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Help!! 130p mod to achieve focus

Russe

By Russe
in DIY Astronomer

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Russe

Russe

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Hi guys,

Please help! I wanted to mod my 130p by inserting longer collimation screws to move the primary mirror up by an inch or do.

Apparently one is to use 50mm or 70mm screws.

My problem: I can't remove the screws from the bottom of the primary mirror!!

How do I get them out w/o breaking anything???

Any thoughts greatly appreciated!!!

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Oddsocks

Oddsocks

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Hi Russe,

It looks from your pictures that the screws you can't remove are threaded studs, in which case they will have been "glued"in place with engineering stud-lock adhesive such as "Locktite"

You will need to remove the mirror clamps and take the mirror off the cell.

Now you need to heat the cell to weaken the glue, the problem is if you heat too highly the paint will peel as well.

I would suggest heating the cell to 100 deg C in the oven for thirty minutes first then try to remove the studs again while the cell is still hot, be careful not to burn yourself.

There may be a cork mat behind the mirror, if this is glued to the cell it will not be damaged by heating at 100 deg C but the glue holding it to the cell will soften and the mat may need re-glueing afterwards.

To remove the studs without damaging the threads you need to lock together a pair of nuts on the thread and then use a spanner on the locked nuts to unscrew the stud.

If you don't have a pair of nuts then you have to use a locking wrench to grip the studs such as "Mole Grips" but this will damage the threads and they wont be useable in the future.

If 100 deg C is not enough you can go up to 160 deg C or 180 deg C but be prepared for paint damage and cork mat damage, you may have to repaint the cell afterward and cut a new cork pad.

When heating the whole cell do not go above 180 deg C or the cell may warp.

If a higher temperature than this is needed then to avoid warping the cell you have to heat each stud individually for a few minutes with a gas blow torch, this will heat the stud to around 400-500 deg C and you need to apply the flame long enough for the heat to travel the whole length of the stud down into the cell, this will soften the strongest of adhesives but will cause paint damage as well so a touch-up of the affected areas will be needed.

William.

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Oddsocks

Oddsocks

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Russe,

Forgot to say, the quality of the steel used by SW is mostly recycled rubbish and very weak compared to the mix used in a Japanese, European or American foundry.

You will find many threads on SGL from people who have sheared off the studs while trying to remove them from the cell.

The use of heat to soften the factory applied stud-lock is vital, don't rely on brute force alone or the result will be a broken stud!

Best of luck.

William.

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Russe

Russe

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Measure the OD of the threaded sections then try a nut of the same dia. just in case they have used something other than 'normal' M4,M5,M6 etc.

Nigel

Got the Loctite today and applied it twice. Not budging yet. Just got some M5 nuts now to counterlock them and get a better grip...

One thing is true - the paint wipes off very easily. Might have to repaint the back.

Got winged nuts too. Hope it'll work. Really don't want to cook it...

The ends of the screws will have to be sawn off I suppose. Mhm... Don't have a metal saw...

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JamesF

JamesF

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Rather than araldite the threads back in for your mod, you could do a nut up against the back face of the cell. That should hold it in position without the need for unpleasant stuff like threadlock and mean it can be undone in the future.

How are you trying to turn the threadlocked parts at the moment to undo them?

James

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Russe

Russe

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Rather than araldite the threads back in for your mod, you could do a nut up against the back face of the cell. That should hold it in position without the need for unpleasant stuff like threadlock and mean it can be undone in the future.

How are you trying to turn the threadlocked parts at the moment to undo them?

James

A nut up?

Sorry, u lost me. How? Like a nut that allows to join to M5 screws?

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Russe

Russe

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I've just tried turning the screw with pliers. Since they don't budge I've just bought a set of M5 screws n nuts and want to put them on the one sticking out of the mirror cell. Then lock them tight against each other and with a wrench turn the lower one might do the trick?

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JamesF

JamesF

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I've just tried turning the screw with pliers. Since they don't budge I've just bought a set of M5 screws n nuts and want to put them on the one sticking out of the mirror cell. Then lock them tight against each other and with a wrench turn the lower one might do the trick?

Pliers are a fairly horrible way to do it :)I'd thread two nuts onto the stud and tighten them up against each other, then put a spanner (ring spanner if possible) on the lower one and use that to turn the stud.

Extending the existing stud by joining it to a new one using a threaded collar might be possible, but could well foul the springs.

James

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JamesF

JamesF

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A nut up?

Sorry, u lost me. How? Like a nut that allows to join to M5 screws?

In your third image in the first post in the diagram of the modded cell you show a 5mm rod glued into the mirror cell. There are times when threadlock is a sensible solution. However, given the trouble you're having because of it's previous use in this situation and that you may well want to change the mirror placement again one day I'd caution against using it here.

What I'd do is screw the 5mm threaded rod into the cell. Then put a nut on the rod and spin it all the way down to the back of the cell and tighten it up against the back of the cell. That should hold the threaded rod firmly in the cell, but is easily reversible if the need arises in the future.

James

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ibraidwo

ibraidwo

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Some late night thoughts...

Wouldn't it be easier/safer to leave the existing screws in place and remove their heads and then use a treaded collar to join your new (threaded) rods to the remainder part of the screws on the mirror (basically extending the existing screws).

This would work as long as the diameter of the springs were larger than the diameter of the threaded collars.

Else, another way to remove a stuck screw is (again) to take of the screw heads and tighten two nuts together on the thread end and use a spanner on the lower screw (closest to the mirror) to try and remove it from the mirror.

Also do check that its not a left-hand thread which needs undoing the other way....

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ibraidwo

ibraidwo

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Ah, a threaded rod I need, not a screw. I was at B&Q, and couldn't find rods...

You can turn a bolt into a rod by cutting off its head. The cleanest way to do this is to thread a nut on it up to where you want the cut, then use a hacksaw against the nut face (to keep the cut straightish) and then unscrew the nut over the cut end, which helps ensure that the thread end has the correct shape.

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JamesF

JamesF

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It should be obvious from where the thread enters the back of the cell which way you need to turn it to undo it. If you've already removed the mirror though, does it matter which way you turn it initially, as long as you loosen the screw? They go all the way through, don't they?

I think they'd have to be pretty obtuse to have used a left-hand thread mind. Mainly they're used to retain pieces that rotate where the normal direction of rotation would tend to undo the nut/bolt/screw. And on gas bottles. Never found out why they're used on gas bottles.

James

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