Hole Pattern Used to Establish Datum

[jimbod20]

Hello,

I wanted to get some feedback. Please reference the attached test fixture drawings. The test fixture mating interface is surface A. My test item "mating part" in the drawing bolts to the fixture via four holes shown pictorially. The test item pilots in datum B.

I need the test fixture to be 'clocked' when mounted on surface A. I use 8 holes to bolt the test fixture to the test table. My test item is clocked via four holes shown pictorially. Fours holes have orientation to A and position to B.

I have defined datum C for clocking via the test fixture drawing 2.

One of the options discussed where I work is fixture drawing 1. My question. What does the C datum simulator look like for drawing 1? Is this a series of planes. I have read Y14.5 standard section 4.5.8. I'm trying to understand what drawing 1 datum C means.

 

[rhmeng]

ASME Y14.5 2009 figure 4-3 (see scn shot) shows what the datum simulator for a cylindrical surface. I think that this applies to what you are asking as far as what the datum simulator looks like. For clocking, I am not exactly sure what you mean when you are clocking with datum C. Datum C will only partially clock the part, it defines a position about where the part can rotate, usually a slot is used to further clock to stop rotation.

 

[greenimi]

Why do you need the clocking for?

 

[greenimi]

Also you did not define/locate the holes for the " mating part mount".
If you do ( and you need to), use the same DRF, A primary and B secondary, then you don't need any clocking because simultaneous requirement will take care of it (clocking)

 

[jimbod20]

Hello,

I was in a hurry writing Friday post.

The sketch is a test fixture. The fixture is bolted to a vibration test shake table and my test item bolts to the fixture. I simply draw the test item mount holes on the test fixture without providing detail mount hole definition. The reference frame for the test item mount holes are A primary, B secondary and C tertiary. I control orientation, position and clocking of my test item with respect to the test fixture.

The test fixture mates to the test table via surface A. I can "clock" the test item to the test fixture via one fixture to table mount hole as shown in sketch 2. I think I need a way to clock the test fixture to the test table. I just want to make sure that motion of the table along one axis results in motion along one (ie the same) axis of the test item. I now realize I need to discuss this issue with test facility. Maybe they can simply mount the test fixture/test item I provide to the table and rotate the fixture about the test fixture datum axis B to clock the test fixture/item to the table. Maybe I just provide a machined 'flat' on the OD of my test fixture to clock the fixture to the shake test table. The flat would provide a way to indicate the fixture to the table.

Back to original post.
I can see using surface A, diameter B and then one hole to 'clock' my test item to the test fixture. TMy designer suggested I simply define datum C as shown in Figure 1. He indicated the interpretation is one hole would be used as the datum. I know this is not correct. The datum is the pattern of holes. I cannot picture what the datum simulator looks like for a pattern of 8 holes?

 

[patdh1028]

The datum simulator for the 8-hole pattern applied RMB will be 8 cylindrical "posts", located at the pattern's basic dimensions, oriented to any higher-order datums in the callout, and which expand from hole-MMC-size to hole-LMC-size (all 8 posts simultaneously expand) or until the simulator makes enough contact with the part to constrain it based on the amount of constraint remaining from datums A and B. In this case since it is called out as a tertiary datum, the DFS would expand to make contact with 2 points somewhere on the cylindrical faces of any of the pattern features and fully constrain the rotation of the part as desired.

Because of the complexity of the DFS for hole patterns I typically use the MMB modifier in any callout reliant on a pattern datum. In that case the DFS is 8 posts the size of the MMC of the hole minus the location tolerance located at the basic dims, no expansion to consider. The DRF @ MMB is referenced to the simulator but the part can "float" within any clearance available between the MMB simulator and the part, it would not fully constrain rotation clocking but would limit it.

From what you're describing though it doesn't seem like that 8-hole pattern matters at all to your current function. If you're using a different pattern to mate the two parts, isn't that pattern going to limit rotation to some degree?

I also want to note that if you make only one hole datum C, and your part is axi-symmetric, you will probably have difficulty determining which hole to use as the datum once the part is made. So maybe your idea of adding the flat is the best course of action.