Factoring in shrinkage whilst cooling

[Joel OF]

At the minute I'm making some very traditional looking gates full of scrolls for some clients who want some new gates on their property to match the design of their existing driveway gates, the picture below shows one nearly finished. This project is presenting me with a series of firsts as these are my first gates, first time of doing this kind of jig work and first time of doing this kind of traditional design.

 

I'm using lightweight 20mm x 3mm (approx 3/4" x 1/8) for the scroll infills to keep the overall weight of the gates down. That's a very springy weight of steel so I also fabricated some rectangular templates (kinda like little windows if you will), from some 20mm x 6mm with internal dimensions that match the dimensions each scroll should be able to fit into. I did this because I knew that I'd never get the 20mm x 3mm material off the scroll jigs without it getting a tad mishapen, so I thought if I've then got these templates as a "second opinion" I'll know each one is okay.

 

Nice idea but I found out after doing 1 row of scrolls like this that the scrolls were then shrinking as they cooled and reducing by approx 2mm in width and height which soon adds up...so I enlarged the template size and tweaked them.

 

So like I say, this is first time time of doing this kind of repetative jig work and I'm wondering if shrinkage is a factor with heavier weights of steel too? Quantify "heavier" however you like. Maybe there's a weight where this stops becoming an issue, or maybe the shrinkage depends on the profile of the steel? The weight of steel I'm working with is bendy enough to be tweaked by hand when it's cold, but I can't imagine some 20mm x 20mm (3/4" x 3/4") square stock being as sympathetic to tweak!

[swedefiddle]

1/8" x 3/4" is no problem to work cold, to fit your layed out drawing.

3/4x3/4 will adjust the same way, just with a longer handle on your tweaker.

 

NO PROBLEM!! :) :)

[JNewman]

In my other trade (patternmaker) we use shrink rules to compensate for contraction of the metal as it cools after it freezes.  Shrinkage is 1/4" to the foot or 2%.  However I have spoken to people who work in commercial forge shops who use 1/8" rules which is 1/8" to the foot which is roughly 1%. I would guess the difference is in the fact steel just frozen is hotter than we would forge it and the fact that even when the outside of steel is frozen  

 

Thickness of the metal does not make a difference unless there is something restricting the shrinkage which there often is in castng but is only an issue in forging if you are using a jig fixture or possibly a die.  The shrinkage still occours but it will show up in a different place.  A good example is a ring, if a ring has a gap and it is restrained from shrinking in diameter the gap will get bigger because the length of the bar is getting shorter.  if the ring is complete and is restricted from shrinking in diameter the steel (or other metal) will stretch as shrinking which will reduce its cross section slightly just like you were to draw it out by forging.  In extreme cases the metal will actually tear,  In a forging situation it would make it very difficult to remove the jig from the center of the ring. 

[ThomasPowers]

As he says a 4' long piece of steel will shrink the same amount from a set temperature no mater what the cross sectional area is so a 1' sq stock will shrink the same as 1/8" sq stock the same length and starting temperature (and ending temperature...)

 

As we manipulate length and cross sectional area a lot in smithing---sort of a definition of the craft; a lot of folks will make a trial piece or pieces recording starting and ending sizes and then using the recorded starting material that gives the end result they like.

[kubiack]

I would think doing a test peice and recording the beginning and ending sizes would be the best solution.

 

I have sceen several scroll patterns which had removable sections which were pin in place. After the scroll was bent the pinned section could be removed which then allowed the scroll to be removed without damaging it.

[Charles R. Stevens]

One of the uri hoffe Blue prints descuses this. If i remember right he quotes a figuer of one hundred thousanths of an inch per degree. As he is using inches I assume that is in ferenhight. For a patern maker dealing in molten steel, thats a bit hoter than us forging by about a third.

[HWooldridge]

Shrinkage can be a factor in fitting forged pieces together.  If you care about the math, the coefficient of thermal expansion for steel is .0000063 in/in per degree F.  Assume ambient temp of 75 degrees F then heating to 2000 degrees - size change is about .145" - so the 1/8" per foot rule is VERY close and sufficient for most any shop application.

 

Thermal expansion is a big deal on railroad tracks and suspension bridges - but can also affect a garden gate pattern if the scrolls are measured hot.

[Charles R. Stevens]

It also maters when putting pickets threw a bar wile building said gate. If you drift a 1/2 hole threw the bar hot then try to put a 1/2" picker threw said hole wile cold you are going to be to tight. Just adjust the math by 1/24 of a foot. Comon practice is to take one upseting blow on your drift (may take more on a big picket)