Short set-up times are decisive for the economic CNC production of simple parts
Almost a fifth of the approximately 100,000 living individual parts that the bookbinding machine manufacturer Kolbus produces in-house were not produced on CNC machines until the beginning of this year. However, an analysis of the range of parts showed that for 8,000 of these components, an economical CNC production was possible with the right clamping equipment. In view of the very small batch sizes, short set-up times were decisive. They can be achieved with just two different HILMA machine vices, which are now used to clamp most of the workpieces.
At the end of 2014, Kolbus had around 17,500 parts for which production on a CNC machine was not economical due to simple machining. Investment planner Sven Kolwey questioned this and came to a different conclusion in early 2015. He found that CNC production for more than a third of these parts would be profitable under certain assumptions.
In his review, he evaluated the 17,500 components according to materials, geometry, dimensions, type of machining and then determined the common features: About 8,000 part numbers require only minimal machining. They only require less complex operations such as simple milling, countersinking, drilling or thread cutting. They all have a prismatic shape and measure a maximum of 500 mm x 400 mm.
Another important common feature was the short running times of the workpieces, which are mainly produced in small quantities of between two and eighty. “With an average batch size of 16 pieces, we have to retrofit several times per shift. Therefore, short set-up times are a significant issue for us. When the machines are down, it costs a lot of money every time”, says Frank Bekemeier, head of mechanical production at Kolbus.
With these specifications, Kolwey and Bekemeier approached the clamping technology specialist ROEMHELD. Since 1999, Kolbus has been using HILMA machine vices on CNC machining centres. In the meantime, a total of about one hundred vices are in use on various machines, many of them for clamping two or more workpieces.
Kolbus manufactures more than thirty different types of bookbinding machines at its headquarters in Rahden, in the north of North Rhine-Westphalia. With an export share of around 85%, the company is one of the world market leaders in its sector. With packaging machines for the production of high-quality luxury cardboard boxes, a second business field has recently been established. More than 200 employees work in the parts production, which extends over several halls on 12,000 square metres.
Kolwey expressly praises the HILMA machine vices used: “We have consistently good experience with the workholding systems, they are technically very mature and ROEMHELD has a lot of experience in the market.”
Karl-Heinz Stötzel, product manager for workholding systems at the ROEMHELD location Hilchenbach, and Frank Millkuhn, technical sales representative of the local sales partner Werner Werner Bock KG, developed, together with Kolwey and his colleagues, a concept which allows to clamp the majority of the described parts with only two different workholding systems.
The hydraulically-operated double workholding systems of the types DCS 80 H and DS 125 H proved to be the optimum clamping solution. The compact workholding system DCS 80 H with a jaw width of 80 mm and a maximum clamping force of 20 kN offers jaw openings up to 142 mm for simultaneous machining of two workpieces. Larger components are accommodated by the double workholding system DS 125 H: Its jaw width is 125 mm, and the jaw openings reach two times 204 mm. The clamping force is up to 40 kN.
All HILMA machine vices are of compact design and mounted on carrier plates. The four smaller models are mounted together on one plate, and the larger versions are each mounted on their own. The carrier plates are equipped with retractable nipples for a zero point clamping system. They can thus be quickly positioned and clamped on the fast-clamping plate, which is firmly attached to the machine table. This allows the vices to be changed quickly and combined as required. The well-designed arrangement of the vices allows longer parts to be clamped using two vices.
The hydraulic oil supply to the clamping slides is provided via deep-drilled channels in the fast-clamping plate and via automatic media couplings between the fast-clamping plate and the carrier plates. Deep-drilled channels in the carrier plates ensure a direct connection to the individual workholding systems and thus to the clamping slides.
Changing the clamping jaws is just as quick and can be done in a few simple steps. The QIS jaw system (Quick Insert System) developed at ROEMHELD offers the best conditions for this. Kolbus is using the jaws in this project for the first time and has already gained good experience in a short time.
The machine vices were equipped with QIS base jaws with magnetic inserts, onto which the required interchangeable jaws are simply attached in a few seconds. This eliminates the need for time-consuming re-screwing when changing jaws. Numerous interchangeable jaws are offered for different machining operations. Customer-specific versions are also available, for example, interchangeable step jaws or interchangeable jaws with prism or with lateral guidance.
Kolbus well received the suggestions from Stötzel and Millkuhn, says production manager Bekemeier: “The jointly developed concept of double clamping is good. Despite their compact dimensions, the vices offer large jaw openings for each clamping point and can be converted extremely quickly thanks to the QIS jaws.”
In the course of the project cooperation, Kolbus presented some further special requirements which ROEMHELD implemented as requested. The customer wanted lateral guides on the base and interchangeable jaws as well as special stops for the fast positioning of the workpieces to be machined as additional facilitation for the jaw change. Also, Kolbus intended to develop and manufacture the fast-clamping plates and special grip jaws – and the clamping technology specialist supported him in this task as well.
The result of the cooperation is impressive: “The workholding systems are perfectly matched to the intended machine. By combining the vices on the machine table, we make optimum use of the travel range of the machining centre,” says product manager Stötzel.
About four months after the start, investment planner Kolwey draws a consistently positive interim balance: “The machine is used to full capacity in two shifts and we are finding significant savings in set-up and production times compared to conventional production. These time advantages are so significant that even with an hourly rate almost twice as high, the entire plant, including the machine, will pay for itself in about 3.5 years. The higher precision of the production also brings an additional quality gain for the components.”
It will still take a while until the programs for all 8,000 part numbers are written and all processes are optimised. It is foreseeable that set-up and production times can be reduced even further thanks to the ongoing adjustments. In the meantime, we will ask ROEMHELD for additional workholding systems to increase the variety of clamping possibilities,” explains Kolwey.
Just a few weeks after the company visit to Kolbus, the investment planner's announcement was fulfilled, reports product manager Stötzel: In the meantime, productivity in mechanical parts manufacturing has further increased, so that further workholding systems have been ordered from us and already delivered to Kolbus.
The Kolbus GmbH & Co. KG offers, according to its own information, the most comprehensive product range of bookbinding machines worldwide. It consists of more than 30 machine types, covering all applications for the production of bound products from the folded sheet. Kolbus sees itself as a classic mechanical engineering company with a comprehensive production. The company originated from a village forge founded by Christian Henrich Kolbus in 1775, which was expanded in 1877 to include a foundry that still exists today. In 1900, the construction of bookbinding machines was added. Today, around 1,000 employees work at the plant at the Rahden headquarters in the north of North Rhine-Westphalia.