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The aid of an engineering brain required please


Horwig

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Hi Huw.

Attached are a couple of suggested sketches for a worm carrier backstop and side thrust stops, both are adjusted just to touch and limit movement of the carrier but not to fully prevent clearance movement in the required directions.

Hollow tipping the nose of the side thrust stops and dropping in a single ball just before tightening up would reduce vertical drag on the worm carrier and allow a tighter adjustment than would otherwise be possible with a simple friction mating.

Regarding jerkiness of the drive, this could still be improper alignment of the worm and gear, have you tried bluing the gear and checking for correct contact between worm and gear?

Clean and degrease the worm and gear and apply fresh engineers micrometer blue paste, reassemble the worm and gear, load up the mount and drive the dec through one or two rotations. Look at the wear displacement pattern in the blue paste remaining on the gear using a loupe or magnifying glass, the point of contact between worm and gear will push away the blue grease revealing the shiny aluminium below. If the point of contact is not central between worm and gear add/remove shims as necessary to bring the worm and gear into alignment, re-apply or re-spread the blue past around a little and repeat until the central axis of the worm is perfectly aligned to the central axis of the dec gear (when I was a young engineer, just starting out, we used to mix chimney soot from the factory foundry, pounded in a mortar and pestle and mixed with regular grease to make our own 'micrometer blue', the boss certainly didn't like to spend out on 'luxuries'....).

http://www.arceurotrade.co.uk/Catalogue/Protection-Grease-Adhesives/Engineers-Blue

Another possibility for the jerkiness is that the mesh pressure is too high leading to friction lifting of the carrier. A lower mesh pressure and carrier backstop to limit lift may be all that is required to smooth the drive.

William.

5af98d138769a_Backstop.thumb.jpg.586c57b38746dc1da15afc7901975316.jpg5af9906d38eec_Wormcarrierreinforcement.thumb.jpg.fbdd2bc757dbaf7666d9830c90c32bcd.jpg

 

 

 

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Thanks Dave and William, a long hard think is required. Your solution would certainly work Oddsocks, but I'm worried that the original design was flawed, It might be better to scrap and start again. I do particularly like the swung block you show Dave, wonder if sandwiching the pivot between two large diameter thrust bearings would help.

Huw

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40 minutes ago, Stub Mandrel said:

Looking at the graphs, as DEC approaches the line and the correcting movement stops, it appears to 'recoil' upwards slightly. This is seen as a reversal and is over-corrected swinging the mount past the line. Exactly the same seems to happen as it then approaches the line for the other direction.

I think the heavy but balanced OTA is wobbling like a slow pendulum, possibly with a period of several guide pulses.

I suggest increasing the exposure time to allow the OTS to settle, consider reducing the aggression and reducing the backlash compensation.

It may also help to unbalance the OT slightly to or use single-direction DEC guiding.

This should greatly reduce the tendency to have the big overshoots.

It may also be worth adding a tunable mass damper at 90 degrees to the OTA to defeat that resonance.

Yes it does appear to be a pendulum effect, the OTA was miss-balanced by about 1/2 Kilo at the top end. I did also play with aggressiveness, took it from 100% down to 20% with not much joy.

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18 minutes ago, Horwig said:

Thanks Gina, but loaded with nearly 34 Kg?

Should be.

Quote

50 Kg payload capacity.

Can't remember what was the most I had on it but I know it was more than the NEQ6 would carry.  I had three scopes on a big, heavy 10mm thick aluminium plate mounted on it.

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On 11/05/2018 at 17:41, Doc said:

Just out of curiosity has the worm and worm wheel been lapped together as if not then errors can occur.

I had a word with the bloke that makes them, yes they are lapped. If you want to lap them further, just use a light oil.

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On 14/05/2018 at 15:10, Horwig said:

Yes it does appear to be a pendulum effect, the OTA was miss-balanced by about 1/2 Kilo at the top end. I did also play with aggressiveness, took it from 100% down to 20% with not much joy.

Mis-balance may not be too bad a thing.

Try making a mass balance to break the resonance. Simply a weight on a stick that has a period near to but not in a simple ratio to the problem oscillations and attach it to the scope so it can waggle in the same plane. If you make the weight adjustable you should be able to tune out the wobble. A simpler but less elegant approach that has been used would be to drape a heavy blanket over the body of the scope - worth trying as at least it could reveal if the problem is one of resonance.

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Thanks Neil, love the idea of the blanket. At the moment everything is stripped out, I'm re working the DEC drive and have removed the OTA, will update the thread when everything is up and running again.

blue.thumb.jpg.6c7c9cfc9aa70984ea1b604dd9229283.jpg

as you can see I'm getting serious

 

Huw

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I had serious problems with Beacon Hill's shoddy wares after months of waiting and repeated failure to supply to spec.

I finally accepted oversized bores and sleeved them to 50mm myself in my lathe using best practice.

One of the worms was eccentric and "graunched" on every rotation. So lapped it with "Solvo-autosol"

Fine lapping would have been pointless given the crap milling of the 11" wormwheel teeth! The 8.5" was fine.

Both wheels needed cleaning of the teeth with a wire brush just to remove the black filth before putting into use!

Firstly I fitted two more radial plug clutch screws to each wormwheel boss to increase grip.

You can't have any slip and still do accurate Gotos or even guide.

The BH worm shaft extensions are very mean in length.

So I added triple clamping screws to the toothed wheel pulley bosses.

Supporting the very mean channel profile worm housings needed some serious cross sections.

So I mounted my worm housings with larger bolts onto square section ally tubing with the motors tucked away inside.

Then the worm housing bearing took to sliding out sideways under slews! The ball [NOT roller] bearings were only held in with varnish!

So I had to drill and fit bolts in the corners of the bearing housings with washers to retain the bearings in both directions.

Balance is a serious issue if the AWR stepper motors are not to stall.

My clutch additions require slacking off to allow complete freedom of movement.

I'm adding supplemental balance weights for OTA balance. More weights to come when the slide rails arrive from IKEA.

P1310904 rsz 500.JPG

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Just in case you think "Johnny Foreigner" is bad mouthing a favourite British vendor:

I'm English [in exile] and a 70 year-old, lifetime, telescope maker.

Here's my 11" Beacon Hill wormwheel which cost me several hundred quid and many months of waiting.

Plus hours of careful machining to make it fit and much of my remaining hair.

The finest available? You decide!

P1260409 rsz ringed RA wormwheel.JPG

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Very worrying if BH are turning out stuff like that now, I wonder what the problem is, looks a symptom of things that have happened to other " respected " names in the past, long delays and poor quality, could be down to health or other issues, I'd like to know before judging.

Dave

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Beacon Hill don't make their gear sets, they are contracted out. The one that @Rusted has shown has obviously been hobbed with a tap, a suitable method but tricky to get the diameter exactly right so that the teeth don't clash as they overlap. How do I know?, I've made hundreds! For large important gears I would precut the teeth positions with a flycutter and dividing head, the tap would then follow faithfully.The biggest potentional problem is the manufacture of the worm, unless this is produced in a one-off operation the concentricity is at risk which will manifest itself as periodic error in use. Some early units I recall had worms turned from stock threaded rod. Hopefully things have been upgraded since then.  ?

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Peter,

You've made hundreds? Why have I never seen them advertised? Have you stopped now?

I was told by BH, amongst many other lame excuses, that his wormwheel maker was in his 90s and didn't want to do it any more.

I know how they're supposed to be made. I thought I'd paid to have it done properly to the highest standard.

Had I known what would eventually turn up I would have made my own the correct way.

I think the 11" I had from BH was lying on a shelf as a reject for half a century.

Perhaps I was seen as an easy touch being on the Continent and impatient to progress on my big GEM?

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Here's an update:

First of all a huge thank you to Oddsocks for his link to the worm adjusters used in the Paramount, if they are good enough for SB etc etc:

1717078591_springplunger.thumb.jpg.adb0798d3dc0999e68b20fe70c1145c9.jpg

Also, this is what the mount looks like now:

defork.thumb.jpg.5eb4fa611cbf3a3db7f1a9cd7447560d.jpg

You don't think I've gone a bit far in sorting the Dec worm problem do you?

 

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  • 2 weeks later...

Results time:

After many days of generating swarf on an industrial scale, I finally have all the new parts made:

bits.thumb.jpg.e4fbfb4a0e9da73f3b50d53cc39ff20d.jpg

I'm not responsible for the round bits you understand!

I decided to rework both axes, and do some work on the OTA as well whilst everything was in bits.

I tried the swing worm as described by Dave above, but it was as I expected, beyond my level of skill to get right, so reverted to something along the lines of the original arrangement, just hopefully better.

Here's the DEC, without the gear wheel

dec1.thumb.jpg.970f3e1ec9e30329508942405bcccfab.jpg

the hinged element is now the whole width of the fork tine, and uses a pair of 8mm point-nose grub screws as pivots, the spring plungers as recommended by Oddsocks can be seen at the bottom.

Here's the same, now with gearwheel:

dec.thumb.jpg.7a2452491da7b188a070a3527f50cabb.jpg

I'm experimenting with a single spring plunger, it seems to be giving enough force.

The RA gear was also re worked, here's the view down the hinge, PEC sensor foreground:

ra.thumb.jpg.3176483216f54252ca3215dd4526f06d.jpg

It took two long days to assemble and fettle everything, and tonight I managed to try it out, first here's calibration.

cal2.thumb.JPG.ac429eb03f35d5f452537981f84d126a.JPG

And the acid test, guiding:

guide2.thumb.JPG.f79157a32c5211594c2be51102e75214.JPG

I'm think I'm fairly happy with this as a first outing, it's far from perfect, but at least guiding is under control in both axes. DEC is still a bit sluggish to respond, but at least it's not shooting all over the shop as before.

It might be that Synta will sell one less EQ8 after all, time will tell.

Can I thank everybody who added their twopence to aid me in this project, in particular William (Oddsocks), who was invaluable with his advice.

 

Huw

 

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Good to see your redesign has been successful Huw.

Have you ‘run-in’ both worms and gears prior to the tests above?

Reading the forum posts on the Software Bisque site it seems they run the Paramounts before final adjustment and shipping with an exercise program, slewing the mount in both axis at moderate slew rates for many hours with around 25kg of added weight to simulate an ota, so that the worms and gears bed together accurately. Now that the geometry of your worm brackets has changed significantly I think you would need to carry out something similar.

I have no idea whether SB do this with light oil, lapping compound or greased worms, I can imagine it would take a long time to lap the gears with regular grease used as the lubricant. I have read on various engineering forums that chrome metal polish and even toothpaste make good lapping compounds for aluminium gears.

It might be worth spending a day just slewing the mount back and forth in both axis at moderate slew rates ‘as-is’ to bed-in the worms and see if the guiding graph improves, if you do this do watch out not to overheat the motors and drive controller though and pause the exercise regularly to allow time for cool-down.

William.

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BH told me to use only oil on my worms/wheels because abrasive would linger and cause rapid wear.

Given the rotation speed, loading and average run time I'd say that was distinctly on the cautious side.

The only obvious danger is "coarse grinding" them together at speed while misaligned and thereby altering the "tooth" form.

Most owners would scrub the worm and wheel clean after any attempt at rapid lapping.

I used Solvol Autosol on mine and cleaned up with a brass brush followed by a toothbrush and thinners then soapy water.

They were soon cleaner than when they arrived in the post!

I used a rechargeable drill but seriously doubt they'd even notice the metal polish unless I ran them together in a bath of the stuff for weeks on end!

The cutting tool marks were still clearly visible afterwards. Enough said?

 

 

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2 hours ago, Oddsocks said:

Have you ‘run-in’ both worms and gears prior to the tests above?

I have indeed, the obsy stinks of toothpaste, I used Arm and Hammer for a couple of hours on the DEC and three on the RA, then full clean before re assembly, it sure makes a mess.

 

Huw

 

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