How the bonus tolerance which we get by Material conditions are used during manufacturing? 1

[sluzzer]

I understood that providing material condition (MMC or LMC),while giving GD&T, gives extra or bonus tolerance. But how this extra tolerance is used during manufacturing?
For example,
When positioning a hole using Positional tolerance with Maximum material condition, we get extra tolerance for positioning, when the hole size is more (i.e., when it is different from MMC). But during manufacturing, the hole is drilled & after that we are measuring whether the hole dia is at MMC or different. Even if it is different, how the extra tolerance which we get from material condition, can be utilized, because the hole is already drilled???!!

 

[drawoh]

sluzzer,

Let's crunch numbers. This will show some context for all these specifications.

I have a housing with a pattern of tapped holes. Your part will be attached to this. There are locating faces on the housing which provide you with secondary and tertiary datums.

The holes on the housing are specified as M6X1, and they are located to positional tolerances of Ø0.4mm with respect to the datums. Your holes must clear these.

Each screw has a diameter of 6mm. Each screw will be located inside a diameter of 0.4mm. It is possible that the screw will project outside of this diameter. There is a process for specifying projected positional tolerances, but it is outside the scope of this post. Let's just add 0.2mm.

Your part must provide holes that clear 6.0+0.4+0.2=Ø6.6mm at the exact nominal positions. There are several ways to accomplish this.

My preferred method is to call up Ø7.8/6.6 at a positional tolerance of zero at MMC/MMB. This explicitly specifies that the Ø6.6 is to be clear. All of the manufacturer's error budget is contained within the additional 1.2mm of diameter allowed for the hole. The manufacture can use this to manage his drill diameter, as well as his positioning.

An alternate specification would be something like Ø7.8/7.0 at a positional tolerance of Ø0.4, with no material condition specified. Again, the smallest hole at maximum positional error clears the 6.6mm diameter required by the fastener. There is no potentially scary zero tolerance and no scary MMC/MMB symbol. There might be a reason you do not want the Ø7.8mm hole to drift 0.6mm away from true position, which my specification above, allows.

Either way, your fabricator must select a drill or punch, and work out how to position it within tolerance.

--
JHG

 

[ModulusCT]

I have to say this... Bonus tolerances are not just for hole. They're for any FoS that has a flatness, perp., angularity, parallelism, position or straightness tolerance.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!