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Hi Jeff,

A few ideas:
-Counter weight the vertical axis using a loop of round belt with a weight
and two pulleys.  The pulleys may provide enough friction to prevent
slamming.

-On the horizontal, consider making a delrin plastic stop collar that can be
tightened down on the shaft, allowing you to dial in the friction.

-Also try a plastic nylon/PTFE linear bearing.  I bet they have more
friction than the ball bearing type.

-Eric

*****************************************
Eric Yuan
Exhibit Developer
Children's Museum of New Hampshire
6 Washington St
Dover, NH 03820
(603) 742-2002
www.childrens-museum.org
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-----Original Message-----

Jeff--

Can you seperate the user from the lens with one more layer of  
mechanicals?  For example, can you use a little chain drive system  
with a crank?  Or selsyn motors?

If one of your axes is vertical, you'd need to be concerned with old  
Father Gravity, too.

How about a magnetic cushion?  Attach magnets of one pole to your  
lens armature, and magnets of another pole on your frame.  If the  
magnets are reliable and strong, an over-exhuberant user could shove  
the lens at top speed, but as long as it's prevented from reaching  
another rigid body  (in this case by magnetic field) for impact, it's  
all good, right?

Just brainstorming.  I have sketch book pages full of doodles of two  
axes sliders--for some reason I daydream about these things every so  
often-- but I have never built one.


--Jason

On Dec 22, 2008, at 10:06 AM, Jeff Courtman wrote:

> I need to build a 2 axis slider to move a magnifying lens around a  
> large graphic (think of 'where's waldo').  I plan on using linear  
> bearings and wanted to know from those of you with experience what  
> might be the best way to control the friction in the movement (so  
> an over-exuberant 7 year old, like my son) can't wind up and sling  
> it from one side to the other.....
>
> Thanks in advance,
>
> Jeff

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