Calculating shrink.

Discussion in 'Home Foundry & Casting Projects' started by diyengineer, Mar 12, 2014.

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  1. Mar 12, 2014 #1

    diyengineer

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    So say you want to cast a 1"x1"x1" cube. How big must you make the sand cast to get as close to this number as possible. Anyone have a good book reference that isn't to technical?
     
  2. Mar 12, 2014 #2

    aonemarine

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    what alloy?? typical shrink is between 1 and 2%
     
  3. Mar 12, 2014 #3

    kquiggle

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  4. Mar 12, 2014 #4

    gbritnell

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    Each material has it's own shrink,iron, brass or aluminum, so therefore each has it's own shrink numbers. When calculating a dimension you have to add shrink, draft and finish and to be safe you have to add a little more. Better to have a touch more than to start over.
    Gbritnell
     
  5. Mar 12, 2014 #5

    Tin Falcon

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    George brings up some excellent points.

    If you want to end up with a 1.00 in cube the casting needs to be bigger than that then you need to add for shrinkage allowance for the pattern.

    IIRC rule of thumb draft is 2 degrees per side or 0.070 bigger at the draft line for a 1" nominal cube.

    Tin
     
  6. Mar 13, 2014 #6

    kquiggle

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    I threw together a quick spreadsheet which will allow you to calculate the thermal expansion for different materials. You can select the material from a drop-down list, enter a length and starting/ending temperatures, and get the results in inches or mm.

    I'm not sure if I got the access permissions right on this, so if someone can try it out and let me know if it works for you I would appreciate it. Any other suggestions for improvement would be appreciated.

    This is a google spreadsheet so if you access the link below you should be able to use it online.

    https://docs.google.com/spreadsheets/d/1iJcmwPv0ejMKzfQp5SV55Wi2T-8CKMmK0MnOVs1OPSA/edit?usp=sharing
     
  7. Mar 13, 2014 #7

    pkastagehand

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    I was able to get the list box to drop and I picked iron, cast and new numbers came up. But I can't seem to change the yellow boxes; initial size, temps.

    Paul
     
    Last edited: Mar 13, 2014
  8. Mar 13, 2014 #8

    kquiggle

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    Paul - thanks for the feedback. After much experimentation and web research I have concluded that Google spreadsheet will not enable the required sharing mode. Consequently, I have created a downloadable spreadsheet (in both LibreOffice and Excel formats); I also made some additional improvements to the spreadsheet.

    You can find the spreadsheets here (scroll down to Thermal Expansion):

    https://sites.google.com/site/lagadoacademy/useful-links#calc

    or more directly here:

    https://sites.google.com/site/lagadoacademy/machining---lathes-mills-etc/charts

    By the way, in answer to the original posting, for cast iron a cube at 2200 deg. F with dimensions of 1.123" will shrink down to 1" at room temperature. Other materials will give other answers, of course (which you can now determine with my spreadsheet!).

    In closing, I will note that I had planned to do something like this spreadsheet anyway, as I am planning to do some experiments with shrink-fitting. This spreadsheet will help me estimate some initial parameters.

    Edited to add this: If anyone downloads this spreadsheet and tries it out, I would appreciate some feedback.
     
    Last edited: Mar 13, 2014
  9. Mar 13, 2014 #9

    Chiptosser

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    Are you sure?

    Don't you mean it to be: .125 per Foot for cast iron.
     
  10. Mar 13, 2014 #10

    kquiggle

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    Chiptosser - thanks for the feedback. I found an error in my spreadsheet (missing 0 resulted in factor of 10 error); I have corrected the spreadsheet.

    Correction: for cast iron (not just iron as previously) a cube at 2200 deg. F with dimensions of 1.0106" will shrink down to 1" at room temperature, equivalent to 0.127"/foot. Other materials will give other answers, of course (which you can now determine with my spreadsheet!).

    Chiptosser, this is closer to your answer, which I assume is based on the rule of thumb of 1/8" per foot. Thanks again for catching my error.
     
  11. Mar 13, 2014 #11

    Chiptosser

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    That looks better!

    That is what the two pattern shops close to me use.

    Now, depending on the surface, more allowance maybe needed.

    If the out side of a part is to be machined, more stock allowance may be needed.

    If it is a vizual surface not needing to be machined, you may want to for allow less.

    Don't forget, for cylinders, to calculate the circumferance and calculate the shrink. That is one easily forgotten.
     

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