Desperate For help before my head turns purple

[UGMENTALCASE]

Morning All,

OK I've been going round in circles a bit and really need some help. I have 1 component, a bracket and a pin. The component is sat down, the bracket is set and doweled off to a base plate when in the correct setting position. The pin is then used to check two holes are in the correct place on the component, so passes through each hole in the bracket and into the component. Now firstly am I right in saying that this falls under floating fastener scenario?

So the hole in the part is 5.75 ± 0.15 positional tol at MMC is 0.4
I need to tolerance both the holes in my bracket and the pin to ensure they fit together.
The customer wants us to work at 10% of component tolerance for our parts. so our holes must be positional to 0.04 at MMC.

Can someone please help me out with the best way to start of calculating this out please?
I've checked numerous examples but they assume the positional tolerance between both parts is the same, and that's when I come unstuck

Please please help me!

 

[greenimi]

Could you post a picture/ drawing, please?

 

[drawoh]

UGMENTALCASE,

Your pin is an inspection tool, right? Are you inspecting the holes in your part and in your bracket?

What goes into your holes at final assembly? This is where the floating fastener case either applies or it does not apply.

How about a sketch?

--
JHG

 

[UGMENTALCASE]

Thanks for the replies so far, attached is a sketch. Yes pin is the inspection tool, so checking whether they have machined the component correctly.

So in the attached sketch you have the component on the left side, our bracket on the right, and pin far right. What we need to do is have our holes in the bracket positioned/tolerance in such a way that the pin can check the holes in the component. An added extra is that our bracket is bushed in the two holes, but if I can fully understand the set up without bushes I'll then tackle the one with bushes
So the examples I've previously looked at tend to have the same positional tolerance between the two parts, however our customer is stating 10% of component tolerance, so positional tolerance of component is 0.4 and ours would be 0.04. This is our starting point.

Thanks

 

[3DDave]

Files with "&" in the name can't be downloaded.

 

[UGMENTALCASE]

Sorted

 

[greenimi]

Two options:
1.) press fit pins (interference fit between the pins and the bracket) into the bracket: pins size 5.2 (basic relationship between the press fit pins in the bracket 58 basic apart)

2.) floating pins (you also need two floating pins): pins size: 5.4, holes in the bracket 5.6
holes positioned basic from each other with Ø 0.04 positioned (gage manufacturing tolerance)

 

[drawoh]

UGMENTALCASE,

If you want the holes in the bracket to clear whatever is inserted into your part, you are looking at the fixed fastener case. How you work this out, depends on what you shove into the holes on assembly.

A part occupying the entire 5.75 hole can be 5.75+0.15+0.4/2 = Ø6.1mm located at exact nominal position. If your bracket is Ø6.5/6.1 with a zero positional tolerance at MMC/B, it will always accept whatever is shoved into the part hole, especially if you apply your Ø0.4 positional tolerance with a projection equal to the thickness of your bracket. Your pin should be Ø6.1, and the end should be tapered to fit, centred into your Ø5.75 hole. This is not necessarily what you want to do.

--
JHG

 

[UGMENTALCASE]

Drawoh I think that is what I'd be going onto as the holes in my bracket will be bushed. So our pin would be a sliding fit on the Bush, and then will need to fit in the component to check it's been machined correctly. It's a touchy feely fixture.

But I'm seeing how we look at it if both were clearance also, just trying to get my brain in to gear!

 

[pylfrm]

UGMENTALCASE,

The goal is to inspect the two position tolerances on "Comp" using a functional gage made up of "Brkt" and "Pin", correct? Also, do the position tolerances have datum references? If so, are they identical? I'm guessing yes, and that the base plate and dowels you described take care of this somehow, but I'm not sure.

You will have to determine whether your gage must reject all bad parts, accept all good parts, or something in between.

When calculating tolerances for a gage like this, quite a few factors can come into play. The sliding fits will allow positional and angular errors in the pins. Errors can arise from the datum features/simulators. Many of these factors are often ignored though.

ASME Y14.43 (Dimensioning and Tolerancing Principles for Gages and Fixtures) may be helpful.


- pylfrm

 

[greenimi]

pylfrm,

You hit the nail in the head. " The goal is to inspect the two position tolerances on "Comp" using a functional gage made up of "Brkt" and "Pin", correct?"
That is my understanding too hence my answer on the conceptual gage design I have provided yesterday.
However, I did not go into the details on how to tolerance the gage as I did consider that too early for this stage of the discussion.

By the way, do you agree with my assessment I have provided yesterday? (the assessment with two options)

OP,

Please confirm pylfrm and I understood your issues correctly (before your head really turns purple)

 

[greenimi]

Also, just per the attached drawing, the position of the holes required are to be measured/qualified just in relationship to each other and not to in relationship to any datums. That was my understanding. If I am mistaken, OP, please advise, and I will stand corrected.

 

[UGMENTALCASE]

Ok I've got me a copy of Y14.43 so that'll take a bit of reading, but seems quite interesting at a glance.
So yes the function is to check the two comp holes using the bracket and pin.
The comp drawing dimensions the upper hole to datum a/b/c. and then the lower hole is tied to the upper hole. So we will need to recreate this. our bracket is 'floated in' and the initial positioning isn't an issue, it's more the location between the hole.
So I've seen an image on the first few pages of Y14.43, showing a pin going through the gage block and into a right angle plate, and being checked with a pin, that's basically this set up, only with two holes.
With this I suppose I'm looking at the affect of the difference in the positional tolerance as well, as the component is 0.4 and we are instructed to go 10% of this for our tolerance, I think that is where I'm tripping myself up.

The principle of this is to reject bad parts, and accept the good ones. So we are doing it in such a way that if the pin goes in, then happy days, if not then bad days. So this is a positional check of the component holes.

 

[greenimi]

RE: "and the initial positioning isn't an issue, it's more the location between the hole. "
I would still stick with my idea provided yesterday. (because I am very stubborn)

The way I said yesterday, I think will check the position between the holes (copy paste from yesterday)

Two options:
1.) press fit pins (interference fit between the pins and the bracket) into the bracket: pins size 5.2 (basic relationship between the press fit pins in the bracket 58 basic apart)

2.) floating pins (you also need two floating pins): pins size: 5.4, holes in the bracket 5.6
holes positioned basic from each other with Ø 0.04 positioned (gage manufacturing tolerance)

What is wrong with these options?

 

[pylfrm]

UGMENTALCASE,

Gage tolerances mean that a gage that accepts all good parts must accept some bad ones as well. Similarly, a gage that rejects all bad parts must also reject some good ones. There is no way around this, but there is the choice of which option to lean towards, and how far to lean. This will determine what direction you apply your gage tolerances in, and which effects you make allowances for.

It sounds like the datum situation is more complicated than I first imagined. It's hard to give a recommendation without seeing the complete feature control frames.


greenimi,

Assuming we're dealing with simultaneous requirements without datum references, and ignoring gage tolerances, I agree with option 1: pin nominal diameter matches hole virtual condition.

For option 2, it seems like you'd accept bad parts due to the substantial angular displacement of the pins allowed by the 5.6 / 5.4 fit. Wouldn't you just want the pins sized at 5.2, and then a close sliding fit with "Brkt" to limit displacement (both angular and linear) to acceptable levels?


- pylfrm

 

[greenimi]

pylfrm,

I agree. My second option will introduce additional uncertainties. And that is because my second option does not follow the rules established by paragraphs 4.5.2 (namely here the basic orientation/location between the datum feature simulators)

4.5.2 Requirements
Datum feature simulators shall have the following
requirements:
(a) perfect form.
(b) basic orientation relative to one another for all the
datum references in a feature control frame.
(c) basic location relative to other datum feature simulators
for all the datum references in a feature control
frame, unless a translation modifier or movable datum
target symbol is specified. See Figs. 4-9, 4-19, and 4-32,
illustration (a).

Further more I agree, I have intentionally ignored the gage tolerances. We have to come to an agreement on how the CONCEPT would look like and THEN see what the gage tolerances are going to be (Absolute tolerancing, Tolerant Tolerancing and Optimistic Tolerancing)

 

[greenimi]

19.3 Gage Tolerancing Policies
Gages must be toleranced. There are three gage tolerancing policies commonly practiced throughout the
world. These policies are known as: Optimistic Tolerancing, Tolerant Tolerancing, and Absolute Tolerancing
(also called the Pessimistic Tolerancing approach).
Optimistic Tolerancing is not an ANSI-recommended practice for gages. It assures that all parts
within specifications will be accepted by the gage. Most of the technically out-of-tolerance parts being
inspected by the gage will be rejected, but a small percentage of technically out-of-tolerance parts will be
accepted. This policy is accomplished by tolerancing the gages from their appropriate MMC or MMC
concept virtual condition boundary so that gage pins can only shrink and gage holes can only grow from
these boundaries. This method subtracts material from the gage so that gagemaker’s tolerances, wear
allowances, form tolerances and measurement uncertainties all reside outside the workpiece limits of size
and geometric control.
Tolerant Tolerancing is also not an ANSI-recommended practice for gages. It assures that most parts
within specification will be accepted by the gage. Most of the parts outside the specification will be
rejected by the gage. A small percentage of parts outside the specifications may be accepted by the gages
or a small percentage of parts that are within the specifications may be rejected by the gages. This policy
may either add or subtract material from the gage MMC boundary or MMC concept virtual condition
boundary since the tolerance is both plus and minus around these boundaries. This means that some of
the gagemaker’s tolerances, the wear allowances, the form tolerances and the measurement uncertainties
reside both within and outside of the workpiece limits of size and geometric control.
Absolute Tolerancing is recommended. This type of gage tolerancing means that gage pins are
toleranced only on the plus side of their MMC concept virtual condition boundary (only allowing them to
grow) and that gage holes are toleranced only on the minus side of their MMC concept virtual condition
boundary (only allowing them to shrink). This has the effect of rejecting all parts not within tolerance and
accepting all parts that are within tolerance except those borderline parts that fall within the range of the
gage tolerance. Part features that are produced within the range of the gage tolerance are rejected as
though they were not in compliance with their geometric tolerance, even though technically they are
within the design specification limits. This is the price we must pay if we choose to accept no parts that
have violated their tolerance.
Absolute Tolerancing is the ANSI-recommended practice of applying gage tolerances so that the gages
will reject all workpiece features that reside outside of their specifications. This is to assure complete random
interchangeability of mating parts in an assembly inspected by these gages. Gagemaker’s tolerances, wear
allowances, form tolerances and measurement uncertainties of the gage are all within the workpiece limits of
size and geometric control. These gage tolerances add material to the gage. The gages are dimensioned at the
MMC limit or MMC concept virtual condition limit of the feature being gaged, then toleranced so that gage
pins can only get larger and gage holes can only get smaller. This policy is based on the gaging premise that
all parts not within tolerance will be rejected, most parts that are within tolerance will be accepted, and a small
percentage of in-tolerance parts that are considered near the borderline between good and bad will be rejected
as though they had violated their tolerance requirements.

The ANSI-recommended amount of tolerance is 5% of the tolerance on the feature being gaged plus
an optional 5% of the tolerance allowed for wear allowance. This recommendation is only a place from
which to begin the decision as to what tolerance will be assigned to the gage. Using the Absolute
Tolerancing method, the actual amount of tolerance chosen will depend on the number of parts the gage
will accept and the number of parts one is willing to reject with the gage. It is a balance between the cost
of the gage and the cost of the rejection of good parts by the gage. The smaller the gage tolerance, the
more expensive the gage and the quicker the gage will wear beyond acceptable limits and begin to accept
bad parts. On the other hand, the larger the gage tolerance, the less expensive the gage. However, the gage
will run the risk of being produced at a size that will reject a larger number of produced parts that are within
tolerance but near the borderline.

 

[UGMENTALCASE]

Evening guys,

Thanks for all the replies, I'm working my way through them. I have another question just to see if I'm following something correctly, if you could help me out?
The attached image shows a picture from one of my books, which just explains MMC and LMC. If for example the DIA 30 was produced at 30.4, does that give an allowable tolerance of 1.6 to the DIA 19mm, am I seeing that correctly? So the worst case positional tol is DIA 2mm. So if one feature is produced at a particular size, does this give the other feature the difference, if I'm making sense?

Cheers

 

[greenimi]

First of all you are working in ISO GPS and not ASME Y14.5 where modifiers on concentricity are not allowed.

The "equivalent" of concentricity of two coaxial diameters/cylinders/surfaces would be position in ASME.

Probably position for two coaxial cylinders in ASME and GPS ISO concentricity are the same (the same concept or same mathematical scheme/definition). I am not very familiar with ISO.

 

[greenimi]

Re: "If for example the DIA 30 was produced at 30.4, does that give an allowable tolerance of 1.6 to the DIA 19mm, am I seeing that correctly? So the worst case positional tol is DIA 2mm. So if one feature is produced at a particular size, does this give the other feature the difference, if I'm making sense"

You lost me here. Please provide more details if you would like more help.