SANDING MACHINES ARE CONSIDERED SIMPLE BY MANY - UNTIL IT COMES TO BUYING ONE
Time and time again we hear that buying the latest high-tech CNC machine was a breeze by comparison, so what makes the selection process such a challenge? Surely all sanding machines are created equal, yet the price difference can be huge – so what makes them so different? The area of sanding is driven by application know- how more than any other machining process in the shop and the expectations from the sander have grown proportionally with the equipment levels found on most modern machines.
Not that long ago, machines were much simpler. The standard twin-head roller/pad machine that dominated the market for so long was manually controlled with fixed cutting speeds and the expectation from users were pretty straightforward as well. They used it mainly to sand solid wood, sometimes a bit of veneer. What else can a sander do? How the goal posts have moved. I often hear:
“We need to calibrate substrate prior to veneering, sand solid wood as well as veneers and sand between lacquer coats on the same machine - can you help?
“Can you sand MFC to prepare it as a base for lacquering?”
“Can you prepare my high gloss parts before buffing? “
“Our veneers are so thin you can read a newspaper through them - can you sand them?”
“We run a print line at 50M/min, can you sand the base coats?”
This is just a small sample of the kinds of questions sander manufacturers get these days and the resulting recommendations will inevitably include electronically controlled segmented pads, frequency invertors, air- jet belt cleaners, brushes and vacuum tables - options that make the earlier sanding machines look prehistoric!
Let’s consider some aspects of the selection process: Once the simple criteria of width-capacity has been established, another basic consideration is whether or not the machine will operate as a stand-alone machine or inline. The two basic types of construction fall into moving table models or constant passline. With the moving table the feed bed is adjusted up or down to accommodate work piece thickness, whereas with the constant passline the feed table remains static and the sanding heads are raised or lowered. The moving table concept is often less expensive and most entry-level sanding machines tend to be of this design, but it’s not suitable for line integration. Constant passline machines have be- come more popular, even when operating as stand alones, as they make it easier to place fixed height infeed and outfeed tables to better support longer or heavier components.
The next selection determines the number and type of sanding heads. This needs to be done taking into consideration the following factors:
• applications
• input quality
• output quality
• required feed speed
Sanding applications usually fall into two categories. When a defined thickness/stock removal/tolerance is needed, it’s usually referred to as ‘calibration sanding.’ An example might be to sand the joints flush on a five- piece kitchen cabinet door. Alternatively when sanding only to upgrade a finish, and where calibration may not be necessary, then this process is generally referred to as ‘fine sanding.’ An example might be sanding a decorative veneer pressed onto MDF.
There is a wide range of sanding head types and machinery manufacturers each have their special variations. But generally speaking, contact rollers (profiled steel or hard rubber) would be used for calibration, and pads would be used for fine sanding. In certain situations softer rubber con- tact rollers can be used for finish sanding, but pads are more usual.
So far, the selection process would have been much the same 20 years ago, but now, particularly with the choice of pads available and in conjunction with touchscreen operation and computer control, we see big differences in the overall capability of the sander.
Perhaps with the exception of straight forward solid wood sanders, where rigid or air cushion pads are still used, the modern finish sanding machine is likely to be equipped with segmented pad technology.
As opposed to one continuous bar providing pressure on the back of the sanding belt, the ‘electronic pressure beam’ comprises a row of individual segments making up the working width of the machine. The segment width varies from one manufacturer to the next. Parts entering the machine are scanned to measure part dimensions and this information is input to a processing unit. Outputs then control when and where the segments are activated and in this way the part is sanded tip to tip, without risk of damage to sensitive veneer or lacquer edges.
The segmented pad, which can be either an air-operated system or using magnetic solenoids is used in three pad types.
The standard, wide-belt pad is simplest and in many cases is sufficient, but the use of an inner chevron belt brings many advantages.
In a wide-belt head, the use of the inner chevron belt enables finer abrasive belts to be used particularly for lacquer work and preparation for high gloss, but its ability to eliminate oscillation lines and create a short scratch length also make it the choice for the high quality sanding of veneered panels and solid wood sanding applications. A shorter scratch length is preferable to minimize the visibility of sanding lines on cross grain for example.
The Chevron belt turns slower than the abrasive belt so the pressure zone is always changing. As the sanding belt advances the pressure is constantly applied on new abrasive.
The cross belt is always a segmented pad and always uses an inner chevron belt. Once again, the fact that the speed of the chevron belt is different from that of the sanding belt, allows for the use of very fine abrasive belts. That and the cross belt’s longer length, make it extremely suitable for high gloss and lacquer preparation. The more common use of the cross belt head is as a pre-sanding tool before the use of one or more wide belt heads for veneer sanding. In this application, by sanding at 90 degrees to the grain, it shears the fibres cleanly and levels any undulations due to wood density variation from early to late-wood and creates an extremely flat part. Similarly, because it’s length prevents premature clogging of the abrasive, the cross belt is useful for removing paper tape or excess glue in the veneer pressing operation. When veneers are jointed by edge glueing, they present parallel glue lines to a wide belt head and this often results in sanding defects from the glue rings that appear on the sanding belt. The cross belt pro- vides the solution to this problem by pre-sanding the part across the glue line.
Of course the obvious application for the cross belt is to sand long parts that are veneered across the short dimension. An entrance door veneered horizontally for example. In this case the cross belt is used to finish by sanding with the grain direction on its own. It should also be remembered that sanding pads, of any type, only perform well on flat parts. For solid wood this normally means pre-calibrating the parts with one or more contact rollers.
By careful arrangement of the sanding heads, more than one application type is possible in a single machine. Heads can run independently and are selected by the program. By controlling the cutting speed the capability of the head increases further. Slower cutting speeds are needed for lacquer, and increasingly faster speeds for veneer and solid wood sanding.
The range of applications helps us determine the number and type of ‘heads’ on our sanding machine. With sanding, so much is dependent upon the abrasive tool and this is a huge subject on it’s own. Manufacturers often use the ‘skip a grit’ rule as indicated below and this will help when we consider the input and output quality parameters.
By understanding the application, and with which grit we must start, the output quality will either determine the number of heads that are needed or the number of passes that need to be made.
Of course a major aspect to the selection process is the budget, and to keep machine, floor space and utility costs down, smaller companies often find it practical to change belts and pass parts for a second time on single or twin head machines, but for maximum efficiency a single pass is obviously preferred.
Recent developments in sanding machine design have also impacted the very frames on which they are made. The reduction of vibration is critical to produce a defect free surface and the latest technique being used is to mould a complete machine base in special vibration absorbing material.
With all this information considered, we start to understand that the sanding machine of old has evolved into a hi-tech sanding centre and that there can be big variations in price and specifications from one manufacturer to the next. There is more to sanding than meets the eye and therein lies part of the problem. Everyone sees and also feels the sanding results differently. One thing is for sure, sanding machines are not created equal and the buyer who considers them all to be the same ‘metal box’ will be in for a big surprise!
Christian Vollmers is the president of HOMAG Canada Inc. He can be reached at Christian.Vollmers@ homag-canada.ca or visit the website at www.homag-canada.ca
