The Mapstar 22" build

[Soupy]

Looks amazing all laid out like that Damian!

[Physopto]

I thought that the mirror sat in a sling? I noticed the 4 contact points on the arms. So sling or just those points?

Derek

[mapstar]

7 hours ago, Physopto said:

I thought that the mirror sat in a sling? I noticed the 4 contact points on the arms. So sling or just those points?

Derek

Just the points Derek no sling 

First point on either side is 22.5° from vertical  then the next two are 45° from that. The points rest (or should do) at the mirrors centre of gravity or just over 16mm  from the rear.

Hope that explains it referring to the pics 

 

Edited March 25, 2016 by mapstar
auto correct spelling

[Physopto]

Thanks Damian.

Must admit I am a bit surprised, after all you measure the mirror with a sling. So I was expecting some sort of sling like support to spread the load of the mirror, instead of at only 4 points! As I said before (probably several times ) you learn summit every day.

How is it going, can't wait for a look see.

Derek

[mapstar]

5 hours ago, Physopto said:

Thanks Damian.

Must admit I am a bit surprised, after all you measure the mirror with a sling. So I was expecting some sort of sling like support to spread the load of the mirror, instead of at only 4 points! As I said before (probably several times ) you learn summit every day.

How is it going, can't wait for a look see.

Derek

The four points or Whiffle tree as it is known is the best support for a large thin mirror. A sling if set wrong can apparently pinch the optics. Here is Mike Lockwood's site with testing details

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

[Physopto]

Thanks I will have a look at that.

[mapstar]

So to the mirror box construction.

Because I intended to attach the lower trusses to the box with a corner piece I designed the box as an octagon which gave me a little more edge clearance for the way I was going to construct the rocker box. This meant a more complex construction than just a square. All the wood was baltic birch ply 18mm thick

Here are all the parts laid up with the cell inside before gluing. The ample use of the biscuit jointer provides a lot of strength. The box is roughly 200mm deep and 680mm square

Strengthening gussets were added to improve the torsional rigidity of the completed assembly. Large holes were just there for aesthetics really, but provide a way of picking the box up. I had yet to cut a rear hole in the box to mount fans if required. It is essential that at least the left and right panels are parallel and square as the bearings when attached would run out if they were even the slightest bit out.

In the four corners I cut a slot out to accept the lower truss mounting points. The trusses would be joined together as in my last build. Having made all the mountings at the same time I used them but in future I have a different design to try. Here's a pic after gluing up with the lower truss mounts loosely fitted, and the top mount for the cell with 8mm countersunk stainless allen cap bolts which were fitted from the rear of the box

Once I knew where the cell was going to be I then went about fitting the two bottom collimation points. Packers were made and the two adjusters bolted in using 8mm stainless countersunk allen cap bolts fromm the outside of the box . All the calculations have to come together so that the cell is centralised in the box

The collimation is done by only adjusting the two lower points with a 10mm stainless thread from the front of the mirror box. Again I'd made everything to do this way back in the past when I made the other scope on SGL

I then made two top pieces (from 12mm ply) to complete the mirror box construction

More on the next instalment 

Edited March 28, 2016 by mapstar
additional information

[estwing]

Lovely stuff mate...keep em coming

[Physopto]

Very interesting reading Damian. Thanks, I see where you pictures of the secondary support came from. You definitely put some effort in to this project.

Derek

[Stub Mandrel]

Great stuff Damian, worth waiting for ;-)

 

Whiffle trees are what they use to attach draft animals to carts to even out the effort. A similar system is used on some locomotives as unsprung suspension.

[Physopto]

Tanks and bulldozers have them as well. Or at least some do.

[swamp thing]

Lovely biscuits Damien  

 

[Soupy]

Biscuits and Whiffle trees... It's a new language this Dob making business! Cracking read so far buddy!

[jetstream]

That is a great mirror cell Damian. Your build is coming along very well!

[mapstar]

Thanks Guys. It's been a long time coming but glad you're all enjoying the read. Amazing what you learn along the way 

Edited March 26, 2016 by mapstar

[kev]

Ive been wondering how this was going.

Nice to see the project continue.  I take it there will be a fan somewhere to ventilate all that glass? Otherwise the coll down could take days!

Keep up the good work! 8-)

 

[astronymonkey]

Excellent work Damien , and that looks like very good plywood that you are using. Did you have many problems with edge splinters as cutting such shallow mitres along an edge on ply can be a pain ?

I wondered where you got the ply from as well and what you asked for? Cheers

 

 

[mapstar]

32 minutes ago, astronymonkey said:

Excellent work Damien , and that looks like very good plywood that you are using. Did you have many problems with edge splinters as cutting such shallow mitres along an edge on ply can be a pain ?

I wondered where you got the ply from as well and what you asked for? Cheers

 

 

Hiya Astronymonkey

I used Baltic Birch Ply from here as I have used them several times. They aren't the cheapest but with delivery included it makes life easy for me. There are other sources close by me. Quite a few on ebay too

http://www.cutplywood.co.uk/products/baltic-birch-plywood-bb-bb-standard-grade?variant=9658998917

It can be difficult to cut without splintering. I find scoring along the waste side with a stanley knife before cutting stops chipping out of the surface, but I also try to cut with the unseen side face down as it's always the bottom side that tends to be problematic.

A good table saw has it's benefits and I use a Kity 1619 that I've had for about 15 years now. Here's a few pic's of the kit I use starting with the saw 2.5Hp induction motor very smooth

The old Pacera pillar drill which I think has been around longer than I have. High/low speed

Home made router table

And the trusty routers

 

 

[mapstar]

Anyway moving on from the workshop tools to continue the build. After I had cut the top I then had to work out where the threaded rod needed to come through so I could put in the bushed hole. Here's the box with in final stages of gluing with the two collimation screws sticking out and the cell top mount clearly visible

So it was then onto sanding and finishing off with a cover 

[Physopto]

32 minutes ago, mapstar said:

Hiya Astronymonkey

I used Baltic Birch Ply from here as I have used them several times. They aren't the cheapest but with delivery included it makes life easy for me. There are other sources close by me. Quite a few on ebay too

http://www.cutplywood.co.uk/products/baltic-birch-plywood-bb-bb-standard-grade?variant=9658998917

It can be difficult to cut without splintering. I find scoring along the waste side with a stanley knife before cutting stops chipping out of the surface, but I also try to cut with the unseen side face down as it's always the bottom side that tends to be problematic.

A good table saw has it's benefits and I use a Kity 1619 that I've had for about 15 years now. Here's a few pic's of the kit I use starting with the saw 2.5Hp induction motor very smooth

The old Pacera pillar drill which I think has been around longer than I have. High/low speed

Home made router table

And the trusty routers

 

 

Hi Damian,

Got the same Kitty saw and Big router. Nice to see someone with the space to fit the moving table to the saw. Mine is still in its packing. Was intended for next house with a proper workshop. At least I know what it will look like

What blade are you using? I bought the finest tooth I could specifically for the breakout problems.

Keep it coming.

Derek

[mapstar]

Just now, Physopto said:

Hi Damian,

Got the same Kitty saw and Big router. Nice to see someone with the space to fit the moving table to the saw. Mine is still in its packing. Was intended for next house with a proper workshop. At least I know what it will look like

What blade are you using? I bought the finest tooth I could specifically for the breakout problems.

Keep it coming.

Derek

Hi  Derek,

I use an 80 tooth blade (cant remember but I think it's DeWalt?) and don't have too many problems. I think its getting ready for sharpening though as it's done a lot of work.

The sliding table is good although the Nylon running bearings seem poor quality, as they develop flat spots on them making the table bump along. I have replaced both of mine once but the same problem has returned. I keep saying I'm going to get something better for them. I picked up a spare beam a long time ago and extension table which are both still in their packaging unused and I doubt they ever will be?

Like you say the table saw with side extension and sliding carriage takes up a lot of space. I also have a kity 1637 planer tucked away under the spotted blanket at the rear of the garage in the shot above. It doesn't get much use nowadays

 

[Physopto]

Yes I replaced my Kitty blade with another make but it is slightly smaller diameter. Not accurate for depth of cut but not a worry as I use a  ht gauge/ test cut. Interesting about the bearings though! My 300mm planer thicknesser is holding up bags of cement at present !

Like Mike said, don't know how you get the time! But good reading.

Derek

[mapstar]

I made a cover which I later added four pads to so I could stack the upper tube assembly on top when it was all finally finished. Here it is shown with the lower truss mounts slotted onto the locating pegs. The bottom left one has a truss pole  end in. The poles were to be the same 22mm dia as the last scope. Again all the metal work was made 3 years ago

I would later add two bushes where the threaded rod came through the surface. I also did the same on the rear with the intention of putting an O ring around the rod inside the bush to give the collimation screw resistance when it is set so it doesn't alter with any vibration during the many nights viewing I would be having with the finished scope ()

The mirror box was then given a final sanding to prepare it for finishing coats.

Upper tube construction was started by cutting out the two rings from 12mm ply which would be joined by 8 spreaders made from 18mm ply and the focuser board

Lots of dust later and I had the two discs ready for the marking out of the dowels for the spreaders

I'll post more pics shortly

 

 

[mapstar]

Having marked them up laid together I put a pair of holes for the dowels in each. The outer diameter of the rings was 710mm to get a sense of scale

I also put in the focuser board dowel holes before cutting out the central disc with a 1.5mm rebate around the inside which I would glue 1.5mm aero ply in.

The spreaders where made and I first drilled the dowel holes in each end jigged up on the pillar drill, and wanting the upper tube to be as light as possible I put a series of holes in them (they also double up as some where to run dew heater cables)

 

The holes were also cut on the pillar drill but as you can see a little scorching is present, this is due to the glue in the ply clogging up the teeth on the hole saw. I picked up a little tip of withdrawing the hole saw from the workpiece and running a wire brush against it to clean it off and gives a better finish.

Once done it was the gluing up. I assembled one half dry and glued the other half making sure everything was square. The glue I use (titebond 3) doesn't have a long open time so with that much to glue up in one go it is a much less fraught affair than doing both sides together. Also in the picture below you may notice two small holes (top and bottom) on the inside of the back up right this is to prevent the hydraulic lock you sometimes get when the dowel is being pushed in to the fixing holes.  

Can never have too many clamps!

Edited March 26, 2016 by mapstar
additional information

[astronymonkey]

It never ceases to amaze me as to how versatile a router can be, and what a nice finish you can get with one. Keep up with the good work and thanks for sharing the details of the equipment you are using. I have a decent table saw but I must get to swapping the blade as its cutting a bit rough these days.

cheers