Balancing Act

For the past year I have been studying Kreige and Berry's Dobsonian book and I'm getting ready to start building my first truss tube dob. The optics are a 14.5" f4.3 primary and a 2.6" secondary. Most of the parts are bought and I just picked up the beautiful European birch plywood last night. 8)

So I haven't even cut any wood yet and I'm starting to get nervous. :S I've been studying the section on balancing the scope. How successful were those of you who have built truss dobs in making a balanced scope on the first try? I'm an pretty good woodworker, but somewhat mathematically challenged. :S

Any advice would be appreciated.

[COLOR="Purple"]Don't you love that birch plywood!

For balance, some folks swear that using a spreadsheet can get everything figured out in advance. I'm not so good with numbers either so I build the OTA, find the actual balance point, and build everything around the OTA.
Bill[/COLOR]

Is your mirror full thickness? If it is then you should really be pretty good with a 'normal' depth mirror box. If it is a lightweight mirror then you may be a bit top heavy.

I agree that balance seems like an untamable beast...I have always used a spreadsheet to layout the torques for the major components ahead of time. The trick with truss dobs are the length of the tubes. In the past, I have always gotten some graph paper and made a scale drawing. This last time I used MS visio to do the detailed layout..it worked like a charm everything came out very close.

After you have a pretty detailed plan the upper cage is a good place to start. Get it as close to done as you can and get an accurate weight on a postal store scale (they'll give you some great looks). If you do that, then you can cut the parts for the lower box with complete confidence.

Hello:
I think a good way to do this is to maintain a spreadsheet, and as you build parts you weigh them and update the spreadsheet. I plan to do this on my next scope. For my last one, I did these calculations on paper, built the truss tube without the bearings, then found the balance point by balancing the tube on the broom stick. The calculations on paper get you close enough to build the rocker box. Also, don't forget to include the weight of your favorite eyepiece! In my case, it's about half the weight of the top end unit.

Cheers
Mike Connelley

This weekend I am at precisely that point in my 16" K-B truss dob. I used the spreadsheet available on the web to get a rough mirror box height then added 2 inches. (Be sure the spreadsheet value used is measured from the BOTTOM of the cell frame to the top of the mirror box/balance-point) I feel much more comfortable with empirical data, so I intend to find the final balance point using an improvised balance beam.

First, I plan determine the CAGE'S balance point and place a pencil mark on that point. Next I will find the fully assembled mirror box's balance point and mark it also. I will then find the balance point of a 2x4 I will use as a balance beam. With the mirror box and cage hanging from the beam by ropes or straps that I can move back and forth on the beam, I will keep the BEAMS balance point on the fulcrum so that the moment arm of the two sides of the beam cancel each other. When the entire assemblage is in balance the distance from the fulcrum to the ropes will equal the distance from the cage's and mirror box's individual balance points (previously marked) to the scope's balance point.

Once the scope's balance point is known, I can trim the box down to the proper height. I figure the moment arm of the removed wood should be negligable.

Does anyone see a flaw in this plan? I guess those perfectionists out there could repeat the process after trimming and make the whole process iterative to the "sixth sigma"

Hi Jim--

I have a PhD in math and I don't bother to calculate the CG--I like Bill and Tom's approach better. With your short (f/4.3) mirror it will not be hard to get your scope balanced. As a rule of thumb, the distance from the center of your bearings to the secondary should be about 3 times the distance from the center of the bearings to the mirror--just build it and find where to hang your bearings when the whole thing is done. Also there's no law that says the center of the bearings has to be right at the top edge of the mirror box. As for the plywood thickness, my advice is to keep it light--1/2" is plenty thick; the bearings should attach with bolts, not screws anyway, and it's the gussets/corner braces (whatever you call 'em) that give the box its rigidity, not the thickness of the plywood. My $.02

Good luck!

George