10" Dob Construction - Aluminium

[tempestwizz]

Driving past a specialty tool-shop the other week, an Australian/Chinese TIG welder jumped into the back of my car! What to do I thought? I know, I'll make a new dob mount for the new10" F5 Suchting mirror I recently purchased.
I have previously built a similar style mount for my 12.5" F5 based on a design similar to that built by Peter Bobroff's for his wife's 16" F4, and a friend's 16" F5. I thought I would try and learn from my earlier mistakes, and document the progress.
My first earlier mistake was to try and use a MIG welder to do the aluminium welding. I guess the concept may have been OK, but the unit I borrowed just did not have the oomph to get good welds, and my results (although still holding together) are not a pretty sight. When you pull the trigger on a MIG, the wire spews out continously, and you have to (try to) do something with it.
The MIG I was using only had 120 amp capability, if I had a 200 amp unit things may have been a little different - but they are around $2K.
As I have since learnt, the more elegant solution is the TIG. It generates a plasma, and you only insert filler metal as required. There is still a learning curve in driviung these things, but considering I have had no training in MIG or TIG, I am quite happy with the results.

The first stage of construction is the mirror box. The idea is to make it detachable from the rest of the assembly, to allow easier and safer transport. The construction is from aluminium and (where possible) stainless steel extras. This is so that the whole assembly can be washed/cleaned under running water and left to dry without detriment. Just a final touch-up to the mirror if required.
The plate that the mirror will adhere to is of 4mm aluminium. One colimation adjusting point is fixed via a small tie-rod end purchased from a bearing supply place. The tie-rod screw into a tapped aluminium plug I inserted in the box frame.
The other two adjuster consist of screw mounted in opposition. ie a screw fixed to the mirror plate tries to pull the plate toward the frame, while another bolt in the frame tries to push the mirror plate away.

I am now working on the sides of the mirror box to shroud the mirror and form the joining mechanism for the OTA, and allow a place to put a front-cover on the box during transport.

Attached are a few pictures showing progress so far. I will update as things progress.

[tempestwizz]

Moving along with construction, I have made up some sides for the mirror box that will shroud the mirror itself. I will be able to affix a top cover plate in addition to a bottom cover plate for when the scope is in transit.

Next I have made up a 25mm tube bracket that interfaces with the top of the mirror box. This is the base of the OTA itself. This bracket is clamped to the mirror box with over-centre clips, and it is located in precise and repeatable position by three dowel pins inserted into this bracket.

It is important that as this project develops, all components are clamped in the correct position during any welding, as the heat distortion can cause twisting that is nigh impossible to correct later.

The attached pics will explain better than words.

[tempestwizz]

Today I built up the top end of the OTA, ie the bit that will hold the spider and focuser.
I needed to do that next as I have to come up with a way of getting the main struts of the OTA in alignment.
Hence, I have made some aluminium pins that will accept the 32mm round pipes that I will use for the struts, and welded them to the top frame.
The top piece I have made octagonal so that I can set the 4 vane spider 45 degrees offset from the focuser. I intend to use a Moonlite focuser with a filter-slide, so the spider cannot interfere with the filter slide if it is offset.
Later I will use some short pieces of 32mm pipe to position the aluminium pins (yet to be made) that will attach to the bottom section of OTA frame.

I could not see how to set up good alignment of the pins on the bottom OTA section without first making the top piece.

Pics attached.
Back to work all week... no progress expected till next weekend!

[MrB]

Looking good.
May I ask what store(Hare&Forbes?) and price for the TIG?

[tempestwizz]

The TIG welder is a UNI-TIG AC/DC 200. Made in China, but with some inferred Australian affiliation. I bought it locally in Fyshwick (Canberra) from a group called M&G who specialise in professional tools of all sorts. I was advised that this unit was not one they would recommend to a professional user, but for the hobbyist should be fine. The 'professional' models started around $4K, but this one ended costing under $1700.... which is less than the cheapest I had been quoted for a 200 amp MIG welder fitted out to do 3mm thick aluminium.
The shop threw in a few Tungsten rods suitable for Aluminium. I bought a pack of 2.4mm filler rods ( must be a couple of hundred by 900mm long) for about $50. I am using a 2.4mm tungsten rod doped with Zirconite. If you happen to physically touch the Tungsten rod on the 'work', there is a big 'splat'. There is a big black spot on the work, and you need to re-grind the point on the rod for the weld to go smoothly again. There are stacks of web-sites on TIG welding. I have absorbed quite a few, and with their advice, things seem to be going OK. (AC and pulse for Aluminium. The tungsten ground point burns into a round ball-shape. The ball should extend about its diameter from the ceramic shroud.) I have found that working toward the plasma works best. An instant-on welding mask is essential. Also a large, truly flat heavy steel table on which you can clamp things flat is a great help. ( mine is not so large).

My delight with this particular unit grows as I use it. I am not a trained welder, but I do have some basic understanding of the principles..... and I am now turning out some pretty good welds. ( some ****ty ones too from time to time to date, but I am thinking I was not getting enough Argon into the weld area, so it turned out a bit pocky). Nice thing about the TIG however is that you can go back over existing welds to 'heal' the bad spots. Something I doubt you could ever do with MIG.

HTH (Hope this helps)

Cheers

[MrB]

Cheers, yes very helpfull. Have Googled the model and now have some reading to do!.
Been toying whether to buy or build, I used to do some stainless work on an industrial TIG(Miller) with all the fruit and I miss it.
Not a welder myself either, was repetitive work so didn't need to know much, but the Miller was nice with all the features(including 100% duty cycle and a wand with water cooling and a thumb trigger etc etc) but they come at a price :(

Thanks, now to the reading.

[tempestwizz]

Moving right along.
I have welded the pins that will hold the OTA struts on both mirror end, and spider end. I have cut the tubes, but (hopefully) they are too long, and I need to mount the primary mirror, and the spider/secondary mirror to testthe length of these tubes before cutting to final length.
I have made up a 27 point mirror support. (It is a Suchting mirror after all and deserves the best!)
The declination rotation will be on two rings supported by roller bearings and teflon brakes. I have made up the rings, the actual mounting position will be determined by over-all ballance. These will attach to the OTA struts (somehow)

Pics attached to show progress. They may explain better than I.

[stringscope]

Looking good Tempestwizz :thumbsup: Mirror and cell components will all be RTV'd together like PB's 16"??

See you Thursday.

Cheers,

[tempestwizz]

I failed to mention a couple of important points.

1. When doing any welding on the assembly, ensure that it is all clamped together as best you can. Aluminium can warp terribly with heat stresses, and once bent, is difficult to re-straighten.

2. The mirror support pieces are made of 4mm aluminium plate. ( something which I have a stock of) The support points were calculated using the program mirror plop obtainable from the Bintel website. Rather than 'point' supports, I am using an 'area' support centred on the 'point'. ie I have calculated the support points, but am using thin o-rings filled with RTV silastic as the adhesion mechanism. A relief hole is drilled in one of the plates, the o-ring is slightly over-filled with RTV. As the support plate is pressed into position, the excess RTV extrudes through the relief hole. After the RTV is cured, ensure that the excess/extruded RTV is not in contact anywhere it shouldn't be.
I have used this method successfully on my 12.5" F5 Parkes mirror, and I believe Peter Bobroff has used it on his 16" F4 Suchting. The RTV requires about a week to obtain full setting strength. Thickness of RTV (which means o-rings) should be about 1-2mm. As the support layer mounting points decrease, the area (diameter) of the o-ring adhesion points should increase to maintain about the same contact area at all layers.

More as it comes to hand.

[tempestwizz]

I've made a little progress since last post. Now it seems that all the things that can get out of alignment need to come together. I have welded the OTA struts, but it took al lot of manipulation and clamping to get them (as close as possible to) square. I had to determine the length of the struts by clamping things together and waiting for a night when some stars could be seen. I found the TIG struggled a bit welding the solid aliminium alignment pins. Things seemed to get very hot, but not quite enough to amalgamate. The welds ended up a bit messy, but I am sure they are no going to fall apart anytime this side of the next supernova.
I have finished the mounting assembly for the mirror, although I still need to attach the bottom cover plate, and fasion a removable top coverplate to protect the mirror while in transit.
I have mounted the brackets to hold the Moonlite focusser and filter slide. It was all part of the juggle while determining the OTA strut length. The filter slide is on the inside of the OTA, but only intrudes about 13mm, and should not obstruct any light to the mirror. I can bring to focus the lens that reqiures the most 'in' travel. For me it is the Takahashi 3.8mm.

The next stage will involve mounting the declination rings. These will need to be centred on the point of balance when things are fully loaded. I still have to fix a plate for the finderscope and perhaps laser pointer on the top ring. I also need to decide if I need a hood above the spider to exclude stray light. These things all add weight to the top end, and hence affect the balance point.

As usual, pics are provided to explain better than words.

[tempestwizz]

As planned, I made up the Declination ring cage. I needed to fix the separation between the rings so that I can later make their supporting points on the RA ring. I made the structure in the form of a cage so that I can slide it up and down the OTA struts to find the balance point when everything is attached to the OTA. I will fix the cage onto the struts near completion of the project.

I have made a protective top coverplate for the mirror box which clips on and off with the same clips as the OTA struts. (Still have to make a bottom one)

Next I have made up the bearing assemblies to support the RA ring. I used some aluminium bar, and have tapped holes through the centre, and across for the bolts to hold the bearings. Bearings were selected mostly by cost! Common sizes are much cheaper that exotic varieties. Then the mounting bolts were selected to suit the bearings.
The idea is that the RA ring will sit on the top of the vertically mounted bearings, and be held central by the horizontally mounted bearings. One of the vertically mounted bearings is mounted on one end of a pivoting bar, with the other end fitted with a section of teflon rod. (MUST be teflon). The weight of the whole assembly causes tboth the bearing and the teflon rod to come into contact with the base of the ring. The teflon acts as a brake to the horizontal rotation action. The pivot point can be moved to apply more or less braking affect to the base. At this stage I have made a guess. If there is too much, or too little braking, I will move the pivot point accordingly - hopefully it may be OK where it is!
I have spent about three days worth of activity setting up the bearings on the triangular base frame, and filing the RA ring to get it round. The roundness is important for the smooth rotational action. I figure I have another day's worth of filing to get it acceptable. Currently, it still binds a little at one section in the rotation.

Pics attached to explain better. More as it develops.