There seems to be a huge amount of voodoo involved in setting up segmented platen machines. It seems the techs and the folks running them just don’t understand the concepts so they just play with the settings until something looks okay. Once issues pop up they play some more until it seems things are better. I am going to introduce some basic principles that will take away some of the mystery.
Segmented platen heads are designed with fingers or segments in the head designed to fire only specific parts of the platen down at one time. The segments are usually around 25mm wide but that varies from machine to machine. They can be fired down via pneumatic pressure or electro-mechanically. The segments respond to a sensor bar on the front of the machine that reads the parts coming in via rollers or infrared.
These segments are behind a thin stainless steel plate with dense felt glued to it. This creates a reasonably flat surface under the fired segments for a more even contact area. Over the felt is a layer of graphite cloth for friction reduction.
There are many variations on this theme. I will start with different head types.
Cross belts are primarily used for removing veneer tape running with the direction of the feed, but they work great for leveling edge banding and slight high spots. These heads are also used for conventional sanding of pieces that only fit in the machine cross grain.
When using the cross belt for removing veneer tape and edge banding or leveling, the cross belt should be set up with very light pressure to skim across the surface at high speed. A wide foot print works best with light over all pressure.
More conventional pressures are used for sanding parts with the cross grain. The trickiest part is setting up the foot print on the edge facing the direction of the belt rotation. Once I elaborate on the actual set up this will be easy.
These heads are a great way to remove edge banding that is sticking up to avoid getting the glue in the later finishing heads. This protects the quality of the sanding and color consistency.
Drums for veneer usually have the ability to pop up and down according to the settings in the computer. These drums are usually fairly soft. These drums are not for dimensioning veneer, but for removing tape and high spots just like the cross belt. Only difference is the drums remove tape running cross the feed better than anything else. Light pressure is an absolute must as most veneer has variation in the thickness of the substrate. Good vacuum hold down is a must with this type of head on veneer. I do not expect a head of this type to contact the entire surface of every part. Setting the pop up and down of this type of head is very straight forward.
Platen heads are the work horse of the segmented platen machine. They are mostly used for conventional sanding of raw wood or between coats on finish. Most parts run through a wide belt machine will have the grain running parallel to the feed. This is the type of head that would sand these parts. These heads all have the ability to pop up and down according to the settings in the computer. Some of them have the ability to add or subtract segments on the sides for more or less sanding foot print. Some just change the percentage of pressure on the segments corresponding to the edges of the parts. Most segmented platen machines are based on this type of head.
Super finish platens work exactly the same as a regular platen head but they use a Chevron belt between the platen and the sanding belt. This belt actually does not usually have “Chevrons” on it, but just angled pieces of felt that help break up the scratch to make a less noticeable final scratch pattern. They do as intended but they muddy the sanding foot print so proper set up is actually more difficult on these types of heads.
Set Up for Raw Veneer
This is what separates the techs from the fiddlers.
The abrasive belt speed will be 12 to 18 meters per second normally.
We need to properly set up the segments to fire to contact the entire footprint with absolutely perfect pressure over the whole surface. No point on the surface should have more or less pressure than any other. We don’t want less pressure on the edges. We want the same as the entire surface without over pressure.
I start just like many techs before me. I lightly pencil mark the entire surface of a piece of veneer and set the height of the machine to the thickness of the part. I only run one head at a time.
I set the footprint to leave a halo of pencil mark all the way around the part. In some machines a bigger number means more sanding and a smaller one less. Some machines use a smaller number to designate a shorter amount of distance passes before the action takes place. You must know your machine. I subtract pressure off the outside segments if available or I will subtract segments if required.
The next step is crucial and this is where other techs lose it. Lower the overall pressure enough that the part will not be sanded the first pass through. Run the part and confirm. Slowly raise the pressure until you see the pencil marks are smeared in the middle of the part and left alone on the outside edges. This is important that you smear it and not entirely remove it. This is how you establish how much pressure you have on the part. You had enough to smear the pencil mark.
Now that you know the exact footprint of your program and exactly how much pressure you have on the part, you can start working the footprint out until you smear the pencil mark on all four sides. You don’t have to totally remove it. Just smear it and now you have nearly perfect pressure over the entire surface.
Once you have your footprint established you can turn the overall pressure up a little bit until you can just remove fresh pencil marks over the whole door. It actually isn’t a bad if you leave a tiny bit behind, as long as it isn’t in the same spot over and over again. Try with different parts.
If your set up is right you will be able to look at the veneer on edge and confirm it is the same thickness all the way around the piece. Your eye can pick up slight variations as you look around the part.
I have run cherry veneer through a machine a dozen passes without burning through with this set up in a quality machine. I have sanded veneer so thin you could see through it entirely and not burned through to the glue.
Set Up for Between Coats
This set up is the key to successful sealer sanding with a segmented platen. I use the exact same methods to set it up. The pressures will be lower and the MPS of the belt will be much slower, usually 5 to 9 meters per second. It really isn’t magic with a proper footprint.
The biggest mistake I see is that the platens with adjustable height are set too low so that the platen is hitting the part without ever firing down. It would be acting like a standard fixed platen. A segmented platen should have at least 1mm of clearance between the part and the unfired head. I usually run a part through and confirm that none of the platen heads are touching without being turned on or fired down.
Make sure to leave enough room between parts on all sides so the platen can retract before the next part comes under the platen. Also make sure when running frames to leave the vacuum off so the abrasive belt is not sucked into drawer boxes or pockets in the parts.
The concepts in this article are meant as a starting point and guide to better understanding how these heads work. Once these concepts are put into use there may be a few tweaks required to achieve a perfect footprint. I have had extraordinary success with this technique.
SurpPrep Abrasive Specialist