Anyone ever see a double positive tolerance? 6

[jaimeshawn]

Anyone ever see a double positive tolerance? (e.g 1.000 +0.005 +0.008)

I've always thought the target value must be between the maximum and minumum value; and was selected by the engineer based on maximizing yield depending on how the part would be made, and on maximizing performance of the final assembly. The tolerances could be symmetric +/- 0.003, or they could be asymmetric +0.003 - 0.000, but I've never seen double positive tolerance, or double negative before.

Did I miss something in GD&T class?

 

[vc66]

Usually double positives and double negatives are used for fits (i.e. slip, locational, press, etc.)

I use them all the time.

Try to search before posting. This has been discussed before.

Here's on example.

http://www.eng-tips.com/viewthread.cfm?qid=212575&page=8

V

 

[TheTick]

I've seen them. I don't like them.

 

[ctopher]

I start seeing them towards the end of the day.

I agree with the others.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[PaulJackson]

It never isn't done like that here... but if a double negative is actually a positive... then isn't a double positive actually a negative???

 

[MikeHalloran]

I've used them when I want to convey different information to different groups of readers, e.g. that the shaft takes a 120mm bearing, but it's pressed on.

It usually results in a call from the shop ("Am I understanding this correctly?" ... or some rude equivalent).

It's not a bad way to:
- get acquainted with the guys in the shop.
- find out they are really working on your project.



Mike Halloran
Pembroke Pines, FL, USA

 

[JMarv]

I'm not that crazy about them, but they do show design intent.

Joe
SW Office 2008 SP4.0
P4 3.0Ghz 3GB
ATI FireGL X1

 

[CoryPad]

Double positive (or negative) are part of comprehensive systems for limits and fits. For example, the ISO 286 ISO system of limits and fits series provides for parts that can be designed with any combination (double positive, double negative, symmetric +/-, etc.).

You can review information on this subject here:

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[fcsuper]

I've used double positive and double negative. sometimes for fits, sometimes because a solid model hasn't been updated to have a nominal within the desired tolerance range on the drawing (yes, there are reasons to do this).

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

I agree with above that you see them a lot with shaft fits, especially ISO ones.

By extension some German drawings seem to use them even on items that aren't a classic 'fit' situation.

They can cause confusion so I avoid them. Also with some CAD systems it can be awkward to achieve.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[handleman]

A lot of this stuff is very industry-specific. I design custom assembly equipment for factory automation. Unilateral tolerancing is really the only thing that makes sense in that context. Check a catalog for dimensional drawings of anything - linear shafting, bushings, servo flanges, spacer blocks, dowel pins, raw steel dimensions... the list could go on and on. If there's any sort of fit involved, you will see unilateral tolerances. I'm constantly amazed at people's ignorance of this common practice.

-handleman, CSWP (The new, easy test)

 

[btrueblood]

Mike's quote:

"It's not a bad way to:
- get acquainted with the guys in the shop.
- find out they are really working on your project."

I would add

- find out if they really read your drawings

 

[dingy2]

Double positives or double negatives blow me away. One would still have to figure out the calculated nominal with both the USL and LSL. Why not just relay the true nominal with tolerances rather than making shop floor personnel use their calculators? It really seems like exercise in math but I am coming from a shop floor perspective.

Dave D.

http://www.qmsi.ca

 

[TheTick]

Most machinists are really good at giving what was asked for. Even better if one asks nicely. Double positive/double negative is not asking nicely.

I create part drawings that reflect the desired geometry of the part. Things like sliding fit or other design intent information belong in the project design file, not on the drawing.

 

[KENAT]

In places that are used to them I'm sure they're fine but places that aren't used to them they cause confusion.

On the other hand, places that are used to them can most likely handle nominals with +-, or max min, just fine.

So, unless I'm sure all potential users of the drawing are fine with them (not just now but in forseeable future) I'll convert to either limits or nominal with +-.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[handleman]

As I said, much of this stuff is pretty industry-specific. I've never had a machinist from any shop we use complain about a double positive or double negative tolerance. If I modeled everything at the middle of the tolerance range, my assembly drawings would look messy - two lines so close that they look like one thick line, ugly dimensions like 29.95, etc.

-handleman, CSWP (The new, easy test)

 

[KENAT]

What's ugly about 29.95?

Not sure I see your concern on the assy drawings either.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[jaimeshawn]

Thankyou, And thank you for the link to the previous discussion.

I hadn't seen the practice before and I didn't remember the practice being part of Y14.5. I thought that the target value was an important piece of information for the machinist; tolerances could be, and in some situations should be, asymmetric, but the target dimension was an important piece of the design.

OTOH I could see that if you just roughed all the dimensions in at 'nominal' and then just loaded the values from the ANSI B3.17 (Shaft Dia and Housing Bore) tables at the time you create the dwg, it could save engr time. It is the equivalent of using MAX/MIN dimensioning. The machinist then choses the target value based on the processes being used.

 

[KENAT]

Jaimes, like folks above have mentioned it's more of an ISO/European thing. If you use 'common' basic sizes of shafts/holes then these correspond in many cases to commonly available tools. At least that's the theory I was told.

I don't believe it's in ASME Y14.5M-1994, though I dont' think 14.5 forbids it either.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[drawoh]

dingy2,

If the guy on the shop floor has a DXF file and is programming a CNC machine, it might help to see what the actual as-drawn feature size is.

I try to remember to convert my +/+ dimensions in SolidWorks to limit dimensions. Either way, the CNC programmer has to pay attention.

JHG