Building a 2-D Counterweight System
by Ed Stewart <stargazer skymtn.com>
Links to alternate designs are at the bottom of this page.
To use plywood instead of metal for the Base Plate of the 2-D CW System, requires the use of high-density plywood
such as Baltic Birch, available at hardwood lumber stores. I recently saw a similar product at Home Depot in a rack next to
the regular grade plywood. It was labeled as 4' X 8' X 9mm (also 13mm and 19mm thickness) from South America. Both of these
products have many thin plys, usually 1mm thick vs. the 1 / 8 " thickness of regular grades.
The only problem I have experienced with these plywoods is that some sheets will have a brittle top veneer that chips-out
easily requiring patching and sanding. Otherwise, they can be sanded very smooth on the edges and the corners rounded with
a router to produce very professional appearing (and performing!) pieces.
The Base Plate dimensions given on the diagrams are for the 8" LX200 optical tube assembly (OTA). For the larger size OTAs,
I would increase the width from 1 .75 " to 2" for the 7" Mak & 10", and to 2 .25" for the 12" plus
increase the thickness from 9mm to 13mm. As to the slot that the carriage bolt rides in, I would not make it the full length
of the OTA, but half the length, since there is seldom any need to be countering weight at the corrector lens end only. If the
need should arise, then the Base Plate could be reversed in its position. By the way, a carriage bolt has a round head with a
square shank the size of the threads, e.g., a 3 / 8 " bolt has a 3 / 8 " square shank immediately next to the head
before the threads start:
Another alternated to the a slot is to drill a series of 3/8" holes to move the bolt to, but this would require removing
the Base Plate for each adjustment.
I am assuming you are an intermediate level woodworker
and therefore will not be explaining some of the saw setups in detail.
Procedures:
- Determine width and length for your OTA and cut the Base Plate to size. The distance needed to span is from the row of
3 small bolts on the bottom side of the corrector lens cell to the matching 3 bolts on the primary cell. This piece will
be attached only to the center bolts when completed. These bolts are provided for attaching accessories and do not retain
anything on the inside.
- With a 10" saw blade installed in the table saw, raise the blade just enough so that when the Base Plate is run over
the blade at 90 degrees to the blade the resulting arch cut down the length of the Base Plate
will just reach within 1 / 16 "+ or- to each edge of the bottom side of the Base Plate (see diagram--End View above).
I realize that this is not a normal cut. I clamped guide blocks onto the saw table to guide the Base Plate accurately and
safely over the blade. This arch cut will create a relief so that the Base Plate will sit firmly onto the round OTA when
clamped into position. If you are uncomfortable with this, then make shallow dado cuts that increase in depth toward the
center of the Base Plate that will provide the same relief to clear the OTA. Any of these cuts require proper set-up to be
performed safely.
- Alternate choice is not to shape the bottom of the Base Plate, but to add two small wooden strips on the bottom side of
the base plate along each of the long sides to elevate the Base Plate enough to clear the arch of the tube.
- Flatten the head of the carriage bolt by grinding until only an 1 / 8 " is left (see diagram--End View with Bolt).
Hint: cutting away most of this material with a hacksaw will save grinding time.
- Install a dado blade set to a cut the width of the carriage bolt head. (I ground off 1mm on 2 sides of the head parallel
to the sides of the square. shank to reduce the amount of wood cut away. In diagram r, this is indicated as 13 / 16 " for
the 3 / 8 " X 5" carriage bolt I used. Set the depth slightly deeper than the bolt head & make the cut from
about 1.5" from the lens end down the Base Plate about 2 / 3 its length-- (8" in diagram--Top View).
- Reset the dado blade to a width for a slip-fit of the bolt's square shank and a depth to cut all the way through the Base
Plate. Make the cut the same length as in Step 4.
- Optional--use a small (1/ 4 ") round over bit in a router to radius the top edges and corners.
- Almost done! Very carefully measure the distance between the mounting bolts on the OTA and drill holes slightly larger
than the #10 x 32 bolt holes in the Base Plate. The bolts originally in those holes will be too short to use. Try to find
#10 x 32 x 1" winged bolts or glue wing nuts onto #10 x 32 x 1" bolts. This way you won't have to use a wrench
to install/remove the Base Plate.
- Sand, fill-in any problems areas, and paint or varnish.
- Put the carriage bolt through the Base Plate with the head toward the OTA and mount the Base Plate to the OTA. Slip a
washer on to the bolt and screw a 3 / 8 " wing nut all the way down the threads to act as a clamp to position the bolt
in its slot. Thread another wing nut upside-down part way down the bolt sufficient for your counterweight's thickness plus
the third wing nut (see diagram--End View with Bolt above).
Making the counterweight:
Many possibilities here depending on what you have on hand or can think of. Some suggestions are:
- have a welding shop cut several different size squares out of 1 / 4 " or thicker plate steel and drill (or, even
better, thread) a center hole. The size would depend on what weight of your accessories you are trying to balance;
- using a small tin can, bolt a 3 / 8 " threaded connector (check where the hardware store has threaded rods) in the
center of the can and then pour in a mixture of epoxy & lead shot. Cut-away the can and remove the bolt from the connector.
The CW can then be threaded onto the carriage bolt without using the wing nuts;
- or use a stack of large "fender" washers;
- or ?
If you need help, I can be reached at <stargazerskymtn.com>
To set-up the balancing of your scope using a 2-D system:
- Load or install all accessories, eyepiece, and any other items onto the OTA
- Place the OTA in the horizontal plane or position (level with the earth)
- Adjust the CW along the horizontal plane using the bolt in the slit until the OTA balances; secure the bolt with the wingnut
- Rotate the OTA 90 degrees until it is pointing vertically or straight up
- Adjust the CW in and out along the bolt until the OTA balances; secure the CW with the two wingnuts on either side of
the CW
- Drawing showing procedures for balancing.
<http://www.amtsgym-sdbg.dk/as/telbalan.gif>
Note: should open a new browser window over this one>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Links to other 2-D CW Designs:
#1--From: Chris Vedeler <ckvedeleraccess4less.net> 
I made my own 2-D counterweight system that won an award at the '97 WesTex star party. It is very simple and only requires a
little JB Weld and some simple things from the hardware store and sporting goods store.
If haven't seen it take a look at: <http://benedict.isomedia.com/homes/cvedeler/scope/guide.HTM>
and go to the bottom where I talk about my counterweights.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#2--From: Ron Wagner <wagnerdma.org>
Another balance system is at my site. Not any better, just different.
<http://www.dma.org/~wagner/balance.htm> Note:
should open new window over this one.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#3--Another 2-D Counterweight Design:
From: Bob Cadloff <bomosympatico.ca>
Here's another 2-D balance design:
<http://www3.sympatico.ca/bomo/astro/jig/railsystem.html> Note:
should open new window over this one.
Woodcraft sells the inner extrusion at: <http://www.woodcraft.com/family.aspx?familyid=1774>
Counterweight Rail System Supplies
I've received a few requests from people asking for specifics on my SCT rail system. All the bits and pieces you need are available
from Lee Valley.
From: Chad Brink <cbrink(AT)dtgnet.com>
Two alternate supplier's for this extrusion are Woodcraft and Rockler:
<http://www.woodcraft.com/> Search on "Specialty
Jig and Fixture Hardware"
& <http://www.rockler.com/index.cfm?cookietest=1&> Note:
should open new window over this one.
Woodcraft has stores in most major US cities.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#4--Another 2-D Counterweight Design:
From: Ric Woods: <rvwoodsjuno.com>
I would like to share my 2-D CW design for my 14" LX200GPS.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
|
