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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?

RE: Positional tolerance of helical threaded inserts

I've previously had it suggested to me that around .014 dia position tolerance (corresponding to around +-.005) is a good guideline for various types of 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.

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RE: Positional tolerance of helical threaded inserts

(OP)
Thanks KENAT. That is helpful. I was given the same suggestion a few years ago by a design manager, but was told recently by a co-worker that my suggestion for a .010" position tolerance was TOO LOOSE. That co-worker told me to call out 0.005" positional tolerance on the threaded insert position.

RE: Positional tolerance of helical threaded inserts

Too loose?

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.

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RE: Positional tolerance of helical threaded inserts

sorry, but what do you mean by "positional tolerance can be reliably achieved on 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

(OP)
The tolerance in this case needs to be very tight because of the small size of the part and the minimization of it relative to the components it contains. Therefore, I need to know how tight I can tolerance the position of the threaded helical inserts. For rb1957, I mean: what can be achieved as far as the position of the pitch diameter of the insert with respect to the part datums without incurring a ridiculous increase in cost or a very high likelihood of non-conformance.
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'd talk to the machine shop that will be making the parts if at all possible.

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.

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RE: Positional tolerance of helical threaded inserts

we "just" tap a hole, screw in the insert, no problems ... ??

another day in paradise, or is paradise one day closer ?

RE: Positional tolerance of helical threaded inserts

that'd depend on the mating part.

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

(OP)
Thanks guys. KENAT, what you are saying agrees with what I have found from other sources. Sandia Labs conducted a test where they found they could achieve Ø positional tolerances of about 0.003 for threaded holes in aluminum. However, in steel they were able to achieve 0.0004! Like you said, on top of that you have to figure in the tolerance of the insert alone. On a final note, I have found that using a jig boring machine is the best way to achieve these tight tolerances.

RE: Positional tolerance of helical threaded inserts

ys... Note.

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

The use of a threaded insert usually guarantees that the shear plane between the two mating parts will pass through the threading of the bolt, not its shank. Meaning the bolt's minor diameter, not its full diameter is the shear load path. Shear-critical joints are not what these fasteners are intended for. The corresponding mating part should have enough "slop" in its hole sizes (oversize) to accommodate the variances in the tapped hole locations.

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

(OP)
There is no significant shear load to transmit through the screws, and micrometer-level slip between the fastener and the joined part is acceptable under CTE mis-match conditions. I just don't want to have a through-hole so close to the edge that it might break through the edge or leave it paper thin. I am working on a very small electronics package. The distance from the hole to the edge is only .022 in. So, 1/64 oversize plus positional tolerance is leaving it uncomfortably thin. I have implemented a solution that thickens the screw boss and lengthens my edge distance on the through hole. Yes, this has a minor impact on the mass, but we are still well under the baseline.

RE: Positional tolerance of helical threaded inserts

for me, in this special instance, i'd drop the oversize allowance.

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

I don't use helicoils, but with keenserts or rosans I usually apply the positional tolerance to the tap drill hole to make inspection easier. The downside to doing this is that it does not capture the error between the inner/outer insert thread PDs and between the tapped thread PD and the drilled hole cylinder, but the total of these errors is typically quite small. If you apply the positional tolerance to the installed insert internal thread, the measurement is based on the thread PD according to ASME Y14.5-2009 section 2.9 (see attachment). Inspecting the installed insert thread PD position also requires the use of a special gage.

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

(OP)
Thanks very much for your valuable input Terry. Neat little animation of the gage. QUESTION: Would you hesitate to apply a .005" positional tolerance to a #2-56 threaded insert referenced to the part datums? The greatest distance to the datums is roughly 1.5 inches. The material is a 99% molybdenum alloy.

RE: Positional tolerance of helical threaded inserts

BriPat-

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

"I usually apply the positional tolerance to the tap drill hole to make inspection easier"

Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists - or am I missing something?

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RE: Positional tolerance of helical threaded inserts

Tap drill size sets the minor diameter of the tapped hole. If it isn't still there, the tap was being used as a drill, which doesn't work well. Not that it can't work, just not well.

RE: Positional tolerance of helical threaded inserts

3DDave, depending on material and thread forming method I don't believe that's true. Tap drill size will be close to thread minor diameter but may not be within the required tolerances.

Explicitly applying the FCF to the minor diameter is sometimes done to simplify inspection, but again strays slightly from the true functional requirement.

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RE: Positional tolerance of helical threaded inserts

I agree with KENAT. I prefer to specify either MINOR Ø or MAJOR Ø (or let it default to pitch diameter, which while more difficult to verify is most accurate).
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

Quote (KENAT)

...Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists - or am I missing something?

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

I was referring to the suggestion that no trace of the tap drilled hole remains: "Well, it makes in process inspection easier but not finished part/receiving inspection as the feature no longer exists"

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

The reason thread PDs are mainly used as the positional locating feature is because when the mating component threads are fully engaged, it tends to produce a self-centering effect about the relative PDs. The mating threads contact at their flank surfaces, and not at the major/minor diameters or roots/tips. The number of engaged thread pitches also has an effect on the positional location between the mating parts. With just a single engaged thread pitch there will usually be significant clearance. But with, say, six engaged thread pitches there will be more of an accumulated interference fit that will result in a greater self-centering effect about the thread PDs.

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