dowel pin tolerance

[vscid]

Hi,

I have a square plate with a bolt circle diameter on which lies a dowel pin hole.
If I wish to call out a positional tolerance for the pin hole, would I tolerance it with respect to the bolt circle or the plate faces?

Thanks!

 

[KENAT]

This needs to be driven by function.

What are your datums?

I suspect datum A is one of the large faces but B & C could vary based on function/design intent.

For instance, if the dowel is used to give precision location to a mating component it may make sense for the dowel pin hole to be your secondary datum. With one of the edges as tertiary datum for orientation and have your bolt circle referenced to it.

KENAT, probably the least qualified checker you'll ever meet...

 

[ctopher]

I don't have mine here with me, but I believe this is called out in the Machinery Handbook.

Chris
SolidWorks/PDMWorks 08 2.0
AutoCAD 06/08
ctopher's home (updated 10-07-07)

 

[TheTick]

Dowel pin w.r.t. plate, bolt circle w.r.t. dowel pins.

One job I had designing machine tools, dowel pin positions were within Ø.002"

 

[vscid]

Kenat,
The pin will be a press fit onto the plate and it is indeed used for precise location.
I understand that the main face (front face of the plate) would be datum A, however how would the hole itself be the secondary datum B?

 

[KENAT]

I'm going to make the assumption that you're working to ASME Y14.5M-1994.

A hole can be a datum, look at fig 3-4. Section 4 maybe better explains the implications of this, maybe look at 4-6 but imagine that the dia 25.5 is a hole instead of a shaft.

For your example the best illustration I can find is 4-24, someone else may be able to find a better one. On the top view your pin hole would be the equivalent of E, with the bolt pattern related to it like the 4 holes are. You'd use one of the plate edges for Datum C in the way B is used in the figure.



KENAT, probably the least qualified checker you'll ever meet...

 

[vscid]

Kenat,

If I do not specify any modifier in the feature control frame, does that automatically mean it is RFS?
Would RFS be less advisable for a precise location dowel pin application as compared to MMC?

 

[KENAT]

Q. If I do not specify any modifier in the feature control frame, does that automatically mean it is RFS?

A.If working to ASME Y14.5 then yes, I can't recall for ISO.

Not sure I'm qualified to definitively answer the second question. However, if this is an interference hole that the dowel pin is press fit into then the tolerance gained from MMC will typically be very small. If this is the hole mating to the dowel pin then depending what fit you have the implications of MMC may be more significant.

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

Yes, general rule #2 states that when no modifier is specified in the Feature control frame then RFS is the default. RFS will provide for a more closely located feature as it ignores the size of the feature. RFS is probably what you'd want to use on both the press fit hole and the clearance hole in the mating part if this is for precise location. MMC is sufficient for assembly purposes such as clearance holes for bolts.

Powerhound, GDTP T-0419
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Inventor 2008
Mastercam X2
Smartcam 11.1
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[drstrole]

If the dowel pin presses into the hole(s) the position tolerance should be RFS, if the dowel pin is clearance the position tolerance between the dowel pins can be MMC.

 

[dgallup]

The principle advantage of MMC other than the bonus tolerance is that QC can use functional gaging rather than an expensive CMM.

 

[TheTick]

If the function of dowel pins is to locate, I would be inclined to use RFS and not MMC.

 

[ewh]

If it is a press fit, use RFS, as MMC doesn't buy you anything and may actually throw the position off slightly because any variation in hole size is non-existent once the pin is pressed.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[drstrole]

Nicely stated ewh

 

[TheTick]

Dowel pin fit could be viewed as "tight side" (piece that holds dowel pin) and "sliding side" (piece that receives dowel pin). On tight side, you almost certainly want RFS. On sliding side, MMC could work, depending on the requirements for alignment. Since hole diameter tolerances for dowels are so small, I don't think MMC can buy you much.

 

[KENAT]

Well thanks all. You confirmed what I'd been thinking about RFS for the press fit based on what someone else had told me.

I now need to delete a couple of MMC symbols from the print I'm redlining right now!

KENAT, probably the least qualified checker you'll ever meet...

 

[CheckerRon]

The way "The Tick" described in the 3rd post (11:40) is the way I have been doing dowel pins lately. RFS on pressed fit hole, and MMC whenever I can on the mating hole for the pin. Since CMM's are pretty common in most good machine shops, the MMC functional gaging aspect of MMC on pressed fit pin hole is rarely needed, and can be a detriment as someone else noted.

 

[weavedreamer]

When you press a pin into a hole, it only means that the c/l's are considered coincident.

The hole locations could have been RFS, MMC, or LMC prior to press.
The assembled pin locations can be RFS, MMC, or LMC post press.