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The "Mess You" Mount


sploo
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So, that Mesu-Mount (http://www.mesu-optics.nl/mesu200_en.html) looks nice doesn't it? It would probably be great for my Skywatcher 300P tube... except it's quite expensive. Could I cobble together something from second hand eBay bits, scraps I have in the garage, and a few other cheap components?

The first job was to make some bushes (from 35mm steel) that will hold the main friction wheel:

01.jpg.2c402c0cfe94ffc127ddb152cdb056fc.jpg

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They bolt onto a 20mm diameter steel rod:

04.jpg.de4bbb0d4fd7463e8a72cb03e5ac977d.jpg

 

A 150mm diameter, 10mm thick, steel disc was drilled to allow the bushes to be mounted:

05.jpg.b40e52d17997a24cb7b2cd3d37cfddee.jpg

 

Then the center of the disc was bored out:

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...and fitted to the 20mm rod, along with a couple of bearings:

09.jpg.9aba323bede8e4478d0496474d2bffbd.jpg

 

To finish the outside of the friction wheel, the rod was mounted in a collet, to ensure the wheel is machined true to the rod:

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Just under 0.04mm (about 1.5 thou) out of round. I'm pushing the limits of my little lathe (and my non-existent metalwork experience), but I hope that'll be OK:

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I sourced a used 1:100 harmonic drive from eBay, and turned a sleeve that will form the driver for the friction wheel:

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In order to mount the harmonic drive (and the stepper motor that will power it) I'll need something to hold them. Time to chuck some 6mm scrap MDF on the CNC:

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It's not exactly pretty, but it will support the stepper and harmonic drive (hinged on the smooth rod on the curved side of the MDF sheets). Hopefully it will allow the driven wheel to be pressed against the friction wheel without undue stress on the harmonic drive output shaft:

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Using the very highest quality "scrap bits of chipboard", carefully sourced from IKEA's skip, the wheel and rod are mounted, along with a lever that will push the harmonic drive output into the friction wheel:

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With the rod screwed down (into a threaded insert in the chunk of pine) the bearing on the lever raises up:

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Here you can see how the bearing on the lever will engage with the harmonic drive output - pushing it into the wheel. This part of the mechanism will give a 10:1 ratio:

21.jpg.34e99401fca2dcdac92dc8d0660cf2cb.jpg

 

The carefully cut (read: hacked to fit) sheet of plywood (glued together from two parts of a jig I no longer needed), supports the lever, rod, and the about-to-be-installed harmonic drive/stepper motor cradle:

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Like this:

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I have some (almost) working code running on an Arduino to control the stepper, but for the moment I removed the stepper and spun the wave generator in the back of the harmonic drive by hand. It was OK lifting a 5lb (2.3kg) weight, on a beam that is 33.3cm (rod-to-string-hole distance). I make that 2.3kg x 9.81m/s^2 = 22.56N, at 0.33m => 7.4Nm of torque:

24.jpg.497b6bd520d9a97cfc8e66ad207fc7c8.jpg


Attempts to get it to lift 10lb (4.6kg) were not successful; the friction wheel slips. I could tell there was a lot of flex in the lever as it had bottomed out against the pine block. I hacked a bit off to allow it to move further (i.e. bend more), and not surprisingly it failed. It did get close to lifting the 4.6kg though, so I might have got away with 3kg (which would be nearly 10Nm of torque):

25.jpg.82eccad66227924e11b4e408dd7e37a3.jpg

The lever was designed to be the same dimensions as some standard metal stock (I wasn't expecting 6mm MDF to survive), so once I take delivery of that stock I'll make a new lever. How/if the rest of this will actually work I don't know, but the above is "only" about 3 months of occasional work (which includes a previous all-wooden prototype to test the theory), so it may be a while before there's much progress to report.

Edited by sploo
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Being a Mesu 200 owner. I loved this. :)

The mount does not need to lift 5Kg or 10Kg...It should be balanced so it can hold the weight on the DEC of 100Kg. If the weight has a counter balance then its not a problem. Hope that makes sense?

 

Dave.

 

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22 minutes ago, Star101 said:

Being a Mesu 200 owner. I loved this. :)

The mount does not need to lift 5Kg or 10Kg...It should be balanced so it can hold the weight on the DEC of 100Kg. If the weight has a counter balance then its not a problem. Hope that makes sense?

 

Dave.

 

Then you're a lucky man, from what I've seen of the Mesu 😀

My 300P is the version on the Dobsonian mount, and although it's pretty well balanced I reckon it needs something around 5Nm of torque to overcome the friction; though granted it's intended to be a bit "sticky". I see the Mesu 200 is rated for 10Nm, so I'm hoping my prototype will be in the ballpark.

I am actually intending to make it a fork mount... for reasons I can't quite yet explain (basically, I don't know better, but it appeals to me).

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Very nice work so far.  Even from the limited views in the pictures I reckon I might recognise that lathe, too.  I may have something very similar in my own workshop :D

James

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6 minutes ago, JamesF said:

Very nice work so far.  Even from the limited views in the pictures I reckon I might recognise that lathe, too.  I may have something very similar in my own workshop :D

James

Thanks.

It's a ubiquitous Sieg SC2 clone (this particular one is a used Clarke CL300). Under-powered and woefully wobbly - and that's just the operator...

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Ah, yes, near identical to mine I think, though I have a speed display and DROs on the slides (which I'd probably much rather not have):

lathe.jpg

Much as I'd like something better, I can't really justify the cost.  I have been meaning to replace the spindle bearings and do a few other mods I've seen recommended, but haven't got around to it yet.

Anyhow I shall look forward to seeing how this project progresses.

James

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1 hour ago, sploo said:

(and my non-existent metalwork experience),

😂

Loving this :)

Does the Mesu use harmonic reduction? Makes sense for zero backlash drive 👍 So then the friction drive stage is just to reduce the torque ripple?

following with great interest- top work Sploo!

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9 hours ago, JamesF said:

Ah, yes, near identical to mine I think, though I have a speed display and DROs on the slides (which I'd probably much rather not have):

lathe.jpg

Much as I'd like something better, I can't really justify the cost.  I have been meaning to replace the spindle bearings and do a few other mods I've seen recommended, but haven't got around to it yet.

Anyhow I shall look forward to seeing how this project progresses.

James

Very similar - though I think mine's a pretty old model so the controls are different. I would like something a bit bigger and more rigid, but, cost aside, the size and weight goes up massively; which unfortunately isn't that practical in terms of space.

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9 hours ago, markse68 said:

😂

Loving this :)

Does the Mesu use harmonic reduction? Makes sense for zero backlash drive 👍 So then the friction drive stage is just to reduce the torque ripple?

following with great interest- top work Sploo!

Thanks. I believe the Mesu is a three stage friction drive; which in theory should be ideal in terms of having no periodic error. The problem with harmonic drives is that they're very expensive to buy (new), so it would make a commercial mount very pricey.

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1 hour ago, sploo said:

I would like something a bit bigger and more rigid, but, cost aside, the size and weight goes up massively; which unfortunately isn't that practical in terms of space.

But it is a giggle when you can take 1/4" depth of cut in hard steel 😊

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2 minutes ago, MarkAR said:

But it is a giggle when you can take 1/4" depth of cut in hard steel 😊

Yea, that would be nice. I'm sure I saw a review of one of the mini lathe clones where a guy with a really nice shop (lots of "big" lathes") forgot and tried to take a substantial cut on the little one. It doesn't go well.

The biggest problem I find is flex and chatter - even after stripping it down, cleaning, and adjusting all the gib screws there's a bit too much flex with anything other than a really light cut (when using steel). I've never actually used a "proper" metal lathe though, so it's hard to judge.

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There is nothing like making your own mount it's extremely satisfying. I have made two Alt/AZ mounts one a clone of the DM6 and the other a clone of the panther mount both work extremely well. I'll look forward to following this thread ad hope everything goes well. If you can take a 1/4" cut in steel you are doing really well, my Myford struggles doing that, in aluminium it's pretty easy but not in steel.

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Tool geometry gets really picky on lighter weight machines. You can often dial out chatter by playing with cutting angles etc and speed and feed rates.

But sometimes just taking lighter slower cuts is the only way.

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2 hours ago, Doc said:

There is nothing like making your own mount it's extremely satisfying. I have made two Alt/AZ mounts one a clone of the DM6 and the other a clone of the panther mount both work extremely well. I'll look forward to following this thread ad hope everything goes well. If you can take a 1/4" cut in steel you are doing really well, my Myford struggles doing that, in aluminium it's pretty easy but not in steel.

With the little mini lathe clones a 0.01" (0.25mm) depth of cut is about your sensible limit. The newer models have some form of overload protection, but the older models (like mine) apparently just pop their motor control board when you overload it and lock the spindle up!

Long term, I'm hoping to motorise both the RA and DEC axes, and maybe even the focuser. For the moment though, if I can simply get the 300P tracking across the sky in order to take a photo of a few seconds I'll be pretty pleased.

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My cheap Chinese lathe breaks its plastic motor pulley when overloaded!!  I did a lot of upgrades on mine but still wouldn't call it particularly accurate.  These are just not precision lathes!

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4 hours ago, Gina said:

My cheap Chinese lathe breaks its plastic motor pulley when overloaded!!  I did a lot of upgrades on mine but still wouldn't call it particularly accurate.  These are just not precision lathes!

I've heard it claimed that the plastic gears in these little lathes is actually a "good" thing; in the sense that it's a cheaper weak link than having to replace the motor or control board; which I suppose is a fair point. Still this one didn't cost me much (second hand), and I couldn't have done what I've done without it so I'm not going to be too harsh on it.

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I got a little time to remake the MDF lever in steel, as originally planned. With no welder (or welding skills) it'll be pinned together with 3mm machine screws:

01-2.jpg.97202ecc36c34382d07886697a1d6635.jpg

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To fix the 12mm rod for the bearing, I drilled to 12mm on one side of the lever, then tapped the other side to an M12 thread. I put a thread on the end on some 12mm rod, and used a saw to create a slot at the other end:

04-2.jpg.7e25c64a7c5fca111a82702322c9642b.jpg

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New lever test fit:

06-2.jpg.fe1212de1498147ce9cb87ff2b3144df.jpg

As is so often the case, once you remove one weak point, you find the next 😉. While it was a little better than the previous version, the threaded rod on which the lever pivots is now deforming the hole in the plywood cover that provides support. I expect it's probably doing the same to the black chipboard too (though hidden behind the large washers). I could make supports using steel, but on inspection of the rod I think the forces are starting to bend it, so I don't think it's going to be a viable solution. To be fair - you don't actually need that much force to bend a length of M8 rod.

It occurs to me that the lever could instead be mounted vertically, and slide upwards (pushed by a threaded rod). It's not the ideal size/shape in its current design, but I plan to look into that when I next get time.

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Updates...

I mounted what was the lever in a wood "slider", and drilled a small indentation in the bottom to locate an M12 bolt (the end of the M12 bolt was turned on the lathe to have a point that roughly matches the indentation left by drilling the lever). The M12 bolt is secured in a chunk of wood that has a couple of M12 nuts embedded. Long term I'd do this differently/use metal, but it's just a test:

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Mounted back on the main body, to push the output of the harmonic drive onto the friction wheel:

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And with a lid, plus the "skin" covering the friction wheel:

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First test was promising; I very nearly got the two 5lb weights lifted, but couldn't quite get the lever all the way to the 9 o'clock position before the friction wheel slipped. Checking the side of the main body, it's clear the force of the bolt was bending the flimsy chipboard:

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Ideally, the top of the bolt cover would be tied to the friction wheel spindle with a single covering skin (under tension it'd stop the flex - basically like a torsion box). But this configuration is just a test, so instead let's clamp a couple of lengths of timber to keep it straight... and success:

05-3.jpg.c9ca696f831fb4e41f9d04150501ebc4.jpg

I checked the weights on a scale, and they come to a total of 4.61kg (just over 10lb). At a 33.3cm (0.333m) distance between the lifting point and the spindle, I make that:

4.61kg * 9.81m/s^2 = 45.22N

45.22N * 0.333m = 15.1Nm

I'm calling that good enough.

The lever (now a "slider") can be much more compact, and I think it should be possible to redesign the stepper + harmonic drive hanger to slide up and down too (rather than hinging).

More when I next get chance to look at it...

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  • 1 month later...

A little more progress...

After the previous torque test it dawned on me that the large (Nema 24) stepper motor in the original design is way over specified (I could never make use of the output torque). It turns out that a tiny stepper (commonly used on 3D printers) should be more than enough. This also made the interface plate between the stepper and the harmonic drive gearbox much simpler (previously the bolt holes overlapped, but were just a few mm different). The only problem is that the output shaft is too small for the harmonic drive's wave generator. So... a simple sleeve solved that:

01.jpg.04e9e16f783a9b084d972927ff9f6f8f.jpg

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Next, the new design for holding the two parts of the friction drive together. The original driver shaft (from the harmonic drive) was 15mm at the interface, but 12mm for the bearings I had. Some new 15mm bearings meant I could make a new - much simpler - 15mm driver shaft, but also this required new bearing blocks:

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The two main parts now fit together by the use of four threaded rods. The bearing blocks on each end of each axis would be joined together to prevent twisting, but you can see the layout below. By torquing the nuts on the four rods the two axes are pulled together:

08.jpg.df88a12cf1231481e0821f41d9a8a447.jpg

 

With none of the planned support to hold the top pair of bearing blocks together, I got the friction drive to manage just under 10Nm before slipping (the photo below is about 7.6Nm):

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Unsurprisingly, attempts to torque down the nuts on the four threaded rods started to split the MDF I'd bodged together for the bearing blocks, so I couldn't match the previous 15Nm result.

I think this design would work (and it's pretty compact and simple). I don't think that having to torque up four nuts is that much of a problem. However, the cost of steel, alum or even engineering plastic sheet of the required size for the bearing blocks turns out to be expensive. Some quality plywood (and making the blocks wider so the rod holes are further away from the ends) would likely take the load without splitting.

However, my second design idea for the above was to just use the four threaded rods as alignment, and instead have a 'U' shaped hanger on the inside faces of the top pair of bearing blocks - the hanger would extend down past the smaller drive rod. At the bottom of the 'U' of the hanger there would be a screw with bearing, which could be torqued up in order to push the bearing into the smaller drive rod, thus lifting the whole bottom axle. That design would allow the use of a single adjustment screw (rather than four nuts) and the load of the hanger could be taken across a larger area of the top bearing blocks - meaning they could be made of a softer material (e.g. plywood). The hanger could be made of thinner strips of steel, which I have already. It's more complicated, but I think it's worth giving it a try... when I get time...

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