79 posts in this topic

"The catch;

There is a problem when you cut a spiral in a tube! After the first cut, the tube relaxes so that the second cut which must be offset
precisely from the first cut becomes impossible because the tube is no longer rigid. So, longitudinal 2x2's are cut to length and screwed to
the inside of the tube to make it rigid."

 

Possible answer.... Putting two, appropiately spaced saw blades on the same shaft, that is, if you are cutting for a two flight helix. I envision using your frame with a piece of all thread attached to the frame as the drive shaft for a carriage that holds a modified skill saw with two appropiately spaced blades. The all-thread and the 16" cylinder are connected via sprockets and chain to acheive the desired carriage travel per inche of cylinder rotation. Acheiving the appropiate travel per travel per inche of cylinder will be the biggest challenge. The first blade has only cut half way around in the cylinder rotation when the second blade begins to cut the second flight, so the problem of tube relaxation and loss of rigidity should be greatly reduced. Also you don't necessarily have to start your cut at the end of the cylinder but could start with a plunge cut an inche or so from the end which would also help with regards to relaxation and rigidity.

Geowizard likes this

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Geowizard,

If you are cutting two flights at the correct pitch, you will use all of the pipe, no waste, and initial pipe can be half the length if you splice the two flights together end to end. Obviously your pitch, blade angle and blade spacing will have to be adjustable variables, as well as carriage travel relative to the cylinder rotation.

RJ

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Geowizard,

Ha! Now I understand. As the saying goes, "A picture is worth a thousand words." My only thought now to keep the cylinder from relaxing and to retain ridgity would be to support the interior of the cylinder. I remember receiving items that I were shipped to me in boxes that had bags in them filled with foam. The foam was the expanding type that had molded around the items in the shipping boxes. I wonder if that may not work for you in this case. In the end you are able to pull the plastic wrapped foam out and use it again if you wanted to try a different pitch for your flights. I've used the A/B mix polyurethane expanding foam. You wouldn't necessarily have to fill the entire interior space of the cylinder. You might put an stove pipe or?? inside the cylinder and just fill the annular space with foam.

RJ

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GW,

The hot wire and PVC rollers are good ideas. On the original idea have you thought about using a router. One advantage to the router is that you don't need  to deal with adjusting the angle of the saw blade and I think, all in all, the end product will produce a smoother finished edge than you would get using a saw.

The advantage of the hotwire is that you could set up several hotwires on the same plane, still using your PVC rollers, and cut multiple flights at the same time by rotating the cylinder through the hotwires. You would probably need a flange on the driven end to rotate and advance the cylinder through the hotwires and when you get to the end where the flange is you just stop the advance but keep rotating until you cut the flights free from the flange.

RJ

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Interesting about the router bit. On the lathe the bits are ground different for plastic for this reason. Talk with a machinist with experience with cutting plastic.

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RJ,

 

I sold my Bridgeport CNC Mill last year. I used the same bits for cutting plastic or steel or aluminum. Cutting speed was the main issue from one material to another. I kept some of the milling bits and lots of nice material to cut. :)

 

- Geowizard

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Abort the notion and hire some labor,  you supply the shovels and crack the whip.

 

Everybody wants to build  perfect machines, ignore the desire.  Keep your mind on the pay.

 

Recovering maximum pay is the answer.

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GW,

I have a 2 ft dia. pipe that I have been thinking of using for a reverse helix cylinder. I am thinking of talking to a local machine shop to see what they would charge to roll a 1/2" square bar into a coil to fit the cylinder. Have you done any checking along this line of thought?

RJ

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If I were to make one on paper as I'll never actually do it with cold hard steel but who says that the helix has to be continous. You could make section's about 2'-0" long and overlap and offset the ends ends while still following the desired helix path. The fall of concentrate from one section to the other may be a plus factor in shaking out the gold,,,smile................. 2'-0" section's could easily be replaced and the short section would fight off any expansion/contraction issues.

 

dick

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GW,

I have a 2 ft dia. pipe that I have been thinking of using for a reverse helix cylinder. I am thinking of talking to a local machine shop to see what they would charge to roll a 1/2" square bar into a coil to fit the cylinder. Have you done any checking along this line of thought?

RJ

There's a guy on another forum who did this using HDPE pipe, or maybe it was PVC.  He cut a section of pipe into a slinky like coil and glued and screwed it to the inside of his trommel pipe.  Can't prove it but I think HDPE is probably more abrasion resistant than steel and might last longer.  Different story if  heavy rocks make it through to the  helix, they would deform HDPE pretty fast.

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Hi Robert,

 

Yes, and there is a youtube video of the slinky method.

 

I have done that. There's an unexpected problem with the slinky!

 

The problem is that when the slinky expands, the coil gets smaller and it "twists".

 

The twist is hard to describe but I will try to explain;

 

The stretching of the slinky causes the spiral coil to rotate. So, looking at the spiral from the end, the normally flat side that would meet the inside of the tube has an angular pitch. It is raised at a significant angle and not flush with the surface. The pitch increases with the stretch.

 

Fabrication becomes almost impossible.

 

Why?

 

There is NO way to press the spiral against the inside of the tube, hold it in position, also hold it flush and drill a hole. You are also having to work against the force of the spiral as it tries to "un-slinky". If you expand the slinky, the diameter gets smaller. You can't expand the slinky, lock down on both ends and begin attaching the slinky to the inside of the tube. It's a 3D possibly 4D fabrication inside of a tube with forces working in every direction in opposition to your objective. 

 

The slinky has to be attached (fastened) to the inside of the tube. I wish anyone that wants to give it a shot the best of luck! :)

 

- Geowizard

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Just how tight(approx spacing between coils)of a helix are we talking about for a given diameter? Now don't give me a pitch /inch type answer as that would just confuse me LOL.

 

I went back and read all of this post and it appears what you are trying to do has already been attempted and in most part failed. I know there are fab shops out there with the capabilities of building this reverse helix trommel as desccribed. If you are convinced this will do what you want it to why not bite the bullet and have it built? I like spending other people's money. smile

 

dick

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Okay then here is a way to "Poor Boy" it. In post #66 I mentioned using helix sections 2 foot long. Go to you local fab shop and get these sections fabbed or do it yourself if you have the equipment at your disposal. Once you have the required number of sections to complete the job be it ( round bar, flat bar. angle, structural "Tee" etc) next make a jig that will hold 2 or 3 helix sections in the correct position on the inside of the trommel and clamp it in place to the trommel with the helix sections captive with in the jig and pressed against the trommel inside wall. This will allow you to drill and tap from outside through the trommel wall  and tap the helix as required. You can then use the last section  secured for jig positioning  and then repeat the D&T operation and jig repositioning until complete. This would be no step for a stepper. NOTE: If you use aluminum for the helix material you could probably use high strength self tapping screw's and not need to "Tap" Now what's the next problem other than the aluminum wearing out but I'm guessing a small one man operation aluminum would last a season. Whatcha think??

 

dick

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There are ways to press the slinky against the inside of the tube using a small bottle jack.

 

Predrill the holes on the barrel/drum/tube at the desired spacing. Line up the spiral - centered on the hole. Jack it into place and screw a short self-tapping screw into place.

 

The protruding screw can be cut off. Screw heads can be countersunk to keep a smooth outside surface.

 

- Geowizard

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True that can be done but without a jig you will never keep the helix aligned to the desired slope/path true.

 

Finding gold is not a problem ,thats a given.

 

dick

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