Postion Tolerance on a Theaded Hole? 2

[humanbone]

Has any one ever heard of a postion tolerance on the minor diameter of a tapped hole? If so how do you inspect that?

To me, it just doesnt make sense because there is very little surface contact to inspect. Does any one have any ideas why someone would do this? Thank You

 

[ctopher]

Yes. Add the FCF with position tol and add "MINOR" to it.
There are gauges to check threads.

http://www.gagemaker.com/api.htm

http://www.toolingu.com/class_class_desc.aspx?class_ID=350250

http://www.rmenergy.com/literature/pipeline/yale_thread_guide.pdf

http://www.sizes.com/tools/thread_screw.htm

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[ewh]

Chris is right. It is specified in para 2.9 of ASME Y14.5-1994.

 

[drawoh]

humanbone,

How would you inspect positional tolerances on anything other than the minor diameter of the thread?

JHG

 

[ewh]

Major or pitch diameters (need correct gauges).

 

[JohnNabors]

humanbone-

If you look on page 1360 of your handy-dandy MSC catalog, at the top of the page you will find threaded hole location gages in English and metric threads. They are threaded plugs with two gaging surfaces. The post is a straight plug ground concentric to the P.D. of the thread and the flat is ground perpendicular to the center axis of the P.D.

The only problem with you using these gages is that you are assuming that the minor diameter is concentric to the P.D. If it is tapped, it most likely is. If it is thread milled you can't make that assumption.

-John

 

[meintsi]

Minor thread diameters are the easiest of the three to check using a gage pin pushed down the hole. No special gages would then need to be ordered.

All machine shops should carry several sets of pins.


Remember...
[navy]"If you don't use your head,[/navy]
[navy]your going to have to use your feet."[/navy]

 

[MechNorth]

Minor diameter is also common when using a CMM to inspect features. Thread gauges will give a good estimate of the center axis of the tap, but are subject to the sizing error of the tap, plus a good thread gauge (plug) is very expensive. Picture that multiplied by a couple hundred or more of the same sized holes on a plate ... $$$. Stating MINOR DIA with the FCF indicates the inspection is to be done using the minor diameter, and this allows either the pin-method for manual inspection as suggested above, or the use of a cylindrical probe on a CMM. The drop-in gage-pin method does not provide any control over the orientation, and most people would only check its location at the surface instead of projecting the tolerance above the surface to see the entire axis. As a result, it could be significantly out of perpendicular to the surface and still be accepted. The CMM method has the advantage of having the cylindrical probe contact the actual inner boundary of the minor diameter, and includes the perpendicularity wrt the CMM bed. While there are errors inherent in this method also, they are comparatively minor and you have significant cost savings over carrying multiples of a large variety of thread gages, installing, removing and maintaining each of the thread gages.

The question of relative accuracy of each tolerancing method (Major, Pitch, or Minor Diameter) of a thread, and the different gage and inspection methods was being addressed by Sandia National Labs a few years ago. If I recall correctly, the preliminary and follow-up results weren't conclusive, and I think they were doing more tests. As I recall, there are some public-domain white papers published on the results.

Overall, consider the cost vs risk when deciding which part of the thread to use as the basis of the positional tolerance. If you can live with a little more uncertainty, you can more easily and economically inspect at the thread minor diameter.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[dingy2]

On high volume projects, I would suggest that a pattern of threaded holes should be reflected with "MINOR DIA" below the feature control frame and also at MMC.

One would make a checking fixture with locating pins made at the virtual condition size which would be the tolerance in the feature control frame beyond (smaller) the the theoretical minor diameter. This feature control frame could (should) reference primary, secondary and teriary datums.

This checking fixture would not only check the location of the holes but also orientation to the primary datum which probably would be perpendicularity.

Fixtures like this are ALWAYS used in the automtove industry by shop floor personnel. They are easy and quick!!

This is the only practical way to check or confirm a pattern of threaded holes in high volume work.

 

[MechNorth]

A checking fixture should be the most economical verification method in high volume projects, but more & more customers want hard data rather than a go/no-go, largely because they mistakenly think it is always better. The reality is, if they aren't doing statistical processing or tolerancing, then go/no-go is usually adequate. The few automotive facilities I've been in (stamping, assembly, and engine castings/machining) didn't use checking fixtures; they all used CMMs. Part of the reason is that if a feature is out of spec, it needs to go through a dimensional inspection anyway to determine what/where the defect is, and determine rework potential. Also, more suppliers (in all industries) are relying on statistical processing and statistical inspection to qualify workpieces without 100% verification. The reliance upon the process needs continuous numeric inspection data as a monitoring tool.
I've always been involved in small/moderate run production, so I've never justified the cost of a gage fixture. In these situations, with large plates, CMM is the only practical alternative.



Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[dingy2]

Hard data (CMM) is used on sample submission in stamping shops, etc but on an ongoing run, is not practical. I would never see in a Control Plan that one should take a part to CMM for positional confirmation.

A CMM confirms centres and not the shape of the hole. A checking fixture will confirm the centre and shape. One could have a CMM confirm a marginally acceptable hole but the checking fixture could reject it. The checking fixture (made correctly)supersedes the CMM.

If one was manufcturing an extremely small run, the CMM is best BUT one must confirm at the top and bottom of the hole and it is difficult if the hole size is relatively small especially contracting minor diameter threads.

 

[MechNorth]

Dingy2, you're right that 100% inspection is impractical if not impossible on large runs. The reality is, however, that large body stampings aren't 100% gaged either. Using statistical process controls, automotive companies have been moving successfully away from 100% verification for some time now. They select pieces for CMM inspection based on a statistical sampling algorithm that meets their quality requirements.

The CMM mechanism itself does indicate the center location of the probe at any given time. The software and algorithms therein determine the surface at the contact point. This extrapolation provides a minor error which is generally accepted by most industries. Using a cylindrical probe in a helical tracing path ensures that you are getting the smallest cylindrical zone that contacts the MMC condition of the minor diameter, and that the MMC cylinder is perpendicular to the CMM bed, which is typically a datum simulator. Additional errors will accumulate based on the length of the probe (deflection from normal increases with length), the scan speed/sampling rate, the surface condition of the probe, cleanliness of the machines surface, ...).

There are several key differences between gaging and inspecting that are driving manufacturing away from gaging and toward dimensional metrology, primarily using CMMs.
- gage fixtures for small parts with gage-maker tolerances are typically very expensive and are limited to one product whereas the cost of metrology equipment or a CMM is spread over a range of products
- gage fixtures for large parts with gage-maker tolerances are awkward to handle, damage easily, are difficult to store safely, and the cost is typically prohibitive whereas an appropriately-sized CMM can handle all workpieces from small to large with nominally the same accuracy and repeatability assuming normal maintenance
- verification of a gage requires dimensional metrology equipment, whereas dimensional metrology equipment requires calibrated artifacts; this puts gages at least one step further away from the national standard unit than the metrology equipment which translates to greater error
- functional gages only check the inner boundary, and do not indicate anything about the outer boundary condition; you would have to do LMC size-checks on each hole as well as doing the gaging, to be complete
- functional gages oly indicate PASS/FAIL at the MMC, with no indication of where the failure originates or data to determine if it is correctable
- functional gages do not provide any data regarding your process, and therefore a dimensional metrology plan must still be established to randomly inspect pieces to obtain the data needed to feed back into the manufacturing process
- functional gages do not indicate when a process has started to vary, whereas dimensional metrology with statistical tracking makes any variance immediately recognizable
- functional gages are typically applicable to one plane only, which means repeated setups, whereas CMMs can inspect all but the bottom surface of a workpiece in most cases
- each pattern of features must have its own gage fixture, whereas all patterns can be inspected simultaneously by the CMM with each pattern processed separately by the software
- functional gaging requires 100% part sampling whereas statistical sampling can be done when using dimensional metrology, which reduces your overall verification and process control costs

Functional gages do have a valuable function in situations where the tolerances are adequate to allow wear on the gage, where the gages are small enough to be easily handled and engaged with the workpiece, and where the variance of the manufacturing process does not impact the workpiece functionality.

My involvement has been in situations where cost reductions, reduction of scrap and rework, and tight tolerances were the rule. A gage-based system had been used for 30+ years for verifying standard tapers and diameters, etc.; warranty and rework costs were high but accepted due to market conditions. Problems became more evident when new (CNC-only) machinists were hired. The gages were not used properly and were abused so that they were constantly being reworked and replaced; the cost of gaging rose sharply. Because the gages weren't being used properly, the critical features were out of spec in reality whereas they "gaged ok". Around that time, the manufacturing-engineering group started to look into statistical process control, and found that they needed dimensional data. They started inspecting the critical features and new gages, and found that the workpieces were dimensionally more accurate than the new gages. I was indirectly involved up to that point, but became intertwined with the project when I indicated that the form, position, etc. that GD&T controlled needed to be verified also. We checked the workpieces and the new gages over time and found some very interesting trends on the manufacturing equipment and on the gages. All gages were scrapped as a result.

I used to think that the traditional contact CMM was the be-all and end-all but I was re-educated as to the reality of their limitations and errors. Each new software, hardware, and probe release reduces the error and improves the functionality of contact CMMs, but there are other systems that make some things a whole lot easier. Vision, laser, ultrasound and other technologies provide dimensional data quicker and with greater resolution and repeatability than contact systems, but are often limited bydepth-of-field to the outer surface.

No system is perfect. In the end, the method of part verification (gage, bench dimensional metrology, automated-system dimensional metrology) that you use should address what your goals are overall, not just at that one point in the manufacturing process.



Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[dingy2]

Mr. Sykes:

Sampling plans are reflected in a Control Plan and use, usually, a sample size of 1 part/hour for position confirmation by the Operator at the job site.

100% inspection is not used.

One cannot do statistical process control (SPC) on positional tolerances. It is not appropriate and the information that 1st tier automotive Customer receives as far as a Ppk of 1.67 or more is more or less "make believe". Over the years, suppliers are dropping the "critical designation" with positional tolerances.

 

[MechNorth]

Hello Dingy2,
I'm not from an automotive background, so what I've experienced are specific situations in stamping, not the trends. What I've read and discussed with automotive people, and seen at casting & machining facilities are my experiences, and again may not reflect the trends but specific instances.
My background is in machining; turning, milling, grinding, jig-boring, EDMing, etc. Workpieces ranged from under 5mm-cubed to plates 6ftx5ftx1.5ft. My preceding comments dealt with a range of machined "critical" features on a variety of machined parts, not stampings. I apologize for any confusion.

You're right that a Ppk of 1.67 is unrealistic for the position of a tapped hole, but the statistical data still gives useful process trends and control data for positions of machined taps. Sampling at a rate of 1 per hour, however, is a convenience rather than a statistical sampling rate. If you always follow the same sampling frequency, you can't correlate other factors that impact quality that occurr intermittently or cyclically throughout the day; there are published works on this topic. These sampling schemes are, however, perceived as a challenge to implement and are not used as often. I wouldn't think that it would be worth it in a stamping, but on a .0002in tolerance on a turned part, it's very useful.

You are also right that "critical" is being dropped, at least that's what I've found outside of the automotive sector. If people want items located better, they use alignment features such as taperlocks, dowels, etc. and adjust their tolerances appropriately for their needs.

As I said in my last post, no system is perfect. In the end, the method(s) of part verification that you use should address your overall goals, not just at that one point in the manufacturing process.


Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[Tunalover]

Folks-
Y14.5 applies positional tolerances to the pitch dia of a threaded feature by default and it does so for good reason. The pitch dia is the most functionally-significant property of a threaded feature because is represents the interface between the external and internal threads. To judge the entire screw thread by the minor diameter (internal thds) or major diameter (external thds) is a recipe for disaster. All it takes is one roque imperfection on ONE thread and the positon of ALL the threads in that threaded feature is misjudged. The pitch diameter expresses the collective effects of ALL the threads forming the threaded feature.

For this reason, EVERY text I own on GD&T states that it is not a good idea to measure positions of threads using the MINOR DIA or MAJOR DIA.

Tunalover

 

[musashi99]

I'm with you tunalover! Need to use truse position agages