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Workholding Clamping Cylinders

0637_appl Bar Clamp with Handle

Securing the Part

The previous chapter discussed positioning cylinders, which accomplish the first of the three steps in workholding. In this chapter we look at clamping cylinders, used to implement the second step.

Holding a workpiece in place while working on it is so fundamental that generations of metalworkers have devised countless variations of mechanical clamps such as the one illustrated (Refer to Chapter 3 for more examples).

These devices are based on principles such as the lever, inclined plane, and screw thread. They are simple and comparatively inexpensive to purchase. But, like virtually every other choice in life, there are tradeoffs. In return for the lowest upfront cost, the purchaser of a mechanical clamping system must accept two key limitations:

  • Modest production runs;
  • No critical tolerances.

Production runs can be limited by the labor-intensive nature Production runs can be limited by the labor-intensive nature of mechanical clamping. For many parts, tolerances are limited by the need to allow for distortion of the part and inconsistent clamping forces from part-to-part due to the variability of manually tightened clamps.

Workpiece An over-located workpiece

Hydraulic workholding overcomes those limitations. The tradeoff is that the purchaser must pay more upfront for a hydraulic clamping system. However, the numbers readily show that for any significant production volume, the cost of using hydraulic workholding is lower than for manual clamping. The lower total cost of hydraulic workholding combines with an increased production rate, improved accuracy and repeatability, less congested fixtures, increased safety, and automation compatibility.

Whether the workholding method is mechanical or hydraulic, the same three fundamental steps must be accomplished: positioning, clamping, supporting. Regardless of how it is accomplished, the positioning step is based on the 3-2-1 locating principle explained in Chapter 2. It’s important to avoid the mistake of over-location. This occurs when there is more than one locating plane or point for any given degree of freedom. Over-location can cause in distortion or mis-orientation of the part, resulting in degraded accuracy and consistency.

"Clamping" is the secure fastening of a positioned workpiece in a fixture. Clamping always requires force transmission through the part. Each clamping force vector should, as nearly as possible, describe a line that extends from the application point of the clamping force through the workpiece to the bearing points and is parallel to the axis of the clamping cylinder and perpendicular to the plane of the bearing points.


Clamping Force

When properly designed and controlled, a hydraulic clamping mechanism prevents the workpiece from moving when machining forces are applied, yet will not cause permanent distortion to occur in the workpiece. As already mentioned, it is important to ensure that the clamping cylinders themselves do not move or distort the positioned part.

The clamping force needed to keep a part in place is determined by the machining forces to be applied to the part. Cutting tool suppliers provide information about the forces produced by each type of tool when machining various materials. Following are example calculations for a face milling operation.

Example

An 8-inch diameter cutter with 10 teeth (inserts) is to be used to machine low silicon aluminum at 3000 SFM (surface feet per minute). How much force does it apply to the part, and how much clamping force is required to restrain the part?

Step 1

Convert SFM to RPM using this formula:

Formule1.gif where RPM = spindle speed SFM = tool surface feet per minute D = tool diameter in inches Inserting the numbers, formule2.gif

Step 2

Determine the material removal rate using this formula:
MRR = (W)(D)(F)(N)(RPM)
where
MRR = material removal rate in in3/minute
W = width of cut in inches
D = depth of cut in inches
F = feed per tooth in inches
N = number of cutter teeth
RPM = spindle speed
For the 3000 SFM specified in this example, a typical tool catalog lists a feed per tooth of 0.008” maximum at a cut depth of 0.1”. Inserting the numbers,
MRR = (8”)(0.1”)(0.008”)(10)(1432) = 91.6 in3/minute

Step 3

Calculate spindle hp: For milling aluminum with a dull tool, a typical fixturing reference table lists a spindle horsepower-to-MRR ratio (generally referred to as “unit power”) of 0.4. Therefore, spindle hp = (0.4)(91.6) = 36.6 hp. It should be noted that this horsepower calculation is for fixture design and not for machine tool horsepower requirements. (A true 40 hp machine, for example, can remove aluminum at more than 200 in3/minute.)

Step 4

Calculate force transmitted from the tool to the workpiece using this formula: formule3.gif where
Cutting force = force transmitted to the workpiece, in lb
hp = the spindle hp calculated in step 3
SFM = tool surface feet per minute
26,406 = 33,000 ft-lb/min conversion factor, with 80% efficiency
Inserting the numbers,
formule4.gif Using a safety factor of 1.5 for hydraulic clamping, the allowance for cutting force becomes 483 lb.

Step 5

Calculate the required clamping force using this formula:

formule5.gif

where Fclamp = required clamping force to resist cutting force, in lb
Ftool = cutting force, in lb
k = coefficient of friction
The coefficient of friction for lubricated aluminum (flooded with coolant) is 0.12. Inserting the numbers,
formule6.gif

This is the clamp force required to keep the workpiece immobile while subject to the cutting tool force if clamping alone is holding the workpiece in place. Other elements, such as positioning cylinders and locating stops, may contribute forces that reduce the required clamping force.

The foregoing calculations determine the required hydraulic system parameters. Recall that the force produced by a cylinder equals the product of cylinder cross-sectional area and hydraulic fluid pressure. Cylinder catalogs list cylinders in terms of their maximum clamping force when maximum rated hydraulic pressure is applied.

In practice, the hydraulic system should be operated at 50-75% of its rated pressure, so clamping cylinders selected for the above example should have a total capacity that is 1.33-2.0 times the required force. That means the total maximum rated clamping force of all cylinders in the example should be within the range 5353-8050 lb. (e.g. three cylinders rated 2600 lb each.)

Force is not the only cylinder parameter of concern. The next section describes additional factors in cylinder selection.


Types of Clamping Cylinders

SA_operation

Single and double-acting

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. They require less valving and plumbing than double-acting cylinders, resulting in a less complex hydraulic circuit and minimal fixture congestion.

However, single-acting cylinders deliver slower operating speeds than double-acting cylinders. Additionally, the spring return means that hydraulic fluid must have an easy return path with few restrictions, and only a few cylinders may retract simultaneously.

DA_operation

Double-acting cylinders use 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. This is important when greater control is required during the unclamp cycle. The tighter control provided by double-acting cylinders is also helpful when timing sequences are critical, because they are less sensitive to system back pressures that can arise from long tube lengths or numerous cylinders retracting simultaneously.

Swing_movement_jpg.jpg

Swing cylinders

The swing cylinder is without doubt the most widely used hydraulic clamping device. The plunger of the swing cylinder and the attached arm rotate 90 degrees in either a clockwise or counter clockwise direction at the beginning portion of the stroke. They then travel down an additional distance to clamp against the fixtured part. Upon release of clamping pressure, the clamp arm raises and rotates back to 90 degrees in the opposite direction to allow for removal of the part removal and placement of the next part.

Most swing cylinders are available in either single- or double-acting form. Stroke length is selected according to the geometry of the part to be clamped. Capacity, the clamping force, is selected according to the machining forces that must be resisted. A variety of arm lengths provides design flexibility. (The System Considerations section of this chapter provides more information.)

SU_model Upper flange mounting
  • Oil connection via manifold or threaded port
  • Fixture hole does not require tight tolerances
  • Easy installation with 3 or 4 mounting bolts
SL_model Lower flange mouting
  • Oil connection via manifold or threaded port
  • No fixture hole required
  • Easy installation with 3 or 4 mounting bolts
ST_model Threaded body mounting
  • Threaded body allows precise cylinder height positioning
  • Threaded oil port connection
  • Threaded into fixture and secured in place by standard flange nut
SC_model Cartridge mounting
  • Minimal fixture space required· No external plumbing
  • Adjoining units can be closely spaced
  • Cylinder can be completely recessed into fixture
98_007.jpg

Pull cylinders

Fitted with a variety of plunger end-effectors, pull cylinders can be used to pull a part into place, pull it down, or push it via a clamp arm fitting. Pull cylinders are offered in single-and double-acting configurations and mounting styles such as illustrated for swing cylinders.

Pull cylinders

Fitted with a variety of plunger end-effectors, pull cylinders can be used to pull a part into place, pull it down, or push it via a clamp arm fitting. Pull cylinders are offered in single-and double-acting configurations and mounting styles such as illustrated for swing cylinders.

CDB_appl_G

Block cylinders

In addition to clamping, block cylinders are used for punching, pressing, riveting, and bending applications. They are available in numerous mounting, stroke, and connection arrangements.

ECH_appl_G 98_008

Pull down clamps

Enerpac pull down clamps allow unobstructed top face machining. Independent horizontal and vertical movement achieve high lateral and pull down forces to hold the workpiece firmly against the fixture. The pull down force is equal to approximately 1/3 of the clamping force.

WP_connections

Positive clamping cylinders

A conventional hydraulic workholding system requires either special valving or a continuous power source to maintain clamping force. There is another way to accomplish positive clamping: Enerpac’s Collet-Lok® system. Collet-Lok combines the benefits of hydraulic workholding with the long-term stability of bolted mechanical fixturing. 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.

A Collet-Lok cylinder is locked by hydraulic pressure supplied to a lower port to drive a wedge upward to mechanically lock a collet chuck around the cylinder plunger. The collet releases the cylinder plunger only when hydraulic pressure is supplied to the opposite end of the wedge via an upper port. For full details, refer to Chapter 4.


System Considerations

Swing cylinders and hydraulically advancing work supports are very sensitive to the oil flow rate supplied. To ensure safe and reliable operation of these elements, the maximum oil flow rate indicated in the catalog and the instructions must not be exceeded. If there is a risk of high flow rates, use flow control valves to adjust the flow rate.

It must be ensured that during the clamping sequence work supports will operate only after the workpiece is firmly positioned and clamped against locators and datums. However, if a cylinder is clamping directly over a work support, the work support should be brought to full pressure before the clamp cylinders operate. This can be done by means of a sequencing valve.

The clamp arms used to transmit force from the swing cylinder to the workpiece are available in a variety of lengths, and users can also machine their own configurations based on supplied dimensions. Because of the cantilever nature of a swing clamp arm, the longer the arm, the less force it can transmit without risk of damage to the arm or cylinder. Put another way, the longer the arm, the lower the permissible hydraulic pressure. Product literature includes graphs of allowable pressure vs. arm length and allowable clamping force vs. arm length for each arm design.

When details such as these are attended to, hydraulic workholding is an extremely reliable and efficient way to accomplish positioning, supporting, clamping, and release of a workpiece.