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Workholding: Mechanical, Hydraulic or Both?

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What is it?

Locating and holding a part in place for machining can be done in a variety of ways. For those just getting started or even those with vast experience in manufacturing, choosing the right method is not always easy. What types of components do I need? How do they work together? How do I know it is going to work?

This and the next chapter of Enerpac University answer these questions and more as we present an overview of mechanical and hydraulic workholding, and detail a combination of the two that incorporates the most desirable characteristics of both.

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Introduction

Workholding is a fundamental task in metalworking. It’s comprised of three distinct steps.
1. Locating (positioning)
2. Supporting
3. Holding (clamping)
Three methods of accomplishing the workholding task are in common use.
1. Mechanical Workholding Components such as bars, springs, nuts, and bolts.
2. Hydraulic Workholding Pneumatic or hydraulic components.
3. Hydraulic/Mechanical: Collet Lok® A unique combination of hydraulic and mechanical components.


Mechanical Workholding

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Mechanical workholding is the oldest and simplest method, and it offers the lowest initial cost. Familiar components used are available in countless variations. The components are easy to manufacture, so they are offered by many, many suppliers and have become essentially a commodity. Mechanical workholding components from many manufacturers are interchangeable and, like Lego© blocks, can be assembled into endless geometries and reused over and over. Typical Components Part positioning is accomplished by use of components such as precision ground pins and spring plungers. Support heads can be used to provide support points required for 3-2-1 fixturing. Probably the most common clamping device is the strap clamp, also called a bar clamp.

In the illustrations below, the clamping is implemented by using a strap or bar together with studs, springs, washers, and a nut or threaded knob or handle to apply force. Clamping force is controlled by torque applied to the nut. The spring lifts the bar away from the part during placement or removal.

03_page2_2.gif Clamping force can be generated by tightening a nut, a knob, or a handle.
03_page3_2.gif Examples of other mechanical clamping devices are hook and toggle clamps.
A cam can also be used to apply force to the bar clamp.

Applications

In general, mechanical workholding is most appropriate when these two criteria apply:
• Modest production run
• No critical tolerances
See the "Cost Comparison" on page 8 
for details on evaluating the size of a production run.
Mechanical clamping can cause distortion of the part because the operator, using hand tools, cannot tighten the clamp to a specific torque with great accuracy. This can result in significant differences in part height and position on a fixture. However, with a rough casting where the required 

finish is not critical, this may be acceptable. Another acceptable example is a large casting, where clamping force is highly unlikely to distort the part.
A “modular” fixture system can be used 
to speed fixture assembly by eliminating special machining tasks. Instead, horizontal and vertical T-slotted or drilled grid plates are used to mount the workholding components.


Hydraulic Workholding

The term power workholding refers to the use of pneumatic or hydraulic systems to perform the supporting, positioning, and clamping functions necessary to machine a part. This discussion focuses on hydraulic workholding, which has more capabilities and versatility than pneumatic.

The three fundamental types of components used in hydraulic workholding are cylinders, pumps, and valves. As detailed in the first chapter, cylinders convert the flow and pressure of hydraulic oil to linear motion and force. Pumps convert power from compressed air or electricity to hydraulic power: pressure to provide clamping force and flow to move cylinders. Valves are used to control the flow path, flow rate, or pressure of hydraulic oil.

Advantages

In order to be productive and efficient, procedures such as positioning, supporting, clamping, and release must be rapid, straightforward, and safe. This is especially true for larger parts with relatively brief processing intervals, making semi-automatic or fully automatic clamping in one fixture a profitable option. Hydraulic workholding is an extremely reliable and efficient way to accomplish these objectives.

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These are the principal advantages of hydraulic workholding:

  1. Time gain – The major benefit of hydraulic workholding is the enormous time saved in clamping and unclamping. Time reductions of 90 to 95% are not unusual. Less machine idle time means more capacity utilization and lower costs.
  2. Accuracy and repeatability – Consistent clamping forces produce precise, repeatable locating and identical processing of each part. Virtually no parts are rejected due to distortion.
  3. Less fixture space – Compact standard components and the ability to clamp in manually inaccessible areas allow optimum use of fixture space. Often, more parts can be clamped and processed simultaneously on one fixture.
  4. Safety – Less operator interaction means less chance of mishaps.
  5. Automation – Automating part of the production process frees operators for other tasks.

Typical Components – Positioning Cylinders

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These are the principal advantages of hydraulic workholding:

The first operation in workholding is to press the part against fixed points and surfaces, as detailed in the previous chapter, "The 3-2-1 Locating Principle." This task is performed by positioning cylinders.

Stroke and capacity

Stroke and capacity are the two performance parameters of concern. Many choices are available, so cylinder stroke length can be selected according to the distance the part must be pushed or pulled. The capacity of a cylinder is stated as the pushing or pulling force it produces when supplied with a given hydraulic oil pressure.

Mounting styles

Mounting style determines how a cylinder mounts to the fixture. There are three styles for mounting, threaded, bolted and flanged.
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Mounting styles

Mounting style determines how a cylinder mounts to the fixture. There are three styles for mounting, threaded, bolted and flanged.

Single- and Double-acting cylinders

These cylinders are available in either single- or double-acting form. Single-acting cylinders deliver hydraulic power in only one direction of travel, utilizing an internal spring to provide plunger return when hydraulic pressure is removed. These cylinders are the simplest type and require the least plumbing (one port), but also deliver slower operating speeds than double-acting cylinders.

Double-acting cylinders

This cylinder uses hydraulic power to move the plunger in either direction, so they are more complex. The payback for slightly more plumbing and control devices is high speed operation in either direction and precise control of the plunger.


Typical Components - Work Supports

Once a part has been positioned on the fixture, the next step in workholding is to provide support where needed to prevent part deflection or vibration during machining.

It’s important not to confuse work supports with the support points and planes inherent in the positioning step! As explained in the previous chapter, rigid support points or surfaces additional to those used in the 3-2-1 locating process must not be introduced.

After a part has been positioned, the work support advances to contact the part with slight pressure, then locks in place. This can be accomplished in these ways:

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  1. Spring advance – The weight of the part compresses a spring behind the plunger. When the part is in place, the plunger is locked
  2. Air advance – Air pressure is used to advance the plunger to contact the positioned part, and then the plunger is locked.
  3. Hydraulic advance – hydraulic fluid is used to advance the plunger to contact the positioned part, and then the plunger is locked.
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For each advanced method, locking is accomplished by applying hydraulic pressure to a sleeve that compresses to lock the plunger in place.

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Typical Components - Swing Cylinders

After a part has been positioned and supported, it must be clamped where needed to complete the workholding process. The swing cylinder is without doubt the most widely used clamping device. As the plunger travels from the extended position to the retracted position, the clamp arm rotates, usually 90°.

The 90 degree rotation of the clamp arm eliminates interference when the part is mounted to the fixture or removed.

Stroke and capacity

Stroke and capacity are parameters of concern, just as with positioning cylinders. Choice of cylinder stroke length is primarily affected by the geometry of the part to be clamped. Capacity, the clamping force, is selected in accordance with the machine cutting forces that must be resisted.
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Mounting styles

Mounting styles provide flexibility to meet requirements of part geometry, fixture space, and accessibility for loading and unloading. Oil feed to flanged and threaded models can be via external plumbing or a fixture manifold. Cartridge models recess into a fixture manifold for an extremely compact setup.

 

Single- and Double-acting

Mounting styles provide flexibility to meet requirements of part geometry, fixture space, and accessibility for loading and unloading. Oil feed to flanged and threaded models can be via external plumbing or a fixture manifold. Cartridge models recess into a fixture manifold for an extremely compact setup.

 

Arm Length

Arm length is a parameter that allows design flexibility. However, the longer the arm, the higher the side load on the cylinder. Consequently, the longer the arm, the less clamping force available. Clamp arms are available in a variety of lengths, or you can machine your own configurations. Product literature provides information on allowable pressure and clamping force as a function of arm length.


Typical Components – Valves

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Valves are the fundamental control devices. There are many kinds of valves, but any given valve serves to determine the flow path, flow rate, or pressure of the hydraulic oil. Enerpac valves are of modular design, which simplify system assembly.

 

Directional Control Valves

They are just what the name implies, multiport valves used to direct hydraulic oil. Single-acting cylinders are usually operated by 3-way valves, while double-acting cylinders are operated by 4-way valves.

 

Sequence Valves

Directional control valves that control the order of operation of various branches of the hydraulic circuit are sequence valves These valves are activated by a sensed pressure. When one part of the hydraulic circuit reaches a preset pressure, the sequence valve opens to permit oil to flow to another part of the circuit. An example is activation of a support cylinder first, then the corresponding clamping cylinder. The sequence valve contains a check valve in parallel with the directional valve mechanism. The result is sequenced/directed flow in one direction (toward the loads, during clamping) and unrestricted flow in the other direction (unclamping).

 

Check Valves

Check valves are used to allow the flow of hydraulic oil in only one direction. A special version, the pilot operated check valve, is used to hold pressure in one part of a circuit and release the pressure upon command. A pilot operated check valve works the same as a regular check valve, but also has an additional port. The application of a low pressure (about 15% of working pressure) to this extra port causes the check valve to permit oil flow in both directions, releasing the contained pressure.

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Flow Control Valves

Flow control valves control the operating speed of hydraulic components by means of an adjustable orifice. They also contain a check valve in parallel with the metering orifice. The result is restricted flow in one direction (toward the load) and unrestricted flow in the other direction (return).

The most common use of flow control valves is to regulate the operating speed of cylinders. When the holding pressure on a cylinder is released, a spring in the cylinder produces a small pressure, causing the check valve to open and thereby providing rapid cylinder release.

 

Pressure Reducing Valves

Pressure reducing valves do just that; they are adjustable pressure regulators, used to provide reduced oil pressure to a secondary part of the circuit. A common use is to control the clamping force of a cylinder.

NOTE: Pressure Limiting Valves perform a more specialized function. They deliver a settable lower pressure to a secondary part of the circuit. However, when the set pressure is reached they do not supply any makeup oil to maintain that set pressure.

 

Pressure relief valves

Pressure relief valves limit the maximum pressure in the hydraulic circuit. For safety, all hydraulic systems should include a relief valve. An Enerpac relief valve does not act instantly. As pressure approaches the set point, the valve permits a very small amount of oil to pass before further pressure rise results in full opening of the valve. Therefore, a relief valve should not be adjusted under conditions different from use. For example, don’t set the valve with a hand pump and then use it with a power pump.


Typical Components - Pumps and Other Power Sources

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In some applications, the pressure and flow of hydraulic oil required to operate various cylinders can be derived from a machine’s hydraulic system. In all other cases, a type of pump or power source must be included in the design.

 

Hand Operated

The most rudimentary power source is the hand-operated pump. While they are a low-cost power source, hand pumps are of limited usefulness because they are slow. Additionally, they can only operate single-acting cylinders. Therefore, hand pumps are best reserved for occasional special situations.

 

Air Operated

The air hydraulic booster is a simple source of one-shot hydraulic power. Here, a large-diameter air cylinder drives a smaller diameter hydraulic cylinder, providing 5000 psi hydraulic pressure from shop air pressure. These devices can provide high oil pressures and flow rates, but are limited by their nature as to the amount of oil that can be continuously delivered.

 

The air-operated hydraulic pump is the next step in sophistication. These products range from compact "portable" units that can be mounted directly on a fixture to free-standing units that include reservoirs and modular control valve mounts. Portable pumps are best suited to single-acting cylinders, while free-standing units can handle single- or double-acting cylinders on small to medium-size fixtures.

 

Electric Operated

The electric motor-driven hydraulic pump, although the highest price source, provides the greatest flexibility and capacity. Desired pressure and flow dictate the required source horsepower, and for larger setups an electric pump may be the only suitable source. Advantages of electric pumps include high delivery for high speed operation; modular design for easy attachment of manifolds, filters, heat exchangers, and controls; and compatibility with automation systems.

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Typical Components - System Components

Cylinders, pumps, and valves are the primary workholding components, but the "little things" count too. For example, pressure gauges are your "window" to the hydraulic system, revealing the status of various parts of the hydraulic circuit.

The accumulator is another helpful device. It is a chamber, divided by a sturdy diaphragm. On one side of the diaphragm is a charge of high-pressure gas. The other side of the diaphragm chamber is connected to the hydraulic circuit.

In operation, the chamber and pressurized diaphragm absorb pressure pulsations and thermal expansion of the hydraulic oil, and provide a small but "instantly" available reserve supply of oil.

Rotary couplers (rotary unions) are unions specially designed to transfer pressurized hydraulic oil from a stationary supply line to a rotating device such as a rotating index table.

Pressure switches, available in both mechanically operated and all-electronic versions, serve to monitor hydraulic pressure. The switch outputs can be used for status indication, safety purposes, or automation interfacing.

Filtering is a must! Contaminated hydraulic oil is the number one cause of failures. Filters can be installed in the high pressure line, return line, or both.

Fluid lines can be of steel tubing or hose. Care should be taken to use only the specified tubing, hose, and fittings in order to assure reliable and safe operation.

Hydraulic oil, the lifeblood of the workholding system, shouldn’t be taken for granted. A manufacturer-specified oil has controlled characteristics, such as viscosity, flash point, and pour point.


Introduction to Hydraulic Workholding

Applications

Power workholding is particularly suited for medium to high volume applications and lends itself well to integration with automated manufacturing systems. Hydraulic workholding is particularly appropriate where critical tolerances must be held. It allows clamping force to be maintained within a 1% accuracy. This type of accuracy and repeatability permits machining to tolerances as tight as ±1 mil.

Parameters Hydraulic Mechanical
Production quantity:
60,000 pcs
60,000 pcs
Material cost per piece:
25.00
25.00
Machine time cost per hour:
150.00
150.00
Fixture cost:
30,000.00
5000.00
Parts per fixture:
4
4
Load + unload time:
20 sec.
240 sec
Run time:
720 sec.
720 sec.
Labor cost per hour:
30.00
30.00
Resulting costs, per piece
Material:
25.00
25.00
Machine time:
8.82
11.43
Fixture
0.50
0.08
Cost of fixture money at 5%:
0.04
0.01
Labor (one 8hr shift, 7hr working)
1.76
2.28
Total cost per piece:
$36.12
$38.80

Compared with mechanical workholding, hydraulic workholding is a classic example of a way of doing something that costs more upfront, yet is cheaper in the long run.

Here is an illustrative cost comparison. It is offered with some trepidation, in that any of the numbers used could be critiqued. However, the important point is that if you work the numbers using other values, you will still invariably find there is a production quantity beyond which hydraulic workholding becomes the most economical option.

Using hydraulic workholding, not only is the cost lower by $2.68 per piece, the run is completed in 18 months, versus two years

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Collet-Lok® Workholding

A unique product from Enerpac combines the benefits of hydraulic workholding with the long-term stability of bolted mechanical fixturing. The Collet-Lok® system provides the speed, precision, controllable clamping force, safety, and automation compatibility of hydraulic fixturing. But in addition, once the part is positioned, supported, and clamped, the cylinders are mechanically locked in place so that all hydraulic pressure can be removed for any length of time. When clamping is completed and locked, the fixture can be entirely disconnected from the hydraulic system, or even if left connected, it is not dependent on the hydraulic supply to retain clamping force.

How is this accomplished? As seen in the cross-section, hydraulic pressure is used to drive a wedge that mechanically locks a collet chuck around the cylinder plunger. With the plunger locked, all hydraulic pressure can be removed. The collet releases the cylinder plunger only when hydraulic pressure is applied to the opposite end of the wedge. The next chapter of Enerpac University provides details of this versatile system.