Deep hole position tolerance

[cjccmc]

I need to make a hinge support for a heavy hatch. The part has to be machined from a single piece of metal. The hinges on the hatch are about 24" apart and will hang/attach to my support with two .50 dia pins, each about 4" long. So my support needs to have two clevises machined into it, one for the upper pin and one for the lower. I would like the pin holes to be .501 - .502 diameter. My question: How "in-line" can I get the final four holes? In GD&T what true position tolerance would I need to specify to make this practical to manufacture.

Initial piece of stock is 4 x 4 x 28. Alum 7075 or Titanium 6-4 depending on how much strength is needed (not final on that yet)

 

[mfgenggear]

true position, The datums must be to the mounting surfaces.
specify hole(s) must be machined & inspected inline.

HTH

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

 

[MikeHalloran]

I think you can call for a projected tolerance zone, at least within a given clevis.
The old school way would be to insist that say a .500" gage pin of 4.5" length slide through both holes in a clevis while neither hole can be larger than .502" dia. GD&T can say that concisely.

Doing the same thing for all four holes may not be realistic, or desirable.
One, a .500" dia x 28" long gage pin is going to be a little flexible, so gravity can affect the measurement. If the nominal diameter were say 2.500", gravity would be less of an issue in inspection.

Two, it's possible to 'line bore' both holes of a given clevis with a single tool in a single XY location, only feeding the spindle in Z direction, so the stated requirement is achievable. But it's not possible to line bore both clevises without moving and reorienting the spindle or the workpiece, so the desired tolerance may not be achievable. Again, easier for a larger bore, where a secondary line bore operation is slow, but possible.

I think the alignment of the two clevises relative to each other can be looser, because the pins are not all that stiff, and probably the clevis ears are somewhat flexible too, so the assembly is to some extent compliant or self-aligning. How much misalignment you can tolerate depends on the geometry of the door and the jamb, the size of the pins, and operational requirements not yet revealed.

You might consider making one of the clevises flexible enough in non-radial directions to align itself, or bolted in place instead of integral, to allow a little adjustment.

Mike Halloran
Pembroke Pines, FL, USA

 

[silveringchap]

If I was given the job of machining this up i would drill & bore one clevis from one side turn it around and setup for drilling & boring the other side but coming off the same machined datum faces as used for the other end. If aligned perfectly, I dont see why this wouldnt work. Only other way of doing it is with a jig, but for a one off, the expense would not be justified.

 

[cjccmc]

This design requires that the end of the hatch furtherest from the hinge (about 4 ft away) close to the same position within a small tolerance. Looking for a stiff mounting with minimal play. Mike's idea of the clevis floating and located after the hatch is positioned is one way, but I'd like to get this done with less assy and shimming work if possible. I first want to evaluate how tight I can make the hinge pin and clevis assy. I think I need to find a gun drilling shop and see what they can realistically do for in-line holes. Just perusing some websites I see .001" per inch for some, others say .001" per foot of depth for hole drift. Bottom line number is how big do the holes have to be to get one long .500 dia pin thru all four holes.

 

[MikeHalloran]

Gundrills are supported by the bore they make.
A big lump of air in the middle doesn't help.


Mike Halloran
Pembroke Pines, FL, USA

 

[mfgenggear]

no vote for gun drill, yes the clevis can be flip & held in line easily.

I am old school, & I prefere to verify the holes are inline with a minimum size diameter pin in both holes.
it is done consistantly this way. This way it is assured it is in line.
a CMM can also verify if the positioning is good, I prefer the pins because I know it will assemble. like a go gage. yeppers.

but for you just specify that it must be inline let the mfg worry how it's done.
but only if you have to have it this way.

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