Project Tol Zone Rule of Thumb?

[dthom0425]

Hi all,

Just a quick question for projected tolerance zones. Does anyone have a general rule of thumb when a projected tolerance zone should be used based off the thickness of the mating parts?

 

[3DDave]

The rule of thumb is to perform the calculations to see if you have coordinated the feature sizes with the expected fits and tolerances on orientation and locations. There's no rule about not having to do this.

A projected zone is a way to make the interface relationship between parts more explicit.

 

[greenimi]

I just used copy-paste from a different discussion:

"It is mostly a trigonometry issue, but it is just a rule-of-thumb. So it becomes a judgement call as to when to use a projected zone. The theory applies down to paper-thin mating parts, but at some point the effective "excess" perpendicularity allowed by a very short projected tolerance zone would allow you to drag the head of the fastener across the surface of "paper." Also, with a very short projected zone, the likelihood of actually being able to produce a part so far out of orientation so as to cause an actual problem is minimal, and therefore not really worth specifying it.

Remember, that rule-of-thumb really only applies in situations where you put the mating parts together and then put the fastener through. If you have a situation where studs or pins are threaded or pressed into one of the mating parts first, and then the mating parts come together, then you would need a projected zone on the part with the press-fit or threaded holes in it and the length of the projection would be as long as the portion of the stud that sticks out (projects) -- no matter how thick or thin the mating part is.

In the case where studs were installed and then parts are assembled together, the projected tolerance zone needs to be specified on the part with the holes that receive the studs -- which is correct. If the drawing in question is the part that already shows the studs in place, then no projected tolerance zone is necessary since, the tolerance zone would default to being the length of the protruding stud, which is exactly where you want it to be.

The projected tolerance zone is a tool you can use to control material that isn't there yet. In other words, you want to be able to control where the stud/bolt/pin is going to eventually be forced to go. So you specify the projected zone on the drawing with the press-fit or threaded holes into which the stud/bolt/pin will get installed.

The reason I even brought up the difference between the situations of mating parts together, *then* installing the fasteners, versus installing fasteners *then* mating the parts together, is that the question was specifically about the "rule-of-thumb" for deciding whether a projected zone specification was needed or not.

I will reiterate. In the case of mating parts together *then* installing fasteners, the general practice (rule-of-thumb) is that you do *not* need to specify a projected zone unless the mating part thickness is equal to or greater than the diameter of the fastener being used. In this type of situation, the projected zone would be specified with a length that is equal to (or greater than) the maximum possible thickness of the mating part. That "rule" is simply a trigonometric effect and judgment call since the theory of the projected zone would apply even with thinner mating parts. So it's not in the standard that way, but it is a general industry practice out of practical necessity.

On the other hand, in the case where you install studs in one of the mating parts, *then* bring the mating parts together, the thickness of the mating part is irrelevant to the length of the projected zone required, and there is no projected zone required on either the clearance hole mating part, nor on the mating part where the studs are already shown in their installed condition. But there *would be* a need in that situation to specify a projected zone on the mating part into which the studs are installed, only this time the projected zone is specified at the drawing level *before* the studs are installed, and the length of the projected zone is the length of the stud that projects (i.e. not the entire length of the stud).
In the case where studs are put in before the mating parts come together, the projected zone is required on the part where the studs are going to go eventually, but on the drawing level before the studs are shown. There is no need to have a projected zone on the mating part with the clearance holes.

Think about this scenario: I'm going to bolt a piece of paper onto a part with threaded holes in it that will have threaded studs put in first, before I attempt to put the paper on. Also, let's say that the plane of the paper and the plane of the part with the studs that touch the paper are our primary datum features. If I do not use a projected zone on the part with the threaded holes, then the tolerance zone for those threaded holes will only apply throughout the length of the (internal) threads. Now let's say that the threaded holes are produced as far out of perpendicular as the position feature control frame with allow.

Now we put the studs in, and for the sake of illustrating my point with words, let's say that the studs are a meter long. If we "project" the out of perpendicularity/position allowed by our non-projected zones, the studs, once threaded into this part, will stick out of the part every which-way, and I will never be able to get my piece of paper (with the clearance holes in it) over the studs.

So again, in this type of scenario, the part with the threaded holes into which the studs will eventually be installed, but at the drawing level before the studs are installed, there needs to be a projected zone specified and that zone needs to apply for a length equal to the portion of the stud that will eventually stick out. Once the studs are in place, if there is a drawing that shows the studs installed, then at that level there is no need to specify a projected zone because then the zone will default to the length of the stud sticking out -- exactly where we want it to be.

 

[SeasonLee]

There is no rules indeed, but you may consider to use the projected tolerance on the following two cases:

1. If the length of the fastener that extends above the surface with the threaded hole is larger than the depth of the fastener inside the part.
2. Normally, manufacturing will cause a larger thread positional tolerance, so the designer need to control the perpendicularity by calculating the maximum angle permitted of the tolerance size and length.

Season

 

[drawoh]

dthom0425,

If you are clamping something thick to a tapped hole, you need a projected tolerance zone.

--
JHG