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Hand stones, better than files.

The use of small hand stones to deburr, chamfer, finish, and precision-fit parts has a long history. Given the many dies, templates, and machined or grouped parts in production today, the need for specialized stones or "files" is greater than ever.

There are more than 100 different combinations of shapes, grits, and abrasives in our own line alone. Thus, a background knowledge of how to finish and fit with stones is increasingly important. The ability to select the correct combination of shape, abrasive, and grit (size) for the job at hand is quite challenging.

For the average toolroom, small hand stones provide versatility and value as finishing tools. They finish to close tolerances, and their hardness allows cutting of hardened steel-unlike traditional metal files. Stones offer a variety of grit/abrasive/shape possibilities off-the-shelf, and they are long-lived if properly cared for.

Common applications include keyway cleaning and fitting, centerpunch touch-up, template dimensioning, magnetic-chuck deburring, precision finishing of dies and molds (reaming irregular holes to specifications), and precision fitting of machine parts.

Hand stones use three basic types of abrasives: silicon carbide, aluminum oxide, and Arkansas. Each has its advantages in specific applications.

Gray stones made from silicon carbide electric-furnace abrasive are harder than any natural abrasive except diamond. They provide fast material removal and serve where coarse finishes or moderate tolerances are acceptable. Such stones are usually available in coarse, medium, and fine grits. In small files, usually only medium and fine grits are available. Stones made of aluminum oxide (A1[sub.2]O[sub.3]) electric-furnace abrasive are brown in color. They serve where there is a need for lesser stock removal but greater dimensioning ability, closer tolerances, and smoother finish. A1[sub.2]O[sub.3] stones generally are available in coarse, medium, and fine grits. Arkansas stones are made from a natural mineral and will vary in color from black to white. (Norton Co grades Arkansas stones by density to ensure consistency from order to order.) Arkansas comes in two basic densities: hard and soft. Soft Arkansas (super-fine grit) is best used where tolerances are critical or a very fine finish is desired. Hard Arkansas (ultra-fine grit) is denser than soft Arkansas, and will provide the very finest finish and closest fit of any stone.

Correct use

The first rule of use is lubrication. Not only will stones deburr and cut best when lubricated, but oiling the stones will improve workpiece finish and stone life.

If oil is not used, the bits of stone and metal generated during the stoning process will clog or "load" the pores of the stone. Over time, this loading will reduce the cutting action of the stone. Soaking stones in oil before use, or applying oil to the surface of the file just before use, will suspend the stone and metal particles in an oil slurry. The slurry protects the stone from loading while actually aiding the cutting action.

Most files are 3" to 6" long and from 1/4" to 1" thick. The basic available shapes (named by cross-section) are square, triangle, round, flat, beveled end, diamond, oval, knife blade (very long, narrow triangle), and half-round. Many of these shapes also are available in forms that taper in width from end to end for use in a variety of tight applications. In addition to these standards, many stone manufacturers will provide custom shapes.

Other types of stones include bench stones (commonly known as sharpening stones), slips, and machine knife stones.

Toolroom applications

Stone selection should be based on workpiece geometry-flat or square files for straight edges, large bench stones for large flat surfaces, and round or oval cross-sections for curves. Select the abrasive based on the amount of material to be removed. Choose grit to provide the desired finish and dimensional accuracy. In many applications, a series of stones may be required-a medium silicon carbide to rough out, followed by a fine A1[sub.2]O[sub.3], to achieve the desired dimensional tolerances, and finishing with a hard Arkansas to final fit the part.

Applications-true grit

In one shop, operators machined a D2 part, which then required deburring and chamfering of its straight edge. A square file suited this task, because of its flat edges and strength. The largest generally available (4" x 1/2") is suggested, because there are no small areas to stone, and the setup provides the best fit in the user's hand. Engineers selected a silicon-carbide stone and a medium grit to get the job done as quickly as possible.

For a steel template requiring precision dimensions, operators used just three files. The task included filing a 1/4" flat, followed by 1/2" of an irregular curve, followed by another 1/2" of flat. Engineers suggested bringing the template to tolerance with an A1[sub.2]O[sub.3] stone. Next, they selected a fine grit, where small amounts of material had to be removed (a coarser grit for more). The best shape for the task was a half-round-using the flat side for the flat sections, and the round side for the irregular curve.

They selected a file with a radius equal to or smaller than the smallest included radius of the workpiece, so that the stone would fit easily into all the curves. After bringing the workpiece to tolerance with a well-lubricated stone, they used a hard Arkansas flat file for the flats, and a hard Arkansas round file for the curves. The Arkansas stone removes very little material, but brings the surface to a precise, smooth finish.

Most die-work and other finish-critical applications will benefit by the similar use of an Arkansas stone in the final finishing step.

For keyways, it's often necessary to clean the workpiece to achieve a proper fit. Use a square-cross-section file starting with a fine A'203 abrasive if the burr is small. If the damage or burr is large, or if the material is difficult to abrade, select a medium silicon-carbide stone, and finish with a fine A1[sub.2][sub. 3]. Select a file that is only slightly smaller than the keyway itself to minimize any rocking motion and possibility of misshaping the keyway.

For a centerpunch, it may be necessary to do point touch-up and maintenance. As with most tools, routine maintenance is preferred over a major repair. Regular stoning of center punches-as soon as they show signs of wear-will provide precision points for every use. Select a triangular or square file, with a medium to fine grit A1203 abrasive. Holding the punch and stone in hand, with the original bevel flat on the stone, slowly draw the punch across the stone while slowly rolling the punch between the fingers. Routine stoning extends tool life and is beneficial in the case of pointed or edged tools.

Real-world mag chucks

Repeatable close-tolerance work with magnetic chucks on mill tables demands absolutely true work surfaces. Periodic use of stones to remove the burrs and raised edges of nicks and over-drills ensures that workpieces lie flat on the table.

Select a large (4" round and 1" high) silicon-carbide combination flat stone or benchstone that consists of two grits, one on each face. (The shape is easier to hold onto while spanning the mill table.) Apply enough pressure to keep the stone flat upon the workpiece and to increase the abrasive cutting action. Using too little force, which means less cutting action, is a common oversight.

Start first with the coarser-grade grit, and finish with the finer. Lubricate the stone and the table properly, because the stock removal from a large work area may easily load the stone. Clean the residual slurry off the surface after each stoning. If left on the table or chuck, the mixture of oil, metal, and abrasive will prevent the workpiece from lying flat.

Stamping-dies maintenance

Routine stoning is a quick and productive method to extending the working life of stamping dies while insuring the tolerances of the parts the dies are used to produce. Because of the numerous shapes and contours of such dies, a number of different-shaped stones may be required for the task.

For more information on hand stones from Norton Co, Worcester, MA, circle 154. by Paul J Champagne Product Engineer Norton Co Worcester, MA
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:finishing tools
Publication:Tooling & Production
Date:Mar 1, 1992
Words:1375
Previous Article:Putting a new face to machine value.
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