Quick answer: Sanding belt splices come in two basic constructions. A lap splice overlaps the two ends, is the most flexible joint, and must run in one direction (follow the arrow). A butt splice joins the ends edge to edge over a tape on the back; it's thinner, stronger, bi-directional, and leaves no mark on the work, which is why it's now the standard. Choose butt for almost everything, sine-wave butt for wide belts and glass, and lap only where extreme flexibility or a harsh environment calls for it.
Lap splices
A lap joint is made by cutting the belt to its finished length plus the overlap – typically 1/4", 3/8" or 1/2" – at an angle to the running direction. The angle varies by manufacturer, from about 30 up to 65 degrees, and by industry convention runs from upper left to lower right when you look at the abrasive side, so the joint angle is always opposite the serrations on a contact wheel for the best performance.
The bottom lap is then run over diamond wheels to strip all the grain and resin off the end, and the top lap is roughed on the cloth side to take adhesive. Adhesive is applied, one end is lapped over the other, and the joint is cured in a heated press.
Lap joints date from the earliest belt manufacturing and are still used in many industries. Over the years they've been refined into a family of variations – different angles, overlap widths, and top skiving, where the grain and bond on the top lap are also removed, fully or partly, to reduce the thickness of the joint. Top skiving is graded as NTS, MTS and FTS – no, medium and full top skive. As a rule, the finer the grit, the more top skive is used, because joint thickness matters more as the belt runs smoother.
Butt splices
A butt joint is made by cutting the belt to length, preparing both ends on the cloth side, and butting them together – no overlap – over a centered joint tape, then curing in a press. The tape can be a woven fabric or a unidirectional film-and-strand construction, in a range of thicknesses and tensile strengths matched to the belt backing and the application; your supplier matches them for you.
Butt joints come in two designs:
- Straight-cut – the original: the ends meet on a straight line at a set angle, normally 67 degrees.
- Sine-wave (sine-lock) – the newer design, cut in a wave that looks like it was done with pinking shears. The interlocking wave lets the maker use a thinner tape without the bending or hinging at the joint that a straight cut can show. The trade-off is a stiffer joint, which makes it unsuitable for the very flexible backings used in offhand polishing.
Lap vs. butt: side by side
| Lap splice | Butt splice | |
|---|---|---|
| Construction | Ends overlap and are glued | Ends meet edge to edge over tape on the back |
| Direction | One way only – follow the arrow | Bi-directional |
| Joint thickness | Thicker (unless top-skived) | Thinner |
| Strength | Good | Stronger |
| Marking on the work | Can leave a shadow | Uninterrupted cutting surface – no mark |
| Flexibility | Most flexible joint | Straight cut: good; sine-wave: stiffer |
| Heat and erosion | Tolerates conditions that degrade a tape | Tape can degrade in extreme heat |
| Reversible to refresh | No | Yes |
Why butt splices took over
Thinner and stronger than ever, butt joints give an uninterrupted cutting surface that eliminates the joint marks and shadows a lap can leave on the workpiece. And because they're bi-directional, a partly used belt can be taken off and put back on the other way round – which helps clear a loaded or impacted surface and brings back some of the belt's cutting ability for a while. That trick alone makes butt-spliced belts more cost-effective.
When a lap splice is still right
Lap joints have to run one way so the work doesn't catch the trailing edge, and that looks like a drawback. But a lap is the most flexible splice there is, which suits applications where the belt has to conform to a radical part shape, and it holds up where heat or erosion would degrade the tape on a butt joint.
Choosing
The splice is one of the most important specifications on a belt, and the right answer depends on the machine, the material and the finish. When in doubt, tell us the application and we'll spec it. Our guide to all five sanding belt joints covers top skive and tape-on-top variations for wide belt sanders. Every joint we make is pressed and inspected in Anaheim – order custom belts in any size with the splice you need.
