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J.T.'s 12.5" F/4.3 Hexapod Dob

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#51 tommm

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Posted 13 January 2019 - 10:26 AM

Yes, on plywood, like baltic birch, I always countersink about 1/6" or a bit more to avoid having the top plys separate. When the helicoil first enters the hole all the force pushing it in is on the top plys where its threads are engaged.  It's a lot of force on those plys in harder wood like b.b. so they sometimes will separate and split.  It also gives room for the two nuts locked together on the bolt to move down past the surface of the b.b. a bit so the helicoil is countersunk a bit below the surface of the baltic birch.  I just use regular nuts for double nutting the bolt before threading into the helicoil. I put a bit of lithium grease on the helicoil and bottom threads of the bolt so they are easily separated. The two nuts are easy to loosen from each other and the helicoil using two end wrenches.


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#52 jtsenghas

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Posted 13 January 2019 - 10:41 AM

...The two nuts are easy to loosen from each other and the helicoil using two end wrenches.

Yes, but note that in this application I chose to bury the insert well into the wood for more tensile strength.  Also, we should differentiate between helicoils and threaded inserts, which come in both versions for metal and wood. Helicoils require special taps when used in metals because their spiral design has an odd thread pitch for the outside diameter.  They can be used in wood but don't have threads that retain as well in the fibers as what I used here.  Threaded inserts for metal have standard machine screw threads for the outside diameter. Threaded inserts for wood have sharper threads at a narrower angle for better cutting and retaining. 

 

Two jam nuts on a bolt work well with any of these provided one is not trying to set the insert below flush. 


Edited by jtsenghas, 13 January 2019 - 08:24 PM.


#53 ckh

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Posted 13 January 2019 - 03:06 PM

Roscoe mentioned in Bill Schneider's build thread that he likes to go oversized and use Titebond glue both as a lubricant and to help anchor things, although, strictly speaking that wouldn't adhere to the brass.

Would the Titebond strengthen the wood around the threads?



#54 jtsenghas

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Posted 13 January 2019 - 07:21 PM

Would the Titebond strengthen the wood around the threads?

I have no doubt it would some  and I think it's a good idea. Titebond really acts as a lubricant for assembling tenons into mortises, so I have no doubt it would lubricate well these inserts going in too. 



#55 Jeff Morgan

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Posted 13 January 2019 - 08:11 PM

A fabrication Tour de Force. Please keep it coming!


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#56 jtsenghas

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Posted 14 January 2019 - 12:24 PM

The Upper Tube Assembly (UTA):

 

The woodworking for the UTA is nearly completed with some of it done just yesterday, and it is only a little heavier than I was targeting.  I intend to show in a couple of long posts here that portion of the process.

 

I've noticed that UTAs on truss scopes are sometimes made a bit too flexible, at least on the lower rings.  I understand the desire to make this structure lightweight, since each pound at this end needs to be counterbalanced by several pounds at least on the lower end for balance.  It's all too easy to overlook, however, that the UTA is an integral part of the truss system and must be quite rigid itself, especially for tension and compression between the three or four top vertices of the struts.  I completely understand why exotic materials such as carbon fiber with cellular cores or end grain balsa wood cores are often chosen on the lightweight builds.  As Jonathan said at least once on these pages: Stiff, lightweight, cheap--choose any two.  I'm trying to choose moderately lightweight, moderately cheap, and very stiff.

 

To make my rings very stiff and moderately lightweight, I opted for a hollow panel construction, similar to that of interior house doors:

 

UTA lightweighting.jpg

 

 

Before I used my router to cut the inside radius of the ring I set the bit depth just 1/8" shy of the full 0.710" panel thickness and made passes at multiple radii to eliminate much of the material.  Full thickness material was left at the locations where trusses and UTA struts attach. Otherwise, I cut within 3/8" of the inside and outside radii.  I didn't bother to clean up the ends of these cutouts since they would be buried in the assembly.  To close things up, I cut rings of the same inside and outside diameters from 3 mm thick Baltic Birch.  These were glued up with Titebond 2 and sanded. The resulting panels were nearly 7/8" thick, but no heavier than 7/16" plywood and nearly four times as stiff.

 

To attach the two rings together, and to provide an easily drilled structure for the wire spider, I used 5/8" wooden dowels. To provide very rigid and controlled connections to the rings I turned the ends down to 1/2" tenons on the table saw:

 

 UTA tenons.jpg

 

As long as one is working with straight and very round stock, perfectly acceptable tenons can be made this way.  The saw fence acts as a depth stop.  The clamped miter gauge keeps things square, making a bit of a V-block with the saw surface. One merely has to spin the workpiece tip gradually into the blade and clean up any rough areas with repeated spinning at various depths.  If one starts with dowels of precisely the same length, the shoulder to shoulder distance on the tenons will also be very consistent.

 

Tool tip:  To precisely tune in the tenon diameter, adjust the miter gauge position rather than the saw height for the final microadjustments.  On a 10" saw, moving the miter gauge 1/4" beyond the top of the blade reduces the blade height by only about 0.006" and increases the tenon diameter by 0.012".

 

I then cut some thin wedges from some scrap maple I had and made saw kerfs the full length of the tenons with a dovetail saw:

 

UTA tenon kerf.jpg

 

Tool tips:  Round stock gets damaged easily in a vise. Putting soft material in the vise jaws or, as I did here, cork-lined jaws held in place with magnets will prevent the damage.  Note also the marking knife.  A good, very sharp knife with a moderately narrow double bevel and a 60 degree tip or less is indispensable for fine woodworking for layout, measuring and marking.  Although high end woodworking stores are willing to sell excellent knives of such profiles (for a very high cost), one can sharpen a basic pocket knife to that profile on a bench grinder.  Even a kitchen knife can be repurposed this way.  My philosophy is to save money on such tools to spend on good tool sharpening equipment, like diamond plate steel sharpening stones.

 

With glue on the tenons and wedges I pounded things securely together.  The resulting glue-up was reassuringly rigid and square in both directions:

 

UTA glueup.jpg

 

The thin plywood material within the rings is only dry fit to help it to relax in that shape between work sessions.  I discovered that the 3 mm thick plywood would have been a struggle to fit to that radius, and was also a bit heavier than I wanted to add to my UTA.  I chose to turn 3-ply, 3 mm plywood into 2-ply, 2 mm plywood by first belt sanding most of one layer away with a 60 grit sandpaper belt and following this with 80 grit paper on my random orbit sander.  This operation took less than fifteen minutes and I was grateful to have my holdfast to keep this thin material against my bench:

 

UTA plywood sanding.jpg

 

(continued on my next post)

 

 

 

 


Edited by jtsenghas, 15 January 2019 - 10:59 AM.

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#57 jtsenghas

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Posted 14 January 2019 - 12:45 PM

Upper Tube assembly (continued):

 

When the glue was dry I flush cut the tenons and wedges with a fine-toothed saw:

 

UTA tenon trim.jpg

 

In keeping with my tooling philosophy, this isn't a high end flush-cutting saw, but rather a cheap fine-toothed saw from a big box store with one face of it polished against a fine sharpening stone to eliminate the set of the teeth on that side.  As a result, this saw doesn't mar the workpiece.

 

Tool tip: Make such tenons about 1/8" long and flush cut them.  Much shorter and it is difficult to keep the saw from breaking out at the end.  Much longer and it is difficult to get a tight fit of the wedge at the workpiece surface. 

 

The sanded surface of the wedged tenon cleaned up nicely:

 

UTA sanded.jpg

 

The V-groove in the above photo is to hold an epoxied piece of angle aluminum to act as a locator for my hexapod ends.  I rough chopped these about 1 mm undersized with a mallet and a 1/2" chisel, and then pared them to size and depth with another of my quick and dirty jigs:

 

V sanding jig.jpg

 

In the above photo I was just testing the jig on a scrap piece.  The final paring was done without a mallet and just hand pressure with the chisel against the four faces of the jig.

 

I made a focuser board out of poplar with a cove on the inside face so that it varies from 1/2" to 5/8" thick or so.  Poplar is among the lightest of the hardwoods and looks a lot like walnut when stained.  I settled on a focuser orientation that will put the fine focus knob downwards on my right side focuser scope:

 

UTA with focuser.jpg

 

The plywood won't be trimmed and fit to the inside of the UTA until after the spider is done.  It will take on more weight when the finishes are applied.  As of now I'm satisfied with the weight of just over 3 lb. for these components.  It looks a lot heavier, but remember that plywood is only 2 mm and the rings are hollow:

 

UTA as weighed.jpg


Edited by jtsenghas, 14 January 2019 - 10:33 PM.

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#58 gnev

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Posted 14 January 2019 - 01:34 PM

For your focuser board did you use a router or your table saw to make the cove cut. Thought I had seen somewhere that running the wood thru at an angle will create a cove. Did you have to special order the 3mm plywood thru a local lumber yard?. How big a sheet was it. Love seeing your workmanship. 

Glenn



#59 jtsenghas

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Posted 14 January 2019 - 04:09 PM

For your focuser board did you use a router or your table saw to make the cove cut. Thought I had seen somewhere that running the wood thru at an angle will create a cove. Did you have to special order the 3mm plywood thru a local lumber yard?. How big a sheet was it. Love seeing your workmanship.
Glenn

Thanks for the kind words, Glenn.

Sorry I glossed over the focuser board details. I first placed the focuser board into position and scribed one end at the inside radius of the ring with it in position using that short sharp knife shown previously. I made most of the cove by running the board crosswise through the table saw cautiously raising the blade only about 0.030" at a time until it reached that line in the middle of the board. Yes, running a board at an angle will create an almost elliptical cove (varying from an ellipse only a little from the blade thickness). At a 90 degree angle as I did here the circular radius matched the blade. I needed about a 7" radius and the blade gave me 5", so I still had a bit of material to remove on either side of the center. I took off most of it with a small block plane which, although it has a flat sole, got most of the curve removed making it somewhat hyperbolic in shape. Finally, I hand sanded it for a reasonably good fit. When the thin plywood is adhered to the inside of this board it should be even stiffer than it is, which probably is already adequate. I don't need heavy focuser components causing a twist to this board and throwing collimation off.

Tonight I'll try to get a photo of this cove and of the way I doweled the focuser board into place with three 1/4" dowels through the end rings. I like using dowels because they are lightweight (about the same weight as the wood displaced) and have low stresses due to their large contact areas. I bored three 2" deep holes into each of the rings and into the ends of the focuser board and glued those last. They still need to be flush cut.

I did have to get 3mm Baltic Birch at a specialty woodworking store. I went to Woodcraft in Toledo, which is about an hour's drive for me. They usually have it in stock according to the salesman there, but I was warned to call to check in advance. They have it available in some very small pieces. All choices of dimensions are listed online at Woodcraft.com, and can be shipped to the local store without shipping charges. I know 12" x 30", 24" x 30" and 30" x 48" were choices. I got 30" x 48" so I'd have enough for the two thin ring layers and the 10" x 44" piece to go around the inside of the UTA (with no allowance for mistakes).

Edited by jtsenghas, 14 January 2019 - 07:06 PM.


#60 gnev

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Posted 14 January 2019 - 05:18 PM

Thanks for the description and source for plywood. I found a website that describes cutting coves using an angled fence. Site also has a calculator for making different size coves and what angle to use.     https://woodgears.ca/cove/



#61 jtsenghas

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Posted 14 January 2019 - 05:27 PM

Thanks for the description and source for plywood. I found a website that describes cutting coves using an angled fence. Site also has a calculator for making different size coves and what angle to use. https://woodgears.ca/cove/

Ahhh... yet another excellent offering by that Canadian electrical engineer and "woodworking machinist" Matthias Wendell.

I've been meaning for some time to make one of his geared finger joint jigs that allows you to dial up just about any thickness of finger joint.

Matthias is my younger mentor. My older one is the excellent Paul Sellers in Great Britain. Paul has helped me to refine somewhat my hand tool layout and woodworking techniques. YouTube can be a wonderful thing....

Edit - It was Mr. Sellers, by the way, who gave me that excellent advice on marking knives in a video in which he snapped off a kitchen knife and sharpened it to just that profile I used in my marking pocket knife.

Edited by jtsenghas, 14 January 2019 - 06:23 PM.

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#62 jtsenghas

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Posted 14 January 2019 - 09:43 PM

Here is what the inside of the focuser board looks like.  As I mentioned in post 59, the dowels haven't been trimmed yet. The opposite end is doweled the same way. I intend to glue the 2 mm plywood to both the rings and the inside of this board and to trim it to match the 2 1/2" hole in that board.

 

20190114_212806_resized.jpg


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#63 Jeff Morgan

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Posted 15 January 2019 - 02:32 PM

Love the tip on taking one side of the hand saw against a sharpening stone!

 

I completely understand why exotic materials such as carbon fiber with cellular cores or end grain balsa wood cores are often chosen on the lightweight builds.  As Jonathan said at least once on these pages: Stiff, lightweight, cheap--choose any two.  I'm trying to choose moderately lightweight, moderately cheap, and very stiff.

 

Currently I am trying the CF route with balsa cores. It looks impossibly light, and it gives me some concern for the rigidity of the finished product.

 

In the event I do need to circle back and build a heavier upper end, my lower end is going to incorporate a couple of empty interior pockets (which are a smaller version of your UTA hollow cores). Should I need some balance help, the plan is drill a 1/2" hole into the pocket, pour in molten lead, then seal with a baltic birch plug and sand smooth.


Edited by Jeff Morgan, 15 January 2019 - 02:46 PM.

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#64 jtsenghas

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Posted 15 January 2019 - 02:50 PM

Should I need some balance help, the plan is drill a 1/2" hole into the pocket, pour in molten lead, then seal with a baltic birch plug and sand smooth.

Careful, Jeff, the melting point of lead is about 620 degrees Fahrenheit.  You'll almost certainly be making those cores out of material that can't handle those temperatures and may either melt or get charred by the lead.

 

I'm planning on making a split steel pipe mold, likely with 1 1/2" pipe, possibly larger, that I'll hold together with hose clamps on a thick steel slab.  I intend to cast cylindrical ingots for a few strategically placed weights.  I'll bore matching holes with a Forstner bits in my plywood, glue these plugs into place with RTV, and cap the holes almost invisibly with wood.

 

You might benefit by making your cavities some sort of symmetrical shape and doing something similar, even if it you use a corresponding mold box that gets somewhat charred.

 

I do have lead already.  I ordered online from Ace Harware.  In-store pickup had no shipping charges, which would have been significant.


Edited by jtsenghas, 15 January 2019 - 03:58 PM.


#65 jtsenghas

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Posted 15 January 2019 - 02:59 PM

Currently I am trying the CF route with balsa cores. It looks impossibly light, and it gives me some concern for the rigidity of the finished product.

Jeff, how is your build coming along? I know you had your inserts made before I turned my own.  Am I gaining on you?

 

I am very interested in your lightweighting methods.  I'm making no such lightweight claims on mine, but it is a bit easier to avoid making it a beast when the aperture is only 12.5".  I believe you owe us a build thread!



#66 Oberon

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Posted 15 January 2019 - 03:33 PM

You can also do balsa core with a thin <3mm ply. This works really well and doesn’t involve exotic materials or techniques.

med_gallery_217007_9394_176955.jpg


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#67 ckh

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Posted 15 January 2019 - 05:42 PM

Balsa filles the entire space? That might be stiffer than hollows.  What was the original size/shape of balsa?



#68 jtsenghas

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Posted 15 January 2019 - 06:02 PM

I presume that is entirely end grain balsa with all the balsa wood grain perpendicular to the thin plywood surface. No doubt that is lighter still than what I did  which is essentially plywood tubes with about 10 mm side walls, about 3 mm surfaces, and several solid pads. My rings worked out to about half the density of the plywood. 



#69 Oberon

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Posted 15 January 2019 - 08:54 PM

Balsa filles the entire space? That might be stiffer than hollows.  What was the original size/shape of balsa?

As JT says, its end grain balsa and you buy it in sheets made for the application in a standard range of thicknesses. I bought mine from a surfboard specialist supplies. 


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#70 Jeff Morgan

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Posted 16 January 2019 - 11:23 AM

Jeff, how is your build coming along? I know you had your inserts made before I turned my own.  Am I gaining on you?

 

I am very interested in your lightweighting methods.  I'm making no such lightweight claims on mine, but it is a bit easier to avoid making it a beast when the aperture is only 12.5".  I believe you owe us a build thread!

 

I think I'll just surprise you all with a post of the final product. If it works. wink.gif

 

In the meantime, I've been following the thread on the modified ball ends with interest. Also getting some great ideas from this thread!

 

You are way ahead of me in the build. Unfortunately, my workshop is the 3rd bay of the garage. Unheated, facing north (why did I ever buy a house facing north?). With 19 days of vacation coming up and minimal travel plans, hoping for warmer temps to get started again.



#71 careysub

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Posted 20 January 2019 - 11:53 AM

Careful, Jeff, the melting point of lead is about 620 degrees Fahrenheit.  You'll almost certainly be making those cores out of material that can't handle those temperatures and may either melt or get charred by the lead.

No problem! 

 

Get low melting point alloys here:

https://www.rotometa...fusible-alloys/

 

You can get dense metals that will melt in a cup of not-too-hot tea!

 

The classic, cheap alloy is Wood's Metal that melts at 157 F and they are currently selling for $10/lb:

https://www.rotometa...ot-woods-metal/

 

It contains lead and cadmium (but if you were planning on melting plain lead anyway that should not be a concern) but since you can melt it underwater, and handle the melted metal with plastic ware, it is easy to arrange to handle all of the manipulation under water so that no contact with fumes or particles is possible. I made telescope weights by melting the ingot and pouring it into a plastic tube, from which I could dispense convenient sized pieces. Never had to touch the metal.


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#72 careysub

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Posted 20 January 2019 - 12:00 PM

Love the tip on taking one side of the hand saw against a sharpening stone!

 

 

Currently I am trying the CF route with balsa cores. It looks impossibly light, and it gives me some concern for the rigidity of the finished product.

It shouldn't. Rigidity is the necessary result of the thickness, and the modulus of the skin (the modulus of the core is a bonus thrown in). It also has excellent crush resistance from the compressive strength perpendicular to the surface.

 

The caveats are that the CF/EGB has negligible strength laterally, and is also soft - it can be punctured easily. A 1 mm baltic birch sheet between the CF and the balsa can considerably improve this. Also all loads need to be distributed over the skin, or else inserts embedded in the core where attachments are intended.



#73 jtsenghas

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Posted 20 January 2019 - 12:12 PM

No problem! 

 

Get low melting point alloys here:

https://www.rotometa...fusible-alloys/

Very interesting! I'll stick to basic cheap lead for myself, though. I am taking precautions against contact and fumes and now have a 1 1/2" x 3" steel pipe nipple to split for a cast. The weights will be sealed inside the structure too. Thanks for the tip, though. 



#74 gnev

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Posted 20 January 2019 - 12:20 PM

When I built R/C planes i had one that was very nose heavy. I used  lead shot that is  used for reloading. Mixed up some epoxy thinned with isopropyl alcohol and poured over lead shot in a built up receptical in tail.  Lot easier and safer than melting lead. Never moved.


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#75 jtsenghas

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Posted 20 January 2019 - 12:27 PM

When I built R/C planes i had one that was very nose heavy. I used  lead shot that is  used for reloading. Mixed up some epoxy thinned with isopropyl alcohol and poured over lead shot in a built up receptical in tail.  Lot easier and safer than melting lead. Never moved.

I like that method and considered doing just that. For a lot of applications that would be both easy and safe.  I chose not to go that route due to the increased density I get with solid lead.  I have to consider that the effective density includes the loss of the drilled wood, too. There are lots of options, though. Thanks. 




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