A34 Comet Mk I Type A - New Build

[Stephen White]

Steve

I'll use some spare roadwheel tyres, reduced in diameter and width. I'll happily post dimensions but this drawing and the sectioned drawing from the Tank Museum may help:



After three days of machining and a huge pile of swarf later, the first half is roughly the right shape. It needs a lot of finishing and the lightening holes to be drilled:









The highlighted area on this section shows why the early pattern idlers were prone to pack with mud and throw the track. Not surprising they were replaced with cast idlers of a more open pattern. The later idlers and the Comet roadwheels went on to be found on the early Centurions.....



Regards

Stephen

[Stephen White]

Here's a lesson in the perils of building to a 1/35th pattern kit. The original Comet idler supplied in the Bronco kit has the six bolts to mount to the hub and six lightening holes.



The later idlers certainly have eight mounting bolts and it simply wasn't likely that the change in idler would have involved a change in hubs. There are very few wartime photos of the original idler and none show all the lightening holes clearly. So I was prepared to go with six. Fortunately, before drilling, I found this wartime document which clearly shows eight lightening holes, which is much more logical:




Subsequently, I checked on the Resicast kit, which is excellent, but which I haven't yet had time to start. It has what I now think is the correct eight holes.





The reverse side. It isn't completely accurate as it would be impossible to turn on a lathe and would have to be made like the original in two pressings but it representative:



One half idler complete, compared with the Armortek late pattern idler:



And here a reminder of the blank which is now a large pile of swarf:



Three more to go....

Regards

Stephen

[Paul Morris]

Hello Stephen.

Well done a lot of work and reserch but it looks like its paying dividends to you.
I know what you mean about piles of swarfe . . .must get back to the J.T. one day .
Anyway congrats on what looks like is going to be another great build.
Cheers Paul

[michael hilton]

Hello Stephen, excellent, I have followed you build from the beginning, some serious engineering, you certainly have an eye for detail. Enjoying every moment....Mick

[Adrian Harris]

Amazing piece of machine Stephen - most impressive.

> I found this wartime document which clearly shows eight lightening holes

But it also only shows seven holes in the hub ...

Adrian.

[Stephen White]

The new early type idler now has its second half:







Regards

Stephen

[Adrian Harris]

A lovely bit(*) of machining Stephen

Adrian.

(*) For very large values of "bit"

[Stephen White]

Thanks Adrian - see you at Tankfest?

Moving now to the drive train, some work on the final drives. The kit doesn't represent the quill shaft covers which are visible at the top of the final drive housings. So bored out a cut out on the outer hull plate and a corresponding section of the final drive housing and turned the two covers from ally bar. 10BA blind set screws to fix. Job done.









Regards

Stephen

[Adrian Harris]

> Thanks Adrian - see you at Tankfest?

To quote Teal'c: "Indeed"

Looks like you're moving along nicely, though that Cent book looks like it'll take a bit of reading. Any long flights coming up

Adrian.

PS: You have a PM about the Comet.

[Stephen White]

A bit mundane, just the assembly of the suspensions. Remarkable only for the ability of Kilopoise to get everywhere.









Regards

Stephen

[Iacopo Di Giampietro]

I had lost this WIP!

Great job, congratulations!

Iacopo

[Stephen White]

The retaining bolts for the final drives are cap heads. The real Comet is covered in hex heads, not least on the final drives. It's important to retain the high tensile cap heads on this unit with all the gearbox stresses but I wanted the final drives to look authentic.

With cap heads:



Solution:

Mill out a rebate to the depth of the cap head:



Sink the cap heads:



Place a cut down 7BA set screw on top. Job done:





Regards and thanks for looking.

Stephen

[Stephen White]

Back to the front idlers. I think I need a course in production engineering techniques. Anyway, all four now complete:



Next stage is the tyres. I decided to use Comet roadwheel tyres, which are bigger in diameter and section. The material is easy to shape on a grinder, as long as it isn't allowed to heat excessively. Not sure what the white hat and yellow coat brigade would say. First step is to achieve the correct section:





Then cut and shave off until a perfect fit is achieved:



I used Loctite 271 as adhesive. The joint needs reinforcement and I decide to pin both sides:





The central flanges of the two halves are obviously thicker together than that of the single kit idler, which would offset the idler from the track centreline. I've therefore taken a risk and reduced the hub flange to take account:



Full set:



Assembled idler:







Clearance for the track horns:



Comparision between the early and late idlers:



It will be worth it, as long as the tyres stay on. NOt sure of the wisdom of taking a part which worked well on the real thing and replacing it with one that didn't. I guess that's one of the mad pleasures of a custom build.

Thanks for reading.

Stephen

[Adrian Harris]

Excellent work as always Stephen

> NOt sure of the wisdom of taking a part which worked well on the real thing
> and replacing it with one that didn't.

Just remember to swear in 1/6th scale when it fails in a prototypical way

Can't wait to see mud oozing through these idlers, just like Steve Stuart's W&P photo.

Adrian

[Steve Stuart]

The solution to the tyres may well be at some model engineering show now you have honed your technique for grinding rubber! You may find some thing smaller that is the right diameter. With my work hat on, I am wondering if micro scooter wheels or in line roller skates might offer some thing of the near or correct size, please let us know what the diameter needed is and I, and, I am sure, others might find some thing!
As Adrian has pointed out extruding mud would be good, but finding 1/6 scale War and Peace mud might be a tall order, you are more likely in my experience with clogged tracks to blow a fuse, namely the one the local Halfords has run out of!
All the best
Steve