Build Project: Dob Equatorial Platform

[SniffTheGlove]

Finished off the segments and sanded today. Also cut my test wedge.

I did not cut the segments with root end chamfered as the circular saw only went to 50deg and not 52 so I did a plain vertical cut instead. I was only doing the chamfer for aesthetic reasons.

Here are the photos

Tomorrow I will cut the north segment support wedges and then dry fit them with the segment to the bottom of the rockerboard platform.

Then I need to work on the south pivot but I need to order a few things from ToolStation like some M12 threaded bar with washers and nuts to make the pivot as my local Focus Shop only does M6 and M6 will not be strong enough and does bend easily.

[Moonshane]

Looking good Sean!

[SniffTheGlove]

Not feeling to good this morning as I have overdone my back as usual (not use to all this standing around witht he chaps doing my patio this week and then doing some EQP work yesterday.

So today I was going to just see how well the segement sits on the top platform and how big to cut the support wedges.

I also looked at the caster wheels I will be using and how they would fit onto the platform. Either use the current fixing holes on the caster fixed framework or remove the M6 bolt from the bearing and then mount the caster to a wedge that sits behind the segment (segment will the sit on top of the wheels).

First pic is the distance of 62.4mm as measured with my digital callipers is the distance from the end of the platform to where the north segment will contact the platform and then in the second pic is the segment in "dry" place with a 37.5deg wedge.

Everything looks good so far

[Moonshane]

looks like it will be a nice fit! don't like the sound of a dry wedge - ouch!

[SniffTheGlove]

looks like it will be a nice fit! don't like the sound of a dry wedge - ouch!

At least it's better than a wet one

Anyway, I cut all the wedges to size to fit the segment and are all in the process of being glued into the correct places. One glue has dried then I shall measure and drill a small piolt hole through the top of the platform, countersink them and then put some small brass screws down into the wedges. Going with brass so it does not effect the compass that will be set into the top platform.

I have also cut out some circular feet, 3 in fact. 2 for the front of the platform and a slightly larger one for the rear of the platform. These will have an M12 stud bolted into place (nut will be countersinked from underneath to provide a flat surface) and then have a M12 coupler nut (much long shank nut) that will be epoxied into the bottom platform to allow for vertical adjustment of the legs. The top of the M12 stud will also have another coupler nut epoxied into place to which will be used with a socket and wrench to do the actual turning for adjustment. I am using a coupler nut as it will provide a larger surface for the socket to fit over and make a snugger fit.

[SniffTheGlove]

OK, my stuff from ToolStation arrived this morning, so I have now started to a bit more today.

I have countersunk and screwed the segment wedges to the segment.

I drew a straight line 8.62cm from the platform edge across the width of the bottom of the top platform, this is where the point of contact will be for the segment. Made sure all was striaght and level then used 4min epoxy to set the segment in place by epoxying the segment wedges to the bottom of the platform using the drawn line to make sure the segment edge sits on this line.

When cured in a few hours I will then drill and countersink and and screw down from the top of the platform into the support wedges.

I need to mark off the correct distance along the top of the platform to mark where I need to drill a 12mm dia hole at an angle of 52deg. This is for the south pivot bolt to come through from the bottom pivot bearing.

To make the pivot bearing support I have some 3in by 2in wood, I will drill a 13mm hole directly vertical through the centre of the block and then cut the block to the correct angle and distance which should make that drilled 13mm hole be on a 52deg angle. At the correct distance from underneath the top platform I shall cut in half and place a cut teflon washer. At this point the reference plane should be from the bottom of the segment and across to the teflon washer thus making the trig angle correct.

[Moonshane]

going well mate

my first one is what I am considering my prototype and I'll almost certainly be making another almost immediately.

I am following a different route next time with no sectors (just a straight piece) and more in the Poncet style.

[SniffTheGlove]

going well mate

my first one is what I am considering my prototype and I'll almost certainly be making another almost immediately.

I am following a different route next time with no sectors (just a straight piece) and more in the Poncet style.

Shane, do you mean the VNS type when you say no sectors?

Well, I brought a 8ft by 4ft piece of ply, so depending how good I make my first I might make a second as I have already cut that segment out for a backup and nothing has happened to my first yet to damage it.

OK, now been out doing a bit more seeing that it is tipping down. I am now in the process of attaching the aluminium runner to the bottom of the segment. I am wondering how to attach it. I was going to put 2 small countersunk screw in at each end of the runner but the aluminium strip is to thin to bed the screw head in so it sticks out a little. Not really a problem as the rubber wheels will not be moving over it but I would like to make it look neat.

It is a question of would I need to replace the aluminium strip sometime in the future, if not I could go straight for the epoxy route and clamp it down until cured. I think I shall have to play abit more tomorrow when it daylight and see what is better. Screw or glue?

[Moonshane]

personally I would countersink the screw and then glue as well. in the future if you need to replace then just add another piece over the top but I doubt this will be needed.

re mine, it is a bit like this. http://web.telia.com/~u41105032/Stepper/platform.htm scroll down a bit for the pics

you just have a ball joint at the back and an angled straight wedge at the front. the angle of the wedges means it follows the correct arc when combined with the ball joint.

don't really know how it works but it does!

watch my thread - seems a doddle to make with all straight cuts - cf the sector version. will probably be the weekend before I can do any more as I have to work outside and drilling/cutting in the dark/rain is no fun.

[SniffTheGlove]

Yes Shane, I see what you mean about the ball joint method. I initial looked at another design that had a ball joint for the south bearing instead of the threaded rod pivot. The guy used a ball joint off a car abd looked really simple as you did not need to cut any angles for the south bearing and the ball joint always floated at the correct angle.

I looked for ages for a similar ball joint but could not find one, the closest I got was Ball Joint - RENAULT ESPACE Mk II (J/S63_) 2.1 TD (J633, J634, J/S635, J/S63D) 01/1991 - 10/1996 88bhp 2068cc J8S610 J8S772 J8S776 J8S778 Cone Size [mm]: 14,5Fitting Position: Front Axle left and rightFitting Position: Upper

I disgreaded this design in the end and went with the threaded rod pivot at latitude angle (Which if all goes well this afternoon I might get it mounted onto the platform. The rain has stopped, the sun is out but my back is crippling me today but I might get up to an hour if I'm lucky and I have the extra morphine.

[Moonshane]

I got some off Ebay (£10 for 5!) M8 Quick Release Balljoints x 5 Trike Chop Race Rally on eBay (end time 05-Mar-11 14:15:25 GMT)

the 'problem' is that the north sector has a much tighter Radius than I cut as the south bearing is right at the point of the alt angle

I can get away with this as I am using the ball transfer units but a straight sector would be better.

BTW I found a good anti-tipping tip on one of the commercial seller's site - they have theirs with the top board at no more than 5 degrees off horizontal at both extremes of travel. I have marked these points off after checking with the Wixey. Seems to reduce the tracking time but I'd sooner have less tracking time than a scope fall off! I'll still get at least half an hour and with a 1-2min 'auto' reset it's no biggie

[SniffTheGlove]

I got some off Ebay (£10 for 5!) M8 Quick Release Balljoints x 5 Trike Chop Race Rally on eBay (end time 05-Mar-11 14:15:25 GMT)

the 'problem' is that the north sector has a much tighter Radius than I cut as the south bearing is right at the point of the alt angle

I can get away with this as I am using the ball transfer units but a straight sector would be better.

BTW I found a good anti-tipping tip on one of the commercial seller's site - they have theirs with the top board at no more than 5 degrees off horizontal at both extremes of travel. I have marked these points off after checking with the Wixey. Seems to reduce the tracking time but I'd sooner have less tracking time than a scope fall off! I'll still get at least half an hour and with a 1-2min 'auto' reset it's no biggie

That should put your tacking time to 40mins with an effective movement of the platform by 10.5cm roughly

To work out the excat tracking movement use this equation...

(2 *PI() * NorthRadius * (0.25 * $B$16)) / (360)

In my case at 10deg will produce a tracking time of 40mins and with a North Radius of 61.02mm the tracking distance of the segment is 10.65cm

I did not go out today as I did not feel good enough even with the meds but I did spend some time looking at the electronics side and I have made the decision now of going down the Arduino route with Stepper Motors.

I have brought a pair of New 0.9deg (400 Step) 4.5v Stepper motors of ebay. With the 400 step instead of the 200 step I will have a bit more control of shaft rotation.

I am now in the process of getting an Arduino UNO Developers kit (comes with a few sensors, switches, breadboard etc) that will allow me to play and understand how it all works then I need o get a Motor Shield that will actually control the steppers.

Also with the Ardinuo, I can add additional components and sensors to add temp recording, humidity recording all to an SD card and then off load all the data and control to a web browser over ethernet/wifi

I also will look at motorising my focuser with the other stepper motor.

[Moonshane]

cheers Sean - that's plenty!

your electronic ability sounds a lot better than mine.

[SniffTheGlove]

your electronic ability sounds a lot better than mine.

Oh no it isn't!

I can program in many languages and have a little understanding with analog electronics but my soldering skills are ^%$^^ (I cook components before the solder flows and always ruin things).

Now as my soldering skill are rubbish I am finding it difficult to search for some Arduino motor shields. The main place I have been recommend to go to has the shields sold out without any idea when the next lot may come in, another supplier has the shield but it is in kit for and need soldering. Oh choices!

Being completely new to Arduino I have asked a few questions in the Arduino forums but like a lot of forums sometimes no one responds to you posting which makes you feel a little downhearted.

At least here in SGL someone will post replies.

[Moonshane]

ha ha - you sound like you know what you mean anyway

I tried my dc motor last night with my dew controller and there's a lot of vibration when it's tuned down in speed. I suppose that's why it's called a pulse width modulator?

I am hoping I can either use something else or 'filter' out the vibration somehow (either mechanically or with another electronic device). everything's a challenge!

[SniffTheGlove]

ha ha - you sound like you know what you mean anyway

I tried my dc motor last night with my dew controller and there's a lot of vibration when it's tuned down in speed. I suppose that's why it's called a pulse width modulator?

I am hoping I can either use something else or 'filter' out the vibration somehow (either mechanically or with another electronic device). everything's a challenge!

I am afriad Shane I can not offer a suggestion to cure the vibration.

Is it the actual motor vibrating stongly or is it just the output shaft.

Do you have a link to the motors you got off ebay, I presume it is the ebay motors that are vibrating.

[SniffTheGlove]

Now onto my next part of the build.

This morning I marked out the CofG on the plaform top and then rechecked all my measurements from that point. All Fine

Marked the drill point on the top of the platform for the hole for the M12 pivot thd rod. Drilled nicely through at 52deg.

Mounted (dry) the underneath pivot block and measured off the correct dimension 6cm down at 77.31cm from the North Segment Centre Line to make sure the reference plane is correct and it is.

In the attached photos you can see first the finished segment with it's wedges epoxied and screw down. The next photo is the underneath of the top platform showing the segment now epoxied and screwed into place with the south pivot block dry mounted.

Last photo is the south pivot block close up.

I have also ordered today...

An Arduino Uno Starter set Starter Kit for Arduino (ARDX) from .:oomlout:.

A 9v mains supply to power the Arduino

A EasyDriver Stepper Motor Driver

I think I will get to have some fun with this Arduino as it has given me a few other ideas.

[SniffTheGlove]

Top Top

Drill your south pivot hole first in the top platform because if you are not spot on then you can adjust the north segment to fit the dimension.

I have just notice that I am 4mm out on my reference plane. I think that when I did the initial 4mm pilot hole for the south pivot in the top platform I was spot on but then when I drilled out the hole by hand to 12mm I must of been slight off and not dead centre.

I am confident it wil;l not make much difference at 4mm but I will continue and see and if it is out when complete then I can adjust the north segment back 4mm by unscrewing and breaking the epoxy bond with some heat off the heat gun.

[Moonshane]

I am afriad Shane I can not offer a suggestion to cure the vibration.

Is it the actual motor vibrating stongly or is it just the output shaft.

Do you have a link to the motors you got off ebay, I presume it is the ebay motors that are vibrating.

hi Sean

it's this one. 12V DC 4RPM 4 RPM High Torque Electric Gear Box Motor on eBay (end time 28-Jan-11 14:25:57 GMT)

works fine at 12v but when when the dew controller is turned down to say 50% it's like a kangaroo hopping! I am sure it's the pulses being modulated (i.e. it's the controller not the motor) and I am looking at other ways to control the DC voltage which will solve my problem.

[SniffTheGlove]

hi Sean

it's this one. 12V DC 4RPM 4 RPM High Torque Electric Gear Box Motor on eBay (end time 28-Jan-11 14:25:57 GMT)

works fine at 12v but when when the dew controller is turned down to say 50% it's like a kangaroo hopping! I am sure it's the pulses being modulated (i.e. it's the controller not the motor) and I am looking at other ways to control the DC voltage which will solve my problem.

Here is a way to test it Shane. Instead of using the dew controller, connect 6v to the motor (either from a mains adaptor like a phone charger or connect up 4 AA batteries) and see how it runs. If it smooth then the controller is an issue else if it vibrates then it's the motor/gearbox.

From the details off the link, the shaft is off-centre which suggests a planetry gearbox so you could run with the PWM at 11v but gear the output shaft down to the necessary rpm you need for tracking, then you can slightly alter the PWM up to 12v to increase the voltage and increase the tracking speed when the battery starts to drain off. Hope that works as this is purly me thinking a solution.

[SniffTheGlove]

Quick update as it's getting to dark in the garage and colder.

I decided to redo the south pivot hole. I just needed to bring it northwards 4mm, so I just used a 11mm drill and slowly rotated it in the hole at 52deg and manually dragged the drill northwards grinding out the hole to elongate it.

Now the measurements are spot on. I have now used weatherproof wood adhesive to set the south pivot block (As per last photo) into the correct position. It will set overnight and tomorrow I will flip the platform over and use epoxy (with glass powder and wood dust) to fill in the elongated hole to the correct size location then put a smaller wedge block (which is already made) in position and glue that as well so there will be a perfect 52deg 12mm hole through the top mounting block, platform and south pivot block.

Again, a reminder to do the pivot block drilling first then place the north segment.

[Moonshane]

hi mate - just done it and it's a smooth as with just 6v via AAs.

I may actually try and create a mini circuit with resistors in between the batteries and the motor. this might work, would just need to do some trial and error or calculations with Ohm's law.

I am investigating ways around this though and will do a full report once it's all sorted.

[SniffTheGlove]

All my stuff has been delivered today (Arduino Kit, EasyDriver, Stepper Motors) and now having a quick play with it making the simple circuits up and understanding how it all works.

I have also attached the top platform south pivot block. It is currently awaiting the adhesive to dry. To keep it in the correct place the M12 threaded rod is located through the top block-platform-pivot block. Should be dry in a few hours when I shall then start on the bottom platform with the north segment support wheels and get them in place. Once that is done I can then measure the correct distance for the bottom part of the south pivot.

[SniffTheGlove]

This Arduino is great. I awalys wanted to be able to do things with my computers to control objects but in the past the serial control cards etc were £xxxx's but since hearing about Arduino a few weeks back I have read quite a bit (though it was confusing) but I am now starting to get to grips and it is great though some little knowledge of electronics does help else you start hanging around electronic geeks as well.

I did a complete tidy up in my dirty garage this morning as it was a mess with tools everywhere. Now it's back to a clean state and lots more room so I can start doing the bottom platform next.

[Moonshane]

hi Sean

I'd really appreciate a copy of the diagram if that's ok with you. I am still struggling.