Milling long thin work to minimize warpage?

[homebrewed]

I have a question about the best procedure for milling a piece that is relatively thin compared to its length. Specifically, a gib strip. I made one out of brass awhile back and ran into a warpage problem -- compounded by a poor work-holding decision. I DID expect some warpage due to manufacturing stress but made an apparently bad decision after I machined one of the long axes of the piece. I flipped the part over, re-installed it in the vise, and then (and here's where I think I made my mistake) I hammered the piece down and machined the other side. Well, when I removed it from the vise I found that it STILL was warped. I had to do a lot of screwing around to get it reasonably flat.

So my question is, what's the best way to deal with the built-in stress while machining? I understand that it probably isn't feasible to simply machine a piece and expect it to come out perfectly flat, but I'd like to minimize the work I need to do after doing the machining.

 

[Canus]

I've often wondered if using heat to stress relieve a piece of steel before machining would help to eliminate warpage that occurs during machining.

 

[Lo-Fi]

You're into some interesting territory there. Heating should help, though you'll have to look up some data on the material to figure out what temp and how long, and it's not always within the reach of a domestic oven (or agreeable with the other half).

Cold rolled is particularly prone moving, being full of stresses after the rolling process. Many a model engineer has struggled making connecting rods and suchlike, though its common to simply "bend it back" after roughing and keep checking as you approach final size.
Even pros struggle on full size stuff. I know one of the loco projects I worked on that had new coupling rods forged, the machinist sent one of the forgings back to be heat normalised four times during machining as it just kept moving around.

 

[Karl_T]

You want your part to come out of the middle of the stock. Each piece will have its own best method, but in general rough the part out to slightly oversize, then final machine.

Cold rolled low carbon steel is the ABSOLUTE worst for warping, great for making bananas. 4140 pre-hard is about the best.

 

[pdentrem]

Even removal of material from both sides as mentioned. Solid fixturing to maintain flatness and proper heat treat if required. We have made items that are really thin metal foils, and even giving them a light sanding to remove burrs cause warping at room temperature. We have to flip them over and lightly sand the opposite side to back them back to flat. Painfull!
Pierre

 

[homebrewed]

From what I found from browsing the 'net, normalizing steel requires temperatures much higher than what a kitchen stove can achieve. It may be possible to normalize brass in your stove but SWMBO may not approve.

For some (many?) items it's really not possible to remove equal amounts of material from both sides and still end up with the part you want. It sounds like if it is critical one needs to start with the right material to begin with. I knew that cold-rolled was pretty bad for warpage.

The tip about 4140 pre-hard is a good one, thanks!

 

[dbb-the-bruce]

My experience with brass, is that if you don't anneal it, it will warp. Not very noticeable in big blocky shapes but very common long flat shapes.

I personally encountered this when machining two gear racks out of 1/8" brass, 6" long, 1/2 inch wide. Cutting the rack on one side of each stick caused them to bow significantly. When I put the two racks back-to-back, there is easily about a .1" gap in the middle. No amount of fixture-ing would have helped. Once released they would warp.

To stress relieve brass, you need to get it up to 480 - 575 F for 1/2 to 1 hour. You can probably come close with a typical kitchen oven, but that's basically the top end of a kitchen oven. It would probably help - but might end your marriage.

A cheap electric toaster oven (yard sale?) could probably be hacked to have its duty cycle extended so that it gets just enough hotter.

The best bet by far, would be to find someone with a muffle or heat treating oven or possibly a ceramics person with a kiln (they tend to be very picky about what goes into their kilns!). You may also be able to find a used muffle oven for a fair price. If you plan to do a lot of metal work going forward, you will probably end up with one eventually.

I'd start with trying an oven.

Here is a great link on the subject. BTW - heat treating worked for me.

-Dave

Academy of Lagado - Brass - Stress Relieving

[Page created Dec. 25, 2012, last update Jan. 5, 2013] [Added additional references Feb. 22, 2013]] Relieving Stress in Brass by Heat Treating - a Small Experiment While milling some extruded brass for the purpose of making tapered gibs for my 7x12 minilathe, I experienced the well known problem

sites.google.com

 

[homebrewed]

Is it fair to say that just about all cold-worked or extruded metals will exhibit significant warping when machined? Or is that an overly simplistic view?

 

[MrWhoopee]

A self-cleaning oven can hit 900 deg. F in the cleaning cycle. Also gives you a slow cool down.

 

[matthewsx]

A self-cleaning oven can hit 900 deg. F in the cleaning cycle. Also gives you a slow cool down.

If you watch the Craigslist "free stuff" section I'm sure you can come up with a used working oven pretty quick.

john