standard callout for "stock size" in drawings 3

[pavvpk]

Hi

What is correct way of specifying stock size for machining print. For example if holes need to be drilled on a 1/4" thick plate.

- Reference the dimension in parenthesis "()"
For example - (.25)/(.250) - depending on level of tolerance looking for stock.
OR
- calling out "STOCK" after the dimension?
For example - .25 STOCK / .250 STOCK

thank you very much,
pavvpk

 

[CheckerHater]

If holes need to be drilled on a 1/4" thick plate (or any plate), you either specify depth, or you say "THRU"

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[ewh]

Being reference, those dimensions have no tolerance associated to them regardless of the number of decimal places.

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

 

[ctopher]

I never call out stock size on a drawing.
Leave it up to the machinist to use available sizes. Don't tie them down to a certain size.
If you call out a .25 plate, and he/she uses .38 plate, does it really matter?

Chris, CSWA
SolidWorks '15
SolidWorks Legion

 

[ewh]

Different companies do it different ways. I have usually seen it as a reference dimension or a dimension with "STK" following it to designate it as an uninspected dimension.
Any size tolerance usually gets controlled by the parts list or bill of materials where the relevant material standard would be included.

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

 

[3DDave]

Stock sizes are frequently subject to tolerances that are specified in the related material spec. ASTM specs for sheet and plate have thickness and other tolerances. One of the trickier things is that material properties, such as strength and ductility are also based on material thickness - typically improving as the material is thinner. In some materials, machining to reduce thickness relieves internal stresses and nets a potato chip shaped part as a result.

In any case, the thickness should be a firm callout and there must be traceability to tolerances for thickness, flatness, et al. The number of decimal places on stock dimensions have no effect on tolerances for stock material characteristics.

 

[powerhound]

Also keep in mind that if you want the material thickness to comply with general rule #1 you should not identify it as a stock size. Doing so means that rule #1 does not apply.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

As per ewh second post with the proviso's that others mention.

Be aware, while the thickness spec on 'stock' material may be ok, it often has bad flatness - especially if at some point it was on a roll of material.

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(probably not aimed specifically at you)
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[drawoh]

pavvpk,

I don't call out stock size. I call out the dimension I want, and I specify the tolerance, parallelism, and flatness. I try to ensure that my numbers are achievable by the stock size.

Let the machinist figure out how to do .250[±].030. There must be a way!

--
JHG

 

[pavvpk]

Thank you all for replies..First I would like to clarify on the the dimension control on stock material I am looking for.
Few colleagues pointed out about - flatness, tolerance issues etc if we specify that thickness dimension as "STOCK" or reference one. But if that thickness dimension needs have precise tolerance/flatness control or surface finish, I wouldn't leave it as stock or reference. I would call out required tolerance specification so that machinist will machine that part to obtain required control.If any stock plate is available with required tolerance or flatness control, I would call that plate purchasing info in the print so that would guide people to procure same or equivalent material - to avoid unnecessary machining cost.

The question here relates to a stock material(with .25 or .250 thickness)& which doesn't require controlled features - either flatness or surface finish..etc.

Looks like ASME Y14.5 doesn't suggest to use or not to use "STOCK" or "(reference)" methods.
Rule 1 only says each dimension should have tolerance (or govern by general note) except ones that says "STOCK" or referenced ones or MIN/MAX...etc.

So can we take it as - use of "STOCK" or "STK" is approved by ASME Y14.5?

Also, I would like to know if you run into any issues by using "STOCK" or "STK" for stock size?

thank you,
pavvpk

 

[JNieman]

Issues with calling out "STK/STOCK" is that sometimes it's bad business to tell people how to make parts, who are in the business of making parts. Unless it is critical to the design of your part, such as grain direction or other structural considerations, just tell them the finished size. If you call out "STK" as a way to indicate an unimportant REFERENCE dimension, that's one thing. To call out ".250 STK" and expect them to hold whatever your applicable tolerance is, say +/- .010, you'll be disappointed. Most I've encountered take "STK" to mean "Start with that size stock and clean it up. Your final dimension is whatever is left, that's it." which means they treat "STK" essentially as "MAX". Not saying it's right. Saying what I've seen in various manufacturers. If the dimension has a tolerance to it, leave off the "STK/STOCK", IMO.

Here, we treat it as a REF dimension (that rule was established before I came here and I have had to reason to challenge it).

The popularity of dovetail vices has rendered "STK" dimensions a PITA for anyone hired to make parts. Yes, the part can be made from 1-1/2" thk bar stock, for example, but they'd rather start with 1-5/8 or 1-3/4 so they can machine a dovetail into it and cut on all 5 remaining sides while it's in a dovetail vice.

I also wouldn't rely on the flatness, straightness, roughness tolerances of stock material. Depending on your supplier, you may or may not be getting material in such condition. Yes, it might have started that way when it was fresh off the rollers from the mill/foundry, but you don't know what you're going to be delivered. It's sometimes cheaper to just get something 1/8 or 1/4" thicker and machine it down than to pay a higher price for stress-relieved flat stock at the 'finish' thickness. Depends on the part of course.

Long story short- there's a lot of variables at play that you don't see if you're not familiar with buying material and making parts from it. IMO it's best, as a designer, not to create expectations of what stock material to use unless it's critical to your design. You're not doing anyone any favor by suggesting they use 1" x 1/2" bar stock. They will look at your part and decide what stock to start with. They likely know much better than you do, based upon their supply chain and in-house capabilities.

*edit* Fixed a couple grammatical issues

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[ctopher]

I agree with JNieman.

Chris, CSWA
SolidWorks '15
SolidWorks Legion

 

[powerhound]

pavvpk,

You're confusing general rule #1 with the first fundamental rule. The rule you mentioned is the first of about 13 or 14 fundamental rules. General rule #1 is one of 4. General rule number one states that form must be perfect when a regular feature of size is at it's MMC. By identifying a thickness as STK, you are releasing that requirement and the form is then subject to mill standards. I think JNieman nailed it perfectly. Every time I've seen a thickness identified as STK it's the engineers attempt to be helpful. He/she also thinks they are saving money by not requiring material to be faced down. The real world scenario is that most of the time, facing surfaces to get them flat and parallel makes other operations go faster and makes for a better looking--and sometimes more accurate--part.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

You mention for machined parts in your OP for sheet metal though it's often entirely appropriate to use 'Stock' - IMO.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
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[2JL]

I concur with Kenat. We only use it for sheet metal parts. Although some machined parts are designed with a specific stock size in mind, we don't mentioned it for cases where the supplier want to start from a different size.
2JL

 

[CheckerHater]

"You are right, and you are right, and he is right too"

Let's break problem into pieces:

1. STK is legitimate abbreviation for "stock" (I don't have ASME on me, but MIL STD says so). No arguing here.

2. Sometimes specifying stock does more harm than good. I remember working in a shop making simple small parts about 1/2 x 1 x 3 inches in size. One week we would cut them from 1/2 x 1 stock, and another 1 x 3. Clearly it 'd be better not to mention stock anywhere.

3. Specifying reference stock size may be helpful with non-machined flat plates and sheet metal.

4. Surface of cold-finished steel is actually strengthened by the process applied to it (rolling, drawing. etc.), so it may beneficial to show on the drawing that certain size should not be machined, but fabricator should make an effort and find stock of proper size.

Just 2 cents


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[sammcc]

an "in house" thing I know, but if we want the job to be made from stock plate we specify "stock" after the dimension, eg 0.250 STOCK or if we want it to have a 'decent' finish we add C/U eg 0.250 C/U for clean up

 

[JNieman]

If you state ".250 C/U" do you expect the finish size to be .250 within a 3-place tolerance? Or do you expect them to start with a 1/4" stock piece and machine it until the entire surface has had material removed and a 'machined finish' left? I've had designers at the /same company/ give me different answers. I've had quality people respond all over the board. I've heard:

".250 C/U" means:
1- Start with a 1/4" stock piece and machine it until it's cleaned up without exceeding LMC/LMB of the 3-place tolerance.
2- Start with 1/4" stock and clean it up until it's machined on both sides, regardless of resultant thickness.
3- Same as above but one side only.
4- Doesn't matter what you started with or what the C/U means, that is a .250 three-place dimension and you have to be within tolerance.
5- "I don't care that much, just gimme a plate that's reasonably flat and smooth, without mill scale/bark and somewhere close to 1/4" or so" (designer)

Such ambiguity is why I say "STK" is over-used and should only be used when it's clear, and means something, though I believe it to be rare.

The other issue is that I doubt many engineers/designers understand that stock size is a NOMINAL dimension. Example: If we order tool steel it will come in .030" oversize as it's assumed we intend to make it clean up exactly at the nominal dim after squaring/grinding/whatever. The dimension may be right-on or intentionally over-sized or under-sized depending on manufacturing process; whether it's a rolled shape, extruded shape, cast plate, etc. I expect that the vast majority of designers/engineers writing "STK" on their drawing have no idea that "3/8 STK" may come in at a wide variety of actual sizes depending on the /type/ of material being ordered.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[sammcc]

When using the phrases C/U, STOCK, STK etc in my opinion these should only be for items where the tolerance is wide open.
You could not specify STK or C/U and then expect a machinist to hold +/-.005"

At least I wouldn't anyway!

 

[DiegoLGraves]

Where applicable our company uses this in the notes: "REFER TO THE MATERIAL SPECIFICATION INDUSTRY STANDARD FOR "STOCK" MATERIAL TOLERANCES." and calls out the material type, such as ASTM A36, in the title block. On the drawing view the material thickness is a dimension such as .250 STOCK. This covers about 90% of what we produce including metal and plastic sheet parts. For machined (not plate or sheet material) we call out the material type in the title block or notes and dimension and tolerance in a drawing view.

Diego