Screw size

[N3ptune]

Hello, I would like to get some advices with bolt sizes for those who have experience.

I want to replace the thumbscrews from my 2" and 1.25" skywatcher eyepiece holder tubes with Nylon thumbscrews.  So I am trying to identify the gauge of the actual screws and it's not obvious, my measurements don't match the metric or imperial charts I found on the WWW. Also, the tube holder holes seems to be wider then required for the screw.

METRIC: (Caliper measurement 3.8mm = 0.1496")

M3 - 3mm / .1181"

M3.5 - 3.5mm / .1378"

M4 - 4mm / .1574"

IMPERIAL: (Caliper measurement 3.8mm = 0.1496")

#5 - 0.12"

#6 - 0.13"

#8 - 0.16"

At home, I have #6 and #8 bolts in imperial, the #6 is too small and the #8 is too big for the hole so I think these skywatcher screw must be in metric this means M3.5 or M4.

To put even more fun into that, the first caliper is the one from the job, an expensive one probably. My own 7$ caliper, full plastic, gives me another reading, 3.3mm instead of 3.8mm

Do you agree my best bet would be to order M3.5 and M4 thumbscrews or did I forget something in the process?

Thanks.

 

 

 

[LightBucket]

They will be standard metric M4 screws....

[Ben the Ignorant]

The tolerances in diameter don't matter, any M4 metal screw will enter any M4 hole tapped for metal screws, but metal M4's are 4mmx0.7mm when plastic screws are 4mmx0.8mm because the more fragile material needs to have thicker threads. A plastic screw will turn a little inside a metal thread and then bind. When I tried to replace the broken nylon 4mm screws in my GSO finder, I found I had to enlarge and tap the holes in the 6mm format, which is identical for metal and nylon screws alike, fortunately.

To make eyepiece holder screws less aggressive, I put a nut and a counternut just under the screw tip, so only a fraction of a millimeter is visible. After that I rub the tip against fine sandpaper to make it smooth and square to the screw's axis. A final polish on a rough kitchen sponge makes it easier on eyepiece barrels.

[N3ptune]

Ok

Well I hope I am not going to have to enlarge the holes..  It's a good idea to sand the tips, i just looked at my 1.25" holder it's not even the same bolts, these ate 2.9mm diameter. Ill try to send the bolts, and I thought of putting an epoxy coat at the end of the tip to make it softer.

Although, I don't know if the epoxy can sustain the pressure of the bolt against the barrel without chipping away.

[Ben the Ignorant]

2.9mm means 3mm because screws and tapped holes need to have a little play to avoid binding. These small 3mm screws are usually brass while the 4mm ones are aluminum in modern scopes. Sometimes steel is still used but it's hard, and dents barrels with a little too much pressure. I don't know about epoxy but solder sticks to steel, and it's soft enough.

Could be it adheres better to naked steel, or nickel-plated steel, or zinc-plated steel? I didn't experiment with that but it would be good to know because steel screws are sold everywhere contrary to brass and aluminum ones. I've been annoyed many times by old eyepiece holders devoid of compression rings, finding a way to secure oculars without scratching them would be a nice relief.

A caveman solution is to wrap a layer of tape around the eyepiece barrel if tolerances allow, and replace it when it's damaged. This would even help a bit with centering and tilt.

 

 

[N3ptune]

Heheh a layer of tape, funny, I already did just that before your last message Ben solder is a good idea also. (didn't though of that) I tried the epoxy solution previously to see if it can hold on chrome. If it's not working ill try with solder.

I have doubt it's going to resist pressure without chipping at -10 outside.

[N3ptune]

I want to conclude this thread today, It's a success at the end of the day and a really good solution I learned from @Dave In Vermont,

GOAL: To replace the existing metallic screws in my 2 Skywatcher extension tubes in order to prevent barrel aesthetic damages induced by the metallic screws.

This should help retain a better resell value for the eyepieces too (I suppose) and it's a great system for tapered barrel eyepieces like explore scientific (Or other eyepieces not working perfectly with compression ring holder devices.)

So I ordered a few size of Nylon screws from scopestuff.com and this is the result. They are strong and it's doing a fine job, eyepieces are stable the same thing has with the metallic screws. I had a very good service at scopestuff.com too.

LINK METRIC SIZE

http://scopestuff.com/ss_nscr2.htm

LINK IMPERIAL SIZE

http://scopestuff.com/ss_nscr1.htm

 

 

 

[Luna-tic]

If those nylon screws work for you, well and good. However, I would worry that I could not torque them tightly enough to hold my EP's without shearing them where the head reduces to the threaded portion. Those are pretty small diameter; re-drilling and tapping the holes for an M6 would give more strength to the screw. By the time I'd try that, I'd as soon just upgrade to EP holders that have a collar inside that the screw turns against.

[N3ptune]

You are right to have worries... I decided to see how strong they are with a strength test (about 10 minutes ago), and they are surprisingly strong even the small ones, it's possible to apply quite a bit of torque (torque I don't even apply with the metal screws)

With the eyepiece they remain solid and nothing is moving, we will have to see in a few observation if the tip remains round or get flat.. But the eyepiece is stranded strongly inside the tube with not much torque.applied.

I did these tests usine my vice

--> I can't take the head off with the strength of my fingers only, and the head will pop out after at least 5 complete turns, which is far, from the torque used on the eyepiece.

Video 34, I can't cut the screw with my weak strength only It will survive abuse

https://drive.google.com/open?id=0B0OZ0oXfzDlFLXJXeTdma0k2WkU

Video 32, with a pair of pliers, 5 complete turns are required to cut the screw.

https://drive.google.com/open?id=0B0OZ0oXfzDlFUG9iRmdDb1JLaUE

Video 33, to be able to cut it with my fingers (with a struggle) I need to turn the head at least 1 full time with the pair of plier.

https://drive.google.com/open?id=0B0OZ0oXfzDlFcEY5b1N4UXFhTVU

I am surprised by their strength, I had to know.

(Warning: the sound of the video can be high)

 

 

 

 

[Luna-tic]

That's good to know the strength is so good for such a small diameter screw. Great idea to exchange them for metal screws than can scratch and/or dent the EP tube.

[N3ptune]

8 minutes ago, Luna-tic said:

That's good to know the strength is so good for such a small diameter screw. Great idea to exchange them for metal screws than can scratch and/or dent the EP tube.

Well it's certainly not stronger then the metallic ones, but I apply about the same amount of torque  and i feel is pushing hard on the eyepiece, it's locked there. I could get a holder with compression ring but I heard the taper barrel of the explore scientific eyepieces is too steep, the copper ring will bend most likely.

That's the reason why I decided to stick with the screw extension tubes.