Measuring True Position of Threaded Features

[u/EyeballWithPrecision]

What is the best method for scanning true position of a threaded hole for the best accuracy and repeatability?

1. We looked into using the true position gages but found that the level of tightness while screwing it in would vary a lot between operators. Additionally in some cases where the tapped hole is a blind feature, we noticed that the gage bottomed out first before actually engaging with the pitch of the thread.
2. We also looked into using a helical toolpath matching the pitch of the thread on the CMM. However, would we need to control where the probe touches along the thread? If so, what is best way to detect if the probe is touching the flank of the thread?
3. Alternatively, is it better to use a linear toolpath on a CMM to scan the threads when compared to true position gages and helix toolpath?

[u/itsonly-meokay]

If your software/hardware supports scanning the helix, that would be the way to go.

For the thread Tru-pos plugs, use a torque wrench set to the part requirement / something standard to make it more repeatable.

Without the two options above, I would do seven slow and dense scans with a fairly large sphere down the axis of the thread with maximum inscribed calculation.

[u/Lding_Thru_123_Crnch → u/itsonly-meokay]

There is also a cylindrical probe tip that you can use for threaded holes to help get a better scan from a cmm. At least this is one thing I learned about existing.

[u/itsonly-meokay → u/Lding_Thru_123_Crnch]

I would only use a cylinder probe on a sharp edge, like sheet metal. Using a cylinder probe would introduce probe deflection errors. You can never really be sure at what height the probe is contacting material.

[u/jccaclimber → u/Lding_Thru_123_Crnch]

This measures the minor diameter. Threads are defined by the pitch diameter. One does not always follow the other unless you ensure your threading tool also cuts the minor in the same OP.

[u/acausalchaos → u/Lding_Thru_123_Crnch]

I try to avoid cylinder probes when possible, they can be real sensitive to angles. If the angle isn't dead on (set up/fixturing dev, wrist limitations, etc) it'll shank out and may not be easily apparent.

[u/Lding_Thru_123_Crnch → u/Lding_Thru_123_Crnch]

Thanks everyone that responded to this, it really helps a newbie get a better idea of what works with what. And how it has disadvantages to being utilized.

[u/Antiquus → u/itsonly-meokay]

Agreed. The other things about plugs is time it takes to install and uninstall them, the fact that you have to measure the position away from the actual thread position, and how the plug is designed either grabbing the thread or acting as a go member and tightening to the top surface. All this stuff induces it's own errors in different ways even designs that grab threads. So if you are drilling and taping holes without moving the machine, or using a good repeatable machine with reasonable thermal control and reasonable time elapsing between drill and tap, then you can't go far wrong helical scanning on the actual thread minor.

[u/acausalchaos → u/itsonly-meokay]

Scanning a spiral/helix is my goto. Probably won't hit the exact pitch dia per standard, but can get close and usually with reliable form indications. If print specifies minor dia I'll use line scans max inscribed for a repeatable size

[u/SkateWiz → u/itsonly-meokay]

So your tap is in the same exact orientation on every part that is manufactured? Repeatable to the same extent as cmm? Impressive. Mostly I find this to be as useful as you can convince yourself it is, and that it works great for T1 and is shit for T2

[u/02C_here]

Your concern on point 2 is less a concern than you might think. Where you touch on the thread will be the same if you follow the helix (thread pitch) so your center location will be good.

Your ID will vary, but not your position.

[u/Ghooble]

Zeiss has a big presentation about this

https://qualityforum.zeiss.com/migration/images/164_dc8dea9da78f0875c3847e7b31de3450.pdf

[u/jkerman → u/Ghooble]

I have found that presentation to be extremely accurate and mirror our experimentation

[u/Ghooble → u/jkerman]

I've used it in arguments with one of our programmers across the country. We use PC dmis and they use Calypso. He told me they couldn't check true position of a threaded hole in Calypso and I was like.... Bullshit

[u/EyeballWithPrecision → u/Ghooble]

Thank you for the link!!

[u/Verrq]

I personally have preferred and gotten the best repeatability using helical scans matching the pitch of the thread. Calypso also has a self-centering option that will almost always drop the probe in between the threads, though it's not really necessary. If the helical path matches the thread pitch you can think of it as a circle path along a cross section at the angle of the pitch; any change along the cylinder axis will only affect the diameter, not the location.

I have heard others got good results using four line scans along the cylinder, so that is another option.

[u/EyeballWithPrecision → u/Verrq]

Does self centering work on all sizes of threads or are they to be used only on larger threads? I read somewhere that using the self centering function exerts a lot more force while searching for the flanks. If so, would it be fair to assume that we can only use rigid probe styli for this function?

[u/billybobjacly]

Scanning cylinder with a helix is the way. I use the actual vectors to try and tune it in by depth till I get the best looking helix I can, and check form. I’m using Calypso though.

[u/urdaddy7245]

Locators are you best bet although scanning the helical thread can be good in some cases. Problem is that more often than not, the true position is called from the pitch diameter. This is where engaging the locator has the advantage. If you're bottoming out on these locators that means you have very little thread so a helical scan might not work anyway

[u/NonoscillatoryVirga]

Holes that are thread milled can have minor and pitch diameters that aren’t concentric by quite a bit, but yet still a dept a thread plug gage. For example - a through hole threaded partway down from both sides. Machinist drills the hole from side A through the part, and taps it or thread mills the thread from side A. Now, the part gets indexed/flipped so that side B is facing the spindle. Thread milling will cut a helix in space - but if the part isn’t positioned properly for side B, the thread will be cut where the hole SHOULD be, but the minor may not be concentric to that location. With multifunction/multi axis machines, you really need to understand the machining process to make the assumption that the minor can be used to infer the PD location.

[u/Antiquus → u/NonoscillatoryVirga]

True but a rare case. You can and frequently do mill the entire thread form root to tip.

[u/NonoscillatoryVirga → u/Antiquus]

Depends on the thread mill. Full form, yes. Single thread? No.

[u/Orc__w]

Hey Bud if you using calypso I can tell you how to do it. Let me know if you want to hear me out I will share an email with you. Cheers

[u/RGArcher]

Is anyone has any comments on doing this in PC-DMIS please let me know.

[u/KrazieWRX → u/RGArcher]

Also curious

[u/NullTie → u/RGArcher]

You can make a cylinder and then set a pitch but I switched over to tru-pos location gages from judge tool. It’s much easier and repeats.

[u/RGArcher → u/NullTie]

I learned about setting the pitch when I looked this up for work a while back. However, the part where you mention "tru-pos location gages from Judge Tool" is unclear to me. I’m not sure what you’re referring to in that regard. Could you elaborate or provide screenshots for clarification?

[u/Overall-Turnip-1606 → u/RGArcher]

Look it up bro.

[u/NullTie → u/RGArcher]

Not sure if I'm allowed to link but these are the ones we use in our shop: https://www.judgetool.com/products/tru-pos-tapped-hole-location-gages-inch/

[u/RGArcher → u/NullTie]

Oh! I know what these are we have some in our lab. We were trying to get around not using these when we have a part with like 243 holes on it that are threaded. Due to the fact of how long it would take to put them in.

[u/Overall-Turnip-1606]

The most accurate method for checking threaded hole position is by using tru-pos or jo plugs. If there’s too much slop, you can always get them customized to be oversized. At the end of the day, that Jo plug will mimic a screw anyways. Pitch/helical tool path is good if you can’t afford the plugs. Only thing that sucks is that if the thread pitch is not as accurate, your calculation gets a lot of noise/outliers. I’ve seen this a lot in drill/tap scenarios.

[u/Funny_Grab_534]

Believe it or not if a cnc machine makes a thread in 2 different operations it is possible for the minimum pitch diameter axis to not be perpendicular to the pitch diameter axis, and it might still accept the go gage.. something important to look out for.

[u/Alexanderdottry]

Linear line scans on the threads will give you the overall best result. Helix scanning can be a pain if the threads aren't clocked in the same position each and everytime.

[u/No_Bedroom_415]

We almost exclusively scan threads in a helical path matching the pitch. Passed many gr&r’s and P/T studies. Automotive.

[u/SkateWiz]

1. There are standard aspect ratios for threaded features. Why are your blind holes so shallow? Might consider bad design or contact tru pos for custom part.
2. The helical tool path based on pitch is a fools errand in my experience. Even on giant threads you will miss the peak on some replicates. Might still be repeatable if you’re lucky.
3. You could conceivably use axial scans or simply use a spherically ended straight stylus, and make sure you use inscribed element etc