How To Pick A Feed Unit

A feed unit is more than just an automatic drill press. It is an automated, modular machining unit usually consisting of only two axes of motion—spindle rotation and linear feed. The feed mechanism can be mechanical, hydraulic or pneumatic, and usually extends the spindle in a quill. A feed unit can typically perform rapid or controlled feed, and provide position control through limit sensors or encoder/resolver feedback for integration into machine control. Feed units can do everything from basic drilling and tapping to more exotic applications such as reaming, combination drilling/reaming, thread rolling, and even twisting.

In the right applications, feed units can be more cost-effective than CNC machinery. But shops that are unaware of all the variables involved when purchasing a feed unit can end up with a poor product application or setup, a slow production rate and, in general, frustrated machine operators.

Here's what you need to know to make the best use of this productive machining alternative.

Different Strokes For Different Folks

Mechanical, pneumatic, hydraulic and leadscrew tapping units each have several different features and functions that perform best when used for certain applications. For instance, when drilling with a carbide tool, a ballscrew feed unit enhances both tool life and hole quality due to its ability to maintain a constant feed rate and chip load. In contrast, a pneumatic or hydraulic piston type unit provides a constant force feed, which may suit the requirements of a particular tool such as a deburring bit or a friction drill.

Another important selection distinction is that retrofit applications almost always have an existing tool for which a unit needs to be selected, whereas a new application may or may not have the tool already specified. If not, the question is whether to select the tool or the feed unit first. While the selection of the first item can put constraints on the second, with the flexibility of some of today's programmable, ballscrew-type feed units, this problem can be avoided. The units can be programmed to simulate any requirements, including even the constant force operation of a piston type unit.

Mechanical Feed Units

The major types of mechanical units are ballscrew feed, cam feed, or gear driven.

The ballscrew unit transmits the feed power from a motor through some form of transmission, such as a coupling, belt and pulleys, or gears. It advances the quill, which is somehow attached to the ballscrew nut. A direct coupling to the ballscrew nut allows for high efficiency thrust on the center line axis of the tool. This also results in minimized deflection—a major consideration with precision tooling.

Ballscrew units are the most flexible of units, with the capabilities of the programmable control largely dictating what can be achieved. High power and precision is a common asset of these units.

In the cam type unit, a cam follower attached to the quill follows the path of either a rotary drum cam or a linear (kidney shaped) cam. As the cam rotates very slowly through some means of speed reduction, it advances the quill in relation to the path that is cut into the cam. Because the cam is driven by the same motor used for the spindle, it is possible to synchronize the spindle speed and quill feed. Varying combinations of belts and pulleys to the spindle and speed reducer are used to adjust for rpm and feed rate needs.

Of all the feed units, the cam type is the least flexible. The cam has a permanently machined path cut into it for the drilling cycle—including rapid, feed and retract sections. This makes the cycle non-adjustable without installing a new cam. But to many high production lines, this method is attractive for rock-solid consistency in part/feature qualifications.

Gear unit is a very general term to describe a unit that has the transmission means, from power source to output, of a gear drive. There are many types, and the gear drive can be used to power the spindle and/or the feed. These units are most often used for tapping. Changing of the tap pitch is accomplished by altering the gear ratio by changing the two drive gears or, with a leadscrew unit, by changing the pitch of the leadscrew and leadscrew nut.

Pneumatic Feed Units

Pneumatic feed units can have the spindle and the feed powered by air. They are very popular, general purpose, and low in cost. The needs for power and installation are also familiar to most production personnel, making them a comfortable unit to buy. On most pneumatic feeds, advancement is performed with a piston style quill. The air both advances and retracts the unit. Some units have a spring return rather than an air powered return. When selecting a pneumatic feed, consider total air requirements (depending on the number of units) and the ratio of on- to off-time.

Some units use an air motor to drive the spindle. These types are compact, but offer relatively low power. As such, these units are often used for drilling plastic, wood and aluminum. The other spindle power choice is an electric motor. The most common variety has a fixed speed AC electric motor and uses different pulley combinations to vary speed. Also popular are DC variable speed motors and AC motors with an inverter to vary the speed.

The feed rate is most often controlled on these units with an additional hydraulic feed control cylinder, adjusted manually from the outside of the unit. The units move forward in rapid until they engage the feed control cylinder and enter into feed for the length of stroke of the hydraulic cylinder, or until they hit a positive stop mechanism on the unit. The adjustment of depth control for blind holes is controlled by adjustment of the positive stop. Depending on the pneumatic control system and limit sensors used, these units may be capable of stopping at mid-stroke.

Hydraulic Feed Units

The hydraulic feed unit is very similar in most cases to the pneumatic units, but instead uses hydraulic fluid for power transmission. The motor also can be hydraulic, delivering great amounts of power for its compact size.

Size-for-size, hydraulic units can deliver much greater thrust than pneumatic units. Due to the low compressibility of hydraulic fluid compared to air, feed and rapid advance are controlled by the same system. Thus the external hydraulic control cylinder is eliminated. But depending on the number of units on one machine, hydraulic feeds can have high flow requirements, and thus require a large hydraulic tank.

Leadscrew Units

Leadscrew units are produced by many manufacturers, which is reflected by the variety of available products. Simply speaking, when the spindle rotates, the tap advances at the pitch of the leadscrew. Changing the tap to a different pitch requires changing both the leadscrew and leadscrew nut, but most operators should be capable of performing the change-over. The five to 20 minutes it takes to change the leadscrew must be factored into the production requirements for this unit type.

Programmable ballscrew units eliminate this manual conversion and allow the simple entry of new pitch value, part number or tap size, depending on how the control is set up. Low speed operations usually require gear reduction of the motor. The most common type of motor used for these units is a reversing brake motor for near instant deceleration and reversing for retraction of the tap.

Selection Considerations

When contemplating a feed unit purchase, three general requirements must be identified: performance/production requirements, cost, and product life cycle.

The amount of idle production time will help determine your performance and production requirements. Is your machine operating 24 hours a day, five to seven days per week? Or is the machine operating only two to three hours a day, two to three days per week? It makes a big difference in the selection of a feed unit.

Will re-lubrication, inspection or replacement of parts be needed? Also, ask yourself how many major- and minor-wear parts are in the unit. Minor replacement parts include O rings, seals and cam followers. Major replacement parts would typically include bearings, spline drive shafts, motors, quills, and so on. Also, note the availability of these parts from the manufacturer.

Understanding your specific needs can assist the feed unit manufacturer in preventing you from over- or under-buying. For example, if you plan to replace a $10,000 machine with a $1200 feed unit, you could be drastically underestimating the feed unit's actual life cycle costs and overestimating its capability. Unrealistic expectations are common and should be addressed before the purchase.

The product life cycle needs to be compared to total feed unit investment costs. If your feed unit is extremely durable for a short-run product, you have invested ineffectively. Consequently, if your feed unit is unequipped for a long-run product, you may not receive enough feed unit performance for the duration of your product without extensive replacement part or repair costs.

Engineering Requirements

In terms of the integrity of the feed unit equipment, it is important to evaluate physical engineering requirements such as size/weight and manufacturing considerations of horsepower, torque, thrust, speed and feed unit environment.

Many purchasers of feed units forget how important size and weight are to both the overall feed unit effectiveness and the machine's stability. Over the years, companies have relied on various manufacturers to design their machines. Now, there is an increased demand to adapt or retrofit equipment or build machinery in-house. With proper preparation and modification of the machine, the feed unit to be integrated can be pre-scheduled. This allows the retrofit schedule to stay on track and minimize "on the floor" engineering modifications. Compromising structural integrity rarely produces positive results and can often jeopardize operator safety.

By taking into account the other engineering requirements of horsepower, torque, thrust and speed, you can begin to filter out the most appropriate feed unit for your needs. All these figures can usually be obtained from manufacturers who have experimental data charts or calculations, though data is usually only available for standard drills and taps. But the feed unit manufacturer is in a much better position to analyze the selection of a unit if the customer has data on tool requirements either from experience, experimentation or data from the manufacturer of the tooling.

It is very useful to have data on the torque, horsepower or even current draw from motors used in the past for a given tool. And you should also take note of the specific process requirements. For instance, a carbide drill will typically take nearly twice the horsepower and twice the speed of a normal high speed drill. Another example is a small but very deep hole, which will require a feed unit capable of peck drilling.

Many times customers are aware of these facts prior to consulting the feed unit manufacturer. However, in cases when information is unknown, it is important to deal with a feed unit manufacturer who will educate you as much as possible during the process for your future reference.

The feed unit environment is also very important. High temperatures, increased moisture, corrosive fluids and dirty, contaminated environments all play havoc on a feed unit. The capability of a feed unit to endure these environments is very important.

Tool Requirements

When it comes to specific tool requirements, keep in mind manufacturing specifics, multiple hole requirements and spindle nose or toolholder requirements.

More specifically: What kind of shank does the tool have? What is the configuration of the spindle nose? What kind of toolholder is required? How is the tool held and adjusted if a tap driver with a specially configured shank is needed? Are quick-change toolholders desired?

Miscellaneous Requirements

Some additional requirements to bear in mind when selecting the most ideal feed unit are operator friendliness, flexibility, precision, power supply needs and your plant's specific production methods.

If change-over is frequent, a ballscrew feed unit will serve you best because it is very flexible and can be relocated to new applications. Also in a repetitive application, a ballscrew feed unit has the useful capability to blend a dedicated module unit with a CNC type of machine tool.

Flexibility in a feed unit often implies the programming capability or ease of the setup of the unit. Ask yourself: Are there many tasks that need to be achieved and do not fit into a canned or pre-programmed type of setup?

Another key element to your feed unit's effectiveness is how accurate the hole features need to be. Here, you may have to rely on manufacturers' recommendations to determine which units are capable of meeting hole tolerances on position and size.

Once you decide on a specific feed unit, you must also evaluate the existing power supplies whether it is electric, pneumatic or hydraulic. Being both consistent with your current power supply and choosing the most energy efficient source will save you the largest amount of money in the end.

Feed Unit Options

A number of feed unit options are available to help further customize production solutions. Options include various types of multiple spindle drilling and tapping heads, toolholders, controls, motors, speeds, coolant application and even programming services. Here are some specific issues to be aware of:

Drill and tap heads come in all different shapes and sizes. Some manufacturers specialize in multiple spindle heads and feed units and ship a large percentage of their feed units with multiple spindle heads as an integrated, single manufacturer solution.

If a manufacturer does not have an integral spindle nose option to attach a tool to the feed unit, then you need to be aware of the available toolholders.

The "coolant-thru" option allows for coolant to be passed through the tool, directly affecting the cutting operation. The coolant can be provided in two ways—either through a rotary union passing coolant through the center of the unit or induced through a front-end gland on the spindle nose.

Standard control features may do the job well enough. But custom programming service options allow customers to identify their parts with their terminology or specialize a program to meet their needs. The controls become familiar to operators because they can understand the operations or data entry conventions with a minimal learning curve.

Feed Units vs. Other Types Of Machinery

There are also many less apparent benefits feed units offer. Their modular construction can be easily adapted to new machinery with minimal setup time. With programmable units, part change-over time is rapid.

And there are other efficiencies to be gained. For instance, secondary operations, which burden a typical CNC machine, can be taken off-line, or previously labor-intensive procedures can become seamlessly automated. Safety concerns can be dramatically reduced. In addition, highly productive technology is brought into a shop at a relatively low cost.

Today, engineers, machinists, purchasers and manufacturers can choose from many feed units and feed unit features. There is no single unit that is best for an entire industry. The terms outlined above should help you better understand what your buying needs are so you can tailor your purchase and receive the most effective results. One thing is for certain: When choosing a system and tooling supplier, be sure the company can offer solid advice, excellent technical support and service as well as a wide variety of feed unit features to suit your needs.