Manufacturing continually evolves as technologies become sufficiently versatile and affordable. When it comes to gear manufacturing, CNC machining allows the performance of multiple operations on a single machine, significantly reducing cycle time. Because broaching is a secondary operation still performed using conventional techniques, many gear manufacturers subcontract broaching to specialty houses when the manufacturing process requires splines or keyways. Because gear manufacturers continue to face long lead times and protracted supply-chain issues, replacing costly, time-consuming subcontract work with in-house capabilities is beneficial to manufacturers and customers alike.

John Gardner, owner of CNC Broach Tools in California, says, “I call it the emancipation of broaching. The manufacturer wants to be able to make their own stuff and do this process on their own. And I find with the advent of CNC lathes and mills where the operations are done on one machine, broaching is kind of one of the last processes done offline.”

duMONT CNC Broaching System
Courtesy of Pilot Precision Products Broaching is an unusual machining process, because it has the feed built into the tool, which effectively makes it a collection of single-point cutting tools arrayed in a sequence. Analogous to the multiple passes of a shaper, a relatively new class of CNC tooling and programming allows broaching operations to be done without the need for a second operation, thus greatly increasingly quality control and reducing time, space and cost.

“At the OEM level, if they are fully invested in secondary operations, the pull side of broaching is still the fastest metal removal rate. You’re never going to beat it in a CNC,” said Marco Morgado, Director of National Business Development for Pilot Precision Products, makers of the duMONT CNC Indexable Broaching System. 

“It’s where you get into the Tier 1 and Tier 2 contractors who don’t necessarily do a lot of broaching that the CNC broaching is very intriguing to them and is our fastest growing segment. In the last two years, we’ve had 42 – 48 percent growth year over year. That is the future of broaching. For many customers, not all. We don’t see linear broaching going away.”

It can’t be emphasized enough that the software program itself is more than half the battle when it comes to tool life and productivity. Despite the technology having been around for more than a decade, Morgado explains that CNC broaching is still relatively novel in the U.S. because “there’s no canned CAD software that has CNC broaching programming built in. That’s the biggest obstacle of entry. We wrote a macro for variable controls to help customers apply CNC broaching. There is not a lot of standard programming out there, but it’s coming.”

Static vs. Live Broaching Tools

Courtesy of CNC Broach Tools

Broach tools for CNCs can be mounted static to the turret face or to a live motorized-slotter accessory that attaches to the side of the turret with live tool capability. Making the right choice for an application depends primarily on complexity — the number of keyways, teeth, etc. — and the quantities one needs to produce. With smaller runs (up to around a few  hundred pieces per month), the static tools are cost-effective. If the run is over several thousand per month, a live broaching tool is a faster, less expensive solution.

Depending on the design, however, a static tool has a greater machine length capability than a live tool, but depending on the material, there is a potential risk for deflection. According to Morgado, “The biggest thing customers have to keep in mind is how much load is being generated in the cut based on the tool recommendation that goes with it. With our products, when we tell you a ¼" keyway insert will go in a #6 holder, it’s because of the optimal rigidity of the #6 holder to minimize deflection in the cut. In broaching, that’s what you have to manage is deflection.”

Pilot Precision Products inserts are standard in stock for inch & metric keyway widths and standard broaching tolerances from H7, P9, D10, C11 and with end chamfer, square, and hex shaping. Spline inserts are custom made in various alloys, geometries, sizes and tolerances and are attached to the insert holder with a set screw. The insert holder, usually with a shaft diameter of 1" or 1¼", is then clamped to a boring bar holder or a VDI boring bar holder with a corresponding diameter on the CNC lathe. For use on machining centers, the holder can be clamped with any tool holder with a cylindrical holder, such as a collet chuck or Weldon holder.

By pushing the insert into the workpiece, the keyway or spline is machined step by step. The cutting speed and the cutting depth must be set depending on the workpiece material. Gardner says, “Ongoing education of the machinist is really a large aspect of what we do here.” Catalog values mostly serve as approximate values and that programming adapted to various factors on site (e.g., room temperature, machine type, machine condition, workpiece quality, coolant) can produce optimal machining results.

Live broaching tools are worthwhile for large quantities and provide cost-efficient production of different geometries. They are a low investment relative to purchasing a separate broaching machine with competitive processing (up to 400­ – 800 strokes per minute). Depending on the material being cut, general speeds and feeds for keyway slotting are .0005" – .0015" per pass at 250 – 550 IPM.

Technological Advantages and Limitations

Blind keyways have always been a challenge in the machining world because it usually involves an expensive, time-consuming EDM operation. To cut a blind keyway in a CNC, most tools on the market require cutting in a relief groove or cross hole for the chip to release. However, in the case of Pilot Precision Products, their round shank tools are 58/60 HRC with inserts that are sintered steel alloy with 13% cobalt heat treated to 72 HRC. As a result, the tool can be programmed to ramp out and leave 30 – 45 degrees back taper.

Another advantage is that by eliminating a second operation, one reduces setup and cycle times. Complete machining of workpieces in one setting results in a leaner production process that increases precision and production speed. While some keyway inserts are available in standard sizes, most manufacturers are equipped to produce a variety of shapes and profiles in carbide or other materials.

This reality means it is cost-effective to manufacture a wide variety of shapes and profiles for keyways, blind keyways, slots, splines, and gear teeth. According to Gardner, “I hear every week from clients who have a part they no longer know how to make. So, they’ll send us a physical part and we reverse engineer it. Spline parts are blowing up for us. We literally have the infrastructure to make a part that has been made in China for 30 years and nobody knows how to manufacture anymore. We hand clients the tooling to cut the spines within three weeks.”

Determining whether CNC broaching is a sensible solution requires considering a host of complex variables. Morgado breaks it down like this: “If you are talking to a customer about doing a long spline over 2 ½", which is longer than our 65mm stroke on our motorized slotter, conventional broaching is going to be the fastest way to go if they have enough volume to make that kind of investment. 

“They can use a round shank tool with their machine cycle, but when you go with a longer-length tool, one needs to manage rigidity in the cut by asking oneself: Is there deflection near the end of the cut with the tool holder? Is there enough clearance inside the ID for the amount of thickness needed to put on the stem?

“It’s a clear-cut variable we help customers with all the time. It comes down to what do they care for the cycle time to be? On a keyway the timing is negligible, but if you are talking about a 3" spline, you there will be a lot of cuts all the way around the part. Now you are talking about a possible 5- to 15-minute cycle time in your CNC, versus if you were pulling a broach, it would be 20 seconds. So, what’s your machine time to the number of parts you have got to make? 

“As a job shop, say you’ve got 100 parts to make. You input your cycle time and it takes 25 minutes to do it with a round shank tool, but you get the finish and everything you need out of it — compared to using wire EDM. It’s still going to be less time than EDM. If you sub it out, you are paying someone a bunch of money, because those spline broaches are super expensive, and they are all custom made.”

There are quite a few factors that contribute to assessing whether CNC broaching is the right solution, but when calculating cycle time, it is worth considering the extent to which long lead times and protracted supply-chain issues continue to affect production. The ability to reduce costly subcontract work and bring broaching operations in-house could be a game-changing solution.

RESOURCES:

Pilot Precision Products (USA): https://pilotprecision.com

PH Horn (USA): https://www.hornusa.com

WTO (Germany): https://www.wto-tools.com/en-us

Benz (Germany): https://www.benztooling.com/en-US

MD Tooling (USA): https://mdtooling.com

MT Marchetti (Italy): https://www.mtmarchetti.com/en

CNC Broach Tools (USA): https://www.cncbroachtools.com