The Need for Feed Units: 3rd Installment
Contributed by John Zagar
Edited by Jo Gardner
Pneumatic and Hydraulic Feed Units
A feed unit is more than just an automatic drill press. It is an automated, self-contained, modular machining unit usually offering two axis of motion-spindle rotation and linear feed. Feed Units can do everything from drilling and tapping to drill/reaming, reaming and thread rolling. The feed mechanism, which usually extends the spindle in a quill, can be driven mechanically, hydraulically, or pneumatically. A feed unit typically can perform both rapid advance and controlled feed and provides position feedback through limit sensors or encoder/resolver feedback for integration into machine control.
In the 1st installment, we covered general variables in mechanical feed unit selection. Mechanical, hydraulic, and pneumatic feed units each have different features and functions that perform best when used for certain applications. When a carefully selected feed unit is optimally utilized, it can offer a rich payoff. Unfortunately, without an understanding of all the variables involved, it is too easy to end up with a poorly applied feed unit, a slow production rate and frustrated machinists.
In the 2nd Installment, we covered mechanical feed units including: ballscrew units, cam feed units and gear units.
Today is our promised 3rd Installment, in which we will cover pneumatic and hydraulic feed units. We decided to break a final segment into a 4th installment, which will cover other general considerations in feed unit selection.
Zagar Pneumatic Feed Units
Both the spindle and the quill can be air-powered in pneumatic feed units, making this the most popular type of Zagar feed unit. Additionally, the power and installation needs of these general-purpose, reasonably priced units are familiar to most production personnel. Low power, compact varieties of pneumatic feed units employ an air motor to power the spindle. These units often are used in light-duty applications such as drilling aluminum.
On most pneumatic feeds, a piston style quill advances and retracts the spindle via air power. Other units employ a spring-loaded return rather than an air-powered return.
A pneumatic feed unit without an air motor employs an electric motor to power the spindle. The most common electric motor used is a fixed-speed AC motor using pulley combinations to vary the speed. Variable-speed DC motors and AC motors with an inverter for speed variation are also popular. The feed rate is most often controlled with an additional hydraulic feed-control cylinder, adjusted manually from outside the unit (Figure 3). The units move forward in rapid advance until they engage the feed-control cylinder and enter into the feed for the length of stroke of the hydraulic cylinder or until they hit a positive-stop mechanism on the unit.
For all pneumatic units, the adjustment of depth control for blind holes is controlled by adjusting the positive stop. Depending on the pneumatic-control system and limit sensors used, these units may be capable of stopping in midstroke.
Hydraulic Feed Units
The hydraulic unit is similar to a pneumatic unit, but it uses hydraulic fluid for power transmission. A hydraulic unit. can reach much higher thrust levels than a like-size pneumatic unit. An electric motor usually is employed. A hydraulic motor, used on some units, delivers a lot of power for its compact size. If a large number of hydraulic units using hydraulic motors are installed on one machine, large hydraulic tanks may be needed to meet flow requirements.
Both rapid advance and feed are powered hydraulically. Because the feed can be controlled by the same system as the rapid advance, the external hydraulic control cylinder is eliminated.
Feed Unit Selection Parameters
Once you’ve gathered information about the various types of feed units available, examine your company’s general, engineering, and tooling requirements for using feed units.
Identify your company’s performance/production, cost, and unit lifecycle requirements. Performance and production requirements vary dramatically from one user to another. Some shops purchase low cost., less rugged feed units and treat them as short life, disposable equipment. But for shops that require units… with more power for longer-run production, this is not a practical strategy. Today’s feed units have been designed for these extended-use applications. They are more reliable and can achieve longer maintenance cycles than ever before.
In determining your needs, ask yourself: Is my machine operating 24 hours a day, five to seven days a week? Or is the machine only operating two to three hours a day, two to three days a week? It makes a big difference in the selection of a feed unit. The amount of idle production time will help determine your performance and production requirements. It also will help determine whether you need a light, medium, or heavy duty feed unit, as well as the type of preventive maintenance schedule you will need for relubrication, inspection, or part replacements. Ask yourself how many wear parts are in the unit, and how many of these parts are major or minor wear parts. O-rings, seals, and cam followers are inexpensive enough to be considered minor replacement wear parts. Major replacement wear parts include bearings, spline drive shafts, motors, and quills. Find out if these replacement parts are available from the manufacturer.
Cost considerations also vary from customer to customer. If the feed unit manufacturer you ‘re dealing with understands your specific needs, it can help you avoid overbuying or underbuying. For example, if you plan to replace a $10,000 machine with a $1200 feed unit, you may be drastically underestimating the feed unit’s actual life cycle maintenance costs and overestimating its capabilities. Unrealistic expectations for a feed unit are common and should be addressed by your feed-unit supplier.
You need a lot of horsepower to drill large holes. Likewise, a drilling unit with a 0.38″-dia. drive shaft is probably not going to drill a 1.25″ diameter hole in steel with a carbide drill. Accurate calculations by both the buyer and seller are helpful in the selection process because they help eliminate improper choices. Be sure the manufacturer provides adequate technical support to demonstrate the feed unit’s capabilities and that the unit can perform all the functions it is supposed to perform. Ask the manufacturer what additional equipment you’ll need to have on hand or buy as an option. The more complete the unit, the sooner you’ll have a machine operating on the floor. The feed-unit vendor should act as your team member and help you with the sizing and selection criteria for an optimal feed-unit choice.
Engineering Requirements. Many feed-unit buyers forget how important size and weight are to both the overall feed unit effectiveness and the machine’s stability. Over the years, end-user companies have relied on various post-OEM machine builders to design their machines. Now, there is an increased demand for adapting or retrofitting equipment, and many shops are opting to build machinery in-house. The engineering of mounting components such as risers, columns, angle brackets, and barrel clamps is of prime importance, whether the machinery is built in-house or by an outside vendor, including the feed-unit manufacturer. For example, a shop may retrofit equipment because its tooling needs have changed. With proper preparation and modification of the machine, installation of the feed unit can be scheduled in advance. This allows the retrofit schedule to stay on track and minimizes “on the floor” engineering modifications. The base and mounting components must be designed to handle the load the feed-unit is to carry. Compromising structural integrity rarely produces positive quality results and can often jeopardize operator safety.
By taking into account engineering requirements such as horsepower, torque, thrust, and speed, you can begin to identify the feed unit that best suits your needs. Most feed-unit manufacturers have experimental-data charts or calculations to show their products’ requirements and capacities in these areas. But the data is normally available only for standard drills and taps. Most often, tooling manufacturers can’t and won’t estimate engineering requirements, because tool material, tool coatings, and coolant application influence the data. Therefore, it’s helpful for the buyer to have experience with the tool and to take measurements of the torque, horsepower, or even current draw from the motors when running the tool with existing equipment. A feed-unit manufacturer is better able to analyze the selection of a unit if the customer has tool data from experience, experimentation, or the tooling manufacturer.
To illustrate how tools can differ in their needs, compare a solid-carbide drill to an HSS drill. A carbide drill typically will take nearly twice the horsepower and twice the speed of a normal HSS drill. unit will operate also is an important consideration. High temperatures, moisture, corrosive fluids, and dirt all can play havoc with a feed unit. Make sure the unit you choose will be able to endure your shop-floor environment.
Tool Requirements. Keep in mind manufacturing specifics, multiple-hole requirements, and spindle-nose or toolholder requirements when selecting a feed unit. To determine spindle-nose and toolholder requirements, ask yourself the following questions:
- What style is the tool shank? Do I need a specially configured tool shank to use this feed unit?
- What style is the spindle nose?
- What style of tool holder do I need to connect the two?
- How is the tool held and adjusted (if applicable)?
- Do I need a driver?
- Do I want to use quick-change toolholders?
Stay tuned for the final installment of the Need for Feed Units on Wednesday July 1, 2020. We will cover other feed unit selection considerations, followed by feed unit options. Zagar Inc. is a manufacturer and distributor of a range of workholding solutions, multi-spindle drill and tap heads, feed units and robotic end effectors. The company’s expertise is machining both ferrous and non-ferrous materials for industries such as automotive, aerospace, electrical, medical, off-road construction equipment, hydraulic, pneumatic and high-production drilling and tapping.