Tuning up a Synta Crayford focuser

Tuning up a Synta Crayford focuser

Most of the earlier standard 2” Crayford focusers as fitted to Skywatcher/Orion and other Synta made scopes have a particular machining fault. Here’s an easy fix. The example pictured is from a refractor, but the same applies to reflectors. If you have one of these its worth taking a close look at it.

If you look at Pic 1 note the area indicated by the screwdriver and you will see that there are two witness marks, one either side of the flat on the draw tube, that indicate where the focuser shaft is bearing on the draw tube

Pic 1 :

clip_image001

This is not meant to be. The shaft should bear equally over the whole flat section. While it only rides on a small area you need to wind up the tension on the shaft quite firmly to avoid slippage, and so you lose a lot of "feel" at the knob.

So here's a simple fix. (you don't need to remove the whole focuser from the scope tube if you don't want to, I just find it convenient to do so, and it reduces the risk of any damage to the scope.

Look at Pic 2 and you will see an Allen Key in the central grub screw. This is the tension adjuster (not the chromed knurled screw adjacent, which is in fact the draw tube lock). Back this off two or three full turns, but don't remove it.

Pic 2:

Now remove the four screws as indicated by the screwdriver in Pic 3

Pic 3:

And Pic 4 shows the thing disassembled, and the draw tube can now easily be removed.

Pic 4:

So now we need to make that draw tube section flat right across its face.

It will help if you have a vice, otherwise get SWMBO to hold the thing firmly for you! When you put the draw tube in the vice, first place some soft aluminium or wood and then a soft cloth between the vice jaws and the actual tube and tighten it just enough to hold the job. Over tightening will damage your draw tube beyond repair.

Also you need a very fine oil stone or whetstone (the type you use to sharpen knives) In Pic 5 I am using an extra fine Aluminium Oxide stone, but Carborundum is also fine, as long as the stone is flat, and I mean flat. Apply a couple of drops of thin machine oil (here I am using sewing machine oil) and work evenly across the draw tube and along it holding the stone flat. But stay in a plane at 90 degrees to the axis of the tube, so that the fine marks from the stone are at right angles to the tube's axis.

Pic 5

You'll feel as the tube becomes flatter as the stone will start to slip on the oil, rather than bite the tube itself.

Pic 6 shows the job about half finished

Pic 6

And Pic 7 is the finished product, after about 20 mins of rubbing. Note that the flat section is now flat and shows an even patina right across its face.

Pic 7

Now you need to thoroughly clean everything so it is absolutely free of any grit or oil. I used some methylated spirit on clean rag. don't forget to clean the focuser shaft, as it will probably have some grunge on it.

Now put it all back together, but don't lubricate anything. This is important as the Crayford design works on friction, so you don't ever oil or grease it. The cleaner and drier the better.

Finally with the whole thing assembled and fitted to the scope, add a diagonal and EP and gradually adjust the shaft tension until the focuser moves smoothly without slippage. (Don't forget that the tension screw is the one that uses an Allen Key, it should now be obvious as to why I told you to back this off before disassembly) If you did the job right, you'll find that you will need a lot less tension on the shaft than previously and the whole thing will feel smoother to the touch.

Disclaimer: I've taken a lot of care with these instructions, and I have successfully completed this job many times, but I won't be responsible if you stuff it up.