Is datum A-B still practical when the corresponding bearing surfaces have different sizes? 2

[rollingcloud]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1695940259/tips/6gorixqgl2rb1_z5rpth.webp[/url]

If the diameter of bearing surface A is smaller than that of bearing surface B, is it still practical to use A-B as the primary datum? The V-blocks supporting them would need to be of different heights. How can we ensure the shaft is flat and straight when it's sitting on the V-blocks? I have a part with a similar shape. However, while my part primarily moves axially (still allow to rotate), I'm most concerned about ensuring the conical surface (datum C) is as perfect as possible since it's used as a sealing surface. I'm wondering if I can apply the same A-B datum concept to my part.

 

[3DDave]

V-blocks for this application need to be augmented with a precision OD bushing with an internal taper and matching flexible collet to conform to the diameter. Either that or break out the micrometer and a calculator and figure out the gauge blocks to prop up the V-blocks to match the difference in diamaters.

 

[geesaman.d]

It's more convenient to manually inspect against an A-B datum line when they are the same diameter, but it's not necessary and in my world, not usually equal diameters.

Worst case you'd need to add gauge blocks to the v-block on the smaller end until the difference in diameter is eliminated. Best case, it's inspected on a CMM that produces a nearly perfect A-B centerline. The trickiest bit is making sure you include an axial stop that ensures the part doesn't move axially when rotating the part in the v-blocks.

 

[Burunduk]

Is it practical to place two collets coaxially?

 

[SeasonLee]

If the diameter of bearing surface A is smaller than that of bearing surface B, is it still practical to use A-B as the primary datum?

It is a common practice for a part establishing a single datum axis from two coaxial features of size with different diameters, many examples can be found on standard Y14.5-2018, pls see Figure 12-9, 12-11, 7-17.

How can we ensure the shaft is flat and straight when it's sitting on the V-blocks?

Gage blocks stacking method : Since both datum diameters share the same axis and are a different size, the V block method for inspecting this part would have to offset of V blocks and gage blocks to provide the proper amount of offset, you can place the gage block stacks under one of the V blocks to account for the offset of the different diameters.

Micro-rise V block method: You may use the Micro-rise V block set(as shown below) adjust one of the v blocks directly to account for the offset in the two datum diameters and then lock it in place, now the part can be mounted on the offset V block and is ready for inspection.

Micro-rise V block:

https://www.penntoolco.com/flexbar-micro-rise-v-blocks/

Season

 

[rollingcloud]

Thanks guys, I should have checked Y14.5 first..