This is a story of a machine and two different ways of getting the job done. Both methods sound very similar at the outset, but the effect on the wood is not.
Recently, I visited a customer in Rochester, NY. I was working for Flex-Trim (my former employer) on brushes for a Quick wood machine and was addressing some of the customers concerns about sanding. The first thing I noticed about the product was the very hard polish on the doors. The brushes were having a really hard time breaking the surface back open. In fact, the orbital sanders were even having a hard time of it! I am way too nosey to leave it alone, so I asked to see the sander.
They have a newer Timesaver 3 head, with a drum, drum, combi-head. The grit sequence is 120-180-220. The required stock removal is .015” total, but this did not include imperfections in the joints on the doors and frames.
I talked to the owner about the polished condition of the doors and he knew it was causing some issues, but his main concern was reducing the amount of time it takes to remove the scratches from the doors with the orbital sanders. The idea was to make the scratches as fine as possible to reduce labor. The reality of the situation was that the sequence was making it harder to remove the scratches.
Let me explain.
When a belt is cutting cleanly, it will cut the part easily to a very consistent thickness (dependent also on the drum hardness). The wood will yield to the drum, with the drum exhibiting some amount of deflection. This deflection increases with the belt wear or sanding overpressure. This increase in sanding pressure also produces a modeled effect on the wood as the drum starts to conform to the grain structure of the part. This will produce high and low spots on the part according to the hardness of the wood, hardness of the drum, and the condition of the belt.
When the belt is polishing and smashing the wood grain, it will create a “work hardened” surface that will exhibit these highs and lows. This effect can leave a very polished surface with deeper scratches still left under the surface. This polished surface is mostly solid with scratches from earlier belts still there. .
Cutting a polished surface is hard, even for a orbital sander with a sharp disk. This means that your hand sanders or orbital machines will have to remove the hard, polished surface by grinding on it until they get down to the low spots and remove the scratch from these valleys as well. More time is required to remove all traces of scratch. Combine this with the inconsistency of an operator with a hand sander and the lack of aggressiveness of most orbital sanding systems, you end up with some areas of grain opened up, and others that will still be closed, because the operator just plain missed it!
The affect this has on producing orbital marks is a subject for another article. (Wouldn’t we all like to know where orbital marks REALLY come from?)
The grit sequence
On paper, the original grit sequence should easily be able to remove the required amount of material. A 120 grit belt can remove about .010” at normal feed speeds, the 180 grit can remove about .004” and the 220 can remove .002” without damaging it. This should be the best choice for this operation. After all, it matches the 70%-20%-10% model for stock removal perfectly. 70% is .0105”, 20% is .003”, and 10% is .0015”. According to all the big time sanding gurus, this is the perfect ratio and therefore the perfect grit sequence for this situation.
The reality of this situation is that the first head is completely maxed out, the second is not, but how long will a maxed out 120 grit belt be able to produce an accurate part thickness for the next head? The second belt will be maxed out in a matter of a couple hours at best. The first belt is sanding material that is reaching all the way to the bottom of the grains and it is creating massive pressure against the resin and producing heat. Both enemies of open grain in the wood and of sandpaper in general. What happens when the uneven joint comes into the first head? It can be as at least as high as the total stock removal (.015”) higher than the top of the part! Now the first head is removing .025” in a few spots on every part.
An optimal grit sequence will never max out every belt in the sequence. The lower grit belts will usually be taking closer to their maximums, but this is due to accuracy limitations in the machines and to the tiny amounts of removal required. If we optimize this grit sequence, then the stock removal ratios that all the “big time” sanding gurus advocate go where they belong, right in the garbage!
I am not one to overload a customer who is still warming up to the idea that a coarser grit sequence will fix his issues without causing more hand sanding, so I decided to recommend a small change to give his machine a better chance of success. First thing we did was to re-setup the current grit sequence. We set the first to take .010”, the second to take .004”, and the last to take .002” (The best set up for this sequence, but not for this application). This took some of the excess pressure from the last two heads and put it back on the first head. This will mean a period of lower belt life on the first head, but because the second two heads are only removing the scratch from the first head, they will survive longer and take the beating better when the 120 starts to fail. Before this change they were taking more material than they realized on the last two heads, so it had to be changed.
The surface of the part became much more open for stain. The wood had a noticeably more powdered and soft appearance, which indicates a lack of pressure and heat. This will not last. Larger changes need to take place to permanently fix the issues.
My starting recommendations for a grit sequence on this machine are as follows: 100, 150, 180. This sequence will handle most of the actual stock removal, but also leave a much more consistent color and an open grain. The 100 grit can remove .017” total, 150 can remove .006” and the 180 can remove .004”.
Now for the set up.
I am going to set up the grit sequence backwards. I know that I need to remove at least .002” to get rid of a 150 grit scratch, and I don’t want to grind down past the base of the 150 grit scratch pattern in order to leave the grain open for stain. Grinding down past this scratch entirely will force my belt to remove more than just the peaks and valleys of the scratch pattern but establish a new base of scratch in the solid material. This action would literally double my stock removal amounts without even knowing it! This is where the overpressure on the belt really starts to smash the grains closed and polishing starts to rear its ugly head.
At most, I will take an additional .0005” to .001” more than the average depth of the scratch. This accounts for inconsistencies in the abrasive. (no abrasive is perfect, but you knew that.) So my last head must take about .0025” to .003”.
My second head must take at least .005“ to remove the 100 grit scratch. I will not worry about taking any extra on this belt as this should be enough. We are not removing anything lower than the original scratch pattern.
My first head will be taking whatever the first two heads don’t. We really don’t know the exact amount. We never did! The differences in the part thickness and joints will change this number constantly. If we actually take .015”, as we wanted to remove with the original sequence. Then the last two heads will be removing about .008”. That is more than 50% of the total! (I think all the old school sanding gurus just feinted!)
Does this mean that we are set up wrong? Or does it mean that we have much more capacity than we are using, and it is set up for the correct scratch pattern, instead of some silly ratio?
Ultimately, the machine dictates the final grit sequence. Although, my customer was hesitant to try it, the machine was well suited to running 80-120-180. This is a radical departure from the original grit sequence, but it takes the machine to a whole new level of performance. :The life of the belts would be dramatically increased, leaving the open finish we desire for the entire life of the belts.
The last head is a combination drum and platen. This allows for two different contact points and a much easier removal of the scratch from the second head. This is especially handy when you are jumping two grits.
The scratch from a 120 grit belt is about .004” depending on the hardness of the drum. This is the maximum for the 180 grit belt and taking that much with a platen just doesn’t work well at all. Having a drum on the last head allows for easy removal of this scratch. The platen only needs to remove about .0005” to get rid of the 180 grit drum scratch. In this configuration the platen can run for months without replacement. Sometimes even without adjustment on the bladder type platen!
The 120 needs to remove at least .008” to get rid of the 80 grit scratch. The 180 removes .004”. That is .012” total on the last two heads for a proper scratch pattern.
Now when we put a part through the machine and remove .015”, the first head only took 20%. ( I can hear the old timers yelling from here! “He’s crazy!!!!!”)
Lets remember the deviation on the parts that inspired a lot of this in the first place. We are removing .030” in some spots, not the .015” over the majority of the surface. Now the first head is removing .018” in some spots and .003” in others. Now, do you see what I am trying to get across? Suddenly the ratio almost fits, but only when we reach the top end of the grit capacity. Who would want to push this to the limit all the time?
The 80-120-180 grit sequence is great for machines that have a drum or the combination head in the last position. It is not well suited to machines that have only a platen in the last position. These machines would do better with the 100-150-180 sequence as it only requires .002 or .003 from the last head. This is much more achievable with the platen. Slower feed speeds will also be required to allow the platen time to work.
The net benefit from either the 80-120-180 or the 100-150-180 grit sequence is a much more open scratch, much less time sanding by hand to remove the scratch pattern left by the wide belt as it will not be polished or hardened at all. Tip of the scratch peaks to the bottom is only .001” and it is the easiest sanding most folks have ever seen. It is typical to reduce cross grain sanding from several minutes a door to under 10 sec. per rail. Swirls are massively reduced if the sander sands only to the bottom of the scratch. (Oops, I am giving away my next article!)
I hope this article will give you some ideas on how to best achieve your finish requirements. Paying attention to the minimums to remove a scratch are just as important as the maximums to achieve an open scratch. Establishing a base scratch with the first head and then removing only that pattern is the key to opening wood grain and keeping it open.
