Translation modifier versus MMB modifier

[trottiey]

I've been reading up on the new translation modifier in the 2009 version of ASME Y14.5. I'm trying to get a feel for when it should be used. At first reading, it sounded like it should be applied to any datum used for clocking. (i.e., on any tertiary or secondary datum used to constrain rotation on a higher precedence datum axis or point per section 4.16.) But after much thought, I started thinking that it's actually the MMB modifier that should be applied to clocking datums. I haven't been able to imagine any practical situation where the translation modifier is more suitable than the MMB modifier.

Take figure 4-19 of the standard for example. If you think about it, the translational modifier is just allowing a datum shift in one axis, whereas the MMB modifier would allow the datum C feature to shift in two axes. At first I thought that MMB might not allow enough radial range relative to the datum B centre, but then I realized that it has to according to the MMB clarification in section 4.11.6.1. So in any practical situation where you have round pegs going into round holes, you would either want to lock down the radial distance as in figure 4-9, or free up datum shift in all directions around datum C with the MMB modifier.

The other example of the translation modifier in the standard is figure 4.32. Here the MMB modifier would again allow more datum shift than the translation modifier, but in this case the two "axes" are parallel. I.e., datum feature B would be allowed to shift up or down until it touches the maximum slot width, and then it would be allowed to shift some more within the position width per section 4.11.6.1. Again, in any practical situation of a square peg going into a square hole, you would either want to lock down the 5 basic dimension as in figure 4.32(a) or give it full freedom with the MMB modifier.

So after all this effort teaching myself the translation modifier, I've come to the conclusion that it is rarely useful. Whenever you are thinking of using it, you should consider using MMB instead. But the biggest lesson I may have learned is that I should probably apply MMB to most of my clocking datums.

I would welcome comments or criticisms on this post. I don't feel all that sure of myself on this, and I'd like to know if I've understood it wrong. For others who are also new to translation modifiers, I found this discussion helpful in understanding what they mean: thread1103-256665

 

[Belanger]

The main difference between MMB and translation is that MMB also takes into account the size variation of the hole (I'm thinking specifically of Fig. 4-19 that you referenced). Notice that the explanation in that picture says that datum C is simulated by an expanding pin -- so in 2009 parlance that is still an "RMB" type of datum. If the triangle were replaced with the "M" then the simulation would be a fixed-size pin of 6.4, which accounts for the 0.1 position but also the 0.1 size tolerance.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[trottiey]

Yes, I agree that figure 4-19 datum C is technically still an RMB datum in 2009 parlance, simulated by an expanding pin. But it's not simulated by a normal expanding pin; it's an expanding pin that can float on a one-axis track. So once it hits one side of the hole, it doesn't stop there. It starts retreating in the track while expanding until it hits the other side of the hole and makes 3-point contact.

The pin will never have to expand beyond LMC, and the track will never have to extend beyond the positional tolerance. The result is that the datum C translation modifier is taking into account the size variation of the hole (and position variation) just like MMB does - but only in along the B-C axis.

 

[pmarc]

Let me just cite an excerpt from Bill Tandler's paper "What’s New in the ASME Y14.5 2009 Standard !" [page 6], which I think very nicely explains the nature of translation modifier concept and the reasons of using it. It also points out some weaknesses of figures shown in standard:

The new “Translation” modifier, which may be appended to a Datum Feature label in a Feature Control Frame,
specifies that the associated Datum Feature Simulator (and its associated Datum) shall be free to translate
toward or away from the primary axis or origin of a partially established Datum Reference Frame during the
Datum Reference Frame constraining step. Availability of the “Translation” modifier allows the Designer to
clearly distinguish between the function of a “clocking” Datum Feature and an “aligning” Datum Feature,
namely between a feature which constrains rotational degrees of freedom by virtue of its location (clocking)
and one which does so by virtue of its orientation (alignment).

Referring to Fig. 4-32 p.74 in the 2009 Standard, lack of a Translation modifier in alternative a) makes Datum
Feature B a “clocking” Datum Feature, in other words, one whose orientation has no impact on roll constraint.
The presence of a Translation modifier in alternative b) now makes Datum Feature B an “aligning” Datum
Feature, in other words one whose orientation determines the alignment of the remaining axes of the Datum
Reference Frame. Note: Both illustrations on the right in Fig. 4-32 are intended to represent a real part.
Unfortunately the two bores are not “squiggly” and therefore do not look very real, but they are understood to
be. Even more unfortunately the bore pattern in alternative b) is not rotated slightly clockwise as it must naturally be if the part in alternative b) is the same as that in a), which is absolutely the intent.

 

[dingy2]

Just going to put my 2 cents worth on this subject.

MMB is the old MMC on datums except it can now apply to datums that are surfaces in addition to features of size. Figure 4-16 on page 61 indicates the meaning to MMB depending upon whether it is a primary, secondary or tertiary datum.

The translation symbol could apply to a tertiary datum that is reflected in RMB. Let's take Fig. 4-19 as an example noting that the datums are in RMB. There is a basic dimension of 57.4 mm to the centre of tertiary datum C. Let's say that we are using a V-cone in a checking fixture to locate on datum C but the hole is not "dead nuts" on 57.4 but, maybe, 57.5 mm. What would happen. Well the V-cone would not seat all the way around the hole. Using the translation modifier, one could move the V-cone towards datum B allowing the V-cone to seat on the actual hole. That's it.

I would not suggest that a Designer place this on a drawing unless they are ready to hear the phone ring and you better have a good explanation of the translational modifier.

Dave D.

http://www.qmsi.ca

 

[trottiey]

In figure 4-32(b), p. 74, what is the advantage of using the translation modifier instead of the MMB modifier? They don't do quite the same thing; MMB would provide more freedom than the translation modifier. But in what situation would the translation modifier be better?

Let's think of some practical situations. Suppose the part in figure 4-32 is a shaft, and there's a timing pin that fits freely into that slot. Suppose that the shaft bearings have enough slop, or the shaft has enough flex, so the location of the slot doesn't matter much. What matters is just that the pair of holes are perpendicular to the slot. The best choice of datum B modifier to ensure fit at lowest cost would be MMB, because it gives you credit for an oversize slot. (If the design intent is to maintain timing accuracy at lowest cost, then the best choice would be LMB, still not RFS.)

Now imagine that instead of a timing pin, there's a press-fit key going into that slot, mating to a part that bolts into those holes. Like an RFS simulator, a press-fit key will essentially centre itself between parallel planes at maximum separation. But by the same token, it will also be unforgiving of position error. This is basically an aligning feature, and the holes should be located without modifiers on datum B, exactly as shown in figure 4-32(a).

When would you want to dimension the part with a translation modifier as in 4-32(b)? You would need a situation where a press-fit key in slot B is free to move up and down relative to whatever bolts into those holes, but needs to stay perpendicular to them. You could contrive a situation with sloted holes on the mating part, but I can't think of anything that resembles real-life.

I would be happy to be proven wrong, so please take your best shot. But right now, I can't think of any real-life situation where I would want to use the translation modifier. I still think that clocking datums should generally use MMB, not translation.

 

[trottiey]

For clarity, I didn't use Tandler's definitions of "clocking" and "aligning features" in my last post. My first post defined what I mean by a "clocking datum," and it basically includes both of Tandler's features. I won't bother explaining what I mean by "aligning feature;" it's not important and you can just ignore those words in my last post to avoid confusion.