Need help fitting timing pulley to dob pivots for Argo system

[mental4astro]

Hi everyone,

I'm looking at fitting the Argo Navis to my big dob. I'm looking at doing the installation using the hardware that came with the system that I bought.

The hardware I've got are:

* 2X encoders fitted with timing pulleys, 36.7mm diameter
* 2X 70mm timinig pulleys, with hubs, and 31.5mm bore. Two locking grub screws are tapped into each hub.
* 2X timing belts.

Azimuth: my idea is to fit the encoder onto the ground board with the timing belt system located in the space between the ground board and the rocker box. My question is how can I fit the 70mm timing pulley around the azimuth pivot which is a 10mm metric stainless steel threaded bolt. I realise I may need to cut a hole into the ground board to accomodate the encoder - that's not a big deal really, and I'm quite happy to do that. Just how do I fix the large timing pulley, :shrug:.

Do I just centre it around the pivot hole of the ground board and screw it into place?

There is not enough clearance between the OTA's mirror box and the rocker box to fit the encoder inside the rocker box.

Altitude: Thing here is I would like to make the encoder a removeable component. Needs to be for transportation and storage purposes. So I'm looking at mounting the encoder directly onto the altitude tangent arm, and make a little coupler to sit on the altitude bearing. The altitude tangent arm can then be readily removed/replaced, and the encoder just slips in and out of the coupler. This is the simpler solution.

The other is to stuff around with timing belts and pulleys on the tangent arm, but this is too complicated.

Any other ideas for the altitude encoder?

Still, my main concern is the azimuth connection.

The first pic shows the 70mm timing pulley. The second pic the encoders with their fitted timing pulleys. The third the altitude bearings of my big dob.

Any ideas, suggestions, techniques, machining places, etc, is greatly appreceiated.

Mental.

[gary]

Hi Alexander,

As you can imagine, we routinely design and manufacture custom encoder installation
solutions for ATM's, so we have a wealth of experience in this regard.

By way of background, the existing hardware you have looks as if was made for
something like a German Equatorial where the pulley with the large bore would have
slid over the shaft. The brackets are not something that we would have ever supplied.

When the edge of the mirror box passes over the vicinity of the Az pivot bolt,
how much clearance do you have from that edge to the inside base of the rocker?

What is the thickness of your ground board, the thickness of the base of the rocker
and the dimension of the gap between them?

I suspect that the low profile design of the mount affords very little clearance for the edge
of the rocker to pass over the top of the existing Az pivot bolt.

If it transpires there is at least 32mm of clearance, consider replacing your Az
pivot bolt with one that is 10mm diameter as before but that extends 31mm
taller with a 1/4" diameter shaft. Buy a new pulley with a 1/4" bore to
fit the bolt. A timing belt would then run to the encoder mounted on a different
bracket to what you have but you would retain the existing small pulley you have
on the encoder shaft.

If there is still insufficient clearance, then things can get trickier, but usually the
next thing to start looking at is what clearances can be gained by countersinking
into the base of the rocker box.

Your Alt axis looks straight forward. A coupler would be fabricated to be screwed
onto the Alt trunnion at the exact center of rotation. The coupler would have a socket
head cap screw for quick release of the encoder shaft. A tangent arm would run
from the encoder and have an accurately milled slot in its far end that would mate
with the shoulder on a shoulder screw. This arrangement would then allow
for the Alt encoder and tangent arm to removed or replaced in a couple of seconds
for easy transport of the scope.

Use the pulleys with the large bores for some other project.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[mental4astro]

Hi Gary,

Thanks for your reply.

Your words have provoked a bit of a rethink of my azimuth idea. With the way I've been thinking, it would prove difficult to engage the timing belt when the ground board and rocker box are coupled.

The thickness of the rocker box is 30mm (15mm hardwood ply was used throughout). Cutting a recess into the rocker box may be a good option, especially if located towards the posterior of the rocker box. That is because the mirror box stores inside the rocker box - see pic. The clearances, both in storage mode and in-use mode, would work if the encoder is recessed. Might only be a small amount too, the recess that needs to be cut. The gap between the mirror box and rocker box may well be in the order of 30mm.

However, I'm not keen on making the pivot bolt smaller than what I already have. Is there a larger pivot you have, or make a bushing to accomodate a larger bore pulley onto the existing pivot bolt? Maybe I misread your suggestion in pivot bolt. Come to think of it, my pivot bolt may actually be an M8 bolt.

Alex.

[gary]

Hi Alex,

In order to provide the best advice, could you also please answer the following -

1) You mentioned the rocker box base is 30mm thick.
Is the ground board also 30mm thick?

2) What is the dimension of the gap between the base of the rocker and the top
of the ground board?

3) Could you please confirm the diameter of your existing Az pivot bolt.
Is it M8 or M10?
What is its total length beneath the head?

4) As we need to check that the rocker box will clear the encoder or its associated
hardware, could you please provide the minimum clearance dimension from the
mirror box to the inside base of the rocker box in the vicinity of the Az pivot bolt?
Minimum clearance should also take into account things such as collimation nuts.
You mentioned that the clearance "may be in the order of 30mm" but I need you
to confirm for me that that figure is the minimum clearance, not simply the clearance
of when the OTA is pointing to the vertical. If you could please let me know the
clearance within a couple of mm, it would be useful.

5) Could you possibly provide a snapshot of inside the rocker box showing
where the existing Az bolt comes through and another snapshot of the bottom
of the ground board also showing the existing Az bolt?

6) Please let me know if there are any other peculiarities with regards the stacking
of the scope in its storage position with respect an Az encoder installation.
As seen in your picture, when the Alt trunnions are removed and the scope is in
its storage position, the mirror box is then sitting on the inside of the rocker box.
How much clearance is available in that scenario from base of the mirror box to the
inside base of the rocker in the vicinity of the Az pivot bolt?


Indeed, you have misinterpreted my textual description of the Az pivot bolt
using a timing pulley scenario. The 1/4" diameter section of such a bolt only
occurs above the inside surface of the rocker box. The section of the bolt that
passes through the wood is identical to what you have at present, either M8 or M10.
The 1/4" section is to accommodate the bore of a pulley.

Though one can turn a bush in Acetal that would allow one to use the existing
pulley, it is more economical to simply buy a new pulley rather than the
machining time and materials to create the bush.

However, if the encoder can be mounted directly by performing some countersinking
in the base of the rocker, then the timing pulley approach will not be needed at all.
Usually in this scenario, I would then advise on a new larger diameter custom
Az pivolt bolt, typically 1/2" (12.7mm). Depending upon the dimensions you
supply for the thickness of the ground board, etc., an off-the-shelf encoder-ready
Az pivot bolt may even fit and I will be able to calculate and advise.

As I advise on custom encoder solutions for ATM's around the world on a daily basis,
I get to see a very broad range of scopes.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[mental4astro]

Hi Gary,

Here's what you've asked for:

1: Ground board thickness is 15mm

2: Rocker box and ground board clearance is 3mm, plus 1mm for the laminate sheet.

3: Az. bolt is M8 & shaft is 65mm. Pic shows bolt used and the inside of of the rocker box' pivot nut:

99438

4: Min. clearance of mirror box over Az. bolt is 16mm

5: Inside of rockerbox:

99439

Underside of ground board pivot bolt head:

99440

6: Clearance between stacked mirror box and rocker box is 30mm:

99441

Whole width of stacked mirror box:

99442

This pic shows the underside of the mirror box, where the posterior of the mirror box is clear of any obstructions from its centre. The black wire is the lead of the heating strap around the primary:

99443

I've also noticed that the pivot bolt turns WITH the rocker box when the rocker box is moved.

This last pic shows the clearance between the mirror box and the rocker box with the trunnions in place and the scope pointing to zenith:

99444

[gary]

Hi Alex,

Thanks. Will have a look for you and advise.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Barrykgerdes]

Hi Alex
As you know I have a good workshop and can make just about anything
I am going to attach an articlle I wrote years ago when I made an azimuth "bearing" out of a rocker box. It won't directly suit what you want but may give some ideas. I fitted encoders to it (the same ones I now have fitted to my lightbridge that uses them with the Argo Navis.

Look at the centre bolt design. I turned it up on the lathe out of scrap.

Barry