Translation modifier versus MMB modifier
Translation modifier versus MMB modifier
(OP)
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: secondary datum MMC, tertiary datum RFS
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: secondary datum MMC, tertiary datum RFS





RE: Translation modifier versus MMB modifier
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Translation modifier versus MMB modifier
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.
RE: Translation modifier versus MMB modifier
RE: Translation modifier versus MMB modifier
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.
www.qmsi.ca
RE: Translation modifier versus MMB modifier
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.
RE: Translation modifier versus MMB modifier