Unilateral VS Bilateral Tolerancing

[tiktak292]

I am wondering if there is any type of drawing that would require the designer to use both Unilateral and Bilateral tolerances in the same drawing? If so what would be an example of one?

 

[olixus]

Unilateral = variation permitted in only one direction.
Bilateral = variation permitted in both direction.

I'm not sure if I'm missing the question completly, but it wouldn't make sense to limit the drawer to only one or the other?

EX: Sometimes interference between two pieces is a big nono, so you are forced to specify a unilateral tolerance. Other times you dont really care about the direction, you just specify the tolerance to have something more specific than the offered machining tolerances.

 

[MintJulep]

Yes.

Any case where some features require one type of tolerance and others a different type.

 

[dgallup]

Unilateral tolerances are much less frequently used than bilateral tolerances. Most drawings will not have any unilateral tolerances. I would think nearly every drawing with unilateral tolerances would also have bilateral tolerances.

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[drawoh]

tiktak292,

If we were working on drafting boards, this may be a meaningful question. I believe all of us are designing on CAD, mostly[ ]3D.

We have to decide how to transition from the CAD[ ]model to the dimensioned drawing. On parametric 3D, features on one part are used to drive features on the adjacent part. I like to set everything to nominal dimensions, e.g. If a 40mm shaft goes into a 40mm hole, I model both pieces at 40mm. If I have a minimum clearance, notice how we must wind up with a +/+ or a -/- tolerance somewhere.

On diameter dimensions, I reset the dimension to limit style tolerances. This is largely because people are rattled by +/+ and -/- tolerances.

Increasingly, people are using Model Based Definition (MBD). This makes the relationship between the nominal model size and the as-displayed dimensions, nasty and critical. The limit dimensions, noted above, actually conceal information. If I show a shaft at 40-0.05/-0.15mm, I am showing the as-modelled size, as well as my required tolerance.

--
JHG

 

[KENAT]

I've done it occasionally. I rarely used unilateral dims (other than 'min' & 'max' where appropriate) but when I do they're typically on a drawing that is otherwise bilateral and/or GD&T on other features.

(FYI if you want hardcore drafting advice in future you could try posting in Drafting Standards forum.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[handleman]

I do it all the time. A more prescient question would be, why would one forbid unilateral or bilateral tolerances? Anyone who can't understand both equally well should not be reading drawings for a living.

-handleman, CSWP (The new, easy test)

 

[redpicker]

We use both unilateral and bilateral tolerances on the same drawing all the time. I never even thought about it.

The way a part is toleranced can affect the way it is manufactured. Consider the length of a part that is produced from bar stock with a required length between 4.750" and 5.000". It must be less than 5.000" to fit the assembly, but as the length of the part decreases, it does not function as well. Below 4.750", the part will not function properly. If the length is expressed as 4.875 +/- .125", you will find the output is centered around 4.875 as the impression is that 4.875" would be "perfect". If, however, you express the length as 5.000 +0/-.250", you will find the output centered as close to 5.000" as the variation of the process will permit. Now, the OD could be 3.625 +/- .031", particularly if the design was based on 3.625 and history has shown that a +/- .031" tolerance was appropriate.

rp

 

[KENAT]

redpicker, unfortunately the effect you talk about, biasing real parts to one end of the tolerance zone is not explicitly supported by drawing standards etc. While some machine shops/machinists will do as you say and keep close to the 'nominal' dimension others will split the difference and still aim for 4.875. Others still might keep as close to 4.75 because it saves a little material.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[handleman]

I agree with Kenat... you can't depend on the behavior of the fabricator to influence the final measurements of parts you receive. You cannot reject in-spec parts because of where they fall in the range you've already said is acceptable, no matter the distribution.

http://eng-tips.com/viewthread.cfm?qid=332690

In my experience it's most common, especially on complex/high tolerance parts, that machinists (especially on non-CNC) aim for MMC and sneak down inside the tolerance to be safe. It's always easier to cut a little more metal off than to add it back on. You definitely don't want to scrap $500 in material and 35 machine hours because you cut a few extra microns out.

-handleman, CSWP (The new, easy test)

 

[handleman]

Sorry, ignore the extraneous linkage....

-handleman, CSWP (The new, easy test)

 

[mfgenggear]

Unilateral = variation permitted in only one direction.(not prefered for manufacturing)
Bilateral = variation permitted in both direction,
or high limit & low limit
example: linear length .350-.300. I use the nominal anyway regardless of the tolerancing method.
Max or Min type tolerancing can be dangerous. because it is human nature to hit close to max or min dim.

I do not Give a plus or minus on my work instruction. I only give high & low limit.
But on the engineering either way is acceptable.
the prefered is Bilateral

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mfgenggear]

In addition to the above the exception to the rule are those close tolerance diameters.
.0005" tolerance and closer. Outside diameter the the high limit is held.
Inside diameters the low limit is held. standard practice for OD & ID grinding.
this because it is very easy to deviate size. examples are if it is an OD," in the undersize condition " or if it is an ID "the oversize condition".
The dimension as drawn by the designer will be held regardless.

the point to this. is for your information. there's more than one option.
and it depends on the type & size of the part, mfg method, fabrication, machining, assembly. all have their methods.
to say one way is better than an other is probably experience or can be very subjective.

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mrainey]

I spent twenty-five years doing cnc programming and process planning in a variety of machining environments. I always aimed for the middle of the tolerance, be it unilateral or bilateral. I don't recall ever getting any negative feedback from customers that had to do with my interpretation of their tolerances.

Software For Metalworking

http://closetolerancesoftware.com