June 2009 / Volume 61 / Issue 6|
Taking it to Titanium
By Bill Kennedy
| Reaming titanium is a challenging task, but new tools can give shops an edge as they battle this difficult-to-machine material.
Courtesy of B. Kennedy
As anyone who cuts metal for a living knows, holemaking is much more than simply making a hole. Brian Hurd, sales manager, North America for toolmaker Mapal Inc., outlined the basic holemaking steps: “You drill to remove material, you bore to establish the position of the hole and you ream to establish the size.”
The last step, reaming, usually is the key to a hole’s functionality. As a hole finishing operation, reaming involves removing a small amount of material, with emphasis on generating a smooth and uniform surface finish within the bore.
Among the crucial factors in determining the best tools and parameters for productive reaming are the properties of the workpiece material. With its mix of unique characteristics, titanium can make reaming much more than simply finishing a hole.Lighter, Stronger
An excellent strength-to-weight ratio is one of titanium’s signature properties. While it is about 60 percent of the density of steel, titanium also possesses tensile strength as high or higher than most steel alloys. Because it has perhaps twice the elasticity of typical steels, titanium provides excellent durability and fatigue resistance in structural applications. A stable, continuous oxide layer spontaneously forms on the surface of titanium, creating a protective layer that provides corrosion resistance and makes the material inert and biocompatible. The melting point of titanium is about 400° F higher than steel and 2,000° F higher than aluminum.
Increased demand for precision hole finishing accompanies the expanding list of titanium applications. However, the same properties that make titanium so useful also make it difficult to machine. Although initial cutting forces are only slightly higher than those involved in cutting steel of equivalent hardness, the material tends to workharden. And, despite its chemical inertness at room temperatures, titanium has a strong tendency to alloy with other materials at high cutting temperatures. The material’s elasticity, a benefit in resisting fatigue failure in structural applications, becomes a liability when machining; the workpiece material actually moves away from the cutting edge as it approaches, then springs back.
Relative to metalcutting in general and reaming in particular, one of titanium’s most significant properties is its poor ability to conduct heat. According to Niagara Cutter, when machining steel, 75 percent of the heat generated is transferred into the cut chips, 15 percent into the cutting edge and 10 percent into the workpiece material. When machining titanium, only 25 percent of the heat is carried away by the chips while 15 percent is absorbed by the part and 60 percent is transferred to the cutting edge.
The concentrated heat significantly reduces tool life. The high-temperature reactivity of titanium leads to diffusion of material between the tool and part, accelerating a tool’s crater and notch wear. The light cuts characteristic of reaming demand a careful balance between taking too much stock and distorting hole dimensions and surface finish, or taking too light a cut and pushing rather than cutting the material, generating friction, more heat and a workhardened surface.
Courtesy of B. Kennedy
The size of the preceding drill or boring tool determines the amount of stock left for reaming. B.J. Grizzle, vice president of sales for toolmaker Superion Inc., Xenia, Ohio, said: “For titanium, the stock removal we recommend is 3 percent of the bore diameter or less. Typically, if there are problems with finish, then we start looking at taking a little less material. However, care must be taken to avoid taking too little material and pushing the material. You’ve definitely got to find the happy medium.”
Claude Farrington, plant manager at Quakertown, Pa,. job shop John Prosock Machine Inc., said when reaming titanium bone screw prototypes, the shop found success by “leaving as little material as possible” for the reamer to cut. Where perhaps 0.011 " excess material was typical when reaming aluminum or steel, the shop left only 0.0005 " to 0.001 " when reaming titanium, and employed dark cutting oil, a heavy-duty, sulfur-containing fluid used to lubricate and cool the cut.Coolant Required
Considering titanium’s poor heat-transfer characteristics, cooling the cut is crucial. Peter Diamantis, plant manager for Amamco Tool, Duncan, S.C., said, “It’s very difficult to work with titanium without some sort of coolant.” He noted that geometry on reamers for titanium made by Amamco is practically the same as any other metal, “but we incorporate coolant holes in the tool to deliver the coolant along the cutting perimeters.” Water-based coolants work well when machining titanium, and soluble oils offer excellent lubricity, according to coolant maker Cimcool. Titanium supplier Timet says sulfo-chlorinated oils appear to be the best cutting fluids for titanium, but water-based oil emulsions are also used successfully, particularly with softer, unalloyed titanium grades.
Cutting speed is the chief heat generator when machining titanium. John Forrest, vice president and sales manager, Tool Alliance, Huntington Beach, Calif., said: “As with a lot of workhardening materials like Inconel and some austenitic grades of stainless alloys, speed and heat can make things much more difficult. On those materials, you want to be on the low side of tool supplier recommendations when it comes to speed, and on the high side when it comes to feed. All the speed does is create more heat, which makes it more difficult to cut and creates workhardening.”
Titanium supplier Supra Alloys agreed with this assessment. Tool tip temperatures are affected more by cutting speed than by any other single variable, according to the company. In a typical case, a change from 20 to 150 sfm with carbide tools increased tool tip temperature from 800° F to 1,700° F. On the other hand, the company stated, feed rates do not affect temperature to as great an extent as speed. In the same case, a change in feed rate from 0.002 ipr to 0.020 ipr increased tool tip temperature only 300° F.
Therefore, if a shop is pursuing higher productivity in a titanium reaming operation, toolmakers recommend applying cutting speeds in the low range of recommendations, increasing feed rate until poor surface finish, a distorted hole or poor final size occurs, then backing off the feed until the problem disappears.
Regarding speed and feed recommendations, tool manufacturers emphasize that individual titanium alloys possess widely different machining characteristics. Unalloyed titaniums don’t have the strength of titanium alloys, but their resistance to chemical attack at lower temperatures makes them useful in many corrosive situations. The lack of structural strength, does, however, make them easier to machine than alloyed grades. On the other hand, addition of alloying elements, especially vanadium and chromium, increase a titanium alloy’s strength and fatigue resistance and dictate slower cutting speeds.
For example, while one shop owner said he generally assumes a Ti-6Al-4V workpiece will take three times as long to machine as a steel part, another said a highly alloyed grade such as Ti-5Al-5Mo-5V-3Cr typically requires twice the machining time of Ti-6Al-4V.Tool Materials and Geometries
Tool materials and geometries must take into account titanium’s gummy and springy nature. “It likes sharp cutting edges, it likes to be sheared, it doesn’t like to be pushed because it will workharden,” Forrest said. “We try to cut this material with as sharp an edge as possible so we shear it more effectively.”
Bill Lahr, tooling engineer at Alvord-Polk Inc., Millersburg, Pa., said titanium generally doesn’t require special reamer geometries, and “if it is a HSS tool, we use a M-42 cobalt grade because of its resistance to heat, higher red hardness, if you will.” According to titanium supplier Timet, while in many cases HSS reamers are satisfactory, carbide reamers allow higher surface speeds or longer tool life.
As for flute geometry, Forrest said helical reamers are preferred over straight-flute reamers, “because the action of the helix is going to give you a little freer cutting condition. Plus, if it’s a through-hole, a left-hand helix will drive the chips and heat forward. The downside is that the left-hand helix doesn’t provide a real positive angle at the chamfer and therefore is not as free cutting. In a blind-hole, a right-hand helix will provide a freer cutting condition, as well as lift the chips up and hopefully out. It gives you a more acute angle at the cutting edge, making it freer cutting.” In addition, the angled path of a helical reamer flute can help minimize the impact of cross-holes and other interruptions in the bore being reamed.
Hurd of Port Huron, Mich.-based Mapal noted that a reamer features circular lands or guide margins that direct the cutting edge through the bore, but because they rub on the side of the hole they can create additional heat. For titanium, he said the guide margins would be narrower than they would be when machining steel or cast iron.
Courtesy of B. Kennedy
In the case of replaceable-blade reamers, tool body torsion generated by cutting pressure on the insert pushes guide pads on the body against the bore ID, positioning the blade to ream the bore. Managing the relationship between the blade and pad is different with titanium than with other materials, according to Hurd. When reaming aluminum, typically “you would set the diamond insert only about 3µm to 5µm above the guide pad,” he said. Because diamond tools do not wear much when cutting aluminum, the pad/blade relationship would change minimally over the life of the tool. However, “if you tried to do that in titanium, you would have so much insert wear so quickly that you would eventually seize up the guide pads,” because the insert would quickly wear down to the height of the guide pads, Hurd said.
Mapal solves the problem by putting a stabilizing and guide feature called an “F chamfer” on the blade. The chamfer resembles a secondary cutting angle, Hurd said, and “basically hones the sharp edge of the insert so that the wear isn’t quite so pronounced in the beginning of the tool life. You don’t get a sudden drop in size through the first few parts, and you get a little more consistency in a high-abrasion situation, such as reaming titanium.” For reaming titanium, the insert would be set about 8µm to 10µm above the guide pad.Stack Strategies
Amamco’s Diamantis noted that many aerospace manufacturers drill and ream with one tool to eliminate the two-step drill-then-ream process. Even more challenging than reaming titanium alone is dealing with stacked combinations of titanium and other materials. “Because titanium is hard, when you stack it with a softer material like aluminum or a composite, the titanium chips tend to scar the walls of the softer material. Depending where the titanium is in the stack, you have to construct the direction of your flutes to eliminate the scarring.”
Diamantis described reaming a through-hole in a titanium sandwich stack, with the aluminum as the “bread,” where it would be beneficial to use the flute helix to cut downwards (left-hand helix applied in a right-hand cut) and push the chips downward. “The reamer in a sense will be cleaning out the hole behind the chips,” he said. “If you used a regular [right-hand helix] reamer, you would be pulling the chips over the newly reamed area, thus scarring the walls of the softer material.” He added that “it gets complicated when you cannot down-cut because of clearance constraints. Then, we have to build the tool flutes to encase the chips and keep them from scarring the walls of the hole.”
Superion Products’ Grizzle provided another approach to consolidating hole-finishing operations. For aerospace applications, Superion offers boring reamers to true a drilled hole position in a workpiece. “Typically, a standard reamer will follow the existing hole, whereas a boring reamer is designed to align the hole as it is reamed.” In materials such as titanium, he said, boring reamers are generally used “more as roughers to get the hole straight, followed by a finisher to finish to size and create the surface finish.”The Hole System
In addition to cutting tool considerations and application parameters, other reaming system elements must also be controlled to achieve optimal hole quality and consistency. Alignment of the machine spindle and the workpiece is crucial. A lack of concentricity between the tool and workpiece can produce chatter during reaming, tool marks during retraction and a conical, eccentric or oversize bore.
The hole being reamed essentially guides the reaming tool, so a straight, concentric drilled hole is a critical part of the reaming system. Bill Shaffer, executive vice president of microedge preparation provider Conicity Technologies LLC, Cresco, Pa., said: “A reamed hole is only going to be as good as the hole that it finishes. A drill with blunt corners will compress the workpiece material instead of cutting it. That workhardens the sides of the hole, maybe only a few microns deep.” The result is accelerated wear of the reamer’s cutting edges (see sidebar on page 56).
For many shops, machining titanium is intimidating. Performing a precision operation like reaming adds to the angst. However, knowledge of titanium’s distinctive properties and the effect they have on the cutting tool can be a great help in recognizing what not to do when working with the lightweight wonder metal. Then, when the “don’ts” are covered, the “do’s” can be applied to boost productivity when reaming titanium. CTE
Courtesy of Conicity Technologies
When reamers go bad
John Prosock Machine Inc.
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