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  #1  
Old Sun 31 August 2008, 03:55
sprayhead
Just call me: Francis
 
sydney
Australia
Polyamide (Nylon) Rack - will it work?

Any thoughts on a Plastic rack? Should give a very smooth motion and low wear with reduced strength compared to steel, but maybe still viable?

Francis
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  #2  
Old Sun 31 August 2008, 04:33
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Things to consider:

Will grit embed into it?

Will it attract electrostatic loaded waste?

Will d/sided tape stick to it?

Will the metal pinion strip off the rack teeth if a car hits an obstacle?

Will d-sided tape hold it in place with fluctuations in humidity? (Polyamide is notoriously unstable when moisture is involved)

But, it might just work. However, I can't see any major benefit over the steel rack. Already we have quite a smooth ride and low wear. We don't have noise and corrosion problems for which "plastic" racks are typically the cure.
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  #3  
Old Sun 31 August 2008, 04:39
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
From http://www.plasticsmachining.com/mag...702/nylon.html
  • Nylon is hygroscopic. It absorbs and releases moisture in response to changes in the humidity of its surroundings. Nylon can absorb up to 8 percent of its weight in water when immersed. If placed in a dry environment, absorbed water will be released. This is typically a very slow process. The rate will depend on conditions of temperature and humidity, and part geometry. Use of water-based coolants during machining will have little or no effect on the moisture content of the finished part. "Wet" nylon tends to be more elastic and flexible than dry nylon.
  • As nylon absorbs moisture it swells. Changes in dimensions due to moisture absorption can be estimated as 0.003 inch/inch for each percent of moisture change. A 1-inch-long nylon bar machined at a moisture content of 2 percent would grow by 0.019 inch if immersed in water and allowed to reach its saturation point of 8 percent moisture. Nylon parts which must be closely toleranced for use in a wet environment may be soaked in hot water until saturated, then machined to final tolerance. The finished part can be stored under water until ready for use, or if stored dry, conditioned by soaking prior to use. Usually moisture absorption in nylon need not concern the machinist, however it can explain small dimensional changes due to changes in humidity which may be important in close tolerance parts.
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  #4  
Old Sun 31 August 2008, 07:00
sprayhead
Just call me: Francis
 
sydney
Australia
...wow, a whole heap of considerations Gerald.

My concern initially is with Roughness of movement and energy losses due to a dirty rack. I intend to "mow" PU and EPS foam and it will be sh&&t loads of the stuff... I mean to the point where the machine will have to be fully enclosed while cutting is happening.

As for electrostatic loaded waste, I know that the EPS waste will have a lot of that, and a plastic rack might be very troublesome there.

I am trying to imagine as well if a rack with a modulus of 1.5 would deal better with dust or not, since there seems to be more area for the bits of dust to spread to.

But, we'll see, I might give it a go.
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  #5  
Old Sun 31 August 2008, 07:45
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Well, the guys who are using modified ShopBots for surfboard shaping don't seem to be saying they have issues with the metal racks & pinions. (nor with the V-rails)
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  #6  
Old Sun 31 August 2008, 08:45
sprayhead
Just call me: Francis
 
sydney
Australia
Yeah, but usually most guys use the standard flute mills (coated with carbide abrasive... SSG, MCM or even plain grit abrasive coatings) and that makes a pretty manageable waste and you can add a DMS (dust management system)

I want to use a large disk, this little guy makes havoc. and it's too awkward to make something effective for sucking dust, plus it would take out the advantages of using a disk.

And also I'd like to make a machine that is able to perform plenty of work and that doesn't have to be groomed too much.
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