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Presetting off-line offers CNC savings.

Tool measurement is a fact of life in every shop that uses CNC equipment. According to Dennis King, Parlec Inc, Fairport, NY, presetting is a good way to perform this function off-line. Significant savings in scrap and rework rates are possible, and CNC machines are put to work doing the thing they do best: cutting parts.

A CNC machine can't make a good part until tool length is known and that data is furnished to the machine control, Mr King explains. Tool length plays a critical role in calculating feed distance: feed depth equals work plane height plus machining depth minus tool gage length. For certain types of tools (for example, boring tools), the diameter also must be known or adjusted. The radius of milling cutters must be supplied to the control if the CNC's diametral cutter compensation feature is to be used.

Three factors affect CNC performance: the age and type of control, the type of machine, and the manner in which the part is programmed. Along with the type of tool assembly involved, these factors determine whether a tool must be preset or simply measured, says Mr King.

The word preset in this case means physically moving the tool tip to a prescribed location. Boring tools generally must be preset, as must tools for first-generation controls and tools for multi-spindle machines.

Second-generation and later controls only require that the tool be measured, Mr King explains. "The difference between the programmed (nominal) tool data and actual tool data is calculated and is inputted to the control memory. This difference is called the tool offset.

"Newer CNC machines have virtually unlimited offset capability, which allows the programmer to write his program with tools of zero length and diameter. When the actual tool sizes are supplied to the control, it will automatically correct the program. This also simplifies tool assembly since, in many cases, it is immaterial how long a tool is--as long as it's long enough."

There are two ways to get actual tool dimensions: use a measuring device or use the machine tool itself. Measuring devices include machinist's scales, optical comparators, coordinate measuring machines, and preset gages or machines. Machinist's scales are relatively inaccurate; comparators and CMMs are better suited to the purposes for which they were designed, says Mr King.

The machine tool is often used, although the technique is time-consuming and the machine is down during measurement. Another drawback to this approach, says Mr King, is that it is difficult, if not impossible, to preset a tool to a specific dimension. With a tool locked in the spindle, access to the backup screw is denied. On multi-spindle machines, a lot of time can be wasted trying to preset identical tool lengths, since the tool must be removed from the spindle for adjustment. And making trial cuts takes time and results in scrap.

Using the CNC machine for presetting also doesn't give operators a good opportunity to inspect tool condition. Chipped or broken inserts can be overlooked, resulting in poor finishes or incorrect dimensions. Tool runout is difficult to detect and even more difficult to correct.

Dedicated presetting or measuring machines simplify the process by overcoming the problems associated with measuring tool assemblies on the machine. Time savings average three minutes per tool on most tool assemblies and eight minutes on each boring tool. Properly designed, presetting machines present the tool to the operator in a position that facilitates inspection for tool runout, chipped inserts or broken teeth, and built-up edge.

Presetting machines run the gamut from simple manual devices with digital scales to sophisticated CNC machines. Priced at $5000, Parlec's Parsetter 160 presetter is a manual machine with digital scales accurate to 0.0005", a measuring range of 10" diameter x 16" long, and a #50 taper rotating tool spindle that provides an accurate measurement datum.

At the other end of the spectrum is the Parsetter 320, with either 24" x 24" or 30" x 30" work envelope and optional CNC and third axis spindle robot.

In between are the Parsetter 240 and 260 machines. The 240 has a measuring range of 12" diameter x 16" long, and with Parset Master control offers 0.0001" resolution. The Parsetter 260 has a measuring range of 20" diameter x 20" long, and uses an optional electronic projector to provide measurements within a very small (3|micrometer~) tolerance band.
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computer numerical control
Publication:Tooling & Production
Date:Jul 1, 1993
Previous Article:Cutting corners smarter.
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