Thru hole dimensioning in deep part 15

[Out0fSquare]

Really need help with this.

I have a cube that is eight inches square. There are four holes to be drilled through the entire depth of the part and the centerline of the holes are dimensioned only on the top view. The holes are 1/4" diameter. There is a drawing tolerance of +/- .010.

The hole pattern dimensioning uses no control boxes, neither does it have any requirement for perpendicularity, celindricity, parrellism, etc. All that is shown is the distance to the first hole from the part edges and the center-to-center location for the hole pattern.

The part was manufactured by a outside shop. Where the holes exit the part on the "bottom" they do not fall within the drawing tolerance as measured from the edges at the bottom of the cube.

I contend that the drawing does not control the exit hole locations and so our inspection department should not flag this as discrepant.

My question is this: What controls the exit hole tolerance for thru holes drilled through thick parts when the holes are dimensioned on a drawing only on the "entrance" view of the hole?

 

[MouseTrap]

Maybe another way to help save the piece is to open up the tolerances on the dimensions locating the hole pattern...say from +/-.010 to +/-.06 as OutofSquare stated the sides don't mate with anything and then maybe the holes could possibly be counter-bored to open up clearance for the fasteners if the holes are not satisfactory in their present condition.

 

[Out0fSquare]

fcsuper, said, "The dimensions established a relationship between the center of the hole and the part edge, period."

Your position is precisely the one for which I'm asking for some supporting evidence. You seem to be equating "center point" with "center axis". And that "edge" equates with "surface". I disagree. What says that "point=axis" and "edge=surface". I see this could truly cause a great deal of dead parts.

Also, using your own "standard", why are you giving me .030, more tolerance from the second hole to the bottom far side, but holding me to .010, on the near side? These sides and hole relationships are drawn perfectly symmetrical. But you didn't say the part has to be symmetrical. Why not?

What if the part made to these dimensions, it's 7.995 across the top, and 8.005 across the bottom [+.010 wider on bottom than top]. The first hole is at low tolerance from the dimensioned "side" [-.010], and next hole is also at its low tolerance [-.010], and make the top "out-of-square" to this side (obtuse) such that it measures an additional +.010, at the far corner. (The part is out of square, side-to-top within tolerance, and the opposing sides are out-of-parallel, side to side, but within tolerance.) All dimensioned features are in tolerance (according to your interpretation), and we've imposed the drawing tolerance to the exit hole centers relative to their dimensioned side, and yet the second hole exits the part .040 further from its nearest corner than does the similar hole that's being checked. Why are you allowing an additional .030 on the far side when these holes are drawn symmetrical in all views?

 

[ringman]

If only one part is involved, why not functionally check it on its related test fixture? Is that a possibility?

 

[vc66]

OurOfSquare said, "What says that "point=axis" and "edge=surface"."

If you have a hole in a part, and you mark the face of that hole with a centermark--it represents and infinite straight line through that cylinder, the axis. It can't be anything else. Any person who looked at the drawing and was asked what that cross represented would say, "that's the center axis of that hole."

As for the edge = surface--If you're measuring from your "edge" and you don't consider that part of the surface, you're going to have a hell of a time trying to measure from and infinitely thin line to an imaginary point which only exist on that face of your part. That's why there are assumptions made, WAY before standards are enforced. Using your logic, normal inspection would be impossible without a CMM.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[ewh]

You can't be absolute about these things. A center point may be locating a hole at an angle.

The edge may be a part of the surface, but it does not indicate whether that surface is out of plane. A dimension from the edge is not necessarily the same as the distance from the surface from which the edge came. It can't be less, but it can be greater.

 

[meintsi]

If you have a hole in a part, and you mark the face of that hole with a centermark--it represents and infinite straight line through that cylinder, the axis. It can't be anything else.

True but not complete, a centermark is a point locational reference only. It defines the point where the axis and the plane the centenline rests upon intersect.

As ewh's example shows, centermarks are commonly used to locate entry points for angled holes.

Remember...
[navy]"If you don't use your head,[/navy]
[navy]your going to have to use your feet."[/navy]

 

[vc66]

Sorry, you're both correct, I should've clarified that I'm speaking to the situation where the hole is shown "perpendicular" to the face in which it is drilled and a 90° angle is implied.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[fcsuper]

Given Outofsquare's intensity, I'll state that this is my opinion based on not seeing the drawing.

First, I would disgree with 90deg being implied because no standard is stated on the drawing which establishes this rule.

OutofSquare, to answer your question directly: The dimension as stated on the drawing makes no differentiation between any of the cross sections of the hole. To say one cross section has more value than any other is to say something that isn't on the drawing: it is an assumption.

Further, even if 90deg is implied (which it is not because no standard is stated which says this) that doesn't mean the hole is created to that specification. The only specification is between the edge and the centermark or between centermarks (per what you described). Given the fact there are no cross sectional preferences on the drawing, this would apply to all cross sections of those features. Where the edge surface deviates from 90deg, so would the hole have to in the same fashion.

Further, it is an assumption to state the view itself is 90deg to the surface being viewed. This is assuming a specification which is not on the drawing, even if ASME Y14.5M is employed. There is no such thing as tolerancing to drawing views themselves. In my mind, the only way to establish an unassumed relationship between the entry surface and the surface of the hole across its many cross sections is to use feature control frames and datums.

You can argue this out all you want. In the end, you need to do what is right for you. If I was in your shoes (which I am not ), I would say (given your description of everything) "Accept parts as is because specification is unnecessarily restrictive. Part will work as is. Action item is to work with the customer to fix drawing to better reflect the functional requirements of the part." This doesn't give my vendor ammo to use on me in the future, I accept the part that I know will work as is, and I fix the problem for the future.

Good luck with your situation. In fact, I hope you've already resolved it in whatever way works best for you, and that this discussion is academic for you at this point.



Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ewh]

Surely if you have a drawing of a cube with three orthographic views, it is safe to ASSUME that the sides are an implied ninety degrees from each other, with or without a stated standard. If not, how do you proceed?

 

[KENAT]

To assume makes an ass out of you and me.

The drawing is incomplete/ambiguous.

When quoting the job either ask for clarification or give your price and state that you're assuming standard drawing conventions per the relevant standard are in place. (I realize it's too late for the OP)

I used to prepare quotes (not for machined jobs specifically) and it always amazed me the stuff that wasn't explicitly stated in the document pack we were tendering to. I'd either ask for clarification or make sure any assumptions I'd made were stated on the cover letter to our quote.

Of course this takes time which from other posts it seems people prepparing bids for drawings don't have. However, it appears other people in the organizations have plenty of time to chase down this issues once the job is accepted and there's some liability.

KENAT, probably the least qualified checker you'll ever meet...

 

[ewh]

I agree, really. No stated standard, no basis for interpretation. Assumptions have a way of coming back and biting you.
However, if you were in a situation where it was imperative that this cube be made, you would have to make assumptions. Some, I feel, are safer than others.
Anyway...

 

[KENAT]

I think I see its hoof twitching.

I've made assumptions for internal stuff but when it's a customer drawing I've normally flagged it, if nothing else explained our interpretation or similar.

KENAT, probably the least qualified checker you'll ever meet...

 

[vc66]

Absolutely. If the vendor drew it, and they also made it, then there's no question. Reject it. If you tell them what it needs to do, and they not only make an ambiguous drawing, but also produce a part that doesn't function the way you need it to, then it needs to be rejected.

All of my assumptions were made based on it being an in-house drawing.

OutOfSquare, I have a stupid question for you. Why are you trying to let the part pass inspection? Either way it's the vendor's fault (bad drawing or bad part). Letting it pass is like me drawing a house that's built on a slab of which the material is not called out, building it on a slab made of jello, putting it up for sale, and then having the structural engineer come in and check everything and say, "Oh yeah, the jello slab, that should be fine, because he didn't specify what material the slab was going to be."

Am I wrong in my assumptions?

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[KENAT]

VC66, re-read out of squares posts.

It appears to be a customer drawing.

It appears the machining was subcontracted to a vendor.

So that's why he doesn't want to reject it.

KENAT, probably the least qualified checker you'll ever meet...

 

[vc66]

Aha, thanks KENAT. I read his second post, which stated that the drawing wasn't that old, and was created by a well-known entity. I assumed that meant a vendor.

Thanks for the clarification. It seems that we've gone so far down, I completely forgot what the original post was asking.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[Out0fSquare]

Correct, KENAT. Our company contracted to make this part amongst others in a bigger job. My customer provided the drawing, and I subbed-out the machining. Inspection rejected the part at receiving inspection.

I'm exptected to respond to a Corrective Action Report (CAR), stating why the error occurred and what action will be taken to "fix it". If some "corrective" action needs to be taken, I must also explain this to the machine shop.

After reading the comments here -- and having considered this from every possible angle (pun intended) -- I'm more convinced than ever that KENAT and ringman are correct. Without any stated standard there are no clear requirements to check against. It's my further opinion that, absent any explicit standard the part cannot be held to dimensional requirements beyond those points dimensioned on the print.

Perhaps this could be taken to a silly extreme, but it's equally irrational to inspect two select, un-dimensioned features for "uniformity", but leave other, equally important, yet un-dimensioned features un-checked.

There's a good chance our customer will accept the part, as-is, in which case no corrective action will have to be stated. I'll know very soon.

I'm certified in ANSI Y14.5 GD&T, so I prefer to see GD&T control frames on machined part drawings. Our customer most assuredly knows about Y14.5, but chose not to apply it on this design. Why should anyone else...

 

[bvanhiel]

I would argue from the negative... Is there a standard out there that permits the kind of interpretation that OutOfSquare is proposing to get the part through QA (that the hole locations only matter on one face). I'm not an expert, but I don't think there is one.

Obviously the standard needs to be added to the drawing, but that doesn't give the machinist license to do whatever he likes. The parts are clearly non-conforming. If the vendor disagrees you can ask him to show you what standard he used to interperet the drawing.

-b

 

[Out0fSquare]

FACT: The drawing gives dimensions for the cube in three views (three dimensions). The hole center points are dimensioned in only one view (two dimensions on one plane or surface). All of the dimensioned features are within the drawing tolerance at the points dimensioned on the drawing. The part is in tolerance, end of story.

Inspection assumed the holes “must” be equidistant from a certain side, top and bottom. This is NOT stated, implied or required by the drawing, any more or less than is symmetry, perpendicularity, or cylindricity. Inspection did not assume symmetry, neither did they impose perpendicularity constraints – only parallelism, and only with regard to the relationship between certain selected features, exclusive of other equally important features.

The part is in tolerance at every point dimensioned on the drawing. What’s wrong with applying this most obvious standard? This is the only standard which passes the, "would it hold up in court," test, and it’s the only one implicit in the drawing. It's only when you start to assuming other “un-dimensioned constraints” that problems arise.

I can see how parallelism is the geometric relationship that's easiest for people to grasp. But for the sake of better understanding, disregard parallelism for a moment and apply the dimensions and tolerance relationships to this part in regard to diameter, perpendicularity, cylindricity and symmetry. You'll find that on the generous side, the exit holes have a linear tolerance at the surface relative to other features greater than +/-.010, but not an unlimited tolerance. On the other hand, if you apply the linear tolerance to all of these geometric considerations, you must reduce the tolerance on some relationships. Either way, it's not an "unlimited tolerance for all features not dimensioned." But the tolerance at points other than those dimensioned on the drawing could vary greatly -- if measured -- depending upon interpretation and assumptions, absent any standard.

 

[vc66]

I'm sorry--standard or not, assumptions or not-- if you drill a hole perpendicular to a face, just so i starts cutting chips, and then turn the drill 89° so that the hole is 89° off kilter, just because the face dimensions of the hole are correct, does not mean that part should pass, or that the machine shop is not at fault. That is the last I will say about that, because it's either here nor there.

You are right (as far as my limited knowledge of standards go). Where there is no standard there are no assumptions. I feel that you're letting the machine shop off the hook, but I believe (now that I know that they didn't also make the drawing) that you're doing it for the right reason. I wouldn't want to scrap parts based on a bad drawing either.

I hope I'm clear in my response. I still think you should chastise whoever made the drawing, if only for the fact of forcing people to think about this topic.

Ok... took my medicine. I'm good now.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[ringman]

Does the part perform as intended/