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

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#26 Lukes1040

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Posted 07 January 2019 - 07:43 PM

Sharp chisels indeed! I like the double edge support for the mirror. Was planning something similar for my larger build, if it comes to fruition

#27 jtsenghas

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Posted 08 January 2019 - 10:26 AM

Notes on my design work:

 

Before I get into more components of this scope, I thought I'd share a little on how I approach the actual designing of such projects.

 

As an engineer educated in the early 1980's I had a fair amount of drafting experience at school and got used to visualizing most 3d objects as 3-view drawings.  I also am comfortable enough with geometry and trigonometry to work out most dimensions from the underlying equations.  At GM in the 1980's I used the archaic CADCAM computer aided design software along with the big symbol keypads those systems employed.  At my current employer I have used AutoCad almost exclusively and went through the training disks in their entirety for AutoCad 2000.  I love how powerful this software is, especially for 3d work, but it is not freeware by any stretch; licenses for each copy cost several thousand dollars.

 

I have dabbled in Sketchup, but not enough to get really comfortable with it, and have also gone through a few basic tutorial lessons in Solidworks at work.  On both of these I'm fairly slow and relatively inexperienced, but I get by.

 

My favorite all-time free CAD software package is offered by a French company "Dassault Systemes" and is called "DraftSight".  It functions almost EXACTLY like AutoCad and can open AutoCad files (with a .dwg suffix), so as a result I LOVE it and have downloaded it onto an older laptop at home.  I haven't tested if files edited by both it and AutoCad are a bit "buggy", but I have drawn almost all my personal projects that don't need only simple hand sketches in DraftSight.  If you want a good version of a free CAD package and have any AutoCad experience, I can't recommend it enough.  It is free, but needs to be registered annually.

 

Here's a screenshot of the entire file I'm using on this project:

 

DS Overview.JPG

 

 

As you can see, I haven't drawn the scope up in its entirety, but I have drawn it in sufficient detail to work out a cut pattern, define critical dimensions, and verify a few clearances.  I worked out a way to layout  the altitude bearings on this scope with 14" inside radii that are offset 2" from the 16" outside radii without setting up the router to trim off the holes to be used as router circle jig centers, for example. 

 

Sometimes, as I did on this project, I print out a full size drawing of critical components to get hole patterns or complicated geometries precisely right.  When such things are large, my employer doesn't mind If I print them on the huge inkjet plotter at work, as long as I keep such things within reason.  That's how I made my altitude bearings for the Tardiscope (with its asymmetrical spokes) and the rocker box  that can fold flat and fit into the OTA turned inside out into a flat box.

 

For personal projects I seldom draw things in 3d to get dimensions in 3d.  For that I generally make sketches like this one for calculating my strut lengths and the compound angles of the struts on this project:

 

 calculations.jpg

 

The advantage of doing such things by hand (and with a free scientific calculator app on my phone) is not limited to keeping my math skills working. By expressing dimensions in terms of formulas I can easily make changes in the shop while the project is in progress, or work formulas backwards to optimize dimensions for available materials at hand.  In this case I verified that I already had sufficient aluminum tubing at hand, decided that the approximately 6.5 degrees the trusses tipped in one direction wasn't sufficient to angle the V-blocks I'll be putting their screw eyes into, and calculated how much to taper the hold-down blocks to clear the ends of the screw eye heads.  I tend to visualize a lot of angles using an analog clock face. 6 degrees is the spacing of each minute mark, for example.

 

When I'm experimenting with various versions of a potential design I keep copying portions of my file across or down on the CAD file and editing the copy.  This makes it easy for me to dabble intermittently with what I call "back burner designs" that sometimes take years to come to fruition.  It's not that i work at a slow pace, it's just that I tend to work on many projects simultaneously in the design phase.  For implementation I generally attack them with monomaniac focus, as is evidenced by the rapid progress of this particular scope.

 

I'll share close-ups of some of the above details as I discuss components of the scope as this thread develops.  I just wanted to share here my methods and to give anyone interested the heads up on that excellent package DraftSight.


Edited by jtsenghas, 08 January 2019 - 04:09 PM.

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#28 tommm

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Posted 08 January 2019 - 10:57 AM

Pretty much the way I work, but I use Turbocad, a lower cost CAD package (was $99 when I purchased it years ago), and after I work out the equations with pencil and paper, I put the them in Excel so I can easily do "what if" scenarios.  For example, I make a spreadsheet to estimate the torques on the OTA, then vary materials/construction to vary mass, rotation axis position, etc. 

 

Do you plan to use a shroud to block terrestrial lighting, and if so, how do you plan to keep it out of the light path of the primary?


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#29 jtsenghas

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Posted 08 January 2019 - 01:20 PM

Pretty much the way I work, but I use Turbocad, a lower cost CAD package (was $99 when I purchased it years ago), and after I work out the equations with pencil and paper, I put the them in Excel so I can easily do "what if" scenarios.  For example, I make a spreadsheet to estimate the torques on the OTA, then vary materials/construction to vary mass, rotation axis position, etc. 

 

Do you plan to use a shroud to block terrestrial lighting, and if so, how do you plan to keep it out of the light path of the primary?

Yes, I use Excel quite a bit for balance spreadsheets and things like that.  I haven't for this project because I haven't worked out all the weights of components though.  I currently plan to painstaking balance it with some embedded lead.

 

Yes I plan on a shroud and haven't worked out the particulars.  I see from photographs that Jonathan has collars of Velcro strips around the lower ring of the UTA and the base ring, so presumably his shroud stretches as a cylindrical shell well away from the trusses.  I considered having a lightweight mid-ring, which is something Mark Cowan threw out there on the hexapod thread as something that may also help with scope assembly.  I'm not really keen on that, though.  I'm considering having some longitudinal bungee cords attaching to button-type anchors to form a hexagonal tube just inside the shroud, too.  If the bungees are attached only at the top and bottom hems of the shroud, and if the shroud is a slightly stretched spandex this might be practical and handsome.

 

Suggestions for a shroud, anyone?  I realize that these hexapods with only a triangle of anchors at each end won't keep a shroud that is tightly fit around the poles adequately out of the light path.



#30 Oberon

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Posted 08 January 2019 - 02:54 PM

I’m really enjoying following this project JT. Great stuff.

 

My shroud attaches, as you say, with velcro at the bottom. At the top an elastic hem simply keeps it clipped over the lower UTA ring (no velcro). Being a stretchy fabric it forms a crude tube which doesn’t intrude into the beam. I don’t need bungee straps and neither, I suspect, will you. However its an easy enough solution to keep up your sleeve.

 

gallery_217007_4913_29456.jpg

 

gallery_217007_4913_72892.jpg


Edited by Oberon, 08 January 2019 - 02:55 PM.

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#31 jtsenghas

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Posted 09 January 2019 - 12:51 PM

Circles, Circles, Circles!

 

As seen in the screenshot of the CAD layout of this project in post 27 above, this project has a LOT of circles.  Because I used 3/4" (actually 0.710") plywood rather than the 1" plywood Jonathan used on Merope I actually had a few more circles and rings to cut than he did to build laminations from more layers.

 

Although I've made a lot of simple circle jigs of fixed radii for a router in the past, it was clear to me that this project really needed a well made adjustable circle jig.  In the past I've made circle jigs from single layers of plywood with a nail for the pivot and two-sided carpet tape to hold the router adequately to the jig.  (I use two-sided carpet tape a lot for quick jigs and fixtures, or to temporarily hold two workpieces together for symmetry).

 

For this project, with its wide variety of circle radii I needed something better.  An online search of circle jigs for sale, as is typical for me, motivated me to make my own jig to save money and gave me a couple of design ideas for a jig of my own.

 

This is what I came up with in short order using 1/2" plywood, some steel dowel pins, a 1/4"-20 flathead screw, and a knob with a 1/4"-20 brass nut:

 

circle jig.jpg

 

In the photo the jig hasn't yet had the through hole for the bit plunged through it, but it should be clear at a glance how it works.  The 1/4" slot down the centerline was made with a tablesaw and had the ends chopped square with a chisel to maximize the range for radius. I would have preferred to have made it from slightly thinner plywood--perhaps 3/8" to increase the depth range of the router, which is limited by the router bits used, but I have a few long solid carbide straight two-flute 1/4" diameter bits that worked well on this project.  It was a matter of using "what-have-you" materials for construction and that is what I pulled out of my "best scrap" bin.

 

Two dowel pins ensure the center pin plate won't rotate in use, and the pivot pin is made from a 1/4" dowel pin, which allows for precise measurements of radius.  I would just add 1/8" to the distance between the near side of the pin and either the near or far side of the router bit depending on whether I was making an inside or an outside cut.  A rigid pin of at least 1/8" diameter, but preferably larger as I used, helps enormously with getting a precise consistent radius.  In the past I've had nail pivots flex slightly or loosen slightly in their holes and give me circles slightly out of round.  I wanted precision, precision, precision for both function and to minimize required sanding.  To that end I also drilled the 1/4" center holes in the workpieces square using a block previously drilled in the drill press setup on my Shopsmith as a drill guide. 

 

Tool tips:  Drilling dowel holes a few thousandths undersized for hardwoods or about 1/64" undersized for relative softwoods such as this plywood allows for dowel pins to be retained if simply hammered in.  All these pins remained tightly in place for the duration of this project.  Additionally, small pieces of fine adhesive-backed sandpaper (torn from a used 5" sanding disk in this case) and adhered to one of the surfaces at the sliding interface of such boards can help to keep the jig from slipping for dimensions in use.  I merely had to turn the retaining nut finger tight and the radius was SET. 

 

The circle jig worked great with my plunge router base.  I just made multiple passes advancing the depth 1/8" per pass by using the convenient stepped turret on my Bosch plunge router base.  For decades I used either of two cheap under-powered routers.  Both eventually wore out. For the Tardiscope in 2014 I invested in a quality router with a two bases to switch between as needed.  I wish I had gotten this tool years before.  It has a gentle soft start, a wide range of variable speeds, and precision depth adjustment.

 

cutting first circle.jpg

 

The circles were smooth enough and consistent enough in diameter that, with careful glue-ups minimal sanding was required.  You can see the "mirror box" being glued here.  I did the gluing in my basement because I was concerned about slow curing in my colder workshop once I let the stove burn out. (Yes, Jonathan, it can't be summertime for ALL of us right now).  I clamped each of my laminations with my Black and Decker Workmate stands and as many clamps as I could find to make sure glue joints were tight,  Where I found some pieces to be slightly warped I placed the concave sides face to face to get very good overall flatness on assembly.

 

circle glue up.jpg

 

I favor Titebond 2 for such work.  It is the most common PVA (polyvinyl acetate) glue in my area and has long served me well.  I bought a gallon of it for my workbench construction project a couple of years ago and had nearly half of it left over.  It has a shelf life of only a few years--more if kept cool and exposed to a minimum of air (which can be accomplished by simply squeezing most of the air out before capping the bottle.  It's not worth risking an important job to stale glue.  I tested mine with scraps and it did admirably.

 

Tool Tips:  Waxed paper makes an excellent guard to keep drips from getting on surfaces you want to keep clean, or to keep your glue from adhering to your clamping tools.  Do NOT  wipe squeeze-out with wet rags as that thins the glue and makes it penetrate much deeper into surfaces.  Either wipe it with dry rags or paper towels or simply wait for it to cure and scrape it off.  Cabinet scrapers make GREAT tools for cleaning up glue after it has cured.


Edited by jtsenghas, 09 January 2019 - 07:35 PM.

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#32 Jeff Morgan

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Posted 09 January 2019 - 01:16 PM

lol.gif

 

That is a pretty serious bottle of TiteBond!



#33 ckh

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Posted 09 January 2019 - 01:26 PM

Great thread. Keep it coming. I need more clamps.


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#34 jtsenghas

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Posted 09 January 2019 - 01:55 PM

lol.gif

 

That is a pretty serious bottle of TiteBond!

Yeah, but it was one of those situations where a 16 oz. bottle was sold for about $6, and a gallon was priced at about $18.  I realized after gluing up all the layers on one section of my workbench that the entire project would take at least three more 16 ounce bottles, so I bought a gallon.

 

If I don't use it all before it gets too stale, so be it.  I certainly don't need to ration it on this project. wink.gif



#35 jtsenghas

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Posted 09 January 2019 - 01:59 PM

Great thread. Keep it coming. I need more clamps.

No kidding, I bought more clamps since this particular glue up.  Note that the Workmate is clamping through the two 2x4's that the bottle is resting on.  That Workmate took care of clamping at least one-fourth of the periphery and really helped me to align the layers.


Edited by jtsenghas, 09 January 2019 - 03:51 PM.


#36 CrazyPanda

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Posted 09 January 2019 - 03:22 PM

No kidding, I bought more clamps since this particular glue up.  Note that the Workmate is clamping through the two 2x4's that the bottle is resting on.  That Workmate took care of clamping at least one-fourth of the periphery and really helped me to aligned the layers.

Every time there is a sale on clamps somewhere, I get some. My wife keeps saying "Why do you need more clamps? Didn't you just buy some?"

 

She doesn't understand. 


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#37 Jeff Morgan

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Posted 11 January 2019 - 02:14 AM

Yeah, but it was one of those situations where a 16 oz. bottle was sold for about $6, and a gallon was priced at about $18.  I realized after gluing up all the layers on one section of my workbench that the entire project would take at least three more 16 ounce bottles, so I bought a gallon.

 

If I don't use it all before it gets too stale, so be it.  I certainly don't need to ration it on this project. wink.gif

 

re the 16 oz bottle, I always fall victim to the ATM fallacy:

 

"this is my last project"



#38 jtsenghas

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Posted 11 January 2019 - 02:20 PM

Altitude Bearings:

 

I've often observed on amateur made dobsonian telescopes that altitude bearings are a neglected component for serious design considerations.  All too often they are undersized, making them more sensitive to balance variation among eyepieces and finders, and also making them a little harder to set up for smooth motion if standard Teflon bearings are used.  Larger altitude bearings made in a "C" shape such as mine are are often made inadequately small in cross section, particularly in the middle where bending moments are greatest when the scope is aimed at low altitude targets.  The result is a "springiness" of motion, which can add to significant vibration observed at the eyepiece at higher powers unless bracing is added to them.  If the scope optical tube assembly is adequately rigid with respect to the primary mirror box these issues won't detract from collimation of the mirrors significantly, but such vibration can certainly detract from the observing experience.

 

Another issue I've observed on various builds on larger scopes is that location of the center of gravity of the actual bearings themselves is often far from the scope axis, due to the asymmetry of "D" or "C" shaped bearings. These are often of significant mass and are typically oriented at an angle of about 45 degrees.  This can result in balance issues with the scope when the scope is aimed near zenith being quite different when the scope is aimed near the horizon.  Completely circular bearings, which are more common on smaller scopes generally don't have these issues because their centers of gravity are at or near the altitude axis of the scope.  Very lightweight aluminum bearings on big scopes often aren't a significant problem in this regard either, if their weights are trivial compared to the mirror box even if their component centers of gravity are a bit off center.

 

All too often ATMers performing balance analyses treat the problem as being purely one dimensional and along the direction of the scope axis, which I'll refer to as the "z" direction of the optical tube assembly.  If we use a coordinate system in which the "y" axis is parallel to the altitude axis, I'm referring to balance issues in the "x" direction, perpendicular to "y" and "z" and pointing straight up when the scope is aimed at an elevation of zero degrees.  Imbalance in the "y" direction is generally less of a concern on a dob, since it results in one altitude bearing merely carrying a bit more weight than the other from, say, a focuser mounted on the side of an OTA.  Altitude bearing imbalances can work in a scope's favor since items such as finder scopes and, to varying degrees with varying angles, the focuser and its contents, tend to oppose the altitude bearings in "x"..  One has to be cognizant of the overall balance of a scope in the x-direction as well as z, which is often all people consider even when using spreadsheets to help with balance equations of torque.

 

My scope will have a UTA (upper tube assembly) that has the center of gravity very close to the optical axis for the wooden components, the secondary mirror and the spider to hold it.  It will, however, have a couple pounds on the top side for the Telrad finder and the 8 power x 50 mm RACI (Right Angle Correct Image) finder that will significantly oppose some of the altitude bearing imbalance.  The focuser, with a Paracorr 2 and some fairly wide angle ES82 eyepieces will be mounted, as Jonathan did, above a truss intersection 30 degrees from the OTA y-axis, so these items will also have torque components about the y-axis of about half the value they would have if they were mounted completely up top.  The total torque of these UTA components about the y-axis from these x offsets will still be less than that of the opposing (very large) altitude bearings I've chosen, though, and to reconcile this imbalance I'll be installing some lead plugs into both the rear ends of the altitude bearings and the ring of the mirror box.  The amount of counterweight required hasn't been firmly established, but should be as soon as I get a couple more days' work in on the UTA.  My hunch is that about five pounds will be required just to get the balance in the x-direction tuned in.  More on that when I get to it in the build.

 

Anyway, getting to my particular design...

 

I prefer very large altitude bearings and would have gone even larger on the Tardiscope if only they would have fit into my packaging of the scope in the storage configuration.  On this 12.5" scope in progress I simply chose to go as big as I could, provided the bearings could nest around my disassembled scope in the car trunk, and still get all major components from two sheets of plywood.   The outer radius on them is 16 inches:

 

DS Altitude bearings.JPG

 

The 34 1/2" dimension is just the rough cut line on my sheet of plywood.  I made the outside radius of the bearings 16", but the inside radius 14".  By offsetting the centers 2" (that's what the 2" line segments are in the above drawing) the resulting parts vary smoothly from just under 2" deep at the ends to 4" deep in the middle.  This 4" dimension was chosen to make sure the bearings weren't too "springy". To make the parts very consistent I drilled two 1/4" holes 2" apart in an oak block in my drill press.  I transferred that hole pattern to my plywood sheet by drilling one hole with that jig (to keep it square to the material), then pinning that first hole with a 1/4" drill bit, and drilling the second hole precisely 2" away.  I repeated the pattern with 9 1/2" spacing across the panel. These holes served as the centers for my circle jig.

 

From the above layout you can see that the pieces were placed 9 1/2" apart, which ensured my holes wouldn't fall on any of the pieces or outside of the 48" wide sheet.  I routered these like my circles with a 1/4" bit and progressive arcs in 1/8" increments of depth.

 

If I had decided to saw flats on the sides of my "mirror box" as Starman345 did on his similar scope in 2015 to mount the altitude bearings, I would have been satisfied with two layers, or four pieces as shown above.  Since I chose instead to carve notches in the altitude bearings for the mirror box ring I wanted more stability and made a couple more pieces out of the end of my second sheet as shown here:

 

DS last two altitude pieces.JPG

 

This decision required me to kick my setting circle off my two sheets of material, but I was fine with splurging for good Baltic Birch for that component for a bit more quality on that.  I'll get to the setting circle in time.  It's presently about 2/3 done.  I like that two pieces were done at 45 degrees to the plywood sheet to increase the number of grain directions on the three layer (2.1" total) laminations.  My youngest sister Annie visited from New York that weekend that most of the routering was done and took this photo of me making those last two pieces:

 

Routing altitude bearing.jpg

 

I did most of this work atop scrap pieces of oriented strand board (OSB) on two benches in my shop for a comfortable work height.  That half sheet of plywood was the most wasteful part of this project for material usage and should help to make clear why I didn't want to squander very expensive materials for these components.  I don't mind having to fill some voids and spend extra time finishing things to compensate for my "utility panel" material choice.

 

To improve the balance of these bearings somewhat, and to add some artistic flair, I chose to bore a number of holes in the bearings after assembly that varied from 1 1/4" to 2" in diameter as shown in this layout:

 

DS Altitude 2 view.JPG

 

I'll have to continue on the next post, as I've run out of room for photos on this one...


Edited by jtsenghas, 11 January 2019 - 08:52 PM.

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#39 jtsenghas

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Posted 11 January 2019 - 03:24 PM

Altitude Bearings continued:

 

After gluing and rough sanding the laminated bearings I used some of my scrap pieces to make yet another jig, a rotary table setup for my Shopsmith.  This used a couple of aluminum brackets I use for such things from time to time to set things up square.  I configured the Shopsmith as a drill press and fitted it with a 1/2" router bit and cranked the speed up to near the maximum:

 

altitude bearing jig.jpg

 

The ability of the Shopsmith table to be adjusted to and from the operator with the rack and pinion arrangement that usually controls table height when the machine is in the horizontal configuration, and to advance the quill feed for measured cutter height adjustments really helped.  I moved the table away from me with repeated passes until the radius on the part matched the outside radius of the mirror box ring.

 

To finesse the outside surfaces to an exact match on the disk sander setup and bore the holes with good symmetry, I attached the two bearings together with two-sided carpet tape.  I love that stuff for quick and dirty setups where screw holes may present problems and clamps would be in the way:

 

altitude bearing disk sanding.jpg

 

altitude bearing boring.jpg

 

I then peeled the two pieces apart, made a 1/4" roundover with my router on all edges except the bottom one that the laminate will be attached to, and sanded away the paper thin mahogany layer of plywood.  As previously mentioned, this is a mere 0.010" thick, chips easily, and doesn't match the birch edges or the Baltic Birch used elsewhere:

 

 altitude bearing sanding.jpg

 

Finally, after boring 3/8" holes through the mirror box ring and extending them into the altitude bearings, I opened the holes in the bearings to 1/2" and installed the 3/8"-16 threaded inserts:

 

altitude threaded inserts.jpg

 

A few notes about this operation.  I had previously purchased 3/8"-16 threaded inserts from the Woodcraft store in Toledo, and these had a finer thread and a shorter thread length of about 1/2" than I wanted to use here.  The tension on the rearmost inserts when the scope is aimed low is expected to be considerable.  When I went to my local Ace Hardware store to buy other components, I noticed they had 3/8"-16 brass inserts that had a coarser thread and a greater length (about 3/4" thread and 7/8" overall length).  I bought six of these and buried them as deep into the bearings as I could without risking disturbing the bearing surface underneath in order to put as much of the altitude bearing into compression in use.  I made an installation tool by grinding down a 3/8" nut to just under the 1/2" diameter pilot hole.  It did well with a long bolt as an installation tool. A small antique square helped ensure that the insert was started nice and square.

 

I've received the black, slightly pebbly laminate to attach to the underside of the bearing from my local Lowes, but haven't applied it yet. I also picked up a pint of contact cement since the last time I used mine was too long ago and it's getting quite pasty.  I wanted to buy black, crystal finish Formica, which goes by the number 909-42 (909 = black, -42 = crystal finish).  I was told at Lowes that the crystal finish has been discontinued, so I ordered the closest match they had in the Wilsonart brand.  This material, in the "standard" finish for Wilsonart, also called "matte" is nearly, but not quite as pebbly as Formica's crystal finish.  It did appear to perform well for low friction and similar static and dynamic coefficients of friction when I rubbed a Teflon block that I had in my pocket against the sample coupon (to the amusement of the salesman).  I'll tell you how it works out.  I also have a trick for how I'll trim it to the bearing up my sleeve I'll share when I get to it.

 

As for Formica and the crystal finish, I had a thread on this forum last month inquiring if others had the same experience.  I wondered if Lowes might simply be pushing a preferred supplier and knew that, ever since Ebony Star was discontinued, Formica 909-42 was known as the preferred material for this application.  An interesting response I got on this was from Danny (Pinbout).  He asked his buyer at work to inquire into this.  The story they got was that Formica was discontinuing crystal finish except in Black and White.  It may be available directly from Formica, or from other dealers.  Lowes, however, is reportedly discontinuing ALL crystal finish Formica. 


Edited by jtsenghas, 11 January 2019 - 09:05 PM.

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#40 Pierre Lemay

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Posted 12 January 2019 - 08:51 AM

JT, I’m just now getting deeper in reading your thread. I really appreciate the way you are presenting it: very good and detailed descriptive text, with paragraphs just long enough to describe the theme being covered and frequent and relevant photographs that are there to support the written descriptions that you provide. 

 

Someone wrote, earlier, how he appreciated detailed descriptions rather than just a bunch of pictures. I think a good technical article, like the one you are writing, needs both. They complement each other. And I would add: you can never have enough pictures. But, having done a few of these articles, I can appreciate how much work you must be putting in the writing of all this. I, for one, am enjoying your adventures and will continue to follow, AND LEARN, from your thread with much interest. 👍. 


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#41 ckh

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Posted 12 January 2019 - 11:36 AM

JT,

 

I guess you intend that the altitude bearings can be removed from base ring?  In this picture:

 

gallery_240847_5047_36793.jpg

 

Is the pivot point where the red circle is?  I don't understand exactly how you moved the bearing toward the pivot point. It looks like the position of the router bit is a fixed distance from the pivot.



#42 ckh

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Posted 12 January 2019 - 12:00 PM

I need a little help with this one too:

 

post-233063-0-79604800-1547237002.jpg

 

The bolt is threaded into the insert and the ground down nut is used as a jam nut?  You're not engage the slots, right?



#43 jtsenghas

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Posted 12 January 2019 - 12:28 PM

JT,

 

I guess you intend that the altitude bearings can be removed from base ring?  In this picture:

 

gallery_240847_5047_36793.jpg

 

Is the pivot point where the red circle is?  I don't understand exactly how you moved the bearing toward the pivot point. It looks like the position of the router bit is a fixed distance from the pivot.

As mentioned in the text, the table can be moved on a Shopsmith, even in the drill press configuration. The tubes with a rack and pinion can be seen at the left. Yes, the pivot is a dowel pin where you circled. With each pass I either lowered the bit or moved the table to increase the distance of the pivot from the fixed router bit. The whole jig was moved.

 

Edit - Maybe it's not clear  but the pivot is not fixed with respect to the upright tubes. There is a hidden plywood panel screwed to blocks through the insert hole on the table. The dowel pin is tight to that lower panel (15/64" hole) and loose to the top board (1/4"hole)  


Edited by jtsenghas, 12 January 2019 - 01:35 PM.


#44 jtsenghas

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Posted 12 January 2019 - 12:35 PM

I need a little help with this one too:

 

post-233063-0-79604800-1547237002.jpg

 

The bolt is threaded into the insert and the ground down nut is used as a jam nut?  You're not engage the slots, right?

Yes. I actually jammed two nuts tightly together with about 3/4" of thread (just under the insert length) protruding.  I then ground them to about 7/16" diameter by spinning the bolt with a drill against a bench grinder. The tool doesn't use the slots, it just pushes. The ground nuts are small enough that the tool can install the insert well below flush. I wasn't inclined to spend $7 on an insertion tool that wouldn't fit as well into the pilot hole anyway.

 

Edit-  I should add that I had a little trouble on a couple inserts with the tool tending to jam tightly enough that on extraction it wanted to unscrew the insert. By removing the bolt with an impact wrench it broke the connection before unscrewing the insert. That happened on two of six, probably from my bottoming out so firmly in the blind hole. 


Edited by jtsenghas, 12 January 2019 - 12:46 PM.


#45 jtsenghas

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Posted 12 January 2019 - 12:54 PM

JT,

 

I guess you intend that the altitude bearings can be removed from base ring?

Yes, I've already pressed rosette head knobs on long 3/8" bolts, Carl. If the bearings weren't removable, I wouldn't be able to fit this assembly into my car. For observing at home I'll probably remove only the trusses and carry everything outside in three trips. That bearing and mirror box assembly will fit through my back door.


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#46 Oberon

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Posted 12 January 2019 - 05:14 PM

Yes, I've already pressed rosette head knobs on long 3/8" bolts, Carl. If the bearings weren't removable, I wouldn't be able to fit this assembly into my car. For observing at home I'll probably remove only the trusses and carry everything outside in three trips. That bearing and mirror box assembly will fit through my back door.

 

Wheels man, use wheels...


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#47 Oberon

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Posted 12 January 2019 - 05:16 PM

JT, I’m just now getting deeper in reading your thread. I really appreciate the way you are presenting it: very good and detailed descriptive text, with paragraphs just long enough to describe the theme being covered and frequent and relevant photographs that are there to support the written descriptions that you provide. 

 

Someone wrote, earlier, how he appreciated detailed descriptions rather than just a bunch of pictures. I think a good technical article, like the one you are writing, needs both. They complement each other. And I would add: you can never have enough pictures. But, having done a few of these articles, I can appreciate how much work you must be putting in the writing of all this. I, for one, am enjoying your adventures and will continue to follow, AND LEARN, from your thread with much interest. . 

THIS!

Nobody buys “how to” magazines anymore, THIS is how we introduce, develop and share skills and experience.


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#48 gnev

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Posted 12 January 2019 - 06:22 PM

With todays throw away mentality and I'll just buy it already built so much skills and experience will be lost. I'm from the generation where fathers with these skills taught you. Whether I liked it or not I had to help my father when he built and repaired things. Surprisingly when I see some things done here I remember being shown how to do these things. It's a shame that so much emphasis is placed on college and the trades are  forgotten as a desirable profession. 


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#49 totvos

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Posted 13 January 2019 - 09:12 AM

Yes. I actually jammed two nuts tightly together with about 3/4" of thread (just under the insert length) protruding.  I then ground them to about 7/16" diameter by spinning the bolt with a drill against a bench grinder. The tool doesn't use the slots, it just pushes. The ground nuts are small enough that the tool can install the insert well below flush. I wasn't inclined to spend $7 on an insertion tool that wouldn't fit as well into the pilot hole anyway.

 

Edit-  I should add that I had a little trouble on a couple inserts with the tool tending to jam tightly enough that on extraction it wanted to unscrew the insert. By removing the bolt with an impact wrench it broke the connection before unscrewing the insert. That happened on two of six, probably from my bottoming out so firmly in the blind hole. 

I had wanted to use threaded inserts on my project but always had trouble sizing the hole and splitting the plys. I see you are clamping while threading. Is that the secret sauce? And rules of thumb for insert holes vs. insert size?

 

And to continue the applause, this is an outstanding build thread!



#50 jtsenghas

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Posted 13 January 2019 - 09:28 AM

I had wanted to use threaded inserts on my project but always had trouble sizing the hole and splitting the plys. I see you are clamping while threading. Is that the secret sauce? And rules of thumb for insert holes vs. insert size?

 

And to continue the applause, this is an outstanding build thread!

Tom, I had mentioned the clamping with the vise on a "What did you work on Today" post of this operation, but neglected to include it here.  When screws or inserts are used in narrow sections or edges of plywood it really does help to protect against splitting to clamp the workpiece solidly. That's an oak 2 x 4 jaw I've added to a classic Craftsman vise. Inserts are generally designed to use standard sizes for pilot holes: 5/16" for 1/4", 3/8" for 5/16" and 1/2" for 3/8".  Sometimes I choose a slightly larger bit (about 1/64 over) to reduce stresses in hardwoods. I've also coated threads in epoxy. 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.

 

Edit - Oh, and Bill mentioned in his thread that it helps to countersink the top of the hole to avoid damaging the surface.  Clearly he has a lot of experience judging by the variety of bagged inserts shown in one of his photos.  He reportedly has used them a lot on speaker builds. 


Edited by jtsenghas, 13 January 2019 - 09:37 AM.



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