Drills Inch Toward Metric

Millimeter by millimeter, metric-size drills are gaining popularity in U.S. machine shops.

As more and more U.S. companies realize the advantages that metric-dimensioned products offer, cutting tool manufacturers must be ready to supply tools for metric machining requirements. U.S. toolmakers began adding metric tools to their standard product lines in the 1970s. While some were responding to the U.S. government’s push for metric to become the national standard of measurement, the majority of toolmakers produced metric tools to meet the demands of U.S. automakers, who were converting their designs and manufacturing facilities to the metric system.

Whether the cutting edge on an endmill is inch or metric may not be critical, but automakers required metric diameters on holemaking tools, particularly drills, to produce parts with metric-size holes. At the time, most U.S. toolmakers had metric drills available only as specials, not as standards. "If our customer wanted a 6mm drill, we would pull a 1/4" standard drill off the shelf, grind it down, and be able to supply it right away," says Phil Kurtz, manager of special quotations at Precision Twist Drill Co., Crystal Lake, IL. "We were still trying to force the metric standards to fit into our inch-standard drill listing."

Today, most U.S. toolmakers can readily supply drills off their shelves in both metric and inch dimensions. "In terms of drilling, tapping, and reaming, metric is already becoming a 50-50 commonplace for a tool producer like us," says Tom Trost, vice president of sales at CJT Koolcarb Inc., Addison, IL.

U.S. toolmakers are using different approaches to meet the increased demand for metric drills. While some start from scratch with new metric designs, others simply convert their drill sizes from inches to millimeters.

Hard vs. Soft

Toolmakers may offer drills made to metric standards (hard metric) or drills converted from inch to metric dimensions (soft metric). Hard metric drills have a totally metric design that includes diameters and lengths in even numbers of millimeters that conform to ISO (international) or DIN (German) dimensional standards. "Drill sizes that are in 0.10mm increments or 0.25mm increments are considered hard metric sizes," explains Joseph Langenstein, senior project engineer at Caterpillar Inc., Peoria, IL. "Taking a 1/4" drill and calling it a 6.35mm drill is soft conversion."

Soft metric implies conversion by calculation, resulting in numbers that are either rounded or nonstandard in the metric system. Although it is a fast, effective strategy for metric changeover, soft conversion generally is considered the exception rather than the rule. The number of decimal places to which the conversion is carried out varies with how tight the standard tolerance is for a particular dimension on the drill. Accuracy may be lost in converting from a standard inch drill size (fractional, letter, or number) to a three-place decimal inch size and then to a two-place millimeter size.

Converting designs to metric often results in higher manufacturing and quality-control costs, because there is not much compatibility between the rules for standardizing sizes of inch and metric tools. Typically, metric drills must originate as metric designs to be cost-effective for the toolmaker to produce.

The Toolmaker’s Perspective

Tool manufacturers outside the United States may find it economically inefficient to produce all of their metric drills in inch sizes. Therefore, their lines of inch drills for the U.S. market, which is just a fraction of their total market, may not be as extensive as their metric lines. "If they’re making a drill to ISO metric standards for the rest of the world, they can do a run of 10,000 pieces," says Brian May, vice president of sales and marketing at Dormer Tools Inc., Asheville, SC. "If they have to turn around and make a smaller run to American [ANSI] standards, it’s not cost-effective at all."

U.S. tool manufacturers face the opposite problem. The majority of their domestic customers use inch drills, while foreign markets demand metric. While it may not be economical to produce drills for the domestic market in inches and for foreign markets in millimeters, U.S. toolmakers have to produce what each market demands if they want to sell drills worldwide. Tool suppliers have learned to accept this inefficiency as a condition of doing business in the world marketplace. "If the customer wants inch, you give him inch; if he wants metric, you give him metric," says David Lewis, staff engineer at Kennametal Inc., Metalworking Systems Div., Raleigh, NC. "You’ve just got to have both products available."

The User’s Perspective

Selling metric drills isn’t a problem for toolmakers, but ordering and using them can be a problem for U.S. machine shops. Many shops are under pressure to switch to metric tooling, even though they’re still manufacturing parts that were originally designed in inches. Therefore, shops often order drills in metric sizes to make inch-size holes. "A U.S. customer may convert a 1/4" drill into a metric size and then order a metric drill," says Phil Kurtz of Precision Twist Drill. He says it is also common for U.S. customers to ask for an inch-size drill to be remarked with the equivalent metric size.

Tom Trost at CJT notes that many U.S. customers calling in for application assistance convert the drill diameters from inches to millimeters but specify the hole depth in inches. This inconsistency may be a sign of an industry in transition from inch to metric. "Because a lot of the metric drills used in the United States are conversions from inch sizes," says Phil Kurtz, "they carry the overall lengths and flute lengths that are exclusive to the ANSI standard." Although ordering drills with metric diameters and inch lengths may cause confusion, hybrid tools remain popular among metric-drill users in the United States. While European machine tools are designed to take drills with DIN-standard overall and flute lengths, most drilling machines in the United States were built to take drills with ANSI standard lengths. Until these machines are replaced by models that take drills with metric lengths, hybrid-tool use will continue in the United States.

Although the majority of metric drills are bought by customers outside the United States, they’re in demand by U.S. automakers, aircraft manufacturers, heavy-equipment manufacturers, engine manufacturers, and machine tool builders. According to Roger Taylor, president and CEO of North American Tool Corp., South Beloit, IL, "Demand for metric tools started off with automotive, but it’s not tied to any one segment of the market at this time."

Growth in U.S. sales of metric drills has been accelerated by more U.S. companies selling and manufacturing overseas. "There are more and more people who are directly dependent on offshore sales and profits," says Taylor, "And most, if not all, of those requests coming in from overseas are to metric specifications."

John Deere, Moline, IL, is an example of a large manufacturer that is selling and servicing its products worldwide. "Since most of the world is metric," explains Bob Noth, manager of engineering standards, "inch-based designs create difficulties when you try to sell or support them overseas." These difficulties prompted John Deere to switch to metric when it recently redesigned its inch-based line of gas and diesel engines. This conversion to metric-based designs will force the company to replace all of the tooling for the engine line with metric tools.

When companies that have an international customer base adopt metric designs and tooling, their suppliers also may be required to work in metric. As a result, many U.S. machine shops are making parts in both inches and millimeters.

Double Trouble

It’s easy to see how working in two different measurement systems can create confusion on the shop floor. Some companies design and manufacture exclusively in metric. North American Tool has inch and metric custom blueprint design capabilities on its computers and inch and metric manufacturing capabilities on its machines. Other companies faced with producing a metric part convert metric specifications to inch, so even if the design drawing is in metric, engineers may talk in decimal inches on the shop floor. "If the customer sends us a drawing in metric," says Precision Twist Drill’s Phil Kurtz, "we convert it to decimal inch to manufacture the tool." Besly Products Corp., South Beloit, IL, also converts back to decimal inch, because all of its equipment is set up off of the digital readouts in inches.

With today’s electronics in machine tools, digital readouts allow operators to convert from inches to millimeters with the press of a button. Machines can even be programmed in inches to cut in millimeters. Although technology makes conversion easy, developing a feel for sizes of metric units is more challenging, and relying on the conversion capabilities of the equipment may only delay the learning process.

Engineers and machine operators don’t become fluent in the use of metric dimensions by memorizing conversion factors and performing calculations; they need hands-on metric training. John Deere found that the best way to train its workers was to give them metric scales and measuring equipment. The company also converted its machine tools so that the handwheels turned in both inch and metric and the scales were based in millimeters instead of fractions of inches.

Of course, cost is a primary consideration in company-wide metric conversion. While capital-equipment changeover may make the cost of conversion seem prohibitive, it pays off in terms of higher productivity, because engineers don’t have to spend time converting and checking data, or worry about error in performing conversions.

But some tool manufacturers believe that U.S. shops wouldn’t convert to metric if they weren’t forced to meet the metric requirements of international companies. "As the larger manufacturers realize they have to make their products metric to sell them worldwide and to have them easily serviced worldwide, then the smaller job shops are forced to follow suit," says Dormer’s Brian May. "Otherwise, I don’t believe the smaller job shops would ever go to metric."

However, even shops that aren’t dictated by the market to make metric-size products are finding reasons to use metric tools. A major advantage of using metric tools is the simplicity of working in tenths rather than fractions. Users find that the increments between sizes are very even and regular. This consistency is especially beneficial to users of metric drills.

A Sizable Advantage

Shops that primarily use drills to create holes for threading may decide to use metric drills to improve thread quality and consistency. "Using metric drills from standard millimeter-based sizes minimizes tool wear and energy requirements while providing maximum strength threads," says Caterpillar’s Joseph Langenstein. The smaller the drill relative to the intended major diameter of the threaded hole, the more energy is needed for tapping. The result is shorter tap life. As the drill size increases, less energy is required to cut internal threads, resulting in longer tap life. A tap drill that’s too large, however, will lead to the production of shallow threads.

Caterpillar decided to switch to metric drills in 1971, just before the company’s overall conversion to the metric system. "Even without going totally metric," explains Langenstein, "we were considering converting to the use of metric drills just to have a better selection of sizes." He recommends using drills that will result in 60% to 70% thread engagement. Figure 1 shows the percentage of thread engagement produced by standard inch and metric tap drills for each thread pitch. The graphs show that metric drills produce more threads within the desired range of thread engagement.

Why do so many inch drill sizes fall below 60% or above 70% thread engagement? It’s very possible that the fractional, letter, and number sizes weren’t designed specifically for tap-drill applications. And if the developers of inch drill sizes had thread engagement in mind, it’s likely that they didn’t know as much about threading as we do today. However, metric drill sizes weren’t designed for tapping applications, either. "That’s just how it happens," says Joseph Langenstein. "If you want to drill and tap a hole, you just have to pick the closest size."

When selecting the closest size to produce the optimum percentage of thread, drill users have more metric sizes from which to choose. CJT’s Tom Trost explains that metric drills are in increments of 0.1mm or 0.004", whereas fractional drills are in increments of 0.016". Metric drill sizes also are more evenly spaced. Figure 2 shows that sometimes two inch drill sizes are almost the same, while often there are significant gaps between sizes. North American Tool’s Roger Taylor adds that there are about one-third as many metric sizes as inch sizes. With fewer drills to purchase, stock, and maintain, a shop can reduce its tool costs substantially.

Increased use of optimum tap-drill sizes also can help shops save money through improved tool life. A shop that substitutes an inch drill for a metric drill to produce a metric-size hole is bound to choose an inch size that is larger than the specified metric size. This may result in threads that are too shallow once the hole is tapped. "The closer you drill or drill and ream the hole to the maximum minor diameter," says Trost of CJT, "the less power you use while you’re tapping and the less wear you create on the tap." Dan Gajdosik, senior engineer at Besly, emphasizes the need for metric drill sizes, "especially on those tap sizes where you need to get the percentage of thread down below 75%." Like many toolmakers, Gajdosik expresses concern about the lack of metric-drill use. "While drill and tap manufacturers certainly see the need for metric drill sizes," he says, "we can’t seem to get this point across to the end users."

Unless a shop is working with metric threads and metric hole sizes, the advantages of using metric drills may not be apparent. "If you’re dealing with inch threads, you may have to jump from a fractional drill size to a letter drill size to achieve what you want," says Dave Zaval, vice president and general manager of Machinery Systems Inc. - Tooling Group (MSI), a distributor based in Schaumburg, IL, "but that same flexibility to adjust percentages of thread sizes is there in the inch system."

It may be difficult to convince a shop to use metric drills to make holes that aren’t going to be tapped. "If you’re finishing the hole in any way, chances are you’re using a secondary tool," says Dave Zaval. "If that secondary tool is adjustable, you don’t need metric tooling." An infinitely adjustable boring head, for example, can bore a drilled hole to an inch tolerance in an inch size or to a metric tolerance in a metric size. Precision Twist Drill’s Phil Kurtz adds, "You typically use a drill just to make a through hole or clearance hole in a part, or a hole for a standard size of fastener."

Tom Trost points out that inch drill sizes often can be substituted for metric sizes. "A number size 18 is 0.1695", or 4.305mm," he states. "If a customer needs a 4.3mm drill, then the difference is 0.005mm, or 0.0002". The reliability of some of the best drills in the world is only 0.0030", or 0.076mm." Finding the appropriate inch drill size, however, isn’t as simple as finding a standard metric drill size. "It can be messy jumping back and forth between fractional, letter, and number sizes, trying to find which is the best to use," says Dave Zaval. "It’s more of a crapshoot."

Inch-based drill charts may be another reason a shop doesn’t realize the advantages of using metric drills. Besly’s Dan Gajdosik thinks that U.S. shops are still buying inch sizes "because of all the charts that are on the walls in shops and supplied by cutting tool manufacturers." Trost of CJT agrees that almost none of the decimal charts on shop walls and in catalogs give all drill sizes in inch and metric. "I recommend that a decimal chart be created that converts inch into metric and metric back to inch, and all the fractional, letter, and number sizes into inch and metric," he says. "Then the customer can make an educated decision on what they’re trying to do with the drill." CJT has created such a chart, listing all of its drills in inch and metric decimal sizes. This may clear up the confusion at the cutting edge, but even the most comprehensive chart may not resolve problems at the shank end.

The Toolholder Dilemma

For the majority of metric tools sold in the United States, the cutting edge is made to ISO or DIN standards in metric specifications, but the shank and overall length are made to ANSI standards in inch specifications. MSI’s Dave Zaval says this is especially true for taps. "In this country," he says, "We see ANSI shanks on just about everything, with the tap diameter adapted to standards for either metric or inch."

Although toolholder manufacturers sell metric holders for imported tools with metric shanks, they primarily sell their U.S. customers holders that adapt to inch shanks. This makes sense, since the prominent shank sizes of drills and taps used in the United States are inch. But some toolmakers think this trend would change if metric toolholders were more widely available in the United States. "Tool shanks and lengths are to inch specifications because we don’t really have metric toolholders in this country to any degree," says Roger Taylor. In the United States, most BT toolholders, which have taper shank ends made to metric dimensions, have front ends sized to accept inch shanks.

Other toolmakers believe that the real issue isn’t the availability of metric toolholders, but the interchangeability of inch and metric tools. Using metric tools with inch shanks doesn’t affect the way the tool mounts to the machine or the dimensions on the end of the cutter; however, compatibility between the tool and the holder may be a problem. Kennametal’s David Lewis points out that a tool designed in the United States may have a 1" shank equal to 25.4mm, but that same tool designed overseas would have a 25mm shank. Therefore, the tools are not interchangeable in a 1" solid toolholder. "The end user must be aware of this whenever ordering tools," says CJT’s Tom Trost, "since a change in toolholder may be required before using one manufacturer’s tool in place of another’s."

Compatibility may be a problem even if the shop is using a flexible hold er like a collet chuck rather than a solid holder that isn’t adjustable. The user typically can collapse a 1" collet to hold a drill with a 25mm shank; to hold it as securely as possible, however, he will need a 25mm collet.

Although metric collets are often required, they may not be needed even if the shop is using drills with metric shanks. The decision to use metric collets depends on how many parts the user has to produce. To make metric-size holes in a long run of parts, a shop should purchase a metric drill and a metric collet for that drill. But if a shop has only a few parts to do, it may be possible to hold the metric drill in the closest inch-size collet.

Flexible toolholders may be the solution for some shops, but users of solid holders still may mismatch metric tools and inch tools. "It’s hard to recognize when you’ve got a metric tool with an inch shank in an inch toolholder," says Besly’s Dan Gajdosik. "Color coding does not work very effectively." Without proper tool identification, it may be difficult to distinguish between a metric tool with an inch shank and an inch tool with an inch shank.

Users of metric tools may be able to avoid these problems by purchasing tools with metric shanks. "For a long time, the concession to go metric meant that our customers would buy metric tools on ANSI shanks, because they didn’t want to change the toolholders," says Dormer’s Brian May. "But now, our sales are more in the full metric than metric on ANSI shanks."

Regardless of whether metric tools are sold with inch or metric shanks, the real issue is marketability. The progress achieved in manufacturing and marketing metric drills in the United States has been prompted primarily by market forces. "A metric drill is the same as an inch drill in terms of how it actually functions," says John Deere’s Bob Noth. "It’s just a question of what your specification is for buying."

Whether or not the United States ever adopts metric as its national standard, most toolmakers and users believe that commerce has already gone metric for companies that do business internationally. After all, metric is the world standard for manufacturing products, and adopting metric designs and tools guarantees access to foreign markets. Many companies will continue to make and use metric drills

Figure 2: Comparison of standard inch and metric drill sizes.