Skywatcher 72ed focus lock screw

[AstroNebulee]

Hi

A couple of questions on my focuser lock screw for my scope. 

1. How tight do you have to have the screw tightened up normally to lock it and would it cause the focus tube to to move out of collimation atall. 

2. Does anyone know what size they are as I'd like to get a replacement one as mine doesn't seem to grip the focus tube well and can still move focuser when engaged. On closer inspection it looks as though there's a bit of damaged threads. I've got a feeling it's an M6 30mm one, though not seem to be having much joy on a search. 

Lee 

Edited September 1, 2022 by AstroNebulee

[Zeta Reticulan]

It might be worth contacting Sky-Watcher about the size of the focuser screw. In my experience the only focusers that lock properly without affecting collimation are high end aftermarket ones like the MoonLite.

[AstroNebulee]

4 hours ago, Zeta Reticulan said:

It might be worth contacting Sky-Watcher about the size of the focuser screw. In my experience the only focusers that lock properly without affecting collimation are high end aftermarket ones like the MoonLite.

Thank you, I'll giver it a whirl with skywatcher and see if they reply. I was also thinking that only the high end scopes had no issues with collimation and focus lock screws. 

Lee

[Zeta Reticulan]

2 hours ago, AstroNebulee said:

Thank you, I'll giver it a whirl with skywatcher and see if they reply. I was also thinking that only the high end scopes had no issues with collimation and focus lock screws. 

Lee

You're welcome. 

[Chefgage]

12 hours ago, AstroNebulee said:

Hi

A couple of questions on my focuser lock screw for my scope. 

1. How tight do you have to have the screw tightened up normally to lock it and would it cause the focus tube to to move out of collimation atall. 

2. Does anyone know what size they are as I'd like to get a replacement one as mine doesn't seem to grip the focus tube well and can still move focuser when engaged. On closer inspection it looks as though there's a bit of damaged threads. I've got a feeling it's an M6 30mm one, though not seem to be having much joy on a search. 

Lee 

I have the same scope. I try to use the focus lock but in my experience it does affect the focus when you tighten it up. I use a bahtinov mask and an asiair to focus. As you tighten up the focus lock you can see the focus slightly adjust.

Edited September 2, 2022 by Chefgage

[SthBohemia]

Within another thread there is prolonged discussion  regarding the sub quality 'screws' as supplied by many of the Asian manufacturers @badhex started it.

Suggestion- Re-tap the hole with a 4BA tap (approx 3.6mm) buy a few cheese head 4BA machine screws from a model engineer supply company, make a mould from silicone rubber or similar product and cast the desired thumb screw head diameter using casting resin. Same technique as 'make your own Bob's Knobs'...

[StevieDvd]

Perhaps your focuser is too loose to begin with.  The lock screw should only need a light touch to keep the focuser in place. As you can see it also has a soft end to avoid marking/damaging the focuser so it won't dig in - bear that in mind looking for a replacement.

If you can find the screw size it may be worth putting a wanted ad in the buy/sell section as anyone who has fitted focus motors may have one left unused.

The ED72 has the old crayford style focuser which tend to slip but can be adjusted to be better - there are vidoes showing how on youtube.

[AstroNebulee]

5 hours ago, Chefgage said:

I have the same scope. I try to use the focus lock but in my experience it does affect the focus when you tighten it up. I use a bahtinov mask and an asiair to focus. As you tighten up the focus lock you can see the focus slightly adjust.

Thank you, that's a great idea and I never thought of that. I have an asiair pro and use my bat mask to focus so I shall try this next time. Though the issue isn't really the focus of is if it throws off the collimation. 

4 hours ago, SthBohemia said:

Within another thread there is prolonged discussion  regarding the sub quality 'screws' as supplied by many of the Asian manufacturers @badhex started it.

Suggestion- Re-tap the hole with a 4BA tap (approx 3.6mm) buy a few cheese head 4BA machine screws from a model engineer supply company, make a mould from silicone rubber or similar product and cast the desired thumb screw head diameter using casting resin. Same technique as 'make your own Bob's Knobs'...

Thank you for your suggestion but moulding my own thumbscrew is not really what I'd like to do. 

2 hours ago, StevieDvd said:

Perhaps your focuser is too loose to begin with.  The lock screw should only need a light touch to keep the focuser in place. As you can see it also has a soft end to avoid marking/damaging the focuser so it won't dig in - bear that in mind looking for a replacement.

If you can find the screw size it may be worth putting a wanted ad in the buy/sell section as anyone who has fitted focus motors may have one left unused.

The ED72 has the old crayford style focuser which tend to slip but can be adjusted to be better - there are vidoes showing how on youtube.

Thank you, I have adjusted the tension on my focuser before using this tutorial https://www.yumpu.com/en/document/read/22870696/adjusting-the-skywatcher-110-refractor-crayford-focuser-teleskop

I was going to check the tension of it again at the weekend. It may just need a small tweak to add more tension. 

I have seen a few thumbscrew bolts online with nulon/rubber tipped ends but wasn't sure on the size, I shall ask in the wanted section to see if anyone had something spare, thank you. 

 

Lee

Edited September 2, 2022 by AstroNebulee
addition

[Mandy D]

Stick a ruler across the thread and estimate the diameter. It'll be a whole number of millimetres, most likely 4, 5 or 6. That will be the M size, e.g. M6. Final check is to measure the length of the threaded section and count the turns. Divide length by turns and that gives the pitch of the thread. M4 is 0.7mm pitch, M5 is 0.8mm and M6 is 1mm. This is standard metric coarse. Skywatcher will almost certainly be using standard metric fasteners.

Or, take it to your local nut and bolt purveyor and get them to try a selection in it to determine correct size.

They also sell nylon screws that won't damage the surface you are screwing onto.

[AstroNebulee]

7 minutes ago, Mandy D said:

Stick a ruler across the thread and estimate the diameter. It'll be a whole number of millimetres, most likely 4, 5 or 6. That will be the M size, e.g. M6. Final check is to measure the length of the threaded section and count the turns. Divide length by turns and that gives the pitch of the thread. M4 is 0.7mm pitch, M5 is 0.8mm and M6 is 1mm. This is standard metric coarse. Skywatcher will almost certainly be using standard metric fasteners.

Or, take it to your local nut and bolt purveyor and get them to try a selection in it to determine correct size.

They also sell nylon screws that won't damage the surface you are screwing onto.

Thank you Mandy, I shall give the measurement a go moro, I always wondered how you work out by the pitch of a thread, now I know 😊. 

Lee 

[Zeta Reticulan]

11 minutes ago, Mandy D said:

Stick a ruler across the thread and estimate the diameter. It'll be a whole number of millimetres, most likely 4, 5 or 6. That will be the M size, e.g. M6. Final check is to measure the length of the threaded section and count the turns. Divide length by turns and that gives the pitch of the thread. M4 is 0.7mm pitch, M5 is 0.8mm and M6 is 1mm. This is standard metric coarse. Skywatcher will almost certainly be using standard metric fasteners.

Or, take it to your local nut and bolt purveyor and get them to try a selection in it to determine correct size.

They also sell nylon screws that won't damage the surface you are screwing onto.

Brilliant! I always wondered how to figure out the pitch of a thread. You are now officially my hero. 

[Mandy D]

2 minutes ago, Zeta Reticulan said:

Brilliant! I always wondered how to figure out the pitch of a thread. You are now officially my hero. 

I guess being an engineer helps! Glad to be able to assist.

[Zeta Reticulan]

19 minutes ago, Mandy D said:

I guess being an engineer helps! Glad to be able to assist.

Thanks again. You can tell I'm not an engineer. I usually feel proud of myself if I can put the OTA in the dovetail properly when I'm setting up lol.

[AstroNebulee]

19 hours ago, Mandy D said:

Stick a ruler across the thread and estimate the diameter. It'll be a whole number of millimetres, most likely 4, 5 or 6. That will be the M size, e.g. M6. Final check is to measure the length of the threaded section and count the turns. Divide length by turns and that gives the pitch of the thread. M4 is 0.7mm pitch, M5 is 0.8mm and M6 is 1mm. This is standard metric coarse. Skywatcher will almost certainly be using standard metric fasteners.

Or, take it to your local nut and bolt purveyor and get them to try a selection in it to determine correct size.

They also sell nylon screws that won't damage the surface you are screwing onto.

Measured the grub screw and it appears to be M5 with a pitch of 0.8 (thread length is 18mm and number of threads is 22) if I counted correctly. Thank you Mandy. Still not seen any nylon tipped thumbscrews yet, seen nylon tipped grub screws or nylon screws. A nylon tipped thumbscrew seems like rocking horse poop. 

Lee 

[Mandy D]

17 minutes ago, AstroNebulee said:

Measured the grub screw and it appears to be M5 with a pitch of 0.8 (thread length is 18mm and number of threads is 22) if I counted correctly. Thank you Mandy. Still not seen any nylon tipped thumbscrews yet, seen nylon tipped grub screws or nylon screws. A nylon tipped thumbscrew seems like rocking horse poop. 

Lee 

Nylon tipped screws seem to be thin on the ground. Try solid nylon, at least it won't strip the thread in the expensive part and they are for pennies. Have a look at this link:

https://www.digikey.co.uk/en/products/filter/screws-bolts/572?s=N4IgTCBcDaILYFYAEA7AngGwPYqQFwAsBXOAIyQGcBjAJwFMB3EAXQF8g

I would go for a pack of 10 or 25 to have some spares.

[AstroNebulee]

26 minutes ago, Mandy D said:

Nylon tipped screws seem to be thin on the ground. Try solid nylon, at least it won't strip the thread in the expensive part and they are for pennies. Have a look at this link:

https://www.digikey.co.uk/en/products/filter/screws-bolts/572?s=N4IgTCBcDaILYFYAEA7AngGwPYqQFwAsBXOAIyQGcBjAJwFMB3EAXQF8g

I would go for a pack of 10 or 25 to have some spares.

Thank you they do seem so rare. I've sent of emails to 5 or 6 companies to see if they do them or can aquire any. 

I have seriously thought about 100% nylon screws but was a little worried if they breakoff in the thread. Or are they quite tough?

Lee 

[DaveL59]

1 minute ago, AstroNebulee said:

Thank you they do seem so rare. I've sent of emails to 5 or 6 companies to see if they do them or can aquire any. 

I have seriously thought about 100% nylon screws but was a little worried if they breakoff in the thread. Or are they quite tough?

Lee 

I've swapped out a number of the metal thumbscrews on my scopes both for the eyepiece locking and finder shoes for nylon ones. Driven to that move when the tapped hole on the TAL100RS focuser body was mostly stripped when I got it. So far I've had no issues with them and can be safe in the knowledge that any overtightening will not strip the metal threads. Add that buying over-length they are easy to trim to size too, also no corrosion from dissimilar metals when things get damp from dew etc.

If you do ever snap one off, then a hot screwdriver tip inserted into the stub should allow you to back it out without needing to strip the unit down. They are quite tough and I've not had any wear out on the threads, but they're also cheap enough to carry a couple spare just in case. I would say tho don't tighten them too hard as that will eventually shred the nylon threads.

[Mandy D]

4 minutes ago, AstroNebulee said:

Thank you they do seem so rare. I've sent of emails to 5 or 6 companies to see if they do them or can aquire any. 

I have seriously thought about 100% nylon screws but was a little worried if they breakoff in the thread. Or are they quite tough?

Lee 

They are quite tough. You are more likely to strip the thread on them, then it is just a case of picking out any remains from the hole they are screwed into or running a tap through to clear the remains.

Tensile strength of nylon is 50 - 90 MPa, mild steel can be as low as 300 MPa. Remember, these are only thumb screws and you are not tightening with a tool, so you will get nowhere near the torque that you might apply to a steel screw of the same size.

[AstroNebulee]

Thank you guys. I've ordered some of them now and will give them a go. 

I wouldn't over tighten one but if so, the idea of inserting a hot tip into one to get them out is brilliant. 

Thank you to Mandy too for the advice on measuring and calculating the thread pitch, feel quite brainy now haha. 

 

Lee

Edited September 3, 2022 by AstroNebulee

[Nickedynick]

@AstroNebulee did you find out the thread size for this in the end? I want to 3D print a bracket for an autofocuser which I was thinking of mounting in a similar way to this setup which uses the holes for the thumb screw and opposite non-used one, but have been struggling to find info anywhere either!

[AstroNebulee]

On 03/09/2022 at 17:56, Mandy D said:

Measured the grub screw and it appears to be M5 with a pitch of 0.8 (thread length is 18mm and number of threads is 22) if I counted correctly.

@Nickedynick I think it's this size as said earlier in the thread 😊. Good luck 👍

Lee