Tolerances - Undimensioned Drawings
Tolerances - Undimensioned Drawings
(OP)
I am designing several new brake formed parts that are defined on undimensioned drawings with a +/- .030" general tolerance. I am not sure if the interpretation of this general tolerance is the same across the aircraft industry (seems like it should be). I need to know the correct interpretation since I have hole patterns used to attach purchased parts and I need to determine the required tolerances to ensure interchangeability. I don't want to add dimensions to hold things tighter than the general tolerance unless actually needed. Additionally, we don't use GD&T (even though we should).
First, considering just a flat part, is my understanding that tolerances do not build-up, in other words, the tolerance between two holes would still be +/- .030" and the tolerance on the length of a part would be +/- .030". If that is the case, then an undimensioned drawing is actually holding tighter tolerances than a dimensioned drawing with 2 place dimensions (because it would be like having every possible relationship dimensioned, really over-dimensioned). Is that right?
Second, what is the interpretation when bends come into play – is there an additional tolerance on the location of each bend? I assume you would have to at least add the angular tolerance (+/- 2 degrees) to determine the tolerance on the length of a part that has several 90 degree bends?
Third, Is the flat pattern inspected before forming and the final part inspected again after forming? Does anyone have first article inspection experience with sheet metal parts? I have heard several conflicting opinions - does anyone work at a company that has an official document to clarify the interpretation of undimensioned part tolerances?
Thanks,
MechDesignR
First, considering just a flat part, is my understanding that tolerances do not build-up, in other words, the tolerance between two holes would still be +/- .030" and the tolerance on the length of a part would be +/- .030". If that is the case, then an undimensioned drawing is actually holding tighter tolerances than a dimensioned drawing with 2 place dimensions (because it would be like having every possible relationship dimensioned, really over-dimensioned). Is that right?
Second, what is the interpretation when bends come into play – is there an additional tolerance on the location of each bend? I assume you would have to at least add the angular tolerance (+/- 2 degrees) to determine the tolerance on the length of a part that has several 90 degree bends?
Third, Is the flat pattern inspected before forming and the final part inspected again after forming? Does anyone have first article inspection experience with sheet metal parts? I have heard several conflicting opinions - does anyone work at a company that has an official document to clarify the interpretation of undimensioned part tolerances?
Thanks,
MechDesignR
Atlanta





RE: Tolerances - Undimensioned Drawings
Or used when lazy.
RE: Tolerances - Undimensioned Drawings
The bottom line is that if you have something that needed to be controlled, and you didn't specify the degree of control, your company will probably lose the argument and eat the cost of anything that was nonconforming to an unspecified requirement.
TTFN
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RE: Tolerances - Undimensioned Drawings
Engineering drawings typically define the requirements for a finished part as shown on the drawing. With formed sheet metal parts it is common to define a flat pattern layout of the part with mold lines, bend lines, etc. It is also common with digital CAD designs to employ "reduced dimension" drawing formats. Where the dimensions and tolerances are defined by a general note or specification, unless otherwise specified on the drawing.
If you need to control the location of a feature like a hole pattern to ensure that the mating part will fit, then you need to put the necessary dimensions and tolerances on a drawing that defines the finished part condition.
The FAI spec for the aerospace industry is AS9102. The FAI is a validation of the part as defined on the drawing. If the drawing defines a flat pattern, then that's what will be inspected.
Good luck.
Terry
RE: Tolerances - Undimensioned Drawings
Is +/- .030 acceptable for the part you are designing? It's YOUR job as the designer to define the tolerances that are required to allow the part to function properly. The person that typed .030 in the tolerance block of your standard drawing border had no idea what you would be designing today.
No, that's wrong.
RE: Tolerances - Undimensioned Drawings
Possible reason.
some drawings where giving basic configuration,
why because it was controlled by a master mylar.
this was done when there where two or more parts being assembled together. drilled & rivited, or bolted.
all the holes where drilled by slaping a mylar up against the assemly & drilled with pilot holes then held in place with clicos(temperorary fastening). to verify edge distance then finished drilled.
check with the assembly if the holes are prematurely drilled it will not match the other details.
if this is not the case the GD&T should be used.
parts should be drawn & tolerance in the formed view.
then it's upto the the vendor to hold the flat pattern.
it is easier to match drill instead of trying to hold super tight true positioning.
HTH
mfgenggear
RE: Tolerances - Undimensioned Drawings
personally, why inspect the flat pattern ? aren't you most insterested in conforming the final part to the eng'n ??
RE: Tolerances - Undimensioned Drawings
Does the drawing refrence a spec for drawing interpretation? ASME Y14.5M is the most common spec in the US. If no drawing spec is given then it's debatable if the interpretation would be the same. In practice the interpretation would probably be similar to 14.5, however that's not legally binding.
"First, considering just a flat part, is my understanding that tolerances do not build-up, in other words"
Wrong - though depending on how the part is dimensioned you may think it's true. The tolerances are applied to the dimension. If say a 2 hole pattern is dimensioned from an edge to the first hole, then another dimension from first hole to second then no stack up between the holes. However, if dimensioned from edge to first hole, and edge to second hole, then you get a stack up.
As mentioned, drawings generally define the finished article and that is what is usually inspected.
+-.030 is a large tolerance for hole patterns, while you'd need to do the math and /or take into account match drilling, it's likely too large.
When you say undimensioned drawings I take it you are using hybrid model/drawing definition as tbuelna says. You still have to fully dimension it somehow. Variability in how +-.030 might be applied/interpreted is one problem with trying to do this too simplistically.
Honestly, it seems like you don't have a firm basis in dimensioning & tolerancing. I suggest you get hold of the 14.5 standard (assuming that's what you work to) and learn some. Also forum1103: Drafting Standards, GD&T & Tolerance Analysis may be a good place to ask questions, though they/we can get a bit pedantic.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances - Undimensioned Drawings
Software For Metalworking
http://closetolerancesoftware.com
RE: Tolerances - Undimensioned Drawings
Technically this is not a complete drawing, and scaling from a plot is usually forbidden unless the plot is stable (on mylar) and verified for accuracy.
Just my 2 cents...
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tolerances - Undimensioned Drawings
match drilling to a mylar was a an old school method of manufacturing & far as I know it is used today.
it gets very difficult to match up more than 3 or more assemblies.
it has been done this way for decades.
the drawing should specify to a master model & mylar.
the main concern is edge distance from holes & edges.
how ever if this Detail is a single piece of hardware
that assemple to one other detail then true position should be good.
HTH
MfgEngGear
RE: Tolerances - Undimensioned Drawings
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tolerances - Undimensioned Drawings
typically we reverse engineer those and insert them into our own drawings, add our own tolerances which are:
0.000 +/- 0.005
0.00 +/- 0.010
0.0 +/- 0.030
we never send out ANYTHING undimensioned, and even though we model everything parametrically, we also never have parts mastered off STEPs/IGES. so yeah we have to dimension every feature of every part. STEPS/IGES only go out as 'reference only' mostly for the programmer/machinist in case they want to pull off some weird dim off of it. sending undimensioned drawings is out of the question as our suppliers would do 'whatever the hell they want' and at the end it would bite us in the arse big time.
as far as what to dimension and how many decimal places it ends up being at the discretion of the designer/detailer and based on application, machine shop capabilities, cost and most importantly experience.
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RE: Tolerances - Undimensioned Drawings
I can understand avoiding iges for machining purposes, but why step? We often use it for a variety of tasks where the native file (or a parasolid) is unsuitable. We do a model verification (comparison) after translation between the native file and the step, and have yet to run into any problems.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tolerances - Undimensioned Drawings
To the OP, I suggest you take a look at ASME Y14.41, it's not particulary user friendly or usefull but at least it's a start for Digital Product Drawing/Model Based Dimensioning (i.e. product documentation based on model with no or incomplete/simplified drawings).
There have been several good threads on MBD over in GD&T and elsewhere in the last few years.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances - Undimensioned Drawings
model verification is fine, but sending a STEP to a machine shop and telling them to do it... dont know about that.
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RE: Tolerances - Undimensioned Drawings
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tolerances - Undimensioned Drawings
Most of the responses to my question were way off target. I will try to explain more carefully exactly what I am looking for. First, I am working for an aircraft company that uses "old school" undimensioned drawings to define brake formed sheet metal parts (the same way things have been done for many decades). These undimensioned drawings go by different names at different companies: PCM, EMD, Mylar, Undimensioned Master, etc. Typically, these "undimensioned" drawings define a flat pattern, show bend centerlines, specify bend angle, direction and radius. They only contain dimensions for features that need to be held tighter than the general tolerance of +/- .030". For example, full size fastener holes would have a diameter dimension. If you aren't already very familiar with this type of drawing, your reply will not help at all.
Where I currently work, undimensioned drawings have a general tolerance of +/- .030" with a note that states "no tolerance accumulation is implied in any series of features". Some people understand this to mean that the length of the part (scaled off of the mylar or measured in cad) would have a tolerance of +/- .030" and similarly, the distance between the centers of two holes would be +/- .030". Other people interpret this to mean that one edge of the part is held to +/- .030" and that the location of a hole is held to +/- .030" and that the length of the part would have a total tolerance of +/- .060" and similarly, the distance between the centers of two holes would be +/- .060". The correct interpretation is important since I am designing new sheet metal parts that will mate with purchased parts (things can't be drilled at instl) – I have to ensure interchangeability by calculating the tolerance needed for my hole pattern. Sometimes a general tolerance of +/- .030 will work for the hole pattern (due to floating nutplates and large holes) and no dimensions are needed. While other times a tighter tolerance is required.
Apparently, the interpretation of undimensioned drawing tolerances varies from company to company. I would like to hear what other people in Aerospace have to say – what is your understanding of the general tolerances on undimensioned parts? What statement is used on the drawing or elsewhere to clarify things? Maybe someone who makes or inspects these parts could shed some light on the subject?
Thanks!
Atlanta
RE: Tolerances - Undimensioned Drawings
If you do post there I suggest you put a link back to this thread. (thread2*266252 but replace the * with -, just as shown near the top of the screen under the title)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances - Undimensioned Drawings
You've asked a very good question, which involves as much "human dynamics" as it does "drafting standards". When working as an MRB Engineer for Boeing, we found that mylar drawings frequently showed the same part in different zones, and that drawing ADCN's were not always incorporated into all views on all sheets properly. In the end, the mylar drawings could be differently "interpreted" by QC depending on what zone and sheet was used as the inspection standard. To get around this sheet metal "check-fixtures" were manufactured, where the part was held against an edge, or snapped onto an inspection pin that picked up on a tooling or fastener hole. Periphery tolerance was scribed on the check fixture.
As a mechanical designer - you must check with the Quality Control department on how they inspect the parts you are mating with. Against the drawing or a check fixture? Sometimes this information is included in the Manufacturing Plan or Travellor..
Good Luck
RE: Tolerances - Undimensioned Drawings
TTFN
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RE: Tolerances - Undimensioned Drawings
some of you may not like this but it was how it was done once & it worked.
Design engineers would draw their designs it was upto the
manufacturing engineer to coordinated with all departments to make it work.
secondly one lone engineer can not make or change company policy. it takes the management to implement change.
busting the op chops is not helping.
RE: Tolerances - Undimensioned Drawings
There are certainly cases where match drilling, or simulating this with fixtures, is perhaps the only way to do things, I don't think anyone is arguing that point.
However, to have parts that were done this way and then by the sounds of it wanting to be able to make new parts to fit them without match drilling sounds tricky.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances - Undimensioned Drawings
all of you spend your time helping engineers in need help.
that you all spend your valuable time helping others.
all of you are much appreciated.
however Kenat the last comment I do not concur.
during world war 2 on all sides. aircraft & military equipment was mass produce.
the reason it could be mass produced is because all the tooling was coordinated to master plasters. all parts could be interchanged & would all ways fit (assy).
so to say it was hammered in place is a demeaning to these people who designed & made this equipment in the past.
the technology is ten times better but in the wrong hands it can make a lot of junk.
does that means it is better ? would I ever hand draw again, The answer would be NO. I like my cad system.
I enjoy the close tolerances CNC equipment can hold.
because it is reliable in the proper hands.
what we are not getting from the OP is what is the function of this part and how it is assembled.
can it be machined from a cnc laser or punch in the flat pattern, in other words how close does it need to be to assemble correctly.
I dislike making these type of tools, how ever one has to weight the quantity & quality required.
In the OP case it might make sense for this part.
Or it may not. I just added 2 cents
With out knowing fit form or function & cost it's hard to say.
RE: Tolerances - Undimensioned Drawings
I'm not claiming that the companies themselves follow standards of this type, I don't know. As an employee of a subcontractor to those companies, I do know that unverified mylars and other aids are subject to audit findings against us.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tolerances - Undimensioned Drawings
That said, there were problems of interchangeability during WWII, which supposedly was actually one of the prompts for the development of GD&T etc. So don't try telling me they always fit. I've worked on some moderately old stuff and worked with folks that worked on even older stuff and they confirmed that hammer to fit, paint to match etc. happened.
Heck, I had a structures prof at uni not so long ago talking about the 'hammer to fit' mentality on the shop floor of a major aircraft company at least as recently as the 1980's so don't tell me I'm demeaning the memory of Rosie the Riveter and her buds.
We had parts at my last employer that were basically made off the tooling and that was how we knew they'd work. It was a problem when the tooling wore out though as even when we got specialist tooling folks in they had trouble working out how the tooling had been made with respect to tolerances etc.
However apologies to the OP, this is way off topic.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerances - Undimensioned Drawings
They were read with high precision boring machines (SYP 7A) with optical viewers to center the crosses (0,35mm line width).
When requeried, coordination and guide holes (crosses) were added for the single sheet metal parts, in the drawings.
Afterwards, machined templates were made to transport those coordination-holes to other assembly jigs. So, the SYP operator first read the position of the holes in the mylar, and then drilled them in the template.
Then the master template was used to transfer the holes for different purposes.
Also the guide-holes for the molds of sheet metal parts were taken from these master templates.
After conforming and calibrated the sheet metal part to the mold , these guide-holes were drilled to the part with the mold guide-holes. These guide-holes were previously coordinated with the master template.
If the part should perform in an assembly, the assembly jig was already set up with the master template.
The guide-holes were for the single parts and the coordination-holes were for the assemblies. Some holes were also guide and coordination ones.
Simple? more or less, it demanded some tuning up.
Just my two cents.
RE: Tolerances - Undimensioned Drawings
Ie a row of rivets may be shown marked off as ref, but at the part fabrication stage, these holes aren't drilled.
Concentric holes through multiple parts were drilled on installation, always. At most, and where possible, one part was pilot drilled to use as a template.
This general practice resolves the OP's issue with 'interchangeable parts'.
This general practice is not compatible with the modern practice of trimming the extra 0.010" of unnecessary edge distance to save 0.00001 lb. Not compatible with CATIA either, you have to use your brain to maintain adequate ED.
And if you look at a 707 or something, they didn't always get the edge distance right...
RE: Tolerances - Undimensioned Drawings
good point I had forgotten about the pilot holes on the most outer sheet (part). thus setting location.
MfgEnggear
RE: Tolerances - Undimensioned Drawings
For sheet metal parts, the dimensions of the flat plate will vary based on the method of forming. A press brake will have a slightly different bend allowance than a punch press or a bench brake. Most parts that are formed and have holes for locating something will be drawn as the finished part and the flat plate drawing will be a function of the tooling involved.
Most of the existing manufacturers using process as described can't make parts that are interchangable. When I ran a shop that did primarily structural repairs, I would pay extra for undrilled parts for Cessna, Piper and Beech. Anything with pd holes was almost guarenteed not to fit.
Companies that have taken the concept of GD&T to hart make much better parts. In the homebuilt industry for example, Vands will make parts that actually are interchangable, and all the holes line up. They are all lasar drilled on CNC, and it works. Funny how the homebuilt industry can produce better products than the established manufacturers, but it's true.
RE: Tolerances - Undimensioned Drawings
"Good to know you got shoes to wear when you find the floor." - Robert Hunter