Lathe turning speeds and feed rates using carbide cutters

Pmedic828

Pmedic828

Active User
Registered
I am a newbie and just received my Smithy 1340 3 in 1. I was wondering if anyone knows of a chart that I can print giving simple lathe speeds and feed rates using carbide cutters. I am not knowledgeable enough yet to know what type of steel i am turning all the time but I just purchased some hot rolled 1/2 inch rod just to play with to get used to the lathe. Many charts that I searched for on the internet stated low carbon, hi carbon, 4XXX, etc. but I don't know which category this steel falls in. Can anyone direct me to a site that is for beginners and not for 20+ year experienced machinists. Thanks in advance for directing me.:))
 
Here you go, check this link and look at the section called "Quick RPM/Spindle Rate Calculations: Lathe, Mill, Drill". The chart shows speeds for HSS. For carbide, increase RPMs by 2 - 3 times (2.5 is the number I use for normal finish, 3x for high finish). Keep the feed rates the same.

Because you'll need high RPMs, carbide is most effective on pieces 1.5" or better. Also, pieces 1/2" dia or below tend to deflect quite a bit because of the increased pressure needed for carbide. Carbide prefers cuts of at least 10 thousandths inch. Lighter cuts prevent the bit from fully engaging the material and the cut will be rough and ugly.

http://www.southbaymachine.com/setups/cuttingspeeds.htm

It's a good bet that your hot-rolled (HR) is low-carbon but there's no guarantee. HR and cold-rolled (CR) are treatment specifications. Hot rolled means the steel (which could be low, medium, high carbon or alloy (aka 4140 etc)) was formed into shape while it was glowing red and allowed to cool. It sags a little while that hot and therefore has surface irregularities. On the up side, it has less internal stress.

CR steel was formed into shape when the temps were much cooler (about 900 degrees) then rammed through presses or dies to give it it's final shape. The surface is much more even that way but, since the cooler outside of the metal was formed while the inside was still much hotter, it causes surface tensions. Often times, CR steel deforms and deflect a good bit while your turning (lathing) it.

For this reason, I prefer HR steel even though the outer surface is irregular.


Ray
 
Another very valuable reference is the Machinery's Handbook series. It is currently like in revision 30 but previous editions will contain a wealth of info and can usually be found pretty cheap. There are also PDF versions that you can load onto your computer too. That book contains speeds, feed, Depth of cut, direction and plunge info to allow for optimized cutting speeds. Once you peruse a few pages you will wonder how you lived this long without one. I bought a new 27th ed. and it has every possible chart or table ypu will ever want. If you dont have one that is an item you should consider putting near the top of the list.
Bob
 
Ray, I am glad you brought up the HR vs CR thing. I generally don't like the A36 HR steel as sold by the local big box hardware stores. I dont get a good finish with it and I always blamed it on the fact it was HR and responsible for frustrating many a new machine owner.
 
Last edited:
Seems that A36 is hit or miss. You could drill a hole and it works-out fine then, drill another in the same piece and it takes twice as long and twice the pressure. -Tells me right away it's "kinda lumpy inside". LOL: It happens to be machinable but don't think it is by design :rofl:. It comes in CR too right or is it all HR? -Can't remember. Whatever, nobody should expect spectacular finish results. -Still though, I make plenty of stuff out of it.

Ray



tripletap3 said:
Ray, I am glad you brought up the HR vs CR thing. I generally don't like the A36 HR steel as sold by the local big box hardware stores. I dont get a good finish with it and I always blamed it on the fact it was HR and responsible for frustrating many a new machine owner.
Click to expand...
 
You must log in or register to reply here.
Back
Top