Creation of Datum Centerplane in a Wedge (flat taper) shape

[jferg1969]

I have a symetrical wedge shaped part and am trying to apply the correct standards for creating a datum plane for it, alot like a V-Block mentioned in other threads. If this were a part with parallel sides I would have a "feature of size" and all would be well, however the taper has seveal dimensions to contol its size and form. Placing the datum on the Basic Angle dimension alone would leave some ambiguity.

Plan A) I apply datum targets (three)to each face of the wedge effectively creating a center plane for which a Datum Feature Simulator could me fabricated. From this plane I will contol position of holes that need to be related to the center of the part.

Plan B) Apply symmetry correctly.

The use of symmetry gets a lot of discussion in various posts, and I would love some feedback as to prevailing use of datum centerplanes in V shaped features.

Thanks,
Jferg

 

[KENAT]

Does the centerline of the wedge truly represent the functional datum?

Or is the functional datum really the mounting holes or some such?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[jferg1969]

Yes it must. The wedge shape is a clamp with the tip of the wedge bearing upon other parts. The holes are essentially guide bushings that ride on shafts. I've attached some views.

The GDT was not applied well originally and the holes are skewed for lack of correct contol. The holes need to be true position of 0.25mm to wedge centerplane. This part starts life as a casting which also had lack of control for the resultant profile of the wedge faces. I'm concerned that without post machining those faces, applying datums to the faces could create a plane that is not truly centered as well.

Thanks!

 

[jferg1969]

Just to clarify, the wedge centerline truly represents the functional datum not because mating parts contact the faces, but becuase the tip width must center between two clamped parts. The tip width having a theoretical center must coincide with the axis of the guide holes within a given tolerance. Since the tip width is not a FOS completely (caliper rule), not having parallel sides, etc., creating a datum plane here appears to be against the rules. In other words, the width can be seen as a function of the length and the angle depending on how the taper is specified, and thus, to which dimension would one attach the datum identifier? For now I'm attaching targets on each face to create the Datum plane I'm attaching a redline of this, it shows the previous datums and tolerances also.

Thanks

 

[cjccmc]

Just some thoughts....

1. do you care about the tip width varying between the 5.35 and 5.10 as you go from one end to the other?

2. making one side of the wedge flat and controlling the other with an angularity tolerance could be another way to do it. (I always worry that a datum target point I call out may just happen to be at a local depression or peak that may not represent most of the surface, but this may be no different than setting a flat surface on a datum simulator and letting the high points speak for the rest.)

 

[cjccmc]

I meant to say, for the flatness and angularity you could limit those more precise tolerances to the area of the tip that matters, don't need to have it that tight elsewhere if you think it would make the part easier or cheaper to build.

 

[CheckerHater]

Since the tip width is not a FOS completely (caliper rule), not having parallel sides, etc., creating a datum plane here appears to be against the rules.

It is not.
If you have an access to Y14.5-2009 book, check out Para. 4.3, especially part (g) and Fig. 4.3 (g) respectively.
The standard allows you to use “complex shape” as a datum and Fig. 4.3 (g) looks very much “wedge-like”.
Standard recommends using Profile to control the wedge itself, and your setup “embracing” the part with datum targets is up to the task.
All together, your redline looks perfectly fine to me. It may need some polishing (I couldn’t find what datum B is), but definitely not illegal.

 

[fsincox]

You know me, if it really is the function it should be!

 

[jferg1969]

Thanks cjccmc. That is a valid approach, but this part (V shape is cast) and I'm hoping to avoid post machining of the faces since they serve no function (other than creating a true center plane datum. I too worry about deriving a best fit plane from these irregular surfaces. I am going to add more control to the casting part to remedy this.

CheckerHater: Thanks as well, I meant to use the tip dimension with a datum identifier (plane) attached like a slot is against the rules because it alone does not define the shape. I don't have a copy of the 2009 version but my 1994 version
has par 4.67 which shows a highly irregular twisted shape.

Im pretty confident they way I have it will work, thanks for the feed back. Any comments on symmetry?

 

[pmarc]

How about the simplest solution - profile of surface without datum references on both inclined faces of the wedge together with basic angle between them?

 

[jferg1969]

It would help control the surfaces but not help defining the center Datum plane. I'm interested in preserving the tip width and let the angle drift, so a using profile would suggest a parallel zone RFS of tip. Right now, the limits of size based on the angle and tip tolerancing of the cast part do allow for too much variation though.

The goal is to specify the hole axes to be congruent to the wedge shape. The resultant wedge shape(from casting) must be used to define the center plane to which the holes get their reference.


Thanks

 

[fsincox]

That is exactly what I thought when I saw the picture. "He is going to want the holes on the center of the wedge!!"