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Understanding Tri-Roll & ITC Thread Comparator Systems

A variable thread gage system such as external and internal thread comparators can be an effective means of monitoring and controlling screw thread fabrication in high production environments.

The tri-roll comparator system provides a fast and efficient means of definitive thread inspection. Also, variable data output makes this gage a perfect statistical process control device. Routinely, the tri-roll thread comparator is used to analyze and distinguish the following characteristics: Functional diameter size, Pitch diameter size, Lead and/or flank angle deviation, Taper, and 120-degree out-of-round (lobing).

The functional size of a screw thread is defined as that diameter which relates the diameter effects of variations in the above characteristics as they apply to assembly with its mating part. In other words, it is the result of the cumulative effect of variations in these characteristics along the full length of engagement. Any variation in these characteristics will always cause the functional diameter of an external thread to increase, and in an internal thread to decrease. This diameter is commonly referred to as the Maximum Material Condition of the product thread.

Pitch diameter size is defined as an imaginary cylinder, concentric and parallel to the thread axis, at which the thread ridge and thread groove are of equal width. Pitch diameter is the predominant factor used in the control of size and fit of product threads. This measurement isolates one thread, disregarding the cumulative effect of variations of functional size. Therefore, it is proper to refer to minimum pitch diameter as the Minimum Material Condition of the thread.

Using the pitch diameter rolls (best wire radius or cone and vee) set the comparator with the same master used for functional size inspection. Place the product thread in the gage locating two full threads from the front. Rotate the part 120 degrees. Record the smallest reading. Repeat the process at the center and back of the thread. Record the smallest reading as minimum pitch diameter size. Record the difference of the three readings in the same plane as taper. Record the rotational T.I.R. as out-of-round.

Lead: The axial distance a screw thread moves when rotated about its axis one complete rotation. On a basic single start thread, the lead is equal to the pitch (1/TPI).

Flank Angle: The angle created by the thread flank and a line perpendicular to the thread axis. This is often referred to as the half angle of the thread.

The term drunken lead is a broad reference to variations in helical path. This must be added to the axial lead variation to obtain total lead variation.

Understanding Differential Analysis: Lead/Flank Angle

Deviation in lead/flank angle is inspected by comparing functional size and pitch diameter size. Theoretically, identical readings would indicate perfect lead and form. In actual practice, functional size is almost always larger than pitch diameter size. If the difference between the two is excessive the lead/flank angle variations inherent in the thread can cause problems with assembly or compromise integrity of fit. The value at which these variations can be termed excessive is usually dependent upon application. Military specifications for safety critical threads limit the acceptable differential to 40% of the total pitch diameter tolerance. However, as a general rule, 50% is sufficient for most commercial manufacturing.

Example: .375” – 24 UNF -3A
Min. P.D. .3479
Max P.D. .3450
Tolerance: .0029 x .50 = .0015 Max. allowable differential, functional diameter to pitch diameter with limit set at 50% of total pitch diameter tolerance.

Important notes:

1. No single gaging method can guarantee that a product thread will assemble and conform dimensionally. USE A GO THREAD RING TO VERIFY ASSEMBLY!!!

2. Avoid the production of thread parts in the upper or lower 15% - 20% of the pitch diameter tolerance. Most disagreements between gages and problems in assembly occur in these zones.

3. Do not turn product parts through the tri-roll as you would a thread ring. This will cause excessive and unnecessary wear on the gaging rolls, as well as prohibit consistent readings. Always lift the lever arm and index the part unless inspection requires rotation of the part.

4. Set roll screws so that rolls rotate without lateral movement.

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