October 04, 2016

 

What causes grain to raise up on boards we've just sanded?



So-called raised grain is due to excessive pressure during machining, compressed softer cells, and then spring-back.

I do not know how many times I hear from woodworkers, both large commercial operations and small one-shop operations, who report that a surface was very smooth, but then a few days later, the surface was no longer perfectly flat. Oftentimes, this “un-flat-ness” shows up after a glossy finish is applied, as such a finish will reflect even the slightest imperfections. So what is going on?

There are two possible reasons why the surface changes.

First is that we have uneven moisture within the wood when it is first made, we prepare a perfectly flat surface, and then we have a moisture change. With the moisture change comes uneven shrinking and swelling. This situation arises on a gross scale. That is, a door will warp, a table top will distort, and so on over a long distance -- a foot or more. Obviously, the cure is to get the moisture uniform within the piece prior to manufacturing and then keep the moisture content from changing appreciably. This is not discussed further here.

Second is much more complex. To understand this second cause, we need to go back to the structure of wood. Basically, wood is made of very tiny cells, much like a mini-soda straw, with a length of 3 to 5 mm and a diameter 1/100 of the length. Within the growth ring for a given year, many species have the fibers that are formed in the springtime with very thin walls and somewhat low strength. However, as the growing season progresses, the cells have thicker walls with more strength. Perhaps a prime example is in pine where each annual growth ring has different color due to the variation of the cell walls.

End grain of southern pine shows the distinct light-colored, softer, weaker earlywood and harder, darker, stronger latewood.

Whenever a knife, sawtooth or even sandpaper passes over the surface of wood, the forces generated can actually compress the softer earlywood cells. This can happen when veneering (especially with the pressure bar that is used to prevent lathe checks), sawing (especially with a dull circular blade), planing (especially feed rolls and pressure bar), and sanding (especially with dull sandpaper).

In essence, the knife or blade and sandpaper particle asks itself “Is it easier to cut this fiber off or push it down and out of the way?” Obviously, the more pressure generated, the more likely that considerable compression will develop and the weaker cells will be compressed.

The compressed cells (remember that wood cells are like hollow soda straws), may recover or spring-back slightly toward their original size (much like a gently bent piece of wood will spring-back to flatness when the pressure is removed). However, with high pressure, many cells will stay permanently compressed. Actually, I should say “temporarily compressed” as with exposure to moisture (liquid such as from water in a finish, or vapor from a high humidity), most of the compressed cells will recover or spring-back close to their original size.

However, remember that the growth rings have both hard cells that were not compressed and softer cells that were compressed. So only the softer cells experience spring-back. The net effect is that a flat surface now develops small “hills and valleys” within the growth ring.

This similar effect can be illustrated with flatsawn lumber where the hard cells are pushed down into the softer cells right underneath during machining. With exposure to moisture, the softer cells underneath spring-back, giving a ripple or corrugated surface.

So-called raised grain is due to excessive pressure during machining, compressed softer cells, and then spring-back. In this case, the effect was so severe that the there is an actual separation between the growth rings.

Practical approach

It is impossible to control the veneer or lumber manufacturing process. Plus, with the presence of water in the living tree, it is likely that spring-back will occur during this initial manufacturing. However, excessive pressure at the end of drying or when planing or sanding can indeed cause the defect in such products. Even hand sanders with dull paper (which means lots of hand pressure) can compress the cells. So, to avoid this defect, first make sure that before final sanding all products are exposed to a very brief water misting or stemming to recover any collapse. Then finally sanding needs to be done with the sharpest paper and the lightest pressure possible.


April 28, 2016

 

Learning to Sand Wood by Experience


The objective of sanding wood is to remove mill marks, which are caused by woodworking machines, and to remove other flaws such as dents and gouges that may have been introduced in handling. The most efficient method of doing this is to begin sanding with a coarse enough grit of sandpaper to cut through and remove the problems quickly, then sand out the coarse-grit scratches with finer and finer grits until you reach the smoothness you want – usually up to #150, #180 or #220 grit.

You’ll have to learn by experience what works best for you.

How Fine to Sand
It’s rarely beneficial to sand finer than #180 grit.
Film-building finishes, such as varnish, shellac, lacquer and water-based finish, create their own surfaces after a couple of coats. The appearance and feel of the finish is all its own and has nothing any longer to do with how fine you sand the wood.
Oil and oil/varnish-blend finishes have no measurable build, so any roughness in the wood caused by coarse sanding telegraphs through. But these finishes can be made ultimately smooth simply by sanding between cured coats or sanding each additional coat while it is still wet on the surface using #400- or #600-grit sandpaper. It’s a lot easier doing this than sanding the wood through all the grits to #400 or #600. (See “What Is Oil?” in issue #154, April 2006, for a more thorough explanation of both processes.)
Only if you are staining or using a vibrator (“pad”) or random-orbit sander does sanding above #180 grit make a difference.
Fine sanding. Sanding finer than #180 or #220 is wasted effort in most cases, as explained in the text. In fact, the finer the grit the wood is sanded to, the less color a stain leaves when the excess is wiped off. In this case, the top half was sanded to #180 grit and the bottom half to #600 grit. Then a stain was applied and the excess wiped off.
The finer you sand, the less stain color will be retained on the wood when you wipe off the excess. If this is what you want, then sand to a finer grit. If it isn’t, there’s no point going past #180 grit. The sanding scratches won’t show as long as they are in the direction of the grain.
Sometimes with vibrator and random-orbit sanders, sanding up to #220 grit makes the squiggly marks left by these sanders small enough so they aren’t seen under a clear finish. Sanding by hand in the direction of the grain to remove these squigglies then becomes unnecessary.


Squigglies
. Random-orbit sanders are more efficient than vibrator sanders, but they still leave cross-grain marks in the wood. I refer to these as “squigglies.” The best policy is to sand them out by hand in the direction of the grain after sanding to the finest grit, usually #180 or #220, with the sander. Doing this is especially important if you are staining.
In all cases when sanding by hand, it’s best to sand in the direction of the wood grain when possible. Of course, doing this is seldom possible on turnings and decorative veneer patterns such as sunbursts and marquetry.
Cross-grain. Sanding cross-grain tears the wood fibers so the sanding scratches show up much more, especially under a stain. The best policy is to always sand in the direction of the grain when possible. The scratching that does occur is then more likely to be disguised by the grain of the wood.
Cross-grain sanding scratches aren’t very visible under a clear finish, but they show up very clearly under a stain. If you can’t avoid cross-grain sanding, you will have to find a compromise between creating scratches fine enough so they don’t show and coarse enough so the stain still darkens the wood adequately. You should practice first on scrap wood to determine where this point is for you.
Three Sanding Methods
Other than using a stationary sanding machine or a belt sander, which will take a good deal of practice to learn to control, there are three methods of sanding wood: with just your hand backing the sandpaper, with a flat block backing the sandpaper and with a vibrator or random-orbit sander.
Using your hand to back the sandpaper can lead to hollowing out the softer early-wood grain on most woods. So you shouldn’t use your hand to back the sandpaper on flat surfaces such as tops and drawer fronts because the hollowing will stand out in reflected light after a finish is applied.
 
The most efficient use of sandpaper when backing it with just your hand is to tear the sheet into thirds crossways and then fold one of the thirds into thirds lengthways. Flip the thirds to use 100 percent of the paper.
The most efficient use of sandpaper for hand-backed sanding is to tear the 9″ x 11″ sheet of sandpaper into thirds crossways, then fold each of these pieces into thirds lengthways. Sand with the folded sandpaper until it dulls, flip the folded sandpaper over to use the second third, then refold to use the third third. This method reduces waste to zero and also reduces the tendency of the folds to slip as you’re sanding.
Hand sanding.
If you are sanding critical flat surfaces by hand, you should always use a flat block to back the sandpaper. If the block is hard (wood, for example), it’s best to have some sort of softer material such as cork glued to the bottom to improve the performance of the sandpaper. (I find the rubber sanding blocks, available at home centers, too hard, wasteful of sandpaper and inefficient because of the time involved in changing sandpapers.)
Block sanding. The most efficient use of sandpaper when backing it with a flat sanding block is to tear the sheet into thirds crossways and then fold one of the thirds in half. Hold onto the block with your thumb and fingers as shown here. Flip the folded sandpaper for a fresh surface, then open up the sandpaper and wrap it all around the sanding block for a third fresh surface.I made my own sanding block. Its measurements are 2 3/4″ x 3 7/8″ x 1 1/4″ thick, with the top edges chamfered for a more comfortable grip. Any wood will work. I used sugar pine because it is very light in weight.
To get the most efficient use of the sandpaper, fold one of the thirds-of-a-sheet (described above) in half along the long side and hold it in place on the block with your fingers and thumb. When you have used up one side, turn the folded sandpaper and use the other. Then open the sandpaper and wrap it around the block to use the middle.
Most woodworkers use random-orbit sanders because they are very efficient, easy to use, and they leave a less-visible scratch pattern than vibrator sanders due to the randomness of their movement. For both of these sanders, however, there are two critical rules to follow.
Random-orbit sanders are easy to use and efficient for smoothing wood. To reduce the likelihood of the squigglies these sanders produce, use a light touch. Don’t press down on the sander. Let its weight do the work.
First, don’t press down on the sander when sanding. Let the sander’s weight do the work. Pressing leaves deeper and more obvious squigglies that then have to be sanded out. Simply move the sander slowly over the surface of the wood in some pattern that covers all areas approximately equally.
Second, it’s always the best policy to sand out the squigglies by hand after you have progressed to your final sanding grit (for example, #180 or #220), especially if you are applying a stain. Use a flat block to back the sandpaper if you are sanding a flat surface. It’s most efficient to use the same grit sandpaper you used for your last machine sanding, but you can use one grit finer if you sand a little longer.
Removing Sanding Dust
No matter which of the three sanding methods you use, always remove the sanding dust before advancing to the next-finer grit sandpaper. The best tool to use is a vacuum because it is the cleanest. A brush kicks the dust up in the air to dirty your shop and possibly land back on your work during finishing.
Tack rags load up too quickly with the large amount of dust created at the wood level. These sticky rags should be reserved for removing the small amounts of dust after sanding between coats of finish.
Compressed air works well if you have a good exhaust system, such as a spray booth, to remove the dust.
It’s not necessary to get all the dust out of the pores. You won’t see any difference under a finish, or under a stain and finish. Just get the wood clean enough so you can’t feel or pick up any dust when wiping your hand over the surface.
How Much to Sand
The biggest sanding challenge is to know when you have removed all the flaws in the wood and then when you have removed all the scratches from each previous grit so you can move on to the next. Being sure that these flaws and scratches are removed is the reason most of us sand more than we need to.
A lot of knowing when you have sanded enough is learned by experience. But there are two methods you can use as an aid. First, after removing the dust, look at the wood in a low-angle reflected light – for example, from a window or a light fixture on a stand. Second, wet the wood then look at it from different angles into a reflected light.
For wetting the wood, use mineral spirits (paint thinner) or denatured alcohol. Avoid mineral spirits if you are going to apply a water-based finish because any oily residue from the thinner might cause the finish to bead up. Denatured alcohol will raise the grain a little, so you’ll have to sand it smooth again. 
Article provided by -  Bob Flexner - Popular Woodworking Magazine

October 30, 2015

 

Making your own Abrasive Belts


Every so often at Abrasive Resource, we get an email or call from someone that is interested in either repairing a broken belt or making their own belts at home and they are wondering if we sell any DIY belt splicing materials. Unfortunately, we do not…and here is why:
This is all done, of course, to insure that the joint will not break and fly apart—which could potentially cause damage to your belt sander or injury to the operator.

Tips to facilitate a longer life for your Sanding Belts:

Sanding belts DO have a shelf life! We always advise our customers to only order the quantity of belts they estimate they can use within a year.

If you have any questions, or need to order new sanding belts—please give us a call at 800-814-7358 or visit our website for standard size sanding belts!

September 23, 2015

 

Choosing the right Fiber Disc for your grinding application


An angle grinder is a useful tool for any shop! You can use bonded grinding wheels, cut-off wheels, overlap discs or standard fiber discs all on this same tool for many different applications in metalworking, woodworking and stone or tile.

Abrasive Resource just lowered their prices on resin fiber grinding discs, so we thought it would be a good time to review all of the different applications where a fiber disc would be effective:

Aluminum Oxide: A general purpose abrasive used for metal or wood. Removes small welds and imperfections along with light stock removal, blending and finishing. Wood applications include log furniture and homes. These AO discs are also used in light-duty metal applications on low alloy steel or when loading is a problem, such as painted surfaces or aluminum. 

Zirconia: The workhorse of the fiber disc world. Zirconia refractures for longer life when used in high pressure applications like stock removal on Chrome and chrome-nickel steel or high-alloy steel. Used for applications that include blending, deburring and weld removal. Generally lasts twice as long as an aluminum oxide disc when used in the proper application. 

Ceramic: A premium abrasive for high heat generating applications such as stainless as it resists glazing and provides long life. The choice of metalworking professionals due to self-sharpening characteristics that provide high stock removal rates. Features cool cutting technology for reducing heat and increasing performance. Ideal for grinding and blending welds. 

Silicon Carbide: Excellent on titanium, fiberglass, plastics, and stone, tile and masonry products. Sharp silicon carbide abrasive grain quickly “bites” to remove any coatings such as adhesives, paint, acrylics, mastics, etc. Aggressive cutting action provides a clean, smooth finish on concrete, marble, granite, and other stonework. 

To purchase any of these discs at the new lower price, please visit the Abrasive Resource fiber disc home page on our website to find the discs best for you!

February 26, 2015

 

What are Non-Woven Abrasives?

 
Traditional coated abrasives are a cloth or paper backing that has abrasive grain “coated” or applied to one side. But a non-woven abrasive is composed of a synthetic substrate onto which a slurry of resins and abrasive grain are deposited. Non-woven abrasives are divided into two categories—they are either scrim backed, which is also known as surface conditioning products or non-scrim backed, which are the surface finishing, or open web products.

Surface Conditioning: Scrim backed materials are manufactured by needling synthetic fibers into a woven base of monofilament threads known as the scrim. It looks a little bit like burlap. This forms a fuzzy textured substrate on one side onto which a slurry of resins and abrasive grit are deposited. Since the material is only coated on one side of the material, it must be used flat with the grit side down—just like a coated abrasive, so it used primarily for belts or discs. These products are well suited for nearly all metals and used for gasket, paint, rust, scale and oxidation removal as well as light deburring, weld blending and for satin or decorative finishes.

Surface Finishing: Non-scrim, open web material is identified by the absence of the woven base or scrim for strength. It is more homogenous than scrim backed material with the resin and abrasive grain evenly distributed throughout. The nylon or polyester fibers are needled together to form an open, spongy mesh, which is then dipped into the resin and abrasive grit mixture. The result is a lightweight and flexible abrasive material that is less aggressive, more non-loading and more forgiving than the scrim backed material. It is very flexible and long wearing. The finishing material is then converted into discs, hand pads, rolls and other abrasive specialties.

Check out both the Surface Conditioning and Surface Finishing abrasives in the Non-woven Abrasives section on our website.

October 30, 2014

 

What do those arrows mean on the back of my sanding belt?


When sanding with a belt sander, you always want to make sure your belt is
oriented properly. Some sanding belts do have a preferred direction, and they
have been manufactured to consider the arrow on the inside. These arrows,
however, are only important to pay attention to when your belts have an
overlapped joint and there is a “bump” to be concerned with. In that case, you
will want the arrows on the inside of the belt to follow the same direction that
the belt is running on the sander. Most belts made in the United States---and all
of the sanding belts we convert at abrasiveresource.com -- are now manufactured with a high strength tape butt joint, and belts made with these butt joints can be
run in either direction . . . so it's OK to ignore the arrows. The tape spliced joints are bi-directional.

Bi-directional belts can be installed either way. The only adjustment you’ll probably have to make is “tracking” to keep the belt centered on the rollers. Hold the sander up, turn it on, and see if the belt either rubs against the housing or starts working its way off the rollers. With the trigger on, adjust the tracking knob until the belt is centered on the rollers. You may have to make a slight adjustment when the sander is on your work piece. If your sander has automatic tracking, you won’t need to be concerned with manually adjusting the tracking.

September 30, 2014

 

Basic Sanders


The basic styles of portable sanders haven’t changed very much over the years. We have some old advertising posters from the Rockwell Manufacturing company in 1964 matted, framed and hanging in our lobby here at Abrasive Resource and they, quite frankly, look a lot like sanders you can purchase today… although they do look much heavier! Sanding is a chore—there’s no way around it. But today’s new models can make it easier:

Basic Portable Sanders:

belts sander
Belt Sanders- Portable belt sanders are made for fast removal of stock. They will quickly smooth down and level a surface, remove paint or glue lines. They are available in several sizes, but the most popular handle belts in a 3" x 18", 3" x 21", 3" x 24" or 4" x 24" size. 





Random Orbit Sanders- Once the surface has been leveled with a belt sander, the next sander to use may be a random orbit disc sander. These sanders use an oscillating action that removes swirl marks. Random–orbit sanders may be palm sanders or have a grip design and are available in either a pressure sensitive adhesive (PSA disc) design or a hook and loop disc option.


Finish & Detail Sanders- Finish sanders typically use a straight-line or vibrating action. They are sold in half sheet, third sheet and quarter sheet models as well as small palm style detail models that are often referred to as mouse, triangle or diamond sanders. Finish and detail sanders are available in all different shapes, but they all have the ability to get into tight places and corners.


Abrasive Resource laser cuts coated abrasive discs, sheets and strips in all different shapes, sizes and grits. If you need a hard-to-find sheet for a specific sander or application, please give us a call at 800-814-7358 and we’ll be happy to troubleshoot it with you!

For reviews on specific sander models, we referenced this article: http://extremehowto.com/all-about-sanders/

August 27, 2014

 

Back to Basics: Cloth Sheets


Most abrasive products, like sanding belts and sanding
discs, are ready to use right out of the box. But other sanding
supplies--like sandpaper or cloth sheets and rolls, usually need
to be cut to size before you are ready to sand with them. And
some abrasives benefit from being “broken in” before sanding
to improve their performance. Here are some “tried and true”
tips for handling cloth sheets:

Most cloth backed sanding sheets have a flexible, J weight fabric backing and
are available in grits 40-400. They are seen as a general purpose abrasive for
wood or metal and can be used by hand or attached to high speed vibrating
sanders. Abrasive Resource also converts custom-sized cloth backed sheets
for specific industrial or artistic applications. 

For more information on cloth sheets, visit the Abrasive Resource website 

or call us at 800-814-7358.


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