Streamlining chair frame production
May 9, 2010 | 7:00 pm CDT

By using a Balestrini Spider CNC machining center for chair components, Frank Chervan, Inc. eliminated 15 operations in the making of a single chair front post.

It takes the Spider, designed specifically for shaping, mortising, boring and other operations needed to make chair components, only seven minutes to do complete machining of a side of a contract chair. That includes one station of the Spider processing a back post, arm piece, side rail and front post, shaping on two sides and putting in a mortise or tenon. (All chair components are put together with mortise and tenon construction and no boring.)
To do these functions separately would require eight operations for each piece, with eight set-up variations and loading and unloading eight times.

"We're using the Spider for items that cause us problems, headaches, lost time and money running them the conventional way," says Richard Terrill, chairman and vice president of operations and development of Frank Chervan. "We're using it as a problem solver within the plant, which helps us and helps the customer."

Chervan makes hardwood chair frames and components from northern soft and hard maple, with 784 active job numbers for items that have been run in the past year. The Bedford, Va., company is working to increase the use of cellular manufacturing, improve lumber yields and reduce lead times. Chervan may have 300 chairs in production at any time, each containing as many as 12 components, with some components requiring up to 25 operations.

"Our customers are demanding that we become completely integrated with their supply chain, which means being really fast and being able to rapidly respond to what they need," says company president Greg Terrill. "That's what we're looking for from that machine. We want to be able to run extremely complicated frames and only run 10 of them if that's what the customer needs."

Several years ago, Chervan installed two Balestrini CNC shapers, a CPCNB6 with six shaper/sander heads and a CPCNB8 with eight heads. These machines, purchased from Solid Wood Systems in 1995 and 1996, can shape and sand contours on two adjacent sides of a part. Chervan also purchased two Balestrini Astra fingerjoint machines. (See story in the January 2000 issue of FDM, page 60.)

The Spider is a five-axis machining center with two independent working tables that may be linked together for longer parts, explains Eric Zinn, Chervan engineer. It has a four-spindle head that takes the place of a standard tool changer. The Spider's major axis is mounted vertically instead of horizontally, like most machines.

"The five axis movement allows a part to be machined on five sides without being repositioned," Zinn says. "This feature can greatly increase accuracy and time. The four-spindle head allows for much faster tool changes than a standard tool changer. The Spider averages three to four seconds per tool change, compared to 10 to 13 seconds on a standard tool changer."

Zinn says that the independent working tables allow an operator to load one station while the other is being machined, and the vertically mounted axis keeps the machine balanced at all times and allows for fast and accurate heavy machining.
"The most beneficial feature of the Spider is its accuracy and speed," Zinn says. "It allows us to manufacture complex parts in one operation without being concerned with repositioning or operator error."

The five-axis capability allows tools to become more versatile. "You can machine one cut with the end of a tool and on the next pass with the same tool use the side of the tool to make a profile cut, and then rotate or tip the tool again to get a chamfer cut. So even though the machine has only four spindles for tools, the possibilities of the types of cuts it can perform are only limited by the creativity of the programmer."

Richard Terrill also says that Chervan is figuring manufacturability into parts, taking high-volume frames and reengineering the design to reduce cost and deliver a frame to the customer that looks like the old frame, but costs less to make.
"Twenty years ago when you designed and calculated the cost of a product you concentrated only on labor, because lumber was relatively inexpensive," says Terrill.

Today both lumber and labor are expensive, so parts have to be designed for maximum yield as well. He says the improved machining capability helps achieve this improved manufacturability.

Chervan uses AlphaCAM software to create programs for the Spider and the Biesse Rover 23.

"AlphaCAM allows us to graphically create tool paths and manage a tool library," says Zinn. "Once all the tool paths are created the file is sent through a postprocessor that is specific to the machine that the part is to be machined on to produce the code for the machine. The Spider uses G-code.

"AlphaCAM allows us to create very long programs, sometimes 40 or more pages, very quickly without manually entering it into the machine line by line. This system has several automated features that save a great deal of time, which allows us to design and machine parts very quickly. It is also easy to make revisions and changes to programs, and you rarely have to start completely from scratch to change a program."

The Spider has also helped reduce lead times. On the chair frame already mentioned, the only premachined part is the back post. Previously, Chervan's scheduling system added work hours and days for each operation performed in a particular department.

"With the Spider, the back post has to come out a day before, because it's shaped," Terrill says. "The arm, front post and side rail can come out the following day, go to the Spider and be run - you've eliminated 10 days on every one of those parts."

Terrill says the Spider can also produce intricate work like a chair back splat (panel). Previously, these components were produced outside by another company, but that required six to eight weeks of lead time for Chervan to supply the lumber, prepare blanks for the outside vendor and then get the finished pieces back. Then Chervan had to mortise and tenon each end. On the Spider all of this activity takes only two days.

Chervan primarily buys carvings such as core legs, front rails and top rails from outside shops. These parts are made from material Chervan supplies, so essentially all they are buying is the labor.

Chervan also delivers more of a completed assembly (although it doesn't finish any components), and even installs webbing on some pieces.

"Customers want short lead times and they want to have the product come in so they can use it immediately," Terrill says. "Most of the product that's leaving here is assembled. We used to send more components."

Although the overall market is down this year, the company is keeping busy. "Our catalog is still doing well and we're adding new items and new lines," Terrill says. "And we're putting a lot of effort into going to trade shows."

Meanwhile, Greg Terrill and Eric Zinn went to Italy recently to see the new Balestrini Idea, a shuttle table machine that does the same processes as the Spider but in much less space, indicating that the company isn't resting on its manufacturing laurels. 

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About the author
Karl Forth

Karl D. Forth is online editor for CCI Media. He also writes news and feature stories in FDMC Magazine, in addition to newsletters and custom publishing projects. He is also involved in event organization, and compiles the annual FDM 300 list of industry leaders. He can be reached at [email protected].