Small to medium-sized jobs demand the accuracy, versatility, and repeatability of flexible part fixturing systems.
Article reprinted with permission from April 1998 American Machinist.
Content © copyright 1998 American Machinist.
Patricia L. Smith
As the trend towards smaller lot sizes and higher accuracy parts continues, many manufacturers are finding that dedicated workholding systems just don’t provide the versatility they need. Modular systems, on the other hand, are best suited to small quantities or infrequent production runs. These and other flexible fixturing systems not only clamp accurately and consistently, but allow multiple machining operations to be performed in one setup, often slashing both production time and cost.
According to Carr Lane Roemheld Mfg. Co., St. Louis, flexible fixtures usually combine basic tooling plates and blocks with precision-machined vises. Quickly assembled into different module configurations, they readily adapt to medium quantities of related families of parts. Combining fixtures on fixturing blocks and cubes in a machining center, these families of parts can also be machined at the same time.
The company’s Hilma Division, for example, makes flexible fixturing systems specifically for small and medium-sized lots. “Our systems answer manufacturing demands for efficient quick-change tooling, while providing a high degree of accuracy,” says Max Saunders, Hilma general manager.
The Hilma Division generally incorporates hydraulic power workholding elements to shorten setup and handling times.
Comparing fixture systems
Carr Lane Roemheld believes that companies look at a number of factors before deciding on a workholding system. Basically, fixturing costs include the investment needed to create the workholder, maintenance between runs, storage, and the cost of repeat usage. For a true comparison, companies should consider lot sizes, the number of times a job repeats, the life expectancy of the fixture, and so forth.
Modular systems have reusable parts, so component cost shold be amortized over the system’s typical lifetime by dividing total component cost by 100. Once initial costs are established, further cost comparison suggests the most efficient type of fixturing for a particular operation based on the overall cost per part. Each type offers advantages in some areas, reports Carr Lane Roemheld.
The company suggests using the following formula to figure the total cost to manufacture a given part:
Run cost is the cost of labor to produce a part. Generally, run costs for dedicated and modular fixturing are about the same, while power workholding lowers labor costs by improving cycle time and reducing scrap.
Setup cost is the labor cost to retrieve a fixture, set it up on a machine, and return it to storage after use. Since dedicated fixturing and power wokholding are faster to set up, they create lower costs than modular fixturing, which requires assembly.
Lot size is the average quantity manufactured each time the fixture is set up.
Initial tooling cost is the total of labor and materials to design and build a fixture. Here, modular fixturing is cheapest because parts are reused, while the more complex power workholding systems initially cost more.
Total quantity over tooling lifetime is the lesser of the total anticipated production quantity or the quantity that can be produced before the tooling wears out. Generally speaking, as quantities increase, the efficiency of power workholding yields the lowest cost per part.
System proves up to the test
One need search no further than Carr Lane Roemheld’s parent company, Carr Lane Mfg. Co., to see the benefits of flexible fixturing. The company uses Hilma systems to manufacture high-precision angle brackets faster than previously possible. Using Hilma vises and its own tooling blocks on a Cincinnati Milacron Maxim horizontal machining center, Carr Lane mills angle brackets with accuracy of +0.0000/-0.0005. Four locating holes are held to ±0.0005 diameter and within ±0.0015 true position. These tolerances previously took seven machining steps and 8 min/part.
Machining time dropped to 3.8 min/piece because flexible fixturing eliminated five setups, along with five transport times and five queuing times.
The system’s quick-change tooling also boosted production by allowing varied configurations to run on existing equipment. An entire system can be quickly assembled and reassembled with reusable clamping units to accommodate changing shapes and sizes of workpieces. Operators swap clamping units between machining centers and quickly “retool” by simply changing vise jaws.
These systems have been very successful at solving the needs of today’s manufacturers,” Saunders says. “We’ve recently added several new products to the Hilma line to provide more options.”
One of these new products is a compact quick-clamping system that provides fast action when used individually or in more complex fixtures. They are available in multiple configurations and sizes suitable for short or long productions runs.
Hilma designed a new base and fixed jaw for its NC clamping system. The changes improve stability and reliability in a variety of tooling, molding, and fixturing applications. The system’s hydra-mechanical or fully powered units work vertically or horizontally, as does a new universal clamp system.
All NC clamping systems can be quickly positioned on machine beds using the external clamping edges and keyways in their bases. Their surfaces and guideways are precision ground, and hydraulics are integrated into the slide for optimum protection against chips and dirt.
New mechanical double-clamping systems with a third-hand function mount on towers singly or in series for clamping multiple workpieces. Ideal as reversible clamping devices, the all-steel designs work well in horizontal or vertical uses, and a fixed overall length maximizes working space.
With simple jaw changes, double clamping systems changeover rapidly and economically to accommodate prismatic or extruded workpieces, castings, and forgings. Clamping force can be preset with a torque wrench for reliable, repeatable clamping action.
Hilma-Roemheld combines four clamping units on a single base for its flexible fixturing cubes. Their one-piece steel body and integrated fixed jaws provide maximum stability and precision for increased production. Available in three base sizes, fixture cubes have clamping points positioned for machining of workpieces in one setup.