Selection and use of abrasives for metal fabrication and welding
Choosing the right abrasive and using it correctly for metal fabrication and welding applications can make all the difference in maximizing productivity and minimizing costs. Yet with today’s tight deadlines, manufacturers are tempted to use whatever abrasive is at hand without considering which is best for the application. Some might even ignore some basic grinding safety rules. The results can be reduced finished product performance, increased grinding and finishing costs, and lost productivity.
Selection of classy abrasives
It is important to realize that abrasive products come in different grades, which some manufacturers designate as good, better, and best (this is the pattern used in the tables in this article, in descending order: 1=best, 2=best , 3 = good).
Metal fabricators who measure total grinding results and want the lowest overall cost for an application use the best abrasive products available for most of their jobs, reserving the use of the medium grade for high productivity applications where only the abrasive cost is important. End users desiring the lowest initial cost are likely to select good abrasives, but should be aware that such a choice may not be economical in the long run.
Fabrication and welding shop owners wishing to determine total grinding results can work with their abrasive supplier to test and compare different grades of abrasive and alternative products from different manufacturers. These tests, performed on-site, reveal the relative cost of abrasive based on price and wear rate and can even measure overall worker productivity based on a worker’s salary. Then the relative abrasive cost can be added to the operator cost to determine the total grinding cost. What these tests generally show is that the most advanced (best) abrasive products generally last longer, work faster, make the best use of worker time, and often provide the most cost-effective choice.
Dismantling a weld bead
The first step in processing a weld is grinding to remove as much of it as possible (see Table 1). The amount of work required for this step depends on the hardness of the material, the type of weld, the skill of the welder, and a few other factors. In most cases, removing a weld bead is more economical with a right angle grinder. For this application, the abrasives used on these tools may be fiber discs, flap discs or smaller versions of these products, used for greater control or to handle smaller work envelopes.
Fiber discs remove excess material from flat and contoured surfaces with a fast initial removal rate. A few tips :
- Use a 60 or 80 grit ceramic disc to quickly remove the solder and produce a depth of scratch that will create a usable surface for further enhancements.
- Use a hard backing pad to remove stock more aggressively.
- Use a product with a grinding aid to reduce heat in the grinding area and prevent burns.
- Use a 10-15 degree approach angle to remove solder effectively and efficiently.
Flap discs are good for removing thick weld beads and excess material while providing long life:
- For general use, the recommendation is a 60 grit ceramic flap disc.
- For higher speed and higher material removal, a Type 29 taper is the best choice.
- When blending and smooth cuts are needed, a Type 27 dish is the best choice.
The advantages of flap discs over a single layer product include:
- Up to 20 times lifespan; fewer disc changes reduce labor costs.
- More homogeneous finish.
- No backup buffer required.
- Easier storage, no looping.
Whether you use flap discs or fiber discs, higher performance ceramic grit abrasives work best when powered by an angle grinder rated over 1200 watts.
Mixing and finishing
The second step is to remove the scratches created by the first step and define the directional scratch pattern of the final product. It is very important to get below the rotating scratch pattern created by the grinder during the disassembly step. If this pattern is not completely removed, deeper grind lines may appear when trying to develop finer finishes. The abrasive for this step, a #3 finish, is a conventional 100 or 120 grit sanding belt of the size required for the part (see Table 2).
For applications requiring a #4 through #8 finish, it is customary that the depth, pattern and direction of the scratches be uniform across the entire surface of the product. To achieve such a finish, it is common to use an abrasive with the same linear scratch pattern as the abrasive used in the previous step, starting with a medium-grit non-woven strip or wheel.
When a uniform directional scratch pattern is not a requirement of the final product, a rotary tool can do the job after making the weld. In this case, two abrasives of choice are a premium non-woven unified wheel or a non-woven surface conditioning disc. Surface blending discs conform to contours and provide lower cut rates with finer finishing capabilities.
Controlled use of the disc at a 10-15 degree angle prevents random scratches. A coarse grit disc provides mixing capability that prepares the surface for polishing or a coating such as paint.
It may be beneficial to switch to a nonwoven product as early in the blending process as possible. For example, trimming, blending and finishing can be done with a single nonwoven product to reduce the number of separate steps (see Table 2 and Table 3). A non-woven product also provides a controlled and sustained finish while decreasing the likelihood of undercuts and gouging. The non-woven type of product also offers cooler action to minimize the chance of warping or fading. Finally, it is quieter and vibrates less.
Security and productivity
Following a few safety rules and general productivity tips can help improve results:
- Preperation. Review all safety rules established by your employer, government agency or other authority:
- Do not grind near loose or flammable clothing.
- Work only in well-ventilated areas and prepare the work area to keep sparks and debris away from your body and bystanders.
- Consider the size and shape of the project when planning grinding processes. On large or bulky parts, take care to avoid overhangs and imbalances. Use a vise or clamps to secure small parts.
- Avoid contact with the spinning disc or backing pad.
- Mounting the abrasive disc. Mount the drive according to the manufacturer’s instructions. When using a disc with drilled holes, do not stick any objects into the holes.
- Using the abrasive disc. Using an abrasive disc isn’t difficult, but using it effectively requires understanding several do’s and don’ts:
- Start the tool right next to the part; bring it up to full operating speed before you start grinding or sanding.
- Always insert the blade into the workpiece at a 5-10 degree angle to the surface of the workpiece.
- Look for any areas of the room that may catch, snag, or jam the disc. Rather than jamming the disc in such an area, work the disc gradually into the jagged surface before letting the edge of the disc bite into it. Excessive punishment of the disc edge will break even the toughest coated abrasive disc.
- When grinding a depression, molding, lip, or heavy weld, start in that area and move the grinder away from it. Do not start on a flat surface and move the grinder into such a contour or feature.
- When using a disc for heavy stock removal, do not use the entire surface of the disc. This may cause the whole tool to shake in a rough, jittery type of action.
- When using a non-round disc and backing pad, avoid tight corners and any place where the disc may snag or snag.
- If vibrations or vibrations occur during use, stop the tool immediately. Determine the problem and fix it before continuing.
- If you drop the tool, replace the buffer and backup disk before continuing.
- After Grinding. Do not put the tool down until the disc has stopped spinning. Do not store or lay the tool on the disc and pad.