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Modified WeMacro vertical stand

 
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1951
Location: Santa Clara, CA, USA

PostPosted: Mon May 21, 2018 9:01 am    Post subject: Modified WeMacro vertical stand Reply with quote

In an earlier thread, Lou Jost discussed the need for a robust rail to accommodate some heavy lenses he had recently purchased. In that thread, I recommended the use of an "idler rail" to help eliminate the torque such heavy lenses would impart on a typical stacking rail. See the thread here:

http://www.photomacrography.net/forum/viewtopic.php?t=35570&highlight=idler+rail

I decided to implement this concept on a WeMacro stand that William sent me to evaluate. The WeMacro stand is an amazing bargain, and provided a perfect platform for putting the idler rail concept into play.

First thing I must say is that I am obsessed with rigidity and stability in the systems I build. I have two basic principles I try to adhere to:

..........1) Minimize potential camera movement due to moments and mounting flexibility

..........2) Maximally-couple the camera mount vs the specimen mount

The basic WeMacro stand does a good job with both these principles, and is useable as-is, but I realized it can be improved without too much difficulty with some modifications and additions. First, here are the shortcomings as I see them:

..........A) The camera mount extends quite far from the rail slider. This creates an uncecessary moment arm. This is exacerbated by the rail mount being single-point mounted to the vertical bar

..........B) There is only a single mounting point for camera. This eliminates possibility for separate camera + lens mounting

..........C) The camera + lens moment puts unnecessary torque on the slider rail

..........D) The long moment, plus large mass of camera + lens, creates a long settling time for any disturbance

..........E) The single angle bracket in the "pitch" plane to couple the stage to the vertical allows unnecessary movements

Here is a pic of the WeMacro stand modified to improve on all the above aspects:



Referring to my original "idler rail" drawing in above link, I placed the slider rail at the top, and idler at the bottom, of the system. The WeMacro rail and stand components lend themselves to either upper or lower mounting, and I quickly realized that I could improve both rigidity and coupling by mounting the rail at the bottom. This arrangement allows an interference fit between the stand bottom and the stepper motor. I chose not to use any physical attachment (bolts, tape, foam) between the two, as this may be counter-productive, but this warrants further investigation.

There were 3 basic mounts/spacers required: a 1/8" to allow clearance of the 48cm rail vs the slider rail end support; a 1/4" to mount the 48cm rail to the idler rail and give it the proper spacing; and a 1/5" x 3/8" to mount the idler rail to the vertical post. All these are identified in the pic above.

Costs for raw material were:

1/8" spacer: ~$1
1/4" spacer/mount: ~$1
1.5" x 3/8" spacer/mounts: ~$1.50 each
48cm rail: ~$27 (see http://www.photomacrography.net/forum/viewtopic.php?t=36843&highlight=arca+extrusions)
200mm idler rail: ~$25

The 48cm rail is a bit too long to be fully useful in the system. It is essentially unsupported above the idler rail mounting point, so if a camera alone is mounted to the rail above this point, its internal stiffness is all that keeps the rail from flexing. If a second point is chosen at or below the ilder rail mounting point (eg a tripod mount or other type of lens mount) then the torque on the rail is reduced and it "may" be useable all the way to the end. Caveat emptor for any use significantly beyond the idler rail mounting point.

Let me know any questions or comments about the above.

Edited to add: here is an inexpensive linear slide rail from Hong Kong that should work:

https://www.ebay.com/itm/200-300-500mm-Miniature-Linear-Slide-Rail-Guide-Sliding-Block-DIY-CNC-3D-Printer/372309356809?hash=item56af5df109:m:mYuwOX7Ly5pnZw45RSlokoA
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Lou Jost



Joined: 04 Sep 2015
Posts: 3241
Location: Ecuador

PostPosted: Mon May 21, 2018 10:24 am    Post subject: Reply with quote

That's an excellent design, Ray.

I think for my massive lenses I would add an aluminum rail to the back of the WeMacro post, rising higher than the upper end of the idler rail, with an L bracket at its top. The horizontal part of the L bracket should be long enough for its tip to be in line with the back of the camera. That tip (or a point near it) would carry a stiff spring connected to the camera, and would be adjustable so that it would bear most of the weight of the camera + lens. That would eliminate both the torque and the mass.

I just now bought the idler rail you suggested, thanks for the idea.
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 1951
Location: Santa Clara, CA, USA

PostPosted: Mon May 21, 2018 12:37 pm    Post subject: Reply with quote

The spring on the backside is a good idea for minimizing the static load on the rail screw, though it doesn't change the dynamic (moving) load. Looking at the THK KR LM Guide document, the dynamic load rating is limiting at ~60% of static load. I'm not sure if the WeMacro or other rail types have the same limitations, but I would expect so.

Edited to add: actually, I just realized that for a vertical setup, the dynamic load is the static load plus the dynamic factors in one direction, and minus those factors in the other direction. So indeed it would make some sense to balance the system for static load in order to balance the dynamic. Otherwise, it would make sense to do the stacking primarily in one direction so the dynamic load during the stack is less than the static.

That said, for pure axial loads, the limits for rails is pretty high. For the KR26, which is similar to the WeMacro in screw size, the limit is ~2kN, which (if my math is correct) is ~440lbs. So if you keep the load axial, as I am showing you how to do with the idler rail, you can move tremendous loads.
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