Boundary Layer (Tesla) Turbine Mark-II Page

The Tesla Boundary Layer Turbine, Mark-II

bigger! prettier! maybe even better!!

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What It Is?...

(c.a., January 2008)

I've been pleased enough with how my six-inch turbine works to start thinking a while back about building a bigger one. In "Tesla lore" there seems to be a near mythical ability turbine with a, depending on where you read about it, 11.5" or 11.75" diameter runner. So, I decided to make one with 11.625" runner discs. And, in fact, began working on it some time before starting a web page to describe it.

This is still an air powered, test bed device, so, although much prettier than the six-inch unit, includes features that make it easy to disassemble and reconfigure. The most significant feature not found on the six-inch turbine is a slotted shaft which allows testing different styles of runner discs and star washers without having to do a complete "glue up" of another runner. The discs and star washers are made with key tabs that slip into the shaft slots, and the runner stack is clamped together by nuts on both sides, which are screwed onto the treaded ends of the slotted portion of the shaft.

Complete construction and testing information for the Mark-II turbine will be made available as work progresses. But, before starting this page, I decided that putting all the construction technique details onto one page along with theory and testing results was getting in the way of telling the story in an easy to follow manner. Hence, I established my Miscellaneaganza! page as a place to collect all construction technique details for reference. The page you are reading now will present the major construction steps of the Mark-II turbine, with the bulk of the details made available via links embedded in this page's text that will open related sections of the Miscellaneaganza! page in separate browser windows (or tabs, depending on your browser configuration) for those interested in more information. That leaves this page free to concentrate on theory, testing, and analysis results without losing them in the details. Future work on my other pages will move towards this presentation style as well, and utilize the Miscellaneaganza! page for details.

Major Construction:

Volute:

(c.a., December 2006)

The Mark-II turbine volute was formed by heating and bending a flat 2.5" wide, 0.091" thick, approximately 38" long acrylic plastic strip into a ring. The plastic strip was bent around a turned, laminated wood form 2.75" thick with an 11.75" outside diameter. The turned ring was screwed onto a 12.5" square piece of 0.5" thick plywood; providing a base to align an edge of the plastic strip against as the strip was bent.

(c.a., May 2007)

Heating the plastic for bending was accomplished using a 1500 W heat gun with an attached wire whose length indicated the distance to hold the gun from the plastic to achieve the appropriate no-rebound forming temperature. The forming temperature distance was calibrated using an oven thermometer. For cast acrylic plastic sheet of the Plexiglas™/Lexan™ variety used here, the no-rebound forming temperature ranges between 320° F to 356° F [1]. Here, the calibration distance was chosen to select approximately 325° F.

Working with the end of the indicator wire next to, and the wire perpendicular to, the surface of the plastic strip, the "calibrated" heat gun was used to maintain the forming temperature at the surface of the strip during the bending process. As the bend progressed around the forming ring, the plastic strip was lightly clamped in place with small wooden blocks and c-clamps to allow cooling without deforming.

The volute ring was closed by clamping its overlapping ends onto the wooden forming ring, cutting though the ends with a thin-bladed hacksaw, and filling the resulting saw-kerf gap with 5-minute epoxy.

Edges and Sides:

(c.a., May 2007)

To give rigidity to the Mark-II turbine volute ring, and provide a place to mount the turbine side pieces, two acrylic plastic rings were turned from 0.236" thick material to have the same inside diameter as the outside diameter of the volute ring and an outside diameter approximately 2.5 inches greater than that. One was attached to each edge of the volute ring using high-strength epoxy glue.

Prior to turning, the rough cut plastic blanks used for the volute stiffening rings, as well as those used for the turbine side pieces, were aligned in a jig and had size 6-32 threaded brass inserts, (the type used for providing metal threads in wood), embedded in 5-minute epoxy in six holes equally spaced at 60° drilled 0.625" inside the final diameter line scribed on the blanks. Before attaching the plastic blanks to be turned, the alignment jig was mounted on the same turning fixture used to spin the wooden blank turned into the volute bending ring.

To ensure alignment of the brass inserts between each plastic blank, the inserts in the other blanks were installed with the first blank in place on the alignment jig, and the alignment screws threaded though its inserts. With the second set of inserts thus aligned with the first, the second set was epoxied in place.

With threaded inserts installed in the two ring blanks, and 6-32 thread holes drilled in alignment with the inserts in a third blank to be used as a turbine side plate, the three blanks were mounted on the alignment jig, with the two ring blanks closest to the jig face.

Before placing the side plate blank on top of the stack, the paint stirrer compass was used to scribe a centered circle on the exposed ring blank face of the same diameter as the outside diameter of the turbine volute ring. The side plate blank was then set in place, the blank stack screwed to the alignment jig, and the compass used once more to scribe a circle defining the finished stiffening ring outside diameter on the exposed side plate blank face. With turning marks scribed, and all three plastic blanks attached to the alignment jig, as a unit, the alignment jig and rough cut plastic blanks were all trimmed using a jigsaw to just outside the visible scribed circle. With that the alignment jig was mounted in a lathe, and all three blanks were turned down simultaneously to their final outside diameter. After turning, a small registration hole was drilled through all three blanks near and slightly inside the radius of one of the 6-32 thread mounting screw holes as an alignment aid for later assembly processes.

Removing the completed side plate blank from the alignment jig and reinstalling the attachment screws through the stiffening ring blanks allowed the two ring blanks to be simultaneously bored on the lathe to identical inner diameter, given by the previously scribed turning limit circle on the outer ring blank face.

To facilitate testing different width runner assemblies, a side plate small enough to slip inside the turbine volute ring was turned on the alignment jig. The outer edge of this side plate was grooved accept an o-ring, (made from thin automotive vacuum hose), to seal it against the inside of the volute.

(c.a., June 2007)

After construction of the stiffening rings and side plates, a stiffening ring was attached to each edge of the turbine volute ring using high-strength epoxy plastic weld glue.

(c.a., July 2007)

With the volute stiffening rings permanently in place, a set of stand offs was constructed to fix the distance the small side plate sits inside the turbine volute ring.

A second full-size side plate was also constructed. Assembly for both the small plate and the second full-size plate included installing 6-32 threaded brass inserts.

Basic Base:

(c.a., May 2007)

The Mark-II turbine base is basically an all wood construction. The turbine volute ring sits on two commercially obtained 8" long pinewood shelf brackets, modified into volute mounting brackets by making rounded cutouts in their corners to fit the volute ring. The volute ring brackets are mounted on a sheet of 0.5" thick commercial laminated pine board 24" long by 18" wide.

(c.a., June 2007)

A close fit of the volute ring to its mounting brackets' cutout areas was accomplished in two steps. First the cutouts were sanded smooth and square using a drum sander mounted in a drill press. Second, a thin layer of sawdust-and-glue mix type wood filler was applied to the upward facing curved cutout areas, strips of waxed paper laid over the wood filler to prevent sticking, and volute ring pressed down into the filler to form exact fit surfaces on the mounting bracket cutouts. This process set the final distance of the bottom of the volute ring off the turbine base plate to approximately 2".

Alignment:

(c.a., July 2007)

A temporary axle was used to facilitate aligning the Mark-II turbine axle support uprights. Prior to using the alignment axle, the base assembly had to be prepared for installation of the axle supports. That process included final placement of the volute ring mounting brackets on the turbine base plate, and alignment both vertically and horizontally of the volute ring relative to the turbine base plate, as well as cutting pockets for insertion of the axle support uprights into the turbine base plate.

Going straight:

The volute mounting brackets were aligned with a straight edge and the pair set centered on and screwed to the turbine base plate at the proper distance apart to accept the volute ring. Two identically marked spacing boards were used to align the volute ring horizontally on the base plate, and a carpenter's square was used to align the volute ring perpendicularly to the base plate. A thin coat of sawdust-and-glue type wood filler was smeared over the volute mounting bracket support surfaces, the filler covered with waxed paper to prevent sticking, and the volute ring set in the alignment fixtures and pressed into the wood filler to create the final aligned volute mount surfaces.

After the wood filler hardened, the volute ring was marked for drilling four mounting holes over each volute mounting bracket support surface with a pattern to make it impossible to mount the volute ring other than as it was oriented at the time the marks were made. Without moving the alignment fixtures, the volute ring was lifted from its mounting brackets, and, using wooden soft jaws to hold it in a large vice, the marks on the volute drilled through and counter sunk for #6 flat head screws. The volute was set back in the mounting brackets and alignment fixtures, and the drilled holes in the volute used to mark the mounting bracket support surfaces for drilling pilot holes for the volute mounting screws. The volute was again lifted from its mounting brackets so the mounting bracket pilot holes could be drilled, then set back down once more and screwed in place with #6 flat head brass wood screws. With the volute ring mounted in place, the alignment fixtures were removed.

Stiffen up:

(c.a., September 2007)

To add rigidity, and provide support for the turbine runner axle uprights, the volute ring and its mounting brackets were removed and the turbine base board, and the base was ringed around its bottom side with 4" wide pine board 0.5" thick. All four corners were then rounded to a 3" radius using a jigsaw followed up by a drum sander mounted in a drill press, and the top edges shaped using a 0.25"x0.625" bearing-aligned Roman Ogee bit mounted in a woodworking router. The mounting brackets and volute ring were then reattached to the reinforced base

Finding your center:

After the volute ring was aligned on the turbine base, a geometric process, involving cutting a cardboard disc to fit the volute ring and marking the disc with perpendicular lines through its center, was used to find a line on the turbine base directly below the center of, and parallel to the face of the volute ring with a carpenter's square. This line provided an index for setting the axle support uprights in place.

During the center finding process, the perpendicular lines on the cardboard alignment disc were used to put vertical and horizontal center-line marks on the volute stiffening rings. As an aid for later alignment of holes in the axle upright supports, one of the horizontal marks on the stiffening rings was used to mark a square-cut piece of cardboard to indicate the height of the geometric center of the volute ring above the turbine base plate.

Cutting in:

To mount the turbine runner axle support uprights, the volute ring and mounting brackets were removed, and a pocket was cut near each side of the turbine base plate, of the appropriate size for a close fit on the end of a 4" wide by 0.75" thick oak board. Lines for the pockets were marked so that their inner edges would align with the inner edges of the under base stiffening boards on either side of the base plate and their long edges would be perpendicular the line previously drawn to indicate the projection of the center of the volute ring onto the base plate.

A router jig was constructed and used to cut the 4" by 0.75" pockets, using a plunge router with a 0.375" straight cutter. The jig was aligned to set the 4" sides of the pocket perpendicular to and centered on volute ring center line, and also aligned with the previously drawn edge alignment marks. This set the wide faces of boards inserted into the pockets to be parallel to the faces of the volute stiffening rings, as well as directly opposite from each other and centered on the center line of the turbine volute ring. The pockets were cut to a depth of approximately 0.25" into the under-edge stiffening boards. A thin, sharp chisel was used to square the rounded corners of the pockets for the final fit of the uprights into their pockets.

After the first pocket was cut, the square-cut cardboard piece with the volute ring center line height marked on its edge was used to determine the proper length for the axle support uprights, and two appropriate length pieces cut from the 4" by 0.75" oak board. The jig was then aligned on the other side of the turbine base plate and the pocket cut for the second axle upright.

After the router jig was removed, the axle support uprights were shaped to their final form and inserted into their base plate pockets. Then, the volute mounting brackets and volute ring were reinstalled on the base plate. The volute mounting brackets were permanently attached with glue and screws, but, the uprights were left free to be removed.

Inner piece:

For preliminary axle alignment, two runner discs with 0.5" diameter center holes were cut from 0.091" thick acrylic plastic sheet and mounted on a 0.5" diameter US standard thread rod temporary axle, with their outer faces separated by approximately 0.5" less than the distance between the outer faces of the volute stiffening rings. With one axle support upright removed, the runner assembly was centered in the volute ring on thin cardboard spacers and positioned on the threaded rod axle so one end of the rod touched the face of the remaining support, allowing the location to drill the axle hole in the support to be marked around the end of the axle shaft. The procedure was repeated for the opposite side upright, and the marked hole positions drilled to 0.5" diameter. The two disc runner assembly was then centered on the temporary axle rod, and the axle rod installed in the axle support uprights to check the preliminary alignment.

For final alignment the runner assembly needed to be able to spin freely in the volute ring. To that end, the ends of the temporary axle rod were turned down to fit in 0.5" outside diameter brass bushings inserted into the 0.5" holes drilled in the axle support uprights.

With the axle able to spin freely, the axle support uprights were clamped to square cut blocks, and the blocks clamped to the turbine base plate. The axle supports could then be tapped with a rubber hammer until the runner discs were able to spin freely in the volute ring without rubbing. Then, since the axle support pockets were cut to align with the inner edges of the under-base stiffening boards, brass shim material could be inserted under the uprights to set their position, and screws used to fix them in place against the shims.

Finishing:

(c.a., September 2007)

The finishing steps for the basic Mark-II turbine assembly included applying a dark mahogony stain and polyurethane varnish finish, adding rubber feet to the underside of the base, changing the assembly screws and washers to brass hardware, adding black-iron gussets between the axle upright supports and the turbine base, and making locking collars to use on the axle ends to retain the runner assembly within the volute.

Polishing:

(c.a., June 2007)

As a rule, projects involving machining and glue-up are fraught with danger for shiny acrylic plastic surfaces. And, constructing the Mark-II turbine volute was no exception. Once the volute ring and stiffening rings were formed and glued together, there were a number of dings, scratches and globs of glue that needed to be cleaned up. In fact, for a number of reasons, the volute surfaces were particularly badly damaged, and required more effort to restore than might normally be expected for a plastic working project.

Restoring the surfaces was a four step process. First, a small sanding drum in a Dremel™ tool was used with 80 grit sanding cylinders to knock down the worst of the excess glue and also to feather out the more serious gouges. After grinding, the surfaces of the volute were dry sanded by hand, stepping up through several successively finer grades of paper ranging from 80 grit to 340 grit.

Once grinding and dry sanding was completed the next step in refinishing was wet sanding. The wet sanding process moved though several grades of paper ranging from 600 grit to 1600 grit. An electric orbital sander was used for most of the wet sanding. It was necessary to make an extended paper mounting block to gain access to the space between the volute stiffening rings with the electric sander.

Once the final grade of wet sanding was completed and the sanding residue cleaned off, the volute ring was smooth and clear, but, did not have the luster expected of an acrylic plastic surface. To bring back the shine two grades of stick-type rub on polishing compound (#3 and #6) were applied to the volute and polished with muslin buffing wheels chucked in an electric drill. And, finally the volute was cleaned and polished with spray on acrylic polish.

Slotted Axle/Keyed Runner:

(c.a., December 2007)

The key-slot axle and keyed runner discs were constructed using a series of jigs employing a Dremel™ tool router with a 0.25" straight cutter. The central element of the key-slot axle assembly is a length of 0.375" diameter US standard threaded rod, which acts as the axle between the axle support uprights. The key slots were cut into a 0.825" diameter 5" long PVC rod, turned from a rectangular PVC block, which was drilled and threaded through its center to allow it to be screwed onto the 0.375" threaded axle rod.

A set of wooden soft jaws were constructed for working with the 0.825" PVC rod in a vice, and utilized first in threading the ends of the rod for 0.825" US standard nuts. The threads are for nuts to compress and retain runner elements on the PVC shaft once the necessary slots and keys have been fabricated.

Again using the soft jaws, a length of 0.375" threaded rod was screwed through the 0.825" PVC rod by turning a wrench against a set of double nuts on the threaded rod. This made the PVC rod ready for slot cutting.

The first of the key slot jigs was a set of guide rails constructed from precision cut hardwood strips that the Dremel™ router, mounted on the arc-cutting jig used in constructing the six-inch turbine, could be slid down, aided by a pair of aluminum angle material rails mounted on the bottom of the arc-cutting jig. This assembly allowed cutting a 0.25" slot on center down the 0.825" PVC rod. The hardwood guide rails were spaced apart by sandwiching them around two spacer blocks, with the spacer blocks sandwiching a 0.25" thick piece of aluminum bar. With the guide rails screwed down to a base board, the spacers between them, separated by the aluminum bar, formed a 0.25" wide slot exactly centered between the guide rails that was used to center the router cutter between the guide rails, allowing the aluminum angle pieces to be marked and attached to the bottom of the arc-cutter jig to keep the arc-cutter jig on center as it was moved along the hardwood guide rails.

The 0.825" PVC rod was centered between the hardwood guide rails by lightly clamping it down on 0.5" steel rods set between the guide rails, and cutting slots to fit over the 0.375" threaded rod in the center of the PVC rod in two 0.125" thick pieces of 0.75" wide steel bar. The steel bar pieces were screwed to the ends of the hardwood guide rails, and nuts tightened against them to retain the PVC rod firmly on center between the guide rails.

A clamping block was screwed to the bottom of the jig so the jig could be set on top of and firmly clamped into a large vice. The height of the Dremel™ router cutter was adjusted to clear the retaining nuts, and the first slot cut.

To cut the second slot exactly 180° around the PVC rod from the first slot, two pieces of the original hardwood spacers and a piece of 0.25" aluminum bar were cut down to fit under one of the 0.125" thick steel retaining bars. The 0.825" PVC rod was loosened and rotated approximately 180° in the guide jig. Then the cut down wooden spacers and aluminum bar were used to align the first slot exactly on center in the jig by sliding the aluminum piece into the first slot, and using the spacers to center it between the guide rails, forcing the slot to center. With the first slot properly aligned, the retaining nuts were tightened, and the second slot was cut.

To lock the runner discs to the axle key slots, the runner disc axle holes were enlarged, on center, from 0.5" to 0.825" diameter to fit over the 0.825" diameter PVC rod. Then, through a series of steps, two 0.25" wide tab slots were cut approximately 0.375" deep into both runner discs' center holes to align with the slots in the 0.825" diameter PVC rod section of the axle assembly. First, using a PVC fixture consisting of a 0.825" diameter disc about 0.5" thick with a 0.25" diameter stob about 0.75" long extending from the center of one of its faces, one of the runner discs was centered around a 0.25" diameter hole drilled in a board large enough to support the disc. After the disc was centered it was clamped down, the fixture removed, the 0.25" cutter of the Dremel™ inserted into the 0.25" hole in the support board, and a piece of aluminum angle material set against the router and clamped across the disc to provide a guide for the router to cut tab slots into the center hole aligned with the geometric center of the disc, 180° apart, and, hence, unavoidably aligned with the key slots in the PVC axle piece.

With tab slots cut in the first disc, the second disc was placed on top of the first disc, and both discs registered together using a screw with nut through one hole pair from the rings of assembly holes near the outer diameters of the discs. The pair of discs was centered over the 0.25" hole in the support board, again using the PVC fixture, with the second disc against the board. Then the discs were clamped to the support board, the alignment fixture removed, the Dremel™ router 0.25" cutter extended into the 0.25" hole in the support board, and the aluminum angle piece set against the router face and clamped to the runner discs. For this operation, besides centering the router over the discs' axle holes against the aluminum angle piece, the angle piece was adjusted so that the router cutter slid cleanly back and forth along the angle piece into both tab slots in the upper disc. Then, without removing the upper disc, tab slots were cut into lower disc. Because of how the discs were registered together, not only are the tab slots in both discs aligned, but, so are the assembly holes in both discs relative to the tab slots.

After the key slots and tab slots were cut, a jig was constructed for cutting a precise 0.25" wide strip of 0.091" thick acrylic plastic sheet to be used for the tabs to extend from the tab slots.

For four passes, the 0.25" strip was rounded on its end to fit the rounded end of the tab slots in the runner discs and cut to length to extend from the tab slots into the key slots on the 0.825" PVC axle slot rod. The four rounded strips were then glued in place in the runner discs to create the key tabs necessary to lock the runner discs in place on the key slotted axle shaft.

The inside diameter of a length of so-called 3/4" PVC pipe fits (a bit loosely) over the 0.825" diameter PVC key-slotted rod section of the runner axle assembly. So, three sections of the pipe were trimmed in a lathe for clamping the runner discs in the center of the volute when the axle was assembled and installed in the turbine axle support uprights.

Bearings:

Rather than use the bushing from the temporary alignment axle, pockets were cut in the volute side of the axle support uprights, centered over the original 0.5" alignment axle bushing holes, to allow insertion of 1.125" outside diameter ball bearings. To cut the pockets, the volute ring was removed, and an electric hand-drill angle boring jig with the angle set to 90° clamped to an upright, centered with a 0.5" forstner bit through the original 0.5" diameter hole, then the 0.5" forstner bit changed out for a 1.125" forstner bit and the pocket cut to a depth equal to the thickness of a bearing. The same method was used to expand the remaining portions of the original 0.5" holes to 0.75" to allow the ends of the axle shaft to extend through their bearings without interference. After the pockets were cut and the holes enlarged, the volute ring was reinstalled.

The bearings have a 0.5" inside diameter, and two 2" long 0.375" US standard thread coupling nuts were turned down to 0.5" over approximately 1.5" of their length to match the inside diameter of the bearings. The turned sections of the nuts were then polished with emery paper and crocus cloth to allow the bearings to slide smoothly on and off the nuts.

Using the wooden soft jaws to hold the 0.825" diameter PVC rod in a vice, the length of 0.375" threaded rod used for the axle element of the key-slotted axle assembly was centered through the PVC rod by turning it with a wrench against double nuts. The double nuts were removed, and the coupling nuts were screwed onto the ends of the 0.375" threaded rod, with their turned sections outward, to provide the bearing surfaces for the key-slotted axle assembly.

To install the axle, on one end of the axle rod the coupling nut is threaded up the shaft until it is near the 0.825" PVC rod section. This allows that end of the axle shaft to be passed fully through the bearing in its support upright, which, in turn, allows the coupling nut on the opposite end of the axle to be aligned with and inserted into the bearing in its support upright. Threading the coupling nut near the PVC rod back down the axle and into its bearing completes installation of the axle. Prior to installing the axle, a 0.5" inside diameter locking collar is slipped onto the bearing surface of each coupling nut to retain the axle between the bearings.

Final Assembly:

To complete the basic assembly of the Mark-II turbine, the center holes in the volute end plates were cut out, on center, to a diameter of 2.25". Expanding the openings provided clearance for the ends of the 0.825" PVC rod and compression nuts on its ends during assembly and spin testing.

The 0.5" inside diameter locking collars and partially turned coupling nuts were removed from the 0.375" threaded rod axle, a 0.875" US standard thread nut was screwed onto one end of the 0.825" PVC rod on the axle, and a regular 0.375" US standard thread nut screwed onto each end of the axle far enough to allow the coupling nuts, with their locking collars, to be reinstalled. On the end of the axle closest to the 0.825" nut on the PVC rod, the regular nut and coupling nut were run up the axle to near the PVC rod. One of the volute end plates was leaned against the volute side of one of the axle support uprights and the end of the axle with the coupling nut threaded up near the PVC rod was passed through the volute ring from the side opposite the leaning end plate, through the end plate center hole, and into the center hole of the bearing in the pocket in the axle support upright. In order, the runner disc spacers, runner discs, and were slipped over the axle and onto the PVC rod and the second compression nut screwed onto its end of the PVC rod locking the runner assembly to the PVC rod. The second end plate was slipped over and both end plates mounted with brass washers and screws. The axle was installed into the support bearings as described in the previous section. With the coupling nuts in place, the regular 0.375" nuts were run down the axle shaft to contact the coupling nuts, then the nuts were tightened together, double-nut fashion, to fix the coupling nuts on the axle. Finally, the locking collars are adjusted to take the play out of the shaft by sliding them up to the bearing faces and tightening them down.

Doin' Stuff:

(c.a., January 2008)

Here I'll be documenting tests of larger runner designs, and such. But, don't expect anything to show up too soon. There is still a lot of work to do with dynamometer testing, and flow analysis on the six-inch turbine. Also, work on the Mark-II turbine to this point has been somewhat just proof-of-concept regarding construction. There need to be a few changes before real work with the new turbine can begin. In particular, the turbine, in its current state, has no inlet nozzle. Also, the threaded rod material used in assembling the key slot axle is notoriously not straight. So, at very least, before taking the Mark-II out for a spin, an inlet will need to be constructed, and a new central axle element machined from genuinely straight rod.

References:

(Clicking reference numbers here takes you to the text location of the reference.)

[1] Heimann, Erich H., 1975. Do it yourself with plastics. Mills & Boon Limited.

Last updated 03August2008
Alan Swithenbank, alans@cuervo.stanford.edu

Contents:

What It Is?...

Major Construction:

Doin' Stuff:

References: