Topping vs Non-Topping Hobs: Decide the Blank-OD Route Before Tool Design

Before requesting a custom hob, establish one point that changes the tool-design discussion: will the hob cut the gear blank’s outside diameter, or will the outside diameter already be finished before hobbing? The Nobeve catalogue labels these two normal-direction profile routes as TOP and N-TOP.

This article gives design engineers and purchasing teams a short discovery checklist. It is not a substitute for a drawing review: the catalogue identifies the two routes, but it does not prescribe one route for every gear, material, machine, or quality requirement.

The direct answer: confirm who establishes the outside diameter

For a standard topping normal-direction profile, the catalogue uses the TOP tag. It says that allowance is to be reserved for the outside diameter and that the hob cuts the outside diameter.

For a standard non-topping normal-direction profile, the catalogue uses the N-TOP tag. It says hobbing is carried out after the outside diameter has been processed to size, and that the hob does not cut the outside diameter.

Profile route in the catalogue	Catalogue tag	Outside-diameter route to confirm
Standard topping normal-direction profile	TOP	Reserve outside-diameter allowance; the hob cuts the outside diameter.
Standard non-topping normal-direction profile	N-TOP	Process the outside diameter to size first; the hob does not cut it.

1. Start with the actual blank-processing route

Do not use “topping” or “non-topping” as a generic preference. Ask how the blank arrives at hobbing:

Is outside-diameter allowance intentionally retained for the hobbing operation?
Or is the outside diameter already processed to the required size before hobbing?
Which drawing revision defines that sequence and the outside-diameter condition?

Those answers determine whether the discussion should begin from the TOP or N-TOP catalogue route. If the route is unclear, mark it for engineering review instead of choosing a tag from an assumed process sequence.

2. Put the decision on the drawing-review checklist

The TOP/N-TOP designation only answers the documented outside-diameter route. A usable tool-design review still needs the gear drawing and the process context that may affect the tool specification.

Provide the following items with the inquiry:

The gear drawing and its revision.
The intended TOP or N-TOP route, or a request to review the route.
The blank’s outside-diameter condition before hobbing, including whether allowance remains.
The machine and workholding condition, including any known spindle, arbor, vibration, or stability constraint.
The production bottleneck that prompted the inquiry, such as an unclear blank route, a fitting concern, or a process handoff.

This avoids treating a profile tag as a complete manufacturing instruction. The source does not establish dimensions, tolerances, cutting parameters, or gear-quality outcomes for either tag; those items need the applicable drawing and engineering confirmation.

3. Keep the request within the correct tool scope

For a custom gear-hob design inquiry, send the gear drawing together with the blank-processing route and the machine/process context. A Nobeve engineering review can then confirm how the TOP or N-TOP designation should be applied to the requested tool.

If the need is to source an HSS hob rather than request a carbide or custom tool-design package, provide the HSS hob specification and sourcing requirements separately. Do not treat the TOP/N-TOP catalogue note as a material, coating, or performance recommendation.

When to pause before releasing the tool specification

Request a drawing review when the outside-diameter allowance is not stated, the blank route changes between operations, the drawing does not identify whether the outside diameter is finished before hobbing, or the machine condition creates an unresolved setup constraint.