position tolerance for hole at end of long shaft

[Jieve]

Hello,

Yet another question, i have a 100mm rod, 20mm in diameter. In One end an M8 thread will be tapped into the center to a depth of 28mm. The surface where the hole will be located has a perpendicularity tolerance of 0.1mm with relation to the primary datum, the cylindrical axis. I want to locate the hole using a position tolerance but am not sure what to use as the second datum (or if i need One). I figured the perpendicularity of the hole's axis needs to be referenced to something. I was think of using that surface where the hole is drilled into as a secondary datum, but then not sure how you would actually inspect this. If the part has to be set On its secondary datum surface, how would a pin be able to be inserted into the hole?

Thanks.

 

[Belanger]

Which are you trying to control -- perpendicularity of the hole, location of the hole, or both? That will answer the question.

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

http://www.gdtseminars.com

 

[Jieve]

Hi Belanger,

I'm trying to control both. A datum perpendicular to the hole axis is necessary to control perpendicularity, and the axis datum controls position then? This was my impression.

Thanks.

 

[fsincox]

Only it it serves your function, A thigh fitting condition with the shaft may not allow the part screwing it to orient to the face. You must understand, and design, what will happen to suit your application.
Frank

 

[fsincox]

Sorry,
“a tight fitting condition with the shaft may not allow the mating part screwing in to the thread to orient to the face”.

The dilemma I always question is, should I target the end area of the shaft in a very long shaft case. Does anyone do that?
Frank

 

[Jieve]

Hi Fsincox,

Thanks for the reply. My question was actually more about measuring technique and a description of the position tolerance than about proper design.

Basically, there are two of these shafts that fit into clearance holes in a base. A plate is screwed into the tops of both shafts, and the shafts act as a guide for the plate to move up and down.

During assembly, the shafts will be slid into the holes first and the plate bolted second. The top threaded faces of the shafts need to be perpendicular to the shaft so that they mate well with the plate.

I would like to locate the holes' position and perpendicularity to ensure that they are accurately centered and straight. For the specification of the position tolerance, in most examples I have seen there is are X-Y location datums and a Z (perpendicularity) datum. To measure that hole location requires inserting a gauge pin or something to determine the actual center, as well as the perpendicularity. However, if that face is specified as a secondary datum, it has to sit with two points against a surface. That surface would limit access to the hole, so I'm wondering if this is a common situation and if so how would make the necessary measurements, or if not, if there is a more common or better way to specify the tolerances.

Hopefully that clears up the question a bit.