Q. Re QF 75mm on Cromwell

[Old Git]

Hi All,

I originally started this thread as a means of confirming some dims for the QF 75mm tube but as I get further into my quest I find I need slightly more than I started with and so it occurs to me that this thread has the potential to turn into a useful aid to anyone who might need to mock up a QF 75mm, or parts thereoff, as a stop gap for some missing parts on a resto. So, initially I've tired to pin down the measurements of the the tube, followed by a Cromwell Mantlet, then the Breech Ring and Breech block, and hopefully finish up with something on the cradle and anything else that comes to mind.

I've got a quick question re the 75mm on a Cromwell. In the drawing below I've marked various parts of the barrel and what I'm mostly interested in is the bevelled segment marked "B". From what I can tell the aperture in the Cromwell Mantlet that takes the 75mm is also bevelled at the rear of the Mantlet, is that correct? I so I presume that this is to take the bevel on segment "B". The next question is how far into the Mantlet does this bevel go, all the way so that the front face of "B" is then flush with the front of the Mantlet or does it only go part of the way into the Mantlet with segment "C" keeping the gun running true from the front of the aperture?

Finally, can anyone provide dimensions for A - E?

 

Thanks in advance chaps!

 

 

Edited May 29, 2018 by Old Git
added a bit at beginning to indicate morphing of thread into a QF 75mm catchall

[Adrian Barrell]

The mantlet hole is not bevelled, it does have a counter bore in the rear face however. Taper B is not wholly contained in the mantlet though, the rear face of the mantlet is roughly halfway down the taper. The tube is supported entirely by the bronze cradle and does not touch the mantlet at all.

I can't easily get dimensions I'm afraid, I'll try again later when I'm in dirty clothes!

Edited March 31, 2018 by Adrian Barrell

[Adrian Barrell]

[Old Git]

WowAdrian,

that's fantastic mate, you're an absolute star!  The counter bore is interesting, it is shownb on the drawings in the manual but it's not very clear so I assumed (yeah I should know better by my age) that it must be a taper to match that at B. on the tube. What is the purpose of the counter-bore? Is it supposed to be padded against an accidental strike ocassioned by an over-active recuperator or is it simply an 'air buffer' of some sort?

The picture of the tube in the cradle is also very informative,  again I was assuming the bevel came all the way past the end of the cradle. One bad assumption leads to another! But these are the areas that aren't well documented anywhere and of course what the manuals do show is based on the idea that you have the thing right in front of you.

Thanks again Adrian, you never fail to impress with the amount of stuff you know and the amount of stuff you seem have laying in the shed! 

[Old Git]

BTW, on the rear of the mantlet, the cut out for the BESA cradle. I see the main 'square' hole goes all the way through for the BESA barrel assembly but can I assume that the cradle only goes into the Mantlet a short distance?

When I was at the TNA recently I copied a report from NWE in 44, WO 348/105, a analysis of  "Casualties in Cromwells and Shermans". If you're interested I can Dropbox a copy to you, Just let me know mate!

 

Pete

 

Edited March 31, 2018 by Old Git

[Old Git]

So, I've pretty much worked out the dims for the tube now and as far as I can tell, if anyone else is interested, these are as follows...

A = 570mm long and dia about 165.5mm (length is measured from Bevel B to the  start of the thread, I think the threaded portion about 102mm long)

B = 100 mm long

C = 350mm long x 137.5mm dia

D = 165mm long

E = 1265mm long (to end of muzzle) x 111mm dia

all dis + or - 1mm

I'm now working up a CAD file for the mantlet. Hopefully, it'll turn out nice again (best George Formby voice there).

[eddy8men]

great work. it's hard enough doing it with all the bits in front of you, so trying to figure it all out from pictures and drawings is something else. well done

[Old Git]

Thanks Rick, to be honest I've learned a lot on this forum and the chaps, and chapesses, here have been absolutely brilliant and very generous with their time and knowledge! So I'm kinda standing on the shoulders of giants. I'm now working on the 75mm Breech Ring and Block, I've got most of the Breech ring figured out and some of the Breech Block. Sitting here trying to work out how the Breech ring looks from underneath where the innards are exposed at the bottom of the slot for the breech block. I think I might actually be able to work this out with a couple of paper cut outs, but we'll see. I could just as easily go mad counting the hairs on the palm of my hand!

Edited April 9, 2018 by Old Git

[Adrian Barrell]

My tube is very buried which is why I can't measure it but ring and block are accessible for photos and dimensions.

[Old Git]

Adrian,

thanks mate I may well have a question or two on the Breech ring and Block, but what you gave me on the Mantlet allowed me to do finish up the following. It's not 100% complete just yet, there are a few details that need adding, including the counter-bore that you mentioned earlier, and the position of the aperture for the MG cradle needs to be adjusted, but if you spot any glaring errors do say so ;-)

 

 

Edited April 10, 2018 by Old Git

[Old Git]

Quick-ish question for the Cromwell expert's. On the 75mm Mantlet there is a bit at the bottom, just below the tube, that sits lower than the rest of the mantlet. I've circled it below. When the mantlet tips backward to elevate the tube is the bottom, forward edge of this little 'tab' supposed to clear the aperture, by a few mm (just coming out over the groove which is there to allow the tube to depress a bit more) or does it just rely on the bevel, on the lower matnlet face, to give it clearance as per the rest of the bottom area of the mantlet?

[David Herbert]

Not sure that I understand the question properly but as I see it the 'tab' is there to prevent a clear path for splash when the gun is at full elivation and the bottom edge of the mantlet would otherwise be higher than the bottom edge of the opening in the turret front, or too near to being clear. It also has the effect of stiffening the mantlet which is weakened by the hole for the main gun.

David

Edited April 13, 2018 by David Herbert

[Old Git]

David,

thanks for that, it all makes perfect sense, especially in terms of preventing splash and adding extra strength to the mantlet where it's been bored through for the tube. It seemed to me that the tab was a perfect match for the same area in aperture and I didn't understand why they actually had a tab in the first place, why not just continue the 'tab' for the full width of the mantlet, especially  as I can't see where it could foul on anything inside the turret and below the MG cradle. The tab seems to be a pefect match for the corresponding area on the aperture, see below, and I just wondered if, when the tube is at full elevation, the leading edge of the tab passed over the rear edge of the aperture in the corresponding area. It looks like it wants to but as you say it would, possibly, leave a gap of several mm through which splash could occur.

Pete

  

[David Herbert]

I thought that was what you meant

I presume that you have noticed that each different gun has a different shape opening in the turret front caused by the different positions of the sighting telescope.

David

[Old Git]

Hi David,

yeah I have noticed that, I'm focusing on the 75mm and trying to ascertain what the mantlet looks like on the, unobstructed, Front face. All the photos and drawings I have show only the rear face. Still, I think I' getting there. I'vemocked up a scale mantlet in carboard and I can't see how the a full elevation of the gun wouldn't leave  a bit of a gap between the bottom of the mantlet and the aperture. I ust check the manuals again to see what it says about Max / min elevation heights etc.

 

Just as a matter of interest, and by way of a sanity check, does anyone have the Height x width x depth dims for the Mantlet? I think it's about 485mm x 558 mm x 90 mm (or 270  mm including trundle mounts). Is that about right?

Pete

[Adrian Barrell]

[Old Git]

Adrian, you're an absolute star mate. Those are views that we just never see and it helps a lot in understanding how these things actually look. It was only recently that I began to realise that the Mantlet was all cast in one piece with the Aroured MG cradle at the front. I had always assumed that the Armoured Cradle was cast separately and bolted onto the Mantlet. This caused me to view the Armoured cradle as part of the external detail, rather than internal, and because of that I designed them separately. As I said earlier, the only photos I could find of the Mantlet were either in situ or laying in a pile of parts from a resto, and all of them tended to show the back view.

 

I'm gratified to see how close I got in my guesstimations, I had some drawings from the Tank Museum whch I had to scale up and I have found that it can be hit and miss with official drawings!. I have found drawings where items are exactly drawn to scale but then other parts on the same drawing are not, it can be frustrating to say the least! Which is why this thread got started. I had a couple of different drawings of the tube which purported to be in scale but the length, and look, of the B measument seemed to be all wrong and it took some poring over photographs to realise that it wasn't my calcs that were off but the actually drawings. Everything else in those drawings would appear to be in scale, except that one area. It makes no sense to me but I'm sure it made some sense to someone in the DTD Drawing office!

So I was a bit leery of what I had deduced for the mantlet. I had the width at approximately 22" and  from your photographs I can see that it's actually 22 3/4". My height measurement was taken from the bottom of the 'tab' and I'd thought I'd got that wrong too until I realised that your tape measure is passing through the middle of the MG aperture which must mean that your height measurement doesn't include the 'tab', and checking my dims I find that I'm almost a perfect match for height in that area. So, overall not too bad for scaling from a set of suspect 1/35 drawings from Bovi.

Thanks again Adrian, the photos and measurments are fantastic, of course my OCD will now make me go back and mod my CAD drawings!

Another question, if I may, from what I can see the Mantlet sits about an inch back from the front Armour plate on the Turret. This allows it room to pivot up and down and, using the bevels on the mantlet, give elevation and depression on the main gun. I know the Mantlet is contained within a 'box' in the Turret but what happens if you're gun forward into driving rain which is hitting the front of the Mantlet and, presumably, running down to collect just inside the turret. Was there a weather seal of some sorts to prevent this, or did they just have some form of weep holes in the turret to flush it out out on to the decks?

Edited April 14, 2018 by Old Git
spolling

[David Herbert]

There are drain plugs in the floor !

David

[Old Git]

Hi David,

thans for the reply, do you mean that floor of the tank or the bottom of the turret (not the turret basket).? Am I right in assuming that there was water egress between the mantlet and the armour plate of the turret? I'm curious because on a recent trip to TNA I came across a scientific analysis on how cold tanks get in Winter and what the effects are on the crew (all part of designing effective CWW clothing for the crews). They analysed various parts of the tank and how Cold weather affected the heating in tanks, especially in tank  with their engines off!  Interesting stuff and it got me thinking about the severe winter of 44/45 and what it was like for the crews and when I realised that there could be water egress between the Mantlet and the turret I wondered if there was any problems with freezing water inside the turrets, and if so did it have any negative impact on their ability to get the tanks in action in a hurry.

[mike25470]

 

I had to chuckle when reading your comment about the unreliability of "official" drawings, there is nothing worse than reverse engineering a design from an old drawing/illustration/photo and producing a CAD model, then comparing it against a newly found reference, only to find they disagree. Having been there myself I can well imagine the sotto voce anglo saxon mutterings that might have crossed your lips on such occasions. Coincidentally, the 75mm gun/mantlet interface has been on my list as well but for another vehicle (AEC Mk3 armoured car), so this thread has been very useful. My thanks to all that have contributed.

I visited Bovington some time ago and measured the 75mm barrel and muzzle brake on the wall display, albeit using the front face of the breech as my reference point, but my dimensions are pretty much in accord with yours. 

Ref your question regarding the breech, I found some interesting pictures of the gun and breech disassembled here, which may be of interest http://www.armourinfocus.co.uk/churchill/restoration/turret/gun/index.htm. I think this page might be an orphan from a migration to a new web site, nevertheless, it san be navigated using the back link at the bottom of the page.

I am not sure how practical it is for you to get to Bovington, but they have the QF 75mm on an instructional mounting in the WW2 tank hall. It has easy visual access to the underside of the breech to take pictures and measurements (within the limits of creaky knee joints etc), but much easier than in the confines of a turret. Unfortunately it does get moved around from time to time, and is generally tucked away in a quiet corner! If Bovingtion is out of range for you, I would be happy to take some photos on my next visit, probably in the next week or so.

regards, Mike

Edited May 14, 2018 by mike25470
Error

[Old Git]

For a couple of weeks now I've been suspecting that some of my dims for the tube on the 75mm are wrong, not all of them but certainly one or two and it's been a bit of a struggle to ascertain where I was out, although I was beginning to suspect it was around section E. The principal reason for suspecting an error was an old chart I found on my NAS which one of my contacts had sent me years ago. It purportedly came from a WWII Soviet evaluation of the Cromwell Tank and when the various labels were translated into English it gave some interesting info, especially on 'Length of Barrel (mm)' and 'Total Length'. As the sum of my dims did not come near enough to these figures I was beginning to doubt myself. The translated table is shown below...

So I did a bit of digging and in the CHURCHILL VII AND VIII INSTRUCTION BOOK. ARMAMENT SECTION Covering ORDNANCE, Q.F. 75MM. MARK V and VA... Fitted to CHURCHILL VII. I found the following information...

ORDNANCE, Q.F., 75 mm., MARKS V AND VA
        
    The 75mm. Marks V and VA guns fire fixed ammunition and are designed for use in tanks as replacement guns for the Ordnance Q.F. 6-pr. 7-cwt., Marks III and V. The barrel is externally similiar to that of the Ordnance Q.F., 6-pr. 7-cwt., Mk. V.
    Firing is by percussion, and obturation s effected in the usual Q.F. manner by the radial expansion of the cartridge case in the tapered chamber of the barrel when the gun is fired.
    It may be noted that, with the exception of the the extractor levers and the firing pin of the striker, the majority of the parts comprising the breech mechanism are identical and, therefore, interchangeable, with similarly named and numbered components of the Ordnance, Q.F. 6-pr., 7-cwt., Marks III and V
    Alternative designs of varous parts are introduced from time to time and incoporated in the equipment, usually to simplify manufacture and thus hasten production. Parts of guns in service may therefore be found to differ in some particulars from those about to be described, but these differences do not usually affect the functioning of the weapon, although they may occasionally concern the user when dismantling or reassembling the weapon. Whenever possible, particulars of alternativr designs have been given.
    Except where stated to be otherwise, terms such as left, right, front rear, upper and lower describe the position on the assembled gun of the various parts when viewed from the breech end with the gun in the firing position.

    PARTICULARS
    
    Diameter, bore ... ... ... ... ... ... ... ... ... ... 2.953 ins.
    Distance to centre of gravity from breech end -
        Barrel ... ... ... ... ... ... ... ... ... ... ... 32.15 ins.
        Barrel with breech mechanism and muzzle brake ... .32.6 ins.
    Gauge, Striker protrusion ... ... ... ... ... ..No. 40 (0.148 ins. to 0.158 ins.)
    Length -
        Barrel, without muzzle brake ... ... ... ... ... ..107.8 ins.
        Rear face of breech to muzzle brake extremities ...118.576 ins.
    Weights (estimated) -
        Barrel ... ... ... ... ... ... ... ... ... ... ... 3 cwts, 3 qrs. 2 lbs.
        Breech Ring ... ... ... ... ... ... ... ... ... .. 1 cwt, 1 qr. 21 lbs.
        Barrel with breech ring and muzzle brake -
            with breech mechanism ... ... ... ... ... ...  6 cwts, 0 qrs. 20 lbs.
            without breech mechanism ... ... ... ... ... ..5 cwts, 1 qr. 16 lbs.
            

The data from Churchill book seemed to confirm the Soviet Gun table so I now knew that I'd made a mistake somewhere. I put it to one side until I could find time to dig deeper into it but, by happy coincidence, I had a PM today from Mike with revised dims which he'd taken from the 75mm tube on display at Bovi.

Mike,

thanks for that very timely piece of info, it saved me a trip to Bovi. For your own information you may wish to know that the threaded section of the tube is 103mm and requires 11 turns to be firmly threaded into the Breech Ring. The Soviet total length (2859.3 mm) would appear to be from the rear of Breech Ring to the end of the Tube, whilst the British/Churchill total length (118.576 /3012 mm) is from rear of Breech ring to end of Muzzle Brake. Doing a quick mental calculation I'd say the portion of the tube, therefore, held within the muzzle brake is, roughly, 85.6 mm 😉

Pete

 

Edited May 17, 2018 by Old Git
trying to fix the crazy formating, especially with the quote function.

[Old Git]

Mike is going to re-check his dims before posting them here and when he does that I shall edit the dims in my original post .

Now I want to move on to the 75mm Breech Ring. I've worked up some basic drawings showing the breech ring from various angles and have worked out some dims, I've be very grateful if some one can confirm these dims and fill in the gaps where they exist.  There are four drawings in total and I'll put them in four separate posts, with their various dims, to keep things better organised. As per the quote from the Churchill manual, "...Except where stated to be otherwise, terms such as left, right, front rear, upper and lower describe the position on the assembled gun of the various parts when viewed from the breech end "

So, first up, Drawing No. 1, Breech Ring, Left-Side Profile

Below are the dims, as far as I can work them out and I'd be grateful for confirmation and/or filling in of gaps where they exist, (i.e. I, L, & O). Also, Bore 1 /1a has me intrigued and I'd like to see a better photograph of it. It appears to be counterbored but to what depth and is it exactly the same on the left side? What is the little slot right below is this a feature in the Breech Ring or does it allude to some part of the gubbins which goes into the bore?

A = 223 mm
B = 399 mm
C = 284 mm
D = 115 mm
E = 120 mm
F = 46 mm
G = 39 mm
H = 35 mm
I = ?
J = 60 mm?
K = 41mm ?
L = ?
M = 38 mm
N =  43 mm - (depth of lug)
N1 = 64mm - (width of lug at top)
N2 = 124mm - (width of lug at bottom, before curve)
O = ?
P = 58 mm

Bore-1 = 32 mm,  (all the way through) counter-bored to 40 mm and depth of xx mm ?

Bore-2 = 141 mm, counter-bored to 165mm, threaded, depth of 86 mm

Bore-3 = 20 mm, (20 mm x 20 mm from top and back face)

Bore-4 = 46 mm, counter-bored on rear face to 68mm and depth of 16 mm

Edited May 28, 2018 by Old Git

[Old Git]

Next up Drawing No.2, Breech Ring, Rear Profile..

O =
Q =
R = 255 mm
S =
T =
U =
V =
W =
W1 = 35m
X =
Y1 =
Z1 =
Z3 =

[Old Git]

Drawing No. 3, Breech Ring, Right-Side Profile

RA = w x d ?
RB = w ?
RC = h x l x d ?
RD = h x l x d ?
Bore-1a =  how does it differ from right side

Edited May 28, 2018 by Old Git

[Old Git]

Drawing No.4, Breech Ring, Top View

F = 46 mm
G = 39 mm
H = 35 mm
P = 58 mm
Q =
R = 255 mm
T = 95 mm
U = 127 mm
V = 114 mm
Y2 =
Y3 =
Z1 =
Z2 =
RA =
RC =
RD =