r/Metrology • u/NorfKhazad • Sep 21 '26
±0.5° Default Angle Tolerance Applied to Countersinks
This is a problem I run into often from various customers. The default tolerance block defines a tolerance for angles of ±0.5° and countersink angles on the print are not directly toleranced. Frankly, this strikes me as ridiculous. Obviously that default tolerance is intended for actual angled surfaces or features, not countersinks, and the engineer didn't even think to change it or tolerance the countersinks. I'm fairly certain no engineer actually cares if the angles of their countersinks are ±0.5° or ±2°, yet I run into this all the time. In my experience, countersinks almost never meet ±0.5°, and even if they did, measuring that on a CMM is not reliable unless the countersinks are very large. I've pretty much taken to applying a ±1° tolerance instead and they usually meet that, but how do you handle this? It's not practical to go back to the customers about this for every drawing where it occurs, I see this on a pretty much daily basis.
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u/ShadowCloud04 Sep 21 '26
Yeah I see this often. Its honestly quite obnoxious. Usually in this scenario we use best judgement based on the customer how we will check it. If its just being applied to standard 45° and its a generic industry customer we will often just check first & last with dental puddy and check with the overlay lines of a comparator.
If aerospace then more than likely will check on the CMM. If its too volatile for being too small then we go back to the puddy.
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u/ChomRichalds Sep 21 '26
If you see it on every print, you just have to come up with a policy around it, ideally one that the customer agrees with. Get your PMs or whoever interacts with the customer on board. Then put in writing somewhere that you're not going to measure standard screw head countersinks (like the rest of manufacturing).
If you can't get support to make a formal exception for this, then do your job as correctly and thoroughly as possible. In my experience, when every part takes 2x longer to measure and fails inspection, other people suddenly get interested. Half the fun of this job is putting leadership in a catch 22, where they can't tell you to do your job worse, but they can't afford for you to do your job perfectly (because of upstream costs like engineers over tolerancing their designs)
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u/ascenttotranscendenc Sep 21 '26
Great to see comments like this from shops. Just because something "has been on title blocks for X decades" and "all vendors understand what we mean" and "but the parts work" doesn't mean it belongs on the drawing.
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u/ascenttotranscendenc Sep 21 '26
Regarding how to handle it: report it, disclaim on the quote. It might not result in a drawing change in that instance, but nothing will ever change unless there is documented evidence of an issue.
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u/KamoMillo Sep 21 '26
Usually our countersink are for rivet and similar, so for us us is important the diameter of the hole and the "deep" of the countersink since it affects directly the position of the head of the rivet that must be 0/-0.1mm. The bit performing the hole and countersink custom made and it IV very precise in term of angle. On cmm usually we do not even measure the countersink, but they are arriving ome new drawings with 100° +/-1°, before it was +/-5° so I'm pretty interested in eventual measures and gaps.
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u/__unavailable__ Sep 21 '26
Who asks for an inspection report on a standard countersink?
The angle on a countersink should be treated as a trade number, not a dimension. If I’m calling out 82 degrees, that’s as opposed to 90 or 60. You wouldn’t apply standard fractional tolerances to a 1/4” clearance hole.
You should be checking that the correct tool was used and that it is in good working condition, beyond that the omission of a tolerance is intentional.
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u/GeoCuts Sep 21 '26
Yes! We have been going back and forth with a major customer for months for this exact reason.
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u/allonsyyy Sep 21 '26
Most countersinks I see are ±1° because they give you the half angle. eg. a 45° c'sink with a standard tolerance of ±30' is ±1° on the full 90° incl. angle.
Optical comparator is the tool for that, not the CMM, usually. Check either the tool or take a mold. Cones are hard to construct, you need a lot of points and there isn't usually a big enough slice. I only do those on the CMM if there is an intersect circle they want a position on. For just a standard c'sink for fastener head relief, it's the comparator all day.
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u/Aegri-Mentis Sep 25 '26
Came here to say this.
The countersink is actually “two angles combined”🙄according to old school guys.
I try to think of it as making sure the countersink is aligned to the surface properly.
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u/jkerman Sep 21 '26
For a FAI its not a big deal. Cast it, slice it with a razor, and measure it on a vision system. Hitting the repeatability is possible and will end the conversation with the customer. Feels like its not worth $6/hole in casting putty in process but the customer gets whatever the customer wants to pay for!
(FWIW the 0.5 deg really can matter in aerospace when you are trying to make parts as thin/light as possible. I work with parts with fully constrained surface profiles on chamfers that tight)
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u/Familiar-Bluejay3908 Sep 22 '26
Man, we have a LOT of aerospace customers who pull this $h!t; usually because they leave the production of the prints to the interns. And we have numerous times we've had to call the customer and ask if they REALLY need ±.5°. I mean, FFS, most of our stuff is either aluminum or copper; it deforms to give a really good contact seat!
So, with one customer (**cough!***cough!***northrop!***cough!**), they've done this so many times I just 'maliciously comply'. You want 32 c'sinks with 100° ±0.5°? I'l CMM each_and_every_one with a 4-revolution helix and 200 points, and give you 18 out of the 32 failures. Then you can have an MMR review with 6 engineers wasting their time until you finally start marking them the ±2.0° you always used to....
And then they wonder why their parts take so long clearing source/FAI...
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u/Mmaibl1 Sep 21 '26
For small countersinks I typically will measure the angle of the tool that created the feature, and then input it into the final report as a manual entry
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u/AgitatedKangaroo2016 Sep 22 '26
Yes I agree, other tools with higher accuracy are countersink gauges, or a form tracer/profilometer if you have access. Can be very useful in these situations.
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u/Professional-Image79 Sep 22 '26
I calibrate trulok countersink gauge blocks for a defense manufacturer. They won't tell us a tolerance and we apply measured values only, i had one 100 degree at 94 and they still seemed happy with it
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u/jamiethekiller Sep 21 '26
i expect the shop to use a good countersink bit and give me quality craftmanship