My Beaut Aussie Cent

Forum for discussion relating to the Centurion
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Stephen White
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Re: My Beaut Aussie Cent

Post by Stephen White » Fri Feb 05, 2016 11:03 am

To address the backlash issue, on Mark's advice I took a couple of simple steps, moving the traverse bracket to bring the traverse pinion into closer contact with the gear ring and added a second grub screw on the pinion, at 90 deg:

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Installed the AK Interactive lenses on the lights:

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Last bits on the hull are the brow pads on the driver's hatches:

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As ever, started with some card templates. I thought I'd experiment with CX5 thermoplastic clay. It's so easy to warm it with a heat gun, mould it to rough shape, then apply a texturing. I tried various materials but finally used sandpaper to impress the pattern of the original Rexine:

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Added the binding cord using fine wire:

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Then painted and weathered:

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Regards

Stephen

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Re: My Beaut Aussie Cent

Post by simon_manning » Fri Feb 05, 2016 1:11 pm

looks good, regards simon.

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Re: My Beaut Aussie Cent

Post by Derek Attree » Fri Feb 05, 2016 3:28 pm

Hi Stephen
The hatch pads are one of the jobs I still have to do on my comet.

Like the look of what you have done there.

Derek
we must stop making stupid predictions

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Re: My Beaut Aussie Cent

Post by Stephen White » Sat Feb 13, 2016 6:56 pm

Dark winter nights, too much rain and time on my hands, how about the recoil? A servo system will always struggle to represent that explosive force as the gun fires. I thought I'd have a go at another approach. My Pz III has a pneumatic system, which was OK but would need some beefing up for the much heavier Cent.

First step is to reduce drag in the bearing. This linear bearing from Automotion Components is an almost perfect fit for the Cent, the OD is perfect and the ID is only a couple of mm off:

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Servos also limit the recoil travel. The real 20 pdr recoils 11 3/4 inches, which is near enough 300mm or 50mm at one sixth. (105mm recoil was 11 1/2 in). Conveniently the new linear bearing measures 80mm long, enough for 50mm of travel and up to 25 mm of bearing surface.

To minimise contact area, I could therefore turn a thin PTFE ring for the front of the barrel and turn a PTFE bush for the back end.

The mantlet required boring out right through to seat the bearing and the mantlet surround needed a minor mod to preserve the full Cent elevation range. While I was at it, I shaped both to the accurate form:

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Some adventurous work holding:

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The bearing block and breech structure needed some support and the new PTFE bearing rings to be turned up to fine tolerance:

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(The brass bush was replaced with PTFE)

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Recoil testing:

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More to follow (if I ever work it out).

Regards

Stephen

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Re: My Beaut Aussie Cent

Post by Roger Sleep » Sat Feb 13, 2016 11:39 pm

Hi Steve,
Have been watching your build, really impressed with the extra work you've been putting in to make your tank look really realistic, look's like it's going to be as good as your Panzer III.

Wanted to install something myself,like what your doing for the recoil to my King Tiger but i am lacking in machinery to attempt it, I was looking into a linear motor to drive the recoil but have not got enough knowledge to attempt the electrical side.
Seen this on youTube (Link below) and thought this would be great for the recoil if you knew where to purchase them from and help with programming software so you could get the recoil speed to suit your tank.

https://www.youtube.com/watch?v=GNwP_iIb_Fw

Regards Roger

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Re: My Beaut Aussie Cent

Post by Stephen White » Sun Feb 14, 2016 6:44 pm

Roger, this great forum is a wonderful place for sharing of knowledge. Thanks for the link, I've had a good look at linear forcer technology for the first time as a result. I love exploring new technologies and solutions. Linear forcers score highly on speed and acceleration but seem to be expensive and to need control mechanisms which aren't easy to implement on our models, at least not for me. That said, I'll continue to look.

Space is another consideration and in order to get sufficient length of stator, they do seem to be longer. The one on the YouTube vid link is nearly 300mm long and I've only got about 70mm to play with plus 50mm stroke.

There is a good table in the data sheet comparing belt drives, pneumatics, linear motors and leadscrews. The recoil case solution needs to be: short in length, high speed and acceleration, moderate force (assuming a linear bearing has been installed), cheap (obvious really) and easy to control. On balance, I'm looking at pneumatics as the best bet but I'm listening and looking still.

Thanks again. All the best.

Stephen

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Re: My Beaut Aussie Cent

Post by Roger Sleep » Sun Feb 14, 2016 9:15 pm

Hi Steve,
keep up the good work, I will look forward in seeing the end result,
Regards Roger

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Re: My Beaut Aussie Cent

Post by Stephen White » Tue Feb 23, 2016 7:20 pm

Some work on the mantlet, including the coax and ranging .50 MG mounting boss, while I work out the pneumatics for the recoil.

The Mark 3 Cent sported the 7.92mm BESA coax MG. On the 14th Dec 1954, a revised design to accommodate the M1919 .30 L3 Browning was accepted by FVRDE. Maintaining two types of MG was logistically unsound and complete standardisation on the .30 was achieved by 1956. To distinguish the change, the Mark 5 designation was introduced.

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In order to increase first round hit probability with the 20 pdr, a .50 Browning MG firing a three round burst was trialled in Sep 1959. Australia ordered the modification kits in 1963. The mod involved replacing the .30 mounting with the .50 and a new aperture was milled and drilled to the right of the former coax mounting to accommodate the displaced .30 coax. The heavier RMG required a counterweight to maintain gun balance. The Type A 20 pdr barrel introduced a counterweight at the muzzle end, while the Type B had a fume extractor (as is seen on 064).

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A new, representative Mk 5 MG boss was made:

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Regards

Stephen

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Re: My Beaut Aussie Cent

Post by Fabrice Le Roux » Wed Feb 24, 2016 12:02 am

Nice work, Stephen.
I had not appreciated just how co-located the two MGs were. Surely this made for a "can o' worms" on the inside with belt feeds, brass collection and little elbow room to clear jams etc?
Did the troopers like the flexibility in action, or was the .50 cal preferred and the .30 cal just "Madge-the-Bridesmaid Allsop"? :) (I realise the reference is probably wasted on anybody under 35, but I did see Bazza, live in London in the 1970s!)
cheers,
Fabrice

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Re: My Beaut Aussie Cent

Post by Kevin Hunter » Thu Feb 25, 2016 11:27 pm

Hi Stephen
Been quietly following your progress and enjoying the recent small details you're adding. You must find these refreshing after the slog of the track bins?
Your most recent exploits on the machine gun boss have caught my interest. I had always assumed that the added .50 cal ranging gun was the one inserted in the "new" aperture between the .30 cal and main barrel. Your post suggests that in fact the .30 was moved and the .50 occupies the true co-axial position? Makes sense I guess, and at least I can make sure I get mine the right way around. Thanks so much for sharing these important details.
The blue lines on your photos....are you using a particular software to measure parts?
Keep up the fabulous work
Regards
Kevin

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Re: My Beaut Aussie Cent

Post by Stephen White » Fri Feb 26, 2016 10:52 am

Fabrice, Kevin hi

This shows the internal mounting arrangements:

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The cocking handle for the .50 is obvious to the left of the receiver, the .30 required a chain and T-bar arrangment to the right of its receiver. Both guns were fired with solenoids. The loader could judiciously override the solenoid and fire the ranging gun on continuous in extremis although it wasn't an apporoved gunnery technique.

In the close-in fighting in Vietnam, the power of the .50 was invaluable in cutting through vegetation and, in the case of rare urban operations such as the Battle of Binh Ba in which 064 took part, assaulting buildings. This had to be weighed against the greater supply of .30 ammunition available both in the turret and stored in a rack on top. The latter was primarily intended for the comd's MG.

I believe David Hay, who was the operator on 064 at Binh Ba, got through pretty well the whole bomb load before they extricated themselves from the village. Bruce Cameron, who commanded a troop towards the end of the Australian operations in Vietnam, reminded me that the barrel of the .50 barrel was shortened to reduce the muzzle velocity to provide the optimum ballistic match between the RMG and the 20 pdr. This is why the RMG barrel doesn't have the muzzle 'swell' normally seen on .50cal barrels.

Fabrice, needs must, in close in fighting the crews used everything available without fear or favour or much regard for the rule book. Weight of fire counted. The main challenge for the operator was to keep the link exit chutes clear and to that end a good one always had a screwdriver to hand to clear any jams. It was the same on Chieftain, which had the same layout.

Regards.

Stephen
Last edited by Stephen White on Fri Feb 26, 2016 11:35 am, edited 1 time in total.

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Re: My Beaut Aussie Cent

Post by Stephen White » Fri Feb 26, 2016 10:58 am

Kevin, sorry, didn't answer your question. No magic software, just my way of assessing proportions, taking account of perspective and relative sizes and positions. Paul Scott has been wonderful in providing key measurements and I work from that.

All the best.

Stephen

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Re: My Beaut Aussie Cent

Post by Fabrice Le Roux » Fri Feb 26, 2016 9:56 pm

Stephen,

Great photograph! My question is answered by the fact that the .50cal cocking handle has been mounted on the left, a configuration I had never been aware of as it is usually on the right! Illuminating!

So together with the shortened barrel, was this .50 cal RMG a special order to Browning Inc or were the modifications carried out locally (in UK or Aus depots)?

Kind regards
Fabrice

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Re: My Beaut Aussie Cent

Post by Stephen White » Sat Feb 27, 2016 11:41 am

The Cent is such a great choice for an Armortek model - much of the operational and technical material is accessible and there is an international community of interest. Following up on Bruce Cameron's comments about the .50, Paul Scott has sent pictures of a rare surviving Australian L6A1 .50 ranging gun held at Puckapunyal:
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    Mike Cecil added that the L6A1 Ranging Guns were modified in UK from standard production M2HB guns and issued as modification kits for the 105mm Cents. Australia ordered a kit adapted for the 20pdr. Mike also notes that the UK changed the nomenclature from Ranging Machine Gun to Ranging Gun in the somewhat forlorn hope of discouraging its use as an MG.

    The .50 RG User Handbook and Gunnery Instruction Notes are available. It appears that the modifications included a shorter, fatter barrel. The rate of fire was decreased and the muzzle velocity increased compared with the standard M2HB gun. The British L11A1 Observer round was developed from the US M20 AP/T. MV for these was 915 m/sec and 874m/sec respectively.

    The short barrel was developed, in conjunction with other modifications to the M2 MG, to decrease the deviation in Mean Point of Impact (MPI) between each round in the three-round burst. The shorter barrel decreased the barrel’s harmonics (also sometimes referred to as ‘barrel whip’) set up by automatic fire - simply, a shorter, (and fatter) barrel has a proportional decrease in barrel harmonics. Combined with the greater time between individual shots because of the slower rate of fire, the L6A1 achieved consistently tighter groupings than the conventional .50 inch M2HB fired at its normal cyclic rate. The L6A1 had an MPI of 1 to 1.5 MOA – much, much better than a standard ‘off the shelf’ M2HB, which is actually designed to have a cone of fire across a target area, rather than the pin-point accuracy required of the L6A1.

    Put another way, you want a machine gun to produce a beaten zone of fire (a scatter of rounds) whereas a ranging gun needs to produce a tight grouping of rounds on target.

    And then with Chieftain we got laser ranging and all was solved. (It wasn't but that's another story). My thanks to all my contributors. Hope it's of interest.

    Stephen

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    Re: My Beaut Aussie Cent

    Post by Fabrice Le Roux » Sat Feb 27, 2016 8:34 pm

    Stephen,
    Thank you. A very informative reply, with very clear and helpful photos.
    Take the rest of the day off... :wink:
    kind regards
    Fabrice

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