Defining Positional Tolerance of Hole Using Surface Datum

[JMorton99]

Hello,

I am new to GD&T. I have just started to read ASME Y14.5M-2009. Attached is a capture of the drawing. It is a square block that is machined out, creating 4 unique planar surfaces. On these planar surfaces there will be 4 separate holes in which replaceable bushings will be mounted. They will serve to guide a drill to precisely drill a hole on a part. So the positional tolerance and perpendicularity is important.

The hole must be perpendicular to the face it rests on.

I have defined each unique planar surface as a datum.

My question is, I know when defining datums you want to define 3 orthogonal planes, but when defining a surface as a datum how do the rules change? Do you just need two orthogonal planes to define it relative to the part and one surface to define it relative to the surface where you choose the hole to be at? Also when is Maximum Material Condition necessary, would this part need?

Thank you all ahead of time for any feedback I may receive.

 

[powerhound]

Welcome JMorton99,

The surface is not the datum, it is the datum feature. That is to say that the feature (the surface in this case) is used to establish a datum but is not actually the datum. Take datum G, for example. The datum plane is a theoretically perfect plane that will touch the three highest points of the part surface. You've done well in that you've actually identified actual part features as datum features as opposed to attaching datum identifiers to centerlines as some folks new to GD&T will do.

Some things to keep in mind are that your perpendicularity tolerance must be a refinement of your positional tolerance so it's value has to be smaller. You automatically get .003 of perpendicularity control with the position tolerance. You also must dimension your features from the datums from which they are referenced. For example, all 4 holes are referenced to datum B but there are no basic dimensions tying them to datum B.

Hopefully others will chime in as well.



John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[3DDave]

What face(s)/feature(s) will orient the part at the next assembly?
Are the holes being drilled going to act together to orient/located another part or act entirely separately?

As it stands, the holes can wander all about the part and in a wide range of directions because only the slope relative to [A] is controlled, not relative to any other feature of the part. While there can be only one solution for simulating [A] there is no limit on the relative orientations of the tolerance zones for the surfaces.

I'm inclined to suggest making the holes primary datums, 4X Individually, and setting the surfaces for the bushings to have a perpendicularity tolerance, as it is usually the hole that guides the bushing, not the flange.

 

[JMorton99]

Hi,

As it stands, the part in the drawing (Call it Part A) will be mounted on an arm. It will be positioned then fastened through the .266D hole. The "MARKED A" holes will then be drilled through the arm and Part A, where pins will then hold the two parts in position.

The part that will be drilled (Call it Part B) has a curvature to it. The 4 holes must be perpendicular to that surface of Part B. The 4 planar surfaces in Part A are a tangent offset of the surface in Part B. That is, the hole axis of Part A are perpendicular to the surface of Part B. Part A will be machined separately, it will not be machined on assembly.

I guess the relative position of the holes to each other is important, but because they are each at different angles in all 3 axis it is difficult to define them relative to each other so they must be defined by common datums.

 

[3DDave]

Dimension the locations where the axes intersect [A] to and [C], the true angle between each axis and [A], and the apparent angle between the (projection of each axis onto [A]) and .

This will locate and orient the holes to each other in a common frame of reference.

You can use a position tolerance that is appropriate for the projection of the holes to the distance required to drill into the parts.


Then the surfaces only need to be precisely perpendicular to each hole individually with a gross profile tolerance to [A|B|C] to keep enough material thickness.

In order of importance - the holes have to hit the mating part where they are supposed to; then the bushings are aligned to the desired holes, then the distance back to the bushings is set by the length of the drill, and the shoulder only needs to be good enough to keep the bushing from falling through the hole.

 

[JMorton99]

Hello,

To follow up on your comment. You are suggesting then that each hole axis would become a datum? So then by your suggesting, the surfaces would then be perpendicular to [A|B|C] OR [XX|B|C] where "XX" is the respective hole axis datum?

This approach would require 1 additional view at a minimum but 3 for better clarity. Is your suggestion a general practice that is used on the field? Have you encountered similar designs as the one I've presented with a hole at an angle in three axial directions?

Thanks.

 

[JMorton99]

Hello,

Another question I had was if it is necessary in this case to add the Maximum Material Condition? From my understanding it adds tolerance as the hole size goes from MMC to LMC to ensure mating will occur within the Virtual Condition. This is for interchangeability. However, for my part there is no other assembly or part that will go through the holes. Only bushings will be mounted into the holes and they will fall in place wherever the hole should lie. Then a drill will be manually be put through the bushings. Am I correct in leaving it out and in the statements I've made? Thanks.

 

[3DDave]

Last first, RFS is the better answer.

First, because the drill bushings should be an interference fit, so greater size doesn't change the location of the drill bushing.

Second, because there is no value in mis-locating the resultant drilled holes in relation to the departure from MMC.

The surfaces should be held perpendicular to the holes within whatever limits the drill bushings require.

If it needs to be tightly controlled, the datum for a hole is marked, in this case, 4X INDIVIDUALLY and the FCF for the holes is also marked 4X INDIVIDUALLY. This is just orientation control

The surfaces can be located with a basic dimension along the axis from A and a profile tolerance to [H|A] 4X INDIVIDUALLY where [H} represents the hole or, if perpendicularity is not an high concern, to A|B|C.

You might include an image of the overall use of the fixture including the arm and pole and perhaps the drill bits, and another image showing what is being attached to the pole using this bracket. It's the bracket that determines how tightly controlled the holes need to be, which will drive how tightly the drill bushings need to be held.