Positional tolerance of helical threaded inserts
Positional tolerance of helical threaded inserts
(OP)
Can anyone tell me what positional tolerance can be reliably achieved on helical threaded inserts--especially the smaller sizes?
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Positional tolerance of helical threaded inserts
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Positional tolerance of helical threaded insertsPositional tolerance of helical threaded inserts(OP)
Can anyone tell me what positional tolerance can be reliably achieved on helical threaded inserts--especially the smaller sizes?
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RE: Positional tolerance of helical threaded inserts
Doesn't mean tighter isn't achievable if required but if I need tighter I go to greater lengths to check with whoever will be making the part.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
RE: Positional tolerance of helical threaded inserts
At the end of the day the tolerance chosen has to be functional (i.e. mating hole patterns/fasteners fit) and manufacturable at acceptable cost.
Your initial question sounded more like addressing the manufacturability aspect.
From that point of view your colleagues comment makes no real sense.
So I assume they were talking about functional requirements?
Without knowing mating parts etc. no one here can really comment on that, ASME Y14.5M-1994 annex B has calculations for mating hole patterns.
Another trick is to avoid using threaded hole patterns and their fasteners for precision location, instead use features such as dowel pin pairs with a corresponding slot & hole. This means the threaded fasteners only have to 'clamp' the parts together so mating holes can be bigger and tolerances looser.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
there's a spec for the tapped hole, so it's not that ... location of the hole CL ? position it as you need ??
another day in paradise, or is paradise one day closer ?
RE: Positional tolerance of helical threaded inserts
Part of the issue here is that I cannot make the corresponding through-hole diameters too large because of their minimized edge distance. As you probably guessed, that is why I need a tight tolerance on the threaded hole position.
RE: Positional tolerance of helical threaded inserts
I generally avoid threaded holes below around .009 pos dia - I've been forced to put tighter and machine shops have claimed they can hit .003 on smallish parts but I don't know that our inspection ever verified that claim.
Assuming installing a helicoil will introduce a bit more tolerance then I'd suspect .005 pos would be pushing it but maybe the right machine shop will say it's easy.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
another day in paradise, or is paradise one day closer ?
RE: Positional tolerance of helical threaded inserts
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
if it has a pattern of close tolerance holes, then you pretty much have to use a drill template.
if it has a pattern of loose tolerance holes, then typical drwg dim'ns and tolerances should handle it.
i wouldn't rely on drwg dim'ns to set a pattern of close tolerance holes. but then i work in a "chop shop" and not in rocket assembly.
another day in paradise, or is paradise one day closer ?
RE: Positional tolerance of helical threaded inserts
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
RE: Positional tolerance of helical threaded inserts
Most often helical coil thread inserts are used in mechanical assemblies [forged/wrought/cast materials], such as gear boxes to accept cover-plates, secondary covers, seal plates, etc.
In these cases, the holes with helical coil thread inserts [all in one assemly or the other] recieve long-threaded bolts, full threaded screws or cap-screws to provide pure joint tension, without shear capacity, IE: optimum joint clamp-up... usually for gasketing/sealing purposes. In addition to these bolts, the same flange, both mating halves, will have several precision-located close-reamed or transition fit high strength shear-pins around/between the bolts [and on internal webs, etc][or sometines coiled or 'C' spring pins], to stabilize the component joint in shear/torque. Between the tension threaded fasteners and the shear-pins, very durable mechanical case/component joints are made.
Regards, Wil Taylor
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RE: Positional tolerance of helical threaded inserts
A mention of an "edge distance problem" by the OP implies that you do have a shear-critical joint. Or maybe you just have an eager designer trying to "tighten up" the tolerances as a general policy, mostly because it sounds good, not because it makes a better connection between parts. But I haven't seen the parts that you're trying to assemble... Is a 1/64" oversize really going to cause an edge distance problem?
STF
RE: Positional tolerance of helical threaded inserts
RE: Positional tolerance of helical threaded inserts
for me, in this special instance, i'd consider slotting the holes to the edge.
another day in paradise, or is paradise one day closer ?
RE: Positional tolerance of helical threaded inserts
The positional accuracy of a hole pattern is highly dependent on the manufacturing method used. Assuming the part is fixtured properly, the accuracy of a hole pattern produced using a mill or jig bore is mostly a function of the machine used. A good quality CNC mill can hold .0002" per inch position tolerance on drilled holes under normal conditions, which allows fairly tight true position to be used for most applications. As wktaylor noted, fixed threaded inserts are really only suitable for fastener installations where clearance holes are used in the mating part(s). If the hole patterns are produced using a good quality CNC machine, I would have no hesitation applying a composite positional tolerance of .010" dia MMC within the hole pattern.
Hope that helps.
Terry
RE: Positional tolerance of helical threaded inserts
RE: Positional tolerance of helical threaded inserts
As long as you are using a good quality CNC machine, the part is fixtured rigidly and accurately, you use machine speeds and feeds that are appropriate, and your machine operator pays close attention to what he/she is doing, then you should be able to hold .005" dia position tolerance between part datum features and the internal thread PD in the base material. But this is not as simple as many people think and it may require running a couple setup parts to get the process right.
I did a bit of research and I found MIL-I-45914 which is a general specification for key-locked inserts (Keenserts). Section 3.8.3.3 specifies maximum runout of .006" FIM between the internal and external thread PDs, which is quite a bit more than I would have expected. You would have to add this insert runout to the positional tolerance of the threaded hole in the part to get the max positional error of the installed insert thread PD. The p/n for a .086-56 UNJC-3B key-locked insert is MS51830-101.
I've attached the relevant sections from MIL-I-45917 for your reference.
Hope that helps.
Terry
RE: Positional tolerance of helical threaded inserts
Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists - or am I missing something?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
RE: Positional tolerance of helical threaded inserts
Explicitly applying the FCF to the minor diameter is sometimes done to simplify inspection, but again strays slightly from the true functional requirement.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional tolerance of helical threaded inserts
I don't care what size drill the machinist uses as long as the resultant thread meets spec.
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Positional tolerance of helical threaded inserts
That's a valid point. But if the deliverable end item includes installed inserts, then none of the tapped hole thread features would be accessible for verification by receiving inspection, right? It is common practice to rely on in-process inspections as a means of quality control of a finished component. The in-process inspection records are delivered with the finished component, and they are reviewed by your receiving inspection as part of your QA acceptance process.
If your primary concern is truly the PD positional location of the installed insert threads, then you need to develop some method to inspect these features after installation. But this will not be an easy task.
RE: Positional tolerance of helical threaded inserts
The minor diameter generated by the tap drill should still be present even if much of the original hole surface is carved out by the tap. It's not as accurate a representation as the theoretical pitch diameter, however that diameter is established with any certainty, and should not be held as a definitive of which way a screw will be located or oriented when tightened.
Screw threads are of some difficulty for position callout - because screws are installed loosely, the threads they go into are really subject to movement in accordance with a Maximum Material Condition reference, but then they have conical alignment when tightened, suggesting a Regardless of Feature Size treatment.
This search turns up an amount of unsatisfying answers:
https://www.google.com/?gws_rd=ssl#q=position+tole...
such as https://www.linkedin.com/groups/Is-any-one-using-B... and http://www.eng-tips.com/viewthread.cfm?qid=374050
RE: Positional tolerance of helical threaded inserts