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IndustryArena Forum > GENERAL MANUFACTURING PROCESSES > MILLING > my milled wax blank is warped
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  1. #1
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    May 2019
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    8

    my milled wax blank is warped

    I know warping can be caused by internal stress, but not sure if that applies to wax (in this case green File-a-wax)
    The part is 10x80x140mm. I taped it down with ~4mil double-sticky tape. I faced it with light feeds and speeds by hand on a Taig. Not it's not a noodle! My similar aluminum plates are not warped.
    The warp is a centered bow. The bow in the finished part is about 5mil, totally unacceptable.
    I tried to flatten it with heavy weights on a 3D printer bed that I raised to 70 c and slow cooled to room temp. That seemed to work at first but then the bow came back by the time I tested it on my surface. I didn't want to go higher temp because if it starts to melt, it's ruined, and I can't find any info on the glass-point of machinable wax anyway..

    The only thing I can think of (other than residual stress) is the squishiness of the tape because it will squish more from downward pressure at the edges than the center. But the tape can't squish that much.

    This is recast wax btw, but the wax was fresh blocks because I needed a bigger plate than my inventory. A bread-loaf baking pan with Baker's secret coating completely releases on it's own, it just falls out because the shrinkage is so high (I slow-cooled that overnight also). From that I band-sawed the pre-blank stock to start with.

    Any ideas would be appreciated. I could start doing experiments but that would get tedious fast.
    tbanks

  2. #2
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    Jul 2018
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    1700

    Re: my milled wax blank is warped

    Hi SP - Yes it's certainly internal stress. This is usually due to how the casting freezes directionally. It freezes from the outside in and creates internal tension and compression due to the shrinkage of the wax over time and distance. Same happens with metal castings. My wife casts beeswax and it freezes slowly when cast in big blocks. Stress relief is usually done in metals by bringing it up to a softening temperature and holding to allow the weakened material to stretch and settle. The issue with wax is thats its an insulator so by the time the middle is warm the outside has melted. Maybe a microwave oven is the go as this will heat from the inside out but it would be touch and go on melting... We melt beeswax in a uwave and it melts from the inside out like a slab of butter nearly same thing. Good luck - Peter

    Aluminium plates can warp after machining. Tooling grade plates are stress relieved thermally or stretched. Some rolled plate grades like 5083-H32 can warp due to the internal stresses due to its rolling creating stressed skins on the outside of the plate. 6061-T6 being quenched and aged will have little internal stress. If you over constrain billets when blocking they can warp, then you have to correct them or stress relieve them. All interesting stuff...

    In metals the usual rule of thumb for stress relief temp is half the melting temp so steel melts at ~1400C so roughly SR=700C, 650C is a typical SR temp. So if this holds for wax "fileawax" melts at about 116C so 58C is the go? In the kitchen oven? We melt beeswax at 60-65degC...

    soak time for metal is 1hr per inch thick, I expect wax to be more than this as its less conductive...

  3. #3
    Gold Member
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    Apr 2004
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    5108

    Re: my milled wax blank is warped

    How was the wax blank cast? Did you melt it in a pot and pour it into a mold? That maximizes internal stresses, since the exterior cools first at a rapid rate, but then insulates the center as it cools and shrinks radically (as you've noticed). Try this - instead of melting it in a pot, put the wax scraps in the pan and put it in an oven. Use the minimum heat that will melt it. Then let it cool down very slowly, without removing it from the oven. See if the blank you saw from that block warps less, and report your results back to this thread. .
    Andrew Werby
    Website

  4. #4
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    May 2019
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    Re: my milled wax blank is warped

    thats for the replies! (sometimes you don't get any)

    After tons attempts to machine the exact mold pocket, etc, I discovered that this wax has a tremendous thermal expansion coefficient. I ended up putting it into a bread-loaf baking pan (no chance of overflow and ZERO adhesion on that anti-stick coating) and cast about an 1", although I only need enough to cut down to 10mm. That left a rough slab much bigger than the finished stock I need but hey, I can recycle the parts cutoff with the bandsaw anyway. So, I put it in the oven for a number of hours at maybe 200 F? I kept it a significant margin below the flash point of paraffin. Then I just turned off the oven and left it overnight. It shrinks what looks like 1/8" or more from the sides so it is literally sliding around in the pan when I take it out.

    I realized this morning that it won't be as much of an issue because I'm going to drill bolt holes and bolt it down in four locations which will flatten the part. At some point as I machine it will weaken the clamping forces and spring back some, but it's wax! The final aluminum parts wont' have that warp (the 12mm 7075 aluminum blank I mention below is the first blank I will use and it is <1mil flat)
    I tried the 3D printer bed process again but going to 100 deg C for several hours. Warp still popped back! I'm just going to live with the bowing I guess.

    Anyway, thanks for the feedback, I think I have a road forward, at least for this project.

  5. #5
    Gold Member
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    Apr 2004
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    5108

    Re: my milled wax blank is warped

    So you aren't planning to cast from this wax blank, it's just a practice run? In that case, don't worry about the warpage. Did you find that the blanks you made in the slow oven worked better than the first time?
    Andrew Werby
    Website

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