Gage to inspect position of countersink relative to thru hole?
Gage to inspect position of countersink relative to thru hole?
(OP)
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?
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?





RE: Gage to inspect position of countersink relative to thru hole?
This doesn't resolve the issue of increased stress etc. but most of our stuff isn't highly loaded.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
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.
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
So why were they designed in a way that makes it impossible for them to be interchangeable?
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
Are you sure a countersink hole is not a FOS?
Season
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Gage to inspect position of countersink relative to thru hole?
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.
RE: Gage to inspect position of countersink relative to thru hole?
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
RE: Gage to inspect position of countersink relative to thru hole?
RE: Gage to inspect position of countersink relative to thru hole?
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.