How the bonus tolerance which we get by Material conditions are used during manufacturing?
How the bonus tolerance which we get by Material conditions are used during manufacturing?
(OP)
I understood that providing material condition (MMC or LMC),while giving GD&T, gives extra or bonus tolerance. But how this extra tolerance is used during manufacturing?
For example,
When positioning a hole using Positional tolerance with Maximum material condition, we get extra tolerance for positioning, when the hole size is more (i.e., when it is different from MMC). But during manufacturing, the hole is drilled & after that we are measuring whether the hole dia is at MMC or different. Even if it is different, how the extra tolerance which we get from material condition, can be utilized, because the hole is already drilled???!!
For example,
When positioning a hole using Positional tolerance with Maximum material condition, we get extra tolerance for positioning, when the hole size is more (i.e., when it is different from MMC). But during manufacturing, the hole is drilled & after that we are measuring whether the hole dia is at MMC or different. Even if it is different, how the extra tolerance which we get from material condition, can be utilized, because the hole is already drilled???!!





RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
I really like to show 0 positional error at MMC. When I do this, I show a sloppy maximum diameter. That maximum diameter provides allowances for the feature of size error, and for the positional error. No shop in its right mind will try to fabricate to my minimum diameter.
This tolerance provides the fabricator an error budget, which they can exploit as they please. All I care about is that I get parts within tolerance.
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JHG
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
If hole is under MMC you can make it bigger to compensate for positional error. Imagine making holes bigger to allow your part to fit over the gage. (providing that holes are not becoming oversized)
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
That said, there is some use for bonus tolerancing in manufacturing, if you know the accuracy and reliability of the process used. For instance, if you know that a certain machine or process is very accurate for size, but not so much for location, then it might be wise to dial the target size up just a tiny bit (rather than just at the middle of the size range). In this way, you can better live with the machine's erratic accuracy for location, and thus get fewer rejects and save money.
The corollary is this: if you discover that a machined hole is out of position after it's made, you can perhaps rework the hole (ream it open further) and salvage the part.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
Using MMC creates a link between the size of a feature and the location of a feature, which manufacturing is free to trade off for the best cost operation. If one can make holes precisely but can't locate them well, then go with large holes. If the hole size is sloppy but the location can be exact, go with little holes.
After a hole is drilled, one use for the linking of size to location is, if the center of a hole that is smaller than maximum is too far from where it ought to be, one can calculate how much larger the hole could be while in it's current position to make the hole location acceptable (location and size tied together,) and decide if a hole that size is allowed.
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
If the GD&T is not speaking to manufacturing, why should manufacturing listen to GD&T?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
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RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
Think about it: Often people can listen in to a conversation that isn't directly intended for them, and still gain from it.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
From everbody's reply, I understood that the bonus tolerance is used for a kind of 'rework'.
I mean, after the hole is drilled, we are checking for its position, if the position is not within the limits, instead of scrapping, we are increasing the hole dia & there by increasing the position tolerance limits & making the part acceptable. Am I right?
So is there any relation between hole (feature) tolerance & positional tolerance, that we should follow?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
But, if you consider what GD&T is telling you, you could decide on tooling, fixturing, QC procedures, and the entire process. GD&T does not specify the process, but it has encoded information on which features of your part can deviate, how, and for what amount. If you can read this information this gives you more freedom and better chances to make good part.
Like drawoh explained in his post, “zero” positional tolerance doesn’t mean “zero” but rather something different and beneficial to you as manufacturer.
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
There are many sources on-line, you can try this one for example:
http://www.mechsigma.com/
Especially this one: http://www.mechsigma.com/newsletters/2004_07_Newsl...
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
sluzzer I don't agree that bonus tolerance is only for 'rework' or passing parts on the hairy edge etc. Used and understood properly it allows manufacturing some freedom to tweak their process to maximize the number of good parts made/ease with which they are made as drawoh, Belanger and CheckerHater have touched upon.
In ASME Y14.5M-1994 appendix B gives some calculations that explicitly relate these 2 tolerances. They make certain assumptions but are generally applicable.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
Ken,
There is no need to take anything to the extreme.
Just tell me who will pick the proper drill / reamer to meet the tolerance, and who will “accept” the part after the hole is finished?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
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RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
This post has sort of gotten side-tracked. The side-track is relevant.
When you get your part back, you want to inspect it, pass it, and then use it, successfully. During the manufacturing process, you do not want to pay for unnecessary precision.
When you apply a tolerance to a hole at MMC/MMB, you are showing that the positional tolerance is less critical if the hole is over the minimum size. If a hole is fabricated at minimum size, out of position, there is some capability to enlarge the hole, and meet specification. There was an opportunity to use the large drill the first time around, pass the inspection, and avoid the cost and time of the re-work.k
Note how in the case of how I specify things, there is no opportunity to use the smallest hole. They need to enlarge the hole to get a reasonable positional tolerance.
Perhaps the term "speaks to manufacturing" is unfortunate. The reality is that you should not care how they do it. Your drawing should not provide manufacturing instructions. It should describe the part you will accept and pay for.
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JHG
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
Just a doubt. Suppose if our drawing should just describe the part we will accept & pay for, then why we need to give Material condition modifier at all? It seems like Material condition modifier aids manufacturing only right? Without Material condition modifier itself our drawings describes what we want, am i right? In that case, if we choose not to use material condition modifier then what problem we will get? is it optional?
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
No. The use of MMC does not aid manufacturing only. (Forgive me, sluzzer, if you were in agreement and were merely posing questions a la the Socratic method.)
If a drawing is to describe the part we will accept & pay for, then it's right up the alley of function/design to say that we'll accept a hole that's more out of position — on the condition that it's a larger hole. That is not a directive of any kind to manufacturing; it's just a statement about the final product.
So I stand by my original post: the use of MMC and its subsequent effect doesn't necessarily speak to manufacturing. But manufacturing knows what is allowed by looking at the drawing, and can then use any method to deliver a good part while saving themselves money along the way.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
MMC is a tool in setting topology limits; limits set so parts that are within those limits are believed to meet the limits of Step 3, below; they should have nothing to do with manufacturing; Manufacturing should know that QA will reject any parts that are outside the limits and try to produce most of the parts to be within them, on a cost informed basis.
Note the lack of gap between steps 4 and 5, a gap that doesn't include manufacturing. It's not that manufacturing has no place in the process, but steps 6-9 can result in an infinite number of ways that are not reflected or controlled by the drawing; If there was a Star Trek Replicator machine that parts just popped out for free, it would not affect the need for step 5 and would eliminate the need for manufacturing input.
How I envision the flow of information:
1) Task defined
2) Concept defined
3) Stress limits, dynamic limits, kinematic limits, and cost limits defined
**Magical step**
4) Dimensions and tolerances that record the magical conversion of functional limits into topology limits
5) QA/QC designs inspection methods to validate/verify parts are within topology limits
6) QA/QC establishes production limits
7) Production/MEs establish production methods to meet/QC limits
8) Infinite divisions of fixturing and detail process planning
9) Cost estimates from fixturing and detail process planning
10) More efforts - raw materials acquisition, rough cutting, fixture acquisition, tool room setup, machine setup, raw part loading, finished part unloading, deburring, cleaning, surface finishing or heat treat or paint, packaging, shipping, customs, receiving, unpacking, storing, retrieving, assembling, QC, final packing, final shipping, and so on. There's not enough room to include everything. Even if there was I wouldn't type it.
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
A1) If the part can be made perfectly there is no need for any tolerance. Since MMC is a tolerance modifier, if there is no tolerance there is no need for a tolerance modifier. Remove all tolerances and find a supplier of perfect parts.
A2) The MMC modifier aids procurement. Without the modifier, procurement will find fewer suppliers or the suppliers will charge more to make the parts. It doesn't help the metal worker at all except to determine how likely they are to make an acceptable part.
A3) You should get what is on the drawing. Without the MMC modifier the parts will cost more, be less likely to be made, and will not function better, if the MMC modifier was appropriately used.
A4) The parts can cost more or take longer to get.
A5) Sure. Everything is optional. Except death and taxes.
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
Let's crunch numbers. This will show some context for all these specifications.
I have a housing with a pattern of tapped holes. Your part will be attached to this. There are locating faces on the housing which provide you with secondary and tertiary datums.
The holes on the housing are specified as M6X1, and they are located to positional tolerances of Ø0.4mm with respect to the datums. Your holes must clear these.
Each screw has a diameter of 6mm. Each screw will be located inside a diameter of 0.4mm. It is possible that the screw will project outside of this diameter. There is a process for specifying projected positional tolerances, but it is outside the scope of this post. Let's just add 0.2mm.
Your part must provide holes that clear 6.0+0.4+0.2=Ø6.6mm at the exact nominal positions. There are several ways to accomplish this.
My preferred method is to call up Ø7.8/6.6 at a positional tolerance of zero at MMC/MMB. This explicitly specifies that the Ø6.6 is to be clear. All of the manufacturer's error budget is contained within the additional 1.2mm of diameter allowed for the hole. The manufacture can use this to manage his drill diameter, as well as his positioning.
An alternate specification would be something like Ø7.8/7.0 at a positional tolerance of Ø0.4, with no material condition specified. Again, the smallest hole at maximum positional error clears the 6.6mm diameter required by the fastener. There is no potentially scary zero tolerance and no scary MMC/MMB symbol. There might be a reason you do not want the Ø7.8mm hole to drift 0.6mm away from true position, which my specification above, allows.
Either way, your fabricator must select a drill or punch, and work out how to position it within tolerance.
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JHG
RE: How the bonus tolerance which we get by Material conditions are used during manufacturing?
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