Will iron rivets expand to fill oversized hole?

[Arthur Jones 2Participant @arthurjones2]

Hi again all,

First of all, thanks for the sensible advice on piston rings a couple of months ago – please see that topic for an update.

Now another problem, this time of my own making.  I am just starting to fit the hornblocks to the frames, which are to be riveted with 3/32″ iron (steel?) rivets, snaphead on the hornblock side and countersunk (hammered from the shanks during riveting) on the frame side.  But due to carelessness on my part, most of the holes on one of the hornblocks have come out perhaps 10+ thou too large (so perhaps 0.105+ instead of 0.093″), and those in the frames perhaps 5 thou too large.  I’ve managed to make the others fairly snug (maybe a thou or two clearance).  At least this mistake has occurred on a rear hornblock (rear coupled wheel axle), not the middle (driving) axle where I guess the bending moment in the frames is the largest and where the strengthening effect of the hornblocks is most needed.

So my question is: should I be worried about this amount of slack before fitting the rivets, or will it get taken up when I start hammering the rivets to expand the shanks into the countersinks, and the parallel parts of the shanks (hopefully) expand?  I guess the alternative is to silver-solder plugs or sleeves into the (gunmetal) hornblock castings then re-drill, but that risks creating mess and irregularities that will require more fettling and may result in more harm than good.

Sorry if this question betrays my lack of formal training in fitting etc. – I’m not a time-served craftsman and am learning by my mistakes (of which there are many!).  Many thanks in anticipation of some wisdom here.

Kind regards to all

Arthur

 

[vintagengineerParticipant @vintagengineer]

Yes they should expand to take up the slack. Are you riveting hot or cold?

[Clive FosterParticipant @clivefoster55965]

Hot riveting they will almost certainly expand enough provided the rivet stays hot enough. Difficult with small rivets.

Cold riveting the results will very much depend on technique and rivet material.

Hopefully you have rather more rivets than you need along with suitable off cuts material so you can make a few joints with both correct and oversize holes that can be dissected to see what actually happens. 10% (ish) of nominal diameter is quite a lot of expansion.

If the test specimens work out OK you can be confident that all will be well. If they don’t you know you will have to find a more engineered solution. As its only one horn block with the problem the worst case solution of making a few oversize rivets is a practical proposition.

Clive

[parovozParticipant @parovoz]

My perspective….. In smaller model sizes iron / steel rivets are significantly stronger than needed for a good solid assembly, copper or brass will work fine too, so there is really no ‘strength’ issue.

If the horn block is a good fit in the frame, it will not move, therefore the size of the hole becomes less relevant, the main point is the ‘clamping’ force applied by the rivet. The countersink in the frame will ‘hold’ the rivet in the planar directions once it is set, bashed into place….

So even if the hole is a bit loose, it will do the job, no one will see the holes once complete, and it will be fine!

However you might need to allow a little more rivet protrusion before setting it down to account for more rivet material filling the slightly oversized hole.

All the best.

[MacolmParticipant @macolm]

If the worst comes to the worst, and if you have a not too worn lathe, it is reasonably easy to turn up free machining mild steel sleeves with three thou or more wall thickness in the short length you would need. Also needs sharp tools of course.

[Robert Atkinson 2Participant @robertatkinson2]

Surely easier to turn up a few rivets with oversize shanks?
Standard practice on aircaft with damaged rivet holes is to use oversize rivets which are readily available.

 

[Arthur Jones 2Participant @arthurjones2]

Hi all,

Thanks for all your advice, and sorry for the slow response.  All responses are valid points!  Parovoz’s point is fine and the countersinking will indeed take up any slack, but in my case the slack is mostly in the gunmetal casting which is not countersunk.  And yes, my hornblocks are a pretty tight fit in the frames and actually need tapping in with a soft mallet.

Actually, I am fairly low on the rivets which are close to 3/32″ (others of that nominal size are actually a thou or so smaller) and don’t want to pay the post on new ones especially if they are not quite the same as the ones I’ve got. So rather than do the experimenting as suggested or just risking it (and before I saw the last three posts from Parvaroz, Malcolm and Robert), I machined sleeves perhaps 4 or 5 thou thick, large enough to be snug fits in the oversized holes but with a reamed hole to take the rivet shanks, and silver soldered them to two of the slightly undersize rivets.  After cleaning-up this effectively makes rivets with a stepped shank, with the diameter near the head being oversize as required.  I reckon that only the lowest two rivets are needed like this as only those two holes are 7-12 thou too large.  I think the others are less critical anyway (they hold the middle and top of the hornblock which are less likely to move anyway and bear much less of the bending moment on the frames) and the slack is only a few thou so I’m hoping the slack will take up when I form the countersinks. The slightly undersize shanks are I think unimportant because, as Parvaroz says, the countersinks will take up any slack.  Good point about making the blanks a bit longer than normal.

This morning my wife and I riveted a “normal” hornblock in as I’m fairly rusty on riveting (went OK but the blanks were a bit long so tended to buckle rather than squash, and will need more filing to get them flush), but after doing one more “normal” one for practice I’ll see how I get on with the one I messed up!

Thanks again all

Arthur

 

[parovozParticipant @parovoz]

OK Arthur !

Sounds like a good plan !

All the best.

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