Bristol Mercury revisited

[kvom]

My wife and I were in Nuremberg for a day 2 weeks ago.  Mayhaps we passed in the crowd.

[Roger B]

Absolutely splendid      With the demise of UK Model Engineering shows have you considered the European ones such as Faszination Modellbau?

https://www.faszination-modellbau.de/en/faszination-modellbau/

[fumopuc]

Absolutely splendid  With the demise of UK Model Engineering shows have you considered the European ones such as Faszination Modellbau?

https://www.faszination-modellbau.de/en/faszination-modellbau/

I will try to do my visit there this year at the very first day, Friday.

[Vixen]

Thank you all for looking in and for the generous comments, they are always very rewarding.

Admiral; We are already planning a longer and more leisurely holiday in Budapest.

kvom; I saw another guy in Nuremberg with obvious workshop withdrawal symptoms, could that have been you?

Roger; The big European shows such as Faszination Modellbau are massive exhibitions, bigger than anything we have in the UK. I would like to get to one, one day. If I were to display an engine, it would mean a very long road or rail journey, my models are too big to fit into a suitcase, so flying is out.

Mike

[Jo]

Roger; The big European shows such as Faszination Modellbau are massive exhibitions, I would like to get to one, one day. If I were to display an engine, it would mean a very long road or rail journey, my models are too big to fit into a suitcase, so flying is out.

Sadly after the end of the month you might find you have to fill in a load of customs paperwork and probably pay for the honour as well   

Jo

[Art K]

Mike,
Sorry to be jumping on the train late BUT, that is some fabulous work there and hopefully I will get to the UK some day to see it in person. One can always hope. Yes, Vienna and Budapest are great cities to visit but try not to drive in the latter. If you miss a turn the streets are all one way the wrong direction.
Art

[mike mott]

Always great to see not only a beautifully crafted piece of model engineering, but also an excellent way to display it.

Mike

[Dave Otto]

I was going to reply yesterday and couldn't think of what to say except for beautiful job, which doesn't seem like nearly enough for a project of the scope and the amazing outcome.
Anyway the time, effort, skill, and craftsmanship that you have put into this project clearly shows.

What an amazing piece of work!
Dave

 

[Vixen]

Part 2       The second Bristol Mercury

When the supercharged Bristol Mercury Mk VIII was introduced in 1935 it was rated at an impressive 840 BHP all the way from sea level to 14,000 feet. An impressive performance from a comparatively small engine. Engine development had by now outgrown the wooden propellers of earlier engines. The Bristol Mercury MK VIII was fitted with a 12 foot diameter three blade, all metal, propeller with controllable variable pitch. The propeller was a first generation variable pitch propeller with only two pitch settings, Fine pitch for take off and landings, and a course pitch setting for economical cruising.



This photo shows the three blade De Havilland all metal propeller fitted to a Bristol Mercury. The aircraft is the Bristol Blenheim light bomber, restored by the ARC at Duxford UK.

The large cylinder, pointing forward in the propeller hub is the spring assisted hydraulic cylinder which moves the three propeller blades to the fine pitch (a blade angle of 14 *) for take off. The three heavy bob weights move outwards, due to centrifugal force, as the engine revs are increased. They move the three blades to the course pitch (a blade angle of 34*) . The pilot can control the timing of pitch change, during climb out, with the pitch control lever, The lever releases the oil pressure in the central cylinder allowing the blades to move to the course setting. Before landing, the pilot moves the pitch control lever to 'fine' and the engine pumps lubricating oil back into the central cylinder to force the three blades back to the fine pitch setting prior to a landing.

My second Mercury engine is intended to be a runner and so will require a suitable propeller. I am planning to build a 1/4 scale De Havilland variable pitch propeller for the second engine. At 1/4 scale, the three blade propeller will be 36" in diameter. A propeller of that size is required to provide an adequate load for the engine but is very large, unwieldy and inconvenient when displaying the engine at exhibitions, so I have decided to fit three short 'stub' blades for exhibition purposes and replace them with the full length blades when the engine is to run.



Bristols did the same, back in the thirties, and fitted short stub blades to the full size engines they displayed (for sale) at air shows such as Farnborough and the Paris Salon

Stay tuned and watch the chips fly

Mike

[Craig DeShong]

Mike, I can hardly contain my excitement at the announcement  of this new build.  It’s going to provide many hours of pleasure for you readers.  I’m getting the popcorn ready as I post this.   

[Steamer5]

Hi Mike,
 Make that 2 !
Planting season here & the pop corn is growing! Not much of last years left, hope I get a good crop as it looks like I’ll need it!

Cheers Kerrin

[mike mott]

Mike I will follow along as well, I am still mind boggled by the first one and the way that you have exhibited it. Being able to watch and share such work being developed and made is one of the great things about the modern internet, and sites like this excellent one!

Mike

[Vixen]

Thanks for calling in guys.

The first part to be made for the 1/4 scale De Havilland Variable pitch propeller is the Hub Barrel, which houses the blade rotation mechanism and the blade thrust bearings.

The full size Hub Barrel consists of a pair of precision forging which are bolted together during assembly. I started with two 3" x 3" X 1" thick slabs of HE 30 grade (6082).extruded aluminium alloy. Six dowel pins and a central bolt were used to locate and secure the stock to my well used sacrificial jig plate. The first operation was to mill the basic outline profile using a conventional end mill. The curved shape was then created with a 4mm ball mill. The ball mill traced out numerous toolpaths, like the height contours on a map, to form the rough three dimensional shape. The spacing of the contours is always a compromise, very close contours result in a smoother object at the cost of hours of extra machining time. The roughed contours can be cleaned up later by filing to clean up the machining and to smooth out the contour ridges

Here you can see the front half being profiled with the 4mm ball cutter, in stages










The centrol hold down bolt was removed so that the centre through hole could be machined




The whole exercise was repeated for the rear half, using a slightly modified toolpath program, to form the flange on the rear face





The shape of the Hub Barrel becomes clearer when the two halves are bolted together. The next stage will be  to machine the three propeller blade root details, followed by lots of hand filing to clean up the machining and to smooth the contour ridges





I learned a lesson, the hard way, regarding the cutting conditions of a ball mill when profiling a curved surface. Initially, the ball mill contactes the stock material with it's outer edge (full diameter). The feeds and speeds need to be similar to those for a conventional end mill. However as the contours approach the upper surfaces, the point of contact moves form the outer edge towards the centre line of the ball mill. The actual cutting speed gradually reduces as the point of contact approaches the centre line of the ball cutter. and is effectively zero on the centre line. Also the size of the flutes, needed to carry away the swarf, also reduce in size. I learned the hard way, that the feeds and speeds need to be progressively adjusted to cater for the different cutting conditions.

Look at this unfortunate four flute ball cutter. If I had been paying more attention and if I had increased the spindle speed and reduced the feed rate at the appropriate time, I may have prevented this cold welding and clogged flutes. Lesson learned.



Stay tuned

Mike

[crueby]

Fascinating work, following along here as well!   

And that clogged ball end cutter would make a great modern-art exhibit...!

[deltatango]

Following and watching in awe!

The hint regarding ball-ended milling cutters has gone into the memory banks - hope it can be retrieved when needed.

David