A new design, the R&P Crayford.
Posted 01 January 2014 - 11:59 AM
Good quality tuning mechanisms in old radio sets used a similar method where reduction gears were employed. Which makes me think... One could also use a tensioned cord to drive the focus mechanism.
I like the leadscrew & half-nut idea too, and was about to suggest it, but was pre-empted by Tim There are spring loading mechanisms for "full" lead screw nuts that eliminate backlash. These are certainly within the capability of an ingenious home machinist.
Alternatively, one could conceive of a 2 speed arrangement where the rack and pinion is really a worm drive with a long worm. For course adjustment the wheel can be rotated, then locked. Fine adjustment would be made by rotation of the long worm. Just a thought - and it should be highly resistant to slip, and backlash adjustable like mount drives. Again, just mental ramblings
Posted 01 January 2014 - 01:49 PM
Posted 01 January 2014 - 02:51 PM
I wish I could remember the thread from a couple years ago, where some CN member showed a drawing of exactly what you describe, Ed. Only using threaded rod to serve as the rack/worm.
Could it have been similar to this: http://clementfocuse...Drive_Print.pdf
Posted 01 January 2014 - 07:01 PM
I like the idea of a lead screw driven focuser. How 'bout a sled focuser with a lead screw, including half-nuts like on a lathe's cross slide? You'd have to put the screw on one side of the slide, of course, so you could get the eyepiece in there.
As an alternative to the half-nut idea (good idea BTW): In my junk box today and came across an old Velmex Unislide with rapid advance feature. http://www.velmex.co...id_advance.html As shown below on my junk box Unislide the rapid advance feature consists of a internal threaded rod that fits onto the leadscrew. A clamp on the outside of the rod allow for rapid changing position and also increases the travel from 1" to 4". This allows for one to keep the minimal backlash feature and have rapid positioning also. In fact in the new version of the Unislide with rapid advance feature a micrometer head is used. http://www.velmex.co...head_stage.html
Posted 01 January 2014 - 08:34 PM
Nice idea about half nuts,you can disengage and push the slide up or down for rapid transit, re engage and fine up the focus.The arrangement can work without poking your eye out on the focusing knob. Dovetail slides are great of course but you have to be an expert machinist to make one to slide evenly, also the pro slides have adjustment slips built in, and the whole shebang is smeared with goop to make it move smoothly. The R&P CF moves unusually freely for an R&P focuser, there is no goop just mechnical freedom as everything runs on ball races.
An alternative to the more difficult to machine dovetail is a box way slide. I found an old Bausch & Lomb microscope focusing mechanism today in my junk box that uses box ways and a helical rack and pinion as shown in the photo below. The slide is made from what looks stainless steel with a rectangular cross-section. The gibs are made from a self-lubricating polymer plastic so no goop is needed for lubrication. Backlash in the box ways is taken up by set screw (grub screw). The helical pinion is held against the rack by what appears to be brass spring strips tensioned by a separate set of set screws. Box ways may not be as elegant as dovetail ways but are possibly easier to fabricate for the amateur with limited machine tools.
Posted 02 January 2014 - 01:59 AM
Posted 02 January 2014 - 04:30 AM
Although I will comment that the standard of workmanship evident in your projects certainly demonstrates considerable expertise.
Posted 02 January 2014 - 11:12 AM
I believe the average amateur can make a leadscrew much easier than cutting a rack & pinion which requires the use much more sophisticated machine tools. IMO it's good to explore different ideas even if sophisticated or different from one's own. I have been looking into all aspects of focuser positioning for some time now and am open to ideas not limited to just my own. As you said there are many ways to skin a cat.
Why post to this list if one does not want feedback?
Insanity: doing the same thing over and over again and expecting different results. ~Albert Einstein
Posted 02 January 2014 - 11:28 AM
Posted 02 January 2014 - 11:59 AM
1. Focusing by friction between the focusing shaft (or a friction material on the shaft) and the barrel (but no R&P).
2. Bearings that the barrel slid along.
From the Wikki Crayford page:
"The Crayford is similar in appearance to a Rack and pinion focuser, but has no teeth on either the rack or the pinion. Instead, a round axle is pressed (for example by a spring-loaded or thumbscrew-tightened piece of PTFE plastic) against a flat on the side of the focuser drawtube, relying only on friction to move the drawtube as the axle is turned. This also presses the drawtube against a set of four ball bearings against which it moves smoothly with minimal friction."
I'm not seeing how this could to considered a Crayford.
Posted 02 January 2014 - 12:29 PM
Posted 02 January 2014 - 06:46 PM
Is it your intention to make an R&P with the tensioning of a Crayford???????? If that is the case, Corect me if I am wrong but I believe it is considered a design flaw to preload an R&P this way. Tooth mesh is not supposed to bottom out and by squeezing it you make it wear incorrectly and it will eventually seize.
He said he preloaded the tube separately from the rack...
Posted 02 January 2014 - 09:22 PM
Posted 02 January 2014 - 09:38 PM
If the intent is to prevent slippage with heavy loads couldn't you just sand blast the shaft and tube areas that are in contact to increase friction or possibly knurl both in one direction to act like fine helical gears?
The helical R&P is an involute gear form. http://en.wikipedia....i/Involute_gear Can't expect just random sand blasting to replicate the smooth rolling motion of an involute form.
Posted 02 January 2014 - 10:13 PM
Are these tubes all the same material? Would I need to worry about galling if I did something similar?
Posted 03 January 2014 - 12:54 AM
I built a 2” Crayford hybrid for an 80mm finder scope that uses a leadscrew for focus positioning . The leadscrew is actually the drawtube that is threaded on the OD. On the outside of the threaded drawtube I machined three equally spaced axial flats the full length of drawtube in which five bearings support the drawtube in the same way as a Crayford. A captured nut threads onto the drawtube and when rotated positions the drawtube for focus. Shown below is the 2" Crayford hybrid. Note the Capitan's wheel focusing knob. This hybrid design evolved from a non-rotating helical focuser I built and shares many of the same parts. The main difference between the non-rotating helical is that the drawtube is supported by five bearings riding in the three axial machined flats and not by the threaded drawtube. The five bearings also prevent the drawtube from rotating when the focusing.
Posted 03 January 2014 - 03:33 AM
Posted 03 January 2014 - 03:51 AM
Posted 03 January 2014 - 08:43 AM
Posted 03 January 2014 - 09:09 AM
Posted 03 January 2014 - 10:30 AM
Posted 03 January 2014 - 03:38 PM
PS I am also a mechanical designer by trade........
Posted 04 January 2014 - 03:39 AM
Posted 04 January 2014 - 03:41 AM