With so little detail, it is hard to know what you are asking for. Some context would help.
When I refer to a pilot fit, I am usually referring to a set of mating surfaces such that one part is positioned relative to the other. The most common examples in my area of involvement are radial fits that line up two parts for the purpose of concentricity. An example might be the head of a pump that has a pilot fit to the case.
The tolerance for the fit is dependent on the requirements for the concentricity of the parts. Tolerance stack up also needs to be taken into account. In a single stage, centrifugal pump, for example, the bearing housing cover is a pilot fit to the bearing housing which is a pilot fit to the head which is a pilot fit to the case. If there was a great need to keep the bearing housing cover bore concentric with the bore of the case (wear ring, for example), then the fit and tolerance for each of these pilots would need to be such that the required concentricity is maintained.
For a pump, in my example above, the case to head fit may have a target of 0.003” loose (on the diameter) with a range of 0.002” to 0.004” loose. The fit between the bearing housing and the head may have a target of 0.004” loose with a range of 0.003” to 0.006”. The bearing housing cover to the bearing housing fit may have a target of 0.005” loose with a range of 0.003” to 0.007”. So, if all of these fits were at the loosest end of the tolerance range, the bearing housing may be non-concentric to the case by as much as 0.0085” ((0.004” + 0.006” + 0.007”)/2). By this, I mean that the centerline of the bore of the bearing housing cover could be as much as 0.0085” off from the centerline of the bore of the case. And this does not allow for any non-concentricity of the bore within the cover or the bore within the case or the bores within the bearing housing. If the allowable non-concentricity of those parts were taken into account, the non-concentricity of the cover bore to case bore could be even greater.
These fits are generally kept loose for the purpose of maintainability. If they were too tight, there would be difficult in disassembly and assembly. The fit targets and the tolerance ranges could be tightened up, but this increases machining costs and will result in parts repairs and replacement for parts that are out of specification for the fits.
Thanks for the great response. I am profusely grateful for it. I understand your analogies in terms of fits and tolerances. I guess what still rankles me is the 'pilot' term in all of there. I have always known piloting to mean something acting as a guide to something else...but when a surface has to act as a pilot to another...what is meant specifically. Does that imply both surfaces must contact each other....does that mean that the intended piloting surface must have tighter tolerances than would normally be required?
I am not sure what to compare the pilot fit to in terms of tolerances I generally use the term as a fit between two parts which is used to align them for assembly. Once the parts are assembled, there is generally a set of bolts to hold them in that position. The pilot is needed because once the parts are fully assembled, it may not be possible to check for proper alignment. In order tog get repeatable alignment of the assembled machine, I incorporate pilot fits in certain locations. Since the pilot fit needs to position the parts with enough accuracy to meet my requirements for the entire machine (perpendicularity, parallelism, concentricity), then I would tend to hold a tighter tolerance for the machining of these area as compared to less critical areas.
But, I cannot say, generically that the pilot fit area is the most critical. The bores for the ball bearings in my example pump are more critical than the pilot fit areas. I will require that the bores for the bearings hold a tolerance within a few of tenths of a thousandth of an inch (+/- 0.0004", perhaps). I will have tolerances for those bearing bores that includes surface finish, diameter, concentricity, taper and out-of-round. The pilot fit between the bearing housing and the head will have a much looser set of tolerances since I mainly need this to position the shaft through the mechanical seal and the rotating wear rings within the stationary wear rings.
In my industry (industrial mixers) the pilot is part of a connecting flange of some kind. Right or wrong, my colleagues refer to the pilot as the round step machined in both sides of the flange. We typically see the male step machined about .002" smaller on diameter than the female. This ensures concentricity and unless you have major corrosion, it's easy to assemble and disassemble.
However straightness is important too. So the flange faces must also be very square to the centerline of the shaft segments.
Sometimes there are features used solely to position parts but not fasten them. One common examble is cross-shaped bosses on automotive plastic consoles whish are used to mate with holes on other parts. The bosses position, but fastening is done at other points.
I don't think any "Tolerance" should be inferred or implied.
A C-face motor is meant to mount directly to some other commodity equipment, like a gearbox, with adequate concentricity,etc of the two shafts. A coupling that can flexibly accomodate parallel and angular misalignment is still usually required to prevent excessive shaft loads.
Note the "face" runout, "rabbet" runout and rabbet diameter tolerances are all specified and necessary.
Thanks for all the contribution. JJ, your illustrative and expansive explanations are well received. In a way, I think I have a better grasp of piloting now. Loosely, it could mean different things, but like you said, the main function lies in providing alignment prior to assembly. In my application, we are actually trying to use it in a more active way, not just passively proving alignment. Again, thanks.
