11 replies to this topic

[John R.]

I am planing a dob mount for an 8 inch f6. 
How do you keep the tube from slipping in the box? My tube box will have a removable top to balance the scope, but what keeps it there after adjustment. I am planing on a close fit with stick on felt to provide a fairly tight fit but the OTA is about 25lbs. so how do you keep it in place. 

[Oberon]

I split my box on one side and used over center clamps. This way the tube can be rotated and positioned where ever I like with the clamps loose, then clamped up really tight.

See below, the split and clamps are on lower side so hardly even visible.

[John R.]

I intended to have the bottom and sides fixed, and then a lid attached with pull tight clips. I was just wondering if the snugness of the felted on all sides box would keep the tube from slipping when turned straight up. 
This is the latches for the top, there will be 4 latches, two on each side. 

 

[Oberon]

Bottom line is you can’t make a close enough fit that will allow you to both fit the tube and hold the tube. You will need to fix it in place somehow with fixtures - glue, screws, clamps, whatever - which is why I recommend making a split box permitting the tube to slide through, find optimum balance, find optimum eyepiece location etc, then clamp. Very simple and very effective, but each to his own.

[John R.]

This is a crude drawing of what I have in mind. The draw tight latches pull the box top tight as it slides into the box. I would felt all insides of the  box. 
 

[Oberon]

Right…I see. Can you put blocks in the upper corners like you have in the bottom? That way the latch pressure is applied on 4 equally spaced points (or sections) around the tube, which will be considerably stiffer than 3 unequally spaced sections.You might want to look at stronger latches with more leverage too, I’ve used a few of these successfully…(type E). Finally anything you can do to spread the loads evenly (like match the curve of the supports accurately to the curve of the tube) will help stiffen and maintain tube integrity.
 

https://www.mcmaster...e-draw-latches/

[Don H]

I made an octagonal cradle for my 11.3" scope to save weight. For your 8", a box should work and won't be all that heavy. My cradle had a round front and back, with felt lining the edges. It was hinged on the underside and held together with shackle bolts on the top. These were easily loosened to allow rotation or to move the tube up and down for balance. They could be fully opened to remove the tube if desired. If you have a router, cutting the circles is pretty easy and will grip your tube around its entire circumference.

 

 

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[John R.]

The picture I’m getting is that if the clamping pressure is enough, friction is strong enough to prevent slipping of the tube. And, that a high surface contact area between the box and the tube is key to that. 
 

Thanks for the pictures and ideas, they are very useful. 

[kfiscus]

I'd suggest getting the box tension just so and forgo all the split box, hinge, latch drama.  Allow the tube to rotate w/ a tiny bit of force.  A lock ring above and below the box would keep you at your balance point.  A lock ring can be as cheap as an embroidery hoop or as easy as a large hose clamp.  A rotating tube allows for viewing by shorter folks.

Edited by kfiscus, 18 July 2024 - 03:45 PM.

[John Bali]

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[John Bali]

Tried to post a picture but for some reason cannot do so.  Anyway on my 6" dob I have a box like your picture shows.  The aluminum tube is a fairly tight fit.  To hold it in place drilled two holes and put in 1/4" wing bolts. They are threaded through the wood. 

 

As the tube fit is tight it takes hardly any pressure on the bolts to hold the tube in place. Very easy to adjust the tube position for best balance.

 

I considered all the suggested ways and went with the simplest one.  Has worked out well. 

 

John

[davejlec]

I've used a piece of closed cell foam on the inside of the cradle on a smaller Dob, a 5 f/5. The cradle is just 2 square pieces of plywood with 4 dowels connecting both pieces near each corner and routed-out in the center for the tube diameter plus foam thickness, required some id. sanding but works -easy to balance and rotate.