PVCu Mount Assay Part I

As promised, here is the first part of the mount assembly. It has actually been a very straight forward assembly process so far. This excludes the axle shafts (including counterweight) which will be M8 studding and covered in a separate (Part II) post when I get to it. These are the components and tools you will need for this part:

Disclaimer: Please don't blindly copy this. I am just documenting the progress of building something, I will undoubtedly miss things out. If you choose to follow my documentation  to make something similar and as a result you trash your telescope, yourself or others around you, I will hold no responsibility, whatsoever - I may even allow myself a little chortle depending on the grade of your idiocy. I do however hope that the following will help others make educated decisions on how to make their own mounts in the future.

Tools:

Dremel / Rotary Tool with a a drum sanding attachment (shown left).


120 grit sandpaper.

Toggle press or decent vice (I sold my toggle press a few years back but my vice work just as well).

PVCu Solvent Cement .



Materials;

Now you should be able to scale this mount up by using different pipe diameters, I have personally been looking at making one with 3" pipe, floor mounted into a concrete screed for a remote observatory idea with a large home-made Dobsonian, but not yet! 

This mount will be fitted with a small 70mm refractor, so I am using 1 1/4" fittings:

PN16 Flange

2 no. 1 1/4" PN 16 flanges.
2 no. 1 1/4" Tee Pieces.
1 no. 1 1/4" 45 Degree Elbow.
6 no. 1 1/4" > 1" Reducing Bushes.
6 no. 1" > 1/2" Reducing Bushes.
A short length of 1 1/4" PVCu Pipe.
8 no. ABEC 7 skateboard / scooter bearings.

Here is a catalogue of PVCu pipe fitting with a guide on gluing etc as well detailed info on many parts.

45 Degree Eblow

Tee Piece

Reducing Bushes

Standard Skateboard bearings

Now, depending on your bearings you will need different reducers, there is lots of information on-line about their dimensions but I have put a pipe diameter table on my previous post Here and a link to some standard bearing housing dimensions, too - I am sure will be able to find a bearing and reducer that closely match each other to suit your own requirements.





Assembly of the mount base.

I chose to use a PN16 flange as the base because I could then bolt it to a wooden tripod easily. You could equally use reducers down to a 3/8" BSP female thread, then thread to your existing tripod. If you have an odd thread size you could reduce down with plain fittings (1/4" is the smallest), drill out and tap to the thread you have on your tripod.

The idea is to use the 45 Degree elbow for the latitude of the polar axis, I live in southern England so this is close enough to be corrected with my tripod's trailing edge. For those of you further or closer to the equator than this is practical to use, you can still use the PVCu pipe method. However, use 2 no. 90 degrees to create a composite angle equal to your latitude - you could even leave the connection between the two elbows unglued and fit thumb screws to give you the ability to change the latitude. Seeing I am never going to take my telescope more than a few degrees north or south I didn't see the point, but the option is there if you wish.

• Measure the depth of the fitting recess and cut 2 no. pieces of pipe twice the length of the recess, for 1 1/4" pipe this was 50mm for mine.

• Paste the recesses of one of the PN16 flanges and one end of the 45 degree elbow. 

• Push one of the pre-cut lengths of pipe in to the flange connection and push the 45 degree elbow on top. Rotate the fittings until tight against the recess stops then align the 45 so it is centrally perpendicular.

• Next up, glue the other 50mm (or whatever length yours turns out to be) pieces of pipe in to the 45 degree elbow.

• Finally, glue one of the Tee pieces to the pipe stub glued in to the 45 degree elbow. You should glue the central stub of the Tee piece. the longest part of the Tee should be aligned perpendicular to the 45 elbow.

Warning; PVC solvents melt the plastics they are bonding, quickly. Locate and align your pipe work quickly and firmly, once located and not moved for a few seconds the bond will become so strong will not be able to change any angles. Be diligent but quick and of course, careful.

It should like this (minus the bearings, we'll get to them next);

Tripod mount, 45 Degrees latitude + Right Ascension axis.

Reducers and Bearings;

I chose to use bearings on all the axis to reduce the friction and therefore load requirements of my stepper motors. The right ascension axis, shown above shows the axis runs through 2 no. ends of the tee piece. Each end must contain a set of reducers and a bearing this will contain the axle which will bolt to the declination-axis-tee-piece.

This design gives an approximately 100mm of space between bearings for the main axis which I feel is sufficient to support even a fairly weighty telescope given that heavy duty MTB hubs are generally no more 75-80mm apart.

The 1/2" Reducing bushes mic'd up with an ID of 21.25mm and the bearings were 21.98mm, I want a press fit, but pressing into that would risk either splitting the reducers or deforming the bearing housing. I used a Dremel and sanding drum to remove a small amount of the material until the ID measured about 21.60mm.

Rusty vice pressing bearings into reducing bushes.

Stub axis hubs; the receiving hub to the declination axis and the top mount that will take the telescope (the actual declination hub) are both on stub axis i.e. they only have a bearing and support at one end. This is the only simple way to assemble the mount. However, it does put more strain on the bearing and puts a higher emphasis on on the bearing seat being complete concentric, In order to help reduce stresses and to help with seating accuracy I chose to use double bearings in these locations, this will:

• Increase distance between bolting faces; reducing the affect of inconsistencies.

• Put the clamping nuts on two different bearing faces; if one face is not perfectly square the problem is not passed to the axis - if clamped on a single bearing the axis would rotate around the uneven bearing rather than the actual axis.

Double bearings pressed in to declination hub.

5 no. Axis bearings and the declination hub, RA hub not assembled yet.

Right, so that is as far as I got today, I dummied up the RA axis with some M8 rod and it is super smooth! I am really looking forward to getting it all assembled and ready for testing the stepper motors and working out the best mounting options for them - I have a few tricks up my sleeve, Hopefully I can check whether they will work soon enough and update the assembly with Part 2 soon, probably next weekend, I seem to have no time at the moment!!

Right is a quick annotation of what has been assembled so far.