Well, thanks for all of the thoughts on this subject.
From all of this I have deduced 2 things.
1. I don't need UN JF position locators. If I am going to use a locator, the UNF version will be adequate.
2. I would probably be wasting time (and money) using a locator anyway. The value is dubious, depending on the particular thread and application.
What I will do is go with probing the holes with a spherical stylus and if/when the production folks complain that I should be using TP locators, I will let them buy one and we can then determine if they make any difference.
I would get the correct socket cap screw and get it turned down on a lathe (the head) screw it in and check it as a stud. like previously stated too many variable using the pitch... or just measuring the minor Ø anyways with that, not truly the threaded hole location. and whos to say all the threads start in the same spot on every part???
Just F.Y.I. ASME Y14.5M Para 2.9 "screw threads" states that "Each tolerance of orientation or position and datum reference specified for a screw thread applies to the axis of the thread derived from the pitch cylinder" Check out
http://trueprecisioncorp.com/ to find a good pitch micrometer. We use these pitch micrometers to accurately measure pitch cylinder size...they work like a charm and you can manually input actual size for MMC calculations if so desired.
Another little trick I have used in the past is to take a GH2 tap and buff it (so it wont re-cut the thread) then screw the tap into the threaded hole, it locates on the pitch dia and you can use the O.D. cylinder of the tap for your measurement.
I would get the correct socket cap screw and get it turned down on a lathe (the head) screw it in and check it as a stud. like previously stated too many variable using the pitch... or just measuring the minor Ø anyways with that, not truly the threaded hole location. and whos to say all the threads start in the same spot on every part???
There is nothing wrong with this method, but what if you have many holes to check on a production part ? Very time consuming. You are correct, there is no way to know where the threads start, thats why the pitch function works so well. whether your first point is on the crest, or half way in between, it doesn't matter, it hits in that same spot all the way up coming out of the hole. making sure you have a proper probe diameter and you are golden....some people in this thread seem to doubt the reliability or accuracy of this method, well, knock yourself out, do it your way...its like banging your head on a wall...
There is nothing wrong with this method, but what if you have many holes to check on a production part ? Very time consuming. You are correct, there is no way to know where the threads start, thats why the pitch function works so well. whether your first point is on the crest, or half way in between, it doesn't matter, it hits in that same spot all the way up coming out of the hole. making sure you have a proper probe diameter and you are golden....some people in this thread seem to doubt the reliability or accuracy of this method, well, knock yourself out, do it your way...its like banging your head on a wall...
+1 I like this method also I think the issue some people run into is the quality (surface finish) of the thread. I imagine like most places I've worked at in this trade most people cut there threaded holes with a tap. and depending on alot of variables (type of material, different taps, machine,ect). The facility I'm at now have two work arounds. Use a thread mill, this usually should produce a nicer thread then a tapped hole and are other work around is tap the hole after its checked. I know the second ones cheating if the print has the fcf on the threaded holes but I leave it up to the customer if there okay with the method its checked. Then I'm okay with it.
O.K......I will flog a dead mule. I worked as a machinist for 20 years before moving into quality. Anyone who thinks that thread milling produces a thread void of irregularity is wrong. Yes.....wrong. You will still have raised edges , frayed edges, etc. Bottom line: there is no clear cut way to check thread location on a CMM, period. You can be approximate. Never exact. Which is what we strive for.
what kind of material were you cutting nuguy?
we are in the medical industy and thread milling titanium for an implant with no raised edges , frayed edges, etc
and as i stated in an earlier thread we are checking the threads with the pitch function to a tolerence of .002" true postion
prior to trying i would not have believed it myself.
it has been very successful for about a year checking 32 parts a day
Please see my earlier response. The process described would produce the most accuracy on the floor, and in quality. And the least cost effective. We machine all types of material, from exotic plastics to Inconel, as well as titanium. While I may be a putz when it comes to this software (I come to read the posts and try to learn, and you guys are outstanding!), I have a decent handle on process and maximizing quality on the floor.
