Widefield Triple Imaging Rig

[Gina]

OAG arrived by Fedex at 0830   Offered it up to my WF triple rig and it seems to fit in nicely   I shall need to check the thread pitch of the M48 scope-side connection to turn the adapter to connect to an M42 lens.  The other side connects directly with the 460EX and there is room to fit a 1.25" filter holder inside the OAG.  A 3D printed part will probably suffice for the filter holder but if that proves inadequate I will turn it out of ali.

[Gina]

Checked the M48 thread with my thread gauges and it matched up at 0.75mm same as the T2 pitch.  As I recall the lathe feedscrew pitch is 1.5mm so a 2:1 reduction is required for the change gears.  The lens thread pitch is 1mm so a ratio of 3:2.  I have the threading tool for the outside but can't find my 60 degree internal threading tool   I need to clear up!

[Gina]

Been mucking about with the lathe   Trying to remove stuck change gears etc.  I've got the change gears but the metal replacement ones have the wrong size keyway   I filed the two 80t ones out to fit the lathe but now I need other sizes.  I would like to use a 40t and an 80t with anything else as an idler gear but I can't get the 20t metal gear off the input shaft   I'll probably try using the plastic gears and resort to the file if I get a breakage. 

ATM I have left the 20t gear on the input shaft and have a 40t plastic gear on the leadscrew shaft with a 60t plastic gear as idler.  Now I just have to find the right size spanner to change the intermediate gear axle plate to get the spacing right.  I'm none too happy using a 40t plastic gear on the output/leadscrew shaft but it'll have to do if I can't get the 20t gear off the input shaft.

I think I shall have to get the 20t gear off the input shaft to get the 3:2 ratio for the M42 x 1mm threads.  That would use a 40t and a 60t for input and output.  The only possibility with the 20t gear is a 30t gear on the output shaft - that would put a lot of stress on the plastic gear teeth   Don't like it.

[Davey-T]

Have to buy a gear wheel puller now Gina, more expense

Dave

[Gina]

Some success   Managed to get the 20t gear off and found a spanner for the intermediate plate spacing adjusting nuts.  Now have 40t plastic gear on input shaft and 80t metal gear on the output shaft with a 50t plastic gear as idler.  Been doing a test thread cutting exercise and I think I have it working alright

Had to hold the reverse lever in gear by hand or it would pop out of gear and lose the correspondence between chuck and leadscrew which must not happen.  I also found that using the reverse drive also lost coordination as the gears disengaged and re-engaged.  So the only way to do it seems as follows :-

1. Set up change gears to provide the correct thread pitch
2. Set main gearbox to LOW gear
3. Move thread cutting tool into the right position to start cutting the thread.
4. Engage carriage with leadscrew
5. Engage forward leadscrew motion
6. After checking that the speed control is at zero, turn on power
7. Set motor direction to Forwards
8. Increase speed to start thread cutting - I found 100RPM was fine with a 2" diameter piece of aluminium
9. Reduce speed to zero at the end of the thread position
10. Back off the cross slide to disengage tool from job
11. Set motor direction switch to Backwards
12. Advance speed to several hundred revs to wind carriage back
13. Stop when just past thread starting position
14. Advance cross slide to previous position and the some more to cut (I used 2.5 minor divisions).
15. Repeat steps 7 to 14 until thread is deep enough
16. Swich off power

You're supposed to be able to use the threading counter but I don't trust it - I prefer to keep the chuck to leadscrew and carriage coordination by not disengaging anything in the drive chain.

[Gina]

Have to buy a gear wheel puller now Gina, more expense

Dave

Could do - or make one

There's this one on ebay but it might be too big.

Or a set of three - Toolzone 3Pc Puller Set 3", 4" and 6" on Amazon for a little bit more.

[Davey-T]

Yes first one is too big, the other one looks ideal and it's got reversible legs for more options.

Dave

[Gina]

Thanks dave   I've ordered the Amazon (2nd) one or rather set of three   Pullers are always useful

[Davie]

You're supposed to be able to use the threading counter but I don't trust it - I prefer to keep the chuck to leadscrew and carriage coordination by not disengaging anything in the drive chain.

Id give it a go with the thread counter just remember to use the same number all the time or youll end up with a multi start thread! its way easier than doing what your doing and if your only practicing youll get your confidence whilst threading bits of scrap . What I usually do is set up a clock indicator on the cross slide and this eliminates any backlash in the lathe, ok it doesnt eliminate it it's just so your not relying on the lathes dials for depth of cut.

[Gina]

Thanks Davie

I have just finished turning one of the adapters but in spite of careful measuring, it doesn't quite fit   Also, I think the inaccuries in this lathe are causing slight variations in the tool position between runs.  I think I'll do some more practice runs.

One problem with these adapters is that it isn't possible to test both ends until the piece is parted off from the stock.  However, the adapter that connects one of the lenses with the OAG can be turned so that both threads are available for testing.

Here are screenshots from SketchUp showing the adapter and a section showing how I propose to turn this from some tupular stock.  The adapter has an inside thread of M42 x 1mm for the lens and an outside thread of M48 x 0.75mm to screw into the OAG.  I have calculated that a thickness of 6mm between reference surfaces will give the right spacing to achieve focus on the image sensor.  By extending the lens thread right through to the OAG end, it will be possible to try screwing a lens in while the part is still attached to the skock in the chuck, and check the thread size.

[Gina]

Been turning the lens to OAG adapter.  Almost ready for thread cutting

[Davie]

Thanks Davie

I have just finished turning one of the adapters but in spite of careful measuring, it doesn't quite fit Also, I think the inaccuries in this lathe are causing slight variations in the tool position between runs. I think I'll do some more practice runs.

Screw cutting isn't an exact science you really need to have something to try them with, sometimes you just need to move the tool forward a few thou to take a wee cut off the back of the threads rather than going deeper.

You can set them up in the lathe again after they have been removed and take another cut if required. Just bung the adapter back in the chuck and true it up, then put the tool close to the adapter and start the lathe up. Start the feed and once your tool is over the thread stop the lathe but don't knock off the feed. You can then adjust the tool using the hand feeds on the tool post and cross slide to line up the tool to the thread.

I hope that's useful if you didn't know how to line the tool up again.

[Gina]

Thanks again Davie - useful info

[Gina]

I've made a discovery that throws my measurements out.  When the 460EX camera is screwed onto the OAG it doesn't go right down to the reference surface - the thread on the OAG is longer that the depth of thread in the camera.  The difference is about 1.5mm so I will need to subtract that from the thickness of the adapter.

[Gina]

Think I've got thread cutting sussed out now   Using the thread dial works fine and is easier as Davie said

The puller set has just arrived so that should help a lot when changing gears.  All the adapters have both 1mm and 0.75mm pitch threads.

I now have the threads cut on the lens to OAG adapter and both a lens and the OAG screw on nicely   Here's a photo.  Next step is to part off the adapter from the stock and clean up.  I'll probably wait until I have the other adapters turned before anodising and do them all in one session.

[Davie]

Well done Gina, looks good

[Gina]

Thanks Davie

[Davey-T]

Well done Gina, give yourself a pat on the back

Dave

[Gina]

Thank you Dave   I'm feeling pleased I have to say   This is my first successful threaded adapter

Here are photos of the adapter and with it fitted between a Super Takumar 135mm f2.5 lens and OAG.

[Stu]

Great stuff Gina, well done!!

Sent from my iPhone using Tapatalk

[Gina]

Thank you

[Gina]

Been out and tested the lens focussing - fine   Although, I found I miscalculated the back focus - fortunately the right way - the lens needs the focus sleeve set at something like 10 metres rather than infinity.  Since these lenses focus by moving the whole optical train in and out, this is no problem.

[Horwig]

Nice one Gina. Just one question, what's the light cone like from the lens for illuminating the oag? I have been forced to resort to a separate guider for camera lenses shorter than 300 mm, granted, my camera is an sx35, but even then my lenses are medium format lenses.

Not that I want to put a damper on your excellent work.

Huw

[Gina]

Thank you Huw   I have checked that the OAG is useable by looking into the lens from the front.  There is just room for the OAG prism between the long sensor side and the light cone.  ie. I can see the light coming in from where the guide camera would go.  Though so far I have only checked the 200mm, 135mm and 105mm lenses - not the 55mm ones.  But the 55mm lenses have a much larger rear lens.

[DIYASTRO]

Hi Gina,

Nice ! You will be feeling pleased with your efforts....and so you should!

Boyd