How would you mill tubine blades?
|James Veitch||10/02/2010 02:19:12|
|16 forum posts|
Ok, I know this will be a bit vague and I apologize for that. I'm just looking for some general ideas on how to approach this problem.
I want to make a small water turbine.
The general design is a two inch diameter turbine that looks like a jet engine.
That is the water will flow through the turbine end-to-end.
The rotating center hub will have three to four blades extending from the center hub. These blades are to be pitched (imagine a short, stubby propeller blade).
The hub with blades will fit inside a tube.
Hope that's not too confusing.
So the problem is: How to mill the blades on a NON-CNC mill?
I'm not looking for perfection, just damned good enough.
Thinking about it for a moment I guess this question can be generalized to:
How would you mill a propeller? Same complex shape except I'm looking to make very stubby blades.
Thanks in advance for any thoughts you might have.
Yours, Jim V.
735 forum posts
Hi James, I make 4"Ø 2 bladed props out of the solid for my hydroplane out of high tensile steel.
If you have an idea of the shape of the blades and the various angles you could machine a series of straight blades.
Set the blank at the root angle of the blades and use a cutter with a rounded end so no sharp corners. Using a dividing attachment mill the number of parallel-sided blades required making sure they are a bit longer than required.
Draw a diagram of the propeller, decide the pitch and measure the angles at various radiuses.
Make some sheet metal templates of the various angles.
Then twist the blades with a suitable tool checking the angles with the templates.
When satisfied turn to the diameter required and with rotary burrs, files etc. create an airfoil section to the blades.
You could set the blank at the angles required on a rotary table and go in steps with a round nose mill but that is very time consuming and will still have to finish by hand.
The material you use will have to be something that can be twisted without cracking.
Some of the turbines are cast or edm.
Hope this might be of some help.
Edited By Windy on 10/02/2010 13:46:20
Edited By Windy on 10/02/2010 13:46:58
|Ian S C||11/02/2010 09:24:01|
7447 forum posts
|If I was making it I would start with a 2" disc say 1/2" thick,bore it and mount it on a mandril.Fit the rotary table at the pitch angle,and cut out the blades,I'd go for six,or eight,just as easy and less metal to move.With the blades say 3/16" thick take it to the bench and file the blades then bend the blades to give the right twist,althoughthat may not be required on such a small turbine.I'm no machinist,I just do it my way,so there will be half a dozen other ideas,some better and the others not much better than mine. Ian S C|
|3119 forum posts|
You could consider the following. Mark out the profiles of each blade on a piece of material thick enough for the final blade set. On the trailing edge of each blade, plunge down a narrow cut. Rotate by a suitable amount and plunge down again, BUT not so far, keeping the edge of the cut close to the intended pitch and so on. Turn over and again repeat the same. You COULD end up with roughly ribbed set of blades, then a lot of filling.
Alternatively you could buy a model ship propellor and mount it in a tube.
|James Veitch||11/02/2010 11:07:29|
|16 forum posts|
Thanks, Windy, Ian and Kwil,
This is really a tough one with no obvious straight forward solution. I agree that there will have to be a lot of finishing file "artistry" involved.
From the dearth of replies I guess the world community of home engineers is looking at this post and gasping. Hope I get a few more wild and creative suggestions.
Thanks all. JV
|David Clark 1||11/02/2010 11:18:59|
3357 forum posts
Try looking at a Tesla turbine.
Flate plates and no blades to mill.
Camden do a cheap book and Google is free.
|3119 forum posts|
With a 50mm 3 bladed left or right handed prop costing around £10 is it really worth the effort to make one ?
|Ramon Wilson||11/02/2010 14:30:48|
|676 forum posts|
James, with your initial question and your last comment in mind ......
All the above replies have been good suggestions and may be the answer to your needs however "How do you mill a propeller without CNC kit?"
Not an easy problem to solve with basic kit - I have no actual practical experience with the following suggestion though do understand the principles involved.
I'm sure you are aware that (for a constant pitch) the angle of a prop blade changes (lessens) as the radius increases toward the tip. To create this effect then the workpiece would have to be rotated as the cutter travelled along the blade in effect - spiral milling. Outside of NC this would require that the rotary device, either a rotary table set horizontally or a dividing head be connected in some way to the leadscrew movement. The latter is quite normal set up on a universal mill but probably not in the average ME's kit.
Having given it a bit of thought - IF - the rotary device could be disengaged ie to freewheel and the workpiece held securely then the rotary device could be made to rotate as the table moves by a bar emanating from the side of the (moving part) table. If this bar was to bear on a fixed triangular plate (or be captive in an angular slot) that is securely held but independant of the table travel the bar would slide up the slope rotating the table as the cutter advances along. This obviously would be some set up and the cuts would need to be gingerly applied but it would give the effect that you seek. Whether it would actually do the job you require is another question. In effect the same priciple as dis-engaging the cross slide leadscrew for taper turning, just different axiis.(How do you spell that?) The angle off the triangular plate would give you the pitch transition you seek - the actual pitch angle to start would have to be set on the workpiece. It would need to be set up so that the cutting forces if possible acted such to keep the bar in contact - I would see this as very much using the side of the cutter - all a bit pricarious to say the least not to mention backlash but that's the principle. This would give you flat sectioned blades the camber is down to hand work .
Should you decide to go this route I would try it on something very soft first - plastic or wood to prove it all out. The 'maths' involved - well I'll leave that to you - or others -definitely not my best subject! - Just don't forget the pics!!!
Whilst I concur that KWIL definitely has a point I hope this might go some way to answer your original question.
Now, back for that afternoon session
Regards - Ramon
|1008 forum posts|
KWIL - I'm sure you are right, and there is such a variety of pitches to allow experiment. 4 bladers are not that expensive either, or you can halve the hub on a 2 blade job. With a water turbine shape, much of the centrifugal load will come off, because I imagine it will be cased/ducted and therefore cut down.
If I were going to make one without resorting to bending, I'd work out the co-ordinates in steps. Then you'd end up with your blade form, but in rough steps - 3 dimensional. Then its a reasonably quick clean up job with a file. Assuming you want high efficiency blades. Its not actually a fraction so difficult as you think, especially with a programmable calculator, but its plain bloody boring, and equally tedious to machine to so many coordinates. Still it does away with the rotary table. Think of it like building a model ship hull by the bread and butter method.
The other way would be to carve what you want in wood, and use it as the basis for a mould for an epoxy glass turbine.
Making a very simple thing complicated?
Edited By meyrick griffith-jones on 11/02/2010 14:38:25
|Gordon W||11/02/2010 15:02:27|
|2011 forum posts|
I don't know if it is at all relevant, but a book called Windpower Workshop by H. Piggot has a chapter on making rotor blades which looks to be useful for this problem. I have looked at making similar ,but don't have the head ,of water, and would go for carving in wood, then casting, or cover with glassfibre.
|Keith Long||11/02/2010 15:26:09|
|790 forum posts|
Your post says 3 or 4 blades which suggests that you don't have a definitive design for this turbine yet. If so I'd look for a simple way of making any sort of prop so you can run some test to see how many blades work best and to get a feel for the proportion of blade to free space that you need. Initially I'd be inclined to cut a prop from flat sheet and then tweak the blades with pliers to get the pitch. If you use fairly thin sheet it would be pretty easy to put curvature into the faces of the blades with a bit of small scale panel beating. Once you have a feel for what shape actually works then you may have different (easier or more difficult) machining requirements. My gut feeling is that the shape won't be as important as you think as you will get a lot of losses from the hub and the inner surface of the containing tube, and these could swamp any gains in efficiency from subtle changes in blade shape.
735 forum posts
An old book that MAP used to sell (ISBN 0 85242 712 3) Model Boat Propellers has a chapter on ways to make marine props simply, also ways to calculate the various angles etc.
It does not answer your question how to mill one but gives the basics of design.
Another very old one The Book of Modern Engines has a volume devoted to all types of turbines
Edited By Windy on 11/02/2010 17:00:08
Edited By Windy on 11/02/2010 17:01:39
|1314 forum posts|
This may seem a bit off centre, but is there anything on the Gas Turbine Builders Association website which might help?
I saw some of their very impressive models at Ally Pally.
Edited By V8Eng on 11/02/2010 19:27:19
735 forum posts
V8Eng mentions the gtba and I found them very good at giving practical advice.
I forgot to mention an article by D.H.Chaddock (21 December 1950) about a turbine blade copying attachment and another one by L.K.Blackmore (ME 31 January 1957) on turbine blade cutting for an experimental gas turbine.
The L.K.Blackmore attachment might be more suitable for your turbine, if you require the article I could send you a copy.
Edited By Windy on 11/02/2010 20:04:37
|William Roberts||11/02/2010 20:47:51|
|27 forum posts|
Hi James have a look at Hydro Turbines they might give you some ideas . Bill
|Stub Mandrel||11/02/2010 21:38:31|
4306 forum posts
You've picked an interesting challenge. When you start, take photos of each stage for us to see!
The following is all guesswork, but reflects the design of an amazingly effective small plastic ducted fan I have.
Water flowing constrained to a tube is effectively incompressible, but there is no spill of the edges, like with a free prop. An ideal blade will still need a greater pitch near the hub than at the periphery, but the pitch can remain constant along the blade (i.e.you don't need curved blades like a prop). If the turbine is designed so the hub is quite large (say 1/3 to 1/2 total diameter) and the width of the blades short (i.e. use 12 narrow blades rather than 3 wide ones), then I'm sure you could get away with many straight blades. If you accept a less than perfect root profile, such as that generated by a bull-nose cutter going across at an angle, I reckon it would work.
I made a radial water pump rotor from aluminium alloy, but it wasn't too effective because of the size of the gap at the edges is too big. I guess the same would apply to a turbine.
You can either try suck it and see, or do some maths based on the volume of water and rpm required.
Good luck, and don';t forget to report back!
|1008 forum posts|
I agree - gas turbines and high speed steam turbines are not the way to go or even draw inspiration thereform The Reynolds regimes under which they operate are just so vastly different.- about the only thing common is that they go round.
To get efficiency at relatively low velocities you need a proper wing/hydrofoil, and you need short fat blades, of as great a chord as possible.(ideally you want a pretty decent aspect ratio, but there is a limit. It also wants, to prevent high pressure tip spillover, to be ducted and you want the duct to be as close a fit to the blade tips as possible. If you can get a hair through the gap its too big. You ain't got a lot of pressure differential at such velocities so you don't want to be losing what you have.
Look at the hydro electric type of turbine - the St Malo tidal turbines. That sort of low speed high efficiency set up.
The point is that the best you can get is 1/2 x density x v^2 x the area x the lift coefficient at whatever angle of attack works best in water. Straight away you can see the biggest function in there is Velocity^2, and v is pretty low, so the only thing thats left to you is to improve CL to a maximum and get as much area as you can.
I've used the aerodynamic term Cl or lift coefficient. What the hydro people call it I don't know, but the numbers are the same in this instance. CL is a reflection of the hydrodynamic shape.
It's actually rather worse than that implies, because CL is also dependent on Re, and Re is greatly affected by velocity and simple physical size. And your velocity is not great and physical size is small. So you NEED every bit of efficiency you can get,
|James Veitch||12/02/2010 00:16:30|
|16 forum posts|
Thanks all for the many comments and ideas.
I may not have a turbine spinning yet but my head is spinning.
I will try to photograph and document my disasters and progress, if any.
From reading some of the suggestions I feel I should elaborate a bit on my idea.
This may help you visualize the problem and generate solutions.
The turbine is to go on the end of a swimming pool cleaning hose. The little turbine will have a central axle that will drive a gear set. The gears will drive a small toy vehicle along the bottom of the pool. (If you have kids or grand children you will know why this needs to be done.)
So, I will have lots of energy that I can deliver to the turbine because a pool pump moves a lot of water. I may even have to modulate the flow to the turbine so I don't over stress it, but that problem is way in the future. With so much water pressure I can suffer some blade inefficiency but part of the fun is to make things that are as nice as we can.
The pool hose inside diameter is nominally 45 mm or a little over 1 3/4 inch.
I'm not an engineer so I need to research things like: number of blades, pitches, everything.
The many suggestions given here by forum members has made me think (always a refreshing change). My goal is to make a small turbine. Milling a turbine is not my goal, but it might be one of several paths. Maybe some brass sheet, a tin snip, some silver solder and a brass hub is the better way to go.
I think it was Confucius who said, "If you have an expensive milling machine in your shed you are going to use it for the damnedest things."
Once again thanks to all, JV
|Ian S C||12/02/2010 09:59:35|
7447 forum posts
|I made a small (about 1 1/2") prop for a model boat by cutting with a hacksaw 3 slots at about 45 deg,brazed in the blades, and twisted them to adjust the pitch.Don't know how it would stand pressure etc. The idea I had before could be done with a hacksaw and file. Ian S C|
|11 forum posts|
use two different size holesaws to form blade profile (no pilot necessary if you go gently)
haggle the waste out as best you see fit twist to suit, primitive but it works with a ml4 and a steel tape(saving for a mike).
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