Bristol Mercury revisited

[Vixen]

Thanks everybody for calling in,

Carving an engine part from a solid billet is fun and a challenge. You need to think the whole process through from beginning to end, before you start cutting metal. It is all too easy to machine away the datum or the means of holding the work for the next operation. The last thing you want is to find a holding clamp just where you wish to place the cutter.

Yes, the bottom bracket does have a split cap. However it is far more convenient to machine it as one piece and seperate the cap with a fine slitting saw after all the holes are drilled and tapped and the outside filled to shape.

J   Looking forward to your next build thread

Mike

[Art K]

Mike,
As usual you managed to make a very elegant part. I always look forward to see what you're up to.
Art

[Roger B]

Excellent metal sculpting as ever     

[Vixen]

Art Roger 

Just a quick update. I cleaned up the machined brackets with a file and abrasive paper to make them look more like castings, including the draft angle. Then I used a 16 thou (the thinnest one in my box) to part off the caps. Finally a quick finish with a disc of Scotchbrite in the Dremmel, to produce a nice even matt finish.




The three little scoops are to catch the oil thrown around in the reduction gearbox in order to redistribute it in the cam gears. Nothing is wasted.





That's it for a short while.

Mike

[Jasonb]

They look good, Being clamps you actually want a gap between the two halves so they pull up tight unlike say an eccentric strap where you don't want a gap.

[crueby]

Love the finish! How did you hold the scotchbrite in the dremel?

[Vixen]

Hello Chris,

I cut the discs of Scotchbrite by hammering a sharpened length of steel tube into the end grain of a wooden block. I find the red/brown grade works best.

The discs are mounted in the same way as a cut off disc. The screws are M1.5 if you need a longer replacement.




I usually mount two discs at a time, but they don't last long if you are against a sharp edge. Bits fly everywhere, so wear goggles to protect the plastic lenses of your glasses.

Ohh, keep the Dremmel speed as low as possible.

Mike

[crueby]

Great tip mike! Good time to use the face shield and apron, sounds like.

[Vixen]

I thought some of you may be interested in the bevel reduction gearbox which fits between the engine's crankcase  and the variable pitch propeller. It drives the propeller shaft at half engine speed.

You can see the reduction gearbox is very neat and compact. It consists of two large bevel gears and three smaller ones which rotate the central propeller shaft. The large bevel gear at the front is static and is bolted rigidly to the bell housing surrounding the gearbox. The large bevel gear at the rear is driven by the engines crankshaft via a large diameter splined coupling. The three smaller bevel gears are mounted on stub shafts which are part of the propeller shaft. The propeller shaft turns at half engine speed.








The central propeller shaft was machined from a length of 3.125" diameter Mild steel (EN8) The shaft is hollow and was made from two pieces, the main shaft which carries the small bevel spider and a smaller tail shaft. The first operation was to turn all the features on the front of the shaft and also the tail shaft. The embrio propeller shaft was then rotated in the chuck to bore the central cavity and turn the rear details. The tailshaft was then loctited in place and the tail shaft outer diameter turned to run truely concentrically with the main propeller shaft





The 40 TPI bearing retainer screw threads were machined on the lathe before The propeller shaft assembly was transferred to the milling machine to machine the three stub shafts which carry the three smaller bevel gears.






The propeller shaft was then transferred to the dividing head for the tapered splines to be machined. The rear of the shaft is being supported by a bearing block. I made the special spline cutter (similar to a gear cutter) from hardened silver steel. The splines were machined in a similar manner to a spur gear but with the added complication of adding the taper half angle.






The bevel gears were based on commercially available stock gears made by HPC. They were generated by HPC on a gear shaper so each gear tooth had the correct taper and cone angle. The stock gears were all modified in the lathe as required for their individual  bearings and mounting requirements










 There are thrust bearings outboard of the bronze bearings on the three smaller bevel gears and two large diameter ball races to support the propeller shaft. The whole reduction gearbox was carefully assembled and shimmed to achieve the correct meshing of the bevel gears.

The whole reduction gearbox runs very smoothly and effortlessly, once I had learned how to hold the gearbox, to avoid nipping my fingers in the gears. The straight cut bevel gears produce a slight tinkling sound when rotated dry. The outer bell housing forms an oil bath for the gears as well as transferring the propellers thrust and torque forces back into the engines crankcase.

Stay tuned

Mike

[propforward]

That is lovely work Mike - very nicely done. 

[Admiral_dk]

Amazing result Mike 

Am I right in assuming that you milled the "The propeller shaft assembly" i two different setups before doing the splines - one where the shaft was parallel to the mill bit (probably mounted in the divider) - and another with the shaft mounted somewhat like you show in the picture where you cut the spines - so you could mill the tree stubs for the small bevel gears ...?

Per

[Vixen]

Hello Per,

The milling of the propshaft used several set ups. A lot of my photos were lost in a PC crash a while ago, so I cannot show them to you.

I started with the propshaft mounted vertically, so that the bulk of the big ring could be machined away and to profile the shape next to the hub. This left three square pegs sticking out. The propshaft was then transferred to the horizontal position, in the dividing head, to machine the three stub shafts from square to round. I used a boring head, with the cutter facing inwards, to bore/ turn the stub shafts to size. The spines were cut with the shaft parallel to the X axis. The half angle was machined by a coordinated move in the X,Y axes.

Mike

[Admiral_dk]

Thank you Mike - that more or less confirmed my suspicion about how you did it - though I didn't thinK much about the reverse drilling head, but it certainly makes the round surface perfect for use as an axle

Shame about the PC accident - they have only happened a few time for me since late 1980's as hardware faults - and a few more from "Error 40" (that should probably be "Error 25" in the Non-Metric world) 

[Vixen]

Hello Per,

You can get lots of satisfaction from devising a way to machine a component using only what you have available in the shop. The alternative is to buy more and more machines and tooling which may never be used again. I prefer to use some nginuity whenever I can, instead of the cheque book.

You only get caught once with a computer crash. After that you learn the lesson about backing up important programs and data. In a way our MEM forum is also a great backup of all our images and a record of all our projects. it's good to be able to look back to find out "how did I do that" and hopefully the information can help others facing similar challenges.

Mike

[steamer]

That's looking sweet Mike!...

Dave