Coated Abrasive Belts: Lap Vs. Butt Splices

Which is Best For Your Particular Application?

Coated abrasive bely splices, or "joints" by which they are more commonly known, are available in two basic constructions: Lap and Butt.

Lap Joints

     Lap joints are fabricated by cutting the abrasive belt to the desired length, plus the amount to be overlapped such as 1/4-inch, 3/8-inch, 1/2 inch, etc. The angle of the cut, relative to the edge of the running direction of the piece, may be at various degrees depending on the manufacturer. The angles of cut are typically  in the range of 65 degrees to a high of 30 degrees, and if one looks at the abrasive surface, the direction of the angle will be from "upper left to lower right". This industry standard was developed so that the abrasive joint angle will always be opposite the direction angle of contact wheel or contact roll serrations, for maximum performance.

     The next step in the manufacturing process is to use special diamond wheels to remove all of the abrasive grain and bonding resins on the "bottom lap" end. The "top lap" is prepared on the print of cloth side by roughing the surface to accept adhesive bonding. The joint is then made by applying adhesive to the prepped areas, "lapping" one over the other and curing the adhesive in a press.

     The "lap" type of joint has been used since the early days of coated abrasive belt manufacturing and still finds utilization in many of today's industries.

     Modifications and improvements during the years have led to a family of lap-type splices incorporating a variety of angles, widths of lap and a top-skiving technique. zin the top-skiving, the abrasive grain/bond on the top lap is also removed totally, or in part, in order to reduce overall thickness at the splice area. Industry terminology generally categorizes the amount of top skiving or grain/bond removal into three groups which are; NTS, MTS and FTS. These abbreviations stand for No Top Skive, Medium Top Skive, and Full Top Skive. As a general rule of thumb , the amount of top skiving is increased, as the abrasive grit size becomes smaller, and the overall joint thickness is more critical for a smoother running abrasive belt. 

Butt Joints

.     The other basic type of coated abrasive belt splices is known as a butt joint. Butt joints are fabricated by first by cutting the abrasive belt to the desired length, and then preparing both ends of the piece on the cloth side to accept adhesive bonding. After the application of adhesive, the two ends are "butted" together over a centered joint patch material and cured in a press to form a costed abrasive belt. 

    Common splice materials used today can be either of woven fabric construction of unidirection on film and fabric strand design. They are available in a range of thickness and tensile strengths to match specific coated abrasive belt backings and application uses. Most abrasive companies will supply their customers the proper combinations according to their requirements. 

    In recent years, butt joint have been subdivided into two distinct designs. The first is the original design, known as a straight cut butt joint, whereas the area butted together is "straight" at a predetermined angle.

    The second and newer design is sinusoidal in configuration and appears to have been cut with pinking shears. An advantage of this interlocking butt splice is that it allows the fabricator to utilize thinner backing patch materials without the bending or hinging at the butt area that is common with straight-cut versions. A disadvantage is that the sine wave joint design tends to be considerably stiffer, making it unsuitable for use on highly flexible belt backings used in offhand polishing applications.

    Butt joints, in general, offer several benefits over lap designs, that have increased their popularity and usage. Along with being thinner and stronger than ever before, butt joints provide an uninterrupted cutting surface that reduces undesirable markings and shadows on the workpiece. Being bidirectional (can be run either way), butt joint belts are often removed from the machines after partial use and reinstalled in the opposite direction. This technique can help clean an impacted or loaded abrasive surface and can, for a period of time, rejuvenate the cutting ability of the belt for prolonged use and cost effectiveness. 

    Lap joints, on the other hand, must be run in one direction only-- according to the directional arrows-- so that the work does not catch or snag the trailing edge. While this may appear to be a drawback, the fact that lap joints are the most flexible type of splice makes them well suited for many applications where comfortability to radical part shape is mandatory. Lap joints are also well suited for use where erosion or heat can degrade a backing patch used on butt joints. 

    Selecting the appropriate belt splice is an extremely important specification in each particular application. For assistance , contact your abrasive supplier for his or her recommendations on which of the many types and designs is best suited for your need.