Clamping

by R. Bruce Hoadley

The object of clamping a joint is to press the glue line into a continuous, uniformly thin film, and to bring the wood surfaces into intimate contact with the glue and hold them undisturbed until setting or cure is complete. Since loss of solvent causes some glue shrinkage, an internal stress often develops in the glue line during setting. This stress becomes intolerably high if glue lines are too thick. Glue lines should be not more than a few thousandths of an inch thick.

If mating surfaces were perfect in terms of machining and spread, pressure wouldn’t ‘ t be necessary. The ” rubbed joint, ” skilfully done, attests to this. But unevenness of spread and irregularity of surface usually requires considerable external force to press properly. The novice commonly blunders on pressure, both in magnitude and uniformity.  Clamping pressure should be adjusted according to the density of the wood. For domestic species with a specific gravity of O. 3 to 0. 7, pressures should range from 100 psi to 250 psi. Denser tropical species may require up to 300 psi. In bonding composites, the required pressure should be determined by the lowest-density layer. In gluing woods with a specific gravity of about 0. 6, such as maple or birch, 200 psi is appropriate. Thus, gluing up one square foot of maple requires pressure of (1 2 in. x 12 in. x 200 psi) 2 8, 800 pounds.  Over 14 tons! This would require, for an optimal glue line, 1 5 or 20 cee-clamps, or about 5 0 quick-set clamps.

Conversely, the most powerful cee-clamp can press only 10 or 1 1 square inches of glue line in maple. Jackscrews and hydraulic presses can apply loads measured in tons. But since clamping pressure in the small shop is commonly on the low side, one can see the importance of good machining and uniform spread.  But pressure can be overdone, too. Especially with low-viscosity adhesives and porous woods. too much pressure may force too much adhesive into the cell structure of the wood or out at the edges, resulting in an insufficient amount remaining at the glue line, a condition termed a starved joint. Some squeeze-out is normal at the edges of an assembly.  However, if spread is well controlled, excessive squeeze-out indicates too much pressure; if pressure is well controlled, undue squeeze-out suggests too much glue. Successful glue joints depend on the right correlation of glue consistency and clamping pressure. Excessive pressure is no substitute for good machining. Panels pressed at lower pressures have less tendency to warp than those pressed at higher pressures. Additionally, excessive gluing pressure will cause extreme compression of the wood structure.

When pressure is released, the cells spring back and add an extra component of stress to the glue line.

The second troublesome aspect of clamping is uniformity, usually a version of what I call ” the sponge effect. ” Lay a sponge on a table and press it down in the centre; note how the edges lift up. Similarly, the force of one clamp located in the middle of a flat board will not be evenly transmitted to its edges.   It is therefore essential to use heavy wooden cover boards or rigid metal cauls to ensure proper distribution of pressure.

clampin-1

 

Clamp time must be long enough to allow the glue to set well enough so that the joint will not be disturbed by clamp removal. Full cure time, that is, for development of full bond strength, is considerably longer. If the joint will be under immediate stress, the clamp time should be extended. Manufacturer’ s specified clamp times are established for optimum or recommended shelf life, temperature, wood moisture content, etc… If any of these factors is less than optimum, cure rate may be prolonged. It’s best to leave assemblies overnight.

Most glue specifications are based on ” room temperature” (70 · F). Shelf life is shortened by storage at above-normal temperature, but may be extended by cold storage. Normal working life of three to four hours at 70· F may be reduced to less than one hour at 90· F. Closed assembly at 90· F is 20 minutes, against 50 minutes at 70· F. A curing period of 10 hours at 70· F can be accelerated to 3 – 1 / 2 hours by heating to 90· F.

Finally, cured joints need conditioning periods to allow moisture added at the glue line to be distributed evenly through the wood.  Ignoring this can result in sunken joints.

When edge-gluing pieces to make panels, moisture is added to the glue lines (1), especially at the panel surfaces where squeeze-out contributes extra moisture. clamping-2If the panel is surfaced while the glue line is still swollen (2, 3), when the moisture is finally distributed the glue line will shrink (4), leaving the sunken joint effect.

Rubbed joints its success and failure

As sprung joints are great for clamping two edged boards together to make a panel, they are disastrous for non clamped rubbed joints. To avoid following the footsteps of magazine articles, I will not go into detail of what a rubbed or sprung joints are.  I believe that you are well passed the novice stage and I don’t feel it would be beneficial in re reading something you already know,  but would rather bring to your attention to something you may not have been aware of or unintentionally overlooked.

There has been a strong emphasis awarded to sprung joints, many articles and videos have been written and produced stressing the benefits of such a joint. While I don’t disagree with them, sometimes the obvious tends to skip us and we continue to apply a certain technique that has been drummed into our heads by an almost hypnotic suggestion through the continual parroting of others, that would lead to disastrous results if we were to apply the same technique using a different application. This issue I feel needs not be overlooked, but addressed in any future articles written on the subject.

The success of a rubbed joint is comprised of only two things, glue and two perfectly straight no gapped edges. A sprung joint as you know has a 32nd hollow in the middle, creating a successful rubbed joint would not be possible. The other point is the type of glue that best suit a rubbed joint would be hide glue. True, you could get away with small thin pieces using ordinary PVA or other quick setting PVA glue, but for a small cabinet or bedside table or even a coffee table, only hide glue in my opinion would be better suited for a such an application ie. rubbed joint.  I have written in my previous posts on the benefits of hide glue so I won’t go into any great depth on the subject here, other than to add, only hide glue as far as I know, has the capabilities of drawing two mating edges together as it dries, forming a good solid join and for that to happen there cannot be any gaps.

Blotch free staining update

I’m jumping the gun a little on this one but I just couldn’t hold out on it.

If you remember I did a post on this topic a few months ago, I was very excited about my discovery then and I’m even more excited about it now.  I have discovered something that actually works, even though I have only tried it on Pine I strongly believe this will work on other timbers that are prone to blotch.

I have made 2 videos on this topic and have demonstrated the steps I took to produce a blotch free stain and have made a comparative look between Minwax oil based against my own version.

I hope you find these videos most informative and helpful but before you watch these videos take a look at the samples and see for yourselves just how well this works.

The stain above I used oak.  Now have a look at the bottom pics.  The one on the left is my homemade brew and the one on the right is Minwax oil based stain.

As you can see the picture on the left shows no blotch, while the picture on the right shows heavy blotching.  What’s more remarkable is how the end grain on the left shows a uniform colour, while the minwax version shows a typical dark burnt like effect.

I usually don’t like to talk on camera because of the little amount of work I actually get done.  However, I made this exception for you today as I didn’t rehearse this and wasn’t sure if it was actually going to work with dry non resinous timber.  So I wanted all of us to find out together if it passe or failed.

Let me know what you think.

 

A good sawbench

I used to use a saw horse for all my rip and crosscutting, but a single saw horse isn’t just wide enough to support your material.   So the choice was to make one more, but that also posed some problems.  I have to kneel on the board which meant thinner boards would bend under my weight and the clutter of it eating my shop space didn’t sit well with me either.  There just had to be a better sawbench and so I devoured the net for ideas.

I looked at Chris Schwarz saw bench, then Shannon Rodgers bench and finally at Tom Fidgen sawbench.  Well that definitely was a winner for me, the bench stood 20 1/4″ x 12 7/8″ wide with a split top and 35″ long.  I like the idea of a split top, it meant that I can safely rip not so wide material.  It has dog holes for clamping and a fence for crosscutting. I made holes on both sides so the fence can be used on either side.  What I also like about this design is that one side legs are splayed and the other is square.  What this means is that you can use the square side as a reference while ripping while the splayed side provides great support to stop the bench from tipping.

It was a no brainer so I ordered his book “Hand crafted Project for the home and workshop” this book is great as it has so many other beautiful projects and none of which I ever got around to building and I bought this book probably about 2 or more years ago. Hopefully this will change as work outside my hobby always seems to get in the way.

As I was today continuing with the build of the planter box I thought it would be great if I showed you just how fast ripping with a handsaw can be.  This video isn’t sped up and no edits has been done to it, there’s nothing to sugar coat hand tooling is what it is.  It can be fast or slow it all depends on you, you are the machine, the driving force behind the tool.

The saw I’m using is a Disston 28″ 4 1/2 point  with hooked teeth.  This type of saw is mainly used for carpentry and works well slightly damped wood.  The timber I’m ripping is Radiata pine 3/4″ thick.  True not very thick stuff so ripping is made easier plus it being Radiata and not hoop pine also makes ripping easier but none the less whatever material your ripping,  your stamina and muscle strength is something you’re going to greatly rely on.

In the first video this is the full rip and in the second video I’m ripping probably  just proud of a 1/16″ from the line.  You can see as I got very near to the end I used my foot to clamp down onto the work.  Not sure if this is correct but it works for me.

Btw today was scorcher, sweat poured out of me like a running tap and sadly it landed on the sole of my LN hand plane and immediately rust formed on it.  It broke my heart.

Air dried vs Kiln Dried

On a freshly cut tree the moisture content can be anywhere from 30%-45%, this is called green wood and some chair makers use this wood for their projects, but for the rest of us the MC is too high to build anything from it, so we, have to dry the lumber before it is of any use to us.  The ideal MC for furniture making is around 8-9%, some say 7 and some say 6, so let’s just say anywhere from 7-9% is ideal.

There are two ways to dry your timber, air drying which is a slower process and kiln drying a much faster forced process.  Let’s look at Air drying first.outdoor-air-drying

Air drying is a lengthy process usually each board takes 1 inch (25mm) per one year to dry, the lumber is stacked off the ground 15-24 inch(400-600mm) on stickers which are placed across the boards and inline to each below, and above in between the boards to avoid sagging to gain even weight distribution.  The stickers serve to allow air flow between each board.  stacks-of-lumber

air-dryingA canopy or preferably plywood with cinder blocks or other heavy items should be placed on top.  The canopy or plywood is there to protect the timber from weather elements, plywood is preferred over the canopy because it allows air flow and protection from the sun.  If you use a canopy you should remove it when it’s not raining.  Once the boards have reached equilibrium with its environment which is usually about 15% outside, you then need to bring it into your shop to acclimate, and continue to dry until they reach the 7-9% MC.  Depending on your own environment this can take from two weeks up to a month or more. g05550art01

Air drying needs to be in an open space not surrounded by trees or other plant life, the ground mustn’t be damp either.  If any of these features are present, then the lumber will continually absorb the MC from its surroundings and not dry properly and mould may start to form, mould is notorious in pine and other woods.  Below is a picture of what I’m referring too.  The picture on the left shows the timber stacked surrounded by other trees.  These stacked timbers will continually absorb large amounts of moisture from the surrounding trees and damp ground.wood-drying-03

The pictures below are ideally the type of environment it should be air dried.

Unfortunately for many of us who are not financially endowed, we have to make do with the surroundings we have.  It just means that once the timber has reached equilibrium with its surroundings, which may well be above 15% and then brought into the shop, we will have to wait longer than usual for the stuff to dry thoroughly or acclimate before it’s ready for use.  It pays to have a high quality moisture meter, a Wagner is a good choice.  I prefer a pinless version which costs anywhere from $200 upto $900.  They are every bit as accurate as the pinned type,  even though some will disagree.  I will do a blog on this for sure.

Air dried lumber is a hand tool woodworkers best friend, it’s easy to work and is more stable than kiln dried lumber because it’s not forced dried through high temperatures, but rather a natural slower process.  As the timber air dries the cells collapse, slowly causing them to compress and stay put, so when air dried lumber absorbs moisture, it doesn’t swell as much hence it becomes more stable.  Many people including myself find kiln dried wood more physically demanding to work with hand tools.

Kiln Drying

kiln

Kiln dried is a forced but controlled process where humidity and temperature is controlled using steam and fans for drying.  The drying process normally takes between 6-8 weeks, because of its fast drying due to high temperatures, the cells collapse quickly rather than slowly as it would with air drying, making it unstable.  Because of this when moisture is absorbed, the cells expand rapidly filling up with water quickly than it would with air dried lumber.  However, the positives with Kiln drying due to high temperatures, any laid eggs and bugs are killed off. There is treatment for mould and insects at an extra cost.

If hot dry air is used, then the surface dries too rapidly and develops case hardening, ‘checks’, so kiln drying requires careful control of both air and temperature.  The idea is to prevent stagnant layer of excessively humid air from lingering around the timber, as in the case of air drying, the air is frequently renewed which prevents this from happening.

Final Notes:

My personal preference goes to air dried lumber as you might have already gathered, having worked with both kiln and air dried, and being a hand tool woodworker, I can certainly see and feel the difference between the two.  The seeing is a visual difference.  Air dried lumber has a rich colour over kiln dried stuff.  Also planning seems to be a lot easier and more pleasurable not requiring so much physical exertion to perform these basic tasks.  Lastly kiln drying can cause a lot of internal tensions which can make the timber brittle.  Cambia which is oven baked to extreme temperatures to change its colour from white which is white ash to black is very brittle.   They claim this makes it more stable and resists any cupping, warping etc. but I beg to differ as once re sawn cupping is inevitable even though the effects are not immediate, but if left to sit freely without pressure applied on top it will cup, this does not work all the time.  When planning you don’t get shavings but more like, I don’t know what you’d call it so here is a picture of it, 36and if dropped will shatter like glass.  So you can imagine, after scrolling for an hour every intricate detail and you accidentally drop it and just shatters, it’s enough to make you cry, and cried I have.  Cambia really is a wonderful looking timber and does take an excellent finish, but doesn’t take screws nor nails well.  Even though it has all the anomalies of staying away from it, I will at every opportunity use it only because of it’s appearance.  Since I cannot afford ebony this is the next best thing to it.  Also anything above 1/4″ thick makes it very strong. The cost; well it’s bloody expensive is all I can say, and I don’t know why, in it’s natural state it’s half the price.  Now they have extended its range to Poplar which looks really nice, imagine that, they’ve made a second rate timber to something attractive.6159

For machine work, obviously kiln dried lumber won’t have any effect on you physically, but none the less you still may experience some chip out, loss of colour and internal tensions on some boards when re sawing.  Just the other day I was re sawing American Black Walnut, and just as I finished the cut, it did with a loud bang as the two pieces separated and immediately cupped, even though this board has been sitting inside my shop for the last two or three years.  Once a board has tension in it, it will remain until it’s released, even if it sat untouched for 100 years.  That’s life.

Here is a video a visual description summary of what I’ve written above.

Sandpaper

Above is an audio of the post below.  Read or listen, whatever is easier for you.

The use of abrasives goes back almost since the beginning of recorded history. Prehistoric men, for example, sharpened their tools and weapons by rubbing them together.

Stone used in building the Pyramids of Egypt which were smoothed with a naturally “bonded” abrasive–sandstone!  Around 2100 B.C. a creative Egyptian engineer mounted a circular wheel on a crude sort of lathe and ground bronze tools and ornaments, launching the art of cylindrical grinding. During the Middle Ages, armour and swords were ground and polished. The first recorded manufacture of coated abrasives goes back to the 13th century when the Chinese used natural gums to bind crushed seashells to parchment.

Sandpaper was made out of crushed shells, seeds, sand, and gum, shark skin, rough horse tail plant when boiled and when dried, is finer than sandpaper which is used for polishing in Japan.

In the 18th century shark skin and sand not combined but as separates were used, Anthony Hay’s Cabinet shop at Colonial Williamsburg used shark skin with great success.

In 1775 Carter wrote in his day book on how to make sandpaper.

“To polish wood – Take brown paper, make it wet with glue, then scatter fine sand thereon, through a hair sifter – Sandpaper is equal to fish skins.”

You could safely say, that sanding is older than hand planing. As I mentioned above, the Egyptians were sanding blocks using sand stones for smoothing.  Hand planes were invented by the Greeks or Romans much later in history, no one really knows who invented it first or when, but my gratitude goes out to them for doing so.

When should you sand?

Whenever you want really, but in the 18th century, depending for whom the piece was being made, surface imperfections like small tear outs were left. If the piece being made was for the wealthy usually this would need to be French polished, so everything had to be free of surface defects, and abrasives were used.  In this modern age of craftsmanship and I’m not including mass manufacturers, regardless of the piece were making, any form of tear out is unacceptable.   I am not stating that we are better than our crafting ancestors since we cannot compete with such high levels of craftsmanship, to stand in their defence they made a living from the craft and, working by hand is laborious and time consuming enough, let alone to hand sand every piece in order to flatten or take out every imperfection would be ludicrous and expensive.  Sandpaper according to Anthony Hay’s cabinet shop cost about 6 pence which equalled to a day’s wage.

If you didn’t know you will now, yes it is possible to make a surface completely flat using only sandpaper starting from a course grit and working your way up.  It may be a dusty option but a solution to really nasty but beautiful timbers.  I guess this would be the only time I would ever pull out my belt sander, but the timber would have to be something so beautiful and unworkable with hand planes, that would force a machine in my hand.  You may ask if there is an alternative and I would say yes of course, scraping but not always will work, however, for 99.99% of the time it will suffice.

As you all know, I make clocks, and clocks are usually elaborate high end pieces. So scraping and sandpaper would be used for such fine work.  The steps I would normally take may or may not involve sandpaper, everything depends on the wood and the type of finish desired.

These are steps I would normally take: 16

Rough sawn surface prior to hand planing.

9                                       Flat and smooth with various hand planes and scraper.  These are two boards edge joined seamlessly in fact you can’t even see the join.

I hand plane all the surfaces until they are flat and smooth, if there is tear out I would use my low angle jack plane that has a 50 degree bevel on it.  This will normally smooth most timbers and, I say atleast 95% of difficult grain I have encountered.  Sometimes when planing Camphor Laurel, even the high angle blade will not make the board completely smooth, so in this case I will use a card scraper.  Even then sometimes with this particular timber, a card scraper isn’t enough, that’s when sanding is your only option left.  You can however skip the card scraper and just go directly to using sandpaper however, the card scraper does the majority of the work thus cutting your sanding time by more than half.  I don’t particularly like using a cabinet scraper in replacement for a high angled plane.  The reason is, a cabinet scraper doesn’t leave as polished a surface as you would get from a hand plane.   To use a cabinet scraper or any scraper for that matter, you need to flex the blade.  I’ve discovered if you take the blade out and scrape that way it works beautifully without the need to flex the blade and the results I get are much better than I would get with it being installed in the tool.  I wonder if that 45 degree bevel has anything to do with it.  This calls for another experimentation.

The beautiful nature of timbers is their unpredictable nature, even though you may work with one particular species, each board are subtly and some extremely different to each other.  I look at timbers like people, you have five fingers not all are of equal length, therefore not all people are the same.  Each person is an individual and if treated with kindness and respect you will get the same in return.  So too with wood, you never force the wood into submission like you would with kiln drying and machinery and then expect it to behave in the long run.  When you work with it with hand tools it will quickly tell you what it likes and doesn’t like, the end results are far superior and the process is a lot more enjoyable.   This is something unfortunately a machinist cannot understand and by the way, a power planer will never create a perfectly smooth and flat board. Only a hand plane can do this.  Having said that I have heard that helical blades will create a smooth and flat surface but I will reserve my judgement on this until I see it, but more importantly feel it with my own hands.

If any of you have upgraded your planers with helical blades I would you like to hear your thoughts on them.  Do they really produce a flat smooth surface without any planing skip marks and snipes.