Tru-Weight Indoor Balsa Frequently Asked Questions
- What makes Tru-Weight Indoor Balsa different from other indoor wood?
In essence Tru-Weight Indoor Balsa is the same carefully chosen and closely graded wood as other brands of indoor balsa. What sets True-Weight Indoor Balsa apart from the rest is the way we cut and process it and even more important the way we do business. We list every sheet of balsa in stock individually. When you place an order you choose each sheet one by one just like you were going through our inventory and picking it out in person.
How is Tru-Weight Indoor Balsa cut?
Tru-Weight Indoor Balsa starts as the most closely graded and carefully chosen balsa logs we can buy. Each log is then examined for the orientation of grain. Logs that don't have even straight grain are not processed regardless of the cost of the raw wood or the low density of the specific log.
Once a suitable log is found it is then examined to determine how best to cut it into the billets
to saw the sheets from. Often the best billet orientation will be diagonal to any of the existing surfaces. We mark this orientation and then trim off all the excess wood to yield a
properly oriented billet. This often reduces the amount of cuttable wood in a log by half. The
actual cutting of the sheets is done on fully automated computer controlled
equipment. Presently cutting is done on a 3 axis Bridgeport series II milling
machine using an 8" tool steel saw blade. For saw cut finish sheets are cut
to the exact thickness desired and only require trimming to width and length and
grading to be ready to for shipment. Surface ground wood is brought to final
thickness on a 18" x 6" surface grinder by taking as little as
.0001" per pass off the wood before final trimming and grading.
What is Surface Grinding?
While the majority of the wood we sell will be sawn only and not surface
ground, surface grinding offers a superior finish and in some cases a stiffer
piece of wood. Surface grinding will be limited due to the time it takes not because it is an inferior process. One point to make clear is that surface grinding is vastly different from sanding. With sanding the abrasive grains protrude from the paper as a series of peaks. These peaks cause the sand paper to put small gouges into the surface of the balsa. Also, sanding requires a certain amount of down pressure to get the paper to cut because the surface speed is quite low. Surface grinding uses a wheel that has been trued with a diamond tool. This makes the surface of the wheel smooth to the touch with sharp edged pores protruding into the wheel. Additionally the wheel has a surface speed of just over 5600 feet per minute.
With these 2 properties combined you get more of a plane type effect than a sanding effect. I have also noticed a very interesting result from surface grinding wood that is over about 5# density. With lighter wood when I surface grind I find a very slight density increase (about 0.1#) and no change in the stiffness coefficient. With the more dense wood I see the same slight bump in density, but I also find about a 10% increase in the stiffness coefficient. All things considered there is no comparison between sanded and surface ground.
How is Tru-Weight Indoor Balsa measures and weighed?
True-Weight Indoor Balsa individually measures and weighs each sheet. This is
quite different from most indoor suppliers that mark the sheet according to the
density of the block it is cut from. This is a very very time consuming process,
but it allows you to actually get the specific density wood that you desire.
Measuring is done using a dial indicator mounted on a granite surface plate.
We take measurements at a minimum of 3 points (both ends and the middle) and average
the reading to come up with the marked thickness.
Weighing is done on an Acculab VI-1mg digital scale that reads to .001 gm and
then rounded to the nearest hundredth of a gram.
Will my measurements of Tru-Weight Indoor Balsa match the markings?
Maybe. The weight, thickness, and stiffness of balsa will vary with the humidity. Because we use very thin sheets that expose much of themselves
to the atmosphere it does not take very long for the wood to change it's moisture content. People in particularly damp climates have reported
to me that their balsa weighs more than it is marked and this is to be expected. If you want to see how this works
here is a test you can repeat. Take a sheet and put it in the oven at say 200 deg F for 20 minutes. Weight it immediately upon
removing from the oven. Weigh it again every 10 minutes for an hour or so. You will see the weight increase as it sits out and
starts gaining moisture. On a thin sheet it will be back to the original weight within an hour or so. A thick sheet will take longer.
You will notice this far more on the coasts and very little in dry climates like Colorado and Arizona. Bruce McCrory in Seattle has reported
a 6% weight change between a sheet immediately out of the oven and the heaviest reading on a muggy rainy day on the same sheet.
Thickness measurements will also vary for two primary reasons. Actual changes in thickness due to humidity and measuring techniques.
It is virtually impossible on this light and compressible a material to
consistently measure by feel. If you calculate the force multiplication
from a 40 tpi screw and look at the pressure that exerts on the anvil of
the mic you will find that altering the torque you apply to the thimble
of the mic by fractions of a gram will alter the force on the wood
between the anvils quite materially. That is why I have gone to a
measurement system that totally eliminates feel from the equations and
depends upon repeatable weight (the ground bar I place over the wood)
and spring tension (the down force of the indicator) in my surface plate
based measuring system. Additionally when measuring light balsa you get
a certain amount of compression of the material if you are not using a
large anvil. I get around this by using a 1-/4" wide strip that covers
the full width of the sheet instead of a small anvil as in a mic or
What do all the markings on the wood mean?
||Thickness is the measurement in inches
Weight is in grams
Density is in lbs/cubic foot
Stiffness if present is based upon the work of Bernard Hunt's stiffness coefficient
ID# Is how we track each piece on the web and in inventory
What's the story with all the variation in the prices?
This is another business decision that sets Tru-Weight Indoor Balsa apart from the rest.
While most vendors charge a set price for wood they also choose the specific
sheets for you. At True-Weight we let you make the choices for yourself. We have
put together an algorithm that factors in the width, length, thickness, density,
and if rated the stiffness to automatically calculate the price of each sheet.
We then list every single sheet that is in stock along with the specifics on
that piece of wood including the price. This allows you to decide for yourself
whether a piece of 4.6 - 5.0# wood will do the job or if you want to spend more
to get that piece of incredible under 4.0# wood listed next to it.
How do I save money by buying more expensive wood than your competitors sell?
Our philosophy at Tru-Weight Indoor Balsa is that you save money because you can
buy the exact wood you need. With other vendors you buy $100 of wood to get 3 or
4 sheets that truly fit your needs. At Tru-Weight you just buy the 3 or 4 sheets
of exactly what you need and end up spending less than half the money.
Additionally, much of Tru-Weight balsa is cut in wider sheets the
1-1/8" that is common. With our 1-3/4" sheets you are getting 35% more
wood per sheet and in 1-1/2" sheets you are getting 25% more wood. All of
this combines to make Tru-Weight balsa the economical choice.
What is the stiffness rating all about?
While density and grain are important, in many parts the stiffness of the sheet is also important. We are all taught to flex test our wood to find the stiffest sheets. The way it has been traditionally done is as a comparison test. You have 2 sheets of the same thickness and you test to determine which one is stiffer. Only problem is this does not let you compare sheets before you buy them and even more it does not take into account how stiff the sheet is relative to the norm for balsa of that density. To help us stiffness better Bernard Hunt and Joe Maxwell have study and written extensively on the subject. Bernard has created a formula that takes into account the thickness, width, length, and density of a sheet and combines that with a measurement of the buckling resistance of that sheet to yield a
comparative stiffness value. What this value tells you is how the specific sheet rates compared to the norm for a piece of balsa of the same dimensions. Armed with this information you can make a decision that light piece of
balsa with a high stiffness rating can replace a heavier piece with a lesser stiffness rating.
There has been quite a bit about stiffness testing and how to best use the information written in the various indoor newsletters like Indoor News and View and a great chapter in the now out of print book on Indoor Balsa by Joe Maxwell. We can not cover all that has been said here. The best we can do is to let you download the freeware program written by Fred Rash based upon the work of Bernard Hunt that we use to calculate the stiffness ratings we apply to Tru-Weight Indoor Balsa.
Download the program
Because it gets more difficult and time consuming to test thinner wood we only apply the ratings to wood .020" and thicker. Additionally, because the measurements depend to a certain extent on the "feel" of the person doing it we have decided to offer the rating in ranges instead of using the exact number the program generates. Here is how we convert from the program numbers to our ranges:
Low < 90
Normal 90 - 99
Good 100 - 109
Very Good 110 - 119
Excellent 120 - 129
UnReal Stiff 130 - 149
Out of This World > 150
This does not mean that poor or average wood is not usable. In the many parts of a plane where the stiffness is not a concern or on planes where you are not needing to save weight lesser stiffness wood can be the perfect choice. The way that we price it you pay a premium for the higher rated wood. So, if your requirements are not such that using slightly more dense or slightly larger sections will not be a problem you can save by using this less stiff wood.
Does real thin Tru-Weight balsa have holes and slits in it?
The short answer is most likely yes.
Balsa is composed if numerous small diameter hollow cells that make up the majority of it's mass.
Interspersed in this mass are relatively large diameter hollow tubes referred to
as Vessels. The vessels are the plumbing system of the balsa tree serving to
carry water and nutrients through the tree. Unfortunately for our purposes these
vessels are frequently larger in diameter than the thinnest sheets. This causes
oblong shaped holes and slits. You will start seeing these holes and slits
starting with sheets around .010" thick and becoming very common as the
sheets get thinner. At True-Weight we try to select wood with the smallest
vessels possible for the thinnest sheets to minimize the occurrence of
these holes. Even with careful selection they still occur and are
considered normal in even the absolute best of wood.
How much are shipping charges?
Shipping costs are based upon the order total. USA rates are as follows:
Canadian orders add US$5.00 additional to USA amounts
|Order Amount|| ||Shipping charge|
|< $50|| ||$6.00|
|$50 - $99.99|| ||$7.00|
|$100 - $199.99|| ||$9.00|
|$200 - $299.99|| ||$11.00|
|$300 - $399.99|| ||$13.00|
|$400 - $499.99|| ||$16.00|
All others add US$10.00 additional to USA amounts