Just started building another new scope. I was very happy with the one I built for Phil Hart so I am now using a 10" F4.5 mirror I bought from 33South a couple of years ago as the basis for another one of very similar design.
Attached images are the 303mm former for the UTA built up from scrap timber and the glue up of the first 2 layers of 1.8mm plywood. A final third layer to go tomorrow night. The UTA will have an ID of about 303mm, an OD of about 315mm and is 385mm long.
Final image is of the 8" F6 I built for Phil. This one should look similar, just a bit on the "plump" side.
Cheers,
Last edited by stringscope; 09-09-2008 at 10:14 PM.
Reason: add image and comment
h0ughy, This is the third UTA I have built using this technique, every time I build one of the formers they remond me of a winery basket press - we use to get a lovely basket press Shiraz from Rockford winery when we lived in the Barossa Valley - sigh!!
On the 8" scope the mirror box is edged with Jarrah. I will see if I can find a more close up image in my files. By edging in this way all the ply end grain is hidden and the exposed edges now present a very hard wood that better resists damage.
Final main layer of ply tonight. Note the outer face has been protected from glue staining and scratching with masking tape.
Cheers,
Ooops, sorry I have just noticed I failed to rotate some of the images.
Last edited by stringscope; 10-09-2008 at 09:48 PM.
Reason: Test addition
Here is an image of the rear of the 8" scope that better shows the Jarrah edging. As the particular piece of Jarrah I used to cut the edging strips from was a fairly light red, I used Jarrah stain as well just to provide a better contrast to the Hoop Pine plywood. Was a real pain to do, needed very careful masking and application.
The circular plate resting on top of the mirror box is the rear dust cover.
For the 10" I will be using a different mirrror cell arrangement.
Last edited by stringscope; 10-09-2008 at 10:20 PM.
Thanks guys. The power of the web, we can share these ideas in an instant - I still find it amazing
I source the ply from a local timber supplier, Turners Timber in Fyswick. I believe they get it from BrimsPly in Sydney. They call it "bending ply" and is 1.8mm thick 3 ply Hoop Pine, comes in 4X8 size sheets. So with 3 layers of ply you get a light weight UTA with 9 laminations and less than 6mm thick .
Here is a link to Dave Gaults website where he describes the ply tube fabrication process much better than I could. This is where I got the idea from, thanks Dave
I found a cutoff from the 8" mirror box and the attached images show how the Jarrah corner pieces fit. I also sanded one corner so you can see the colour difference. There are additional bracing pieces glued on the inside to provide additional rigidity and strength. I did make a couple of test pieces first. When subject to the breaking test, the glue joins remained intact and the wood gave way.
Mark, this is the Upper Tube Assembly - tube component, yes, the diagonal cage. Re overlap there are 3 thin layers of plywood and the joins are placed about 120 degrees apart.
I guess I should have included some additional detail in my first post:
Objective is to build a fully featured 10" Dob that basicaly only takes up one passenger seat in a car for transport and a similarly small volume for storage.
This then determined the use of a truss tube design and most importantly the ability to "nest" the UTA inside the mirror box. The "nesting" requirement has driven the use of a formed ply tube for the UTA (to minimise the diameter) as opposed to the more traditional cage design.
Large diameter altitude bearings and an azimuth bearing diameter no larger than the mirror box width, result in a small/compact rocker box. The altitude trunions will be removable without tools if required.
This is only a small 10" scope and weight/size control is not as critical as for larger scopes so compared to a larger scope, an oversize mirror box is of no great concern.
I have attached some images of the 8 inch taken torwards the end of construction, before the altitude trunions were fitted, that show the concept.
Third layer glued up OK so I glued an additional 40mm wide ply band around the base of the UTA tonight. This provides some additional thickness for the truss attach points. I will probably stain this like I did with the 8" scope.
I should also have mentioned the former was covered in Gladwrap before placing the first layer of plywood. This is to ensure the plywood tube doesn't become permanently glued to the former, nearly happened to me the first time I did this .
I am using moisture cure polyurathane glue BTW.
A couple of pics showing remasking and clamping for the 40mm strip.
I noted Dave's comments on removing the 3 disk former from inside the tube. As I am only making short tubes I can get away with 2 disks so I decided to make split formers. Removal is really simple. Undo the screws attaching the supporting disks and the former splits in half lengthways. Remove the lose half and then carefully pull away the half that is lightly pinned with 12mm brads. Attached pics probably explain this better.
When you attach the first ply layer, it has to be held against the former without using clamps so you can apply glue and then the second layer. The method Dave used and I have copied is to clamp it down and then use 12mm brad nails at about 25mm spacing to the butted edges. Seems to work OK.
I agree with Dave. Very light compared to other simple solutions. I am sure CF is lighter, I just haven't been brave enough to work with exotic materials
This ply is much easier to bend around the long axis of the sheet. This means the largest diameter tube you can make without edge joining sheets is about 350mm. But you can make one 8ft long if you want
I used the same 1.5mm Hoop pine ply for the SC wall on my 16" and made a
tube 440mm bending along the long axis. I'm buying another sheet to
make a hat box for the SC. So it can be done, you can also get it 0.7mm
3 ply aparently.
I used the same 1.5mm Hoop pine ply for the SC wall on my 16" and made a
tube 440mm bending along the long axis. I'm buying another sheet to
make a hat box for the SC. So it can be done, you can also get it 0.7mm
3 ply aparently.
Lovely job on the 8" Ian
regards,CS
Thanks Rob. That is good to know you can bend it lengthways, where do you get the 0.7mm ply from BTW??
UTA has now come off the former. Focuser and truss attach points fitted. I am leaving the masking tape on for the moment to protect the ply surface. 2.6" secondary and 4 vane mount has been ordered from Protostar.
I found some 3-ply that is just under 1mm thick (yes, 3-ply!) at Hearns Hobbies in Melbourne. Amazing stuff, I had no idea ply could be made that thin. I used it for the light baffle on the rebuilt UTA that I recently posted here (image re-posted). It came in a sheet that was about 150x30cm. Being so thin it bends very easily.
I also found a very nice sheet of 1.5mm 3-ply there, which I used for the non-load-bearing sides of the mirror box on my 14". It is an octagon; the sides, top & bottom are 8mm ply, & the rest are this thin stuff. The box is 20cm deep. Pic also attached. This would also be a good material for baffling a UTA.
I love your UTA Ian; it looks incredibly strong for the weight.
- sigh!!
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