Dovetail bar help

[deadsimple]

Hi there,

Here's a silly question for the record books:

I received the top half of my new EQ6 mount today (the other half coming tomorrow, silly couriers!) but in the meanwhile I've been exploring how to put everything together once I have the whole thing.

I have an 8" GSO Dob. Got myself 230mm GSO tube rings, plus a 13" Skywatcher dovetail bar (product page (http://www.skywatchertelescope.net/swtinc/product.php?id=117&class1=5&class2=501)) came with the EQ6 shipment.

Question: how in the world do I attach the rings to the bar? Are they incompatible? Do I have to buy a different bar? I feel completely gumby that I can't work out such a simple problem.

I've attached some photos of the two parts in various positions.

Any ideas?

[Spike]

gday mate you should have a couple of hex head setscrews/bolts. One goes up through the central hole at each end ov yr dovetail bar - the holes between yr allen head pair each end - and inta the threaded holes at the bottom ov each ring. ya can pinch em all up a bit with those allen head ones after if ya like too.

[deadsimple]

I don't have any. Is that meant to come with the rings or the mount/bar? I assume the rings since that's what I'd be screwing them to?

[Whizgig]

I think they come with the mount not with the rings as you would use the ones from the standard rings for the mount I would think.

[monoxide]

i think the bolts that hold the altitude hubs on fit the tube rings, atleast they did with my 6" dob, you can cover the holes left from removing the hubs with a bit of tape.

pretty crude but if they fit, it should get you back in action

[Wavytone]

You'll need 2 x 6mm metric capscrews (try Bunnings).

In the last photo you have the rail upside down with respect to the tube. The four smaller screws threaded in the rail are provided so you can adjust the alignment to get the optical axis perpendicular to the dec axis, if that matters to you. If not, remove these smaller screws.

Cheers...

[deadsimple]

Ah that makes sense. I thought it was just for other types of rings.

Is that what you know will exactly fit the ring? 6mm is indeed the size of the hole on the bar but the GSO ring's hole is 4.7mm in diameter. I assume something around 4.7 would be what to go for?



But how would the mount manufacturer know what size screws I need to screw onto the exact tube rings I might have?

Sorry I'm not very good with hardware bits and pieces :)

[Whizgig]

well I think they are all a standard size used by the mounts but not sure.
so you would take the ones from the mount that you are getting tomorrow and use them as they come with them to bol on the standard rings to the standard mounting unit.

[Wavytone]

4.7 mm...

Well if they aren't already threaded, cut 6mm threads into the holes in the GSO rings. For a 6mm thread it is normal to start with a 5mm hole, 4.7 should be close enough that a 6mm tap will start and cut into soft alloy.

For this you will need a 6mm metric tap and the handle to grip it, not overly expensive from Bunnings. Don't buy an entire set of taps and dies if you don't need to.

[deadsimple]

Actually they are .. I just can't easily find what the outer diameter is.

I'll see if the rest of the mount comes with screws/bolts tomorrow ... and see whether they fit. Here's hoping ..

[Spike]

if ya havent got those hexheads its because they come with the rings which it seems yaint got yet.....and as someone says the allen heads can be used for ultrafine aligning but thats almost irelevant except if ya obsessive;) fr gawdsake dont go listening t people tellin ya to start tappin an drillin, these are allstandard fittings regardless of length of bar and scopering diam:thumbsup:

[deadsimple]

You mean the mount, not the rings? Now I'm confused :)

Well looks like I'll be waiting a couple more days for the other half of the mount, it currently has no status on the courier's tracking page other than picked up at the store's address last week. What were the couriers thinking :(

[Spike]

Sorry mate I shoulda looked at yr photos again, just didn't want ya doin somethin idiotic like drillin an tappin which someone obviously with out any more clues than you was suggestin;)

Those round bits that hinge in half is what were talkin about, they are the rings that go around your scope tube an they have that little squared/blocky bit on their castings which has the tapped hole in it that the hexhead bolts that shoulda come with the rings go into like we said before
Get onto whoever sent you them rings an say wheres me flamin ring bolts mate!:mad2:

[bojan]

the bolts are in Bunnings... :-)

[deadsimple]

I had to look up 'tap' in google to figure out what that was about. When I hinted that I have absolutely no wood/metal-work stills, I wasn't kidding :)

Yep I understand that much, needed to know that to actually order the rings last week :P

I might give bunnings a try first as it's quicker than waiting for an interstate shipment of bolts. If I can't find a suitable pair (of bolts!) then I'll hassle the seller.

(btw, the courier company currently has no idea where the second half my shipment is. great!)

[deadsimple]

Bunnings indeed had what I was after.

6mm wide, 30mm long "high tensile" screwcaps worked like a charm.

Thanks guys.

[g__day]

In that last picture your dovetail is upside down - it won't grip in an EQ mount if the wide end faces the O-rings!

[deadsimple]

Yeah don't worry, someone corrected me earlier in the thread.

Thanks anyway.

[bojan]

BTW guys,
There is no need to buy dovetail profiled bar for some lighter applications.. For camera and lenses (up to 500mm) I am using standard Aluminium U-channel, 40mm, cut to 25cm length.
It does the same thing as dovetail bar, but costs much less (even in Bunnings).
Yes it is square, but it still goes nicely into the slanted side of my EQ6 mount head, where the screws are holding it in place.
I just had to make sure the bottom sits flat against the mount head in the beginning - gravity helps here a lot.. But since the groove is formed by the pressure of screws, I do not have to pay any special attention to it any more.

[Spike]

Jeepers creepers friend if it works for ya then fine but I certainly wouldnt recommend it. Those tapers and the width etc are to stop ya load from being able to cantilever out of the recess if say your scope/gear is pointing/hanging out overboard in specific directions regarding dec an RA- it cant "ride up" against the taper so to speak in these positions an flip out but I guess ya saying for real light applications and the fact the screws do create definite indents/craters well then ya half-safe. My stuff nah wouldn't risk it custom bars are easy to machine Spike.

[bojan]

Fair enough.. and you gave me a good idea: I will drill a half-way-through holes where the craters are formed.. so the screws will sit nicely into depressions.. so there will be no way for this to fall off :-)
So no risk at all.

On image is Rubinar 10/1000, mounted as discussed earlier.
Solid as a rock.

[BC]

Hi,

Just picking this thread up again now that I have acquired an EQ6. I was going to pose the same questions but most are answered here.

One concern is that 6mm bolts seem so small to be holding a 10” newt and possibly a camera etc. Surely bigger bolts would be the go? Do any of you drill out to a bigger size and tap the hole?

I imagined that the block on the tube ring would sit inside the channel, not sit bridged over it. It doesn’t seem quite so secure where the bolt isn’t holding metal to metal.

Bruce

[bojan]

This is my concern as well.. so I still use my 10" newtonian (13kg) on dobson mount.
Some people modified EQ6 head to be like Losmandy's.. I am not sure about that either.
Anyway I will not risk it with current and supplied dovetail bar, it looks too flimsy to me, like OK for up to 5kg but not much more that that.

[bmitchell82]

Like the old saying is Bigger doesn't mean better.! For instance have you ever looked at fishing line (i know not astronomy related) Dacron (aka fireline/wire) is really fine yet extremely strong as compared to a similar monofiliment ... look at 100 lb fishing line. you could mistake it for rope!

As for your 6mm bolts look or ask for the manufactures specs on what shear strength the bolt has and that can tell you if your bolts are worthy of the effort.! 10 - 20kgs is light talking newtons 20 kg would be close to 200 N and consdering that engineers usually talk kN... not N to give you a grasp on the magnitude of things. some may or may not agree but without looking into it too much ide say your bolts will be sufficient!

[bojan]

Well, it not about how much the screw can take before it bends or breaks.
It is about (elastic) bending and, consequently, vibration or swinging or mechanical/positional stability and resistance to wind gushes etc.
Bigger is always better (in astronomy, that is..)

[bmitchell82]

that is right elastic deformation depends on the material mainly. Have you ever heard of the term... "Damm these butter screws". you can have the same screw the same size one will just mash itself to bits the second you put a load into the head, the other will torque down so hard its not funny.

Yes it may be experiencing elastic deformation but it will only be in the axial movement not in the lateral so much this is where your bolt will fail due to shear stress and often will lead to a "catastrophic failure (material fractures compleately)" not just "fail(fall into the plastic deformation)".

If you wish to stop your gear from swaying in the breeze then increase the moment of inertia of your matierials or as you said make it bigger but at what cost? Heavier, Bulky, akward cheap or Light, Slim, and costs more! Aluminium is light but not so strong, Aluminum Alloy is a whole different ball game. as you then move to the atomic level and put in place defects to stop the slip of atoms from happening.:thumbsup:

Bigger is only better when it comes to aperature! Slim small light is what is needed everywhere else!