Sanding rules to live by.
Advanced abrasives and high-efficiency sanding equipment are just two factors which have increased the quality and productivity of finishing operations in the woodworking industry today.
However, for all this technology and expertise, problems and challenges still occur. Left uncorrected, sanding problems can result in significant losses of time and money for the furniture and wood products manufacturer.
You can troubleshoot your sanding techniques for increased productivity by paying close attention to such parameters as coated abrasive product selection, grit sequence, grain size and machine parameters. Such attention to detail will reduce unnecessary rework, decrease production downtime, and improve the quality and consistency of product finish.
While you can't always have a coated abrasives expert on hand, it can be helpful to remember a few solutions to some common problems.
Q. How can I decrease the cost of my widebelt sanding operations without affecting the quality of finish "in-the-white" prior to color?
A. First, machine setup must be correct. A three top-head intermediate widebelt machine should be set up to employ the "60-30-10 rule." This means that head #1 should do 60 percent of the dimensioning work. Head #2 dimensions 30 percent, which will upgrade the finish left by head #1. Finally, head #3 should only upgrade the finish left by head #2; this will actually account for only 10 percent of the stock removal.
(Tip: When setting up a widebelt sanding machine, it is advisable to use a panel that is at least 75 percent of the width of the coated abrasive belt. For example, with a 50-inch by 103-inch belt, use a panel that is 38 inches or more in width.)
Q. What causes burning of the workpiece when sanding?
A. There are several different factors - and combinations of factors - that can cause burning of the workpiece: using excessive pressure, using too free a grit size, running at too slow a feedthrough speed, using a worn coated abrasive belt, expecting too much stock removal from the grit size you are running, using excessive running speed of the coated abrasive belt and not using oscillation on edge sanders when platen sanding.
The best corrective action in all of these cases is to remember what causes burning - it's temperature and time at the point of sanding. Determine what can be changed on your sanding application to minimize heat generation. Exercising good common sense saves money.
Q. What can be done to eliminate the shiny streaks coming off a widebelt machine that has a platen head as a finishing head?
A. These streaks, which are parallel to the panel edges, can be seen but not felt. They are even more apparent after stains have been applied. Generally, these streaks are caused by the graphite-coated canvas used to cover the platens (the canvas is used to add lubricity to the back of the coated abrasive belt, minimize frictional heat, and induce finish).
Sanding the canvas using grit 100 or 120 will remove all the loose graphite particles that would otherwise have adhered to the back of the coated abrasive belt and then transferred, in the form of shiny streaks, to the workpiece. It is advisable to sand new covers as well as rejuvenating worn covers. This does not effect the productive life of the graphite-coated canvas, as one might think.
Q. What axe the criteria for choosing between cloth or paper widebelts?
A. Cloth-backed coated abrasive widebelts will better withstand the shock caused by thick, tapered or warped workpieces; paper belts are more likely to break. Cloth belts are also used on machines that are in fair-to-poor condition.
Paper widebelts are less costly, generate a better, more consistent finish, and, with today's designs, can produce nearly equal belt life.
When testing paper vs. cloth on widebelt applications, it is advisable to decrease the belt tension gauge pressure by approximately 20-25 percent. The reason: cloth will stretch very slightly, while paper will not.
Q. How do we minimize or eliminate the swirl marks that are created when using a DA (dual action) air-powered disc sander?
A. To achieve maximum benefits from DA disc sanding applications, it is important to realize that these portable tools will create a finish that is approximately one to two grit sizes finer than a coated abrasive belt finish. For example, DA sanders using grit 150 will produce a grit 180-220 finish.
The free running speed of these tools are 10,000 to 12,000 rpm. The tools need adequate air pressure (90 pounds) and air volume (18.5 cfm). When these conditions are met and the operators let the tools do the work (in other words, they refrain from applying too much pressure), swirl marks are generally not a problem. It's also important not to waste air supply or restrict the flow at the tools or the fittings. Using shorter hoses is a common recommendation.
Q. The belts on our drawer sander break prematurely. What could be the cause?
A. On drawer sanders, it's advisable to use as little belt tension as possible to run the coated abrasive belt. When installing a new belt, apply only enough belt tension to allow you to move the belt by hand across the platen area. Run the belt for a few minutes and increase the belt tension slightly; this will take up the "stretch" that has occurred. This will enhance belt life by retarding premature belt snagging and breakage.
REMEMBER THE BASICS
When approaching any sanding problem, it's important to understand the "cause and effect" relationships in the sanding process. For example, a machine adjustment may temporarily solve the problem at hand. However, it may also create a different problem entirely. This is particularly true when making frequent adjustments on multi-head widebelt machines. While the following guidelines may seem obvious, they have a direct effect on your ability to use your coated abrasives productively and economically.
* Don't choose a coarser grit than is absolutely necessary to achieve your
required finish. The patterns produced by coarse grit abrasives must be removed as the finish upgrade approaches your desired sanded surface.
* Don't skip more than one grit sequence when upgrading your finish. When two or more grits are skipped in a sequence, an excessive number of finer grit belts must be used to remove the previously imparted coarse grit pattern. This can be very costly.
* Feedthrough speed has a direct relationship to stock removal rate. Slower feedthrough speeds allow for greater stock removal, and vice versa. However, it's important to remember that slower feedthrough speeds can generate more heat.
* Finish also has a direct relationship to feedthrough speed. Faster feedthrough speeds produce a finer finish with less heat. The opposite is true using slower speeds.
These examples, while hardly exhaustive, demonstrate how an understanding of the sanding and finishing process can help you to identify and rectify sanding problems. To optimize your sanding results, work with your coated abrasives manufacturer to ensure proper product selection and usage.
Bill Beatty is industry specialist, production wood markets, for Norton Co. of Worcester, MA.
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|Title Annotation:||sanding techniques for furniture and wood products manufacturers|
|Author:||Beatty, William Alfred|
|Publication:||Wood & Wood Products|
|Date:||May 1, 1997|
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