This blog has lately become my ATM build diary. I have built various telescopes, a large, Goto equatorial mounting and am now working on a dome on a raised, octagonal platform. Images are now kept small to cater for those still on slow connections. Click for enlargements. NOTE: If you are faced with a long series of posts and slow downloading just click on a single post title. The title is a direct link and will open just one post. Now you can click Newer Posts or Older posts to taste.
As an aid to truing the run-out of my 11.5" wormwheel I added a low friction disk between the wheel and cylinder. PTFE [US Teflon] is a low friction material which is readily available on eBay[UK].
I order a piece 20cm x 30cm in 3mm PTFE [8" x 12" x 1/8"] on Friday afternoon and received it in the post on Monday. Even ignoring the weekend that is a delivery speed record!
Marking a thin circle on the the snow white sheet proved slightly difficult. I tried felt pens, pencils and Biros in the compasses without much success. In the end I used a large pair of screw adjusting dividers to scribe the circle. This was then filled with a magic marker pen and rubbed away to leave a thin, sharp line. [Engraving style.]
The image shows the disk with the cylinder lifted clear for the photo.
I then sawed out the circle using a hand fret saw. Cleaning up the sawn edge was going to be time consuming. So I hung the disk on the expanded jaws of the lathe. Being slippery and floppy the PTFE would not stay in place. PTFE is not remotely as rigid as materials like Perspex, for example. I added a 2" V-belt pulley with a sunken face held by a live center in the tailstock. A piece of sponge trapped between the pulley and the PTFE kept it safely in place without slippage.
Turning it required a very small, sharp pointed tool. Even this flexed the material and left a radius. So I backed up some coarse emery paper with a batten. Then used that to quickly obtain a neat edge using a high lathe speed to keep the plastic straight by centrifugal force.
The brass sleeve I used to adapt the 60mm diameter bore to my 50mm shafts was originally [and deliberately] left too proud but was quickly turned down to be flush.
The large wormwheel is now beautifully stabilized and runs perfectly true thanks to being pressed against the 7" /180mm diameter cylinder. The PTFE adds very little friction and this will actually help to stabilize a long and heavy OTA. The brass sleeve rests on the top bearing to provide the upward thrust necessary to bring the wormwheel, PTFE disk and cylinder into intimate contact. In fact I had no need to use 3mm / 1/8" PTFE but decided I liked the stability it offered. The rest of the material will be handy for Dob bearings. My earlier 2mm thickness was too thin to allow CSK wood screws to hold it fast.