It’s like a plague to all the wide belt users everywhere. My machine won’t sand level! It’s tighter on one side than the other. Why won’t this stupid thing stay square?
Let’s define square in a wide belt sanding machine.
It means that the table is square and parallel to the machine frame, or the moving machine frame is square and parallel to the table, AND the heads, shoes, hold down rollers, and platens are square and parallel to the table.
The machine has a lifting mechanism that either lifts the table in relation to the frame, or in a constant passline machine the lifting mechanism lifts the frame of the machine so the table stays level with infeed and outfeed conveyors. There are variations on this theme.
If the jacking screws in this system are in good working condition the lifting action will be very accurate and consistent corner to corner. The screws are machined to very high tolerances. Even a smaller jack screw can hold very high pressure, literally tons of force. They can’t just jump a thread unless they are completely destroyed. They are usually timed to one another via a chain running around each jack screw, although there are a few exceptions.
If these jacks are so strong, how do they end up out of time?
I’ve heard endless speculation on how this happens.
Most folks just figure that someone ran something in too thick, but that is rarely the source of the issue. Running something in too thick can damage the machine, but it has absolutely zero chance of defeating a healthy jack screw. It can however, dent or bend the bed plate, breaking internal welds and destroying the bed if the situation is severe enough. This is not common but if the heads aren’t running and the pressure in great enough it can do unbelievable damage.
Since the jack screws are so strong, the only real way to get them out of time is to skip links on the jack chain. This is where you stop one or more screws and the others keep moving. There are a few ways to achieve this. One is a loose or stretched jack chain and/or worn sprockets. This can be from wear or repeated abuse. The second is damaged and under-lubricated jack screws.
The third way is to close the machine on a part that is still inside the machine. This is usually the culprit and it can be extremely destructive. I have seen machines nearly completely destroyed by operators who fail to check to make sure the machine is clear before moving the machine to a new part thickness. The reason this is usually the culprit is because the jack screws and chain must be moving in order for the chain to jump links on the jack screw sprocket. One or more jacks must meet so much resistance that the screw stops and the chain pops over the sprocket. Closing the machine with parts still inside will cause stretched jack chains and damaged jack screws so the more in happens the worse it gets.
It is absolutely critical that the machine operator visually inspects the machine for parts still inside by looking through the gap between the heads and the conveyor to be absolutely positive that the machine is empty before setting the machine to a thinner size. They must do this every single time.
It is actually quite rare for heads to move on their own. Most sanding heads have locks to keep them from moving. Most shoes and hold-down rollers have locks or have enough friction to avoid movements over time.
If your machine is set up by a competent technician the table/machine frame will be set to certain reference points and that technician should be able to teach your staff how to check these reference points. This means that the moment and out of level condition is noticed this is the very first thing to check. If everything was right, and then it wasn’t, the worst thing to do is to align every head, shoe, roller, and platen to the new bed orientation. Instead the table/frame must be re-leveled to the reference points and then the machine can be back in service in less than an hour. I’ve seen customers do days of work destroying the calibration of every part of the machine instead of spending an hour fixing the real issue.
This article is a warning that you ignore at your own peril. Machines are damaged all the time by closing them on pieces still inside of the machine. It is completely avoidable and it costs companies millions of dollars in lost efficiency dealing with poor quality sanding and repairs.
