Noobie question - how bad are blue chips?

I'm struggling up the learning curve, slowly. After initially having trouble with brazed carbide tools when I was first starting out, I gave up and worked with HSS for a while. I'm giving the carbide tools another go. Two recent jobs - one involved facing (fly cutting) a small rectangular block; the other turning a new gib locking screw; both cases I used a brazed carbide tool. I doubled the SFMs as I've read here and other places. Got a nice finish in both cases (hooray).

Depth of cut was varied 0.035" - 0.050" for the fly cutting operation, 0.015" - 0.045" for the turned screw body. Fly cutting was hand fed (no power drives on the mill); screw body feed was I think 0.003" per rev. Both SFMs and DoCs were greater than I've used for HSS tools and I crept up to the higher DoCs.

In both cases, I got dark blue chips - most of them in the facing operation, some in the turning operation on heavier DoCs.

Question is: how bad are blue chips for tooling and/or machine? Is this a sign of too rapid material removal or normal/ok for carbide tooling? If bad, am I at too high a SFM or too big a DoC?

Materials were "plain old" CRS stock (1018 or who knows what?)

In general, blue chips are GOOD if you're cutting with carbide. It means the heat of the cut's friction is transferring into the chip rather than the tool or the workpiece.

AC,
Your numbers sound good and the blue chips are a good thing. It means the heat is going into the chip and not into the part or the tool. Of course, you don't want them too blue. Ideally, I think you want brown into light or med. blue. Really dark blue is a no-no. Your tool might not last too long like that.But, it's a trade off. It sounds like you are cutting dry too, which is OK. If you are getting blue chips with oil or coolant, that ain't so good either. Hope that helps.

Paul

Edit: Sorry Pixman, you beat me by a few seconds

When using carbide on steel, dark blue chips are a must. If it isn't turning blue its going way, way, way too slow. You do have a very wide range from the time you start getting blue chips, to the time where the insert actually has too much SFM. Although it varies with DOC and such, I think its about 150SFM to over 1000SFM. So lots to play with in there.

With HSS its a fine line between a good speed and a speed that'll burn the bit. Usually its better to run slower SFM with a higher DOC as its what causes the least reduction in tool life. 100SFM for HSS is usually the "standard" but it varies with the type of HSS used, proper grinding and if coolant or oil is used, which helps a fair bit with HSS(not so much with carbide..).

Are Blue Chips Bad?

Only in aluminum and magnesium!

I like them blue and shaped like 6's and 9's

Gosh, no. When running carbide, if they ain't blue, the lathe ain't gettin' into it hard enough :p.

HSS is another matter. I can make blue chips with HSS, but the cutter doesn't last long.

FG

You ought to be able to push up the feed 2x or 3x with those DOC's.

Push on the carbide until something makes a bad noise, then back off.

FG

to answer the questions, I'm at about 200 SFM, and machining dry except...

the thread I cut for the gib lock screw- used oil and had to go much slower SFM and DoC or I would have threaded right through the head before I could disengage the leadscrew! Got a crappy finish. Not sure I see any advantage to using carbide for threads and usually can't get to the higher SFMs. I will probably remake the screw at some point and do the threading with sharp HSS.

I got a good amount of dark blue chips on the facing operation, so perhaps was a little heavy handed there.

Thanks all for the info and putting my mind at ease.

Finegrain - the lathe is an old Atlas (1942) restored to fine working order but I'm afraid to push it much harder as I might find myself having to make replacement parts for the machine. A "bad noise" would surely be the compound snapping off or some such. The QCTP and much improved rigidity that it gave is the main reason I tried the carbide again... and was pleasantly surprised.

Blue is fine, and I'll add a little theory to the mix.

I wouldn't necessarily go by color, go by SFM and tool wear.

Also, turning and milling, though similar are different animals, basically, interrupted cut, and NON-interrupted cut. Whats the difference?

Since you running dry, on the non-interrupted cut(turning), I woudn't want to generate too much heat. This is where coolant comes in, your in the cut and in the cut, heat can build up pretty easily, and you can get a very hot part, unless you find that magic balance of feed/speed/DOC/chipbreaker/coating.

In general, coolant needs to be where it needs to be at sufficient pressure to eliminate steam pockets, and this is a whole lot easier to do on a lathe than a mill. The goal is to keep everything at a constant temperature, not necessarily the same temp, you just don't want things to go hot/cold/hot/cold, this is how you can get some insane SFM while turning, that you could never get milling.

So why is that? Carbide can take heat, lots and lots and lots of heat, what it can't take is thermal cycling. That is why you mill dry with carbide. (Unless you are one of the few that has the equipment and the tools to get the coolant where it needs to go on a spinning tool). In the case of milling, dark blue and smokin' hot is fine, as long as your tool isn't getting too hot, and your material is staying cool, and your tools are lasting.

Ran a bunch of 4340 today, Milling off about 30lbs on each part. Open the doors and it was like a sauna, the tools were fairly warm to the touch, 140 degrees maybe, and the part was still room temp. Piles and piles of dark purple chips.

Now don't try to get blue with any kind of stainless, it turns a nice golden color, try convincing the scrap guy that a barrel of golden chips is "stainless".

The coolant/non-coolant Interrupted/non-interrupted thing works on the lathe also. If your banging through some soft jaws or hex stock, run it dry until you are down to round.

blue chips are REALLY bad if they fall down the front of your shirt!

Little sharp blue chips shaped like 6s and 9s help me get up my steep driveway when it is covered in ice. I have used ten gallons so far this year and could have used twenty gallons more. Gary P. Hansen

geez, Gary, don't that tear h*ll out of your tires?

Ok, nobody else said it, so I will:
Blue chips taste similar to regular yellow chips. Great with salsa, guacamole, and a Margarita. My "sure fire" Margarita recipe is given below:

1 blender full of ice
1 Cup Cuervo Gold
1/2 Cup Curacao, Triple Sec, or (if you've money to burn) Courvosier
1/2 Cup Rose's Lime Juice.
Blend until the ice is slushy. Share with someone whose standards you want to lower.

Variations:
1) Use Blue Curacao for a very pretty blue drink.
2) Use a combination of honey and fresh squeezed limes in place of the Rose's.

Warning:
These are not your volume-bar Margaritas, though they taste wonderfully innocuous. They are pretty lethal to your sensibility. Woozification factor is about 1.8. Have a designated driver if you are the designated drinker.

Jim

OK Milacron, you can lock the thread now.

Are you using negative rake tooling or positive? Negative is great if you've got a 10,000# CNC machine, not so good on WW2-era hardware. For your Atlas, I'd definitely recommend positive rake, like TPMG, or at least something like a CCGT/CCMT. Stay away from the CNMG/TNMG negative rake stuff.

FG

Is this the Home Shop board? I'm going to puke if I hear the "You can't use a negative rake holder" one more time.

An N series holder is only 5 degrees negative and can hold a vast array of inserts, 99% of which give you POSITIVE cutting action. You can end up 25 or more degrees of POSITIVE rake on a NEGATIVE tool holder, due to the chip breaker/T-land.

And the difference between a *P** and *C** is what 4 degrees of BACK relief, that has no bearing on how the TOP of the insert is presented to the work piece. Necessary in a bore, but not on an OD cut.

Are there negatives to a "negative" tool holder, some, they are bulky and not very well suited to smaller boring bars, the holders are not cheap. You can run into a rubbing situation when turning from chuck to tail stock, if the DOC is not kept small. I've heard that the bottom of the insert is not supported as well due to the two sided nature of the insert, though the lumps and bumps in the chip breaker are made to support the bottom side, I've never pushed one THAT hard.

Positives, TWICE the bang for the buck, two sides!!!!! They are abundant, and available in tons and tons of grades, chipbreakers, coatings, and sizes.

The only problem with blue chips as you describe, is if one should land on unprotected skin right after leaving the cutter. Keep body parts away from the hot rain.

FG - rake is ~0 deg ... using brazed carbide tools for now in a QCTP. The tool holder presents the tool at level.

My first attempt with these brazed carbide bits was in the LTP with the old Armstrong holders that came with the lathe. They hold the bit up at an angle (16 deg I think I read somewhere). The bits weren't ground with enough clearance and cut poorly.

When I feel I've mastered the brazed carbide, I'll probably be looking to get set up with insert tooling. By then, maybe I'll have a lathe with a little more meat on it's bones.

And thanks for the recipe, Jim. Always on the prowl for a good margarita. Will have to try the Corv. for a variation... will tell the wife it's just for her.

Blue chips or yellow - doesn't matter if the margaritas are good!

Thanks all for the helpful advice.

Aeronca, I think what Finegrain is referring to, is losing the tip of the carbide. The cutter will give up before you compound will. If you are breaking carbide tips you are running too slow (or lack of rigidity) and if your burning tips you're rpms are too fast. I found that the temperature and color of the chips is about right when one bounces into your ear and sticks so you can hear the sizzle. Some call me Carlos.

"The only problem with blue chips as you describe, is if one should land on unprotected skin right after leaving the cutter. Keep body parts away from the hot rain."

Amen. I caught one of these the other day as I reached for the carriage wheel, right in the upturned palm. This was about 3/8" DOC and 335rpm on a big engine lathe. So hot it stuck in stead of bouncing or falling as I slung my hand. It was so hot the skin turned white. Took two weeks for the scab to fall off and now it looks as if I have a nice little 9 shaped scar that will be with me a while.