STK - Reference or Not? 1

[InvoluteSpline]

The case I'm about to present below is easily solved by having our customer update the drawing to remove any discrepancies, but sometimes that is easier said than done. Also, the drawing follows the ASME Y14.5-1994 Standard.

Material is defined as Acrylic Sheet per MIL-PRF-5425. The part itself is round and the outside diameter is defined as "Ø.50 STK". Given the part needs to be produced from sheet material, no "STK" material will remain after manufacture. Looking beyond the simple issue of making a round part from a square block of material, is it possible to produce and inspect a part according to this drawing as-is? Can "STK" be treated as reference? I read on another thread that it is more like a soft requirement.

 

[djhurayt]

There should be an industry standard and what the tolerance is for 0.50" nominal thickness material.

As for example in the steel industry, when I spec out a part that is burned out of 3/4" thick plate, the drawing will call the material as" 3/4" STK, STEEL". The part may actually be 0.750" thick (and forgive me I don't remember the exact tolerance range), or it could be anywhere between 0.746" and 0.758" or what ever the actual tolerance value is. And not it is not necessarily symmetry about the 0.750 nominal.

 

[ewh]

Ø.50 derived from a .50 STK sheet is not stock. STK tolerances would only apply at possibly one discreet location on the finished product, unless the intent is to hold fabrication to the STK sheet tolerances for the entire part; those tolerances should be clearly spelled out.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[jasoncwells]

I'm a little confused about using a flat sheet to make a round bar.

Stock is essentially reference.

When you say that no stock material will remain after machining, then a stock annotation probably should not appear on your drawing.

From ASME Y14.5:

items identified as stock, such as bars, sheets,
tubing, structural shapes, and other items produced to
established industry or government standards that
prescribe limits for straightness, flatness, and other
geometric characteristics. Unless geometric tolerances
are specified on the drawing of a part made from
these items, standards for these items govern the surfaces
that remain in the as-furnished condition on the finished
part.

 

[drawoh]

InvoluteSpline,

A fundamental principle of drafting is that you specify what you will accept. You don't tell them how to do it. When you describe a feature as stock, you are telling the fabricator to not manufacture the feature. Do you really care? Your drawing should call up a size, tolerances and surface finish.

Can you specify a diameter, tolerances and surface finishes that are within the material stock specification?

--
JHG

 

[InvoluteSpline]

Thank you everyone for your replies.

I should clarify, I am on the aerospace manufacturing side and do not have the authority to change the design/drawing. Rather, I have been provided with a drawing that clearly contradicts itself by calling for sheet material while defining the OD of the part to remain as stock. I am curious as to what the interpretation of the term "STK" is (i.e. reference, soft requirement, hard requirement). In this case, can it be interpreted as reference, thus not violating any part requirements and successfully passing a first article inspection?

 

[looslib]

Is the part design in-house or are you manufacturing the part to someone else's print?
If in-house, go to the designer of record and get their reasoning for why reference STK when specifying a rod from sheet.
If external design, you still have the right to question the drawing to be sure your company produces the part that the customer is asking for.


It is a lot cheaper to buy acrylic in rod form than cut and then turn a piece of sheet into a rod.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[pylfrm]

InvoluteSpline,

"STOCK" (or "STK" I suppose) following a dimension generally means that the requirements for the dimensioned feature are defined in the material specification instead of on the drawing. If it were considered reference, you'd be free to ignore it. That is certainly not the intention when the notation is used correctly.

Basically you have a drawing with an invalid requirement. Applying a flatness tolerance to a cylindrical surface would be invalid in a similar way. How you handle such issues with your customer is beyond the scope of drafting standards. It could possibly be covered in your contract though.

Just out of curiosity, does the drawing have a general tolerance that would apply to the dimension if "STK" hadn't been specified? If so, what is it? Also, how many pieces are to be produced?


pylfrm

 

[3DDave]

There should already be an acceptance inspection sheet for the part. See if "STK" is on that sheet.

 

[InvoluteSpline]

Pylfrm,

"STOCK" (or "STK" I suppose) following a dimension generally means that the requirements for the dimensioned feature are defined in the material specification instead of on the drawing. If it were considered reference, you'd be free to ignore it. That is certainly not the intention when the notation is used correctly.

This is what I was after.

Yes, it would default to the tolerance block (±.010). Low quantity, 5-10 pcs.

 

[ctopher]

I never call out "stock". Let the shop use what can. If you call out "stock", this can mean this is the dim you intend to use on the part; therefore the stock tolerances can vary among suppliers.

Chris, CSWP
SolidWorks '17
ctophers home
SolidWorks Legion

 

[3DDave]

It would not default to the drawing tolerance block. It would default to the material specification tolerance. This is seen in some ASTM specifications. The annoying thing is that many times those tolerances are based on the width of the as-processed material coming from the factory, so it's not clear what applies to a random piece that's been cut loose.

 

[jasoncwells]

I stand corrected. STOCK is not essentially reference. You might still inspect that an I beam you purchased is the size you specified. But the dimensions and tolerances of that I beam will not be per your drawing, but per the material specs. pylfrm is correct.