### Author Topic: Frustrations designing simple linear motion system  (Read 1635 times)

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#### hernejj

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##### Frustrations designing simple linear motion system
« on: January 18, 2015, 08:59:33 AM »
My latest project is a very small/simple machine to fill very small containers.
The problem I'm trying to solve is this: I have a 6"x6" tray that contains small
containers in an x,y grid. Think of a miniature ice cube tray. I want to design
something that will be able to position the tray underneath a filler so I can
automatically fill the holes with water/sand/whatever. So essentially I need to
design a simple 2-axis linear motion system. Something like the movable bed some
3d printers have.

Now, I want to keep this project as simple/cheap as possible while still
retaining the accuracy needed to hit the ~0.4" targets spaced ~0.5" apart. My
idea so far is to use a stepper motor with a pinion gear to connect the platform
via a rack (linear gear?). This seems like it would work quite well, and be
cheap and simple. Taking only about a single axis at the moment, the tray would
sit on two parallel rods that allow motion only in the axis direction and the
rack would be fixed to the underside of the tray in the middle, parallel with
the rods and axis of motion. The stepper motor would be fixed to a chassis under
the tray. It would have a toothed gear (pinion gear?) fixed to its shaft
that would directly drive the rack, thus, moving the tray.

Ok, with all of that out of the way, here is the 1st problem I am facing. I
cannot, for the life of me, figure out a simple method of linking the rack and
stepper motor.

Here is the motor I was thinking about using:
http://www.robotshop.com/en/5v-2oz-in-unipolar-stepper-motor.html

It has a 5mm flatted shaft. So I need a gear with a 5mm hole.

Here is a cheap rack, and it even comes with a gear. The rack comes with a gear,
but no information on the size of the center hole. It appears to be too small
though.
http://cgi.ebay.com?ViewItem&item=380918372103

I could use the following, but I'm worried about it slipping since there is no
set screw. Maybe some blue Loc-Tite would hold it? Also, it is module 1.0 and
the rack is 0.5:
http://cgi.ebay.com?ViewItem&item=231161739468

Here is another option, but It is for a D shaft, will this fit on a flattened
shaft? Also, the teeth do not look very long, I wonder if this will mesh with
the rack?
http://cgi.ebay.com?ViewItem&item=331352172996

The teeth on this one look better, but I cannot find one in 5mm??
http://cgi.ebay.com?ViewItem&item=231365752261

Alternatively, I've seen metal gears with set screws on various websites, or
universal mounting hubs. But these items cost upwards of \$8-\$10 each. Do I
really need to spend 200% the cost of the motors on chunks of metal/plastic to
link them to a simple plastic rail?? What do others do in this situation?
GAaaah!! Help

2nd problem: All of these gears have a different Pitch/module.  The rack I found
is module 0.5. The most promising gear (1st one listed above) has a module of 1.

See my frustration ??  Is there anywhere I can buy a simple shaft mountable
gear and rack that will mesh and fit some small stepper??

Thanks for your time.

#### mklrobo

• Supreme Robot
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• From Dream to Design at the speed of Imagination!
##### Re: Frustrations designing simple linear motion system
« Reply #1 on: January 21, 2015, 04:25:36 PM »
Hello!
I think I understand the logic of your process, but I still do not understand the filler part.
If you could explain the process in more detail, maybe I could offer assistance? Thanks.

#### bdeuell

• Robot Overlord
• Posts: 189
• Helpful? 15
##### Re: Frustrations designing simple linear motion system
« Reply #2 on: January 21, 2015, 10:24:07 PM »
For a "very small/simple machine" you have a lot of questions...

i think i can answer a few questions...

loctite (referring to the common threadlocker not the brand) is designed to work with metal fasteners not plastic, I would not recommend this to mount a gear.

A "D" shaft is the same as a flattened shaft in principle, but be warned that the location of the flat is not standardized. Also some motors may have a very small flat or i have seen shafts flattened with a file and are not likely to fit a "D" shaped hub, these flats would be intended for a setscrew.

it is also possible to modify your gears/components to fit your needs. the success will depend on your performance requirements, tools available, skills/craftsmanship, and many other factors.

Without knowing the intent, environment, or design parameters of your application I can not say weather you need to pay the extra \$ for metal gears. Plastic gears can cerently be sufficient and even preferable in many applications.

Also consider the ease of sourcing parts and getting it to work. if you pay twice as much for your gears but you know they will work and what the specs are vs you buy five gears because you don't know what you are getting and none of them work.

the pitch/module must match

SDP-SI is a great source and has many small gears and small components but the price will likely be higher.

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