Sparks from carbide, speed and feed for lathe with cast steel part?

python50

python50

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Hello,

I am trying to machine a 6" round cast steel part on my little Grizzly G4000 lathe. Due to the smallness of my lathe I have to take very small depths of cut (that's okay, I've got time). Overall I need to remove about 0.25" from the diameter of a 0.5" wide section of the casting to make it fit in an assembly.

I'm using carbide tooling [1]. I'm currently in the process of running compressed air to my machine so I can cool the part. Feed rate is approx 0.005"/rev.

I have found that at 120 RPM I get no sparks, but the torque is too much (at only 0.002" DOC) and causes the 1/8" wide drive belt to slip. At 300 RPM I get some sparks and no slip. At 600 RPM I get lots of sparks; no slippage either. I think I could have gone deeper and used faster feed, but I was afraid of overheating my tools without coolant.

First question is, are sparks okay? As best as I can tell they are okay so long as I keep the tooling cool.

The second question is what should the speed and feed be for my flimsy little lathe? I have done a calculation below, let me know if it is sane for my setup.

Code: 
According to [2] for cast iron with carbide 400 SFM would be my (rather optimistic) goal.
This yields a target RPM of 255 which is roughly 300 RPM. With 0.005"/rev that gives a feedrate of 1.5 IPM.

Using turning horsepower the calculator on [3] with 300 RPM, 1.5 IPM, 6" diameter, 0.005" DOC, 1.4 HP/in^3 [4], gives 0.2 HP of spindle HP required. 

My lathe is 0.75HP. Assuming I don't run into the drive belt torque limitation [5], I should be able to cut 0.005" DOC or maybe even 0.010" with coolant.

[1] Grizzly G5640 Lathe Bit Set
[2] https://littlemachineshop.com/reference/cuttingspeeds.php
[3] http://www.custompartnet.com/calculator/turning-horsepower
[4] Manufacturing Processes Reference Guide. Page 157. Table of unit horsepowers for turning.
[5] There is also mention of a clutch in the G4000 parts diagram; this also could be cause of my slippage. I need to investigate further.
 
I would keep the speed down where there are no sparks, and perhaps tighten the belt to eliminate slippage.
 
Wow! I am impressed. I've yet to see sparks on the lathe. See them to often on the mills.
 
Sparks are primarily from over speeding, which is likely to reduce tool life; an occasional spark, perhaps OK, but lots of them, not so good.
 
Depending on what type of inserts you are using, that may be part of your problems. A small lathe works better with really sharp inserts, and a positive rake angle. I am guessing that 120 rpm is the minimum speed for your lathe. If not, try going slower yet. You also might try high speed steel tooling, again sharp and with positive rake, which requires less power to take a cut.
 
Hard turning always sparks and the chips come off glowing red, if not you are doing it wrong (-:
 
Last edited:
Wreck™Wreck said:
Hard turning always sparks and the chip come off glowing red, if not you are doing it wrong (-:
Click to expand...

I'll find out soon enough. I have some 4140QT (medium hard) turning to do soon. Most of my hard turning has been with stainless. Taking as big of a chip as possible to avoid work hardening. Turned some parts in 316 the other day. I thought I was doing 416L, and I just couldn't understand why I was struggling. LOL. I made the parts, but I had to use a vertical shear to get a passable finish. I may try a round shear next time just to see what it does. I'm sure that surface is hard now. Fortunately stainless was all that was speced. Not any particular alloy.
 
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