Chapter Two - The Basics of Design

Chapter Thirteen
Wood Shop 101
Compared to kitchen cabinets or fine art cabinets, a telescope lives a
tough life. It will be carried outside and subject to thermal stresses, pulled on,
tugged on, covered with dew and perhaps even rain or frost. A telescope needs
to be well made to keep from getting pulled apart over the years, and especially
well finished to keep the dew out for years.
It’s pretty safe to say that the most used construction material in all of
history is wood. Wood has found its way into telescopes since the earliest days
of telescope making, and is still a mainstay of telescope making. There are
thousands of books and magazines devoted to woodworking, but in this book
we’ll focus on working with wood from the ATM’s point of view. Before we get
into those details, the most fundamental question is why should we use wood at
all?
To begin with, wood is plentiful, cheap, easy to find and easy to work with.
The home improvement centers all around the US carry ready-to-use lumber in
various sizes, plywood sheets and structural shapes such as 2x4s and 4x4s.
Because wood is so widely used in the construction industry, the material and the
tools are easy to obtain. The system used to talk about wood sizes is quite
arcane and I don’t really know where it comes from: a finished 2 x 4 is not two
inches by four inches. That’s the size of the “unfinished” piece that hasn’t been
surface planed on all four sides, This operation takes off some rough wood on
each surface, making the finished nominal size 1 ½ by 3 ½. Likewise,
dimensional lumber boards called 1 by whatever are ¾ inch in the 1” direction,
and ½” smaller in the named dimension.
Aside from cost, and convenience, the strongest point in the favor of wood
in ATM construction is that some wood types are a good vibration suppressor. I
must quickly add that some wood is awful in this regard. Certain spruces,
cedars, exotics like mahogany, and others are used in musical instruments
because they resonate well. The best wood for vibration suppression is plywood,
because of its alternating grain structure. Chip board, particle board and other
composite woods are also good vibration suppressors. They need to be carefully
sealed with paint, varnish or a laminate like Formica ®, because they are much
more prone to damage from water than plywood or solid wood, but they are good
at vibration suppression.
Remember, though, that woods are not as strong structurally as metals or
composites. This can generally be compensated for by using bigger pieces, but
the hardest woods are still no match for aluminum. The modulus of elasticity for
oak is 2 MPSI (million pounds per square inch), while aluminum is 10 MPSI.
Also, woods are anisotropic, some strongly. This means that the properties vary
with or across the grain. Table 3 in appendix A shows the properties for Douglas
fir: along the grain, it’s virtually as strong as oak at 2.0 MPSI. Across the grain,
it’s about 1/10 as strong. This makes challenging to design – you need to know
the direction from which the load will be applied. Plywoods are a way around this,
as they alternate the grains from layer to layer, giving a finished piece that is very
dimensionally stable, with more uniform engineering properties. Because of
these factors, you will not be able to get quite as good strength to weight ratio
with wood as you will with metals, and not be able to build the highest
performance scope or mount.
The Material
Wood is broadly classified into two main types: softwood and hardwood.
Generally the softwoods come from conifers – pine trees – and the hardwoods
come from deciduous trees. This classification leads to the silly situation of
having hardwoods like poplar that are easier to work than some softwoods.
Density would be a better way to classify woods, but the hardwood/softwood
nomenclature is so entrenched we may as well just get used to it! Wood is
available as solid lumber, or as plywood. Plywood is available in different grades
from the structural kinds made with little or no thought to appearance (C-D grade
softwood plywoods), through appearance grades (A-B) to hardwood plywoods
used in expensive furniture. The combination of letters means there is a surface
that is better than the other – the A being the best – and you can orient it putting
the best face forward. The hardwood plywoods are sold as the species used,
such as oak plywood, cherry, Baltic birch, and so on, followed by a grade (A-B)
and perhaps other information. The inner layers are not necessarily the same
species as the outer layer, so be aware of that if you are looking for a specific
plywood. The number of plies varies and there is even lumber-core plywood,
which uses strips of hardwood faced with a veneer of the appearance wood.
For ATM use, plywood is a good material. It’s found in every home
improvement center in the country, or can be easily obtained. Softwood
plywoods in sizes up to ¾ inch are adequate for much construction, as you might
use in a Dobsonian rocker box. In addition, many of the chains carry hardwood
plywoods that are very attractive, such as the Baltic birch plywood. Hardwood
plywoods can be significantly stronger than softwood plywoods – with a modulus
of 2 MPSI for oak plywoods vs. 1 for soft pine plywoods.
There is a rapidly growing field of engineered wood composites that is
producing composites stronger than this by creating composites of wood with
epoxies, and even stronger materials like metals. They tend to not be home
improvement center materials and are therefore harder to find, but can offer
higher strengths than oak. This still leaves them weaker than aluminum.
Designing with wood is no different from designing with metal, if you follow
the principles outlined in the sections above. If you’re designing for a certain
minimum strength in a portion of a mount, you’ll increase the dimensions of the
piece until you get the results you want. The result you get may look very
different from a metal piece: beams will be bigger, tube walls will be thicker, and
so on, but if you’ve designed for the strength you need that’s what happens.
Cutting, shaping and joining wood pieces.
For anything other than the smallest pieces you’ll cut, a power saw is
really the best way to work wood. Unless you’re extremely muscular with arms
like Popeye. A long, straight cut in a piece of wood is hard to do otherwise.
Power saws and handsaws fall into two basic types: rip saws and crosscut saws.
Rip saws are used to cut wood along its grain, crosscut saws cut across it. The
difference is the set of the teeth on the blade, with crosscut saws having less
sideways offset from tooth to tooth than rip saw blades. Ripping is a good
description of what these saws do; the offset teeth separate the wood’s fibers like
ripping a cloth apart. Combination blades do both kinds of cut, but purists think
they don’t do either as well as a dedicated blade. For cutting plywoods,
combination blades are the best choice, and designated plywood blades are
usually fine-toothed. Carbide tipped blades will outlast regular steel blades many
times over, and are worth the extra cost. You’re striving for a smooth cut that
doesn’t require further work before fastening into a tight fitting joint.
Figure 1 - Circular Saw, the basic power saw, courtesy Porter Cable
Figure 2 – Compound Sliding Miter Saw, Courtesy DeWalt
Figure 3 - Table Saw, Courtesy DeWalt
Telescope components, Dobsonian rocker boxes, English mount yokes, or
other straight-edged parts don’t need to be cut with a table saw, although a table
saw is certainly convenient. A circular saw is fine, as long as a bench or other
method of supporting the board being cut is available. To get straight edges that
join well, you’ll need to use a straight edge to guide the saw along. The same
hardware stores/home improvement centers that sell plywood typically sell
aluminum or steel dimensional metal. A piece of angle aluminum, or even a long
straight piece of flat aluminum, will make a nice straight edge. Clamp it to the
board at both ends at a fixed distance from the line you wish to cut (and
remember that the cut has a width associated with it so that you need to measure
to the closest or farthest point of the blade tooth, depending on which way you’re
cutting). Then make the cut while maintaining some sideways pressure against
the straightedge you’re using. There are devices that will attach to the circular
saw and run along an edge that will help you guide the saw along the line you’re
cutting. Don’t forget that if the edge the guide is running along isn’t straight and
square to the top of the board (what woodworkers simply call “square”), the cut
you produce won’t be either. That’s why a straight edge of shop aluminum can
make a straighter cut on your finished piece – it doesn’t depend on the previous
cut of the wood.
A router is a very versatile tool that many woodworkers already own – or
want. A router is like the cutting head of a milling machine in that the bit spins
and is moved along the work to shape it. In fact, this type of shaping trim pieces
is called millwork. Routers are available as fixed base or plunge routers; some
routers are available with both bases as an option or in a kit. The fixed base is
generally cheaper, and can be mounted to a router table or used free hand. Best
results for wood joinery are usually obtained with fixtures.
Figure 4 - A Plunge Router can be lowered into the work while powered. Courtesy Porter
Cable
Cutting Circles for Use as Bearings
Circles are not hard to cut out. In fact, given the right tool, they are the
second easiest thing (to a straight line) that you can cut. The right tool is
something with a small cutting tool and a method of pivoting the work piece
around a single point. For wood disks, I use my router table. I drive a nail
through the work piece (this will become the center) and then drive the nail into a
hole previously prepared at the desired radius distance from the router bit. I use
a 1/4" carbide straight bit, and most of the time the radius is measured to the
nearest approach of the cutting edges. This will produce a circle whose outside
diameter is twice the radius I've marked.
Cutting a circular cutout with a precise inside diameter is marginally
trickier. If I need to cut a circle with a precise inside diameter (such as a ring with
an inside diameter equal to a snug fit on the outside of a piece of Sonotube) I
measure to the farthest distance of the router bit's cutting edge. This piece will
be cutout from the solid blank and could be damaged when the cut completes
and the work piece starts flopping around. To prevent this, I nail a piece of scrap
to the top of the work piece with nails just lightly gripping the work piece beyond
the edge of piece I'm going to cut out, or, at a minimum, beyond any place the
router bit could hit. Hot melt glue also is a good way to temporarily join the work
piece to the one on top.
Figure 5 - Measuring from the edge of the router bit to the circle radius.
In both cases, the router bit is positioned just above the table top, usually
1/4 inch. Figure 5 shows measuring for a 12" diameter disk, with the router bit
positioned to get the farthest reach of its cutting edge. A nail is used to start the
hole for this diameter. I then put the axis nail through the wood blank, tap it
lightly into the pre-drilled hole in the table with a hammer so that the wood just
barely touches the top of the router bit, and start the router. Once it is running, I
press the nail into the hole, usually with a few taps of a hammer. This forces the
blank onto the table and causes the router to cut a small, blind hole in the piece.
Now I spin the piece into the router bit and complete one rotation, turning that
blind hole into a circular groove. Turn off the router, raise the bit another 1/4 inch
and repeat the above operation. The third pass will put the router bit through the
top of a 3/4 inch work piece. The result will be very smooth, like a good router
cut should be, and quite round. A plunge cut router could probably do this in one
pass, but my table approach wouldn't be the thing to do (it's hard to plunge an
upside-down, table mounted router!).
Figure 6 - After the last cut to size of a plywood disk.
An alternate approach is to mark the circle that will be the finished
dimension on the piece of wood to be cut, using a beam or other type of
compass, and rough cut it with a saw. A saber (jig) saw, scroll saw, or band saw
will work. You don't have to be accurate; trim to within the router bit's diameter of
the marked circle, then use the router to trim to the exact line. This is done by
driving a nail through the exact center of circle (which you already marked with
your compass) and into the router table so that the innermost approach of the
router bit edge to the center will cut to the edge of the marked circle. This should
only take one pass with the router, and no fooling around with raising the bit and
repeating the cut.
Cutting a semicircle is a little harder, but probably easier than cutting a
complete circle exactly in half. Cutout a wood blank, square or rectangular will
do, that you can fit the semicircle on. Then attach a piece to this that the pivot
nail will go through. A small tab of wood will work, but so will a tab of sheet
metal, as seen in Figure 7. It's important to remember that the circle will be cut
with respect to this point as its center. The advantage to doing this is that you
can repeatably cut less than half a circle. You can see that if you extend the
curve over a larger size, you can cut ¾ circles, or any sector you desire.
Figure 7 - Cutting a semicircle
There are, of course, other methods of cutting circles in wood using
anything from a table saw to a band saw. If you have one of these fixtures and
are familiar with it, by all means use it. If not, the router can be an incredibly
versatile tool in the shop and you should have one even if this is your excuse. It
is also an excellent tool for trimming Formica laminate from bearings, fiberglass
skins on composite parts, and even door skins on foam bearings.
A router is also valuable for other tasks, though, especially joining boards.
A router makes an excellent way to form the main woodworking joints, a dado or
rabbet. Both of these are grooves cut across a piece, the difference being that a
dado is a cut groove with wood on both sides of the cut, and a rabbet is a groove
cut on the end of the board with no wood on one side. Most ATMs join their
wood by gluing and screwing square board edges to each other directly, what
woodworkers call a butt joint (Figure 8). A dado joint (Figure 9) is many times
sturdier, and while it does take more work the improvement can be dramatic. A
dado can also be cut with the hand circular saw used to cut the plywood to size,
by moving the guide piece to allow multiple cuts in the channel that you want
removed, then removing the rest of the wood between saw cuts with a
woodworker’s chisel. A rabbet is not as strong as a dado, since the wood is
supported on only one side; however it provides more glue surface than a simple
butt joint due to the recessed piece’s surface being exposed to the end grain of
the rabbetted piece.
Figure 8 The simplest joint - the butt joint
Not convinced it makes a difference? I guarantee that anything you build
with a properly made dado joint will be stiffer than anything made with simple butt
joints. Make even a simple book case this way and you’ll see. Woodworkers
invented these joints centuries ago, before the invention of power tools, when
each of these joints had to be cut by hand. If plain butt joints were good enough,
would they have made the extra effort?
Figure 9 - The dado joint. A rabbet is formed when the groove is at the end of the board.
Your final piece can’t be strong if your joints are wobbly, and no amount of
glue and screws can make a poorly cut joint work perfectly. What makes a joint
proper? The sides of the dado need to be perpendicular to the face of the wood,
and the bottom of the dado flat. The board going into the joint should almost be
strong enough when assembled with no fasteners. That means the width of the
groove must match the thickness of the mating piece closely – within 1/32 inch or
less (about 0.8mm). If anything, the groove should be slightly undersized, so that
it’s a force-fit to get the mating board into place. Router bits are available the fit
the nominal plywood thicknesses quite well. The router leaves a straight edge on
the cut, and a flat bottom, making a tight joint. A dado blade set on a table saw
tends to leave an uneven surface on the bottom of the dado. It can be smoothed
with files or chisels or used the way it comes off the saw.
Top end telescope makers that build large Dobsonians are starting to use
finger joints in the cases. The Astrosystems Kits are built this way, but many
home builders have adopted the approach. Finger joints and their elaborately cut
cousins, dovetail joints, were developed for drawer faces that were pulled on
thousands of times over the life of a chest of drawers. Dovetails are the ultimate:
the shape of the cut pieces resists the strain, but box joints are very strong in this
application, too.
In addition to these joints, there’s a staggering variety of ways to join
wood. Mortise and tenon joints are widely used, and also rely on the strength of
a cut piece (the tenon) bound in a receiving slot (the mortise) in another. These
can be drawn together with hardware or wooden wedges, for disassembly, or
glued in place. Wooden dowels and the newer wooden biscuits also get used
widely. Wooden biscuit joinery is a new invention in the world of woodworking,
where many of the techniques have been used for centuries. It was invented by
an engineer and home cabinetmaker named Herman Steiner in 1955. The
system relies on thin, elliptical plates of compressed wood (usually beechwood)
and a cutter that cuts a semi-circular groove in the pieces to be joined. The
biscuit (more properly, the plate) is slathered in glue, put into the groove, and the
pieces assembled. The biscuit then swells as it absorbs the moisture from the
glue, wedges in place, and forces the glue to cover the entire joint. This is a
strong joint, but requires special items – the biscuits and the tool. Therefore, I
only mention it as an item of passing interest for the ATM. If you’re a tool junkie
who has to have the latest gadgets, you’ll end up owning one if you don’t already.
Figure 10 Biscuit (plate) joiner. Courtesy DeWalt Tool
The glue in a joint, by the way, is what holds it together. If you use wood
screws and glue, the screws merely serve to clamp the piece together while the
glue dries. Wood screws by themselves make a weaker joint than good
carpenter’s glues. The glue chosen for a joint is an important component, too.
There are yellow carpenter’s glues at home improvement centers, and white
glues at these and other stores. The yellow glues are stronger and worth using.
The yellow and white glues do age, and form weaker bonds after a while.
They're cheap enough to replace for new projects; replace your glue if it’s a
couple of years old. Two part epoxies are even stronger, and may be used to
advantage, too. Old time woodworkers used a glue pot with animal hide glues.
These are hardly ever used today, but some shops have them.
For joining wood in ways that are supposed to come apart again, there are
a few options. Tee-nuts are a type of metal fastener that will accept a machine
screw. They are shaped like a cylindrical barrel that has a piece of metal across
the top, like a “T” in cross section, and the top metal has some teeth cut and
shaped in it to help hold it into the wood. To use one, you drill a hole in the piece
large enough for the barrel and then tap the Tee-nut into it with a hammer. The
two boards are lined up, and a machine screw, with a washer to help spread out
the load, is put in from the outside. Obviously, the joint needs to be laid out so
that the force from the screw tightening draws the Tee into the wood and the joint
together. Tee nuts are found at most hardware stores and home improvement
centers.
There are also other fasteners that the home assembled “knock-down”
furniture uses. This furniture like the (often) Danish pieces made from fancy
woods, such as teak veneer plywood, and some office furniture. These use
fasteners that require some more machine work, but that present a nicer looking
piece of finished hardware. The screws are often called “Euro screws” and the
hardware they lock into is a “Cam lock”. Woodworking supply stores carry these
items. If you want to keep your telescope in your living room and have visitors
admire it, this is the stuff to use.
Joints made this way will not be as strong as fully glued dado joints, but
they are completely serviceable joints with the advantage that they can be taken
apart. In joints that don’t require high strength, such as a mirror cell, or lens cell
attachment, they can be quite handy.
A hand operated drill, or one of the cordless drill/screwdrivers is probably
the most commonly owned tool, and is great for tightening screws. The variable
speed drill is handy for driving screws, and will also drill holes quickly without
stalling or binding in most woods. There are combination bits that allow you to
drill a pilot hole for a wood screw, the non-threaded shank and the countersunk
head, all in one motion. This speeds up the work of getting a wood screw
installed by a large amount.
Measurement and layout
Because its properties make it tend to change size in differing humidity
conditions, and tend to make it cut unevenly, wood is not typically specified to
small tolerances. Layout with a tape measure or ruler calibrated to 1/16 inch is
usually fine. The exception to this is making tight fitting, precise joints like the 45degree joints in a picture frame, a miter cornered box, or a dado joint. You
simply need to be aware of things that beginners often overlook: make sure the
wood is stable before you cut it (let it sit in your shop for a while to absorb
moisture or dry out), there is an inside and outside of every saw cut, and there is
a thickness to every cut you make. Measure to the length you wish to cut, mark
a precise line with a sharp tipped pencil or felt-tipped marker and align the saw
so that it will take away wood from the scrap side of the cut, not the side you
want. That is, if I want to cut a 16” length of board and, I need to position my saw
so that the blade is outside of the 16” mark, and the innermost tip of a saw tooth
hits the measured mark. The tips of the saw’s teeth will cut a wider path than the
body of the blade, and I need to line up my work so that the full desired length is
preserved and not taken up with the saw blade’s thickness. The width of a saw
cut, referred to as a saw kerf, can be measured by making test cuts on a piece of
scrap wood, and the width used when laying out dimensions on a board to cut.
Finishing wood
Wood is subject to attack by moisture. Wood pieces will swell with
changes in humidity or after rain. These changes can rip a structure apart: the
ancient Egyptians quarried stones by driving wood wedges into cuts they chipped
into the block. They’d then pour water over the wedges and let them swell,
creating forces that broke off tons of rock. As an organic substance, wet wood
provides plenty of food for fungi and other plant life that will eventually destroy
the structure. For purely functional protection, a couple of coats of exterior paint
are hard to beat. Exterior grade softwood plywoods will often have areas that
look better filled in with wood putty or wood filler, and then sanded and painted.
The paint color isn’t very important from the standpoint of thermal management,
but a dark color helps preserve your night vision if stray light hits the tube. A
dark color may also cool quicker, too, as discussed in the chapter on
thermodynamics.
The surface of the wood will need preparation for the finish. Sanding
should be done with the grain, at least in the coarser grits of sandpaper (100,
220) as you get to the finer grades (400 and finer), an orbital motion is fine.
Sandpaper does work better when wrapped around a sanding block, even a
simple piece of scrap wood, and a block is easier on your hands, too. Power
sanders are widely available in both belt and pad styles. The belt sanders are
aggressive enough to change the wood’s thickness and surface shape when
used with coarse belts (60 to 80 grit). Pad sanders seem to be best at finish
work. Neither is really necessary, but will make life more convenient for you,
especially if you do more projects. Remove the dust from the sanding with a
vacuum or rags.
An alternative to paint, for telescopes made with furniture grade wood, is
varnish. For the kinds of moisture that telescopes will be exposed to: dewing and
the occasional sprinkle, frost or light snow, a polyurethane varnish is hard to
beat. This looks best on hardwoods, hardwood plywood, or veneers, not on
softwood (pine) plywood which tends to yellow under the varnish. A cabinet
grade telescope will really attract attention at a star party, not to mention from
neighbors who see it! Shellacs will make a nice finish but require much more
work, with many more layers applied.
Some folks have used epoxies, and overlaid a wood structure with
fiberglass. There are inherent difficulties with doing this, which come from the
expansion and changes in the wood causing the glass to delaminate. Boat
builders do this type of work, and books in this field may help you learn more.
Personally, I don’t think it’s worthwhile. In the thicknesses of wood most ATMs
would use, the wood is plenty strong enough, and a few thin coats of
polyurethane will protect the surface as well as the fiberglass overcoat.
References and further reading:
1. APA – The Engineered Wood Association, Plywood Design Specification,
1998, Tacoma, Washington,
2. APA – The Engineered Wood Association, Design of Plywood Pallets for
Rack Storage, 1995, Tacoma, Washington,
3. Burch, Monte, The Home Cabinetmaker, 1981, Popular Science Books,
Times Mirror Magazines, New York, NY.
4. De Cristoforo, R. J., The Table Saw Handbook, First Ed, 8th Printing, 1988,
Tab Books, Blue Ridge Summit, PA.
5. Duginske, Mark, Band Saw Handbook, 1989, Sterling Publishing Co. Inc.,
New York, NY.
6. Spielman, Patrick, Router Handbook, 1983, Sterling Publishing Co. Inc., New
York, NY.
7. Sherman Whipple, ”A History of the Biscuit Joiner”, published online at
http://www.huntfamily.com/metz/bj_history.htm ,Whipple, Sargent & Associates
Strategic Services, Hingham, MA