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MMC to Datuam Which is Hole Axis
- Thread starter appvid
- Start date
appvid,
Datum[ ]B is a feature of size. Its size tolerance is not vastly tighter than the positional tolerances it is being used to control. If you were to build a fixture to inspect this part, you would capture datum[ ]B with a pin 14/13.99 in diameter. This would locate your part precisely at MMB/MMC, as specified. It will allow slop when the datum feature is at 14.1mm[ ]diameter, also as specified by MMC/MMB.
If the datum did not specify MMB/MMC, you would have to measure the hole and find the centre. MMB/MMC affects your inspection procedure. I have not used a CMM. I would guess that a CMM would find the hole centre fairly easily, but that is not what you need on this drawing.
--
JHG
Datum[ ]B is a feature of size. Its size tolerance is not vastly tighter than the positional tolerances it is being used to control. If you were to build a fixture to inspect this part, you would capture datum[ ]B with a pin 14/13.99 in diameter. This would locate your part precisely at MMB/MMC, as specified. It will allow slop when the datum feature is at 14.1mm[ ]diameter, also as specified by MMC/MMB.
If the datum did not specify MMB/MMC, you would have to measure the hole and find the centre. MMB/MMC affects your inspection procedure. I have not used a CMM. I would guess that a CMM would find the hole centre fairly easily, but that is not what you need on this drawing.
--
JHG
pmarc
Mechanical
- Sep 2, 2008
- 3,247
appvid,
Maximum Material Boundary (MMB) modifiers specified after B and C in the positional feature control frames do certain things and in general, and under certain circumstances, provide more tolerance for inspection as well as for manufacturing.
Without going into the details of how they really work at the moment (you can read about it in para. 4.11.9 in Y14.5-2009), quite common approach taken by CMM operators in situations like yours is to verify the part as if these modifiers were simply not specified at all. If the controlled pattern of holes meets the "simplified" position tolerance requirement, it will also always pass the requirement with M's.
However, if the inspected pattern does not pass the "simplified" callout, it will not necessary mean that it does not meet the requirement with M's.
drawoh,
I don't want to be picky, but I have to comment on 2 things from your reply:
1. Datum B is not a feature of size. Datum feature B is a feature of size. Datum B is axis derived from datum feature B.
2. Based on the drawing provided it is impossible to say/calculate Maximum Material Boundary of datum feature B as the drawing (at least the portion we see) does not specify any relationship between datum features A and B. That missing relationship must be taken into account because datum feature B is referenced secondary in both positional callouts, with A being primary.
Maximum Material Boundary (MMB) modifiers specified after B and C in the positional feature control frames do certain things and in general, and under certain circumstances, provide more tolerance for inspection as well as for manufacturing.
Without going into the details of how they really work at the moment (you can read about it in para. 4.11.9 in Y14.5-2009), quite common approach taken by CMM operators in situations like yours is to verify the part as if these modifiers were simply not specified at all. If the controlled pattern of holes meets the "simplified" position tolerance requirement, it will also always pass the requirement with M's.
However, if the inspected pattern does not pass the "simplified" callout, it will not necessary mean that it does not meet the requirement with M's.
drawoh,
I don't want to be picky, but I have to comment on 2 things from your reply:
1. Datum B is not a feature of size. Datum feature B is a feature of size. Datum B is axis derived from datum feature B.
2. Based on the drawing provided it is impossible to say/calculate Maximum Material Boundary of datum feature B as the drawing (at least the portion we see) does not specify any relationship between datum features A and B. That missing relationship must be taken into account because datum feature B is referenced secondary in both positional callouts, with A being primary.
pmark,
[ol]
[li]Okay, datum feature[ ]B it is.[/li]
[li]This looks like an example out of a textbook, rather than a serious drawing. The datum feature hole is nominally perpendicular to datum[ ]A. The only possible error is perpendicularity, which would have to be accounted for by reducing the diameter of the pin. This illustrates the importance of using accurate features to define FOS[ ]datums.[/li]
[/ol]
--
JHG
[ol]
[li]Okay, datum feature[ ]B it is.[/li]
[li]This looks like an example out of a textbook, rather than a serious drawing. The datum feature hole is nominally perpendicular to datum[ ]A. The only possible error is perpendicularity, which would have to be accounted for by reducing the diameter of the pin. This illustrates the importance of using accurate features to define FOS[ ]datums.[/li]
[/ol]
--
JHG
- Thread starter
- #5
Thank you Both..
Yes this is something out of Textbook. Some drawings at my work shows something very similar to this picture.
I had same thought that , it is of no meaning having MMC with Datum B, if Datum B(CENTER HOLE AXIS) is not defined its own position with respective to Primary Datum.
And
From the above drawing as it is...
If I have to have a measuring fixture to inspect the part, pin size of 13.99 MMC making sense to me. If I check my part on CMM it is not.
Thanks again drawoh and pmarc
Appvid
Yes this is something out of Textbook. Some drawings at my work shows something very similar to this picture.
I had same thought that , it is of no meaning having MMC with Datum B, if Datum B(CENTER HOLE AXIS) is not defined its own position with respective to Primary Datum.
And
From the above drawing as it is...
If I have to have a measuring fixture to inspect the part, pin size of 13.99 MMC making sense to me. If I check my part on CMM it is not.
Thanks again drawoh and pmarc
Appvid
Correct me if I'm wrong (but pre-2009 at least) I thought that a cylindrical datum feature created a datum reference frame on the centerline that is free to rotate about that axis. If for some reason that it is not desired to allow that DRF to freely rotate then an additional clocking datum is required.
Bruce
Tunalover
Bruce
Tunalover
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