Reinforcing 12 Inch deep pattern for green sand?

Discussion in 'Sand Casting' started by Mark's castings, Oct 9, 2021.

  1. Melterskelter

    Melterskelter Gold Banner Member

    Mark, if you have the cover off the worm, it would be very interesting to see a pic of that worm. I am curious to see the angle at which the worm teeth crests intersect the rotational axis of the worm. I should probably be able to deduce that simply knowing it is a 28:1 drive, but that is too much thinkin. :)

    Denis

    Added: I got curious to see if it this self-locking feature was discussed somewhere and found this site:

    https://roymech.org/Useful_Tables/Drive/Worm_Gears.html

    If you scroll down a ways, you will find that self locking is laid out in a section and graph by that heading. Not surprisingly it is dependent on both the lead angle of the driving gear and the friction coefficient between the driving gear and the driven wheel which often are different metals like bronze on steel. Interestingly, steel on steel has the highest friction coefficient of the combos discussed. The Dutton Lainson WG2000 uses steel on steel and a 40:1 ratio. That is the winch I use. I think it is also true that the finer the pitch of the driven wheel the lower the lead angle of the driving gear and the less freewheeling tendency. One other factor that could cause a fine-thread -driven-gear winch to have reduced self-holding would be multiple-start threads on the driving worm. Like I say, lotsa thinkin…
     
    Last edited: Oct 28, 2021

  2. I did look at dismantling it but I'm not sure how to do it without damaging it as the housing is a single casting, maybe extract the input shaft and punch the wheel so it displaces the rubber seals. The rubber seal is 1 & 7/8" diameter so that would be the maximum wheel size. The NMRV-30 is the smallest standard size gearbox you can buy.

    Edit: the input shaft has a rubber cover on the far end so it should be possible to remove that and gently punch the shaft and bearings out to free the worm wheel.

    Edit 2: The output shaft can be removed and installed from the opposite end to make a left hand unit, that would let me have two gearboxes on the spool shaft: twice the lifting power and hopefully twice the load holding capability.

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    Last edited: Oct 28, 2021
  3. Melterskelter

    Melterskelter Gold Banner Member

    Mark, I was keying while you were keying. See added portion on my post just above.
     
  4. The matchplate pattern is almost ready, hours of filling and sanding with three coats of epoxy resin on the plywood with the primer now drying before a fine sanding to smooth the surface out.


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    Last edited: Nov 1, 2021
    DavidF and Tobho Mott like this.
  5. The flask is taking shape, the 75mm/3" thick cope on top still has to have some reinforcing bars welded in to keep the core in place. I did some quick calculations about the weight of the green sand, if the green sand has a specific gravity of 2.0 then with a volume of 0.0706 cubic metres the weight will be 141 Kilograms :eek:. I have an 80:1 worm drive on the way to replace the 28:1 unit so at the least it will lift the drag off the cope and let me get the pattern out.

    flask assembly.jpg
     
  6. mytwhyt

    mytwhyt Copper

    On your cabbing machine, a diamond dish would go well next to the diamond wheel..
     
  7. A wheel with a concave 2 inch wide rim?.

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  8. mytwhyt

    mytwhyt Copper

    I forget how long it's been since I did any cabbing.. A concave wheel is a much better idea than the dish shape I remember..
     
  9. It'll have a couple of 8" flat discs on the end which have a soft padded surface and then something like an abrasive charged leather or cloth so it has a fair bit of give or compliance. If the flat discs or the wheels were hard (like plated or sintered wheels) then it's much harder to use as it tends to put flat facets on the gemstone rather than rounding of the surface
     
  10. mytwhyt

    mytwhyt Copper

    You got to turn the stone constantly to not put a flat spot on it when working on a flat wheel.. I always used the leather disk to polish whatever I was cutting.. A little tin oxide mixed with water to charge the damp leather disk.. Tin oxide will polish most stones you cab..
     
  11. The Ali Express 1:80 worm drive turned up today, I had to specify a 9mm input bore and once I removed the coupling off the motor shaft it turned out to have a suitable keyway and key already fitted. The motor flanges were incompatible the old was 6 bolt and the new was 4 bolt to the gearbox but I was able to modify it so the motor could bolt on by drilling the 5mm threaded flange holes out a bit so the bolts could go into the motor threaded holes. So with a bit of grunting it was back up on the shed purlin and lifting 40Kg of steel at a stately pace and then holding it fine with no unwinding of the gearbox. I've knocked off for tonight but I'll see how much it can handle tomorrow. No photos this time as the gearbox is physically identical to the old one.

    $60 Australian delivered:
    https://www.aliexpress.com/item/100...37-0&pdp_ext_f={"sku_id":"12000021368829127"}
     
    Chazza likes this.
  12. So I've been trying to ram up the sand mould today using the matchplate pattern and the outside mould is perfectly fine, the inside sand is breaking off at the bottom, first time it was too loosely rammed, second time round I'm not so sure. Both times it broke at the two horizontal steel reinforcing bars so they may be contributing to the breaking. I'm cutting a vertical hole with a tube that could contributing to the break when I push down to the pattern, also the pattern may not be smooth enough down the bottom of the hole and the rough finish is gripping the sand too well.

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  13. Melterskelter

    Melterskelter Gold Banner Member

    So your procedure is to pack the cope and then bore the sprue? If so, indeed the downward force may be stressing the sand. However, I would think you would see a blowout pattern at the sprue cut if that were true. Still may be contributory.

    How close do those gaggers come to the drag surface? From the pic it looks like 1/2" or so. That may be too close. If they came within 1.5 to two inches it would be close enough. They may be causing a fault to develop in the sand. They also may conduct rapping vibration down to that level. If anything, I think I find myself getting into trouble from too much rapping as opposed to too little most often.

    Most likely trouble maker is some roughness or undercut of the draft at the bottom. I am often amazed to find such defects when sand just won't behave. I get the flashlight shining tangentially and, sure enough, there is some small defect. Those deep down fillets could be great candidates for being cast in epoxy. Gravity and surface tension don't lose their concentration and focus. They always cooperate make smooth uniform "perfect" fillets. A very generous fillet in that area is very desirable. I love casting fillets. I just cock the pattern 30 to 45 degrees, pour (usually with a syringe for easy appliction) and watch as the epoxy flows out smoothly. A few bubbles will form and I zap them with microbursts from my gas torch. 8 hours later---a smooth, uniform, and, in my opinion, beautiful fillet. Then repeat for the next intersection, etc. The only down side is the time delay which can be mitigated in a 90 to 95 degF room.

    All that said, you are very close to success. Nice looking flasks, spreader bar, and lifting setup.

    Denis
     
  14. Hi Denis, it feels pretty close to me too, the horizontal bars are an inch off the bottom of the pattern at the moment. For now I'll re-sand the pattern in that area as the primer had some texture to it after spraying, then I'll graphite it again put down a layer of cling wrap film on the bottom, last of all I'll cut the pouring hole after removal of the pattern. The lifting rig is great, if I did it again though I'd use a 40 size or larger worm drive.

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  15. Petee716

    Petee716 Gold Banner Member

    I had a situation once where I had to hold a large sand core in the pattern from below whilst I lifted the pattern. I did it by means of a slightly undersized piece of 18ga sheet metal under the core attached to a wire that went up through the top of the core and protruded through the top of the pattern where I tied it off. By all means see if you can overcome your breakage problem by reducing friction, undercuts, tight fillets, etc, but if all else fails you might consider placing a thin plate at the bottom with a wire, strong cord, or even a rod welded on to aid in the core lift.


    Pete
    PS: super nice lift!
     
  16. That sounds like it would work every time!, I'll try it if the other ideas don't pan out
     
  17. So I did find some internal roughness in the pattern and sanded it with 600 grit and re-graphited the surface again. I phoned Peter my foundry Guru and he suggested flipping it over before removing the pattern so gravity tends to hold stuff together rather than pull it apart. Doing this let me rap all over the pattern with a rubber mallet until I could hear the hollow sound of a small gap and it took a fair bit of rapping to do it. After that it was almost an anticlimax: the pattern slipped off easily. Cutting the pouring hole caused several cracks even when placing it in the centre of the sand, it's almost certain to have caused the earlier failures. I'll have to make a suitable removable pattern rather than cut the sand.


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    Last edited: Nov 25, 2021
  18. Melterskelter

    Melterskelter Gold Banner Member

    Very nice! I can guess at just how good it felt to cleanly pull that cope. That inverted pull occurred to me, but then there is the problem of making a controlled flip of the cope so you could drop it into the drag. How did you do that?

    Denis
     
  19. I was worried it would break off until I realized I had all the steel reinforcement inside it, I had both hands on it to flip and steady it, one toe on the up-down switches of the control box.
     
  20. Melterskelter

    Melterskelter Gold Banner Member

    So, once you had the cope sand out of the pattern and it is standing narrow side up, I was wondering how you flipped it so it was hanging narrow side down and ready to lower into the drag. It is the flip that seemed difficult as there is little on the cope to hang onto while it is flipped.

    Denis
     

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