There are several factors that affect a widebelt sander’s ability to hold a close thickness tolerance. When a sander is new, with proper operating procedures, any rigid orifice type machine should reasonably hold plus or minus 0.005-inch tolerance. If a machine has been specifically designed and has the right characteristics to hold a close tolerance, thickness tolerances of plus or minus 0.0025-inch are achievable. However, within a short period of time (less then one year), certain wear factors require machine maintenance procedures along with proper operating procedures in order to continue to obtain tight thickness tolerances.
Side-to-side head adjustments
Individual sanding heads must be adjusted parallel to the conveyor belt and for best sanding result, to each other. The degree of this parallelism should be within a total of 0.001 inch. This can be achieved and maintained with a set-up device.
Care must also be exercised to make sure that the outboard spacer and its contact surfaces are dust free. The holddown bolt must also be tightened with a uniform tightening force. A torque wrench may be required to accomplish this.
Check the conveyor belt
Many conveyor belt factors can have a dramatic effect on a sander’s ability to hold tolerance. First of all, all conveyor belts must be calipered (dressed to a uniform thickness). New belts must be calipered/dressed and then redressed approximately every six months to eliminate wear variances.
The conveyor belt hardness can have a major impact on a sander’s capability to hold tolerance. For maximum tolerance holding capability, belt hardness should be 85-90 durometer or more. The top surface pattern of the conveyor belt also impacts tolerance capability. A smooth-face belt results in optimum tolerance holding capability. Be advised that belt hardness and top surface type can impact part-feeding capabilities. Smooth-face hard belts have less feeding grip then softer, rough textured.
Contact drums are critical
Contact drums are one of the most critical components in a widebelt sander. For optimum dimensional control, contact drums should be steel or hard rubber. Because they wear unevenly, they must be periodically ground true. Also, for optimum dimensioning capability, they should be running at belt speeds of 5,000 to 7,000 sfpm. As a general rule of thumb:
• the runout should be less than 0.0005 inch T.I.R. (measured when the drum is running in its bearings;
• the balance should be as fine as possible (within 2 grams). Displacement should not exceed 0.0002-0.0004 inch at the operating rpm.
• the wall thickness should be sufficiently heavy so the drum will not deflect when a heavy cut is made.
All widebelt sanders have inherent backlash (slop) in their thickness setting devices. Thickness setting changes are accomplished through the use of jack screws rotated either by means of a worm and worm gear device or a chain and sprocket arrangement. Therefore it is essential that thickness changes are always made with the final adjusting movement in the same direction. If this is not done, side-to-side thicknesses can change by as much as 0.015- 0.020 inch.
• Top machines: On most top-sanding machines, the conveyor bed is moved up and down relative to the sanding heads to adjust thickness settings. Care should be taken to not to go past the required setting.
• Bottom machines: Almost all bottom-sanding machines are built so that the conveyor feed system hangs from the adjusting jack screw arrangements. This means that the final movement of the adjustment should be in a downward direction. If the set point is missed, the conveyor feed system should be raised up by at least one full handwheel turn and then slowly lowered, taking care not to go past the required setting.
• Machines with entire top frame adjusting: On some sanding machines, the entire upper frame assembly is moved up or down instead of just the feed bed. In those cases, the final movement of the adjustment should be in a downward direction. If the set point is missed, the entire top frame should be raised up by at least one full handwheel turn and then slowly lowered, taking care not to go past the required setting.
Inherent machine design deficiency
Not all widebelt sanders are designed for optimum dimensional thickness control. In fact, some machines are purposely designed and built with yielding orifices for optimum results when sanding sealer or thin face veneers. If close thickness control is one of your requirements, make sure that you purchase a machine with the proper characteristics to optimize those requirements. If you find that your machine has thickness control design deficiencies, it may be possible to modify in the field and gain some improvement.
|Belt thickness variation & grit differential (cloth backing)|
|Grit differential||Total thickness||Estimated wear|
|20||0.092 inch||0.030 inch|
|24||0.078 inch||0.025 inch|
|36||0.070 inch||0.020 inch|
|40||0.066 inch||0.018 inch|
|50||0.058 inch||0.014 inch|
|60||0.053 inch||0.012 inch|
|80||0.046 inch||0.010 inch|
|100||0.037 inch||0.004 inch|
|P120||0.035 inch||0.003 inch|
|P150||0.033 inch||0.002 inch|
|P180||0.032 inch||0.015 inch|
Abrasive belt wear differential
Abrasive belts change in thickness across their width due to uneven wear. This uneven wear can be the result of uneven part feeding or a differential in the cutting characteristics of multi-density products. The coarser the abrasive belt’s minerals, the greater is the potential for uneven wear. The following belt thickness data reflects information that I have personally gained by measuring various used belts across their entire width. New belts will generally measure within 0.001 or 0.002 inch across their entire width.
Source: Howard Grivna is the president of Sanding Systems Consulting Inc. and author of the book, "The Principles of Wide Belt Sanding & Data Reference Manual.” For information, call (218) 678-2929 or visit SandingSystemsInc.com.
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