Jeff Simons, technical service engineer
3M Abrasive Systems
edited by Richard Mandel, senior editor

A coarse surface conditioning disc can be used to refine grindlines, remove welds or prepare surfaces for paint.

Leveling the weld with a flap disc or fibre disc in a right angle grinder is typically the first step in carbon steel finishing.

For of leveling and small weld removal in confined areas, try a small (3-in. diameter), grade 50 quick-attachment abrasive disc.

Disc Size Carbon Steel 4 1/2-in. 8,500 rpm 7,000 rpm 5-in. 8,500 rpm 6,000 rpm 7-in. 6,000 rpm 4,500 rpm 9 1/8-in. 4,500 rpm 3,500 rpm

Table 1: Optimum tool speeds to maximize cut rates and life of fibre and surface conditioning discs.

Poor grinding can compromise even the best welds. Fortunately, improved abrasive minerals and tools have appeared on the market in the last few years that make grinding and finishing welds easier and faster. For example, discs containing ceramic abrasive grain are becoming more widely used in industry as they provide longer life and more consistent finishes when compared to traditional aluminum oxide discs. New disc constructions offer increased life and improved productivity, with durable backings that can enhance the discs cutting ability while decreasing the likelihood of premature edge wear. Such a backing, when used on quick-change discs, avoids potential delamination of the mounting button.

When selecting a grinding abrasive, first consider the type of metal to be processed, since many abrasives are specially developed to meet the finishing challenges for specific metals. A second factor is weld size, which in turn dictates abrasive grade and size – typically, the larger the weld, the more aggressive the grade of the abrasive required. Choosing the finest effective grade will reduce subsequent finishing steps by removing stock efficiently while minimizing dwell time and heat build-up.

Finally, consider the final desired finish. Different paint systems and industry specifications call for different final finishes and aesthetic appearances.

Carbon steel weld blending and finishing
When blending welds on carbon steel to achieve a paintable finish, a two-step finishing procedure is standard practice. However, when paint systems call for a finer finish, a three-step finishing process may be better. Typically, the first step is to level the weld with a grade 50 fibre disc or flap disc using a right-angle grinder. In this process, the abrasive disc should contact the workpiece at a 7-degree to 10-degree angle, using more of the abrasive discs surface to maximize disc life and reduce gouging. On the final pass over the workpiece, reduced pressure applied to the grinder will minimize the scratch depth.

The next step is to blend scratches in the repair area, using a coarse surface conditioning disc on a rotary tool. If this is the final abrasive step, the disc should refine the grind lines quickly while producing a paintable finish. To extend the life of the abrasive product and to help keep consumable costs down, the disc should sweep the workpiece edges to reduce snagging of the disc. The resulting finish is acceptable for many welding applications, such as fabricating farm or industrial equipment. A finer finish and a more randomized scratch pattern – commonly desired in automotive or appliance finishing – can be achieved by substituting the surface conditioning disc with a finer grade fibre disc, such as 80 grit, on a rotary tool.

A third step, which will leave the workpiece with a finer finish that can be painted with greater ease and less paint, is to use a random orbital sander fitted with the same grade abrasive as used in the fibre disc of step two. The random orbital action eliminates the pattern from the previous step. Start the sander flat on the workpiece to avoid gouging.

For small area weld removal and leveling, or when working in confined areas, an alternative is to use a small (3-in. diameter), grade 50, quick-attachment abrasive disc. Follow the disc with a surface conditioning disc of the same size or smaller.

Stainless steel weld repair
There are four basic grinding, blending and finishing steps to produce excellent stainless steel finishes after welding. The first step is to level the weld with a fibre disc, grade 60 or finer, on a right angle grinder. Using a finer grade makes it easier to remove the grind lines later. Take care to minimize the grind area, which can reduce time in subsequent steps. Remember that parallel scratches will be easier to blend during finishing, so always try to align the grind line scratches with the grain line direction of the metal.

Next, set the directional grain pattern with a pneumatic wheel and a grade 120 coated abrasive belt. Feathering on and off the work surface helps to reduce the occurrence of undercuts. This can be a challenging finishing technique, so practicing operator technique on a scrap piece of metal is recommended.

To generate a #4 finish, change the belt on the pneumatic wheel to an A-medium surface conditioning abrasive. For a #3 finish, use an A-coarse abrasive instead. An optimum running speed for a 5-in. diameter pneumatic wheel is 1000 rpm. Use the longest stroke comfortable to you and try to ease off pressure at both ends of the stroke. A few slow passes should restore the finish. The final finishing step is to blend the transition lines using an abrasive hand pad. To make the finished area look its best, apply a stainless steel cleaner to remove debris, streaks and resist fingerprints. These steps will successfully produce #3 and #4 stainless steel finishes.

Operating speed
Check the speed of the grinder to ensure that you do not exceed the maximum operating speed (MOS) of either the abrasive used or the accessories attached to the tool. If the optimum tool speed is not followed, you wont get the maximum value out of the abrasive due to excess heat generation, extra wear and overall poor performance. Exceeding the MOS can also cause the disc or backup pad to break apart and cause injury. When grinding with abrasive fibre discs, the following speeds are recommended:

Abrasive safety