Planning Your Workholding System
Successful powered workholding does not just happen. Like any other manufacturing process, it must be carefully planned. But that does not mean that you need to be a hydraulics engineer to implement a powered workholding system. Designing a system involves nothing more than the common-sense application of a few basic workholding concepts.
Applications for power workholding fall into two categories: Retrofits to replace and upgrade clamping on existing fixtures and new fixtures designed from the outset with power workholding. In both cases it is imperative that you keep in mind the forces that can be generated by power workholding devices. A single device, small enough to hold in your hand, can generate five tons of clamping force. If you are replacing existing manual bolt and nut clamping or toggle clamps, make sure that the fixture or machine tool base will withstand the forces. Don't risk damaging a machine bed because you tried to tie a 10,000 pound clamp into a T-slot that would only withstand 5,000 pounds of force.
Using power workholding does not in any way invalidate the principles of sound fixture design. The 3-2-1 concept as it relates to the location of the workpiece in three planes is just as applicable when using power workholding devices as when using manual methods. Workholding devices should be positioned in such a way as to ensure firm contact between the workpiece and locating buttons, pins, or surfaces.
Begin the planning process by asking yourself the following: What do you want your system to accomplish? What sort of operation is it going to be used for? What clamping "speed" is appropriate for the speed at which your production line runs?
You should select "realistic" cycle times . . . the shorter the cycle time, the larger the power source you will require. For example, a pump with a 1/3-hp electric motor may be satisfactory to reach clamping pressure on a given system in three seconds. However, to accomplish the same task in one second may require a pump with a 1-hp electric motor at a considerable increase in both initial expense and operating costs. So before you specify "instantaneous" cycling, be sure the increased clamping speed is really worth the higher costs for your particular installation. Ask yourself if you can productively utilize the seconds saved.
With this in mind, let's proceed step by step through a plan of attack for designing your system.
First, determine the nature of the operation to be performed, the number of parts to be processed per cycle, and whether operations will be performed on more than one surface of each part. Also determine the time that should be allowed for part loading, unloading, and clamping.
Consult your machine tool file to determine the available work space on the machine table, bed, chuck or other surface, as applicable. Be sure that the space available will accommodate the part or quantity of parts to be processed according to your manufacturing work-flow. If not, revise your plan.
In the initial phases of system planning, include adequate measures and devices to ensure the safety of workers and equipment. For more information, see the safety section on the back inside cover.
Prepare an outline of the sequence of events that will take place during the manufacturing cycle. This will assist you in determining the types of special sequencing valves that you might need, as well as any external control (such as a tie-in with machine controls) that your application may require.
Calculate the cutting forces generated in the machining process and note the direction that these forces tend to act on the workpiece. If you are planning a retrofit of a manual clamping system, you may use the torques presently being used. However, it is recommended that cutter forces be calculated as a precaution in such a case to ensure that workholding devices are sized to provide an adequate margin of safety. The operation manuals of many machine tools contain tables that list machining forces or simple formulas for calculating these forces. But if you can't find the information, give us a call. We'll be glad to get you started.
Plan your fixture(s) with positive fixed stops to resist the majority of the cutting forces and to ensure correct location of the workpiece, using the primary part locating features.
Step 5: (Optional)
Normally you need to calculate the forces required to overcome workpiece weight and friction and to move the part into position against fixture stops. However, with our systems, this is an optional step, thanks to the two-stage design of the VektorFlo hydraulic power sources. The low-pressure high-flow first stage will move clamping devices into position around the workpiece and generate sufficient force to settle the workpiece against fixture stops before high-pressure clamping forces are generated. Additionally, in many applications, the nature of the fixture itself will ensure that the part is located closely enough to eliminate the need for positioning devices as a separate fixture element.
After you have determined the machine cutting forces, it's easy to calculate the clamping forces required to hold the workpiece on the fixture or machine table. Again, a simple formula is all you need to arrive at an answer for the materials you'll be working with. Give us a call if you need help.
Determine where clamps should contact the part to hold or support it securely and to avoid interference with machine operations. If clamps cannot be located so as to avoid interference with manufacturing operations, it will be necessary to use an external control device to move the clamps out of the way as the need arises during the manufacturing sequence. This will require that electrically actuated valving be used to control the offending devices separately.
Determine the type and number of workholding devices you need - based on the total clamping force required and clamping positions you've selected; on the size, strength and shape of the part; and on the machine operation.
To help determine the capacity of the power source you'll need, add up the total oil displacement requirements for the devices you have selected. Then choose a power source with equal or greater capacity and determine if it will operate the system within your clamping time constraints by working out the following formulas:
(Device cap.) ÷(L. P. flow) = Position time
Where . . .
L.P. flow is low pressure pump oil volume expressed in cu. in. per minute. Device cap. is total device oil capacity expressed in cu. in. Position time is time to position expressed in decimal parts of a minute.
To the result obtained above, add the result of the following calculation to obtain total estimated clamping time.
[(Sys. cap.) ÷ (H.P. flow)] x .01 (Sys. op. press ÷ 1000) = Pressurize time
Where . . .
H.P. flow is high-pressure pump oil volume expressed in cu. in. per minute. Sys. cap. is total system oil capacity, the workholding device capacity plus the internal volume of any associated tubing, hoses, manifolds, etc. (For small systems, the plumbing volume may be so small as to be negligible. However, for systems with long runs of tubing or hose, their volume may be of such magnitude as to materially affect the time it takes for operating pressure to be reached.) The expression .01 x (sys. op. pres. ÷ 1000) takes into account the slight compressibility of oil and system elasticity which influence the length of time required to pressurize a system. Pressurize time is the total time to reach pressure expressed in decimal parts of a minute.
If total estimated clamping time is not within the cycle time requirements you've targeted but is within device limitations, a larger power source is required - one with greater capacity. Select such a source and repeat the above calculations to ensure that it will provide the clamping cycle times required.
If the total estimated clamping time in the initial calculation is significantly less than the time allowed, your initial power source selection may have been too large. In such a case, select a smaller power source and repeat the above calculations to ensure that it still provides the clamping cycle times desired.
Additional factors you should consider when selecting a power source include shop floorplan and/or machine layout and your own preference for the type of power source (shop air vs. electric).
If desired, large electrical power sources may be used to supply several workholding systems, each operating independently at several machines. In this case, the timing and sequence of operations for each individual system must be calculated as shown above in order to arrive at a size for the power source.
Select valves and other control components to accomplish the sequence of operations you outlined in Step 2. See the valve section of this catalog for guidance.
Select appropriate safety control mechanisms. All VektorFlo electrical power modules have a hydraulic pressure switch as standard equipment to ensure that consistent forces are maintained at all times. However, when a power source is used to power several separate individual systems, each system should also have its own pressure monitor.
Finally, select the plumbing components required to connect the power source to the valves and devices. Simply review your system specifications and layout to determine what you need in terms of ratings, sizes, and lengths.
Call us for help. Our application engineers do not design fixtures. Their job is to help you use hydraulic clamps successfully, Whether you are retrofitting existing fixtures, need an idea (concept) for clamping a new part or want a quick review of your design we stand ready to help VektorFlo customers.
Call 800-992-0236 toll free for everything you need in workholding . . . Discover how easy, economical, and efficient power workholding can be - with one toll free call. We'll be glad to answer your questions, provide concepts or advice, and give you a quote.