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How to use hot-pressed cutoff wheels.

How to Use Hot-Pressed Cutoff Wheels

Hot-pressed cutoff wheels are used to remove gates and risers from castings because they are economical. They offer fast, high production, resulting in time savings. Also, there is no work-hardening of the part.

Wheels with depressed centers permit very close cuts, leaving a 3/8 in. pad for easy removal by portable grinder. Advanced design cutoff machines reduce operator fatigue. Higher horsepower gives freer, easier, faster cutting. Hydraulic assist machines dramatically improve cutting rate.

Hot-pressed cutoff wheels are a mixture of resin bond and abrasive grain. Unlike cold-pressed wheels, however, they are pressed between heated platens, which cause the phenolic resin to flow more freely around each grain particle, creating a much tougher and safer wheel.

For heavy service on large gates and risers, high pressures are applied; hot-pressed wheels are strongly recommended over cold-pressed wheels, provided the horsepower is adequate to utilize one.

Hot-pressed wheels are used on two types of cutoff machines. With the chopper-type cutoff machine, the operator pulls the head down and exerts the required pressure on the handle for maximum cutting efficiency. Machines with hydraulic assist offer greater productivity with increased wheel life.

The swing frame-type cutoff machine is balanced on trunnions and mounted on roller bearings. It is brought to the work in the case of large, extra heavy castings.

To get the most out of the cutoff operation, first select adequate horsepower. Horsepower requirements for various size wheels are shown in Table 1.

Next, select the largest wheel diameter practical to create the largest arc of contract permissible. Keep the greatest number of abrasive grains in contact with the wheel. Maintain full rpm speeds and make daily checks for reduced spindle speeds to belt slippage, etc.

Select the right type of wheel for the application. Use the hardest grade possible, in concern with proper horsepower, wheel diamter, rpm and condition of equipment.

Problems and Remedies

Wheel Striking Guard--This may be due to swarf buildup. The back flange can bend on foreign substances built up behind it. The operator may be applying excessive or uneven pressure, deflecting the wheel and generating micro multi-cracking, causing permanent flapping.

Wheel Fraying and Burning O.D.--Lost rpm can cause dragging and tearing action (belt slippage). Check V-belt pulleys for wear (ten years use or longer). V-belt should be pulling on each side at the top of the V section. Pulleys cause grinding wheels to burn through lack of power. Check to see that there is a belt on each pulley. Two out of three gives only two-thirds of the power required.

Also, do a voltage check. Do not assume there is no such thing as reduced voltage--look for high amperage and fuse blowing. Check for electric feed to the machine that is too long.

The wheel grading may too hard. Horsepower may be insufficient, or the gates and risers may be too large for the equipment and wheel diameter.

Wheel Wobble--Check for bad spindle bearing or excessive play. Also, always keep wheels in original cartons and on original skid. Cutoff wheel skids should be reinforced to stop wheels from bending under their own weight. Keep wheels in a dry area with even temperatures. Prolonged dampness will both weaken the wheel and cause warpage and breakage.

Check for bent flanges and for chinks in flanges. Excessive nut tightening springs the flanges. Flanges must be relieved and be at least 25% of wheel diameter. They also must be identical in diameter and radial bearing surface to avoid bending pressures and stresses in the wheel structure. In many cutting-off operations, better results may be obtained by using flanges larger than the minimum requirement. See ANSI B7.1-1988.

The operator may be bumping the wheel into the work to start the cut, rather than first scoring the area to be cut to get a run pattern. Scoring enables the operator to feed the wheel into the work at the proper angle. This is especially important when using a swing-frame machine. Otherwise, deflection will occur with resulting warpage or breakage.

Wheel Breakage--In addition to the above, breakage may be caused by wheel overspeeding--check spindle speed. Wheel pinching and binding in the cut may occur, or the workpiece may be improperly supported, braced or fixtured. Lack of rigidity, either in the wheel, the machine or the work, can lead to breakage. Excessive heat in the workpiece because of grinding heat causes the cut to close in, with resulting wheel pinching and/or breaking.

If these recommendations are followed and each operator makes a pledge always to follow the ANSI B7.1-1988 Safety Code, operations should be easier and safer.

Table : Horsepower Requirements by Wheel Diameter Size
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:metal castings industry
Author:Wagner, John G.
Publication:Modern Casting
Article Type:column
Date:Jun 1, 1989
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