The Economics Of Converting Manual Mills To CNC

It's no secret that CNC retrofits of older 1 to 5 hp manual machine tools increase productivity and profits for machine shops. By automating manual operations, a CNC retrofit typically turns out parts 75 to 300 percent faster than a manual machine. It also assures repeatable accuracy; makes complex parts more easily; and reduces scrap, rework and manufacturing costs.

In function and price, a CNC retrofit for a manual mill fills the gap between a $2,000 digital readout (DRO) retrofit package and a $50,000 to $60,000 two-axis CNC milling center, but increasing capabilities of newer CNC retrofits are narrowing the gap between them and new machines.

For instance, a CNC retrofit control package recently introduced by Mitutoyo/MTI Corporation (Aurora, Illinois) approaches the functionality of a low horsepower CNC milling machine at about one-fifth the cost. Called MILLSTAR, this CNC retrofit package includes a control with Windows 95 compatibility, choice of shop or G-Code program language, advanced geometry calculations, and glass linear scales. According to MTI, it enables a manual mill to turn out low to medium volume work at the same rates as a full CNC milling machine of equal spindle horsepower.

Connectivity to Windows 95 software makes programming user-friendly and intuitive for machine operators, freeing operators up for more complex jobs. This helps shops cope with the skills shortage, especially programmers, and the need for operators to program on their own. Shop and G-Code language, likewise, facilitate programming by operators. Part prints in G-Code can be downloaded directly to create the program, saving time and eliminating another error source.

The company reports that high-precision glass linear scales provide better positioning accuracy than rotary encoders: This is why they are offered as extras on many CNC retrofit packages and built-in controls. The difference becomes especially important on older mills where there may be more play in the table ways. Positioning repeatability of a retrofit CNC control with glass linear scales matches that of the controls on a full CNC milling machine.

This retrofit package includes servo-motors with adequate power to move the table at optimum feed rates for heavy cuts, even on large 5-hp manual mills. Because the CNC part programs will be calling for higher material removal rates than are customary for manual work, under-powered servomotors are likely to hold the operation back.

A DRO-equipped machine provides no such automatic table movement, but rather clear, more readable positional readings. The operator must dial in table feeds and stops, and start each cut as if on a manual mill; there is still a possibility for error.

Advanced geometry calculation saves both programming and running time for complex shapes including angles, radii, and irregular pockets. During programming, the operator inputs the known linear and curved measurements and designates the type of cut. No calculations are necessary on the part of the operator. The machine's geometry calculator computes the unknown measurements and the required tool path. During operation, moreover, complex cuts are made at the same removal rates as simpler cuts. The control "thinks" fast enough to keep the cutter running at full speed. If a shop's work involves a lot of circles, hole patterns, arcs and pockets, the additional programming efficiency of canned cycles may justify a retrofit.

Large memory for routines means that routines can be retrieved and used on another job. The larger the memory, the more routines available for the next job. The MILLSTAR's memory can store hundreds of routines, depending on their length. Similar parts do not have to be reprogrammed; only new dimensions need to be inputted. The system recognizes the part and updates the routine with the new data.

Manual override enables the operator to run a one/off or prototype job as usual. Where there's an applicable routine or subroutine, the operator can pick it up from memory. Where something unique to the part needs to be done, the operator switches to the manual mode, makes the cut, and returns to program mode.

The benefit of thee-axis readout in a two-axis CNC retrofit is precise control of height/depth at lower cost than full three-axis control. The company is launching a three-axis CNC control at IMTS 98. However, current two-axis units will be upgradable to three-axis control.

Owners of old manual milling machines confronted with today's capacity crunch have at least three choices for increasing throughput without buying a new CNC machining center. They can retrofit an existing mill with a CNC control package or a DRO, or they can buy a new manual mill with a CNC retrofit package in place from the outset. MTI reports that a substantial number of MILLSTARs are going onto brand new manual mills.

CNC retrofit packages vary widely in capability, standard features, extras, and cost. Each shop's choice should be dictated by specific requirements including workload and its complexity, volume and lot size, delivery requirements, and availability of capital and skilled labor. It is a good idea to spend no more than you can recover in a year out of increased throughput and labor savings in programming and chipmaking.