Lathe depth of cut vs feed amount, how to balance.

[BrassCat]

Hello the forum. Beginner question concerning cutting on a lathe. I can increase the amount of material removed by making a deeper cut. I could also remove more material by using a greater feed. Not changing the rotation speed. Obvious I should not overpower the lathe or get the tool too hot. But where is the balance between depth of cut vs thickness of the feed. The sweet spot ??
BrassCat.

 

[WobblyHand]

In many ways, your lathe will tell you if you are cutting at a greater DOC, depth of cut, than it is currently capable of. Adjusting your lathe, tightening your gibs and things like that may extend it's capabilities, but in the end you will be limited by your lathe. Start out with light cuts and slow feeds, and some cheap stock, like aluminum, and experiment some. If you take small steps, your lathe will communicate with you. If you take bigger steps, your lathe may make a lot of noise, or chatter, or worse. This is not to scare you, but to encourage you to experiment some. Use some mechanical empathy - don't abuse your machine - but try stuff. It may be more, or less capable than you think. Only one way to find out. Be safe, don't do stupid stuff, but do stretch your legs and make chips. It won't take long for you to find the limits of your "platform". Then try to stay under them, or make additional adjustments as required.

Keep your tool on the exact center, and start cutting!

 

[markba633csi]

Good advice above, you will find that cutting steel is slower and puts more strain on the machine than cutting soft metals
like aluminum and brass. Use slow spindle speeds at first, and with steel take modest cuts and use oil or cutting fluid.
With some belt drive lathes the belt will often slip if you try to take too heavy of a cut
Keep your cutting tools sharp and the machine's gibs as snug as possible while still allowing smooth travel with some resistance.
Gentlemen, start your spindles!

 

[GrifterGuru]

As above, your lathe will let you know if you are pushing it too far.

As for materials, every material has a "feel" to it. Steel, stainless steel, brass, bronze, aluminium (to name a few materials) all have a "feel" to them. If you experiment enough with each material you intend to work with, you will get a feel for the material which will help guide you in the future.

You also do not mention what type of lathe you have, which plays a considerable part in what advice you will receive, instead, only receiving "general" information without any chance of being specific.

 

[Huub Buis]

Your stock and lathe can handle more forces in axial (towards the chuck) direction. So faster feed could be an option. But is will also give a rougher finish because you cut faster (coarser "thread"). The chip will be wider and not so easy to break.

Once you cut deeper than the tool nose radius, cutting deeper will not increase radial load. So changing the cutting depth a bit will not lead to more tool/stock/lathe flexing. That is why cutting deeper than the tool nose radius is recommended. Cutting deeper also gives a longer narrow chip will break more easily. However my hobby lathe doesn't like deeper cuts so I rarely cut deeper than the tool nose radius.

Increasing cutting depth and decreasing feed rate will have no impact on tool life. You cut slower but deeper so less passes are needed. The slower feed probably will give a better finish. The chip will be longer, smaller and break more easily.

In the end, I do not care much about faster material removing. A better finish and tighter tolerances are more important for me. A heavy load on the half nuts will wear them out faster and I want my half nuts to last long.

Just try different feeds, speeds and cutting depths to find out what your lathe and your stock like the most.

 

[Red96]

I found that all the advice about 'feel' doesn't really help much when you're starting out, as we don't yet know what it 'feels' like when cutting good or bad. I found that starting out using a feed & speed calculator to at least get in the ballpark was very helpful. I'm on a small machine (10x30), so I go a little under the calculator results to be conservative. That has started to train my brain on how the cut 'feels', as in all the advice above. I put a shortcut on my phone to Little Machine Shop's speeds & feeds calculator, and use it all the time. Tell it your diameter, material, HSS or Carbide, # of flutes, etc., and it will spit out a recommended spindle rotation speed and a feed rate in inches per revolution.

LMS Speeds & Feeds Calculator

 

[G-ManBart]

A heavy load on the half nuts will wear them out faster and I want my half nuts to last long.

You should only be using the half nuts when threading, so that really shouldn't be a consideration. When threading you'll usually run into issues on the tool end long before the half nuts are seeing much of a load.

 

[WobblyHand]

I found the feeds and speeds calculators are just barely ok for beginners. I found them to often be aggressive for beginners. Still need to listen to the machine it's talking to you, but you have to be paying attention. The signs can be subtle, but once you go beyond the limits, it's obvious something is wrong. Either the surface quality declines, or the lathe stalls, or makes different sounds. I don't push them to breaking (at least not yet) but they've been pushed gently to the edge of chatter or major deflection.

 

[G-ManBart]

Hello the forum. Beginner question concerning cutting on a lathe. I can increase the amount of material removed by making a deeper cut. I could also remove more material by using a greater feed. Not changing the rotation speed. Obvious I should not overpower the lathe or get the tool too hot. But where is the balance between depth of cut vs thickness of the feed. The sweet spot ??
BrassCat.

There isn't really an easy answer. It will vary with the machine, the tool post/holder/tool/insert along with things like how far the tool is extended, what size shank the tool is, etc, etc, etc.

After using a machine for a while you'll get an idea of what ballpark DOC and feed rates give you the best blend of stock removal and surface finish for a given tool/insert setup.

One thing to consider is that sometimes things work the opposite of how you think they will, so don't be afraid to completely go in the other direction. You might think a slower/faster feed rate will work better but at some point it will flip and start to get worse....until you get experience you could just be starting after the point it's gotten worse and need to make a big swing the other way, so don't be after to go both faster and slower (same goes for shallower/deeper) when trying to find a good combination.

 

[mksj]

Feels is a factor after you have dialed in the other factors such as machine size, material, cutting tool (HSS/Carbide), geometry of the cutting tool is key as well as if you are doing a finishing cut or hogging material. You are limited to the DOC based on the machine rigidity/Power and the cutting tool. The nose profile/geometry also dictates the minimum cut dept.. SFM is calculated off of these and feed is a bases of the material type and if it work hardeneds. Calculators and insert specifications are based on rigid high Hp turning. You also need to look at the surface finish, and last but not least the machine feel if you are pushing it too hard. David Best has a book on the different insert holders as well as some general guidance's, but you need to do you homework first and provide a bit more details on what you are using and the materials.