Why Osborn Products?

  • ISO 9001:2015 Certified
  • AS9100 Rev D Certified
  • Aerospace Quality Gages
  • Precision CNC Machining & Grinding
  • Made in USA
  • Established in 1956

The Tri-Roll Comparator System

Operating Instructions

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).

Functional Diameter Size

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.

Inspecting Functional Diameter Size

Using the functional Type 3 rolls, set the comparator with a master set plug manufactured to the desired pitch diameter. Class W tolerance is recommended, however, a standard class X master is acceptable. For greatest accuracy, use the stated pitch diameter on the long form certification. Place the product thread on the gage locating the full length of the functional rolls. Oscillate the thread slightly to insure proper seating. Index the part in the rolls 60 degrees. Record the largest reading as maximum material functional size.

Pitch Diameter Size

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.

Inspecting Pitch Diameter Size

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 and Flank Angle

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.

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.


  • No single gaging method can guarantee that a product thread will assemble and conform dimensionally. USE A GO THREAD RING TO VERIFY ASSEMBLY!!!
  • 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.
  • 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.
  • Set roll screws so that rolls rotate without lateral movement. See attached instruction card for proper calibration procedure.
Tri Roll Operating Instructions
Mounting Gage Rolls

First, clean rolls, pins and setting plug thoroughly. Gage rolls are furnished in sets of three for each specific diameter-pitch combination and are interchangeable on the applicable frame size. The rolls should be placed on the pins with the marked faces out and the unmarked faces resting against the ground surfaces on the frame. The No.2 roll is mounted on the movable upper arm. The No. 1 and No. 3 rolls are mounted on the fixed lower arm. For right hand threads the No. 1 roll is mounted on the outer end of fixed lower arm. The No. 3 toll is nearest the frame body. For left hand gaging simply reverse the No.1 and No. 3 rolls.

With the rolls mounted correctly on the pins, the locking screws are turned clockwise to tighten. Types 3, 4 and 5 rolls should turn but not float longitudinally. They may be locked so they will not turn for ultra-critical inspection. Locked rolls, if required, should be repositioned periodically to avoid wearing flats on the gaging surfaces.

Multiple lead threads of less than 5 degrees helix angle may also be inspected with standard rolls. Information is available upon receipt of product thread specifications.

Setting the Tri Roll Thread Comparator

The comparator is set with a single master plug gage, preferably of W tolerance. To engage the plug for setting, depress the operating lever at the back of the comparator and place the setting plug (entering from the front of the rolls) between the No. 1 and No. 3 rolls. Do not engage the setting plug by inserting from the side as to do so might nick or damage the rolls. Rotate the setting plug approximately turn to be sure it is seated properly.

Setting the Dial Indicator

The dial indicator is held in place by a positive clamp which encompasses the indicator stem. A standard socket wrench is used for the clamp screw. Preload the dial indicator by at least one full revolution to provide an adequate operating arc for the indicator pointer. To do this, place the setting plug in the rolls, loosen the clamp screw and adjust the indicator until the pointer has made one full revolution before coming to zero. With the indicator pointer on zero, tighten the clamp screw. The indicator bezel can be used for line adjustment.

Setting the Stop Screw The stop screw, located on the top of the frame behind the indicator, is used to prevent damage to the rolls or the indicator by restricting the motion of the operating lever. With the setting plug in place, advance the stop screw until it contacts the operating lever (use special stop adjusting tool). Raise lever and remove setting plug. Lower lever until it again contacts the stop set screw. Retract stop set screw until pointer travels twice the part tolerance (part tol. .005 travel on indicator should be .010). Recheck indicator setting with setting plug.

The comparator is now set to inspect the part. Enter parts from the front as you did the setting plug. Rotate slightly to seat and read deviation from master on indicator.