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Sanding pads - hard or soft?

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How about some clarification on the use of hard and soft sanding pads? Is there anything more to it than 'use soft on bark"? When does one use each?
TYIA
 

Steve Worcester

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(Sorry, I got your email but neglected to respond)

One way I would suggest is to use whatever mandrel you like, then vary the density by using an interface pad.
Use a thick soft pad for anything where you have an undulating or uneven rotating surface, say natural edge, or warped form. I also use a soft pad when I can't quite get the piece recentered.

Use a medium or hard as a sacrificial pad or when you want to remove more material. The problem with that is that most hardwoods in north america have a lot of quick growth between the early wood and late wood and they are varying density. You sand even across that surface or for too long and you get a surface variation that amounts to a ripple between the rings that can be felt as you pass your fingers over it. But the only way around that is to cut cleaner and have less sanding.

Perhaps lesser thought of is to run the sandpaper in the opposite direction of the spinning wood. It will create more scratches but also remove more material. Then spin it in the same direction with that grit to lessen the scratch pattern.

One issue that will come up is the soft pads don't last as long. There are bigger bubbles in the foam and when removing the sandpaper, eventually you rip the hook and loop layer right from the surface. They can be repaired a few times using solvent based contact cement, but I would not recommend CA glue or GOOP (or it's cousins) as they are not as flexible and will alter the density and flexibility of the pad.
 
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Hard backing makes for cooler sanding, because the squirm in a soft pad under the same pressure (DON'T!) will create additional heat. It is also more long-lasting, because stiffer adhesive resins can be used, and they do not throw rogue grit as easily because of cracks in the adhesive caused by flex. I find Steve's argument that they promote ripples over annual rings spurious. They bridge those areas, rather than follow contour like softer backings, removing the high points before contacting the low, just as a gouge or plane. If you use a supported sander, you can actually work areas so soft they take fingernail impressions without dishing them in.

The argument for reverse sanding is another of the strange myths turners toss around. Since the paper is rotating under power it produces an arc of a radius determined by disk size/speed and piece rotation. Sanding from nine to noon makes an arc in this approximate direction /, sanding from six to nine an arc similar to this \ . Reversing your drill or your piece will still yield the same patterns, only at different times. When you take the final hand sanding on your static piece, sand along the grain. Set with water, see if you have a "nap" and sand against it if you do.

If you're going to static sand in your lap, use softer backings. Hard backs are simply too aggressive. That's a plus when the piece is rotating, but it can cut pretty deep if you're careless on a stationary piece.

Technique? When sanding on a static piece, sand from sound wood into unsound, or into the air. Only go halfway into either, so the paper won't tilt around the corner or dig in.
 
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For bowl turning, I like using power-lock discs for course sanding, cuz the extra firm backing seems to help transition into the bottom of bowls better without creating a flat/nipple. Then I switch to a softer pad for 180G and up.

Hutch
 
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interface pads.

Got some interface pads from vince (VincesWoodNWonders.com) The firm ones (orange). with slite use deformed so the foam under the material holding the velcro became smaller. It did this on both interface pads I used.The velcro material then is unsupported leading to some cracking of the sandpaper at the perifery. It also makes removal of the sandpaper harder without tearing the velcro from the backing material. (The same cracks I get with oversized sandpaper (I.E. 3 1/8" o a 3" interface pad). I asked Vince and he sent some replacements with a med firmness. Haven't used them yet. Is it normal for this deformity to occur????
He also told me to use the firm ones when getting out any tool marks (now how did he know I have "an occasional" tool mark?????), and then can use the softer ones for sanding out previous sanding marks. This has helped me out alot . Gretch
 
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Michael Mouse has got it right except he forgot to point out that sanding with soft discs or by hand can cut into "punky"spots, thereby creatlng depressions which are difficult or neary impossible to remove by sanding.

I found this out in the worst way early on in my woodturning and purchased a hard-backed "system."
 
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uneven sanding

Michael Mouse has got it right except he forgot to point out that sanding with soft discs or by hand can cut into "punky"spots, thereby creatlng depressions which are difficult or neary impossible to remove by sanding.

MM mentioned this a several months ago in another thread-a fact I was unaware of. Gretch
 
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Vince does a nice tutorial on his web site about sanding. I use a firm pad almost exclusively. I found out that a 120 grit disc on a firm pad cuts faster than an 80 grit pad on a soft disc. Soft discs are worthless on a random orbit sander. I prefer the firm pads from Vince with the quarter rounded profile. They work a lot better on the inside of the bowl, especially in the transition area.

I am aware of two different soft interface pads, the 1 inch thick ones, and Vince's thinner ones. I don't know if there is much difference in how they perform, other than one compresses more before it is riding on the firm pad. If the soft pads have any advantage to the firm ones, it comes in the transition area where it is easier to get a soft one to conform to the curve, but even then, there will be areas with more pressure or less. Most of the time, if I angle the disc properly, it will fit into the transition fine. The sharper the transition is, the more difficult it is to sand out. If it starts to bounce, I slow the disc down a lot, and ease up on the pressure. Just how much pressure to use is hard to define, but most of the time, just the weight of the drill is enough, but too much for soft and punky woods.

As to a softer pad having the potential to generate more heat than a firm pad, I haven't noticed it, and would think it would be the other way around. Higher speeds, both wood and sanding disc, while using more pressure will generate more heat. To prevent dips and round overs, you have to use a very light touch, and never let the disc stand in one place. You have to support the sander with your hands, rather than let it rest on the wood. For natural edge bowls, I will sand the even part with the lathe spinning, and for the natural part, I will turn the lathe off.

For sanding near the rim, I prefer to have the disc spinning away from the rim of the bowl. On the inside, I have the drill turning counter clock wise, at the 9 o'clock position, on the outside, still the same spin on the drill, but have the drill at the 7 o'clock position working on the down side of the pad. If you have the disc rotating into the rim, you can get catches.

robo hippy
 
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As to a softer pad having the potential to generate more heat than a firm pad, I haven't noticed it, and would think it would be the other way around. Higher speeds, both wood and sanding disc, while using more pressure will generate more heat. To prevent dips and round overs, you have to use a very light touch, and never let the disc stand in one place. You have to support the sander with your hands, rather than let it rest on the wood. For natural edge bowls, I will sand the even part with the lathe spinning, and for the natural part, I will turn the lathe off.

Your observations are counter to generally accepted physics. FWIW, http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html

More deformation, more resistance, etc. Don't press.
 
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"Rolling Friction
In mechanics, friction plays a major role both in the laboratory and industrial worlds. Friction is the resistance to the sliding, rolling, or flowing motion of an object due to its contact with another object. Rolling friction is caused primarily by the interference of small indentations formed as one surface rolls over another. This is the idea behind the frictional forces involved with wheels, cylinders, and spheres. In the ideal case of the wheel, we must first look at the forces acting on the wheel. In pure rolling motion friction is required to start, stop, change the motion of a wheel. Below we can see the frictional force necessary to begin motion, and get the wheel moving at a velocity v. In pure rolling motion, friction causes the wheel to catch and stops the sliding and slipping motion; for example when a car spins its tires, slipping is taking place, thus the frictional force works to stop the spinning out and causes the tires to catch and begin pure rolling motion.




The frictional force, f, the force required to slow the wheel produces a torque which tends to decrease the angular velocity, w. (The normal and gravitational forces produce no effect because their line of action is through the center of rotation.) However, the surface could not possibly have such an effect on the wheel once the wheel has achieved pure rolling motion and constant angular and linear velocity. Zero friction occurs only for horizontal motion at constant velocity, but it is non-zero for any case in which acceleration is occurring parallel to the direction of motion of the center of mass, as when the object is rolling-without-slipping up or down a sloped surface. If we consider the rotation as being about the center of mass of the object, then the frictional force must be in a direction to provide the torque necessary to decrease or increase the angular velocity, depending on whether the object is accelerating or decelerating, respectively. Note that the friction can be in the direction of motion (rolling downhill) or opposite to it (rolling uphill). In pure rolling motion there is no sliding or slipping, thus the contact points have no relative motion (no relative velocity). This results in a frictional force of zero. Therefore, the wheel will roll forward with constant velocity, v = Rw, where R is the radius of the wheel.




In the actual case of the rolling wheel, the free-body diagram is much different. Both the wheel and the surface will undergo deformations due to their particular elastic characteristics. At the contact points, the wheel flattens out while a small trench is formed in the surface. The normal force is now distributed over the actual contact area rather than the point just below the center of the wheel.


Furthermore, the wheel takes on a sort of plowing motion resulting in increased deformation at the front of the wheel, while the rear of the wheel undergoes little deformation which results in the majority of the normal force being located at the front. When the wheel and the surface deform there is a minute amount of slipping, but the majority of the force is due to static friction. The overall rolling friction results in a force at the center of the wheel and is parallel to the surface of contact, and is represented by the equation:



This resultant frictional force is still rearward acting (tending to contribute to the angular velocity), but the new distribution of the normal forces creates a net torque negating the rotational contribution of the friction and causing an overall deceleration of the wheel’s forward velocity. Also, the major source of energy losses during rolling motion is primarily due to the deformation that occurs when the two surfaces are compressed and relaxed. This continual process generates heat in both the wheel and the surface."

I agree.
 
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I don't see how a rolling tire on an axle applies to an abrasive disc on a drill. For me it is more like starting a fire with a stick and a bow to make it rotate. You need the right combination of pressure and speed to generate the heat to produce an ember. You can press as hard as you want, and if you are not spinning fast enough, you won't get an ember. You can spin as fast as you want, and if you do not have enough pressure, you still will not get a coal.

The softer pad, unless you really bear down on it to the point where it is totally compressed against the firm disc/mandrill, will not generate as much pressure/heat as a firm pad with the same pressure.

robo hippy
 
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Individual acceptance does not change reality. Hope the excellent diagrams of the source and direction of distortion in the referenced URL explain the phenomenon for Gretch and Robert. Way to demonstrate it for yourself, of course, is to load the soft pad in a working direction on a static surface (static versus rolling friction) and see how it distorts with pressure.
 

Steve Worcester

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While a soft pad will conform better to a corner, it will also work better on a warped surface. A hard pad has no room for deflection (like a car across ruts) and will bounce across the surface. A soft pad has better rebound and will better conform to the surface in some cases. A soft pad will not work as effective on a random orbit because it twists more with the RO action in each direction.
 
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Hmmmmm. Still does not quite translate to me.

The soft pads will deform. There is as much deforming from torque as there is from compression. Old soft pads will have a line around the middle of the pad where the foam breaks up from the twisting of the pad due to friction/resistance as the pad spins. From the explanation, deforming puts more surface on the wood as it spins, and more pressure will put more abrasive/more distortion into play. How ever, when using the softer pads, you use almost no pressure. I never have had heat problems with the soft pads, on the abrasive disc, the pad, or the wood. I have had some with the firm pads. So, is it that the amount of pressure makes as much difference as the compression/distortion?

robo hippy
 
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Read the other part of the URL and you'll see the standard friction equation. Coefficient of friction times normal force. The distortion effectively increases the normal force in the areas shown.

Other disadvantages, as you mention, are the fatigue of the mandrel material from continual flexion-extension. It can't move until it overcomes the local static friction, therefore it deforms. Then it expands somewhat as the forward force exceeds the static friction, becoming rolling or sliding until it encounters an area of increased coefficient or experiences increased pressure which deforms it, and so forth. Note the deformation also comes from change in workpiece contour, or contour mismatch, which is why you really can't put three inches of foam into a two inch space without penalty, something Steve did not mention.

Then there's the paper, whose backing is not as flexible as the mandrel to which it's attached. It can't keep up with the size changes, nor stop them well enough, and so makes those little folds and lumps we all love, not to mention kicking grit loose, because the cement isn't even as flexible as the backing on the paper.

No free lunch, surely. Cheaper fare, in my experience, when using more rigid paper and backing. People who support the sanding disk on the work will always have more friction than those who support the sander independently, which is another way of decreasing flex and friction.
 
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I don't see how a rolling tire on an axle applies to an abrasive disc on a drill. For me it is more like starting a fire with a stick and a bow to make it rotate. You need the right combination of pressure and speed to generate the heat to produce an ember. You can press as hard as you want, and if you are not spinning fast enough, you won't get an ember. You can spin as fast as you want, and if you do not have enough pressure, you still will not get a coal.

The softer pad, unless you really bear down on it to the point where it is totally compressed against the firm disc/mandrill, will not generate as much pressure/heat as a firm pad with the same pressure.

robo hippy
I agree with you. I prove it o'er and o'er again... daily. Speed and pressure causes heat. You get the pressure with a firm pad, and a lot less pressure with the soft pad, because you'll ruin it, applying too much pressure. :)
 
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I agree with you. I prove it o'er and o'er again... daily. Speed and pressure causes heat.

Perhaps you rushing to complete an urgent order? Speed over surface does not affect friction. Increase in the normal force (pressure) does. When you're trying to prevent or cure problems, take Physics over perception every time.
 

Steve Worcester

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Perhaps you rushing to complete an urgent order? Speed over surface does not affect friction. Increase in the normal force (pressure) does. When you're trying to prevent or cure problems, take Physics over perception every time.

I'm not a big fan of physics, I guess I mostly can't wrap my brain around it.

Maybe when we spin the disk faster, we inadvertently push harder?
 
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Sanding is more complicated than simple friction, there is also cutting action going on. Friction calculations should apply quite well to worn out sand paper but cutting with good sandpaper is a different animal.
 
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I'm not a big fan of physics, I guess I mostly can't wrap my brain around it.

Maybe when we spin the disk faster, we inadvertently push harder?

BINGO! If you're supporting the disk on the work, you probably do. Support the sander on the toolrest and you don't have to.

Of course disk or work, makes no difference, relative velocity is relative velocity. Look at those folks with those screaming wind sanders. They don't burn the place down.
 
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Vince is redoing his web site and said he would be down for a few days. Don't know how long that is though.

As far as the high speed sanding, there was an article in Woodturning desigh a while back about some one who used a Makita grinder that spun at 10,000 rpm. He slowed it down to 6,000 for sanding his bowls. Well, I dug out my high speed angle drill (3200 rpm) and tried it out again at full speed. I am back to sanding at slower speeds, in the 600 to 1200 rpm range.

One problem I had is that my bowls are warped, so I can not support the drill on a tool rest, I have to use my arms and body as a spring loaded pressure bar to feel the wood as in moves in and out. This makes it extremely difficult to keep a feather light touch. Without that light touch, you get over heating on the abrasive, pad, and wood.

It is easier to sand at high speeds on flatter forms, with little transition area. Almost impossible to do with deeper forms with a sharper transition. The slower speeds work just as efficiently as the higher speeds, and don't throw the dust about as much as the slow speeds do. You do not have to push the drill/sander into the wood, kind of like when you are cutting with your tools, just let it cut at it's own pace.

robo hippy
 
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