Cromwell Mk IV Build

[enginepeter]

IMG_9535.JPG (142.93 KiB) Viewed 2837 times

So... I have nearly finished my scratch build Cromwell so I thought I would share the story and a few photos with you.

Having read 43RecceReg's posts about a Ludwig kit I decided to take the plunge and also buy one. This prompted an email to seeking further details. I heard nothing for a few days and in the interim I started researching the Cromwell on the Internet and found some measurements as well as basic dimensions. My thought processes then started to making one with a metal body for strength and I started to sketch out some ideas.

I guess my email to Ludwig disappeared into the the trash box in the sky as I heard nothing and by this time had decided to make the body from sheet alloy, even scaling the thicknesses of the armour.

I decided to bu some T34 metal wheels, swing arms, Panzer IV tracks and the motors. This was where Forgebear proved to be very helpful in providing some extra track links.

Once in possession of the motors I then completed my rough plans, drifting away from scale making it a couple of mm wider so the motors would fit inside the body with a small clearance.

The first thing I did was to machine out the spokes on the T34 wheels and then fill them with epoxy. I then made a small form cutter to shape the epoxy to look like a Cromwell wheel before turning new hub caps, and drilling innumerable holes. I made a small jig to simplify this. The holes in the hub caps were to be later filled with dummy bolts.

Thank goodness I only had to 'adapt' 10 wheels!

[Model Builder 4]

Nice job on the wheels Pete I've used a set of t34 wheels on my Cromwell build but modified them to look ok'ish using styrene discs

Cheers, Lee.

[enginepeter]

After the wheels my attention turned to the drive cog and idler. The Panzer drive wheel did not look the part and I thought that if Ludwig could get them made there is no reason why I could not manufacture two. The first problem was to get the pitch right to match the tracks and this meant it was not a case of copying a scaled version of the drive from the Tank Museum Cromwell. After a bit of research and reading through a gear cutting book, I ended up making a wheel 50mm in diameter with 20 teeth. This was a bit bigger than scale and meant the drive cog had minimal clearance with the nearest road wheel - but it was enough (just)

To simplify the manufacturing process I decided to cut the teeth as a ring and then braze two of these to a core/hub. The steel for the teeth was held in a home made dividing head to cut the slots using the milling machine, before the rings were sliced off using the lathe.

Getting the flat inside the hub to match the flat on the engine drive shaft proved a challenge. To get round this I drilled a normal hole and then cut a small 'half-moon' piece of steel that was then brazed to the wall of the hole.

Yet again numerous small holes were needed for the dummy nuts.

The Idler wheel was a simple turning operation and was finished off by brazing short sections of bar to make it look like an original. Again 2 hub caps were made. I spent more time thinking how to make the tyres. I ended up getting some sheet rubber (water pump gasket material) and made a two piece die set to punch out the tyres in the vice. There were a few failures but I ended up with 4 useable tyres.

To adjust the track tensional two eccentrics were made to copy the originals. This involved a lot of filing to get the shape right before drilling holes for the dummy nuts. The body of the eccentric pieces would be held in the tension position by grub screws through the floor of the tank.

[enginepeter]

For the suspension I opted to copy the Ludwig design. With hindsight this was a mistake as adjustment to balance out a tank loaded with motors, battery etc proved difficult, using springs of different wire thickness and trimming their length.

I used the T34 swing arms but these did not fit the gap between the inner and outer walls. To fit they were thinned using the milling machine.

Next the swing arms were held in a jig so the the holes could be drilled and then threaded for the bolt to hold one end of the spring.

[enginepeter]

Before starting on the hull side plates I started on the two key pieces for the front and back of the hull.

Rather than fabricate them I decided to machine the front and rear from solid alloy. This would give strength to the front of the tank and provide a solid mounting for the sides and bottom plates. This would provide rigidity, front mounting points for the side walls as well as the floor and top. It also enabled a simple design of track tension with enough metal to thread for grub screws to lock the eccentric adjusters for track tension.

The block of alloy was mounted on the mill and I started to produce swarf. There followed a series of operations to shape the front, and cut the recesses for the side walls, top and bottom plates. Holes were drilled for the tension eccentrics and threading for the grub screws. The holes/threads for securing the wall/floor were drilled later after these pieces were cut and fitted.

Similarly a thinner block of alloy was machined for the angled bottom rear, with 3mm plate being used for the rear floor to provide a solid mounting for the motors. The remaining floor wold be cut from 1.25mm alloy.

[enginepeter]

The next stage of the build was to cut out the sides and floor. The top of the hull was left until the rest of the hull was assembled to get a precise fit.

A plan was drawn for each part which were then cut out and part drilled. Starting with the inner walls they were fitted to the front. Parts were clamped together on a flat surface where a set square was used to check accuracy, before the holes drilled for the securing screws. A precise spacer and the rear of the hull were used to ensure alignment. Two bars of alloy were cut to fit the bottom corner of each side to secure the sides/floor as well as provide a strong mounting point for the suspension swing arms.

Once the inner sides were fitted the reap panel and then the floor were fitted.

Spacer bars were cut to separate the inner and outer wall and the tank was now beginning to take shape.

The motors were trial fitted to check clearances before drilling holes for the securing bolts.

[enginepeter]

Spacers to fit the rear of the side walls were machined. To get the correct positions for the bolts, the layout was draw using CAD, printed then glued to the piece to be used as a template.

The rear of the engine bay was machined from solid and the same technique was used for the hole drilling.

An angled support was made to help strengthen the lower rear, fitting neatly between the two motors.

The upper sides of the engine bay were made from brass so I could solder the various spacers and supports to them before starting on the front of the hull.

The glacis and front top plate were made from alloy, in scale thickness to the original.

The swing arms and springs were fitted. They would be adjusted later when the tank was fully loaded.

Finally to complete the hull the top deck was cut then filed to get a perfect fit.

[enginepeter]

Originally I intended to make the whole tank from metal using styrene to complete the detail. However as the build progressed I started to think about using 3D printing.

This resulted in the depletion of my bank balance and countless hours being spent in learning how to create the 3d files for printing.

I decided to print the turret and started with the turret walls. The 6 sides were printed being held in position by the base plate. This base proved a little demanding to get right, making due allowance for the recoil and traverse mechanisms.

A recoil unit was purchased but it proved necessary to print a new elevation quadrant as well as a mantlet. In future I might make the elevation and recoil gears from metal to improve their movement.

The turret top was mainly printed but some items were made from brass so as to get the wall thicknesses nearer scale as well as providing some strength.

[jimbo_wa]

Great skillage on show - impressive work!

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[midlife306]

Brilliant


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