Cutting Keyways - Broaching, Keyseating, Wire-Cut EDM, Shaping, & Milling

When attaching coupling, gear, or pulley hubs to the shafts of either the driving or driven equipment, a key is commonly used to engage two keyways (cut into both the shaft and the hub) to help prevent rotation on the shaft and to assist (to varying degrees) in the transmission of torque. While specific keyway dimensional standards can be found elsewhere (see Active Coupling Bore and Keyway Standards - What Are They & Where Can I Find Them?), this post will focus in-depth on the prevailing technologies that leading coupling manufacturers use to insert keyways into coupling hubs. This post will also briefly cover cutting shaft keyways as well.

Keyway Broaching / Spline Broaching 

Broaches Waiting to be Sharpened

Broaches (pictured above waiting to be re-sharpened) are a fast and cost effective way for cutting standard keyways and splines. Broaching works by pushing a series of cutting teeth through the hub quickly, with each tooth sticking out a bit further and removing a bit more material. 

This allows a lot of material quickly, and can generally allow a keyway to be cut in one to three passes. (While there are several variables used to determine an appropriate depth of cut, 1/8th of an inch or 3.2 mm is generally about how much material can be removed per pass.) 

Like other cutting tools, broach cutting teeth do dull and need to be sharpened (as seen in the tool room photo above). Broaching machines can be either horizontally or vertically aligned (both horizontal and vertical type broach machines are pictured below), and can either be used to push or pull the broach through the part.

Horizontal Broaching Machine

The strength of the broaching machine is generally rated in tons, and, in concert with the broach and hub material being cut, will determine how much material can be removed with each cutting pass.

The main drawback of broaching is the initial upfront cost of the broaches themselves, which can quickly run into the tens of thousands of dollars and do require periodic maintenance. Furthermore, time can be a second drawback if a needed broach is not on site and not readily available and in stock from a supplier. (In these cases it may take weeks to months to have the broach manufactured.)

While it is cost effective to have broaches for common and high volume keyway sizes, it is generally cost prohibitive to secure broaches for custom and unique keyway requirements. Furthermore, very large bores (broadly defined as those greater than 1 inch or 25 mm) also quickly become difficult to broach given how large the broaching equipment must get to cut a keyway in just a few passes. For custom and large keyway requirements, coupling manufacturers will generally turn to keyseating, shaping, Wire-cut EDM, or milling (all covered in depth below).

Vertical Broaching Machine

In addition to the traditional vertical horizontal and vertical broach machines (vertical machine at right is broaching torsional coupling hubs), leading coupling manufacturers may employ highly customized high volume broaching solutions that offer minimal setup time and high repeatability. 

(Note: All photos in this post are from Lovejoy's Downers Grove, Illinois & South Haven, Michigan facilities, but, proprietary custom equipment photos have been excluded for competitive reasons.) 

Though high volume broaching equipment is unlikely to be available in your local machine shop, if you are a large volume original equipment manufacturer, it is important to know that this technology exists.

Big picture, broaching offers a quick, effective, and precise solution for cutting keyways and spines assuming upfront cost of broaches can be justified. 

Keyseating

Traditional Keyseater

Keyseaters, which are also known as "nibblers", employ a similar cutting concept to broaches in that material is removed through a series of tooth cutting passes. The major difference with keyseating is that it uses only a single cutting tooth which makes a multitude of passes (versus a broaching which is generally a single pass with a significant number of teeth). 

With each successive cut, the cutter on the keyseater moves out just a bit more, allowing a bit more material to be removed in the next pass. This process continues to repeat itself until the depth of cut meets the specified coupling hub print drawing or specification. 

As a point of reference, a typical keyseater depth of cut may be 20 thousandths of an inch (versus 1/8 of an inch for a broach), and a single keyseater cutter may cost a few hundred dollars (versus thousands for a larger size broach). Keyseating a large keyway may take 1-2 hours (versus perhaps 15 minutes for a similar size 3 pass broached part). 

While keyseating does take longer than traditional broaching, it provides great flexibility in that a unique size single tooth cutters can not only be procured quickly, but they can also be quickly cut or modified at an onsite tool room to meet a unique keyway demand. Additionally, keyseating serves as an excellent backup to broaching should a broach or broaching machine ever unexpectedly go down or break.

CNC Keyseater with Indexing Table

For cutting multiple large keyways into hubs, something users of very large couplings (generally metallic gear, grid, and disc couplings) often request, an indexing table can be added to a keyseater. Customers can specify multiple keyways in any orientation, but they are most often requested as two keyways 180 degrees apart.

Without indexing capability, it would be extremely time consuming and difficult to precisely manually reorient and fixture a coupling hub for multiple keyseater keyway cuts. The table allows for a single setup.  

Pictured at right is a best in class large CNC indexing keyseater that is capable of cutting keyways for coupling hubs through a size 15 gear coupling. This unit was produced by Frömag of Germany, which is one of the leading keyseating machine manufacturers. (Note: Frömag purchased Mitts & Merrill, the major US keyseater manufacturer, in the 1990s, which solidified its keyseater marketplace leadership position. The traditional, non-indexing, keyseater pictured above the Frömag unit was manufactured by Mitts & Merrill.)  

Wire-Cut EDM

Wire-cut EDM Machine

Wire-cut EDM (Electric Discharge Machining) is reserved for small quantity runs where either precision is require or other cutting options are not readily available. Traditionally Wire-cut EDM machines have been heavily used by tool and die makers, and work by basically slowly shocking or evaporating (non-technical terms) material away through a spooling copper wire that gets close but does not directly touch the component being cut through the electrical sparks.

Cutting rates will vary by the equipment used, but to give a ballpark speed, a standard machine may run at ~12 square inches per hour. (i.e. - If a part is 6 inches thick, the Wire-cut EDM machine will be able to cut two inches deep per hour.) 

While slow, one compensating factor of Wire-cut EDM machines is that they can be run "lights out" (without an active operator), which is particularly useful given how long they take to cut a part.

Wire-cut EDM is a viable solution for most small to mid-size coupling hubs, but very large coupling hubs will not fit into most Wire-cut EDM machines. (The Wire-cut EDM machine pictured above may be able to handle a size 5 or 6 gear coupling hub, but certainly not a size 15 like the keyseater pictured in the previous section.) 

Prototype parts (to get proof of design approval before ordering an expensive broach), rush orders (where a broach or keyseater is unavailable), and unique one off requests (where keyseating or shaping will not work and/or the cost of a broach cannot be justified) are all excellent reasons to cut a keyway using a Wire-cut EDM machine.  

Shaping

Vertical Keyway Shaper

Shaping is used largely for cutting blind keyways (keyways that do not go through the full length of the part). While coupling shaft keyways generally do extend through the full length of the shaft, it is much less common for coupling hubs to have blind keyways. 

(Note: Broaching and Wire-cut EDM are not designed to handle blind keyways.)

Shaping and keyseating use similar principles, both leverage a single-point cutting tool for cutting, but one major difference is that shapers are not guided down through the cut on a fixed post. As such, unguided shaper cuts are prone to being slightly less accurate (via deflection) than keyseater cuts. (Be sure to have and use proper inspection gauging to ensure cut keyway meets specified tolerances.)

Horizontal Keyway Shaper

Cutting blind keyways using shapers also requires and additional step not required for previously discussed cutting methods. This additional step is pre-cutting a relief at the end of the projected keyway. This relief is needed to allow the metal chips, created by shaping, to break. Reliefs can be created either by pre-drilling a hole or by cutting an internally groove on the inside diameter for hubs.

Pictured above right is a vertical keyway shaper set up to cut a key on the outside diameter of a coupling shaft. The shaft (not shown) would be held and clamped straight up and down in a vertical orientation, with the shaper also cutting on its downward stroke in a vertical direction.

Shown at left is a horizontal keyway shaper set up to cut a blind keyway on the inside diameter of a hub. The hub (not shown) would clamp horizontally, with its flange extending past the fixture to the left and the shaper cutting horizontally.  

Milling 

Similar to shaping, end mills provides an opportunity for partial length straight or tapered shaft keyways to be cut. (Woodruff cutters also allow for Woodruff keyways to be milled.)

While popular on external shaft keyways, milling is much less popular for cutting the inside diameter keyways on hubs because you can't mill it using traditional straight milling techniques. (The other side of the coupling gets in the way, which you do not want too mill through.) Rather, to mill an internal hub keyway, a specialized 90 degree angular milling head would need to be used. This is not common.

When milling keyways, similar to shaping, particular attention should be paid to ensure corner radius and all tolerances fall within specification. If not within specified limits, the coupling-key-shaft joint may have key fit problems and/or introduce unintended imbalance into the power transmission system (see Shaft Keys & Keyways - "Be aware of the little details”).

Summary

There are a wide variety of solutions available and used to cut internal coupling hub keyways. While many variables come into play when selecting a keyway cutting solution, broadly speaking: broaching is for speed, keyseating is for large keyways and specials, Wire-cut EDM is for specials, shaping is for blind keyways, and milling is primarily reserved for shaft keyways (not hub keyways). 

Many coupling users completely entrust their preferred coupling manufacturer with finish boring and keywaying all hubs, comfortable that the manufacturer can provide quick turnaround deliveries on breakdown orders. As a coupling manufacturer that differentiates itself based on superior service levels, we certainly appreciate, respect, and support this approach (see How to Get a Replacement Coupling Fast).

Other end users and coupling distributors draw comfort by having additional rough stock bore (small unfinished bore) hubs on the shelf locally that, given an emergency, could be quickly finish bored and key components at an area machine shop. For such individuals or organizations, it is important to not only hold the raw inventory for an emergency, but to also understand and preemptively develop an emergency finishing game plan that is based on the availability of equipment and gauging at local machine shops. 

Should you have benefited from this article, please let us know by leaving a comment on this post below. Should you have any further questions, please ask those here.