Widebelt sanding is a misunderstood asset in the woodworking industry. It is a powerful tool that can cut production time, staffing needs and render a finished product that is finer than a trained human hand can produce. When widebelt sanders are properly used, the finished product will not only pass the "two-foot test," it will pass the one-inch test.
The diversity of widebelt sanders makes choosing the right machine for the job complicated. Using a widebelt sander is completely different from using a sander for planing material. The buyer needs to understand the fundamentals of horsepower, shore hardness and grit.
There are formulas for calculating the horsepower needed for specific types of production. Research must be done and confidence in the manufacturer's recommendations is a must. Close estimation of the sander's horsepower needs is essential to ensure proper specific performance of the sander. Higher horsepower isn't necessarily better. The cost of the sander increases with higher horsepower specifications and too much horsepower will increase production costs long term with respect to energy consumption. On the other hand, too little horsepower increases production time in that several passes will be required, or worse, the machine might stall intermittently. Pertinent to horsepower needs, the manufacturer will also need to know how much material is being removed, what species of wood is being sanded and the pass width of the largest item that will be put through the machine. Horsepower is calculated on seven variables. If we change any one of them, the horsepower needed also changes. The most common aspects are feed speed, part width, amount of removal and species of wood.
Shore hardness is the rating used to determine how hard the drum is relative to steel. The higher the number the closer it is to the hardness of steel. On the accompanying chart are examples of shore hardness, the type of work recommended and acceptable grits for use. The finer the sanding grit that is being used, the lower the shore hardness needed, so that the pressure from the drum on the material accommodates to the finer grit.
The chart shows how specific shore hardness drums and grits of paper work better for different types of sanding work. The figures given are for optimum performance of the drum and abrasives, related to abrasive life and quality of finish.
The subject of grit brings up the concept of sequencing. What grits will be used and in what order of progression must be defined. A grit may be skipped if the machine is configured with drums. If pads are a consideration, more attention will have to be paid to grit sequencing in that a pad has the tendency to close the grit on the sanding paper, rendering a slightly finer grit capability than was intended by the manufacturer of the belt.
The basic rule of thumb is the wider the part, the coarser the grit and the faster the feed speed, the more horsepower will be needed. Bear in mind that for pre-sanding and finish sanding much less horsepower is required because finer grits are removing very little material.
Sanding vs. planing
When considering a machine, a decision must be made on the amount of material removal. Abrasive planing is different from sanding. Abrasive planing is the removal of material and is done with steel drums without consideration for the smoothness of the finish. Sanding is the balance between horsepower, shore hardness and grit sequencing to accomplish the job at hand. It is a finishing process done with rubber drums and variable grit sequencing. Understanding the difference between these two concepts of product treatment has an enormous effect on the type of machine needed to do the work.
Abrasive planing is using a machine with a steel drum backed by a lot of horsepower to remove large amounts of material. This can be hard on the material to be removed because the abrasive virtually has rocks imbedded in it. We are talking about grits below 60, and these can pull a lot of the softer grain out of the material as opposed to cutting it down. Remember that abrasive belts are a cutting tool by nature, similar in theory to any blade or bit. On the other hand, sanding is the art of properly preparing the material to accept some type of finish in a uniform and even fashion. Sanding is generally removing stock amounts of 1mm, which translates to approximately 40 thousandths of an inch. At these rates we are using multiple heads and grits to remove the material.
Grit sequencing examples
By using the chart as reference, we can start to visualize proper grit sequencing for the proposed work to be performed. At this point let's make note: It is physically possible to put any grit paper on any shore drum, and many people do. One of the unfortunate aspects of widebelt sanding is the belief that if it's making dust and the product feels fairly smooth, it must be doing the job. In some cases this might be correct, but usually with a little more understanding of what is going on in the machine and how to maximize a machine's sanding potential, you can achieve better results with a lot less wear and tear on the equipment and increase the lifespan of those expensive abrasive belts.
A couple of examples of grit and shore hardness sequencing could be as follows:
- An 80 to 85 shore calibrating drum with 80-grit paper to remove 0.6mm;
- Followed by a 40 to 45 shore hardness drum with 120-grit paper to remove the 80-grit scratch, which requires a removal of 0.20 to 0.25mm;
- Then a 30 shore hardness drum with 180-grit paper for a removal of 0.10 to 0.15mm to remove the 120-grit scratch, to end with a flat product finished to a 180-grit finish. This has become very popular in the cabinet business for frame preparation of varied species of wood.
Now, the same product but needing a little more overall removal of material to even the frame components might look like this:
- An 80 to 85 shore drum with 60-grit paper for a removal of 0.80mm;
- Followed by the same 40 to 45 shore drum with 100-grit paper to remove 0.30 to 0.35mm;
- Finishing up with a 30 shore drum with 150-grit to remove the 100-grit scratch at 0.20 to 0.25mm. This stops us at 150 grit. If we want a 180 finish, we need one more pass on the third station with 180 grit.
Using the chart for reference will help specify the type of drums and number of belts needed to perform a specific style of work.
It is only possible to skip so many grits to get from one end of the machine to the other. More belts might be needed than originally thought. Most widebelt manufacturers offer a multitude of configurations to fit any number of applications. The buyer might be able to specify a machine with fewer belts, but configured with different combinations of drums and pads in the same unit to allow greater flexibility in the overall performance of the machine.
The finish grit needed for a particular application will determine how many belts the sander will need to do the work. The finer the grit finish, the more belts will be needed to do the job. If production time allows for a primary run with a subsequent change of grit paper on the different heads, fewer belts will be needed.
Platens, or pads as they are some times called, require a higher amount of maintenance for optimum performance. Graphite, felt and often foam rubber are used in layers on a rigid bar, or a thin steel plate in the case of pneumatic and segmented units, to create the working surface of the pad. By and large, platens are more for veneer work or sealers and UV coatings and the like. Many people use platens to help minimize the cross grain scratch on doors and frames of any type where there is grain running perpendicular to the stiles.
Note that cross grain scratch is a wholly subjective opinion, and remember that there are as many opinions regarding it as there are craftsmen to do the work. Whether using a platen or a drum on a widebelt sander, there will be cross grain scratch. The compromise is what degree is acceptable for your shop, and how much more work or how many more machines do you need to produce an acceptable finish for your product?
Many manufacturers build off-the-shelf machines that range from 16 to 53 inches in width. By keeping in mind the basic premise of how widebelt sanders work, getting the most from a machine and the abrasives is a matter of balancing your budget with proper features to perform the work you need done. Keep in mind that most manufacturers have a wide range of available options and features to accomplish any sanding task you may require.
We need to remember there are a lot of variables to consider for our machine and that just because the machine makes sawdust doesn't mean we are getting the most out of our investment.
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