It seems counterintuitive that a rotating cutting tool or workpiece can result in the creation of a hexagon, square, Torx, octagon, keyway, serration, involute, spline or other polygon form. "I get people looking at the tool, and they're like, 'I don't care what form you put on the end of this, it's going to be a circle when it's done,'" said Steve Derbin, president of Polygon Solutions Inc.

The Fort Myers, Florida, toolmaker offers rotary broaches and rotary broach toolholders and states that the principle that makes this process work is the 1° angle of the cutting tool to the centerline of the workpiece. The angle of the tool causes it to shear the workpiece with a chiseling or scalloping effect as the broach is fed into the part to the depth required. The toolholder has a live spindle that holds the broaching tool, allowing the spindle to spin freely within the toolholder. In a lathe, the spindle is driven by contact with the rotating workpiece. Either way, the tool is only contacting the workpiece at one point during the process, Derbin added, noting that rotary broaching is an extremely fast and accurate way to machine polygon forms, which is why the process is preferred in the medical and aerospace industries.

Polygon Solutions offers a variety of rotary broach toolholers. Image courtesy of Polygon Solutions

The broaching method, which can be performed in a matter of seconds to a 0.0127 mm (0.0005") accuracy or better, has been around for a century but that doesn't mean all metalcutting professionals are familiar with it, he noted. "Every now and then I get a call from a guy who's been machining 50 years and he's never seen this before in his life."

Derbin explained that Polygon's GT series rotary broach toolholder — its flagship holder — for lathes, mills and turning centers does not need adjustment for fast and simple setup, and has sealed bearings to avoid the need for greasing maintenance, unlike brands that need to be greased every hour or so. "And if your guys on the shop floor are not greasing them, then you're going to ruin the bearings and start making bad parts quickly."

Venting Pressure

In addition, the company's rotary broaches and toolholders come standard with a pressure-relief vent hole system. With a 1 mm (0.039") vent hole through the broaches and out the side of the holders, air and coolant or cutting oil can escape, Derbin said. Not relieving the pressure can have negative consequences. "If you're doing a blind hole, you can get some kind of vapor lock going on and it puts a lot of pressure on your machine."

Genevieve Swiss Industries Inc. in Westfield, Massachusetts, also offers tools and holders for rotary broaching through its partnership with Swiss toolmaker PCM Precision Tooling Ltd. GenSwiss offers a pressure-relief vent system as an option when the application is appropriate, said Scott Laprade, applications supervisor. "If you have water-based coolant or oil in the pilot hole that you want to broach and you have a very tight fit between the hex and the pilot hole, you're going to encounter a situation where it becomes like a hydraulic ram. It'll just lock up."

For example, when producing a 6.35 mm (0.250") hexagon socket where the maximum allowable drill diameter is 6.30 mm (0.248") and the broach tool is 6.36 mm (0.2505"), one contiguous chip around the hole perimeter will be pushed in and raise the pressure level significantly, he explained. "In a situation like that you're going to want to use the vented broach."

A diagram of a rotary broaching toolholder with the part and broach centerlines. Image courtesy of Genevieve Swiss Industries

Laprade added that, if possible, it might be wise to use a machine that has more Z-axis thrust capability to provide more horsepower on the stroke or shifting to a partial form instead. Partial form broaching enables removing chips and displaced material between indexes, which can be the main reason for rotary broaching being unsuccessful.

He explained the steps for producing a partial form. Spot, drill and chamfer the pilot hole, and then push the partial form into the hole, producing a third of the form per hit and indexing the chuck/spindle between each pass and often clearing the displaced material with a drill between indexes. "You're making a third of the form at a time and just pushing it in, breaking the material. You push that material in again and do as many passes as it takes in order to get the finished form."

Machine Matters

As its name implies, GenSwiss focuses on tools and accessories for Swiss-style machining. "GenSwiss was founded to make the Swiss machinist's life easier," Laprade said, noting that the company introduced a holder for making it easier to rotary broach on a Swiss-style machine 20 years ago.

Outside the Swiss world, its rotary broaches and holders can be used on turret lathes, milling centers and even manual equipment, he added. "I've personally broached on a Bridgeport manually just by cranking either the table or bringing the quill down. You can do it just about on anything where you have either the capability of spinning the workpiece or spinning the holder. Something needs to be rotating."

While rotary broaching generally eliminates secondary operations where the workpiece is moved from one machine to produce the needed form in another, such as an EDM or punching machine, Laprade noted how rotary broaching in a Swiss machine can eliminate any additional cycle time. By performing the process on the sub-spindle side after the part has already been completed and parted off, it is essentially free time because the front side machines the next part while the sub-spindle broaches.

GenSwiss offers rotary broaches with a dished cutting face and optional vent for relieving hydraulic pressure. Image courtesy of Genevieve Swiss Industries

Polygon Solutions reports that its standard broaches are made from M-2 HSS but may also be produced from harder materials such as M-42, powered metal T-15 and PM M-4 HHS. Coatings such as TiN, TiCN and TiAlN can be added for additional strength, longevity and durability. Derbin recommends M-42 for broaching 4140 steel, for example, and PM M-4 for 303 and 304 stainless steel. Polygon also offers a proprietary PS-60 material for broaching exotic materials such as Inconel, titanium and 17-4 stainless steel.

Laprade noted that PCM manufactures its broaches with a specific Swiss formula of tool steel with an 8% cobalt content that is not available from mills in the U.S. "There's some kind of secret sauce in there that we just can't quite figure out."

Tool Guide

A pre-drilled pilot hole is needed for rotary broaching, and the rule of thumb is that the hole is 2% to 3% larger than the cross flats of the form, Derbin said. Because chips curl down the hole being broached, with a hexagon form curling six individual chips, a hole smaller than or the same size of the cross flats would cause one big chip to be pushed down the hole, causing the machine to alarm out or the broach to chip.

"Then you want to put a lead-in chamfer larger than your major diameter, larger than your cross corners," he said, "and the pre-drilled depth needs to be one and a half times the broaching depth. That way you have room for the chips to accumulate down there. Go back in with your drill and drill out those chips when it's done."

To avoid chipping the broach as it enters the hole, Derbin recommends running the spindle at a slow speed, such as 50 rpm, or completely off and touch down into the lead-in chamfer maybe 0.508 mm (0.020") and then going to full spindle speed instead of hitting it head on. Adding a small corner radius, such as 0.127 mm (0.005") can help prevent corners from chipping as well.

A rotary broach brake can be used for multiple-pass broaching. Image courtesy of Polygon Solutions

Rotary broaching is not considered a high-speed operation. "You don't want to risk getting it going too fast and chipping the broach," Derbin said. "I find that 800 rpm across the range is really good. Now, if you're in mass production and time is crucial, you could bump it up."

Laprade noted that rotary broaching provides some "wiggle room" when it comes to machining parameters, altering them to improve the surface finish or extend tool. The highest spindle speed might be 2,000 rpm, but only for small-diameter tools similar to some other machining operations. "Small tools would be a little bit faster rpm, lower feed rate, and bigger tools would be a slower rpm, higher feed rate."

Proceed With Caution

The more corners the form has that's being shaped, the easier it is to rotary broach, Laprade said. Compared to a hexagon or octagon, a square has less tangency between the final form and the pilot hole, meaning there is more volume in the corners to be removed.

He added that GenSwiss, which performs rotary broaching in-house to manufacture some of its products, is knowledgeable about which applications are suitable for the process and which ones are not. For example, an application with a cross-hole that's aligned to a flat on a hex form is difficult to do with a rotary operation because the user does not know where the position of the hex tool in the holder is going to be. "It's not something that's controllable. Usually in a situation like that, we advise a punch tool to get a repeatable and stable process."

According to Laprade, rotary broaching a material as hard as 50 HRC is possible, but tool life suffers and dictates how deep the broach can go and the pilot hole size. "Usually, when someone comes to me and they want to broach hard materials, I give them the rule-of-thumb answer to drill a pilot hole as big as possible and still make a part that's going to be acceptable. That's going to give him the best chance of success."

A properly sized pilot hole is critical to successful rotary broaching. Image courtesy of Genevieve Swiss Industries

Derbin concurred that material hardness is one of the main limitations to rotary broaching and tends to shy away from recommending the process for metals harder than 40 HRC. In addition, the form to be broached cannot be too deep. "On larger forms, we can go up to 1¾" deep. That's about it. On smaller forms like ¼", we can go 1" deep."

Broach size is another limitation. The Swiss world is known for broaching small forms, but Polygon offers a heavy-duty holder that enables broaching forms up to 50.8 mm (2"), Derbin said. "So, we're doing some gears for people."

For manufacturers looking to start rotary broaching and want to produce a range of forms and sizes, he noted that Polygon offers a kit with a toolholder, seven broaches and a carrying case. "The kit is a great option for them because they're getting a pretty good discount."

Polygon also offers an alignment brake attachment that allows the broach to be orientated or clocked to the same position for each part, Derbin explained. "If the broached form needs to be clocked to another feature, this is no longer an issue."

Whether it is used to create a drive socket, fastener, bone screw, mating key, proprietary drive or other feature, the form to be rotary broached must be a symmetrical polygon, Laprade pointed out. "You can't have any irregularities to it."

However, when the application is appropriate, rotary broaching can really shine in its ability to machine a polygon quickly, accurately and without a secondary operation. "It's a really amazing operation," Derbin said.

For more information from GenSwiss and Polygon Solutions about rotary broaching, view video presentations at https://qr.ctemag.com/1ta6f and https://qr.ctemag.com/1ta6h