Westbury Seal by Vixen

[Vixen]

No I was suffering from Brain fade back then
Jo

It seems to have given you a long white beard as well, which is worrying  Hope the effects diminish with time.

Mike

[Vixen]

In this instalment for Vixen's den I will show some more machining operations on the Seal's little crankshaft. The part machined crankshaft was repositioned in the machine vice to give clearance for machining the excess material of the two crank throws at one end. A 6mm diameter, three flute carbide cutter was used.




Again I used ESTLcam V11 to generate a 'peel' toolpath. The stepover for each spirol was 10% of the tool diameter, i'e. 0.6mm. with a feed rate of 150mm/min. I am sure I could have gone faster and removed the material at a faster rate, but I went cautiously as the there was such a large workpiece overhang. The small stepover appeared to put little pressure on the workpiece or cutter. A steady stream of silver colored shavings (not chips) were produced instead of the overheated brown or blued chips I normally expect.




One side completed and the other side marked with marker pen. I would hate to get it wrong at this stage. I didn't want a pair of left handed gloves by mistake.




The second side was quickly repeated giving four crank throws in the required positions.




The outer edges of the end counterweights were chamfered at a 23 degree angle while the bar was in the lathe. I wanted to repeat these chamfers on the inner counterweight; but obviously I could not use the lathe. Instead the part machined crankshaft was transferred to my mill with the 4th axis set-up and tiltable spindle. The 4th axis unit is actually a redundant EMCO Compact 5 lathe with a stepper in place of the spindle motor. The tailstock centre is particularly useful for long workpieces like the crankshaft

The number 14 sign, shows this machine was once part of the rehabilitation workshop in Her Majesty's Prison at Parkhurst on the Isle of Wight, home of some really hard nuts.




I prepared a short program to rotate the crankshaft back and forth through an angle of 200 degrees as I gradually advanced the cutter to produce a nice chamfer on either side of the central counterweight. These chamfered ER25 closure nuts were particularly useful to prevent the toolholder touching the workpiece.




Here is the part finished crankshaft beside the crankcase.

Will it fit?? I do hope so.

I will leave the part finished crankshaft in it's present state for as long as possible before machining the crank journals. Hopefully, this will allow any internal stresses to relieve themselves before the final stage.

Does anyone have any ideas for a simple accelerated aging process to speed up the stress relief process? Hot and cold cycles perhaps, but how hot and for how long?



In the next installment I will describe the flywheel and centrifugal clutch which will be needed for my 1/6 scale Bren Gun Carrier.

Stay safe

Mike

[scc]

Beautiful work

[gadabout]

Mike, wonderful as always!
Am interested in the clutch as I would like one for when I use my Seal in the boat.
Mark

[Vixen]

Will it fit?? I do hope so.

Yes, the crankshaft does fit and spins freely.



Stay in and stay safe

Mike

[Vixen]

I intend this little Westbury Seal engine to power a 1/6 scale model of a Bren Gun Carrier. Therefore, I need to consider the transmission system at the same time as the engine build. I have already designed and built a couple of successful i/c engined tracked vehicles (tanks) so have a fair idea as to what is required.

For a small model tracked AFV, a good reliable centrifugal clutch is essential as is a two speed (plus reverse) gearbox. The engine needs to be capable of reliable operation at two speeds. A reliable tick-over (idle) and then a more or less  constant speed in the middle of the power band. Unlike a racing buggy car, there is little need for a wide engine speed range and throttle response.

I cannot run my engine until it is completed and I cannot find any performance details. Therefore I have had to make some design assumptions. I guess the engine should be capable of a reliable tick-over (idle) at about 2,000 RPM and will produce enough useful power at somewhere between 3,000 amd 4,000 RPM. This will need to be confirmed by bench testing. I am planning on a centrifugal clutch which starts to engage at about 2,500 RPM and fully engaged by 3,000. Due to the uncertainty, an easily adjustable engagement point is required.

The common two shoe (think brake shoe) clutch relies on a single spring to hold the two shoes out of engagement until the engine speed is high enough. This style of clutch can be adjusted only by changing the spring for one of a higher or lower spring rate. However, finding a suitable selection of springs can be difficult.

I chose, instead, to go for a four shoe non-pivoting design which makes for easier adjustment of the engagement point. Below you can see the component parts of my centrifugal clutch. The four pegs on the flywheel rotate the four clutch shoes. Four springs between the drive pegs and the clutch shoes hold then in until sufficient engine speed is reached. The pressure applied by the springs can be adjusted by four grub screws in the drive pegs. If necessary the four springs can easily be replaced with others of different spring rates. I am still waiting delivery of my first guess springs.




Here you see the four holes for the drive pegs being machined in the back face of the flywheel




The clutch bell was machined from EN8 steel bar. Turning the thin walls of the clutch bell proved to be interesting as the clutch bell rang like a bell, (what else?) eventually I found a new insert and a speed and depth of cut which did not make the clutch bell chatter.  I used a thin parting tool to remove the part finished clutch bell from the stock. Actually I only parted part way and completed the job with a hack saw. The clutch bell was reversed on the chuck to finish turn the outside face. The live centres helped reduce ringing (chatter) during the finishing cuts.








Fortunately I found a nice piece of paxoline (phenolic resin/ paper/cloth) round bar, left over from some previous high voltage project, waiting in the stock bin. The four clutch shoes were all machined in one go. The outside diameter of the shoes being exactly the same as the inside diameter of the clutch bell i.e. the clutch shoes engaged. The pockets for the springs were bored before the individual clutch shoes were removed from the stock. I used two slitting saws together to achieve the required slot width. The paxoline was easy to cut, but produced a lot of dust and an awful smell. It was very abrasive and quickly over heated the slitting saws. A slower rotation speed helped. I parted the four blocks from the stock with a long end mill. I didn't have a long enough reach parting tool, besides, the interrupted cut would have been a problem.








Here you can see the centrifugal clutch being assembled and mounted on the rear of the engine







I weighed the individual clutch shoes, they each weighed 4 grams. Not a lot you would think, but wait until you calculate the centrifugal force. This can be calculated from Mass x Radius x rotational speed, squared:

At 2000 RPM the centrifugal force is 0.358 Kg
At 2500 RPM the centrifugal force is 0.550 Kg
At 3000 RPM the centrifugal force is 0.805 Kg
At 3500 RPM the centrifugal force is 1.1 Kg
At 4000 RPM the centrifugal force is 1.4 KG

Doubling the engine speed produces a four fold increase in centrifugal force.

I will start with a set of springs of about 0.5 Kgf These will hopefully keep the clutch shoes disengaged during tickover (idle) speed while still producing sufficient contact pressure on the clutch bell at operational speed to drive the transmission and Bren Gun Carrier model.

Fun times ahead

Stay in. Stay safe

Mike

[Vixen]

Don't miss the previous post.

This is the 1/6 scale model of the Bren Gun Carrier I hope to build.







I think the caption to still photo of a full size Carrier should read, " it's not going fast which hurts, it's the suddenness of the stop"

Stay in , stay safe

Mike

[Admiral_dk]

You are doing great with this build Mike 

I have never seen this version of the centrifugal clutch, but it is interesting and I hope it works as planned 

[samc88]

Interesting project. The been gun carrier is one of my favourite vehicles, some of the systems on it are brilliant, such as the steering system which warps the track for slight turns

Sent from my G3121 using Tapatalk

[crueby]

Looking forward to seeing the clutch operation, very interesting how that works (have used machines with them, never new how they worked).

 

[awake]

Agreed - all of this build is excellent, but the clutch is especially interesting. Eager to see how it works!

[Vixen]

I have never seen this version of the centrifugal clutch, but it is interesting and I hope it works as planned 

Looking forward to seeing the clutch operation. 

The clutch is especially interesting. Eager to see how it works!

Ha ha, me too. I also hope this clutch works as intended.

Stay in and stay safe

Mike

[gadabout]

Mike,
 So very nice!!!
You might need to watch how close the flywheel runs to the block as initially mine was just kissing the camshaft end bush and I had to skim some from the back of the flywheel, yours looks close but might just be the camera angle too
regards
Mark (in awe)

[Roger B]

An interesting clutch design  Do you have a lathe that will reach those speeds for testing? That is one of the advantages of my little Proxxon.

[fumopuc]

Hi Mike.I do like the clutch.