Clement Focuser

[Stub Mandrel]

40 minutes ago, Horwig said:

What about a pair of nuts that can be tightened against each other till slack is taken up, and then locked down, would that work?

Normal nuts will just 'lock' - think how locknuts work.

You can get 'split' nuts where a grubscrew pushes the two halves apart and nuts that have a second nut that screws into them to adjust their separation.

You can buy plenty online, including delrin (plastic) ones that are actually very good and hard wearing.

A ridiculously cheap solution that should work well (I did one co this as an experiment) is a plastic fix-it block, use an ordinary M6 screw to cut a thread in the single hole (not a tap) it will then be a nice shake-free fit on M6 allthread. The two other holes can be used for mounting.

 

[Stub Mandrel]

https://www.ebay.co.uk/itm/3D-Printer-T8-2mm-Pitch-8mm-Lead-Screw-Anti-backlash-Brass-Nut-Shaft-Coupling/222730437201?hash=item33dbc4c251:m:mLw37lEyNbVAsHiMD1BvyYA

 

[Gina]

Backlash compensation in auto-focus routines is quite simple really.  The code checks which direction the focuser needs to move and compares it with the last move.  If the same it just carries on but if different it adds the backlash amount to the distance required thereby eliminating the effect of the backlash   Q.E.D.

[Horwig]

1 hour ago, Gina said:

Backlash compensation in auto-focus routines is quite simple really.

What worried me is that the amount of backlash might vary with the loading of the focuser, ie, the angle the scope point at

[Gina]

 I don't think so.  You may get flexing in the drive but I doubt the backlash would vary.

[Horwig]

50 minutes ago, Gina said:

 I don't think so.  You may get flexing in the drive but I doubt the backlash would vary.

Makes sense, but I'd still like to cover my Bottom (edit: my  original term was more earthy, and four letters long, but profanity filter does a wonderful job) and make the mechanical system as fault free as possible before applying software to iron it all out

3 hours ago, Stub Mandrel said:

You can buy plenty online, including delrin (plastic) ones that are actually very good and hard wearing.

I have some nice blocks of Delrin, that sounds like a nice solution, something like this?

[Stub Mandrel]

1 hour ago, Horwig said:

Makes sense, but I'd still like to cover my Bottom (edit: my  original term was more earthy, and four letters long, but profanity filter does a wonderful job) and make the mechanical system as fault free as possible before applying software to iron it all out

I have some nice blocks of Delrin, that sounds like a nice solution, something like this?

Yes, just like a bronze one, but chunkier and bronze ones tend to adjust by opening up, rather than squeezing closed - I don't think it makes any difference.

[Horwig]

Thanks for that Neil, will go with a split delrin nut, as to the rod, I have some M6 x 1.0 stainless, a 10 tooth and 25 tooth T2.5 timing pulley and belt, so that should give 2 microns per click for a 200 pulse per rev motor, or 1 micron if I microstep, that will keep the figures simple.

Ali on its way for next Wednesday, this is going to start moving quicker than I thought. The 0.1mm stainless foil is from HK, so will take a lot longer, but at least I can start on the major machine work.

 

Huw

 

[Horwig]

Merry Christmas! Working in the man cave on Christmas day is forbidden, but Boxing day is another day, so I've made a start.

This is the adaptor sleeve that will hold the Wynne corector to the focuser, the brass insert holds the corrector nice and snugly in the adaptor, and the flange on the end of the Wynne will be fixed to the flange at one end of the sleeve.

For my previous focuser, the mk3 shown here, I used the Orion Optics Wynne corrector as my drawtube, with a clamp holding it in the ballraces of the focuser:

Not sure if the force of this clamp was responsible in part for the strange star shapes I've been getting, hopefully the new arrangent will be kinder on the optics.

Ali for the bulk of the new focuser allegedly arrives tomorrow.

For the flexible elements, I've ordered 0.1mm thick stainless strips, selected after playing with strips from a feeler gauge.

The Clement focuser uses spring steel, will be interesting to see how stainless performs, anybody have any thoughts on the comparison?

H

 

 

[Horwig]

Metalwork is coming along, here are the top and bottom plates:

And these will be the side bits between the hinges:

What initially looked like a nice simple focuser to construct is proving to be quite a lot of work. I now have 72 m3 holes to drill and tap

[Stub Mandrel]

19 hours ago, Horwig said:

I now have 72 m3 holes to drill and tap

In alloy - yeuch...

WD40 makes a good tapping fluid in aluminium alloys, clear the flutes out after each hole.

Sorry if I'm teaching you to suck eggs.

[Horwig]

1 hour ago, Stub Mandrel said:

In alloy - yeuch...

Agreed, and the trouble is, 2.5mm drills are fragile! Since I had the milling machine still set up to machine the 45 degree cuts, I started drilling those holes first. I used peck drilling, and stopping to clear flutes often, trouble was the top surface might have been perpendicular to the bit, exit surface was not, I broke a bit in virtually my first hole, and guess what, I can't get it out, oh bother, it's dug in well into the soft alloy, can't even drift it out. That hole will have to be re-drilled 3mm+ away, so much for trying to make a professional looking product.

Anyway, all other holes drilled, now I have to tap them all !!!!!

The stainless steel strip is finally here, hope that is going to be easy to drill.

Don Clement charges about $1000 - $1350 for his focuser, it's starting to look like a bargain

Huw

[Stub Mandrel]

2 hours ago, Horwig said:

I broke a bit in virtually my first hole, and guess what, I can't get it out, oh bother, it's dug in well into the soft alloy, can't even drift it out. 

Order some alum off ebay - very cheap, used in middle eastern cooking.

Make a decent solution and soak, the steel bit will corrode and flake away enough hat you can easily extract it without the alloy being affected.

Always drill stainless steel in one go, DON'T PECK - it work hardens.

[Gina]

That's useful information

[Horwig]

51 minutes ago, Stub Mandrel said:

Order some alum off ebay - very cheap, used in middle eastern cooking.

Make a decent solution and soak, the steel bit will corrode and flake away enough hat you can easily extract it without the alloy being affected.

Always drill stainless steel in one go, DON'T PECK - it work hardens.

Thanks for both tips, well worth knowing.

fraid it's too late for the grunged up hole, probably well out of shape, won't both trying to rescue it, another hole won't be the end of the world, well I hope anyway.

 

Huw

[Horwig]

Here's a test assembly of the metalwork:

movement is quite nice but I'm not sure if it is absolutely without give when pressed hard to one side, might use three lead screws to make sure there's no sag.

[Stub Mandrel]

Imp[ressive! I hope it performs as well.

[Horwig]

I haven't given up, honestly, just taking my time.

Here's the drive system and motor test assembled on the base plate, with the anti backlash nuts mounted on the moving plate. Have not fitted the side flexible elements to it yet, that will be the next stage.

 

[Stub Mandrel]

@Horwig I see you have a brass compression ring insert.

I want to fit one of these, but I'm not sure how thick they normally are. Do I use 0.5mm brass and have it quite flexible or 1 - 1.5mm and have it more or less rigid (but springy)?

[Horwig]

2 hours ago, Stub Mandrel said:

Do I use 0.5mm brass and have it quite flexible or 1 - 1.5mm and have it more or less rigid (but springy)?

I have a two thicknesses of brass in use, the one in the photos here is 0.5mm, I also have an adaptor to take this 84mm fitting down to 2inch, and that has 0.3mm brass, the 0.5 was just too rigid to bend smoothly on a 2 inch diameter.

Hope that helps

 

Huw

[Stub Mandrel]

1 hour ago, Horwig said:

I have a two thicknesses of brass in use, the one in the photos here is 0.5mm, I also have an adaptor to take this 84mm fitting down to 2inch, and that has 0.3mm brass, the 0.5 was just too rigid to bend smoothly on a 2 inch diameter.

Hope that helps

 

Huw

Helps a lot, Huw, I already have the 0.5mm ring, it just needs the edges finishing nicely. The knack to a smooth bend is to slowly work it around a slightly smaller cylinder, over several passes,

[Horwig]

55 minutes ago, Stub Mandrel said:

Helps a lot, Huw, I already have the 0.5mm ring, it just needs the edges finishing nicely. The knack to a smooth bend is to slowly work it around a slightly smaller cylinder, over several passes,

There's that word again 'slowly', I'm not that good at it

[Horwig]

And here it is assembled and carrying the imaging train:

It's quite fiddly to set up the drive train, there are two separate tension adjustments needed, but when it's just right everything is nice and smooth.

It lifts the 3.2 Kg train with no problems and feels very rigid, the next stage will be to set it with the load horizontal and test for backlash.

 

Huw

[Stub Mandrel]

Serious stuff! I look forward to seeing it attached to a scope (I trust it isn't for a 150mm dob...)

[Horwig]

9 minutes ago, Stub Mandrel said:

I trust it isn't for a 150mm dob...

No it's for this one, 40 cm f3.5: