Date
1 - 20 of 27
What is the diameter and thread on the Pasil.4 polar scope?
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Tom Blahovici
Hi
I'd like to make a plug for the place the polar scope goes into the 600e mount. I figure I might as well get some use on my new mini lathe. I'm thinking of delrin or brass. Anyways, Once permenantly polar aligned, I should remove the polar scope since it potentially gets in the way or wires. I'd like to if the opening Thanks, Tom |
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Doing a drift align would be a lot less work. (Grin) On Wed, Jun 17, 2020, 2:52 PM Tom Blahovici <tom.va2fsq@...> wrote: Hi |
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R Botero
Tom I have a Pasil on an 900GTO and will send you my measurements later (hoping they match the 600).
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R Botero
Tom Roberto |
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Tom Blahovici
Hi
Perfect. Thanks very much! This will be my first time threading.... Best regards, Tom |
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Len Fulham
Tom,
"I might as well get some use on my new mini lathe. I'm thinking of delrin or brass." For your adapter use brass or aluminium, Threads in delrin are harder to do than expected unless everything is set perfectly. Are you experienced with lathe use & threading? if so read no more. Otherwise there is a simple way to remove and return your piece (e.g. to test fit) without losing thread registration. This assumes there is a flat face or ring which registers against the face of the chuck or (less good) chuck jaws. Having progressed your thread to the point you wish to test it, back off/withdraw the cutting tool but leave everything engaged; then use a felt pen and make a line on the side of the workpiece onto a chuck jaw or face of the chuck. Now you can take it out of the chuck, play with it, test, then return it to the chuck in the same position and make further cuts as required. Accuracy is improved by tightening the chuck using each key position sequentially, and the same when you rechuck it. Don't rub the pen mark off! You should be able to place the piece back in the chuck to the same depth and square (flat surface register) and the felt pen mark will let you have the thread back in alignment with minimal change. Make some test passes with the cutting tool backed out and progressively feed in until cutting is resumed. It is easier to do this than just hope your part will fit ( if your metrology is first class, then no worries). Not a machining forum so we shouldn't expand this too much. Cheers, Len. |
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Joe Zeglinski
Hi Len,
Interesting short cut with testing the part, but I would
be concerned about “repeatability”.
Tom would have to not only mount the part, back into the lathe chuck
in two directions, he would also have to position the lathe cutter blade
precisely in the original thread groove, and resume the lathe travel worm.
Somehow, I feel that there are too many variables,
with a greater chance of the final threads ending up, cut too thin and thus
loose fitting, if all isn’t repositioned precisely, after a part
test-fit. It may be OK for a smooth bar operation, but threading resumption
might be less forgiving, no matter how careful one is.
Joe Z.
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Len Fulham
Joe & Tom,
I understand the concern, but provided there is a positive length location (registering to surface of Chuck) AND the cutting gear train is kept engaged (so nothing can move independently( = picking up the cut) AND an alignment mark is used to control rotational position, then very accurate and repeatable threading operations can proceed. In most circumstances the alignment mark does not need high precision - an error of 0.5mm in rotation on a 40mm dia 1mm pitch thread corresponds to a error at the cutting tool of about 0.004mm (0.0002inch). With quality equipment, e.g. my precision Pratt Burned chuck, which probably cost more than the whole mini lathe, this is achievable. But even cheap chucks which may have noticeable runout are usually quite repeatable. Even if the error is 5 times worse it still corresponds to only 0.02mm (0.001 inch). Just needs a bit of care to be clean and attentive to detail. Otherwise, what should a beginner do - at some point it has to come out of the lathe and be mated to the mount - if it does not fit or is too 'snug' - throw it away and start again? or put it back in and take another little cut? Tom, ideally you should learn to measure and specify threads properly, micrometer and three wires etc, but I have had commercial CNC cut products (thinking Celestron) which do not fit. Maybe before your first real part, you should make a 1/2" thread to fit a 1/2" nut as an exercise; get to know your machine. I have done what I described above and it works. Good luck, Len. PS Its never luck, care and attention to detail wins. |
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Tom Blahovici
Sounds like I my have to rethink this...I certainly didn't think about having to try it and reposition it in the lathe.
I guess I will need some practice. Thanks |
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Speaking as a hybrid hobbyist/professional machinist, I would add that threading can be one of the trickier machining efforts to get right. Especially when you are going to be making two different threads that you are going to want to have very accurate in reference to each other and with the shoulder that the polar scope is going to seat against. And threading up tight to a shoulder is tricky and usually requires a modified threading tool. Testing the fit of the threads on the mount and the polar scope will also be tricky because you will most likely have to remove the part from the chuck and then replace it and re-register it with the threading tool. If you have a compound slide combined with a big illuminated magnifying glass, that can help to get the threading tool re-registered with the partially-cut threads when re-mounting the part in the chuck. However getting the piece precisely-square in the chuck again (.001") isn't trivial at all. I assume you have a dial-micrometer or something with a magnetic base. On my (German) Prazi lathe which does not have split carriage nuts, I made a manual drive handle that allows me to manually turn threads at a slow rate, which gives me a very precise feel and ability to stop the tool at precise positions. And no risk of over-driving the cutter and ruining the part or breaking the threading tool. YMMV. If you get the two threads and the outside shoulder even slightly out of square with each other (more than .001" and .1 degree), you will create problems and inaccuracies for yourself when mounting the polar scope on the mount. You will be able to adjust the PASILL polar scope to compensate for pointing errors in one position but when you rotate the polar scope, your alignment will go out. And the tiny PASILL alignment set screws are in direct contact with the glass reticle. Over-tighten them even a tiny amount and you crack the glass. You don't want any play but even the slightest over-tightening will likely ruin the reticle. This sounds like a challenging but potentially-fun project however you are still going to want to perform a careful drift alignment with the mount so you can accurately test, align and evaluate your machined adapter. And if your goal is to only use this thing once (or once a season) for a permanent polar alignment of a mount on a pier, I really don't see where you will be gaining much from this project, except honing your budding machining skills. And no matter which way you go, you are still going to have to perform a careful drift alignment if you want to do some serious astrophotography. Even if you invested in an official AP polar scope bushing adapter. Depending on your mount threads, I might have 3 or more of these adapters in one of my spare-parts boxes and I could simply mail you one. I hope this helps. On Fri, Jun 19, 2020 at 10:31 AM Tom Blahovici <tom.va2fsq@...> wrote: Sounds like I my have to rethink this...I certainly didn't think about having to try it and reposition it in the lathe. |
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Tom Blahovici
Hi
No, this is much simpler than this. I want to use my polar scope this summer since I will be using the scope as a portable. Thus having the polar scope is simple. The scope is however permenantly mounted on a pier on a 6 foot deep concrete block. I do this from the fall until summer. When I do the polar alignment there I do not use the polar scope. I drift align with Pempro. The scope is removed. I just want to machine a plug to go where the scope was. With the polar scope there, there's too much of a risk for it sticking out and catching a wire . All I need is a plug. Heck, a cork painted black would do...lol. I just wanted to try my luck machining the plug. Tom |
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Ah. Got it! That will be a LOT easier! I think I have some of those in my parts bins too. On Fri, Jun 19, 2020 at 3:52 PM Tom Blahovici <tom.va2fsq@...> wrote: Hi |
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Tom Blahovici
Hi
Well it turns out that the diameter of the threads is very different than the Passil4 on the 900. Which is strange...The diameter of the threaded part is 1.016 inches and the threading is not 24tpi. My thread gauge goes from 32 then to 40 tpi and it is somewhere in the middle of that. So I assume it is metric. This is the losmandy polar scope which I believe comes from China and then adapted by AP. So I guess I will need a metric thread gauge. Tom |
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The PASILL polar scopes are made by Kenko in Japan. Yes, metric threads. On Sat, Jun 20, 2020 at 3:43 PM Tom Blahovici <tom.va2fsq@...> wrote: Hi |
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However the various adapters that AP makes adapt the Kenko polar scope to the English threaded holes on various AP mounts. They make more than one bushing adapter. I don't know which one you have. On Sat, Jun 20, 2020 at 4:02 PM Christopher Erickson <christopher.k.erickson@...> wrote:
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Jumping in here.....I also have a larger metal lathe, milling machine, etc.in my home shop. Your mini lathe is setup for ASA screw sizes and it does this with a lead screw that allows the appropriate settings for those type of screws which are spec'd by TPI.. Cutting Metric threads can't simply be done on a mini lathe without some fiddling around and cheating. Either changing out gear ratios and/or changing to a metric lead screw is required which I doubt your mini lathe has that option. Repeating to get deeper cuts using the threading dial also won't work for metric as it isn't calibrated or sync'd for those type of metric screw pitches. There are ways to cheat to get it close but takes a lot of experience to get it right. Cheating also won't give the precision you need to make this project work.
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My 1963 Colchester Student lathe cuts both metric and imperial threads but most lathes of that age did not come equipped with such gearboxes. Modern lathes should cut mostly metric threads unless specifically ordered as imperial (which I assume will be the case for US customers). As Robert says, there are tricks to cut metric on imperial and viceversa. It’s all part of the learning curve if you are learning to machine on the lathe. You can use additional reduction gears - changewheels -on the back of the machine. For an old (1954) Boxford I had (identical to a Southbend), a single conversion gear on the back would allow me to turn 90% of metric threads with an 8tpi lead screw.
See lathes.co.uk - the bible of all things to do with machining - for historic references and specifics about accessories. I feel you don’t need the adapter for your Pasil right away, try to make one! Turning on a lathe or milling is an addictive a hobby as visual/imaging amateur astronomy. If you are in desperate need of one, you can always click a button and buy it... Roberto |
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I agree that practicing with your lathe will open a world of things you will be able to accomplish. I decided to get my lathe when I was quoted a very high cost to make a lens cell for a Zeiss flat field lens for my 6" APO refractor. I probably put as much into the lathe as what it would have cost me for the custom part but than had a tool for future projects. That was about 15 years ago and I figure I have saved many times over custom building one off projects for astrophotography and race cars since than....for myself and friends. Luckily I already knew how to use the lathe and milling machines from my original high school and college classes where I was going to be an industrial arts teacher. That never happened as I became an electrical engineer....but those were valuable skills I learned. I consider that lathe to be my most valuable tool....it gets used a lot! |
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Tom Blahovici
Hmm, since the mount comes from 2004 or so. I highly doubt that it is metric. So I have ordered a new gauge which has both and more measurement options Tom |
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I believe the AP side is English and the polar scope itself is metric and the AP bushing adapter is metric on the polar scope side and English on the mount side. So all you will need to do is English threads to make a threaded plug for the mount. For a plug I would use Delrin plastic (easy to machine). For a plug, the threads don't need to be precise or tight. They can fit a bit loosely and still work just fine. On Sun, Jun 21, 2020, 9:23 AM Tom Blahovici <tom.va2fsq@...> wrote:
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