Carbide vs cobalt steel endmills for hard mill scale & flame-cut steel.

[Just a Sparky]

TL;DR I'm sick of Chinesium Easy-Bake endmills and I'm ready to upgrade to some real cutters.

Some of what I do involves machining hot roll drops for simple projects. Making 'T'-nuts, jigs, etc. Trouble is the mill scale on some of it is remarkably hard, especially on pieces which have been flame cut. My files like to skid/glide without cutting when trying to break some of the upturned scale along the edges after milling.

For a little Burke No. 4 with a top speed of ~900RPM and not the *most* rigidity in the world, would cobalt steel do well enough for my needs? Or would carbide be worth the extra investment? I'm mainly worried about shattering carbide endmills if a good vibration happens to set up. Would irregular flutes be worth considering for spindle speeds this slow?

 

[Milland]

Cheap 4fl carbide, CONVENTIONAL cuts, grind a lead-in such that you start with soft metal. Cut deep to get well under the HAZ and let it break off as the chips are made.

Find your optimal feed rate to go with your max spindle speed, keep everything as close-coupled (short and stout) as possible when it comes to tool stickout, spindle extension (if any), and part fixturing. The stiffer the setup, the longer the tools will last (generally).

I'd start with 3/8" or 1/2" endmills, when cutting a long distance raise and lower the spindle or knee (or whatever) a little when possible to spread the notch wear along the cutter edges.

After you've gotten some experience at it, coated tools will help as will quality brands, but you might as well use CN carbide until you figure things out.

 

[cudafamily]

Go to your nearest Home Depot or Lowes and get some Muratic Acid. Immerse parts in an acid resistant container and in about 10 minutes or less that hot roll coating will be gone. Don't forget proper safety equip and do outdoors. Use a bench grinder to hit any flame cut areas.

 

[gbent]

Go to your nearest Home Depot or Lowes and get some Muratic Acid. Immerse parts in an acid resistant container and in about 10 minutes or less that hot roll coating will be gone. Don't forget proper safety equip and do outdoors. Use a bench grinder to hit any flame cut areas.

This will work, but you are not told how badly muratic acid will rust anything in its vicinity. Muratic acid is diluted hydrochloric acid. The vapors will seep through sealed containers. Never store or use muratic acid anywhere (50' outdoors, never inside) around steel you don't wish to have a brown patina.

 

[Gazz]

Use a sharp cold chisel to remove gross slag from flame cuts and then grind the edges. As noted muriatic acid will dissolve fire scale and as also noted use outside and be mindful of the soak times.

 

[dgfoster]

And you are referring to flame cut pieces and not plasma cut—-right. Plasma nitrites the steel. Cutting that is really bad. I would only try to cut a plasma surface after grinding. Maybe someone else knows the secret to cutting it.

Denis

 

[lucky7]

Hmmm, I regularly machine plasma cut discs. HSS, even T15 won’t touch them, but carbide set to an aggressive doc and slightly slower than usual sfm does fine. Ok, the inserts don’t last as long as usual but job gets done.

L7

 

[cyanidekid]

Go to your nearest Home Depot or Lowes and get some Muratic Acid. Immerse parts in an acid resistant container and in about 10 minutes or less that hot roll coating will be gone. Don't forget proper safety equip and do outdoors. Use a bench grinder to hit any flame cut areas.

generally a bad idea, for the reasons others have posted, but also, not generally worth the toxic mess (most places don't sell HCl anymore, at least not around me). as noted the OP is dealing with flame cuts and the acid won't remove the heavier slag from that either, so scale easily removed with a pickle isn't the real toolkiller anyway.

also depends on the size of the parts, but chip with sharp chisel followed by angle grinder with Cubitron wheel (bench grinder is for tools) and black coarse scochbrite is good if you must cut on thermal cut parts.

no, cobalt isn't gonna stand up to flame slag for one second. Milland's got it right id say.

bottom line is flame cut drops suck. you don't want slag or scale anywhere near the mill, unless it is unavoidable (cutting large heavy plate, structural shapes, or doing work on complex pieces necessarily pre-processed by thermal cutting).

just buy some cold rolled for those T nuts, really just not worth messing with drops!

 

[Just a Sparky]

You know, I didn't even think of that. These drops are from a place that primarily does CNC plasma and laser cutting of plate steel. I'll bet they are in fact plasma edges, not flame cut.

Thanks for the advice. I guess I'll just stock up on some more sizes of cold-rolled. Hot-rolled is mainly a cost and convenience thing - it's only $0.50/lb for drops so it's no big deal to grab odds and ends for nothing when I'm there. Whereas anything cold-rolled cut to length is around $3.25/lb plus a $5.00 cutting fee unless I want to take home a 22 foot stick and cut it down myself with my portaband in the parking lot.

I suppose in the end expensive steel is cheaper than the time and tooling required to make a cut through cheap steel. Especially if a carbide endmill breaks off.

At least now I don't feel so bad for having such difficulty with my cuts last night.

 

[tnmgcarbide]

i use zircon flap wheels on angle grinders if there is significant scale.
nothing but grinding will endure slag. it is hard and tough . carbide and
m42 or t-15 are brittle , and won't stand up to the sand-castle that is torch-cut
slag.

also, sometimes you can get "under" hot roll skin by climb-milling , or
just use a beater cutter to club the surface away if you have the material to spare.

 

[carlherrnstein]

I once had to machine plasma cut 304l I had to take a angle grinder and grind out the slag at the start/stop point. Plates were 1" thick and 12" diameter the slag would make the insert shatter. The most important thing is to cut deep to get under the slag or else you will kill your endmill/tool there are windmills that have a radius on the corner. They may last longer.

 

[pavt]

X3 use an angle grinder, a $1 grinding disc is the cheapest way to ensure your expensive tooling has a long life.

 

[Milland]

If you can get larger drops so that you're cutting a relatively smaller amount of "HAZ'd" perimeter, I'd still go with that over paying much more for the steel. But you've got to judge where the time/cost of tooling balance is.

For sure I wouldn't buy strips of plasma cut steel as then most of the metal is hard.

 

[Just a Sparky]

So why is it that band saws handle mill scale just fine, while machine tooling really struggles with it?

Surely the teeth on a band saw blade aren't harder than a carbide endmill. Band saws will even do 304 bone dry as long as the work temperature is kept waaay down. (Light pressure, low FPM in granny gear so it doesn't harden.)

Is it just a matter of cutter surface area and tool pressure?

 

[Milland]

Because you lucked into an Adamantium-tipped blade. Never change it, they're rare as hens-rockets.

 

[Just a Sparky]

Just ordinary Milwaukee/Makita portaband blades in variable speed saws.

That trick has saved our butts a couple of times on shutdowns where the dopes at the supply house sent out stainless strut and nobody caught it until the old feeders were cut. Luckily we *had* a variable speed saw on hand, otherwise we would have been up sh*t creek.

What is it with supply houses anyhow? Give them the part numbers and take pictures and they still get at least one item wrong on every other order. Ask for one set of reducing washers... and you'll get one washer. Pay for early morning courier... and it shows up after lunch every single day for a month. Order threaded rod for a cable tray job... and sit on your thumbs for five weeks because it got back-ordered and the PM signed an exclusive supply contract with that house. Sometimes you're better off just borrowing a company vehicle and a company card on company time and going to Home Depot to get the job done instead of playing the suppliers' stupid games.

Anyhow, back to our regularly scheduled programming...

 

[pavt]

What is it with supply houses anyhow? Give them the part numbers and take pictures and they still get at least one item wrong on every other order. Ask for one set of reducing washers... and you'll get one washer. Pay for early morning courier... and it shows up after lunch every single day for a month. Order threaded rod for a cable tray job... and sit on your thumbs for five weeks because it got back-ordered and the PM signed an exclusive supply contract with that house. Sometimes you're better off just borrowing a company vehicle and a company card on company time and going to Home Depot to get the job done instead of playing the suppliers' stupid games.

Anyhow, back to our regularly scheduled programming...

There was a guy over on HSM the other day with a commercial shop, new lathe... ordered some valenite tooling. Received 1000 toilet seat covers. At least they were on time.

 

[pavt]

Is it just a matter of cutter surface area and tool pressure?

Yeah I think so, done that lots of times myself. Helps to recall that the very tips of the teeth are almost file hard. And always use a good name brand blade. I used to like the Morse 1x144 bi-metal with 14TPI.

 

[cyanidekid]

So why is it that band saws handle mill scale just fine, while machine tooling really struggles with it?

Surely the teeth on a band saw blade aren't harder than a carbide endmill. Band saws will even do 304 bone dry as long as the work temperature is kept waaay down. (Light pressure, low FPM in granny gear so it doesn't harden.).

I think its a matter of how many teeth are actually in the scale, if you cut hot rolled with a bandsaw, you may dull a few teeth, but there are so many others it goes through fine. my 12'1" Kalamazoo has good blade life, my Makita battery powered mini-bandsaw, not so much. far fewer teeth, and stainless kills it quick. its also about feeds and speeds, with the hand held you can easily be rubbing and work hardening it, with a gravity or hydraulic downfeed its a bit less likely. with 2,3,or4 flutes, a HSS end mill doesn't have a chance.

oh, and you are paying .50 a pound for flame or plasma drops???!!! wow, have I got a deal for you... ill sell you ALL my steel scrap for that, most of it completely free of slag...

 

[Limy Sami]

You haven't told us what speed you're running the HSS cutters that are wearing fast.