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  1. #1
    spock is offline Hot Rolled
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    Default machining polycarbonate

    Looking at some lathe work, part gets pretty thin walled (.187) during turning. Is this stuff pretty stable, or is it bad to crack in thin sections? How does it machine?

  2. #2
    Bellindustries is offline Cast Iron
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    Not to prone to cracking, harmonics are a huge problem as well as heat. You have to run dead sharp tooling, uncoated. How deep are the bores, as breaking the chip is also a problem. This stuff will pick up and amplify any harmonics in your machine, especially long bores with thin walls.
    Terry

  3. #3
    form_change is offline Aluminum
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    As a general tip there are some hydrocarbons that polycarbonates don't like and will stress crack. One place I worked the designers specified loctite on some bolts holding on some PC blend parts. Next morning the parts had cracking around the bolt holes as the operators had not been told to keep the stuff away from the plastic.
    It does machine alright though - reasonably ductile. I think one of my books suggests around 3 times the cutting speed for steel as a starter. Important thing with machining most plastics is to keep them cool as they will soften at temperature. I'd suggest making a few test parts to see how they react to your machine coolant before running a big batch, particularly if you have sharp corners (or threads) involved.
    This is all general stuff as without having played with the grade you are going to use I don't know the ins and outs

  4. #4
    Hdpg is offline Stainless
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    Quote Originally Posted by form_change View Post
    As a general tip there are some hydrocarbons that polycarbonates don't like and will stress crack. One place I worked the designers specified loctite on some bolts holding on some PC blend parts. Next morning the parts had cracking around the bolt holes as the operators had not been told to keep the stuff away from the plastic....
    Even if they did keep the Loctite liquid away from the polycarbonate that is no guarantee; I found the vapors are enough but it just takes longer for the cracks to appear. That lesson cost me a few thousand dollars replacing parts that had already shipped to the end user, and a lot of embarassment.

    If you don't know whether your coolant is safe stick to soapy water as a coolant/lubricant.

    I do know Shell Dromus B is safe because we have used it to machine polycarbonate for decades.

  5. #5
    Limy Sami is offline Diamond
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    I got a lot of help with polycarbonate from the forum on this thread.

    http://www.practicalmachinist.com/vb...=polycarbonate

  6. #6
    appliedproto is offline Stainless
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    Quote Originally Posted by spock View Post
    Looking at some lathe work, part gets pretty thin walled (.187) during turning. Is this stuff pretty stable, or is it bad to crack in thin sections? How does it machine?
    Melts quick, and as said before me, uncoated polished cutters are best. Low RPMs and heavy feeds when drilling, and you can blast through it quick when milling. It will absorb coolant and might discolor with long exposure to any fluid. It's not as fragile (actually it doesn't crack easy, and won't chip) and is more friendly than acrylic, which melts REAL quick.

  7. #7
    706jim is offline Stainless
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    Plain water seemed to wrok best on acrylic for me. Maybe it would be worth trying on polycarbonate.

  8. #8
    Davis In SC is online now Titanium
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    Oil is the enemy of PC... Even oily fingerprints will crack and craze PC...

  9. #9
    Hdpg is offline Stainless
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    Oil is the enemy of PC... Even oily fingerprints will crack and craze PC...


    If by PC you mean Polycarbonate, also known under the Tradenames Tuffak and Lexan this it not correct. The windows on CNC machines are most often polycarbonate and they withstand prolonged exposure to oil and coolant; they can be washed with solvents such as Varsol without any problem and sprayed with WD40.

    But don't try using Acrylic also known as Perspex or Plexiglas or Polymethylmethacrylate. Acrylic will craze when exposed to a wide variety of liquids and only soapy water can be considered really safe. Acrylic will also shatter readily with an impact but polycarbonate is very impact resistant. Polycarbonate can be cold bent like a metal without problems although it has considerable springback; acrylic is impossible to bend cold.

    However, it is possible to get polycarbonate in a form that will behave a bit like acrylic and this is the abrasion resistant polycarbonate. In its normal state polycarbonate will scratch more readily than acrylic and it is not as easy to polish out fine scratches. This is undesirable in glazing applications and some polycarbonate is available with an abrasion resistant coating. This coating is much harder than polycarbonate and is brittle. When coated polycarbonate is bent the coating will craze. Stress cracks will also initiate around drilled holes with coated polycarbonate and the coated type is not as impact resistant as uncoated.

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