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# How to measure gear start of tip relief diameter?

## How to measure gear start of tip relief diameter?

(OP)
Hi,
Can we measure the start of tip relief diameter of a gear involute profile? My gear suppliers say the machines can not measure exact tip relief diameter. They have got a Tokyo technical instruments make gear machining machine and a Wenzel gear tester machine. How true it is? What is the alternative? Thanks.

### RE: How to measure gear start of tip relief diameter?

By Tip Relief, are you referring to the break or radius on the edge where the gear face and outer diameter meet?

If so, I've not seen that measured directly, at least on a profile diagram. The scale of a profile diagram blows that out of proportion.

If it's critical you may need to develop another way of measuring that radius.

### RE: How to measure gear start of tip relief diameter?

Tip relief (or tip profile modification) is a very slight, tightly controlled minus deviation from the involute profile near the tooth tip, but is not part of the tip break. The start of the tip profile modification (STM) is also the end of the active profile (EAP). Tip profile modification is critical for high PLV gears to reduce dynamic tooth loading, TE and noise. The exact shape/form of the tip profile mod is also very important. The transition from the active profile to the tip modification must be smooth and provide continuous curvature. With high-performance gears a parabolic geometry is often used, but I don't recall seeing a simple constant radius geometry used for a tip profile mod.

The profile tolerance limits of the tip mod are defined and inspected just like the active profile is, including considerations like slope. The EAP and STM are usually defined in terms of degrees roll on the engineering documentation. But determining the exact location of this transition point on a single tooth flank of a specific gear is not likely possible. All you will know from the inspection chart is whether or not the measured tooth profile falls within prescribed tolerance limits.

As for the start of the tip break, it should be fairly easy to determine with some accuracy since the transition shape/form at this location is not controlled in curvature or slope. Tip breaks are usually only defined by length/width. Since they are often produced by hand the shape of the tip break surface can be irregular. If you look at the K chart shown below you will see the abrupt deviation of the profile curve where the tip break is located.

### RE: How to measure gear start of tip relief diameter?

Nameless
Terry covered it very well. If you look at the gear charts provided by Terry. It is a simple measurement with an Involute profile Inspection machine.
The machinist must be between the band with the tip off.
The edge break can also be recorded if needed. This is usually all the requirement. And is a common method. It is based on degrees of roll converted from specific diameters. For example TIF , P.D., and the start of Tip off. To the major DIA break. Depending on the style of gear grinding machine special dressing tools are required. Unless it can be CNC programmed. Like Terry said the modifications are for high RPM gears. !ike Turbine planetary reduction gears or gear boxes. It would normally not be required for low RPM. Unless it is needed for tip interference reduction. But be advised it takes years of experience personnel. To manufacture these type of gears. Due to the complexity of manufacturing and having basic knowledge of heat treating and serious in depth manufacturing experience.
HTH
Mfgenggear.

### RE: How to measure gear start of tip relief diameter?

Sorry, but in my previous post the K chart shown did not include the abrupt excursion of the profile curve that is typical from the start of tip break to the OD. Below you will find a better example.

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