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Any tips for reassembling focus helicoids?
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PostPosted: Wed Oct 06, 2021 12:48 am    Post subject: Any tips for reassembling focus helicoids? Reply with quote

My Sun 80-240mm f/4's focus assembly needed new grease. I was able to separate the focus helicoid and clean the two halves with no trouble. But now I simply cannot get the two to mate again.

I marked the point at which the upper and lower pieces separated so that's not an issue. And I've disassembled many helicoids before and have almost always been able to successfully get them back together.

However, most of the lenses I've worked on have been primes with fine threaded helicoids. The Sun is different because it has wider threads, and that may be why I'm having difficulty.
As far as I can see, the threads are in good condition and the barrels aren't bent.

When reassembling, I generally rotate one half in the "wrong" direction in order to find the starting point where the two halves should join. In the past, gentle persistence has usually been enough to rejoin a helicoid. Occasionally, I've resorted to applying light pressure to help get the pieces started back together.

For the Sun, I've heated the female half in an attempt to make the opening a bit larger. And I've tried to mate the halves first without grease on the threads, and then with. No success.

Determining if the halves are aligned properly while attempting to rejoin them is difficult. Member visualopsins suggested: "Tip -- place half on flat surface rather than trying to screw together holding both pieces." in this thread:
But that hasn't worked for me, either. The two halves just won't bite / engage.

Any suggestions?

my lens:

PostPosted: Fri Jan 21, 2022 2:46 am    Post subject: Reply with quote

place one half on sturdy flat surface

approach with other half at a tilt and hook other half onto lower half threads, then tilt into position, screw helicals together.

(or something like that Smile)

An anecdote: I had a similar problem solved after a good night's sleep on retry -- I had been turning the helicals in the wrong direction.

PostPosted: Fri Jan 21, 2022 2:29 pm    Post subject: Reply with quote

Multi-thread helicoids can be a bit a bit difficult sometimes. Rolling Eyes

I have found indeed that placing one half down and lowering the other works much better, trying to mate them whilst holding both in your hands will be nigh impossible. What may help alignment is first make sure the table is level, place the bottom helicoid half on the table, and lay a bubble level on top of the top helicoid half (e.g. on a stiff flat plate/card across the opening).

Pressure/force should really be avoided, it won't do any good; you will simply damage the thread ends. Instead, this is all about meticulous and careful alignment.

In the end, for problematic ones I have found simply patience to be the only solution.

I often leave the very ends of the threads un-lubricated, and with good alignment and very sensitive rotation in the wrong direction, you can often feel a faint "click" when all the threads engage; then immediately rotate the threads together. Note this "click" is much more subtle and faint as compared to a single start thread!!

Be careful with that helicoid you have there; that's an alu-alu combo which you really shouldn't try to screw together too far without any lubrication. Aluminium on aluminium galls easily when dry, which will damage the threads and you might not even be able to get them apart again.

Again, patience, and Good Luck! Smile

PostPosted: Fri Jan 21, 2022 9:17 pm    Post subject: Reply with quote

I've disassembled a number of lenses and inevitably I'm confronted with this problem. There's no easy way I've found. Persistence definitely helps. One thing I do that I think helps is I'll turn the top piece ion the opposite direction from the threads (ie, if right thread, then turn the piece to the left), and while doing this, I listen and feel for a slight click that lets me know the threads have dropped into place. Then I'll turn the top piece in the correct direction and, with a bit of luck, I'll have engagement. And then I hope I've engaged the right threads, or else I get to do it all over again. I usually check to see if the right threads have engaged by checking focusing and focusing marks at infinity.

PostPosted: Tue Apr 12, 2022 5:01 pm    Post subject: Belated Bump! Reply with quote

visualopsins wrote:
. . .

RokkorDoctor wrote:
. . .

cooltouch wrote:
. . .

Like 1 Thank you!

Your advice is appreciated and I apologize for my delayed response! Somehow I lost track of this topic.

Ah, these stubborn lenses really are a test of patience! But I'm glad to know I'm not the only one who has endured such bliss. Wink
If I ever succeed, I'll let you know.