Newsgroups: comp.robotics
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From: nagle@netcom.com (John Nagle)
Subject: Re: Basic mechanical questions
Message-ID: <nagleCr37Dx.K8u@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <derossi-070694212106@magic-macip-63.genmagic.com>
Date: Wed, 8 Jun 1994 16:34:44 GMT
Lines: 74

derossi@genmagic.com (Chris Derossi) writes:
>Rotation I can do with just about any motor. Speed isn't really
>that important and I can achieve accuracy by using switches to
>sense when the arm tower is aligned with each of the seven locations.

       Continuous position sensing is preferred.  With limit switches,
you don't get any position indication until you've arrived, so you tend
to overshoot.  It then matters how fast and which way you're going.

>What I need most are ideas about the non-motor components of the
>motion mechanics. Let's start with the vertical motion first.

>One person recommended that I use a threaded rod as the center of
>the arm tower. 
>I have a bunch of really basic questions about this.

>+ Where would I find the threaded rod? 
      Stock Drive Products 516-328-0200
      Berg 516-599-5010
           Get catalogs from both these outfits, and most of your
	   mechanical questions will be answered.  
>+ What is it called? 
      Threaded rod or threaded stock.
>+ What kinds of parameters do I need to specify (like thread 
>  spacing?)? 
>+ Can I get one that is 3' long or more? 
>+ How do I secure the top so that it is free to rotate (okay, 
>  I can probably figure that one out)? 
>+ How do I attach the threaded rod to the stepper motor?
>+ What is the item called that gets threaded onto the rod?
>+ How do I mount the subassembly to this piece?
>+ How much weight can I expect to move?
>+ How fast can I expect to move?

      Take a look at the "High helix lead screw assemblies" in the 
Berg catalog.  These are specifically designed for positioning 
applications.  There's a threaded rod, a smooth rod, two end plates
with bearings, and a follower assembly that rides along the rods.
Lengths from 6" to 18".  Price around $50 each.  Choice of
Teflon or chrome steel ball bearings, and Delrin or stainless/nylon
follower assembly.   Berg also has all the shaft attachments, motor
mounts, limit stop assemblies, etc you need.

>Another approach is what I call the elevator approach. This
>involves lifting the subassembly by some kind of chain or
>cable. The subassembly would ride either in some kind of shaft
>or would have some kind of guide rails. More basic questions:

>+ Can I rely on gravity to pull the subassembly down?
>+ How do I accurately "pull" a cable without slipage?
>+ If I use a chain, where do I buy chains & sprockets?
>+ If I use a cable, can I just let the cable wind around the
>  shaft of the motor?

      Berg also has a whole line of precision chain and sprockets.
Chain is available in metal, plastic, nylon over steel cable, etc.

>My next problem is the horizontal motion of the hand. It's
>another issue of linear motion. I need to move a distance
>which is at least the diameter of a CD. I've seen parts that
>look like this:  |^^^^^| with teeth in a row. Meshed with a
>gear, the part would move linearly. The problem is that I have
>only seen this part in lengths of one or two inches. And I don't
>know what it's called or where to shop for them. But it would
>do the job perfectly. So, basic questions, part 3:

     That's called a "rack".  Lengths to 6 feet are available from stock
from Stock Drive Products.

     I'd go with the lead screw assemblies for as many axes as you can.
You can probably bolt this thing together out of parts from the Berg
catalog with little if any machining.  

					John Nagle
