Getting reaming right: Drilling Performance

Appropriate scenarios for applying a reamer when finishing holes.

Courtesy of Komet of America

Cutting rings from Komet have an adjustable diameter range and can be accurately adjusted with special cutting ring holders. This compensates for wear, ensuring the correct adjustment to the exact bore dimension, according to the company.

When machining a hole exactly to size, reaming is often a machinist’s best choice. It typically cannot be used to straighten holes, and can only remove a limited amount of stock, but when done right, reaming can be a fast, highly accurate process.

Performing reaming correctly, however, requires good preparation. Important factors include leaving the exact amount of stock after drilling; knowing when to ream twice, use a multifunction reamer or use another tool entirely; and specifying the right reamer for the job. Regarding the latter, many different styles of reamers are available—including adjustable, chucking and spiral flute reamers—and in many cases a special is the best tool for the job.

When reaming, preparation of the hole is what’s most important, according to Ryan Bysterbusch, group leader of design engineering for toolmaker Komet of America Inc., Schaumburg, Ill. That’s because a reamer enlarges a drilled hole to size by removing a small amount of stock and does not correct a hole. “The reamer follows the hole,” he said, but noted that a reamer helps to slightly improve straightness if the tool has front-cutting capability.

Courtesy of Alvord-Polk

A selection of chucking reamers from Alvord-Polk, including (above top) a straight-shank, straight-flute reamer; (above middle) a taper-shank, right-hand, spiral-flute tool; and (above bottom) a titanium nitride-coated, straight-shank, right-hand, spiral-flute reamer.

Josh Lynberg, president of tool supplier Monster Tool Co., Vista, Calif., concurred that a “fair-quality” hole is needed before reaming. For example, he noted that when reaming a 0.240 “-dia. hole with a 0.250 ” reamer, the hole will not be straightened if it tapers by 0.005 ” or more.

If the hole shape or location needs correction, Will Nestor, a Phenix City, Ala.-based application/project sales engineer for toolmaker Mapal Inc., Port Huron, Mich., recommends boring. “But reaming will typically give you a quicker cycle time than boring,” he said, adding that a reamer is more commonly applied to achieve the proper hole size than impart a fine surface finish.

The surface finish obtainable when reaming generally depends on the workpiece material. The range for cast iron is 50 to 80 rms and 30 to 60 rms for steels, and a PCD reamer can impart a finish as fine as 8 rms in aluminum, according to Bysterbusch.

Sometimes a hole is bored prior to reaming, but that’s not mandatory. “Any hole that meets the minimal required amount of stock for the reamed size needs no other preparations before reaming,” Lynberg said. “If the finished hole has an extremely tight tolerance and the machine tool being used to create the hole is not of sufficient accuracy or rigidity, a bored hole helps keep the reamer aligned with the hole axis, assisting the machine tool to keep the hole straight.”

Material Issues

The amount of stock remaining for reaming depends on hole quality and diameter. A rule of thumb is 0.010 ” to 0.015 ” should remain after drilling for reaming, except for small diameters, such as 1⁄32 “, which should have 0.003 ” to 0.006 ” of material for reaming, Lynberg noted. “A poorly drilled hole may need a little more material in order for the reamer to ‘clean up’ the hole walls,” he said.

Nestor provided a general DOC range from 0.0039 ” to 0.0098 ” for Mapal’s guide-padded reamers and up to a 0.0394 ” DOC for a brazed-PCD fixed reamer, depending on the workpiece material. “For an aluminum engine component application, we’ve been able to remove up to 1mm per side in the hole,” he said, adding that a larger DOC is possible for a reamer with end-cutting geometry incorporated into the cutting edge.

In addition to aluminum, the stock allowance for magnesium is usually greater than it is for steel, cast iron, titanium and nickel-base superalloys, according to Bysterbusch. If the chip is too thick when reaming the latter materials, too much heat enters the chip, making it ductile and difficult to break, which creates a chip-removal problem. “You’re better off removing as little as you can.”

Others provide more hole-size ranges. Kevin Morrison, chief tooling engineer for Alvord-Polk Inc., indicated that the Millersburg, Pa.-based manufacturer of multiple-flute solid reamers recommends 0.003 ” to 0.006 ” of stock for reaming holes up to 3⁄32 ” in diameter, 0.008 ” to 0.010 ” for holes larger than 3⁄32 ” to ¼ “, 0.012 ” to 0.015 ” for ¼ ” to ½ “, 0.017 ” to 0.020 ” for ½ ” to 1 “, 0.020 ” to 0.025 ” for 1 ” to 2 ” and 0.030 ” to 0.035 ” for holes larger than 2 “. “It’s a sliding scale,” he said. “It doesn’t even work out as a percentage.”

Morrison added that the range is 0.002 ” to 0.004 ” for a hand reamer. “That’s about all the human body is geared up to do,” he said. “We don’t have the mechanical advantage to take more material out.”

Although some don’t consider material workhardening to be a significant concern when reaming, Nestor noted that it can create challenges. It’s desirable to take a light DOC, for example, when reaming titanium. A light DOC keeps chips thin for ease of evacuation when machining, for example, a small-diameter hole. “Machinability plays a big role in how you ream,” he said.

Sizing the Hole

When too much stock remains for reaming after drilling, end users have the option of reaming twice. Monster Tool’s Lynberg suggests first applying a smaller reamer followed by a reamer of the required size. “Drilling to open up a hole usually causes drill failure due to unequal and uneven stress along the drill’s cutting edge and is not recommended,” he said. Lynberg added that plunge milling is also an option if the proper size endmill is available.

In addition, a step reamer with a roughing and finishing diameter might do the trick. “It depends on the material,” Bysterbusch said. “Once you get into some of the tougher steels, we have to look more in detail at the application.”

Nestor noted that another option is applying a fine boring tool. A fine boring tool has at least three guide pads and is capable of a significantly greater DOC than a reamer. Bore location corrections can also be achieved using a fine boring tool with a cycle time comparable to a guide-padded reamer, he added.

Part volume also dictates the acceptable solution. “You have to remember the customer always wants to decrease cycle time,” said Donato Pigno, product specialist for Komet. “He’ll typically use a roughing tool and a finishing tool.”

Alvord-Polk’s Morrison suggested that a core drill, which is a cross between a reamer and a drill, can solve the problem when there’s considerably more material to remove than is recommended. A core drill is designed to remove a large amount of material from a hole, but it will not produce a hole by itself. “There are times when a core drill imparts a suitable-enough surface finish that end users use one to finish a hole,” he said.

Tool Considerations

When specifying a reamer, size is the only feature an end user needs to be concerned with unless he’s reaming an unusually deep hole that needs extra tool length, which calls for a special, according to Lynberg. “A properly manufactured reamer will do its job regardless of the material being cut.”

Courtesy of Monster Tool

Size is the only feature an end user needs to be concerned about when specifying a reamer, according to Monster Tool.

Although Bysterbusch noted that “full-blown specials” constitute about 40 percent of Komet’s reamer sales, the company’s fixed, monoblock standards are “semispecials.” That’s because standard blanks are ground to size to meet a specific customer’s requirements once the company receives an order. The standard range is from 6mm to 110mm but specials cover a wider spectrum. “We ream everything from 1.5mm to over 300mm,” he said.

Size tolerance plays a role in selecting the style of reamer, according to Mapal’s Nestor. While a fixed tool is appropriate for a larger tolerance range, “any time you’ve got a ±5μm or tighter tolerance on your diameter,” he said, “you want an adjustable reamer.”