Aluminium tube and threaded inserts for truss Dobsonian
I am ringing around for 1 1/4" aluminium tube for the trusses for the 18" Dobsonian project. Two parts to my question:
1. Threaded inserts: Does anyone know of a local supplier for the threaded inserts described in Kriege's book? If not, and I have to purchase from Obsession Telescopes (which I'm happy to do if necessary), will there be enough tolerance for 1.6mm tube.
2. Aliuminium Tube: All I can find so far is 1 1/4" (32mm) diameter, but 1.6mm thick. Now Kriege's book, suggests 0.049" (1.24mm) thickness walls. My two local suppliers are Smart Aluminium and Williamstown Metal Merchants. I am looking for local suppliers so I can pickup, rather than delivery from interstate, and these two are on my side of Melbourne.
I got some threaded inserts from Hills Clearance Centre in Keysborough
Cambria Rd (cnr Bridge Rd) 3173
Ph: (03) 9238 2555
I am not sure if they have any as large as you want. I know they have 25mm but you may not need 1 1/4. Most people feel the Kriege design is overbuilt. Others with more engineering expertise could advise you there.
Thanks Rod, I'll call them today. Yes, I have been told that the Kriege design is over-engineered. But I always err on the side of caution, but happy to go with 1" (25mm) tubes if enough ATM's convince me
Steve, You'll find a formula in Kreiges book, that gives a ratio of pole diameter to truss span of about 1/80 . So for under 2 meter length truss tube , 1" is fine. Wall thickness does little for stiffness but rapidly adds weight as you increase , which means larger mirror box to cope with higher side bearings, hence large rocker box etc., so I would use the 1.2mm wall.
Another advantage of using 1" tubes is that they will fit the new Moonlite aluminium split blocks, so using splits on the bottom and ball joints on the top makes for a very quick assembly both before and after you finish your scope.
I'm going to use this system on the 14" F4.75 binoculars I am building. http://www.focuser.com/cgi-bin/dman....cgi&category=5
I'm also going to use their Delrin threaded tube inserts for the cage spacing tubes. I decided 20 X metal 1" threaded tube closures needed will add too much weight to my top end.
Thanks Mark, yeah I've decided to go with 1" tube. Unfortunately three companies I have contacted today in Melbourne only have 1" tube with 1.6mm walls. This mean the ID is 0.16mm smaller than a tube with 1.2mm ID. I think the tolerances on the Moonlite's Delrin threaded tube inserts will cater for this small amount, would you agree?
Also, I've looked again that the Moonlite truss parts. I would need 4 x TC100D for the secondary cage, and 8 x TC100S for the Mirror box.
Or would you use the 8 x TC100A for the mirror box?
And Mark, I as I may have mentioned to you over the phone last week, I have a habit of over-engineering things - a habit of mine which can get quite obsessive and unnecessary.
I 'd be using these for the bottom end as they hold all your poles at the right angle to drop on your cage. Use the twin ball/ socket pairs for the top. Too easy.
Heres a link showing a minimal 16" project: its always good to see how people handle functional weight extremes.
Capral Aluminium in Sydney have 1" OD X 1.2mm wall , I'm not too familiar with Melbourne suppliers. You'd find that you'll need to machine the inserts for 1.6mm wall. You could also send Moonlite an off- cut as they'll machine for $1.50 each, though obtaining some 1.2mm would be the best outcome.
I got moonlite to machine for a 1.6 mm pole they charged $1 each
my scope 12" dosn't have very long poles though .
after the fact I noticed another rack with a thinner wall tube
it was marked as 25.4 mm 1.24 mm wall or something close
so it is around if you hunt for it.
good comany to deal with took about 10 days to do the custom order
3 days back to aus
I got moonlite to machine for a 1.6 mm pole they charged $1 each
my scope 12" dosn't have very long poles though .
after the fact I noticed another rack with a thinner wall tube
it was marked as 25.4 mm 1.24 mm wall or something close
so it is around if you hunt for it.
good comany to deal with took about 10 days to do the custom order
3 days back to aus
cheers graham
How did you find the "fit" when when you actually put the machined parts into your 1.6mm-walled tube? Was it a good fit?
What keeps these parts in the tube? Are they just a very tight fit? Or does the action of screwing in a screw into the thread tighten it up?
Smart Aluminium has 1" (1.6mm) in stock at $20.80 per 6.5m length and they will cut it for me.
The fits fine steve .. they make some pretty nice focusers from scratch so
a little fine tuneing of there tube inserts isn't a problem.. few differant ways of secureing them reading there advice here http://www.focuser.com/cgi-bin/dman.cgi?page=trussconn
I'm half way thru an 18in dob build as you know( using Kriege's book) and have on hand a set of moonlite twin ball/socket ends (for top and bottom) and have gone with 1 1/4in tubes. The fit is just a tad loose but a little bit of 5 minuite areldite will fix that problem. The instructions suggest putting a screw thru the tube into the derelin as well. Although I dont have these its a good suggestion to have split blocks on the bottom end so the poles are held upright, making your job of attaching the upper cage easy, otherwise its a two man job.
I got the threaded inserts from obsession and they are worth it.
Just to throw you a "wabbly" I did not consider the 1in tubes, I know Kriege's dobs are overbuilt but you are talking about a smaller diameter and thinner tube with the 1in comparred to the 1 1/4in tubes, too risky in my humble opinion.
When you build the upper cage and see the size of the bloody thing you will appreciate the larger tubes!!! Krieges book explains the mathametics of a smaller diameter which scarred me into the bigger tubes.
Have you tried Capral in Clayton? On Dandenong Rd, near Blackburn Rd. This is where I get all my aluminium. Unfortunately, they don't seem to have a catalogue of extrusions on their website, I was looking for some solid round bar around 120mm a couple of days ago...
When you build the upper cage and see the size of the bloody thing you will appreciate the larger tubes!!!
Krieges book explains the mathametics of a smaller diameter which scarred me into the bigger tubes.
*And yet the formulae he gives in the book suggests one inch tube for an 18" F4.5* . I think its really a case of people go for 1 1/4" tube to visually match the over built top end. For a 60" tube span on an F4.5 18" , 1" tube easily satisfies Kreiges 1/80 rule.
My attitude is to make every square mm of a telescope justify its existance. The primary enemy of large scopes is thermal mass , as you don't always have time to give your scope a long cooldown. Building your scope becomes a compromise on how heavy you are going to build vs the winds you intend to observe in.
G'day Mark, agreed with your comment but is that not with a slightly thicker walled alu tube? Or does the wall thickness not matter too much? I was thinking since the conversion from imperial to metric cant be matched exactly (ie the components available in OZ are usually a little undersized when trying to match whats in the dob book) would it not be wiser go go slightly bigger?
One thing I did not notice before is that Steves mirror is a F4.5, mine being a F5 so that will definitely make a difference.
Hi Mike, If your pole length is shorter than 80" then 1" will do according to Kreige. I've used a 100" FL 18" F5.5 'scope with 1" tubes and couldn't fault it. I had a look at Kreige and he doesn't really discuss the wall thickness in relation to his formula.
Thickness is not so important as diameter, but if you were going to 1.6mm wall tube in 1" , you might as well use 32mm X 1.2 which would probably weigh similar. From memory the problem with long thin poles is resonant vibration in a breeze, though I've never seen it myself. He generally refers to cases of a classic 20" F5 scope. I certainly wouldn't use 1" poles on a 20" F5.
I got some threaded inserts from Hills Clearance Centre in Keysborough
Cambria Rd (cnr Bridge Rd) 3173
Ph: (03) 9238 2555
I am not sure if they have any as large as you want. I know they have 25mm but you may not need 1 1/4. Most people feel the Kriege design is overbuilt. Others with more engineering expertise could advise you there.
Hope that helps,
Rod.
Hi Rod, I rang them today and described the picture of one in Kriege. Yes they do have them in stock, so I'll charge up the e-tag and drive to Keysborough next Monday.
Graham, I see you have used the double ball-joints on both ends. I'm almost convinced with the Moonlite connectors, but it will add up pretty pricey. 8 x USD$25.00 + ($16 for machining) = $200 plus. My second option is the Obsession truss tube clamps at USD$120. Actually I don't think they look too bad, and I'd have to make the mirror box clamps.
Picked up the threaded inserts at Hills today. Bought a handfull of 1" ($1.20 each) and 1 1/4" ($1.00 each). Bargain!![/quote]
Glad it worked out Steve.
With the upper truss assembly I used the offset bracket design described by Krieg and Berry. They are cheap and easy to make. If you use their design, you have a loose wing nut or knob which is not much of a hassle - you can attach it with a short chain to the upper cage.
I am experimenting with keeping my struts in pairs, held together with a captive bolt. I haven't fully de-bugged it yet but happy to send a photo if you are interested.
