Metal fatigue and clock making

[Ian S CParticipant @iansc]

John, how to stop a crack in sheet metal eg an engine cowl or similar,  drill a hole at the end of the crack, it my need a patch to strengthen it, but the hole stops the crack. Ian S C

[RJWParticipant @rjw]

Hi Terry, I know exactly what you mean about cutting dovetails ;&gt)

Having cut more than my fair share of them for various jobs, I personally just find them a total PITA to mark out and cut compared to a general 90 deg joint.

 

 Visible dovetails on a drawer joint is one thing, hidden dovetailed swaged joints in a clock frame just seems  a touch of overkill to me if I’m honest.

I would say one exception to this, would be where the clock frame itself is being used to hang the assembly on a wall, and then have to support a couple of 5+ kilo lead weights too, dovetails would probably then add more strength to the frame to help stop it twisting.

These days I look for the easier option than making work for myself,
especially when the joint isn’t going to be seen, plus the time saved
gives me more time for a coffee break skive!

Just my own opinion of course, others may feel differently.

 

Hi Ian, in my experience, drilling holes to stop cracks doesn’t always work, as quite often the crack will simply propogate in other directions from the hole.

It ‘may’ be ok as a stop gap measure under certain circumstances, but it’s little more than a temporary cost cutting bodge in reality.

I’m well used to seeing holes drilled and patched on the alloy panels of club aircraft, but such repairs have as a rule, generally been confined to non critical areas, I shudder to think of any such repairs than may have been done (it’ll get it to its next CofA etc) that aren’t visible on a ‘walk round’!

 

Such a ‘mod’ even with an added patch doesn’t  restore the original integrity of the structure which has suffered the crack, and it doesn’t rectify what caused the crack in the first place, and can in fact cause further problems, because a rivetted patch can cause an affected area to be too rigid locally when it should have some flexibility, and will then actually flex around the rivets!!

 As an added angst, you then have further distortion and self inflicted ‘stress raisers’ generated in the structure from patching such a fracture, caused by the simple act of  drilling and riveting the piece, but this is taking us well away from Clive’s original question now.

 

At the end of the day, Any material which has cracked, whether from reversals or vibration, has suffered catastophic structural failure due to exceeding its original design limits and is of no further use for its intended purpose, which brings us back to Clive’s clock frame as a case in point, where bending the steel cold, caused the inherant limit of elasticity of the steel he was using to be exceeded, and it snapped.

 

As John1 stated early on Clive, if you’re still running with the original bent steel version of the frame, get your blowlamp out and heat it up – before, during and after the bending routine!

 

Wishing Everyone a Happy Christmas and a Wonderful New Year.

 

John

 

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[TerrydParticipant @terryd72465]

Hi John (RJW),

 

The only reason I suggested the dovetail is that it seems as though Clive wants to create an authentic looking reproduction as John Wilding did.  The part to be made would be the wall frame which is cantilevered and the two bends are very sharp with no real radius to mention and any bending would of course involve an external radius which would not look very authentic, hence the original instruction by JW to saw cut and bend repeatedly until such a 90degree bend was obtained.

 

My suggestion of the dovetail was because it is strong (the clock and its weights hang from it) and it involves no welding which was Clive’s wish.  I agree with you about the need for accurate marking out and cutting with wood joints when cabinet making, although with practice it is not onerous, I can cut a row of dovetails faster than most people can set up a dovetailing jig.  However metal is much more forgiving and the angles of the dovetail can even be guessed as the tails are marked from it.  It only take half a dozen cuts with a junior hacksaw and a few strokes of the file for the joint to be ready to assemble.  Any minor inaccuracy in the cutting is taken up by the swaging process which also acts to lock the pieces without any heat treatment..  Especially as we are only dealing with 1″ x 1/8″ bar.

 

With a minimum of practice such joints can be produced faster than you could get the steel up to bending temperature with most propane blowlamps.

 

 

Best regards and seasons greetings,

 

Terry

[Ian S CParticipant @iansc]

  Yes John, only tempory repair, most of the aircraft were aerial topdressing machines, high hours, and operating of rough farm airstrips, with only miniuts in the air before returning to the strip, to be loaded (overloaded) and off again.  Aircraft were often in once a week for their 50 or 100 hr inspection, were as privately owned planes often did’nt get to 50hrs before they came in for annual inspection.  The 185 was never designed for that sort of work, and we went on to use the A188 Agwagon. The back bone of the industry was, and is the American designed, New Zealand built Fletcher, and its turboprop successor the Cresco. These aircraft are stressed to better than + -8g.

                     Ian S C

[TerrydParticipant @terryd72465]

I’ve just included the only photo I have of a dovetail joint  on a simple oscillating engine made for my grandson, the majority of my models were destroyed by fire.  The joint is between the upstand and the base. This was not a swaged joint but was designed for silver soldering.  The photo shows it after soldering and pickling but before final cleaning up and assembly of the engine. 

 

The whole process including silver soldering took about 10 minutes. Not bad when compared with using screws with the drilling and tapping (not to mention possibility of broken taps) etc,

 

 

 

Best regards

 

Terry

 

 

[Ian S CParticipant @iansc]

Just checking the firures, and I see the the numbers for g loading has been bought down from the American test figs to +3.56 and -i.42, the cocpit section was tested on  the first airframe to 40g. We still seem to have a thriveing aircraft industry in NZ, one vertion of the Cresco a light transport, used for parachute jumping. Another a light military reconnaisance aircraft. Ian S C

[TerrydParticipant @terryd72465]

Hi Ian,

 

We’ve come a long way from Medieval clockmaking

Terry

[RJWParticipant @rjw]

Ian, do you think the G loading was reduced because the certification bods got wind of all the patches and rivets ramping up the AUW beyond load limits <LOL>

Sorry couldn’t resist, but Wow, 8g’s limit on an airframe loaded with nasty chemicals is awsome even if few pilots around could sustain those figures and stay conscious, but stitched up with alloy plasters too makes the mind boggle.

I’m always amazed that wings don’t get more frequently torn off crop sprayers …. or perhaps it’s just that we just don’t hear about it! 

 

When you think about it, metal fatigue rears its ugly head in just about everything we do at some point!

 

 

Terry, Nice joint!

I have quite a few of Wilding’s books, but not for that particular clock, so I was mainly using my imagination to visualise the joints, and dovetailing would certainly add mechanical strength to the frame.

I’ll have to have a rummage through some of my clock books to see if any photo’s illustrate just what types of jointing methods were used back then.

I must confess to not being too interested in iron frame and lantern clocks, they’re a bit too ‘agricultural’ for my taste, and probably why I never bought that particular book.

 

Although the majority of my clock experience has been repairing or making replacement bits for what was already there, I do have a few skeleton clocks I’m making and slowly getting my teeth into, but they’re all brass frames with pillars.

I may get around to posting some pic’s up at some point.

I’m sorry to hear about you losing your models that way, all that graft and attention to detail going up in smoke must have been a soul destroying experience!

 

Regards

 

John

[Ian S CParticipant @iansc]

No John, I think they are being a bit more honest, the company that did the design and initial build was trying to sell the FU25 light ground attack aircraft to anyone who would buy it, it was a similar design, and a Japanese co bought that and built some.  So they were trying to impress. I think the only fatigue problem with the Fletcher is with the fin attachment, the problem has shown up as the hp has increased from 225hp to 750hp.  I think it has caused one crash. Structual failure may have caused some crashes, but regular inspection seems to cover most problem areas. Ian S C

 

 PS We’d better quit while the goings good or we’ll get grounded!

[TerrydParticipant @terryd72465]

Hi Guys,

 

Several threads seem to end up as ‘aeroplane’ threads, would it not be a good idea to start a thread based on this theme for those interested and those of us who aren’t too bothered could ignore it.

 

Seasons greetings

 

Terry

[RJWParticipant @rjw]

Hi Ian, a very off topic reply; we’re already grounded, been stuck under a foot of snow for a month, anyway, will probably see you in the new ‘Aviation related’ topic that someone has kindly started!

 

John

[Gordon WParticipant @gordonw]

I don’t know what metal fatigue has to do with the original question. Wasn’t it about bending a bit of black bar ? Drilling a hole at end of a crack is a standard technique, good enough for cracked cylinder blocks, cracks in chassis etc. etc. If welding a crack in thinnish stuff, start at crack end ,this closes the crack and stops it spreading, probably not used on machines carrying lots of people, esp. above ground.

[Clive CasselParticipant @clivecassel81348]

Dear All

Many thanks for your responses. For my next attempt I shall partly saw through bar then heat to red and then bend. If that fails I shall dovetail cut and join.

Cheers

Clive Cassel 

[Author]

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