Pinning Foundry flasks

Discussion in 'Sand Casting' started by HT1, Oct 15, 2020.

  1. HT1

    HT1 Silver

    Olfoundryman let it slip in one of his videos that he recommended a shrink fit for flask Pins,

    I hate to admit this, but while I have shrink fit bearings, I have never had to do all the drilling and sizing , google search lead me to several sites filled with Algebra, I have a shakey HS education, so if one of you college educated Bucks could break it down Barney style that would be awesome!!!
    I intend to use 1/2 inch pins ,
    what size should is drill out the holes to???
    , what size should I ream them to

    ( I understand the Machining, the Math is just insane to me)

    Thanks in advance

    V/r HT1
  2. OMM

    OMM Silver

    Most metal shrinks(or grows) 0.001 per square inch per 40°C.

    So, if you put the half inch pin in the freezer at -20°C you would have about a 0.0005" shrink from room temperature. If you use dry ice at -80°C you would shrink it 0.0015"

    Now if you warmed up the female location with a blow torch to 100°C that hole would get bigger by 0.0015.

    So the process would be use an old half inch reamer that is undersize by 0.001-2" then make your decision if freezing the pin in the freezer will make it small enough. I don't think it will. But warming up the female will do it.

    I done a lot of freeze fitting. I'll see if I can find the video.
    Edit; couldn’t find the video but I found a picture. This is a 3 inch pin that had been soaked in dry ice. The bottom plate was bored to 2.995”. The pin just dropped in and as it warmed up there was a 0.005” interference fit. The frost wasn't on there five minutes earlier when the pin dropped in. That's just moisture being pulled out of the air.

    These two plates of steel have a pin in each corner. The pins have to be so perfectly machined perpendicular to have this slide up and down hundreds of thousands of times. The top bushing is 3.001". These plates are about 7 feet long 3 feet wide. From one end to the other if they're out of parallel more than 0.050 over the entire length, the pins jam. This is why old foundry man suggests C shaping the other female pin location. Unless he has a machine shop doing it for him, if just one pin is not perfectly perpendicular, as you drew the top flask from the bottom it would definitely jam. This is why he C shapes one pin location.
    Last edited: Oct 15, 2020
    Mark's castings likes this.
  3. HT1

    HT1 Silver

    Thank you just what I needed
    but still scary, I think my drill press has way too much runout to pull this off
    I could always go Myfordboy and use loose pins, but that is a little inconvienient for most of my operations

    Professional flasks use adjustable guides for the pins , which allow you to get some insanely tight flask fits without having to sweat a dropped flask binding , it's better then C shape on one side .

    Ive seen several different designs, some are critically tight, making them hard to work with by hand , others are barely better then wooden boxes. it's pretty common to see the guides on one side very loose , and only getting tightened up when a project with tighter tollerences come along,

    technically since 90% of my work is flatbacked, I could use most of my flasks with no pins at all, but that would be embarasing


    V/r HT1
  4. Al2O3

    Al2O3 Administrator Staff Member

    I realize there are all kinds of pin and flask alignment systems but on relatively small flasks there really is no need for there to be more than one close fitting pin and hole. The other can just be a slot and pin. These were my favorites. Simple and so forgiving as far as fit and moving from one match plate to another and set of flasks to another, yet accurate. You do need the one end to have a relatively stiff pin. These bushings fit 3/4" hardened pins that stepped and threaded on one end, so they could be replaced/mixed/matched across flasks. The bushing could just be moved from one match plated to the other weather they were wood or other. I'd think one set would suffice for your purposes. In fact, I would think you could just cut a slot in the shelf on one end of your flask and maybe have a screw on plate to adjust the slot width......same pin diameter on each end.


    HT1 likes this.
  5. HT1

    HT1 Silver

    those are sweet!!! Got source???

    Thanks in advance

    V/r HT1
  6. Al2O3

    Al2O3 Administrator Staff Member

    Pretty sure I bought them from Freeman but that was 20+ years ago and upon a quick look they don't list them of flask at their online store. Upon quick search:

    I'd imagine most foundry supply business (like Lancaster) would have them.....just a matter of how much work to ID them and whether they'd sell to you without an account. Sorry, I've been out of circulation too long. Maybe a little better internet sloothing could turn up a set for you?

  7. Math aside, doing shrink fits on a drill press will be dicey. You need to hold accurate hole size within tenths.

    A couple thoughts: most drill bits drill oversize if you simply chuck it up and drill a hole. But if you first drill a hole a 64th undersized, then without moving the work chuck up the desired bit and finish the hole, the bit behaves like a reamer and will make a hole much much closer to the nominal size of the bit.

    The coefficient of thermal expansion of Aluminum is close to TWICE that of steel.

    For a 1” hole 40 deg C in steel will give a .0004” expansion and in AL will provide .0009” to mix metric and Imperial systems.
    HT1 likes this.
  8. Billy Elmore

    Billy Elmore Silver

    On some of our old Disamatic machines we had to freeze shafts over night and heat the bushings they slid into to get them to go together. If you held the shaft in your hand for a few seconds it would not fit anymore. Crazy tight tolerances! Swedish and Danish engineering at its finest!
  9. For those new to shrink fits, doing shrink fits is more than just cooling a shaft and warming a bushing or housing. You have only seconds to mate the two parts. You have to have things lined up RIGHT and it helps a lot to allow some lead in tolerance to get the assembly to match up. Then you have to have a stop set so that the shaft drops right in to the desired depth immediately. There is no tolerance for fiddling to get the depth right. As soon as the hot part touches the cold part things start to seize up. Shrink fits have their place for sure. But, unless one has proper machine tools and experience, I'd avoid it. I've done a modest number of shrink fits and I can tell you it is always a bit nervous-making as having any hang-up means you'll be machining the shaft out of the housing or bearing or you will be grinding or turning the bushing off the shaft. No fun.

    In addition if folks are shrink fitting steel shafts into aluminum flasks, the differences in expansion can easily mean that a nice solid shrink fit at room temperature of the dissimilar metals will loosen up when the flasks heat up a couple hundred degrees or more after pouring.

    Using set screws, pins, staking, or threading might be better suited for most of us.

  10. HT1

    HT1 Silver

    Knowing all that I wonder if that might be some of the reason the OLfoundyman's next video is delayed
  11. Chazza

    Chazza Copper

    An interference fit is much easier to make; as a rough rule-of-thumb allow the male part to be .001" bigger than the hole for every inch of diameter. If things seem a bit sloppy, use Loctite shaft and bearing mount to hold it in place,

    Cheers Charlie
  12. Billy Elmore

    Billy Elmore Silver

    Honestly though...making things that tight will make it more difficult to draw. We used to have to set up all the snaps and every molder wanted their snap set up a different way. As long as the snaps kept the molds aligned and the castings did not have shift we let them dictate how they wanted them. We used double round pins that we could slide the bushing in and out to loosen and tighten as much as we wanted.
  13. Sounds like an interesting idea, but I really cannot imagine the setup. Pictures or diagram would be helpful.

  14. Billy Elmore

    Billy Elmore Silver

    I'll try find the one we kept and take a pic..if I cant find it I'll draw it. We made the guides ourselves due to wear. Its hard to imagine someone lifting a 50 pound mold so often it wears out steel guides but they did. We replaced them with in house made cast iron guides that the guys say woked better.
  15. Billy Elmore

    Billy Elmore Silver

    Here is a pic off the web similar to what we used. upload_2020-10-19_17-42-12.png
    Melterskelter likes this.
  16. Sorry to be dense, but what is the advantage of two pins vs a single pin? Either could have an adjustable bushing.

  17. Rocketman

    Rocketman Silver

    Two pins is more reliable at keeping the matchplate from rocking when withdrawing it from the mold, for a cleaner draw
    Melterskelter likes this.
  18. Billy Elmore

    Billy Elmore Silver

    Two pins keeps the plate from tilting side to side when drawing. We also had the squeeze jolt machines which helped keep everything stable and the heavy jobs ran on the roto lift molding machine which lifted the molds and let them rotate on round pins. All machines used a foot controlled pneumatic vibrator while drawing. I will go out to the old storage shop later and see if I can find some of the old equipment.
  19. HT1

    HT1 Silver

    You Civilians, In the Navy we hand rammed, lifted and flipped snap flasks, even the light weight Mag's where Manly, and that was not what we had, the Heavy duty so they would not wear out
    I'll admit I would not want to have to do 14X14 in those anymore. I'm considering pricing a set of Fremont or Hines 12X12 with 3 inch drag and 4 inch cope, honestly I'm afraid they will quote me a grand for a single flask.

    I can burn up alot of wood flask before that becomes a money saver

    V/r HT1
    P.S considering ancient beat up snap flasks are going for $200 on Ebay, I'm really scared of new flask prices
  20. DavidF

    DavidF Administrator Staff Member Banner Member

    Don't know why your worried about it when your more than capable of making your own.
    Now if your making so much money that its cheaper to buy them than to spend your time on them.......;):D

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