Bizarre event with a Sherman

[Adrian Harris]

> Not sure there would be enough meat in the sprocket ring though

Actually, not at all dissimilar from the inner ring you posted earlier

Adrian

[Fabrice Le Roux]

Adrian
CS works like a charm. Loctited the screws. No play or creep in the assembly. The main dynamic force is rotational rather than outward thrust. Plenty of meat left in my ring, mate! Fnarr fnarr!
cheers, Fabrice

[Adrian Harris]

Just to prove not all threads are dead and buried...

I've been recently working on the sprockets of this M4A3 Sherman:

and this thread reminded me of some changes which could be made to the fasteners.

The gear cases are quite badly scored from the bolts heads on the inside sprocket ring, as this Sherman was built and run with the stock M3 HEX fasteners holding the sprocket rings in place. One side has quite a bit more damage than the other.

These have been successfully repaired today with Durafix aluminium brazing sticks, in the same way I did the Sherman Prototype gear cases.

In order to prevent any repetition of this scoring, I have countersunk the mounting holes of the two inside sprocket rings, so that M3 CSK fasteners now sit flush with the face of the ring.

For the outside fasteners, I decided to try and replicate Fabrice's trimmed down fasteners.

This turned out to be a bit more involved than I expected, as I decided to do it on the CNC lathe. That way I could guarantee that they would be near identical.

To start with, I had to make up a 2.5mm HSS drill holder for the turret, so that I could drill and tap a work piece holder for the M3 thread needed to hold the CSK bolts whilst they were having their heads reduced.

Once the work holder was complete, it was mounted in the CNC lathe and the actual work could begin.

Each M3 CSK bolt was screwed into the work holder and tightened down to prevent it being unscrewed by the cutting tool. As these are turret based, I don't have any choice in how the machine cuts, so couldn't do it against the thread.

Then I ran the program which reduced the head diameter from 5.8mm to 4mm.

There is only one tool in the lathe which I felt was robust enough to cut these HT fasteners, hence the gradient on the back of the work holder to allow access by the diamond shaped insert. The cuts were taken in 0.1mm steps, in order to minimise the cutting forces needed.

I'm very happy with the results, as I feel they look much more like the real thing than a standard CSK or CAP fastener.

Tomorrow, chores permitting, I hope to put a small countersink in the outer sprocket rings, so that these fasteners sit at the correct height.

Adrian.

[Fabrice Le Roux]

Hi Adrian,
Definitely worth the work, but I recall that I ended up countersinking the inner ring and leaving the outside proud, like the original cap screws. Old photo but worth a 1000 words:

Kind regards
Fabrice

[Adrian Harris]

The plan is to add just a very small countersink to the outer ring, mainly to centre the bolts, so they will remain proud of the surface.

Adrian.

[Fabrice Le Roux]

Hey! We could call this zombie topic..
"The Walking Thread"
Hurrah for us Allied model makers, we are a minority.
Good luck with the rebuild.
cheers, Fabrice

[Adrian Harris]

"The Olive Drab Green Mile" with "Dead Thread Walking"



Did the remaining 13 screws today, plus straightened out and filed all the burs from the sprocket rings.

Also received the length of 8mm brass bar I needed to complete the pivot pins, so they're done as well.

Adrian.