DIY Fork Mount for Widefield Imaging Rig

[Gina]

On 22/06/2017 at 12:31, Chriske said:

Gina,

what software are you using to make these drawings..?

Sorry, I missed this before.  You already know from our conversations but for the benefit of others, it's SketchUp Make 2018.  This is the "free for personal use" version of the software.

[Gina]

Collected together the metal parts of the fork mount and they weigh over 8Kg.  For a payload of around 4-5Kg this seems OTT.  And this is not including some of the heavy aluminium plates or the fork itself.  It's mainly the pillow block ball bearings.  I think this mount would be more suitable for the Esprit 80ED scope.  OTOH I guess it could do for both the Esprit scope and widefield rig.

Edited May 30, 2018 by Gina

[Horwig]

3 hours ago, Gina said:

Collected together the metal parts of the fork mount and they weigh over 8Kg.

WOW, what kind of axle dia. have you got for the polar axis, have you seen the range of small bearings on the Technobots site, even the Igus plastic/nylon bearings would probably work for a 5 Kg load:

https://www.technobotsonline.com/bearings.html

 

H

[Gina]

The axle is 1" OD in that version.  I already have a selection of ball bearings much smaller than those huge pillow block ones and I'm planning to use these smaller/lighter ones - they are quite adequate for the payload of the dual widefield imaging rig.  I can design a 3D printed carrier which I think will be quite adequate.  Thanks for the link.

[Gina]

I have a 10mm steel rod that came out of a 3D printer that would be suitable as an RA axle.  The Dec axle is simply cylindrical ends to the 3D printed clamp for the cameras.

[Gina]

The fork.

[Gina]

Found another, shorter shaft I could use for the RA axle.  Only trouble is the bearings I have are metric and this is imperial size and a smidgen bigger than the bearing ID.  OTOH I'm not sure a ball bearing is needed - a printed Nylon bearing would probably work well enough.

[Gina]

Also found a piece of aluminium tube about 20mm OD and 500mm long.  Aluminium is easy to cut to length.  Can't remember what I bought it for but it's been lying around for years.  I could print collars to fit it to the 25mm ID ball bearings I already have.  Still overkill probably but nowhere as bad as those huge and heavy pillow block bearings.  This would be a better axle as the shorter one is just chromium plated steel.  The kit will be covered but not damp proof.

[Gina]

New fork.  If I were to change the nozzle in my Titan printer to 1.2mm (largest available for Volcano hotend) it will take around 3 hours and consume over half a 1Kg reel of filament.  212m of 1.75mm filament.

[Gina]

Decided to change the design of the fork to remove the RA drive disc and add that as a separate print.  The result is half the time and half the filament.  Also extended the part that attaches to the axle tube to give additional support.

[Gina]

Printing has now reached the bearing holes but really needs parts cooling which my Titan printer doesn't have yet.

[Gina]

Filed the bearing holes out so that the ball bearings could be hammered in which has enabled me to test the fork.  Found a couple of design changes required.  With the present design I was able to pull the sides out to get the camera clamp stub axles into the bearings.  This shows that the sides are not stiff enough and also that the bearings need clamps to hold them in.  ie. semi-circular ends to the forks and semi-circular clamp pieces so that the thing can be assembled!!  More thinking and design required but that's what prototypes are for.  I shall add webs to stiffen the sides.  A couple other things need attention - the hole for the axle is too small and the tube was a bit too long.

Edited June 1, 2018 by Gina

[Gina]

Here is the fork model ready for printing.

[Gina]

The current print has served a useful purpose in advancing the design of the fork for the mount.  With the dual rig mounted the Dec axis is not balanced but I'm not sure this matters.  It means gear drive would be possible since backlash would not cause a problem and Dec axis accuracy is not important as operation will be manual, to centre the object in the FOV, after which the declination will be fixed during the imaging run.  To improve the balance would involve an offset camera clamping bracket and longer forks and reduced rigidity.

[Gina]

With the dual imaging rig, this fork mount is turning out a similar size to that needed for a telescope but with a much lighter payload.  I may leave printing until I have my Giant 3D printer working again - the hotend heater block broke and I'm getting a new one made.  Probably within a week or so.  There shouldn't be too much of a hurry, as long as I get this mount working for the autumn.  OTOH everything takes much longer than expected so I won't be hanging about ?

[Gina]

Now to look again at calculations of accuracy etc.

1. The fork is nearly 300mm wide so a 300mm diameter RA drive pulley would be appropriate.
2. Circumference of 300mm pulley = 300 x Pi = 942mm.
3. The pulley can be a bit bigger to give easier calculations eg. circumference of 1000mm give a diameter of 318.3mm
4. If I use GT2 timing belt that will be 1000 / 2 = 500 teeth.
5. I have 20t GT2 timing pulleys so ratio will be 500 / 20 = 25:1
6. If I were to use a NEMA17 stepper with 0.9° stride angle and 16x microstepping, each micro-step is 0.9 / 16 = 0.05625°
7. Earlier I worked out that 0.001° would be ideal for a 200mm lens but I doubt I'll go above 105mm with this rig. so 0.002° is fine.
8. To accomplish this resolution a further reduction is required of 0.05625 / 0.002 = 28.125 - say 28:1

Hmm... that's more reduction than the final drive - I think a motor-gearbox may be indicated.  I have a NEMA17 1.8° with approx 100:1 gearbox which would give a resolution around 0.001° but it's got the wrong size shaft for the small timing pulleys I have in stock.  Pulleys are available to fit the motor.  The little 28BYJ-48 motor-gearbox might be powerful enough but insufficient resolution (5.6° stride angle and approx 64:1 gearbox) it would still need further gearing down.  I can see why worm gears are used in mounts

Edited June 2, 2018 by Gina

[Astrobits]

Just a word of caution to anyone planning to use plastics for load bearing components. It is unfortunate that plastics not only flex but creep under load. That is, under load, they deform over time from the original shape and hence the location of the attached components move with the deformation. This could spoil any highly accurately aligned setup.

Different plastics have different creep characteristics. Polyethylene (LDPE ) has high creep while polypropylene (PP) is much lower.  For example, you should not make bottle crates from LDPE as when stacked the bottom ones will distort relatively quickly while  PP is o.k. for this application. Adding fibres ( glass, carbon etc. ) reduces creep significantly but doesn't reduce it to the level of metals.

Another feature of plastics is that they can absorb oils making them more flexible, so make sure that you keep oils away from plastic components where it could cause a problem.

Hope this helps.

Nigel

[Gina]

Yes, I am aware of creep.  The main frame will be reinforced with metal.  Retaining good PA is the main thing.

[Astrobits]

Yes, Gina, I was sure that you were aware of creep. My post was directed to those who might be reading your thread and, being unaware of creep, planning an all, or mostly, plastic system via 3D printing.

Nigel

[Gina]

Just checked the backlash in the gearbox of the 100:1 gearbox-motor and it's of the order of a degree.  That won't do at all ?  Think I'll have to stick with timing belts.  Maybe a triple reduction.  5:1 and 6:1 in conjunction with the 25:1 final drive.

Smallest MXL with 5mm bore is 15t from MotionCo and largest is 120t giving a maximum ratio of 8:1.  I could use MXL belt and pulley for the final drive but already have 20t GT2 plus a length of timing belt spare from 3D printer making.  MXL pitch is 2.03mm so similar but not quite the same as GT2 which is 2.00mm.

[tekkydave]

Could you use a stepper driver with a 256x microstepping mode? That would give you another 16:1 to use.

[Gina]

For small timing belts and pulleys, MotionCo seem the best.  There are 16t GT2 pulleys on ebay but biggest seem to be 80t - 5:1.  A 16t GT2 for the final drive and two pairs of 16:80 would work.  I'll have to total up the costs either way and see which is best.

[Gina]

5 minutes ago, tekkydave said:

Could you use a stepper driver with a 256x microstepping mode? That would give you another 16:1 to use.

That's a thought - just a different stepper driver module, I guess.  Do you know of one Dave (I don't really want to use a Duet board ).

[tekkydave]

The Duet uses TMC2660's but I don't know how much they cost to buy. There maybe cheaper ones by TMC that do 256x.

[Gina]

Those drivers on the Duet are SMDs - I don't think I want to cope with those - they've got a lot of legs.  Bad enough with 8 leg SMDs ?  Ideally a unit to replace the ubiquitous A4988 drivers.