Horn plates bowed when adding rivets

I’m in the process of making the horn plates for my 3” traction engine. All was going well before I started to add the decorative round head rivets around where the boiler would be riveted up in the full-scale engine. The bearings, crank shaft, spectacle plates, etc all fitted and ran well and I’m fairly sure the plates remained flat after I’d drilled and countersunk the holes for the rivets because the other plate I haven’t riveted yet has the countersunk holes ready, and is still flat.

However, when I added the decorative rivets, the once flat horn plate has now a pronounced 1/8” bow in it away from the rivet heads (convex on the inside of the plate), mostly from the double line of rivets across the middle of the plate.

On the outside, the rivets look good, it's just that the plate is now bowed.

When installing the rivets, the plate was drilled and then countersunk on the inside. Each round head rivet was then inserted into the hole, the head placed on a rivet snap and the inside was riveted over until flat and filling the countersink.

My questions are:

1. Why would adding decorative rivets cause the plate to bow like this
2. What might be the best suggestions for correcting the bow
3. How to I prevent the other plate bowing in the same way

Any suggestions and ideas would be truly welcome.

Many thanks

Martin

I would think that hammering the rivets in a line across the plate with the plate being weakened by the rivet holes has allowed the plate to bend slightly, all the hammer blows would be on the same side of the plate. My suggestion is to apply suitable pressure to the plate to bring it back to straight using some form of press. An 1/8 of an inch isn’t very much but I realise it is annoying that it didn’t remain flat.
Dave W

You don't need the rivits to be structural so no need for a full depth CSK. Kist a touch with a 60deg ctr drill will be enough or even a quick twist with a taper pin reamer if you have one that size. Cut the rivit shanks to be equal to the thickness of the plates and a couple or 3 blows to a Ctr punch will be enough to expand them into the holes.

The expanding rivit CSK head will have pushed the sheet outwards and given the distortion that you are getting.

Not sure how well this will come out but the lighter marks are single Ctr punch marks and you can just make out the outer 1/8" dia of the rivits on my 2" Fowler

Edited By JasonB on 28/02/2020 19:33:59

Jason is correct, that's one hell of a countersink! For the dummy rivets on my 6mm thick hornplates I just did a couple of turns with a taper pin reamer and a couple of clouts with a hammer per rivet.

Neat riveting though; I always seem to end up with hammer marks in the wrong place.

Andrew

Thanks for all your thoughts. As Dave W suggested, I've tried pressing the bend out and that has worked surprising well! One main press across the double row removed most of the bow, and then I did the same again in a few more places and it's now as flat as I can hope for. Nothing more than a gnat's hair under the straight edge now.

I'll take your suggestion Jason for the second plate and hopefully that one will remain flat without needing to resort to brute force.

Thanks for the complement Andrew. I should admit to carefully chosen photos, because there were certainly a couple of rivets drilled out and re-done when the rivet snap moved and left a nasty dent across the centre of a few otherwise nice round heads!

I would suggest the side you hammered has stretched and is fractionally longer.