Sunday, April 4, 2010

Of Acme Threads and Diamond Dust

Skippy – hey Buzz – it’s finally April – nearing Uncle Sam’s initial harvest date – er um Tax Day -

Buzz – yup; lots of snow in the Northeast really hammered costs, productivity and profitability in February and March. Of course, that will all be just a dull memory by the time the tax implications for that roll around NEXT year –

Skippy – we’ve had some questions coming in across a variety of topics, and thought that this one might be of interest to a number of the new users of the “old timers” tooling systems –

“We have a variety of pipe and profile tools that have been around since the dark ages (60’s and 70’s). Our engineers keep designing around them, but although still ‘functional’, we’re having a lot of trouble getting these tools to seal (leakage) and to come apart at ends of production runs. They have something on them called “Acme threads” and we’re wondering why we are having so many disassembly issues?”

Buzz – ah, Acme Threads – strong ‘translation’ threads that were used to help convey large standardized tooling pieces together to assemble flow paths for polymer. These flow paths were on the way out generally from screw tips and breaker plate areas towards spiders/pins/shells for tubing or profile tooling and or adaptation between the gate and custom die lands. Not only good for tooling assembly back in the day, but known in some circles for generating a fair amount of industrial ‘diamond dust’ as well.

Skippy – ‘translation threads’ and ‘diamond dust’?

Buzz – yes; first, the translation of rotational energy (spinning) into horizontal movement allowed the operator to precisely align precision flatly ground faces or sealing surfaces between two pieces of tooling creating a sealed flow path between the two pieces. By CAREFULLY conveying the two pieces of tooling together until the very flat ground sealing surfaces were touching, the tooling could seal between the two surfaces without having to torque tools together with a variety of bolts.

Skippy – sounds easy enough – what sort of problems are these folks facing?

Buzz- well, by design the Acme threads are perpendicular to the flat sealing surfaces and by nature do require having a small amount of clearance in the threads for things to move smoothly. As with all threaded objects used in heated plastic tooling, a bit of molybdenum disulfide or copper-based 'anti-seizing' compound on the threads is always a good idea. Anyway, to continue, the flat section beyond the threads of the male portion of the tooling must extend further than the receiving flat area near the end of the threads area on the female portion so that the two don’t bottom out on thread early with the sealing surfaces not having coming into contact and creating the seal by touching each other. Also it is a good idea that the outside “corner” intersection be undercut and or knocked off at the outside edges of both the sealing face of the male and female portions. This additional clearance allows the two sealing surfaces to make contact BEFORE the corners can connect or binds up prior to the sealing faces touching (which never should).

Skippy – sort of like in the picture where "A" bottoms out against "B" before we run out of threads to look more like "C"? (Click on the picture for a larger view)

Buzz – yes and note further that when the sealing surfaces need to be “freshened’ by grinding flat again, they have to remain perpendicular to the Acme threads and the same rules to “corner interference” have to be observed.

Skippy – sounds like a robust system of assembly – so what about the “diamond dust”?

Buzz – well, Acme threads aren’t the easiest threads in the world to generate on tooling, and require a fair amount of design planning and tooling execution. These kinds of assembly threads are “designed for the ages” and as such, once successfully implemented are committed to tooling that is going to be around for a long time. Often this tooling is made from materials that will ultimately be hardened and treated to become long lasting tooling. Occasionally the tooling will be hardened almost into “spring steel” to be resilient against plastic and operator maintenance as well as ‘abrasive wear’ -

To continue; try as they might, most extrusion companies utilizing Acme threads aren’t able to avoid some small amount of leakages or incomplete clean ups in tooling over the life of these tools – and plastic (particularly something that will burn and degrade like PVC) eventually finds its way into areas of the tooling in or around the flat sealing surfaces and threads. Sometimes, edges are “shoe-shined” during clean up, sometimes a small leak occurs, and a small amount of PVC will turn to carbon and coat a small area inside the threads or clearances area. It is this small amount of carbon flake that may eventually cause a BIG problem.

Skippy – how can a small flake of carbon cause any trouble?

Buzz – well, remember that the goal of the “sealing surfaces” is to be conveyed towards each other slowly until they are in intimate contact – there is no room left in this impossibly small space left between two surfaces that are “flat” and so, plastic even under pressure takes the easier path out – though the intended flow openings in the tooling – not between the two flat sealing surfaces. However occasionally, a couple of circumstances (read moons coming into alignment) can conspire to create a very unique set of circumstances – and interesting by products -

Say that an operator is extremely busy and hurries the process of assembly – rather than slowly turn the tools sealing surfaces toward each other, they insert a bolt into the portion they will be rotating into place and “spins it up” like starting an old fashioned crank on a Model “T”. Beyond the scope of this discussion, threads are ramps enabling accumulation of leverage – and so as the operator spins the tooling, they are progressively adding energy while increasing the speed of the rotating mass; the heavy metal parts spin faster and faster, gaining inertia . . .

form a picture in your mind now, that at the same time, a small flake of carbon, previously hanging innocuously in a corner of the tooling, falls down between the quickly approaching impossibly flat sealing surfaces – and suddenly the moment of ‘impact' occurs.

Skippy – what happens in those milliseconds?

Buzz – well – huge torque is going to be applied from the accumulated inertial mass in the rotating piece. This is applied to the impossibly flat surfaces with a small piece of carbon trapped in between. As the crushing pressure of the accumulated spinning mass begins to focus in the single point of contact represented by the carbon, the carbon begins to be squeezed and smeared. The torque continues to carry the moving tool forward towards having the sealing faces touch, and yet, we still have inertia tearing forward with energy having yet to be expended and absorbed. The carbon, which by now has been transformed into a low quality industrial diamond, is now caught between the face and is continuing to be pushed and ground inexorably onward by forces in the rotating “spring steel” still hurtling forward. The diamond dust is harder than the steel, and as such is encouraged by the onslaught to plough into the steel faces, displacing steel material to the sides and perhaps behind it as it is gouged further and further into the seal face(s). Eventually the spring rebounds, but with not enough force to unlock, having spent some energy during the diamond formation phase, and rebounds back, and then forward again, back and forth like a SlinkyTM until finally coming to rest. Again, all this happens in only a fraction of a second . . .

Skippy – and so here we are with the two faces perhaps near enough but not really touching each other as intended with a small bit of diamond dust, ground like a fallen meteor skipping across a Kansas corn field embedded in one or both of the faces?

Buzz – you’ve got it. Sadly, the damage billing isn’t ready for final tally yet – eventually, we have to take the tooling apart, and it is going to take Herculean effort – not to mention a huge “cheater bar”, perhaps our burliest operator and a heat source to get the steel spring twisted enough toward “off” to encourage the spring to back away by crushing AGAIN over the diamond dust as it is unwound – it is not uncommon during the disassembly to find plastic leakage, and perhaps even MORE carbon trapped in the tooling against the next assembly.

Skippy – the tooling will have to be cleaned up again, and the sealing surfaces are likely to require grinding and lapping to be brought back to “flat and perpendicular” again as well –

Buzz – yes – most houses go through much of this process, without really even realizing it has been going on – you can bet that when you walk into an extruder and look at the sealing surfaces on Acme threaded assemblies and see ‘eyebrows’ gouged into the faces – diamond dust was being made along with plastic parts – and not of much positive value at all . . .

Just our two cents –
Skippy and Buzz

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