Question on Perpendicularity 1

[KENAT]

I got a call from a vendor about a drawing I checked a few months ago.

I’ve attached a VERY simplified sketch giving you a rough idea of the situation.

On the prototype, which was copied from a similar older drawing, the central bore was (incorrectly) identified as datum and the bottom face was perpendicular to it. (The real part has some other diameters coaxial with the central bore so it was unclear what the datum was – small iso

http://files.engineering.com/getfil...b7-4751-9435-2e2de688d2e5&file=iso-perp.tif).

When I checked it I changed it to make the bottom face the datum, as this is where it’s mounted, and made the hole perpendicular to it – Rev A.

The vendor is saying this doesn’t suit the CNC program & machining methodology they’d come up with based on the prototype drawing and will cause them trouble inspecting it and possibly meeting the required tolerance and hence cost will likely increase. They’ve asked it be changed back the make the central bore the datum and have the end face perpendicular to it. Proposed rev B.

I’ve looked at 6.6.4.1 in ASME Y14.5M-1994 and am inclined to think that per (a) Proposed Rev B would be a correct callout although I’ve usually seen it done (and most of the figures in the standard are done) it as per rev A.

So, am I missing something, does making the change have some consequence I’m missing? Is my vendor selling me a line? Also am I missing something in thinking it’s virtually equivalent in terms of keeping the bore and surface perpendicular to each other.

Any input/suggestions appreciated. I have to send him something today, preferably within about 3 hours time, so any input would be appreciated. If you read this after that time feel free to put your 2C in as at least it will help educate me for next time.

Thanks.

Your file's link is:

http://files.engineering.com/getfil...3db7-4751-9435-2e2de688d2e5&file=iso-perp.tif

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

Wow I had about 8 emails about this thread in my inbox this morning.

Xplicator, you seem to have got a pretty good understanding of the situation!

Drawoh gave me most of the answer I needed. I knew something wasn't right just couldn't put my finger on it. It was that fact that the axis was significantly longer than the face. Hence I'd changed the 'angularity' of face to bore from around .08° to around .03°, this caused most of the problem.

Part of the problem too was the the vendor had set up their machining process with the bore being the primary datum, based on what the proto drawing said. They'd quoted accordingly, as Xplicator pointed out, probably not an ideal procurment process.

As regards the machining issue, basically I don't care how they machine it as long as it meets the required tolerances. The Datums, as I understand it, should reflect function and support inspection more so than directly support the manufacturing itself. That said it is important to bear in mind manufacturabilty of the part.

While generally I'd agree to Xplicators last paragraph and resist efforts to make a drawing overly match process etc, in this case the problem was mostly our fault. The vendor had initially been sent what was effectively an incomplete drawing and done his best to make to it (I don't know if he'd had any contact with the designer to clarify on the proto). I'd then done a slightly half a$$ed effort to correct/improve the drawing and unintentionally tightened a tolerance. So in this case the vendor wasn't really being unreasonable.

So in summary, I kept the basic scheme as rev A but relaxed the perpendicular tolerance to better match the value on the proto. (the axis is almost 3 times as long as the face is across so I made it .003, although I suppose If I'd gone across the diagonal of the face .002 would have been more correct, oops)

Thannks all,

KENAT, probably the least qualified checker you'll ever meet...

 

[axym]

It looks like a satisfactory conclusion has been reached, but I'll throw a in a few thoughts to muddy the waters further:

There are other factors that can influence how advantageous the different versions are from a manufacturing and inspection standpoint.

If the hole is specified as perpendicular within .003 to the datum face,

-there is no form control on the face (it could have flatness of .25 and still act as a datum face)
-rocking on the datum face could be an issue
-the hole's actual mating envelope must be found in order to find the hole's axis. The AME is the largest perfect cylinder that just fits inside the hole (i.e. snug fitting gage pin or diameter of perfect expanding mandrel)
-there is no form control on the hole


If the face is specified as perpendicular within .001 to the datum hole,

-the face must be made flat within .001
-if the datum hole is referenced RFS, the datum will be the axis of the hole's AME (back to the snug gage pin)
-if the datum hole is referenced MMC, the datum will be the axis of a fixed MMC-sized gage pin (and there may be slop between the gage pin and the hole, causing datum shift)


Some of these effects may be minor, depending on what other tolerances there are on the hole and face (size tolerance on hole, flatness tolerance on face). But you're dealing with fairly small tolerances, so who knows.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

The full drawing of the part has additional controls which address at least some of the issues you brought up axym.

Thanks.

KENAT, probably the least qualified checker you'll ever meet...