Started work on a power riddle

Discussion in 'Foundry tools and flasks' started by Mark's castings, Jun 10, 2022.

  1. This is what I have so far for a vibratory riddle: a 1m x 1m frame with some woven stainless steel architectural mesh welded to it. The frame is 1" x 1.5" steel box section so the sides aren't too high so I can only shovel a bit at a time into the middle but it should be much better than manually sieving the green sand. I'll attach a variable speed DC motor to the corner to couple the good vibrations in two axes and hang it from the back of the shed awning from ropes....should be good enough for a test. The mesh has 6mm holes and is woven from 3mm x 1.5mm flat bar: it cost me a carton of beer a while back. While the hole size is a bit on the larger side the vibrations should give an effective smaller hole size hopefully like it does on mechanical sieves.

    riddle 1.jpg


    riddle 2.jpg
     
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  2. The riddle is now fully welded with the mesh is tack welded onto the frame in addition to having self drilling screws every 10cm or so to hold the mesh to the frame and reduce possible fatigue to the tack welds. I gave the frame and mesh a thick coat of enamel paint to reduce rusting in future. Each corner had a small shackle welded on and it was hung from the shed awning via ropes on each corner: it does couple vibration to the roof so I'll have to add some coil springs to the ropes. There's a treadmill motor driving an out of balance weight on the corner with temporary G clamps reefed up tight. So the thing really works....it sieved some super dry green sand with lumps like concrete due to the excess bentonite of about 10% and the remaining large unsieved lumps move downhill out of the way ready for the next shovel full. There are lumps the size of a small pea in the output sand but they shouldn't be too much of a problem when wet. This is way easier than having to manually sieve sand with my dodgy homemade sieve that causes blisters.


    Input green sand:
    green sand 2.jpg


    Sieved output sand in a matter of seconds:
    output sand 1.jpg


    Riddle frame suspended from ropes:
    riddle setup 1.jpg



    Vibrator treadmill motor setup:
    riddle vibrator.jpg
     
    Last edited: Jun 20, 2022
  3. Tops

    Tops Silver Banner Member

    Looking good. Could you hang a one size smaller mesh underneath to catch the 'peas'?
     

  4. I was just idly thinking today that I could rest sheets of finer mesh on top of the 6mm mesh to get a finer result. A weld on the motor bracket has cracked so today I bought a whipper snipper with a steel flex drive shaft to salvage and hopefully have a remote motor driving an eccentric weight.
     
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  5. Made a start on the vibrator unit which will be a out of balance shaft unit: Two triangles of 15mm thick aluminium were cut to size and bored to 40mm to fit a pair of small tapered roller bearings. I'll size the shaft so that they can bolt either side of the sieve frame and secure the mass with a grub screw. The shaft will have a shoulder to touch the inner race and a bolt on the end with a machined washer against the inner race to set the preload of the pair. The steel flex shaft from the whipper snipper has a plastic sleeve that I'll stuff in some plastic hose to make it flexible and attach it to the motor with a sleeve and grub screws.

    vibrator 1.jpg


    flex shaft 1.jpg
     
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  6. crazybillybob

    crazybillybob Silver Banner Member

    Mark,
    Love the Chinese work boots you're sporting in the photos! :D
    This is a nice build sir! keep up the good work.
     
  7. Thanks for the encouragement, yes the Japanese riding boots are great on a concrete floor, just the right amount of padding. Did some more work today, got the mounting bolts drilled and by pure luck the holes are a very snug fit for the bolt shafts. If it fails anywhere it'll be the bolts or the mounting holes. So the spindle shaft is very snug, needs gentle tapping to move the inner bearing races along the shaft. The shaft has a hard shoulder for one race and a brass cap bored for the 17mm shaft diameter, this will let me set the preload with the cap bolt, the other end is bored with a grub screw to accept the flex shaft....looking good so far, I expect in use lots of Loctite will be needed to keep everything from loosening.

    vibe 3.jpg

    vibe 2.jpg


    vibe 1.jpg
     
  8. So the flex drive and eccentric weight unit is up and running now. There was a lot of vibration coupled to the roof of the shed via the support ropes so I bought some long tension springs and cut them in half to get four springs about 150mm by 25mm in size. They greatly reduced the vibration coupling though the whole thing is quite noisy in operation. There's over an hour of operation time now with the flex drive and the only mistake I made was there's not enough air gap to swing a decent weight: a steel cylinder cut in half and attached with screws to the rotating shaft. This means I have to run some higher revs to get sieving action, I just vary the supply voltage until the sand clods start to jump around. My sand was a bit too wet so I had to use a stick to play "Whack a mole" to break up the larger lumps. So far it works well, about ten times better than manually sieveing sand, maybe a rotating barrel would be better if not quieter in operation with a few weights to break up any balls of sand that would form in a barrel. The flex drive plastic core is now inside some 5/8" garden hose and works fine. The motor is just clamped to a heavy object nearby but will later need an enclosure for dust protection. All in all it works pretty well but will need some time to complete once it's proven reasonably durable. I'm getting black oil off the top roller bearing which I suspect is the outer race flogging out the aluminium plate, so maybe some loctite is in order for it's first clean and service.

    power riddle 4.jpg

    riddle motor flex drive.jpg
     
    Last edited: Aug 13, 2022
  9. Just an update on the power riddle: Today I made a steel stand from bits and pieces lying around and mounted the riddle on a frame at about 1.2 metres or 4' off the ground so I can get a large plastic tub underneath. It's just tied with rope for now but already I notice it's a lot quieter and the stiffer mounting seems to couple vibrations and let the support springs vibrate like plucked guitar strings. I used some polythene sheeting to guide the green sand into the tub and protect from wind blowing it away. It all worked smoothly and let me blend my existing stock of green sand with the new batch of 140Kgs more than doubling the amount. I was able to process about 250Kg minimum in under 20 minutes or as fast as I could shovel it into the riddle. It's a bit of an awkward height but I'll need the clearance once the tub is mounted on wheels to move it. It's slowly evolving and beat sieving by hand by a long shot although a barrel sieve would be even better.

    power sieve1.jpg


    power sieve2.jpg
     
    Last edited: Jul 12, 2023
    Rocketman, Tobho Mott and Tops like this.
  10. Melterskelter

    Melterskelter Gold Banner Member

    Why riddle sand? I do riddle it as a first step in ramming up a mold, of course. But that is not your intent here.

    Denis
     

  11. I'm relying on advice from a guy who used green sand in volume and relied on a power riddle, not mulling to prepare the sand: you sacrifice mulling fineness for speed of processing, so far, riddling gives a fairly fluffy sand that rams down by 1/3rd in volume. In the earlier photos you can see the more than half full bin with dry sand in it, once the moisture level is correct, the same damp sand fills the blue bin completely.

    The riddled sand gets shoveled into the flask and rammed without facing sand or further riddling. I'm getting some finer woven mesh that's specifically used on vibrating tables soon and the sand should be completely lump free. It's a good proof of concept, way better than hand sieving but again a rotating trommel would be even better.

    The agricultural grade bentonite "fines" need about three trips through the riddle in active foundry use before the tiny bentonite lumps disappear where a proper muller would likely break them up in one session.
     
    Last edited: Jul 17, 2023
  12. Melterskelter

    Melterskelter Gold Banner Member

    Well, I will be interested to see how this works for you. I have barely any trommel familiarity and no experience with trommels. But I watched a couple youtubes provided by commercial manufacturers and was impressed with the sophistication of their design. I see that for sorting of material a brush is routinely used to clear the screen. In that application the brush is outside the drum barely poking through the screen. It mkes me wonder though if in your application a second brush inside the drum poking trapped lumps might be useful as a way of breaking them. Another way to break lumps might be to use two long coarse gears that loosely mesh and are spring loadsd. The sand could be dropped through them to crush the hard lumps as a pretreatment. Similarly counterrotating spring-loases drums would pre-crush lumps.

    Here was a very well made commercial (million dollar?) unit. Reviewing its feature list might make a fabricator consider points that might not be evident from the outset. Obviously, it is way more complex than you would need. But, the trommel-specific design issues will be similar.



    Denis
     
  13. That machine in the video does show some good ideas like the external yellow cleaning brush which I hadn't considered. If I were to encounter a large quantity of suitable mesh for free I might consider making a rotating screen drum about the size of a washing machine and put in a few solid rubber balls or similar to break up the inevitable sand lumps. The existing power riddle was made around the sheet of mesh: about 1 square metre, it could really be 60 cm or 2' square and still be big enough. The riddle needs regular attention with a shovel to wipe the balls that form into the mesh to break them up and it has areas where the sand piles up in the corners. That said, it's way better than manually sieving the used sand and easily ten times faster, any bits of metal from casting or the inevitable leaves that blow into the sand get removed by the screen. It helps blend dry green sand ingredients before it gets spread on a tarp and the water added.
     

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