the larger structure, if a fastener will not sit flush. These plates are meant to be interchangeable, which will not necessarily be possible if the countersinks are opened up to fit one particular corresponding part. So, my question: How you do you inspect the position of a countersink relative to the thru hole?

 

[KENAT]

Not a direct answer but, to overcome this issue when forced into this situation (pattern of fixed-fixed csk fasteners) we usually oversize all the CSK in the drawing to allow for this. Essentially the CSK dia becomes max screw head dia (to sharp edge) + pos tol of csk + pos tol of threaded hole on mating part.

This doesn't resolve the issue of increased stress etc. but most of our stuff isn't highly loaded.

Posting guidelines faq731-376

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

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

 

[MikeHalloran]

The bottom margin of the countersink will tell you how well they are aligned.
Usually they're machined in the same setup, so serious eccentricity is not usual.

It sounds like you've got a tolerance problem between the countersinks and the tapped holes in the other parts. Nobody should be machining (opening up) parts at assembly; that gets expensive fast and carries other risks.

Consider floating captive nuts vs. tapped holes in the 'other' parts.



Mike Halloran
Pembroke Pines, FL, USA

 

[illini8181]

KENAT - We have oversized the countersinks, but not as much as the sum of the positional tolerances. This part is actually very highly stressed.

MikeHalloran - unfortunately we do not have room for nuts. What do you mean by the bottom margin? Our vendor does not machine the countersink and thru hole in the same setup. I am not sure why yet.

 

[powerhound]

If I'm visualizing this correctly, I can't imagine a scenario--except for Utopian perfection--where all of the countersinks will be entirely engaged all at the same time. There will always be uneven loading, no matter how badly you don't want there to be. Not only do you have the misalignment of the countersink to the thru holes, there is also going to be positional error in the tapped holes. I know you said this design has been in use for some time, possibly to preclude the possibility of someone mentioning the design flaw, but the design is bad and there probably won't be a forthcoming answer other than matching up parts as your vendor is currently doing.

Am I correct in my conclusion that the countersinks themselves are carrying the stress?

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

 

[illini8181]

powerhound - you are correct in assuming that the countersink carries the stress. I agree that the design is flawed. I would like to change it. However, there is resistance because "this is what we've always done". If I am not able to change it, I would like to be able to inspect the position of the countersinks relative to the thru hole, since they are drilled in separate steps and since the tapped hole is templated from the thru hole. However, it sounds like no one has done (or heard of) a similar inspection of a countersink to a thru hole?

 

[SeasonLee]

Please ref to Y14.5-2009 page 122 Fig.7-26.
Positional tolerances can be used to locate coaxial features, use the thru hole as a datum to define the CSK hole position.

How about the perpendicularity of the tapped hole? You may consider the projected tolerance callout on the tapped hole.

Season

 

[MintJulep]

These plates are meant to be interchangeable

So why were they designed in a way that makes it impossible for them to be interchangeable?

 

[powerhound]

A countersink is not a feature of size so using position to control their location is out of the question.

illini8181,

There are more things to consider besides the location of the countersink to the thru hole. In fact, that relationship is non-functional. What you care about is the relationship of the countersink to the threaded hole. You have to consider the location of the thru holes, the location of the threaded holes,and the location of the countersinks. For the loading to be even, the countersinks and threaded holes will have to be perfectly aligned. That will not happen.

MintJulep got it right. You have no chance of creating consistently interchangeable parts. You might get lucky every once in awhile but that's all it will be...luck. I also have a sneaking suspicion that the drawing for these parts will allow unusable parts to pass inspection. Not that it matters functionally but how are the countersinks controlled?

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

 

[MikeHalloran]

The usual way to measure the position of a countersink is to push a ball against it, and measure the ball's position.

What I mean earlier was that you can get a rough idea of what's going on by inspecting the intersection of the countersink surface and the through hole surface. If they're aligned, the intersection should be a circle in a plane parallel to the part face. In this case, the intersection will be an ellipse in a tilted plane.

As alluded, if you want almost interchangeable parts, the countersinks and the tapped holes should be located from the same exact fixture. ... which you had better not misplace. ... maybe you already did.





Mike Halloran
Pembroke Pines, FL, USA

 

[SeasonLee]

powerhound

Are you sure a countersink hole is not a FOS?

Season

 

[powerhound]

Season,

Yes, I'm sure.

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

 

[SeasonLee]

Sorry, powerhound

I thought it’s a counterbore, so I gave the Fig.7-26 example, my mistake.

Please ref to the photo attached and the thread below for the measuring method mentioned by Mike Halloran.

http://www.eng-tips.com/viewthread.cfm?qid=284005

Season

 

[powerhound]

I'm completely on board with that measuring method. It gives a very close approximation of where the countersink actually is. Depending on the manufacturing method, different sized balls could yield different results though. That really shouldn't matter in 999 cases out of 1000 though because their purpose is usually just to recess the fastener below the surface of the material being secured, NOT to bear some high degree of stress or locate anything.

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

 

[illini8181]

Powerhound - I agree that the location of the countersink to the thru hole is non-functional. However, the tapped hole is drilled based on the location of the thru hole. Once the cover holes/countersinks are drilled on the fixture, the cover is placed on the part. The centers of the thru holes are found and used to locate the tapped holes. So, since both the countersinks and tapped holes are drilled to the thru holes, I thought it would be good to be able to inspect the countersinks to the thru holes.

I agree that there seem to be all kinds of issues with this design. I am relatively new to this project, and so there are things I do not understand that probably have a reason (which may or may not be a good reason) that is buried in the past. It sounds like I should probably do more work to dig up those reasons before going much further.

SeasonLee/MikeHalloran - thank you for the measuring method. On a side note (I'm going to reveal some ignorance here), I know our vendor has a laser tracking system for inspection, I just don't know much about its capabilities. I actually don't know much about part inspection in general. As you may have figured out from my posts, the information flow between my company and our vendor is not very good. Also, I have minimal experience. Does anyone know of a good book on the basics of inspection? Or a website with good information? I would like to make an effort to understand much more thoroughly how our vendor inspects these parts, but first I should probably learn more about part inspection in general.

 

[MikeHalloran]

One thing that may be illuminative is to set up a surface plate and a height gage and try to inspect something yourself. I suppose that would make a lot more sense if you'd ever seen an experienced inspector do it.

So, do that. Ask your vendor to let you watch his inspector measure your parts. Don't try to bluff the inspector; tell him or her that you don't know anything, and ask them to please explain the process as they go along. An occasional question may be necessary to prime the pump, but once you get someone talking about their job, they tend to keep talking, and good information will flow by faster than you can capture it. ... so pay attention.

Bring a box of doughnuts by all means, but don't take them or any other food into the inspection area, and don't even touch the surface plates, much less rest your coffee on them.




Mike Halloran
Pembroke Pines, FL, USA

 

[illini8181]

Mike, thanks for the commonsense advice! I'll do that.

 

[pmarc]

illini8181,

If I could add something to what was already said:

1. Unless I misunderstood something, I find your statements about interchangeability somehow contradicting. On one hand the plates are intended to be interchangeable, but on the other hand the tapped holes in a "larger structure" are drilled based on actual location of the thru holes in a particular plate. This doesn't sound like search for interchangeability of components. What am I missing here?

2. How is the countersink dimensioned on a drawing? Is it done just by defining a directly toleranced included angle and a directly toleranced diameter (similar to what is shown in fig. 1-39 of Y14.5-2009)? I am asking, because this method does not control "symmetry" between the countersink angle and the axis of the thru hole. So if your primary concern is to control this type of relationship between two features, profile of surface control with relation to axis of the hole applied to the countersink may be much better choice.