Equatorial Platform - New Build

[Stub Mandrel]

A new project! I'm making an equatorial platform for 52 degrees, so OK both for here and South Wales with a few mm of tilt and most of the UK with minor adjustment.

I want to combine 3D printing with other techniques.

Here's the South bearing, the pin is a push fit in two ball races and will be glued into a 3D printed bracket on the base.

I'm printing the second of two sectors now, in two minds about facing them with aluminium strip, if I do I will need to pre-bend the strip as it's very springy!

I found it a devil of a job to find simple instructions for the size of the sectors.

If you use inclined sectors, imagine a horizontal line from the lowest  point of your north bearing arc to the south bearing. The length of this will set the proportions for other dimensions, I decided on 600mm.

The diameter of the bearing sectors = bearing separation * SIN (polar angle)

The sectors need to be angled perpendicular to the polar axis

[Chriske]

What scope is it intended for..?

[Stub Mandrel]

10" Dob (restored since this pic):

[Chriske]

A DarkStar. Good scopes..!!

[happy-kat]

That's a good project, not seen one built for a while on here. I guess you won't be using the kitchen timer design.

[Stub Mandrel]

2 minutes ago, happy-kat said:

That's a good project, not seen one built for a while on here. I guess you won't be using the kitchen timer design.

Going from scratch to suit what I have handy.

[Chriske]

Neil,

Did you rebuilt that scope completely using other materials, or is it still the same tube and rocker as in the picture..?

[Stub Mandrel]

Same tube and base, I'll look for a photo...

Utterly strange, I don't seem to have a photo of the whole thing!

I need some pics for my book so I'll drag it out when the sun shines...

[Chriske]

You'll need to lower the scopes COG to have that platform work properly.
The polar axis must run through the COG of the scope.
 

[TerryMcK]

Dark Star is a great name for a telescope brand

[Stub Mandrel]

1 hour ago, Chriske said:

You'll need to lower the scopes COG to have that platform work properly.
The polar axis must run through the COG of the scope.
 

So say many websites.

Why?

Physics say that the COG must remain within the footprint defined by the three bearings for stability. I'm not aware of any unique physics that applies to dobsonian scopes.

[Stub Mandrel]

Plus if there is a tip hazard I can just pop a couple of counterweights (or lifting weights) in the base.

Also, rather than fool around with clamps, locating the feet etc. I'm just going to put three friction pads and a pivot on the platform so it will be spin a nut and swap over the upper part of the base.

[markse68]

8 minutes ago, Stub Mandrel said:

Why?

I guess it makes it easier for the platform motor but if it’s got enough torque? Stu uses one with his OO dob- don’t suppose it’s aligned? 

Good idea making it so that you just swap it out for the ground plate of the dob mount 👍

[Stub Mandrel]

OK, let's assume (wrongly) that the CofG is at the pivot. This is worst case as actually it is much lower due to the weight of the base.

It's under 700mm to teh pivot.

At the mid point of the platform the polar axis will be 52 * TAN 300 = 383mm up, lets say 300mm to allow for the bearing being below the platform (actually by far less than 83mm).

So the CofG will be less than 400mm above the polar axis.

Lets lean the platform to its extreme of 7.5 degrees.

Actually no, let's go to 10 degrees for a margin of safety.

The CofG will be 70mm to one side of the polar axis, which means we need the triangle of stability to be at least 150mm wide at this point.

The north bearings will be 350mm apart, so at the half-way point the triangle of stability will be 175mm wide.

 

So bearing in mind that (1) the CofG will be lower than this, probably by 100-200mm (2) I will probably mount the scope closer to the north bearings to put more load on them  (3) the platform would have to run past its end stops or be leaning over significantly to reach such an angle I don't expect any stability issues.

 

[Chriske]

You could indeed add some weights in the base of the rocker to have it all perfectly balanced.
That's all there is to.

[Stub Mandrel]

You have made me paranoid enough to do a more accurate calculation!

 

The tube is balanced so the CofG is effectively at the pivot (I have a magnetic rail for balance weights).

It's actually 690mm to the pivot.

The tube weighs about 15.5kg* and the base bass weighs about 14kg.

The baseboard means the CofG of the base part is rather below half way up it (this will remain the case as the baseboard of the platform will have similar mass, being rather bigger, if slightly thinner, and made of ply rather than lighter MDF.

My estimate CofG of the base is at 300mm or lower, done by suspending it from a handle.

Distance between the two CofG's is 290mm.

Position of the combined CofG is 152mm from the base's or 452mm up.  Add 80mm for the pivot height = 532mm.

At the mid point of the platform the polar axis will be 52 * TAN 300 = 383mm up

So, the CofG will be 149mm above the polar axis.

That's much less than my original guess of 400mm.

With the bearings 180mm either side of the polar axis, the triangle of stability will be 90mm wide at the mid point.

Working backwards from a tipping point of 90mm lean gives an angle of 37 degrees.

 

37 degrees is way outside the expected envelope for using the scope, even if the base is placed on a slope (I will be fitting adjustable feet for polar alignment).

 

 

*So really I should just put it on my HEQ5 which is rated 18kg for visual (I wasn't planning on guiding and long exposures...)

[Chriske]

2 hours ago, Stub Mandrel said:

You have made me paranoid enough to do a more accurate calculation!

🤭

[Hals]

Having absolutely zero woodworking skills I'm watching this build with interest. I've looked at quite a few designs and frankly I'm not up to making one. Period. I know my limitations. Think I'll have to go down the commercial route and buy one somewhere. Good luck with this project you have my admiration.

[Stub Mandrel]

1 hour ago, Hals said:

Having absolutely zero woodworking skills I'm watching this build with interest. I've looked at quite a few designs and frankly I'm not up to making one. Period. I know my limitations. Think I'll have to go down the commercial route and buy one somewhere. Good luck with this project you have my admiration.

I don't like working with the brown stuff if I can avoid it. I think some other people's builds will be much prettier.

Hopefully, it won't involve more than cutting out some very simple shapes and drilling some holes.

The 3D printed sectors have worked, I printed clips to hold strips of 15mmx2mm aluminium in place and they a seem to have worked in combination with clear gorilla glue and some strategically placed clamps. That means I don't have to machine any fancy curves in wood.

I've ordered a NEMA 17 0.4nm stepper, hope it's up to the job, should be OK if geared down, or I may go for belt drive.

[Stub Mandrel]

Some progress:

Pulley for 1.5mm timing belt, printed with 0.1mm nozzle:

Upper platform with pivot and sectors:

Jigged with platform horizontal to measure correct angle and height for rollers (rather than trying to calculate it):

[sploo]

On 19/05/2020 at 20:55, Stub Mandrel said:

I printed clips to hold strips of 15mmx2mm aluminium in place and they a seem to have worked in combination with clear gorilla glue and some strategically placed clamps. That means I don't have to machine any fancy curves in wood.

 

That's a very clever solution. I'm a chippy rather than a metalworker, but smoothly bent pieces of alum should be a much better surface on which to run.

[Stu]

On 18/05/2020 at 14:42, markse68 said:

I guess it makes it easier for the platform motor but if it’s got enough torque? Stu uses one with his OO dob- don’t suppose it’s aligned? 

Good idea making it so that you just swap it out for the ground plate of the dob mount 👍

Errr, indeed, I just plonk it on and use it! Perhaps given how spindly it is and after my latest incident I should add some weight for stability!

[Stub Mandrel]

Rats.

Complete my Mk2 bearing block, the unpowered one with a 'top hat' roller. Offered it up - and realised it needs to be angled sideways by ten degrees.

Now printing a 'tilted' version.

The roller and the other combined roller/pulley have a shaped hole to suit a pin of 8mm precision bar with a flat milled on it. This lets them run in two skateboard bearings and means they can be assembled/disassembled in place if required by just lifting the pin (and unmilled section stops the pin dropping right through).

 

[Stub Mandrel]

Progress... and some miscalculations...

The platform is materially complete (aside from levelling screws and the electronics). The good news is the 3D printed pivots appear to be strong enough.

I mounted the scope and found two problems:

• The scope starts to tip when it gets near the final extent of its travel.
• The roller doesn't have enough grip to move the scope at this point as the load on it become 'light'.

I have realised this is down to two things - surprisingly not my C of G estimates (which tipping the scope on its base showed to be fairly close to correct).

• I've made my sectors far to big! The platform can tilt through over 33 degrees, typical platforms rotate through just over 15 degree to give an hour's tracking. It's taht extra tilt that's the root of the problem
• The scope is pivoting on its teflon pads, rather than on the rollers which are further apart. Tightening the bearing would help BUT would then make the azimuth adjustment too stiff.

Two solutions, one of which is obvious:

• Restrict the tilt to around 15 degrees by fitting stop switches.
• Use four teflon pads, to give the maximum width of support for stability, just in case.

 

 

[Chriske]

Prin

1 hour ago, Stub Mandrel said:

The good news is the 3D printed pivots appear to be strong enough.
Restrict the tilt to around 15 degrees by fitting stop switches.

PLA is strong enough indeed to do this kind of job. You did a good job there making it thick enough...
And yes 15° is about the max you should aim for.
A knurled axis will help you improve grip on the sectors.

I always build to track no more then 15°. After a session I return to my starting point every time again. But in my case it's a bit a different story. It's not a regular eq-platform. I have my own version( a Box-mount).  Still, no more the 15° of tracking is my 'rule'.