Build Project: Dob Equatorial Platform

[Moonshane]

hi Sean

tough luck. I bet you'll sort it one way or another. mine is stalled by waiting for a pwm unit from China (hopefully here soon) and some machined/tapped bits for the drive rod. I am hoping if they arrive this week I can get the thing tested this weekend.

I'll update my thread when I do. everything else is done now including all my wiring and a battery charging point etc.

[SniffTheGlove]

hi Sean

tough luck. I bet you'll sort it one way or another. mine is stalled by waiting for a pwm unit from China (hopefully here soon) and some machined/tapped bits for the drive rod. I am hoping if they arrive this week I can get the thing tested this weekend.

I'll update my thread when I do. everything else is done now including all my wiring and a battery charging point etc.

Well done Shane.

Like you said before, the first one is the prototype!

I'll have agood look at it tomorrow as the support wheels would of glued into place by then, then just make sure the platform works manually before working out the motor.

Another thing I noticed was that there is some small lateral movement (rocking) of the scope when touching the focuser.

[SniffTheGlove]

Update:

I in fact moved my segment support wheels right out to near the outer edges of the arc, now the wheels will hit my aluminium runner screw heads when it has reached 60mins of tracking, this makes sure that the platform will not travel passed the edge and also as the wheels are spread out more therefore the "wobble" should be reduced when touching the scope.

Also, I am now going with the tangent are threaded rod method of moving the platform, playing around just before tea time showed it was incredible easy to do and push no strain on the stepper motor.

I shall have the current supports for my limit switches and will drill 12mm holes through them to mount the threaded rod, I will use am m12 coupler nut to connect via a aluminium strip to the platform. I have epoxied a lego wheel to a coupler nut to enable the stepper motor to transfer it's motion. I tried to drill some 12mm threaded rod to fit my stepper motor shaft but I failed several times until I thought about epoxy and a lego wheel. I hope to have a go with this in the morning fixing it all up.

PS, When I thought the top platform was not level and off skew a bit, infact it was not, it was just one of the support wheels was not fully up against the segment, now they have been taken off and reseat further out the platform is fine. Phew!

[Moonshane]

great work - adaptability is the name of the game

[SniffTheGlove]

adaptability is the name of the game

Now you have me thinking of Bruce Forsyth singing "life is the name of the game and I wanna play with you" Hohum

[JamesF]

Now you have me thinking of Bruce Forsyth singing "life is the name of the game and I wanna play with you" Hohum

You should worry. I keep hearing Roy Castle singing "Collimation. Collimation, it's what you need..."

James

[Moonshane]

Now you have me thinking of Bruce Forsyth singing "life is the name of the game and I wanna play with you" Hohum

much better than my version!

[Moonshane]

"Collimation. Collimation, it's what you need..."

James

If you want a scope that's best, if you want a good star test...etc

[SniffTheGlove]

Update:

Today, I have completed the paint jon, all black satin exterior.

I have tried to sort out the tangent drive. The threaded rod is in place through some 18mm plywood supports, one end capped with nyloc nut, the other is a coupler nut connected to the stepper motor. The actual support to move the platform is a coupler nut with an alumiuin strip. It move nicly when twisting the thread rod by hand but usless with the stepper.

The stepper just does not seem to be able to rotate the threaded rod at all so will have a good look tomorrow.

When moving

[SniffTheGlove]

Update:

I am getting very frustrated now!!!

The motor is turning the threaded rod which is moving the coupler nut that tehn attaches to the platform. It moves forwards and backwards fine UNTIL some weight is added to the platform then the motor just stalls (you hear the stepping action but the shaft does not move), you take the weight off and it starts moving again.

People have done this with lesser steppers motors from computer hard disks or printers. I get a heavy duty one and it OR me is useless!!

[Moonshane]

Hi Sean

my motors are DC but also geared. I wonder if gearing it would work?

[SniffTheGlove]

Hi Sean

my motors are DC but also geared. I wonder if gearing it would work?

Had another go again and tried using an portable electric Drill, I put the chuck around the threaded rod and pressed the trigger and it moves the platform but struggles with the scope on the platform. I have the scope back as far as it does so that the CofG is over the 52deg latitude line of sight but the Drill still struggles then. Take the weight off and you can move it all with your little finger.

I really thought I had got it right but no! I am going to think about this one for a while as I not prepared to start spending more money to to try things out now.

Now how much was that EQ6

[Moonshane]

hi Sean

could it be that the back of the sector is touching the top of the north wedge when the weight is on?

maybe a couple of pieces of teflon against some kitchen worktop end laminate might work??

[SniffTheGlove]

Shane, I only have 3 contact points between the top and bottom platform.

1 is the south pivot which is a M12 threaded rod through wooded blocks. The rod is only screwed into the bottom platform thus allowing the south pivot to pivot.

The other 2 contact points are the 2 support wheels on the north segment and these are touching the segment square on and rotate freely.

There is no other pieces of wood catching anyway. I have even had my 14year old Son stand on the platform whilst I twiddle and fiddle.

I works fine with about 20Kg (Rocker Box) on the platform and the stepper motor can just work with that load but as soon as more weight goes on the platform becomes much harder to move.

Now I have just resigned myself to the fact that motorizing it is not practical on "this build" so I have stripped the motor and electronics off and just placed it in my Obsy with the scope on and I shall for the moment nudge the platform slowly to manually track.

I shall wither look at doing it a different way with the segment and plywood I have left over or save my pennies for an EQ6 Pro and get a nice refractor for Astrophoto work and use the Dob for visual.

[Moonshane]

bummer. don't give up mate. let's see how my motors go. the overall spend on two motors and the pwm controller was about £30 so assuming mine works then this is an option.

it might be worth adding your problem to the yahoo eq platforms group eqplatforms : Eqplatforms as they really know their stuff.

[SniffTheGlove]

Thanks Shane,

The real answer is motor with more torque (like your geared ones or the Maplin ones I first looked at until I went to the dark side with the Arduino)

Anyway, I shall wait to see how yours work Shane then gwet the details off you.

[SniffTheGlove]

I have been outside for an hour now and thought that I would actually see how the platform worked when manually moving it. WOW it works.

I pick Sirius as it was the brightest object in my 32mm EP, the went down to my 97.mmEP then I just applied some pressure to the side of the platform to gently move it and Sirius stayed in my 9.7mm EP for 4 mins doing this.

OK, it's a bit of hard work using my foot to give sidewards pressure whilst looking through the EP but I am happy at least the damn thing does actually track, now all I have to do is wait for Shane to check out his motors then I should be a happy bunny.

PS, I also got Sirius in my modded SPC880 and again giving pressure I was able to track for a min, OK the image moved around the screen as my pressure was not a sustained amount but at least I managed to get 1 min of Sirius footage.

I **** footage as the focusing was rubbish but at least I managed my first attempt.

[SniffTheGlove]

I spent the weekend giving my garage a complete spring clean ($ car loads to the skip) and now have much more room to work with, so today I stripped down all the old bits of electronic gear that I had scattered around the garage to scavenge the motors and gears out of to see if I could cobble something together.

I will try tomorrow to build a working system from afew old printers as well and see if I can either build a pulley system ot use the gears to rotate the M12 shaft.

I have also looked on ebay for gearsets etc but the trouble I am having is that my stepper motors are 5mm dia and with a plain round shaft so I do not know how to fit gears onto the shaft apart from a lot of epoxy/superglue which is not ideal.

[Moonshane]

hi Sean

If that doesn't work and mine do (should be getting the machined parts for the drive train in the next day or so I'm promised!), I have a spare set of (the same) motors which I was going to use on a future platform. I could let you have them at cost to save some time?

[SniffTheGlove]

Thanks for the offer Shane.

I know I have read about the CofG being in the right place else the motor has a lot of work to do but I can not believe that that I have got it so wrong.

Playing around a little this morning between household jobs with the platform which is now back in my nice clean and more open garage and using my 16Kg test lump (old defunct heavy duty car battery) I am getting to know the platform better.

I need to replace the rubber castor wheels as they are already flat spotting from having the scope on the platorm for a few days so I am looking to replace them directly with some metal bearings, then then should help aleviate the flat spotting and also provide less inerita to get the weight platform moving. So off to ebay or some other online store.

[Peter Drew]

Hello Sean.

Have you tried operating the screwed rod drive manually with the platform fully loaded ?, this would give you some idea of the required torque from the motor. It would also be worthwhile seeing if you can stop the motor shaft by hand as an effort comparison. Like you, no doubt, I would have expected that a combination of the low speed motor and the mechanical advantage of the screwed rod would have been adequate for the job. Replacing the rubber wheels would be a good step forward as there is considerable rolling resistance due to surface deformation when loaded, metal bearings will be much better in this repect, if you get stuck on sourcing these let me know as I might have something suitable to hand.

[SniffTheGlove]

Thanks Peter,

I have tried the following...

Total scope weight is approx 40Kg, the rocker box was weighed with a strain gauge at approx 20Kg, the OTA again weighs approx 20Kg. So 40Kg in total.

First off was the platform resting on 2 rubber 50mm dia caster wheels which can handle 40Kg each. The stepper motor was centrally mounted between the 2 castor wheels and the north segment rested upon the stepper motor shaft which had been sleeved in rubber to give some grip.

With no load on the platform, the stepper worked fine and rotated the platform.

With a 16Kg old battery located at the northern end the stepper would be able to move though with some strain.

With a 16Kg old battery located at the southern end the stepper would be able to move though easily.

With the scope on the stepper just tried to step (ie audible step) but the shaft did not turn.

Now, onto a threaded rod drive with the stepper motor connected directly to the threaded shaft.

With a 16Kg old battery located at the northern end the stepper just tried to step (ie audible step) but the shaft did not turn.

With a 16Kg old battery located at the southern end the stepper would be able to move though easily.

With the scope on the stepper just tried to step (ie audible step) but the shaft did not turn.

This is what I have done so far and it's getting all confusing to what did work and what did not.

I have found a ebay supplier selling some bearings that fits my potential needs. I am going to use the M12 threaded rod for connecting them to a new support wedge just behind the segment so that the segment will sit on the bearings. I am going for 12mm dia so the threaded rod can go through the id and be bolted to the wedge. The od is not important but I wanted the thk to be nearly the same size as the 18mm segment thickness, so I have found these 12mm id, 37mm od, 17mm thk 62301 2RS RUBBER SEALED BEARING 12 X 37 X 17mm on eBay (end time 29-Mar-11 11:22:24 BST)

These should do the job of replacing the rubber wheels.

Now onto the new driving mechanism, yes I can stop the stepper motor rotating by holding the shaft but I have tried this with another stepper motor from an old printer and this to can be stopped by hold the shaft with finger tight force appied. I think this is the case with all stepper motors (I have read some EQ Platforms that have used incredible small stepper motors from computer hard disk drives and these and not high torque motors) unless you pay £40 plus pounds for real high torque stepper motors but then I would need more expensive Stepper driving controller board for the Arduino to handle the hight amperage. My stepper controller EasyDriver can handle .750ma per phase.

[SniffTheGlove]

Awaiting the bearings to come in the post, hopefully tomorrow. Then I can get the new support in place.

I am going to go with some M6 threaded bar as I have loads for the driving mechanism, just need the bearings in place to know where to mount the motor.

[ponytale]

sean here are the pulleys i will be using for my dob setup , my stepper is also 5mm shaft size , i will probably use the 4mm bore pulley and slightly polish the shaft with emmery and slightly ream the pulley to make it fit the shaft ,i hope this is of some help, Tony

motionco_Pulleys & Belts_Stock Aluminium 3mm HTD Timing Pulleys

[SniffTheGlove]

Update:

I have a new full length wedge support and made damn sure everything would be level with no load. I measured a distance between the 2 platforms as 11.50mm on all 4 sides using the south pivot as the reference end.

I drew a pencil line on the support wedge against the north segment, marked off the 8cm distance either side of centre which is my 1 hour (15deg) travel line, then marked off the over-run and placed the mark for the bearing drill hole.

I then measured the distance of between the id and od of the bearing then marked this dimension on the support wedge, then I subtracted 2mm to compensate for the scope load (incase of some sag and movement of the bearing) then predirlled a 4mm pilot hole followed by a 12.5mm dia hole to take the M12 threaded rod. The id of the bearing is 12mm.

I then proceeded to bolt the bearing with spacer washers to the support wedge.

I know have placed the support wedge into it's position on the bottom platform and settled the top platform down, got everything level and seated correctly and then checked for platform rotation. Sweet, looks great. I just nbeed to cut the top corners of the support wedge off to allow for the platform swing at the end of tracking.

To see how well everything worked with a load on the platform I did the following test.

I used a pull strain gauge to measure how much force is required to move the platform from it horizontal position to the end of it's tracking position. The strain gauge was connected to the front and centre of the top platform and then pulled horizontally inducing rotation.

With no load it requires 750grams of force to induce movement

With a 16Kg load directly on the edge of the northern end it requires 2Kg of force to induce movement

With a 34Kg load directly on the edge of the northern end it requires 4Kg of force to induce movement

The loads used was a 16Kg car battery and a 18Kg Concrete Block.

As my scope is approx 40Kg and it will be sitting further back towards the south end I conclude I need 4Kg max of force to keep the platform moving.

I am now going to see if my stepper is up to the job. I have broken down some old battery operated power tools and scavenged the gears etc and found a gear which had a 4mm id and an od of approx 12mm, then I drilled out the 4mm id to my shafts 5mm dia and super glued it onto the shaft.

I have a larger gear approx 24mm with the same tooth pattern at the smaller one with an id of 6mm, which I have epoxied onto a 6mm threaded rod.

Now I am going to either have a go at build the drive mechanism out of 6mm and see how it goes else I will do one in 12mm threaded rod and somehow connect the 6mm rod into the m12 rod.

I am feeling a lot happier now.