Gundrilling tools properly applied create impressive holes with very close diameter tolerances and roundness, good surface finishes and — of course – holes much straighter and deeper than those accomplished with a twist drill.

However, when the drill is used long enough to become dull, chip formation will change, hole quality will deteriorate, and eventually the gundrill will fail. It’s important to stop drilling before failure to avoid the risk of scrapping the part or trying to retrieve a piece of broken carbide out of a deep hole. Gundrill failure also eliminates any chance to resharpen the tool.

Gundrills typically have a slight back taper along the head, so after each sharpening there is a minor change in drilling diameter.  When the drill has been sharpened too many times, the head length and wear pads are too short to guide the tool, and it needs to be replaced.

Most types of gundrills are excellent candidates for resharpening, which can be done approximately eight to ten times.  In most cases resharpened gundrill performance will be similar to that of a new tool. Even coated gundrills can be resharpened, since the coating on the wear pads that burnish the holes while drilling remains, which is often the primary value of the coating.  Twin-flute tools and those intended for ultra-high-feed applications with chip breakers below a coating usually require OEM resharpening.

In a majority of cases, however, a dedicated gundrill grinding system enables shops to accurately resharpen gundrills in-house to save on labor, transportation and tool availability costs. Additionally, the understanding of the gundrilling gained by sharpening drills will encourage improvements in how the tools are applied.

Sharpening a gundrill will take less than 15 minutes with a good grinding system that includes step-by-step instructions and is often performed by a gundrilling cell operator while the machine is drilling with other tools. The best grinding systems also provide information about tool performance before sharpening. Familiarity with normal wear patterns will enable a shop to adjust drilling parameters, extend time between resharpening operations and increase productivity and profits with higher feed rates, improved quality, better machine utilization and fewer unexpected failures that disrupt production.

A dedicated gundrill resharpening system includes a manual tool grinder, a fixture designed to hold the drill for grinding and a digital inspection camera to view and store magnified images of the drill tip. The images enable users to perform measurements and identify problems without taking the tool out of the fixture. Users can then learn to identify the causes of extreme, uneven or unexpected wear or chipping and adjust speeds and feeds to optimize their cutting processes. A shop also can use saved drill images to communicate wear modes to the drill manufacturer, who can provide advice on ways to limit wear and extend tool life.

Given the ease of use and the significant process optimization opportunities the process provides, many shops should consider adding in-house gundrill resharpening capability to save time and maximize their tooling investment.

Machining complex contours inside a drilled hole at any appreciable depth is extremely difficult, and, in some applications impossible. Instead of shops having to attempt to generate such contours with a boring bar that ultimately results in excessive chatter and loss of accuracy, UNISIG has systemized the entire process with its B-Series deep hole drilling machines, bottle boring tooling, CNC control and engineering to ensure a reliable and productive way to successfully produce those deep internal contours that were otherwise thought to be impossible.

The UNISIG process utilizes a Boring and Trepanning Association (BTA) drill to counterbore a highly accurate pilot bore, which is used to support a bottle boring or chamber boring tool. Much like a Swiss-style machine concept, the workpiece and the tool are both supported for rigidity and accuracy. Bottle boring uses CNC actuated axes to then expand and contract the bottle boring tool’s cutting surfaces to create an internal profile with chip-forward discharge to eliminate tool breakage.

UNISIG B-Series machines combine powerful drilling and precise contouring in one machine tool platform that also includes preloaded drive systems, dynamic spindle control and X-axis servo-driven tool actuators. Internal profiles can be programmed in much the same way as external contours using programming similar to conventional turning centers or CAM programs. Built-in process monitoring and data table to manage tool dimension and wear offsets further ensure accuracy.

The B-Series reliability makes bottle boring deep holes a durable process regardless of the depth of the bore. Whether boring holes three feet or thirty feet, the process is repeatable and productive. UNISIG B-Series deep hole drilling machines series are available for bottle boring holes from 2 inches to over 16 inches in a wide variety of materials, including stainless steel and exotic alloys such as Inconel.

UNISIG’s bottle boring technology opens entirely new areas for design engineers to explore in a broad range of industries. Critical machined features for aviation and aerospace landing gear, oil and gas production and hydraulic smart valving and actuators previously thought too difficult to machine can now be produced on a single machine.

The burgeoning electric vehicle (EV) industry also continues to push the boundaries of design and innovation in its quest for lighter components and weight reduction for high-acceleration and torque. The production of high-speed, high-efficiency power trains, electric motor components and rotor shafts will also prove potential candidates for bottle boring with UNISIG B-Series machines.

To learn more, contact UNISIG about its B-Series machines and bottle boring capabilities.

As surgical instrument manufacturers pursue greater throughput while facing increased labor costs, automating the medical instrument manufacturing process has become a necessity. However, integrating automation into the gundrilling process for drilling deep holes in extremely precise surgical instruments in lights-out operation is a major engineering challenge requiring more than simply pairing a robot with a deep-hole drilling machine.

The right machine, tools and process must all come together to create small holes with extreme precision in difficult-to-machine materials such as titanium and surgical stainless steel. More importantly, the entire system must flow from a unified concept where the whole is greater than the sum of its parts.

To meet these challenges, UNISIG developed its UNE6-2i-750-CR dual independent spindle gundrilling machine. The UNE6-2i is capable of gundrilling hole diameters ranging from 0.8 – 6 mm in part lengths measuring up to 30 inches with depth-to-diameter ratios from 20:1 to more than 100:1. The machine has a maximum combined drilling speed of 28,000 rpm and a 3,000 psi (207) bar programmable flow-based coolant system with dedicated pumps for each spindle to ensure precise coolant pressure control.

Automating hundreds of cycles of precision manufacturing, however, is not possible unless the overall operation is considered from the outset. Surgical instrument manufacturing is a sequential process: parts must be loaded into the machine in a particular way for specific operations that happen in a specific order.

Workpiece length, shape and configuration determine where it is gripped by the robot when loaded into a machine, moved from spindle to spindle for drilling, residual cutting fluid removed, and returned to the pallet. Where a part is gripped impacts where it is clamped for drilling to ensure accuracy. Every variable along the process chain must be considered and accounted for, and the calculus is detailed and complicated.

Then there are unique customer needs and requirements. The equipment and process must accommodate a variety of part families and hundreds of parts to increase runtime and efficiency. Operators must be able to change over part types and programming without calling in an automation specialist, and the entire process must be controlled from a central interface. Add to the mix that everything must be packaged in as small a footprint as possible, and the scope of the engineering challenge comes into focus.

UNISIG’s approach to solving these problems, however, results in targeted automation that enhances the existing benefits of gundrilling, ensuring a solid foundation for reliable process-wide automation.

At its core, the automated UNE6-2i is a purpose-built machine with automation embedded in its design, not added as an afterthought. Flexibility and adaptability are maximized by a harmonious, interdependent mechanical, software and operational planning scheme.

To meet size constraints, a 6-axis robot was embedded in the machine with a pallet system on the backside of the machine, allowing easy operator access from the front to setup the machine without compromising ergonomics. The configuration enables quick setup changes between prototype and proving operations and full production runs.

The robot automatically repositions the workpiece from the front of the first spindle into the rear of the second spindle without operator input. The process of drilling a part from both ends in a single-piece flow is unique to UNISIG. Workpieces with enlarged features on one side are loaded from the rear of the collet, solving a common problem in gundrilling medical surgical instruments with full automation.

Control of the UNE6-2i and a computer are consolidated into the Human Machine Interface (HMI), a menu-driven touch screen system for easy, intuitive operation. Training and operator engagement with the system is significantly reduced due to user-friendly UNISIG controller menus and prompts.

UNISIG’s comprehensive and integrated approach to automating medical part manufacturing is a vison that sets it apart in the industry. It’s more than drilling the impossible hole. It’s a commitment to understanding and to the research that drives continuous improvement and innovation for automated part production at its full potential.

Stress relieving, when it follows the button-style barrel rifling process, is a critical firearm manufacturing operation. Button rifling draws a tungsten-carbide cold-forming tool through a drilled and reamed barrel blank. The button compresses and moves the barrel wall material to create rifling grooves in the barrel bore without actually cutting the metal. As such, button rifling requires application of considerable force and induces stress in the barrel walls.

Stress relieving processes will vary based on many factors, but in general it involves heating a rifled barrel to a high constant temperature that is below the alloy steel barrel’s critical temperature (usually ~1300˚ F). (Higher temperatures change the composition/grain structure of the metal and are considered heat treating.)

In the stress relief process, the barrels are loaded into a vertical rack in an industrial furnace. After the furnace reaches about 200˚F, the furnace chamber atmosphere is replaced by nitrogen from a nitrogen gas generator. The temperature in the furnace is gradually raised to around 1050˚F and held there for an hour. After that, the furnace stops firing and the chamber temperature decreases slowly and steadily, still with a nitrogen gas atmosphere.

The entire stress-relieving cycle encompasses about 16 hours, and it is important that the cooling process occurs in the nitrogen atmosphere because rapid cooling can result in excessive humidity and condensation on the barrels, causing surface oxidation. When the barrels reach a safe handling temperature, they are removed from the furnace and treated with a rust inhibitor.

Two furnace types – sealed vacuum furnaces and non-sealed furnaces – are available for the stress relief process. A non-sealed furnace displaces air from the furnace chamber during purging and consumes a continuous flow of nitrogen while operating. A vacuum furnace, in contrast, draws the air out of the chamber and then replaces it with nitrogen.

The vacuum sealed furnace conserves nitrogen because it doesn’t require continuous purging. The maintained steady-state environment of the sealed furnace enables better control of variables that affect barrel quality and cycle time, and maintenance of consistent temperature during stress relieving in a sealed furnace enables the barrels to be heated and cooled off faster.

A non-sealed furnace will typically cost from about $50,000 to $100,00, while a vacuum furnace may exceed $500,000. Either type of furnace requires a nitrogen source and other ancillary equipment, as well as barrel racks, a rust inhibitor dip tank and other ancillary equipment.

In most cases, it is preferable for a barrel maker to work with an established heat treating provider than to perform stress relieving in-house, especially when initially setting up the operation. Partnering with a heat-treating specialist that uses sealed vacuum furnaces for stress relieving will make expert advice and process guidance immediately available and provide access to high-capacity furnaces as a barrel maker’s markets expand. While stress relieving operations with an unsealed furnace may be a lower-cost option, expanding output and maintaining process consistency can pose problems.

A barrel’s finish will affect its performance as well as its looks. Before stress relieving it is important to remove oils left by prior machining operations. Temperatures of the stress relief process can bake any contaminants on to inner and outer surfaces of a barrel, as well as onto the furnace chamber. Stress relief process capacity is also a consideration, and care should be taken to not overload the furnace. Excessive mass can stall the recommended heat cycle.

The goal of stress relieving in barrel production is producing consistently high quality barrels in a reasonable amount of time with no rejects due to process inconsistencies. Barrel makers that want to deliver best in class products are well-served by seeking out and utilizing experienced, established heat-treating specialists to handle stress relief services in a reliable and economical way.

In a routine drilling operation on a milling machine or drill press, a drill’s cutting edges rotate against a stationary workpiece. The opposite is true in holemaking on a turning machine where a stationary drill advances into a rotating workpiece. Either of these drilling methods produce sufficient reliability and hole quality for a wide range of applications. However, other tactics are necessary to produce more exacting tolerances and larger depth-to-diameter ratios.

Drill and workpiece rotation has a major influence on a hole’s concentricity – a key measure of drilling accuracy. When the drill alone rotates in a horizontal setup common for deep hole drilling, accuracy will vary as gravity acts on the drilling tool. A rotating drill can produce sufficient concentricity in relatively shallow holes, but performance will suffer as holes become deeper and less forgiving tolerance wise.

On the other hand, because the direction of gravitational forces relative to the workpiece constantly changes when the drill is stationary and the workpiece rotates, that arrangement can produce holes approximately twice as concentric as the rotating drill approach. While shops can perform rotating-workpiece deep hole drilling on a turning machine, a dedicated deep hole drilling machine using what’s known as counter-rotation will net much better results.

Benefits of counter-rotation

A drilling setup that involves both the drill and workpiece rotating in opposite directions will balance out drilling forces, which are never static in a constant net direction. The balanced forces keep the drill from drifting for a much more concentric hole. With the right equipment and setup, counter-rotation is possible for smaller gundrilled holes as well as larger holes drilled with BTA tooling.

In counter-rotation testing, UNISIG drilled a ¼”-dia. hole in a 30″-long, ¾”-dia. OD, 4140HT steel workpiece. This 120:1 depth-to-diameter application is one typically found in the production of power transmission shafts or aerospace linkages.

Drill drift at the 30″ hole depth was measured via ultrasound. With a rotating drill and stationary workpiece, drill drift was 0.026″; a stationary drill and rotating workpiece exhibited 0.015″ drift; and when both the drill and the workpiece were rotating, drill drift was only 0.009″. It should be noted that results will vary due to many factors, including material, depth-to-diameter ratio and the specific tooling involved.

Dedicated deep hole drilling machines

Even with careful application of drill and workpiece counter-rotation techniques, typical machining centers – if equipped for counter rotation – usually do not have the alignment capabilities needed to consistently produce high-quality holes of 20:1 or greater depth-to-diameter ratio. Superior alignment is critical in maintaining concentricity.

In dedicated deep-hole drilling equipment, the machine base, rotating bearing groups and spindles as well as tool and workpiece supports are all designed with alignment as a first priority. Deep-hole drilling machines also emphasize control of other machining and environmental factors such as consistent temperature maintenance.

Some machines not originally engineered for counter-rotation operation can be retrofitted with a secondary counter-rotating group, but the alignment processes needed to make the arrangement work will be challenging and expensive. Additionally, a machine originally designed to employ counter-rotation will be manageable for nearly any operator. Dedicated deep hole drilling machines, for instance, include operator interfaces that provide detailed process information and maximize control over drilling parameters to enable accurate, efficient and repeatable production.

Basic application guidelines

Every deep-hole drilling application is essentially unique. However, general application guidance for counter-rotating operations includes allowing one-third of the total drilling speed to come from workpiece rotation and two-thirds from the drill. Operating parameters can then be adjusted to maximize drilling speeds and accuracy.

Such counter-rotation techniques provide a way to achieve accuracy and production requirements in deep-hole drilling and are especially effective when drilling holes of 40:1 depth to diameter ratios or more. Counter-rotation generates higher levels of concentricity that enable use of optimal feed rates while also extending tool life. The result is production of more parts per hour with fewer tool changes.