Thread: Best Mild Steel for Machinability

I'm sure it's been asked, but a quick search doesn't return anything.

What is the best mild steel for machinability?

Specifically we are milling some fixture plates, roughly 1in x 18 x 18. Punching a large hole in the middle, drilling some bolt hole patterns, doing some surfacing, and some side work - about 75% of the material is removed by the time it's done. The only functional requirements are a couple tight bores and a flatness callout (within .001in constrained), but I would really like them to be shiny/pretty as well, at least within reason.

We are currently making similar parts out of A36. The material seems to have acceptable dimensional stability, but we are having a really hard time getting good surface finish, and we are struggling with tool life a bit on some of the smaller (<.050) tools. I've cut a lot of other steels, but don't know enough to confidently recommend something that will be "better" IME 1018 likes to warp, and I think 4140 will cut too slow.

Originally Posted by boosted

We are struggling with tool life a bit on some of the smaller (<.050) tools.

Spindle speed. You need more, most likely. If you can use a speeder that's stiff enough not to compromise tool life, it may help with the smaller cutters.

Originally Posted by Milland

Spindle speed. You need more, most likely. If you can use a speeder that's stiff enough not to compromise tool life, it may help with the smaller cutters.

I guess that's fair. We're not too far off though.

20k spindle. I'd like to run our .055in at 450sfm, but at spindle max we're pushing about 300SFM. If anything I'd expect that to help life a bit.

OK, that is a faster spindle than I anticipated on a machine doing larger steel work.

I do think that (again, with required stiffness) that getting closer to ideal SFM is still a good thing. But since you're so close you're likely better off looking at improving coolant (maybe increasing pressure and nozzle positioning if that's possible?), and testing some different endmill brands and flute geometries to see if something works better for you.

Best Mild Steel for Machinability

12L14 wins hands down IMO.

Caveat: It's hard to get a "nice and shiny" look unless you use cermet.

Dunno if that matters to you.

Originally Posted by ChipSplitter

Best Mild Steel for Machinability
12L14 wins hands down IMO. Caveat: It's hard to get a "nice and shiny" look unless you use cermet. . .

My 1st choice too. I don't know about 'shiny', but a beautiful 'satin' finish is easy to get.

12L14 rusts like a bitch. I wouldn't use it for fixture plates unless it's plated.

I was thinking 12L14 would be really nice but will it stay flat? I've never used it in an application where warping was a concern.

Also, these parts are going to be used in grinding machines where they will get hit with flood coolant all day. I imagine if they have good coolant that will help, and if they have bad coolant it won't make much difference? Tell me if that's wrong though.

One challenge you might have here is finding a free-machining steel in a flat/plate product.

Most plate I've found in your dimensions (or larger) is A36; there are some 1018 sources for 'mild steel'. After that, I found 4140/4340 and some other alloys.

4140 ore hard for fixtures here they turn out beautiful
Don


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Pre hard sorry for typo


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There is free machining steel plate, Freemax 15 is one I have used. You said punching a big hole, did you really mean punching, or just making? If you are getting the plates burned out, have them burn the big hole.

Originally Posted by boosted

I'm sure it's been asked, but a quick search doesn't return anything.

What is the best mild steel for machinability?

Specifically we are milling some fixture plates, roughly 1in x 18 x 18. Punching a large hole in the middle, drilling some bolt hole patterns, doing some surfacing, and some side work - about 75% of the material is removed by the time it's done. The only functional requirements are a couple tight bores and a flatness callout (within .001in constrained), but I would really like them to be shiny/pretty as well, at least within reason.

We are currently making similar parts out of A36. The material seems to have acceptable dimensional stability, but we are having a really hard time getting good surface finish, and we are struggling with tool life a bit on some of the smaller (<.050) tools. I've cut a lot of other steels, but don't know enough to confidently recommend something that will be "better" IME 1018 likes to warp, and I think 4140 will cut too slow.

Why are you worried about surface finish on a fixture plate?
Get A36 annealed, and start hoggin.
Tool life, well that can't be attributed to the A36, that has to be a limitation to your rpm's or bad coolant or maybe the toolpath could use some tweaking?

As for 4140 cutting too slow..... you can literally run it at the same speeds and feeds as A36 if you're using carbide. 4140 is nothing special.

Originally Posted by Mtndew

Why are you worried about surface finish on a fixture plate?
Get A36 annealed, and start hoggin.
Tool life, well that can't be attributed to the A36, that has to be a limitation to your rpm's or bad coolant or maybe the toolpath could use some tweaking?

As for 4140 cutting too slow..... you can literally run it at the same speeds and feeds as A36 if you're using carbide. 4140 is nothing special.

If surface finish is something you want, a good blanchard grind surface with the crosshatch looks very nice IMO.

Here's a machinability chart for ya. It's got the basics on it. Plenty of other ones out there if you need.

If you're after a good surface finish and stability, you're probably going to be after 4140. If machinability is more important, you'll want to look into freemax 15 as gbent mentioned.

Machinability Comparison Chart - provides percentage scale of machinability of various metals indexed on 1212 carbon steel: carbon steels, alloy steels, stainless steels and super alloys, tool steels, gray cast iron, nodular cast iron, alumunum alloy

I agree with 12L14 mentioned above.

If you can vary your material requirement, check out Mic-6 aluminum plate. Widely used for subplates.

You're going to bolt this thing down to control the flatness, right?

12L14 machines very easily as said, but it rusts just by exhaling on it. Would that be a problem for you?

The plates get bolted down on the last operation and in service, which makes the flatness achievable without too much crazy. They are fixtures for a customer, not me. As for the cosmetics - I've found that with many customers sending parts that look nice is one of the best ways to get to the top of their preferred vendor list. Very few end users actually measure incoming parts, but almost all of them will give it a quick visual inspection.

Anyways, thanks for the feedback. So far 4140 prehard is on the top of my list.

I do a fair amount of work with A36, if you want a shiny finish such as facing you need to run FAST when taking a light cut, well into the 4 digit SFM numbers. I run air blast and often have a shower of sparks flying. I've never been able to get tools to hold up with coolant at the speeds I run in steel, likely due to thermal shock.

303 stainless looks pretty
Mark