How flat is flat?

[firedog239]

Concerning pans and griddles, how flat is considered flat ? Are we talking dead flat or is a 1/32" wobble considered OK? I've got a #14 round griddle with app 1/32" wobble. I can turn it on my stove so it will sit steady so it's not a big deal. I just want to know what is considered within normal limits, Thanks

[C. B. Williams]

Interesting question. Casting is not the most perfect way to manufacture anything metal. All cast "parts" if needed to be very close tolorence must be machined. I have never seen this question before. Personally, I would think 1/32", is within a degree to be considered FLAT, especially in a large piece. Remember, a heat ring is about 1/8" which is 4 times the amount you asked about. As I have talked about before, heat expands metal. When the molten metal cools it contracts. With a complex shape, varying stresses occur in many directions as the contraction occurs. This results in a contortion of the original mold pattern. Gosh, what a difficult question!!!!

[C. Perry Rapier]

Mike, welcome to WAGS. C.B., I think you did a good job on that question, you bein an architect an all. Just the nature of cast iron is a little sloppy as opposed to something thats been machined.

[moosejaw]

Yes, good answer C.B.

[Duke Gilleland]

When you consider the nature of the beast, I think we are very lucky to find as much "flat" (old) iron as we do. Whole lot of abuse out there by those who don't see the "beauty" :'(

[Steve_Stephens]

I have a NOS slant ERIE skillet with label still in the pan that has a slight bow upwards so it was cast that way.  I am happy to get pans with no more than 1/32" and am delighted to have them perfectly (Perfect!) flat.  The flatness is important to keep fluids from running to a part of the cooking surface you don't want it to and to keep hot spots from developing if certain spots of the cooking surface are in direct contact with an electric or wood top stove.  When checking flatness I use a drafting triangle to check any crowns or bows or just any good straight edge.  I had a Wapak 14 skillet where the cooking surface bowed up from the top of the pan and bowed down from the bottom.  The center of the pan was much thicker than the edges of the cooking surface.  So much for quality control or pattern making to perfection.

Steve

[g0028069]

Is there any correlation to the size of the skillet?  Meaning as the skillet gets larger is there likely to be more chance of the skillet being bowed?  Are all large skillets (12 and up) bowed?  Is this only applicable to vintage skillets?  I asked about vintage skillets, as I have a newer Lodge that is nice and flat, now if it was only ground.

[C. B. Williams]

Any large skillet is more likely to warp, for two reasons. One, it is usually larger than the eye therefore it is heated more un-evenly than smaller skillets, this causes un-even expansion and contraction thus warpage. Two, in the manufacturing process any large piece is harder to keep "true" than any small piece, simply because simply because expansion and contraction is relative to the size, so, if a 1 inch piece expands 1/100 of an inch, a 12 inch piece expands 12/100.

[g0028069]

I think the expansion is closer to .000058 of an inch per inch per degree of Faherneit change for cast iron.  So with a 400 degree temperature change on a 1/4 of inch thick skillet bottom there is approximately .006 of an inch of movement (the thickness of 2 sheets of copy paper).  

However, how much distorsion can be introduced through normal use?  I can't imagine that a #11 or #12 is going to have 3/16 of an inch bow in the bottom from normal use.   Steve mentions that he is pleased with no more than a 1/32 of an inch of warpage on a large skillet.  I have never used skillets larger than a #10 on the top of a stove because it is inconvienent and the heat becomes uneven.  So most of the use that a skillet this large sees at my house is inside of the oven and not on the stove top.  Now I have run into a prevelant theory that any large skillet is just bowed and that is the way it is.  Therefore the folks in WAGS who collect and see lots of large skillets should be able to validate if all large skillets are bowed.  I don't want to collect bowed skillets if I can get flat skillets.      

[Steve_Stephens]

So with a 400 degree temperature change on a 1/4 of inch thick skillet bottom there is approximately .006 of an inch of movement.      

Skillets are more like 3/32" or a little more in thickness on the bottom.  Go ahead and break your best skillet and check the bottom thickness.  I bet you would be amazed at how thin the pan really is on the good, older pans.

When I see sellers stating the pan "sits flat" it really doesn't tell me anything other than there may be no wobble when set on a surface (that may not even be flat?).  There could be 1/4" upwards warp and the pan will still "sit flat" and have "no wobble".  I like to check with a good straight edge.  Getting sellers to understand exactly what you want to know is near impossible unless you run across an engineer or machinist selling the pan.

My observations over the years on thousands of skillets do not show me that larger skillets are more bowed or warped than smaller ones although, once you get down to about a No.6 size skillet, the pans do seem to be warped less frequently.  I think most warpage comes from rapid and uneven heating and cooling.  My mom had an early small TM 10 Griswold skillet that was bowed unevenly downward which I would have to saw was caused by uneven heating at one or more times.

If you sight across a pan's surface you can usually see any bows.  If I can't see any up or downward crown or bow or high or low areas the pan is good enough even without measuring although measuring with a straight edge may still show some bowing.

A pan's bottom should not extend in any part below the heat rim.  Or, is a flat bottom skillet I like to keep any bow less than 1/16" and I think that is reasonable if the bow is uniform over then bottom and not concentrated in a smaller area.

You can find skillets with Perfectly Flat bottoms in any size and can fine NOS pans with slight bows.   I'm not sure that any shrinkage at the time of manufacture when the pan cools is the cause of bowing when new but it could be.  Would be interesting to see the pattern and mold that my slant/E No.8 skillet was made from/in to see if they had the same bow as the pan does.  If not then the pan did bow during cooliing as it still has the original paper label on the bottom.

If you are buying sight unseen you have to impart what you want to know to the seller of the pan and not ask questions such as "does it sit flat" or "wobble".  That won't always give you an accurate description of the pan's flatness.

Steve

[Scott Sanders]

In regards to flatness, has anyone ever tried or had any success in re-heating and trying to remove any bows or warps??  Before I started using cast iron skillets or knew about their care, I tried to straighten out the bottom of an old skillet with a rubber mallet.  Don't try it, that method doesn't work. ;D ;D ;D

Scott

[Troy_Hockensmith]

There was a fellow on here for awhile (darryl) that was gonna experiment. He was gonna use heat and a press. I don't think he ever gave feedback on the results. He was the one that set up a super duper electro cleaner using an arc welder.  

[Greg Stahl]

Darrel did do it and it worked well and removed the bow.  He reported back that the first several cracked or re-bowed, unless he cooled them slowly.  THe other issue however, was the significant scaling and that he would have to work that out.  Never heard back after that, as he moved to WI (I believe) and did not renew his membership to WAGS, but has emailed me several times saying that he was going to renew, but I have not heard from him in about 8-12 months.

[C. B. Williams]

Brik: Thanks for the expansion and contraction factors on cast iron. I didn't bother looking them up and was only using 1/100 as an example to point out the fact the figures are all relative to the size. In either case, cast iron, not being very flexable will not tolerate much movement. Therefore, larger pieces are more apt to have problems.  

[g0028069]

I appreciate all the help on this issue.  I don't think there would be draft on the bottom of the skillet pattern.  I would expect shrinkage at the last point that was still molten when the skillet was cooling.  What you may be seeing is the lack of the sand being tightly compacted in the cope or drag.  I suspect it was shrinkage during cooling since the sprue can no longer feed the shrinking skillet as the gates have quickly cooled and hardened.  I guess to expect a certain amount of "rocking" or imperfection in the larger skillets and what amount is acceptable will be a personal decision.

[Steve_Stephens]

I have noticed that the majority of early, gated HANDLE GRIDDLES seem to have a slight twist or warp to them but not so many of the early skillets.

Cast iron pans do shrink when cooling but I think they cool pretty evenly since they are surrounded by the sand mold.  Since they cool slowly and evenly I don't think there are many stresses that build up in the iron.

When trying to flatten a bow in an old skillet you have to heat the pan up VERY HOT but I don't know how hot is very hot.  If a scale appears on the iron it is because it was heated more than a dull cherry red.  The iron, when cast, is much hotter but is surrounded by the mold which occludes oxygen which would cause a scale.  Also, you have to cool the flattened skillet very slowly over time to keep stresses to a minimum.  Still, I think that some significant stresses will build up in the iron when cooling.  For all practical purposes I consider iron to be non-repairable as for warps and bows.

Not so with cast aluminum.  It's a maleable metal and very warped bottoms can easily be pounded back into very flat surfaces.

Steve

[miniwoodworker]

Steve, what about carbon steel? I have a couple of skillets that I've cleaned up for backyard cooking. One cost a buck, the other was free "because it's so rusty". Both cleaned up well. However, the bottoms are warped. Any tips on those?

Thanks, in advance.

Lee

[Steve_Stephens]

You may not get those carbon steel skillets flat again Lee but it can be done if you shrink the metal where it is bulged.  Auto body people have the tools and knowledge to do that or you might try heating the bulged areas some with a torch followed with a cooling swipe from a wet rag and repeat as necessary.   I haven't tried it but did try shrinking metal on a car fender with a metal disc on an angle grinder.  The disc was somewhat wavy and would produce even heat where you wanted it.  You would follow up using the disc with a wet rag.  Worked pretty well and I don't see why something similiar would not work on your skillet.

Steve

[miniwoodworker]

Thanks, Steve. I'll give it a try after the holidays. With all the stuff that has to be done in the next few days, I doubt that I'll have much time to spend in the shop before then.

Thanks, again.
Lee

[Steve_Stephens]

I don't think you have to or want to heat the bulged areas too hot.  Maybe 150 to 200 degrees but I really have no idea.  Just don't want you to start heating to red hot or very hot.

Steve