by GD&T Basics on February 14, 2021.
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This video is in response to a question that we received on our question line from Tim. Tim’s question is as follows: “We tend to use true position on threaded holes. Is this valid? If so, what is the proper way to measure it, and what is it “actually” measuring? Is it the true position of the Pitch Diam?” To begin answering Tim’s question, we will refer to our Geometric Dimensioning and Tolerancing standard.
The ASME Y14.5 Standard, section 2.9 Screw Threads states, “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. Where an exception to this practice is necessary, the specific feature of the screw thread (such as “MAJOR DIA” or “MINOR DIA”) shall be stated beneath the feature control frame, or beneath or adjacent to the datum feature symbol, as applicable.” To paraphrase the standard, the geometric tolerances and/or datum references apply to the pitch cylinder diameter unless otherwise specified.
The thread pitch diameter is one of three diameters that are properties of each thread. The major diameter is the largest diameter of the cutout required for an internal thread, while the minor diameter is the clear opening for an internal thread.
The ideal tool for locating threaded holes is a flexible hole location gauge, known as a centerline hole location plug. These plugs cost somewhere around $100 each and can be purchased in different sizes depending on the thread to be located. The threads on the plug are slotted and designed to engage the hole threads at the pitch cylinder diameter. The unthreaded part of the plug features a cylinder of known diameter (often ¼”) coaxial to the thread pitch cylinder. Each plug will have a diameter and runout tolerance.
To use the centerline hole location plug, it should be threaded partway into the hole. We recommend that it is not tightened against the part surface to minimize the chance of introducing measurement errors. The cylinder portion of the plug can then be located with a CMM or other measuring device. The reference cylinder’s location and orientation will be “identical” to the thread pitch cylinder location and orientation within the centerline hole location plug’s tolerance.
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For example, consider a series of threaded holes in a rectangular plate. Position tolerance is called out in a feature control frame as Ø.016 and referenced relative to three mutually perpendicular datum planes A, B, and C, respectively. Datum plane A is defined as the plate’s broad surface, and datum planes B and C are plate edge surfaces. The holes are located by basic dimensions from the datum edges. The thread callout is 5/8-11 UNC-2B thread. We can visualize each hole’s tolerance zone as a cylinder with a diameter of Ø.016 inches, having an axis perfectly perpendicular to datum plane A. We can then use basic dimensions referencing the hole locations to datum planes B and C to define the holes’ true position. The axis of the tolerance zone is perfectly aligned at the true position. There is no special note stating that the “MINOR DIA” or “MAJOR DIA” is to be measured in this example. Per the standard, the pitch cylinder diameter must be used to evaluate the hole location and orientation, using centerline hole location plugs.
We often observe one measurement error using a “best fit” gauge pin to locate threaded holes when this procedure is not called out on the drawing. A gauge pin that fits snugly into the hole is selected and used to establish the threaded hole’s position and orientation. However, this procedure engages the thread minor diameter, not the pitch cylinder, and it is only permitted when the note “MINOR DIA” is added below the feature control frame. Due to the specialized gauges required to locate threads from the pitch cylinder, it is good practice for engineers and designers to use the “MINOR DIA” note for internal threads and the “MAJOR DIA” note for external threads where possible.
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Click Here For More InformationScrewdriver said:Some further information for yourself and others who are interested. It's a custom made trailer for a niche application with a mostly flat deck, about 4m long and 2m wide, 700kg gross weight. Due to the requirement for unusual cutouts in the trailer floor, I have a long thin area in each corner, 600mm long and 150mm wide and I'm trying to come up with a jacking design to level the bed that will fold up into that area.
The question is how to build a tailer levelling jack cheaply and the (wrong) answer is to go stainless. I am also wary of suggesting small diameter threaded rod as a support without knowing the precise design. Smaller diameters may be fine for vertical loading but the whole point of a levelling jack is to work on a slope. Since he is asking me, I am happy to share how I would do it without necessarily agreeing with any preconceived idea regarding materials choice or size.
If I am wrong, he may ignore the suggestion.The OP is asking a question AND giving an answer.The question is how to build a tailer levelling jack cheaply and the (wrong) answer is to go stainless. I am also wary of suggesting small diameter threaded rod as a support without knowing the precise design. Smaller diameters may be fine for vertical loading but the whole point of a levelling jack is to work on a slope. Since he is asking me, I am happy to share how I would do it without necessarily agreeing with any preconceived idea regarding materials choice or size.If I am wrong, he may ignore the suggestion.
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Some further information for yourself and others who are interested. It's a custom made trailer for a niche application with a mostly flat deck, about 4m long and 2m wide, 700kg gross weight. Due to the requirement for unusual cutouts in the trailer floor, I have a long thin area in each corner, 600mm long and 150mm wide and I'm trying to come up with a jacking design to level the bed that will fold up into that area.The previous strategy has been to have a very simple jack which slides into a square socket that you pull out, rotate 90deg and slide back in for transport. The issue is that with various teams using the trailer the jacks tend to go walkabout and before you know it there's none on it. A permanently integrated solution is far more desirable. Furthermore, the requirement to pull it out and reinsert makes it a two man setup, and the simple system of different height holes makes fine tuning difficult. A threaded system which can be cranked up by one man to a variable height is badly needed.Something like this would be perfect:Hence my trying to find a suitable threaded rod to make it work. My only reason for suggesting stainless was so that it doesn't rust and seize up on the underside of the trailer. My plan would be to extend that bolt that goes into the trapezoidal nut out to the left and right and have it running in a slot to carry any vertical and horizontal load so that the threaded bit is only required to pull the pivoting arm backwards and forwards without bearing any lateral loads.
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