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Weekly UpdatesFraming squares, chalk, tape measures, levels, piano wire, and a stout hammer are the basic tools for an experienced welder. The good welders have amassed a collection of special tools gathered over the years that makes their jobs easier. It is the evolution of these special tools into a logical system of table-based components, all designed to interface with each other that makes a modular fixturing system practical.
The concept of modular fixturing for welding is not new, but there are still objections to it in some shops: "It's neat stuff but my boss will never go for it; it's too expensive."
When compared to a single, dedicated fixture, a modular fixturing package at first glance looks expensive. However, from a modular system any number of fixtures can be built. When comparing the costs of dedicated against modular fixtures, typical break-even points are reached at six fixtures. There have been a number of cases where the system paid for itself on the first job.
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Get Shorty
Another objection has been "Modular fixturing is for production and all my work is short run."
Where volume is high and the mix is low, a dedicated fixture may be the best solution. With short runs and constant change of part mix, dedicated fixtures can become expensive. Lead times suffer, prime manufacturing space is consumed storing fixtures, and staying current with engineering changes is next to impossible.
A modular fixturing package can build fixtures as needed. After the job is finished, the fixture can be re-assembled for the next job in minutes.
"Modular fixturing is for short runs and all my work is high volume production," is another reason cited for resisting modular fixturing.
It's been proven that using modular elements in a dedicated fixture can reduce lead times, improve quality, reduce fixture costs, and minimize the impact changes have on the fixture.
Modular elements have far more precise features machined into them than may be required in a dedicated fixture. However, these precise features provide an attachment point for smaller, simpler adapters. When an engineering change comes through, instead of sending the entire fixture back to the tool room to be re-machined, the small adapter can be replaced with another. In the future, when the product reaches the end of its life, the fixture can be disassembled and the major elements reused in another.
"We make our own fixtures for every part," is often heard as an excuse to avoid investment in a modular fixture.
This complaint only makes sense where production runs and the product life is long, where engineering changes are rare, and where specials don't have to be produced quickly to compete. But, a case can be made for using base elements of a modular fixture to provide quick, easy, and accurate points to attach special part-specific fixturing elements. It eliminates the need, on large fixtures, of having to make a special base for every fixture. A shop can use a modular fixture setup to save time, money, and storage space.
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Hard Times
"We hard-tool our prototypes," is sometimes stated for not moving to modular fixturing.
With engineers now working with CAD systems, changes can be fast and furious. With these changes comes the need to modify the hard-tool to accommodate the latest part design. The need for prototypes, and related changes, is clear. Modular fixturing implements the changes with ease. The same goes for prototypes, where the welding process is not always predictable. Locators and clamps are often shifted to compensate for weld draw. Changes are easier with a modular fixture. When a good part is produced, the modular fixture can be measured. This information can be given to the tool designer with details about the clamping and locating points needed for welding. The transition from prototype to production can be almost seamless.
Also heard in some shops is, "Welding robots can only be used for high volume jobs."
On the face of it, conventional wisdom dictates that a robot can't be used for short-run welding jobs. But, the robot doesn't know that. Put a part where it should be in front of a robot and it will produce a perfect weld. Robots weld parts about four times faster than a human, with repeatable accuracies. They don't take coffee breaks and never call in sick when the weather is perfect for a round of golf.
Because of the high costs of tooling, robots often sit idle when there are no high production-run jobs. Programming costs can drive a robot into an early retirement.
Modular fixturing can place the parts exactly where the robot expects them, every time. Quick and easy fixture rebuilds within [+ or -] 0.005" are possible.
With all of the component geometries available in 3D solid models, computer-based programming can be done off-line. It also is possible for a setup operator to physically build the fixture and then teach the robot the part. When the operator is satisfied with the program, he hands-off the robot to the machine operator who completes the rest of the short run. In a two-station robot, the workers trade places all day, setting up and running parts with lot sizes in lots as low as four pieces.
ROI
For a shop, the objection is often, "I'm a job shop and I have to put the cost of fixturing in my price."
True. Historically, fixturing was viewed as part-specific and therefore charged to the job. However, modular fixturing should be considered an investment in capital equipment. With the purchase of a press brake, shear, or an iron worker, the cost of the tool isn't included in a job quote.
Renting a modular fixturing system is a way to avoid sticker shock. Try before buying. The rental costs can be charged to the customer. Then, at the end of the job, the rental can be converted to a purchase with credit being given for a portion of rental payments. With rent-to-own, rent-and-return, or initial purchase, once a system is installed, it is often a manufacturing life-changing event.
Here is what to demand in a quality modular fixturing system:
* It starts with a good base. The table should have a five-sided weldment made from 1"-thick high-tensile strength, low-carbon steel, and ribbed and gusseted for strength.
* The surfaces should be milled to at least 0.004" flatness, with bores on 4" centers across the top and around all four sides located to within [+ or -] 0.001".
* The tables should be hardened to 55 [R.sub.C] using the nitriding process which prevents spatter from sticking.
* Table sizes can range from 40"x40" to 6-1/2'x13'.
* Where larger surfaces are needed, standard cast steel angles can be mounted to the sides and serve as outriggers. Tables can also be joined together.
"How do you get started with modular fixturing?" is often the next question once the advantages of a modular fixturing system become apparent.
The best start is to choose the area in the welding department that has the most problems, such as getting the jobs done on time, finished goods quality, part rework, excessive costs, lack of fixture storage, keeping up with engineering changes, loss of welding skills as older welders retire, and maintaining a competitive edge in the market. Then, contact the experts in the field and have a CAD design created for one or more of the parts to be welded. From this, a kit can be defined to get started.
Compared to older methods, modular fixturing can double production and quality. Where the need is great but the budget isn't, systems can be rented. On-site training will ease the transition. Once a system is installed, there won't be any regrets. Bluco
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