7 replies to this topic

[rmorein]

The slow motion cables are attached by bolts which are not captive, rendering them vulnerable to loss. So I decided to purchase extras, knurled M4 stainless and little knurled nuts that complete the tightening process. The original bolts thread directly into the worm shaft and declination shaft without any problem.

 

The brand new replacements do not. They go about 6 turns and bind, more than one would expect for a "wrong bolt" error. They're usable in a pinch.

 

Both old and new bolts are very close to  3.89mm major diameter.  The pitch, not the diameter, seems to be the issue. What is going on? Is this the cottage industry in action?

[LuscombeFlyer]

M4 bolts come in .5 and .7 pitches. Sounds like your replacements are the wrong pitch.

[rmorein]

A rare choice. No stock of thumbscrews on eBay or Amazon.

[YourNotSirius]

The first question that comes to mind is how old is the mount?

If it is from the 1960s then the thread pitch may be the old JIS standard.

If 1970s plus then the are a different pitch.

 

Find someone with a thread pitch gauge to read the threads then go to McMaster-Carr and look under metric thumbscrews. They have them. However, they do NOT have the old JIS threads. Those are more difficult to find. They are out there. I have seen them. I just do not remember exactly were. lol

 

A last resort would be to get a modern tap and cut new threads for the newer thread pitch.

 

FWIW

 

Q

[apfever]

Modern standard 4mm bolts have 0.7mm per thread.  The old JIS 4mm bolts have 0.75mm per thread.  That extra 0.05mm per thread will jam up quick. 

I'll check my Mizar floppies this afternoon. 

[rmorein]

The first question that comes to mind is how old is the mount?

If it is from the 1960s then the thread pitch may be the old JIS standard.

If 1970s plus then the are a different pitch.

Made 1972-1980

[apfever]

My Mizar SP mount does not use floppies. The slo-mo are knobs held on with around 1/4" set screws. 

[deSitter]

The slow motion cables are attached by bolts which are not captive, rendering them vulnerable to loss. So I decided to purchase extras, knurled M4 stainless and little knurled nuts that complete the tightening process. The original bolts thread directly into the worm shaft and declination shaft without any problem.

 

The brand new replacements do not. They go about 6 turns and bind, more than one would expect for a "wrong bolt" error. They're usable in a pinch.

 

Both old and new bolts are very close to  3.89mm major diameter.  The pitch, not the diameter, seems to be the issue. What is going on? Is this the cottage industry in action?

The obsolete JIS thread found on many classics from Japan is 4mm x 0.75. That has been completely replaced by M4 which is 4mm x 0.70. Finding JIS screws gets harder every year. You are better off tapping your own from screw blanks.

 

-drl