Generally speaking, a drum is used for stock removal and a platen is used for finish sanding; however, drums are also used for finish sanding in some applications. A rule of thumb would be, if you need to remove more than 0.003 to 0.004 inch, you should use a drum, otherwise a platen may be used. The difference between the two is also seen in the finish. A drum will produce a short scratch pattern, but it is deeper on a given grit. A platen will produce a longer scratch that is not as deep. You really need to determine stock removal requirements and desired finish to decide which will fit your needs better.
They are mostly related to power consumption and media costs, which are lower with a planer/sander. An abrasive planer will use high horsepower motors with very coarse sanding belts. The latter can be expensive, and usually need to be replaced two or three times a week (depending on the type of product being sanded). On average, a planer/sander uses about half the horsepower of a conventional abrasive planer, and the cutter inserts will last several months. These replaceable carbide inserts have four sides/cutting edges; one edge will produce around 150,000 board feet of product. An abrasive planer uses 24 or 36-grit belts, which leave very deep scratches. To remove these scratches, an additional sequence of 60-, 100- and 120-grit sanding belts must be run to achieve the same finish as that produced by a planer/sander running one knife head and one sanding belt.
When sanding veneered panels or sealer/lacquer, utmost control is required. To accomplish this, the platen is made up of individual segments; each of which receive sanding pressure separately (pneumatically or electronically). These segments are controlled by a CNC controller that, along with a sensing unit, can be programmed to activate only when needed. By doing this, you have the ability to conform to the irregularities of the panel to prevent sanding through the sealer or veneer.
Used primarily in veneer tape removal applications, the cross belt sander is designed to run across the grain of the wood. Because of this design, veneer tape is removed with one head, whereas two heads are needed with other wide belt methods. Cross-belt sanders also are used on long panels in which the grain runs in the narrow direction, such as desk tops and front panels. In processing these, the cross belt is located on the out feed of the machine, so the scratch pattern produced by the belt goes with the grain.
Hold-down shoes in a wide belt sander are similar to chip-breaker shoes in a planer. They are used to control the part as it passes through the machine, prevent dubbed or sniped leading/trailing edges, and to allow for shorter parts to be run. Specific uses are: short or narrow parts, parts under 1/4 inch thick, veneered panels, or any time you need to hold tight tolerances.
There are two circumstances. The first is when you need to run parts that are shorter than the distance from the infeed hold-down rolls to the outfeed hold-down rolls; these parts may slip without using vacuum. The second situation is when sanding very thin or flexible parts; parts with a thickness of less than ½” will have a tendency to bow and curl, while flexible product can be lifted into the sanding head. The vacuum belt assists the pressure rolls to flatten and hold the parts during the sanding operation.