As the demand for fabricating higher-value materials increases, a three-step process will guide operators through the fastest, most cost-effective way to achieving customers' requirements.
In North America, the use of stainless steel continues to grow in popularity, due to its many advantages for consumer and industrial products. Architects, engineers and contractors increasingly specify stainless steel for use in construction, appliances, food processing equipment and the medical field. As stainless steel usage grows, proficiency in finishing stainless will take on increased importance.
For fabricators, the ability to fabricate and finish stainless steel products, may increase business and improve profitability. The most effective fabricators follow a three-step process for stainless steel finishing projects. The first step is to work with the customer to define and document the finish required for the project. In the next step, the fabricator must choose the correct abrasives and power tools to be used on the job to achieve the required finish. Finally, the fabricators must create and execute the mechanical finishing process plan in order to complete the project and satisfy the customer.
STEP 1: DEFINE THE FINISH
The finish of the base material and treatment of the welds should be discussed in detail and agreed upon with the customer. Generally, material finish is specified in grit, i.e., the particle size for the last step in abrasive finishing. (Stainless steel material specifications are defined in the sidebar, p.20.)
The treatment of the welds also should be defined with the customer. Weld treatment options are:
- Remove weld spatter and discoloration only;
- Remove weld spatter, discoloration, and weld ripple;
- Remove weld spatter, discoloration, and weld ripple, plus grind weld smooth with base material;
- Remove weld spatter, discoloration, and weld ripple, plus grind weld smooth with base material — producing all welds pit- and crevice-free.
Once the customer and fabricator have defined the material finish and weld treatment, a sample swatch can be created to use as reference. The sample swatch can be as simple as two pieces of stainless steel square tubing welded together and finished per the aforementioned specifications.
For mechanical finishing of stainless steel, discs and belts are the most popular abrasive mediums. Abrasive discs are used most often for weld treatment; belts are used for material surface treatment. The notable exception is the Dynafile, small portable belts for treating welds in hard to reach locations. Table 1 identifies typical mechanical finishing steps for weld treatments, along with the appropriate abrasive.
STEP 2: CHOOSE ABRASIVES, TOOLS
Flap discs have significantly reduced the steps required for stainless steel finishing. In the past, operators used a two-step process for weld treatment, consisting of a grinding wheel for the first step (material removal), followed by a resin-fiber disc for the second step (uniform scratch pattern.)
Increasingly, flap discs are replacing the older two-step process with a single step that requires only one abrasive product. Layers of coated abrasive are arranged on a backing pad in a fan-like arrangement. In addition to reduced processing times, flap discs are less prone to gouging and scratching the workpiece (very common with grinding wheels.) Gouging and scratching can add significantly to downstream polishing times.
For finer weld finishing, a wide range of new products is available to improve surface finishes and reduce polishing times. These newer products include surface conditioning flap discs, combination flap discs, and unitized disc. Surface-conditioning material is a nylon web impregnated with abrasive grit. The material is available in coarse (50-80 grit), medium (100-150 grit), fine (180-220 grit), and very fine (240-360grit) textures, and produces a matte-like finish when applied to stainless steel.
Combination discs are a single backing pad with coated abrasive and surface conditioning combined in an interleaf pattern, to remove the weld and polish the surface at the same time. Unitized discs consist of layers of non-woven material impregnated with abrasive grit. These are available in coarse through fine grits, and are excellent for producing fine finishes on stainless steel welds.
Abrasive belts — As discussed, belts are generally used to process larger material surface areas. The two main categories of abrasive belts used in stainless steel finishing are “coated” and “surface conditioning” products.
Traditionally, zirconia-coated abrasives belts have been used for stainless steel finishing. However, the introduction of newer ceramic-grain belts encouraged more stainless steel finishing. The newer ceramics offered by coated abrasive manufacturers, such as VSM Abrasives (www.vsmabrasives.com) and Norton Abrasives (www.nortonabrasives.com), require less pressure, last longer than other coated abrasives, and cut cooler. The last of these is particularly important for stainless steel finishing, as heat generation is an obstacle to effective finishing.
Belts made of surface-conditioning materials are generally applied after the coated abrasives do their work. The surface-conditioning belts' texture does more than remove material. The belts generate the appearance of a linear brushed finish on the material.
When sequencing abrasives, fabricators should work from coarse to fine. Skipping more than one grade level when moving to finer grits tends to give the material an uneven choppy look. When blending welds and large surfaces, using longer strokes will result in a more even appearance.
Power Tools — While the angle grinder is still the workhorse for stainless steel weld finishing, linear belt finishing tools are required to achieve the higher level stainless steel finishes that frequently are specified.
Power tools are available as pneumatic or electric. Pneumatic tools offer the advantage of being lighter, and therefore, easier for the operator to hold and handle. On the other hand, electric power tools offer the advantage of more power and variable speed control.
The ability to control the speed of the tool is essential for finishing stainless steel. As the surface finish becomes finer, the speed of the tool must be reduced to achieve a uniform appearance in the surface finish. Therefore, fabricators attempting No.4 finishes (and finer) need to have variable-speed angle grinders and linear belt finishers in their workshops. By reducing finishing speeds as the surface finish becomes finer, abrasive consumption is reduced also, so fewer abrasives are needed to complete the project.
New grinder models, such as the WE14-125VS from Metabo (www.metabo.us), offer variable speed, vibration reduction, quick wheel change, and a powerful 12-amp motor. The yellow dial at the bottom controls the speed. This 6-in. unitized disc must be run at 3,000 rpm (a normal grinder runs at 11000 rpm) and will grind and polish the corner weld in a single step.
For linear finishing and blending welds with adjacent pre-polished material, variable-speed burnishing machines such as the flex machine below allow operators to achieve desired finishes in stainless steel more easily. It is important to note that a linear scratch pattern (frequently specified for stainless steel finishes) cannot be achieved with an angle grinder: the angle grinder will leave swirl marks in the material. Only a burnisher can achieve the desired finish.
A relatively new class of power tool is gaining popularity for polishing pipe and handrails. The pipe sander wraps 270° around the workpiece and quickly, and easily, accomplishes the necessary finishing. The pre-tensioned arms apply even pressure to the workpiece to produce a uniform finish.
The Dynafile is another tool required for most stainless steel finishing jobs. The tool accepts small portable belts and excels at accessing hard-to-reach areas. Dynafiles are typically used where access to use the grinder or polisher cannot be gained.
The final step is to create the mechanical stainless steel finishing plan and complete the project. This involves pre-planning the actual work steps in order to complete the required finish. Each step should list the power tool, abrasive type, and grit that will be used.
STEP 3: EXECUTE THE PLAN
Nowhere is preparation more important than in the area of mechanical polishing. By carefully planning the project, hours of needless re-work can be avoided. If fabricators are attempting a finish they have not achieved in the past, they may want to consult with their abrasives and power tool distributors for guidance.
The following points should be considered during the planning process:
Order pre-polished material when possible;
Protect the material with a plastic film coating;
When possible use TIG welding;
Minimize weld diameter and spatter;
Pre-polish parts that have difficult access points;
Educate employees about care in material handling, as stainless steel scratches easily.
What follows is a sample of a polishing plan created for a stainless steel housing. The housing walls are 11-gauge 304ss, the flanges and end plates are quarter-inch thick 304ss. The requirement is a No.4 finish, with all welds ground smooth and flush with the base material.
Mechanical Finish Designations for Steel Materials
Order all 11-gauge, 304ss and quarter-inch thick 304ss material to be pre-polished to No.4 finish;
Keep protective plastic on pre-polished material
Employ the appropriate tool and abrasive defined for the specific weld treatment, location, and finish summarized in Table 2.
The three steps critical to successful stainless steel finishing include defining the customer's requirements; planning the sequence, which includes choosing the abrasives and tools that will be used; and executing the plan. By adopting this approach, fabricators can fulfill their customers' requirements in the shortest time and at the lowest possible cost.
|Removal of excess weld, spatter and discoloration||Grinding Wheel |
Resin Fiber Disc
Abrasive Flap Disc
|Creating a uniform scratch pattern in weld||Flap Disc |
Dynafile Coated Surface
|Weld treatment||Metabo 4.5-in. variable speed grinder||Flap disc ceramic grain, 80-grit, then 120-grit unitized disc 6A medium|
|Hard-to-reach areas||Dynafile||Belt ceramic grain 80-grit, then 120 grit, then medium surf conditioning|
|Blending weld areas with base pre-polished material||Flex Burnisher||Medium Scotch Brite belt|
No.2D finish — a uniform, dull silver gray mill finish that is applied to thinner stainless steel coils, the thickness of which has been reduced by cold rolling.
No.2B finish — a bright cold rolled mill finish commonly produced in the same manner as No. 2D, except that the final light cold rolling pass is done using polished rolls.
No.3 finish — A semi-polished surface achieved by finishing with the equivalent of 80- to 120-grit abrasive. This finish has a pronounced grit line.
No.4 finish -Also called brushed, directional or satin finish A No.4 finish, characterized by fine polishing grit lines that are uniform and directional in appearance. The final abrasive used in the process is 150- to 220-grit abrasive.
No.6 finish — Polished finish achieved with the equivalent of a 240-grit abrasive. Finer grit lines and higher reflectivity than No. 4 finish.
No.7 finish — Highly reflective surface obtained with the equivalent of a 320-grit abrasive. Minimal grit lines.
No.8 finish (or “mirror finish”) — Produced by polishing with at least a 320-grit belt or wheel finish. The part is then sisal and color buffed to achieve a mirror-like finish.
Chris Stone is the owner of Lehigh Valley Abrasives, which supplies premium abrasives to the metal working industries. Contact him at 908-892-2865, or visit www.lehighvalleyabrasives.com.