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Home made Dobsonian Base.


Moonshane

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One tool I would recommend buying when building the base is a sash clamp, or two. They're not really needed, my previous two bases were made without them, but they do make constructing the box part of the base so much easier. They don't have to be expensive either, mine cost about £8 each.

Buy Axminster Aluminium Sash Cramps from Axminster, fast delivery for the UK

I hadn't thought about using a drill as a lathe either. It's not a bad idea really. I'll have to try it when I get round to sanding my side bearings.

I'm sorry to read about you all being made redundant too. I've been unemployed for the last few months so I know how you all must be feeling, although I was self employed so I didn't get any redundancy pay.. In the long run it could be a good thing for us all though. I've been using the time to go back to college and gain some qualifications. I think the main thing to do is to try and keep yourself busy.

good call mate re the clamps. thankfully, my father in law has a box full of these and I'll be able to borrow some so will be very useful.

cheers for the encouragement re job hunting. let's hope I/we get something soon

Mmm, interesting idea using a drill as a lathe. A couple of points spring to mind though -

1. Challange #1. Make sure the drill is well fixed to prevent any movement during working on the piece.

2. If the diameter of the piece being lathed is large, this is likely to put a huge strain on the drill motor at start-up due to the large inertia. (the drill is designed to turn relatively small diameter tools with low inertia).

3. Getting the piece to be completely square wrt the drill axis of rotation is difficult, and becomes more so as diameter increases.

Not saying it's impossible, but I once tried to lathe circular plastic sheet material using a drill and all the above created challenges/difficulties.

Oh, and wear goggles !! I recall an incident at school woodwork class once where a schoolmate failed to attach the cherished wooden bowl he was making onto the lathe correctly. Next thing, the bowl was through the window and ended up 20 yards across the field. Not pretty.

all good points Kevin and you are spot on. might be better make a wooden arm of the circle radius and then cutting the piece in half and bolting a sander down to the workbench. always more than one way to skin a cat!

I've used a drill for sanding small parts.

ouch. sounds painful! :)

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Mmm, interesting idea using a drill as a lathe. A couple of points spring to mind though -

1. Challange #1. Make sure the drill is well fixed to prevent any movement during working on the piece.

2. If the diameter of the piece being lathed is large, this is likely to put a huge strain on the drill motor at start-up due to the large inertia. (the drill is designed to turn relatively small diameter tools with low inertia).

3. Getting the piece to be completely square wrt the drill axis of rotation is difficult, and becomes more so as diameter increases.

Not saying it's impossible, but I once tried to lathe circular plastic sheet material using a drill and all the above created challenges/difficulties.

Oh, and wear goggles !! I recall an incident at school woodwork class once where a schoolmate failed to attach the cherished wooden bowl he was making onto the lathe correctly. Next thing, the bowl was through the window and ended up 20 yards across the field. Not pretty.

I, too, would be very reluctant to try to use a drill to round anything as large as the bearings or the base. Most drill chucks do not run true enough to accomplish adequate rounding, to say nothing of the difficulties of fixing the drill and the wear and tear that the materials would cause to the motor and to the chuck and possibly to the motor armature.

The best tool for this - probably even better than a lathe - is the hand held router. With a central pivot point and an offset from it to the inner edge of the bit set at the desired radius, geometry and the power of the tool will see you through. I've done this quite a few times. In a shop filled with all tools, the router would be my tool of choice.

The most important part of Kevin's post though is the caution to wear eye protection. The cost of an accident is far too dire to be cavalier about this protection.

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To some extent the measurements are academic as they need to be designed to suit the tube for which you are making the base, the position of the balance point / length of the tube and of course observer height.
Of course you are right, Shane. I was asking about dimensions so that I might get a better idea about why you chose them and a better sense of the overall proportion. There are a lot of variables to consider and I thought that a discussion of dimensions might shed some light on the considerations and educate us about your decision making process. I know nothing about 6" Newts (and very little about any other scopes) and so have no idea of the length of the OTA or about balancing weight among mirrors, finders, focusers, eps, etc.
My main concern is always stability and I feel anything less than 12" at the base is basically unstable.
I'm surprised that a foot print of only 12" on a side could be deemed adequate, but then I'm thinking about the bulk of my 8" dob.
groundboard

a circle with feet set at 120 degrees and as far out as possible. this could be any shape really but a circle remains hidden at all times. e.g. a square would have corners sticking out when the rocker is turned. (toes, dark etc). teflon pads on the upperside 1.5"x1.5"x3mm fixed above the feet. mine is 13" diameter.

All makes perfect sense. I'm in the throes of making a triangular sub-base to fit under the manufacturer's circular. The motivation is to use all-thread rods through nuts fixed in the base at each intersection to be used to level the base so that I can use a digital angle gauge to get more accurate bearings. I move my scope about with a small handcart with the OTA bearings sitting in the carriage. Of course I brace the OTA against sudden shifting. Adding the extra bit of weight for the triangular sub-base doesn't concern me, but I wondered if moving it about while on the bearings is a bad idea.
Rocker

basically four rectangles and a square base. square piece at the bottom to match the groundboard plus maybe 1/2".

sides need a circular section cut to match the bearings attached to the tube. needs to have a large enough curve to be able to fix teflon pads at 70 degree points from centre of arc. as mentioned match the height to the observer's height. I literally got him to hold the scope up to his eye while seated at about the mid point and measured to the ground.

front piece should be measured to reach as high as possible but to allow the scope to drop down to a few degrees.

back piece high enough to provide support and low enough to let the primary end of the mirror pass plus a few inches for balancing if required.

Carriage construction seems straightforward, but I'm curious about why you the proportions you did. It seems tall to me, but you said that the dimensions were custom fitted for someone seated while viewing.
alt bearings

just circular sections maybe 1.5-2x the diameter of the tube. they can be half circles too. obviously need to be matched to the circular sections on the sides of the rocker.

The scope appears to allow you some leeway in choosing the pivot point on the OTA. I will have to examine my Orion to see if it can be changed. I thought, perhaps, that the engineers at Orion had done the geometry and physics and chosen the optimum pivot point making assumptions about the average weight of finders, eps, etc; therefore I had not considered possible mods. I would like to know how you chose the pivot point and whether that decision influenced the size of the alt bearings or the height of their pivot center above ground.
hope this makes sense. really, like an equatorial platform, you need to match the dimensions to suit your own circumstances - one of the main advantages of making one yourself and the reason why I suppose, manufacturers use more 'generic' dimensions?
Yes, of course, your post makes great sense. My curiosity and lack of knowledge - and maybe a bit of obsession and compulsion on my part - prompted my questions.

Thank you for taking the time to share your results with us and for taking our questions. You've helped me learn a great deal.

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I might be getting confused about what a lathe does, it's been a few years since I last used one, but I was thinking more along the lines of making a jig for the circle to spin on and using the drill, with some sort of attachment, to sand the wood. Instead of attaching the wood to the drill.

Something a bit like this but with a drill instead of whatever it is they're using.

Sanding Jig for Circles

I've got a router to cut the circles out. I was just thinking of using the drill as an easy way to finish them off and make sure both bearings are exactly the same.

Edited by scogyrd
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I might be getting confused about what a lathe does, it's been a few years since I last used one, but I was thinking more along the lines of making a jig for the circle to spin on and using the drill, with some sort of attachment, to sand the wood. Instead of attaching the wood to the drill.

Something a bit like this but with a drill instead of whatever it is they're using.

Sanding Jig for Circles

I've got a router to cut the circles out. I was just thinking of using the drill as an easy way to finish them off and make sure both bearings are exactly the same.

You are correct. I was afraid that someone might try to center a piece on the shaft of a drill and shape it. The approach that you depict would work. The tool shown is a rotary bench sander and would be ideal - much better than a drill or a drill press (Is a 'pillar' drill the same tool?). If you have no bench sander, then perhaps a belt sander on its side and affixed to the workbench would do just as well. I've used a belt sander in this way and see no reason why it wouldn't work for the job at hand. The last tool I'd use would be a hand drill and then only as a last resort.

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Hope you don't mind me suggesting ... but you might find that thin aluminium sheet works better than an LP. Its easy to work and will give you a better more rigid base. The buttery smoothness of Al is really nice - and it won't shatter at lower temperatures.

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You are correct. I was afraid that someone might try to center a piece on the shaft of a drill and shape it. The approach that you depict would work. The tool shown is a rotary bench sander and would be ideal - much better than a drill or a drill press (Is a 'pillar' drill the same tool?). If you have no bench sander, then perhaps a belt sander on its side and affixed to the workbench would do just as well. I've used a belt sander in this way and see no reason why it wouldn't work for the job at hand. The last tool I'd use would be a hand drill and then only as a last resort.

I wouldn't fancy doing it with a hand drill either but I've got a drill press so I'll probably use that if I do decide to have a go. I reckon it might be easier to use that than it would be to fix my sander to my workbench too. My sander is a funny shape and I can see it being a right pain to get (and keep) everything square. TBH I don't really know though. I'll have to see what I think is best if I do decide to do it.

Thanks for the advice btw.

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hi Kevin, yes, I totally agree.

as long as the teflon pads are at about 70 (not 90) degrees apart (imagine a 70 degree piece of pie from the centre of the bearing radius) then the shape of the cut out at the top of the side bearing does not matter. effectively, be it on a curve or a V, it's in the same place. so the angle of the V would need to be 110 degrees to ensure they are 90 degrees to the alt bearing surface.

Scogyrd did this on his thread but not sure what his angles were.

I've just noticed this and my V was 90 degrees and the teflon pads were 90 degrees apart as well, and I'm pretty certain the alt bearing sat on the teflon at 90 degrees too. I can't tell at the minute because my scope no longer has the original side bearings on it and I've removed the teflon from my last base.

edit: I've just checked the plans for my dob and the bearings definitely sat on the teflon at 90 degrees.

Edited by scogyrd
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I wouldn't fancy doing it with a hand drill either but I've got a drill press so I'll probably use that if I do decide to have a go. I reckon it might be easier to use that than it would be to fix my sander to my workbench too. My sander is a funny shape and I can see it being a right pain to get (and keep) everything square. TBH I don't really know though. I'll have to see what I think is best if I do decide to do it.

Thanks for the advice btw.

Just a bit more . . .

You can use a drill press but you need to be very careful about how much pressure you apply. Drill presses are not designed for pressure applied at 90 degrees to the armature. You should expect to take very small amounts off the edges of the bearings and make multiple passes of ever lighter pressure. I ruined a drill press (admittedly an inexpensive Asian one) in this pursuit. That's why I suggested a belt sander.

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I might be getting confused about what a lathe does, it's been a few years since I last used one, but I was thinking more along the lines of making a jig for the circle to spin on and using the drill, with some sort of attachment, to sand the wood. Instead of attaching the wood to the drill.

Something a bit like this but with a drill instead of whatever it is they're using.

Sanding Jig for Circles

I've got a router to cut the circles out. I was just thinking of using the drill as an easy way to finish them off and make sure both bearings are exactly the same.

this is a much better idea. just to be clear I agree with a router being the best too for the job (and have one) but was trying to think of ways to do this if you don't have one. yes, any kind of sander on the side would be good with this sort of jig - or just buy a used router for £25-30! :)

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I've just noticed this and my V was 90 degrees and the teflon pads were 90 degrees apart as well, and I'm pretty certain the alt bearing sat on the teflon at 90 degrees too. I can't tell at the minute because my scope no longer has the original side bearings on it and I've removed the teflon from my last base.

edit: I've just checked the plans for my dob and the bearings definitely sat on the teflon at 90 degrees.

this is certainly possible. I think it's due to the relationship between the depth of your curve and the diameter of the bearing. according to Kriege and Berry the optimum spread is 65-70 degrees although this assumes a shallow section of the curve.

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this is certainly possible. I think it's due to the relationship between the depth of your curve and the diameter of the bearing. according to Kriege and Berry the optimum spread is 65-70 degrees although this assumes a shallow section of the curve.

In my view, the decision on where to place the teflon supports is independant of the curve depth and diameter of bearing. I think it's angular position that is important. The decision is, I believe, driven by the following considerations -

1. the material used to make the bearings and the structural integrity of the circularity (ie. is the bearing a solid circular piece, or a hollow ring such as a section of pipe or similar). A ring would be more prone to deformation (ie. losing its circularity) than a solid piece.

2. the weight of the OTA being supported. Heavier = more likelihood of deforming the bearing, depending on the answer to point 1 above.

I guess the overall aim is to use contact points that provide the lowest pressure at the point of contact, without deforming the bearing. The position obviously needs to be wide enough so that the OTA doesn't "roll off" the supports when the altitude is adjusted.

For my next 6" Dob (currently being planned) I was intending using 110mm drain pipe end caps for the bearings (larger ones are available on-line so may consider these instead). Couple of quid each and would not require any fabrication on my part.

Edited by Astrokev
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this is certainly possible. I think it's due to the relationship between the depth of your curve and the diameter of the bearing. according to Kriege and Berry the optimum spread is 65-70 degrees although this assumes a shallow section of the curve.

I've just got back from the woodyard and tried marking the sides up like they said and while I admit I might be doing it wrong I can't see how the teflon would be at a right angle to the bearing if the v isn't 90 degrees. I asked my Dad how he'd do it too, he designs jet engines for Rolls Royce so I think he knows a thing or two about bearings, weight distribution etc., and he said the best way to do it would be to make the cut out 90 degrees. He also thinks a V would be better than a circle because it's only got 2 very slim points of contact and they'd be much bigger, in comparison, if you used a circle.

Did you use your router to cut out the circle in the sideboard btw? I had a go at it the other week and while the bit I cut out was perfectly circular the sideboard I wanted to use wasn't, it was wider at the edges so my bearing was a lot closer to the centre of the board than it was the edges, so if you've got any tips for cutting it out it'd be much appreciated.

TBH I've not decided whether to make the cut out circular or a v shape yet so I'm going to do both and see which one I like best. I was going to try building it today but, typically, it's started to rain here.

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I've just got back from the woodyard and tried marking the sides up like they said and while I admit I might be doing it wrong I can't see how the teflon would be at a right angle to the bearing if the v isn't 90 degrees. I asked my Dad how he'd do it too, he designs jet engines for Rolls Royce so I think he knows a thing or two about bearings, weight distribution etc., and he said the best way to do it would be to make the cut out 90 degrees. He also thinks a V would be better than a circle because it's only got 2 very slim points of contact and they'd be much bigger, in comparison, if you used a circle.

TBH I've not decided whether to make the cut out circular or a v shape yet so I'm going to do both and see which one I like best. I was going to try building it today but, typically, it's started to rain here.

For a V cut-out, the cut-out forms a tangent at the point of contact with the bearing. Therefore, if you want the teflon pads to be spaced at an angle of 90' on the bearing, the cut-out logically needs to be cut at 90' also. It's basic trigonometry if you think about it. Think of it as a quadrilateral, with the points of contact always being at 90' (tangents). The V angle and the angle between the pads as measured from the centre of the alt axis must always add up to 180'.

As Shane previously suggested, if you want the pads to be at 70', then the V cut out needs to be at 110'.

(2 x 90' + 70' + 110' = 360').

Again, whether you choose a V or a circle cut-out doesn't make any difference regarding the amount of support given, providing the points of contact are tangents to the bearing.

hope this makes sense.

Edited by Astrokev
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I think it was a case of me misunderstanding it and trying to overcomplicate things. Sorry for hijacking the thread and thanks for all the advise.

It's still raining here so you can all blame me for causing it. If I hadn't bought the wood yesterday we'd have all been guaranteed clear skies for at least another week.:)

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Hope you don't mind me suggesting ... but you might find that thin aluminium sheet works better than an LP. Its easy to work and will give you a better more rigid base. The buttery smoothness of Al is really nice - and it won't shatter at lower temperatures.

this seems like an excellent suggestion but I thought that there was a fair degree of stiction with smooth surfaces?

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In my view, the decision on where to place the teflon supports is independant of the curve depth and diameter of bearing. I think it's angular position that is important. The decision is, I believe, driven by the following considerations -

1. the material used to make the bearings and the structural integrity of the circularity (ie. is the bearing a solid circular piece, or a hollow ring such as a section of pipe or similar). A ring would be more prone to deformation (ie. losing its circularity) than a solid piece.

2. the weight of the OTA being supported. Heavier = more likelihood of deforming the bearing, depending on the answer to point 1 above.

I guess the overall aim is to use contact points that provide the lowest pressure at the point of contact, without deforming the bearing. The position obviously needs to be wide enough so that the OTA doesn't "roll off" the supports when the altitude is adjusted.

For my next 6" Dob (currently being planned) I was intending using 110mm drain pipe end caps for the bearings (larger ones are available on-line so may consider these instead). Couple of quid each and would not require any fabrication on my part.

no reason not to at all. the principle according to Kriege and Berry is that the smaller the bearings the higher the sides need to be (good when it comes to a 6" tube) and the less friction there is in the bearing teflon combo. again this might be good but I prefer more friction (albeit less 'stiction'.).

again the 65-70 degree spread of the alt bearings is just the optimum according to their research. obviously any other angles can be used but there will be differences in characteristics. :)

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I think it was a case of me misunderstanding it and trying to overcomplicate things. Sorry for hijacking the thread and thanks for all the advise.

It's still raining here so you can all blame me for causing it. If I hadn't bought the wood yesterday we'd have all been guaranteed clear skies for at least another week.:)

hey mate, this is an open thread and all comments and contributions are welcome and useful! :)

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here's the last of the finishing bits and the whole thing set up. used it the same night and it's excellent! he's delighted with it.

added some iron on contiboard end trims to the alt bearings. work well with the 3mm teflon pads added and nailed in place.

also added the keeps and some flocking as a liner to reduce friction and possible damage to the finish when he applies it.

as you can see, being a tidy worker is not one of my strong points!

hope this inspires others to have a go even if they do it their way :)

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ps one unexpected advantage of this design and size of scope is that as you remain seated in the same position throughout the whole of the observing session, and the alt bearing is positioned just where your hands are, I found that you can drive it through space like a car using the alt bearing as a steering wheel!

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  • 5 months later...

Shane,

Resurrecting this thread...

I'm currently looking to buy a Dob mount for my recently purchased used OoUK Newtonian... The obvious choice of buying their £200+ mount does not appeal to me :icon_confused:

If you supply these in kit form, I can probably do the assembling, gluing, tacking etc.

So how about it, sell these mounts!

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