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Ideas for machining Large Aluminum Aircraft Propellers form a raw forging.

mhajicek said:
I hate to say it, but it sounds like you need to hire a consultant who knows what he's doing and has been involved in similar work before.
Click to expand...

Yes I am well aware of that. I already said that in the original post.

Whether I hire a consulting team, hire a company to do the full turnkey system for us, or decide to try to take it on myself I should still do my best to understand every aspect of this project and the challenges that are involved. That's why I posed in this forum.
 
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mhajicek said:
I hate to say it, but it sounds like you need to hire a consultant who knows what he's doing and has been involved in similar work before.
Click to expand...

Yup, it's obvious they are planning on undercutting existing manuf.

That blade config has been around for a long time.

As far as fixturing and locating, the print should tell you that.

Anything more than this, you pay me for the consult.
 
Couple things I'd suggest:

1) Make sure to have the raw forgings X-rayed or otherwise comprehensively NDI'd before cutting anything. I wouldn't leave this to the forging company, I'd take ownership of ensuring there's no internal cracks or inclusions.

2) Mill-certs on the material. Keep, track, worship. You'll want these if something goes wrong. Might be worth having an independent lab confirm alloy and hardness. Know where the material is coming from, some stuff is crap.

3) Yes, stress relief during cutting will be an issue. Don't let anyone try to cheat a part in by cold bending, saw the part in half and scrap!

4) Learn about prop failure modes, the better you know how things go wrong the better you'll help prevent stuff falling from the sky. FAA bulletins like this can help: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-37e.pdf

5) Make sure a Haas is really accurate enough to cut the hub end without babysitting. You'll want to avoid chatter, so tooling and speed/feeds is important. This may be one of the times you'll want to set a procedure and tooling and then never deviate. You have to make sure tooling providers don't change cutter geometry without your OK and retesting. You'll need an appropriate tailstock for the rotary table if that's how you go.

6) Get coolant tested for mercury or gallium contamination. Both are poison for aluminum and should never be near any parts that are going on an aircraft.

7) I'm just some random guy on the internet, don't take anything I say as gospel.

Good luck!
 
I'm not a guy with aerofoil machining experience, but I do have some experience machining forgings in a production setting... If the part is supplied as a complete forging, then this is a 3-op part at minimum. Most likely 4 OP's.

So if I understand your post correctly, the alignment of the OD of the blade shank, tapered bore, and flange OD are critical. Then, you get a .010" form tolerance on the rest of the aerofoil shape, correct?

And you & your company have no equipment, no experience machining these, and very little technical resources to make this part currently? Did the customer give you any target pricing, and an explanation as to why your company was asked to pursue this project? I don't mean to be rude, but this smells fishy...

Are they offering to pay for any development costs, including at the bare minimum, upfront payment for fixture & tooling design? Sometimes larger customers will make this commitment up front, with the expectation that they "own" the tooling, and can re-claim it, should they choose to pull the job from you - which is how they justify paying you up front in the first place.

My last shop I worked in was a Teir supplier to the heavy truck industry, and much of our tooling & fixturing was paid for upfront, by the customer. We also saw some brand-new, never installed tooling & fixturing get repo'd as well, when the customer decided to not give us any orders for the production. So it happens, just FYI...



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I'll let you chew on that, but as to how to process the part... At 70" by 20" rough size, that's a big part. I also don't see anyway to process this part in one setup either. Even done on a single machine, in multiple setups would be a stretch.

Did they give you the expected production volumes? Are we talking 10 a month, 100, 1000, or 10,000? Also, a target price? And how many working days/shifts in a month will you have? (5) day work week, (1) shift to make it happen? Answering these questions, you can now see the "box" that you have to work within.

For example, 20 working days a month, 40 hours of production time, at 75% efficiency, means you can produce 150 blades a month, if you can produce a single blade in 4 hours of cutting/operation time. That means you'll need to produce 7.5 blades a day, without scrap, per month, if it takes 4 hours to complete a blade. How many machines will you need to make 7.5 blades a day, that have a 4 hour cycle time? My guess is one lathe, and 3-4 mills using the below illustration. Reduce the cycle time, and you can reduce the number of machines. (But you can't buy only 1/2 a lathe, or 2.5 mills...)

One thing you didn't mention in the 1st post, was the required surface finish of the aerofoil section. Can this be hand-finished? (Your post seems to indicate that it can be...)




OK, so now - how to machine it... Does the forging come supplied with any type of pre-formed hole of the tapered bore on the flange side?

If so, put a bull-nose center in this pre-formed hole, and grip the other end (aerofoil shape) in a set of custom machined jaws, and turn the blade in a lathe. Rough the round O.D.'s first, then hold these diameters with a steady-rest to machine the tapered bore. Bring the center back into the tapered bore, and then finish machine the outer diameters.

You'll clamp on these finished diameters to mill the aerofoil, but first you'll need to support the far end of the part. If you truly have 1/2" stock on all sides, I'd try to put a center in the far end of the part. To do this, you'll need to clamp on the finished diameters, and drill the center in the end of the forging. Since we're talking about aluminum, you may get lucky and be able to do this without supporting the far end where you're drilling the center. If you have to support it, then this will most likely need to be done on a mill. Clamp on the finished diameters, support the far end, then drill the center.

Next, load the part in a 4th axis on the mill & install the center. Mill the aerofoil profile. You'll have to cutoff the boss where the center was drilled. You'll have to remove the tailstock, and place a nested support under the blade, and then mill the boss away.

Hand finish to blend toolmarks.






That would be my rough draft.

There are many ways to skin the cat, but you first need to get answers to the production related questions down first. That will frame your budget, time constraints, and help guide you on selecting a process & machinery.

If you all get this work, best of luck.
 
patrickpnw said:
I have been asked to source machines and come up with concept fixtures and processes for machining large (77" long x 19.5" wide) Aircraft propellers for the company I am working with.

First off I am not by any means an experienced CNC operator, or programmer. I am a manufacturing engineer with experience programming and running machines 5 years ago when I was in college, and spent the last 3 years supporting CNC operations with minor offset tweaks and things like that. With that being said I am trying to push hard to buy a turn key machine and have someone else do all the programming, tooling, and fixturing for us. This is obviously the ideal solution but it also is going to cost about twice the cost of doing all this work in house.

I would like to get some ideas on how an experienced machinist, programmer or engineer would approach this project.

I have attached a basic picture of the final blade shape to this post for reference. The blade will come to us as a raw forging with about .5" excess material in all areas. The taper bore (blade root) must be within .001" of the blade shank and the blade butt OD must be within .002" of the taper bore, and the rest of the blade is all based off the axis of the taper bore and must be within .010" of the taper bore axis.

Right now we get blades from a machine shop that cuts a rough shape out of the forging and the blade is ground down by hand to shape, so I don't necessarily need to machine down to net shape but as close to net shape as possible without scrapping blades is the goal.

First thing I need to figure out is how to fixture this thing. It makes sense to me to cut the butt and taper bore features first then locate the airfoil off of those key features but I have talked with a few applications engineers that say it would be best to cut the airfoil then locate the butt and taper bore based off the airfoil. I have also heard that the blade stress relieves and moves around a lot as it is cut which makes it difficult to accurately cut the airfoil to the taper bore and butt features.

Second what kind of machine or set up would be best for this part. I have seen a few videos of pinch milling on youtube and that looks like the perfect machine for this but they are in the $2 million + range. I am considering a Haas VF10 with a rotary table mounted horizontally with access the the blade butt so I would rough out the airfoil then use a 90 deg head to locate the butt features then re-fixture the blade locating on the taper bore and finish the airfoil with light passes to reduce and flex from tool pressure.

I don't know what to do or if anyone on here can help with the little information I provided. I am basically trying to get as much info and ideas in my head and sort everything out as I go.

anyone with any suggestions or ideas please feel free to chime in.

View attachment 208389
Click to expand...

Starrag builds machines for this purpose. I have a buddy who works for them (I almost let him recruit me at one time).
 
Jashley73 said:
I'm not a guy with aerofoil machining experience, but I do have some experience machining forgings in a production setting... If the part is supplied as a complete forging, then this is a 3-op part at minimum. Most likely 4 OP's.

So if I understand your post correctly, the alignment of the OD of the blade shank, tapered bore, and flange OD are critical. Then, you get a .010" form tolerance on the rest of the aerofoil shape, correct?

And you & your company have no equipment, no experience machining these, and very little technical resources to make this part currently? Did the customer give you any target pricing, and an explanation as to why your company was asked to pursue this project? I don't mean to be rude, but this smells fishy...

Are they offering to pay for any development costs, including at the bare minimum, upfront payment for fixture & tooling design? Sometimes larger customers will make this commitment up front, with the expectation that they "own" the tooling, and can re-claim it, should they choose to pull the job from you - which is how they justify paying you up front in the first place.

My last shop I worked in was a Teir supplier to the heavy truck industry, and much of our tooling & fixturing was paid for upfront, by the customer. We also saw some brand-new, never installed tooling & fixturing get repo'd as well, when the customer decided to not give us any orders for the production. So it happens, just FYI...



----- ----- ----- ----- -----



I'll let you chew on that, but as to how to process the part... At 70" by 20" rough size, that's a big part. I also don't see anyway to process this part in one setup either. Even done on a single machine, in multiple setups would be a stretch.

Did they give you the expected production volumes? Are we talking 10 a month, 100, 1000, or 10,000? Also, a target price? And how many working days/shifts in a month will you have? (5) day work week, (1) shift to make it happen? Answering these questions, you can now see the "box" that you have to work within.

For example, 20 working days a month, 40 hours of production time, at 75% efficiency, means you can produce 150 blades a month, if you can produce a single blade in 4 hours of cutting/operation time. That means you'll need to produce 7.5 blades a day, without scrap, per month, if it takes 4 hours to complete a blade. How many machines will you need to make 7.5 blades a day, that have a 4 hour cycle time? My guess is one lathe, and 3-4 mills using the below illustration. Reduce the cycle time, and you can reduce the number of machines. (But you can't buy only 1/2 a lathe, or 2.5 mills...)

One thing you didn't mention in the 1st post, was the required surface finish of the aerofoil section. Can this be hand-finished? (Your post seems to indicate that it can be...)




OK, so now - how to machine it... Does the forging come supplied with any type of pre-formed hole of the tapered bore on the flange side?

If so, put a bull-nose center in this pre-formed hole, and grip the other end (aerofoil shape) in a set of custom machined jaws, and turn the blade in a lathe. Rough the round O.D.'s first, then hold these diameters with a steady-rest to machine the tapered bore. Bring the center back into the tapered bore, and then finish machine the outer diameters.

You'll clamp on these finished diameters to mill the aerofoil, but first you'll need to support the far end of the part. If you truly have 1/2" stock on all sides, I'd try to put a center in the far end of the part. To do this, you'll need to clamp on the finished diameters, and drill the center in the end of the forging. Since we're talking about aluminum, you may get lucky and be able to do this without supporting the far end where you're drilling the center. If you have to support it, then this will most likely need to be done on a mill. Clamp on the finished diameters, support the far end, then drill the center.

Next, load the part in a 4th axis on the mill & install the center. Mill the aerofoil profile. You'll have to cutoff the boss where the center was drilled. You'll have to remove the tailstock, and place a nested support under the blade, and then mill the boss away.

Hand finish to blend toolmarks.






That would be my rough draft.

There are many ways to skin the cat, but you first need to get answers to the production related questions down first. That will frame your budget, time constraints, and help guide you on selecting a process & machinery.

If you all get this work, best of luck.
Click to expand...

Thank you for giving a good answer and some good ideas to think over. The is nothing fishy about this project. My company already has a contract to manufacture these blades and has done this for some time. We are just looking to bring the machining capabilities in house to save cost and improve quality. The machine shop we work with currently does a pretty terrible job and the blades require 40+ hours of hand grinding to finish. I we are able to bring the machining in house and get down to maybe 10 hours of grinding and polishing it would be a huge win.

Production rates will always vary I would like to have the capability of machining 1 blade per 8 hour shift. This will well exceed our current predicted rates and allow us to add a second shift to double our capacity.

This will definitely by at least a 3 op job. We have a lathe on the way that will be used for all the shank and fillet features. I am hoping to for from 1 or 2 set ups on the mill to the lathe then to final grind.

Right now I am most worried about harmonics and vibration. I am thinking if I have this blade supported by the butt and tip it is going to flex, wobble, chatter, and ring like a bell if I am trying to take a decent depth of cut in the middle of the blade.
 
0921170853[1].jpg


This is how it was done in 1940. This is just a picture of the machine that did the hub end of the prop. I think these were probably doing the P40 props prior to the US entering WWII.
 
patrickpnw said:
My company already has a contract to manufacture these blades and has done this for some time. We are just looking to bring the machining capabilities in house to save cost and improve quality. The machine shop we work with currently does a pretty terrible job and the blades require 40+ hours of hand grinding to finish. I we are able to bring the machining in house and get down to maybe 10 hours of grinding and polishing it would be a huge win.

<...snip...>

Right now I am most worried about harmonics and vibration. I am thinking if I have this blade supported by the butt and tip it is going to flex, wobble, chatter, and ring like a bell if I am trying to take a decent depth of cut in the middle of the blade.
Click to expand...

40 hours of hand finishing is insane. No wonder airplanes cost so darn much... Again, I know nothing about propeller blades, but I can't imagine anything more than 2-3 hours max hand finishing... Maybe I'm just ignorant, but even 10 hours seems crazy... IF the cycle time requires longer finishing times, consider running the finishing cycle overnight when the machine would normally be sitting.

Flex in the middle of the blade would depend on how thick it is, and how aggressive you're being.

I know this sounds crazy loopy, but I'd be tempted to put this on a small vertical, double-column machine, with a 4th axis & tailstock mounted on the table. I'd then bring in the 90* head, with horizontal spindle being parallel to the 4th axis, and then use a big disc-mill, or facemill with aluminum specific inserts to rough out the part. The big cutter would allow the 90* head to clear the part.

I'd rotate the 4th axis, while feeding the machine in the X axis, while the Z axis rises & falls to machine the shape. I know that sounds crazy unconventional, and I feel like this would put less pressure on the part. It would also let you remove stock evenly around the part, so part movement as material/stress is removed, should be kept pretty low. I also wouldn't worry about harmonics with 90* head & big cutter either. There's some trick things you can do with large diameter cutters to reduce harmonics - even for dampening harmonics of the part itself...

This type of machine won't be cheap though. Probably 700k minimum for a new Japanese/Taiwanese fixed-rail machine with right-angle head - add 75k for the 4th axis. Add 50-70k for tooling & fixturing to be safe. So that's a lot of coin...

The upside to this setup though, is that it would give you enough table space to have the 4th axis & tailstock mounted on the "far side" of the table, nearer the toolchanger. You could have another setup nearer the operator, that was used for drilling the center before the blade machining, and then for removing this boss after blade machining. The fixturing in this station could be kept low-tech, but you would need a skilled operator who knew how to clamp the part without moving it.



Just a few more random thoughts. Let us know if you have more questions.
 
This sounds to me like a job that's begging to be run unattended/lights-out. Putting on a second shift for a part with that long of a cycle time seems crazy to me...

Optimize for best possible surface finish with cycle time of 8 hours to minimize hand finishing. 1 cycle started first thing in the morning, 1 cycle started last thing at night.
 
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