# Milling a slot



## neil_1821 (Oct 5, 2018)

A rather vague and simple title, however I have a query on what would seem to be a simple operation. 

I need to mill a slot is some cold rolled square bar, just enough for a wooden inlay, roughly 20” in length. My dad has said that there is a large possibility that by breaking the surface tension on one surface it’ll most likely warp like a banana. Is there a heat treat process that can be used to prevent this? Any other ideas or advice are welcomed!


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## WOB (Oct 5, 2018)

Stress relieving the bar will prevent warping.     20" is too long for most hobbyist's heat treating furnaces, but you could put it in a fireplace or BBQ pit and let it heat up and cool down overnight.   A propane weed burner or a big oxy-acetylene rig with a heating tip.  You need about 1000 deg F. 
Spot heating with a torch might cause warpage due to uneven heating, but once done, the bar could likely be straightened.  I would simply buy a new bar of hot-rolled steel and avoid the problem altogether.

WOB


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## neil_1821 (Oct 5, 2018)

Yes I was thinking I’d end up buying a new bar of hot rolled steel to avoid this but wasn’t aware if there were any alternatives. For the price I might as well buy some new material


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## nel2lar (Oct 5, 2018)

Sometimes the subjects are so brief and lack of information of what is actually going to be done. There are many ways to skin a cat but if we do not have enough information how can we start. Sending you down a road with the info given so far is about worthless. You do not need a mill to make a slot in metal. Drill holes in a line and start working out the waste with a metal file to form the slot. Good luck and remember there are more than one way.
Nelson


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## Charles Lamont (Oct 5, 2018)

Width & thickness of bar material, width & depth of slot? Warping might be a problem or not. Nelson, it is not the process that causes distortion, it is inherent in the material. If the metal has internal stresses from the rolling process then removing the surface only on one side alters the balance of the internal stresses.


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## neil_1821 (Oct 5, 2018)

To give slightly more context the square bar is roughly 1” and on one side I’d need to remove around 6/8 at around 5-10mm deep. 
I suppose the best thing to do would be to try it and see how the metal reacts. If it does warp I’ll have to cross that bridge when/if it comes l


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## nel2lar (Oct 5, 2018)

I had a bad habit of anticipating what might happen before starting and found more than not there was nothing to worry about. And furthermore it is just a piece of metal and there is always another one to start over. 
Good luck
Nelson


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## mcostello (Oct 6, 2018)

Could You make up a rectangular hollow box out of parts?


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## nel2lar (Oct 6, 2018)

Yes, If you have a welder you could make a square tube of any size.


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## neil_1821 (Oct 6, 2018)

I do have a welder but I’m not sure that’s the way to go for this. The square material is for the outer frames of a 5” gauge locomotive and I wouldn’t be too confident that a tube box section would bear the weight. 
The reason I wanted to mill a slot is because the original had a metal plate, wood and then another metal plate on the back. As I’m trying to replicate it as close as possible, machining the wheels from bar stock and creating square headed bolts I thought a wooden inlay would be the best way to go about it. I think it’ll be a case of trial and error and see what happens.


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## abby (Oct 6, 2018)

If the prototype frames were a timber sandwich , so to speak , why not make the model in the same way ? laser and high pressure water jet have become the norm now for cutting out steel loco frames , both are readily available and cheap here in the UK.
Dan.


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## Dalee (Oct 8, 2018)

Hi,

Couple of things that you might do.

First, buy hot rolled material instead of cold rolled material. The cold roll/finish process is what puts the internal stresses into it. And hot rolled material has far less internal stress in it. Or buy stress relieved cold finish material, like say 1144 stress proof. These are the least effort methods of dealing with warpage. But some might cost a bit more.

Sometimes we are stuck with no choice but to use what is at hand. Then we need to think about maybe doing extra steps to deal with undesirable results like warpage. How to deal with warpage? Sometimes judicious spot heating after machining can remove built up stresses. Not my favorite way personally, but I've seen it work and have done it a time or two myself.  Or just straightening things back out with a press or vice solves the issue. And yes, you can use a straight edge and your eye to get things straight. It gets done a lot.

And finally, try and understand what you can get away with. And don't be afraid to get away with it if you can. This is why we have tolerances. Never be afraid to use your tolerances.


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