Use of M modifier for interference fits?

[Madhu454]

Hi All,

Genarally we use M modifier if the functional requirement is assembly.

If it is a clearnace fit, we can have a 'M' modifier for positional control.

Question
1) Can we have M modifier if the functional requirement is assembly and the desired fit is interference fits??

Please do the needful.

Thank you.

Madhusudhan Veerappa
Mechanical Engineer

 

[PaulJackson]

Madhu454,

Whether the fit is clearance or interference is not the only question.

Whether the ideal function is worsened as the feature location and/or orientation deviation is permitted to increase is also a primary consideration of whether or not to use a tolerance that is variable relative to size.

Slip and press fits speak loudly to function but it is function that must drive the strategy of how much tolerance is permitted.

If the ideal function is worsens as the feature location or orientation deviation is permitted to increase, I seldom use variable tolerance limits.

Paul

 

[flash3780]

The maximum material modifier will provide a cost savings for press fit components since it allows for hard gaging to be used. The bonus tolerance will be an order of magnitude lower than the position tolerance for most press fits, but the hole can be checked with hard gaging.

If this is a locating pin, you probably want to use a projected tolerance zone as well, otherwise the allowable perpedicularity error could cause fitup problems. The projected tolerance zone is a functional requirement, and it will save you a huge headache when you go to stack your parts.

 

[ptruitt]

Flash3780

For the record, when you said "...stack your parts", you were referring to tolerance stack-ups. Right?

Peter Truitt
Minnesota

 

[drawoh]

Madhu454,

My favorite application of the M modifier is to specify the smallest possible hole that will clear the mating fastener or feature, specify a positional tolerance of 0(M), and to specify a generous maximum diameter.

I do not specify MMC/MMB on threads and on accurate holes, because the clearances are not significant compared to the positional tolerance the fabricator can achieve. There is no bonus for making the hole the maximum size allowable.

Perhaps you have some other application for the M modifier?

JHG

 

[PaulJackson]

The maximum material modifier will provide a cost savings for press fit components since it allows for hard gaging to be used. The bonus tolerance will be an order of magnitude lower than the position tolerance for most press fits, but the hole can be checked with hard gaging.

I totally diagree with the premise that hard gaging saves $$$... gaging costs $$$ as well. Press fit's typically need predictable control for size to manage the assembly interference... whether they should be checked with an attribute gage for location depends upon function.

If additional location or orientation deviation due to size is permissable in a feature's function then go ahead design the feature with a variable limit tolerance... but if works best when it is perfectly oriented or located and worse as it is tolerated to deviate then variable limit tolerances are non-functional!

I don't want the critical features of the machines that I trust my life to at 30,000 over the ocean checked with attribute gages if they shouldn't be functionally.

Paul

 

[flash3780]

For the record, when you said "...stack your parts", you were referring to tolerance stack-ups. Right?

Haha, yes. I didn't mean put them on top of one another, I meant when you do a tolerance stackup.

In my opinion, tolerances are arbitrary if you don't do tolerance stackups. Stackups (hot & cold) are one of the major contributors to the quality of a design.

 

[drawoh]

PaulJackson,

A thought about production gauging.

Take the case that I have a plate with two or more press fit holes in it. The holes have a tolerance of +0.01/0mm, and the positional tolerance is Ø0.2mm.

I test the holes' diameters with a go/no-go gauge.

If it passes, I lower the part onto my position tester, which consists of datum references, and a set of pins 0.2mm undersize, located exactly on position. If there is interference, I am assume that the positional tolerance is being violated.

Does this sound reasonable?

Note how MMC/MMB is useless in this scenario.

JHG

 

[PaulJackson]

Reasonable?

How to size a go position gage for a tolerance given RFS is not reasonable. Whether or not the position tolerance should or should not be specified as a variable tolerance depends entirely on the function of the hole's position.

If the function of the assembly is not worsened as the location of that moves from zero to Ø0.2mm then the specification should include a (M) modifier and you could use your set of 0.2mm undersize pins.

If the function however is sensative to the position deviation and 0.2mm is a limit chosen to reflect the maximum deviation that the assembly's function will tolerate... then using the 0.2 undersize pins will permit diameters that are at LMC to have a position deviation of Ø0.21 (0.01 OTOL)

Some may choose to make the pins Ø0.19mm rather than Ø0.2mm undersize to account for LMC sized holes whose position deviates greater than the limit Ø0.2 but that is not the point.

I am not very fond of attribute gaging because if quantities produced are significant then controlling the process with goal post devices is dangerous(sampling less than 100%) and expensive (gage purchase, maitenance and product quarentine re-checks if sampling less than 100%). If quantities are small then use of the devices is just plain expensive.

I am aware that variable tolerance allowances are dis-regarded when statistical conformance predictions of geometric tolerances are figured! That is a shame and in such instances I support the use of attribute go gages to refute the condemnation from the predictions... but... there is a better way.

The varible portion of tolerance can be included in the prediction and size can be adjusted accordingly to optimize (minimize) the potential defect in both simultaneously. If you want more info search the web for 'Ppk' 'Cpk' 'MMC' 'Position' etc.

My point is specify it according to its function and inspect it according to its specification.

Paul