# Cutting Speed derivation

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Somewhere, over the years, it seems I picked up some sense of what cutting speed is.

However, when asked 'what is cutting speed?', the best I could come up with was it is related to tensile strength of the material, blah, blah, blah. (almost literally I'm afraid)

Anyone have a good description of what cutting speed is?

Lurker

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It's the rate of movement at the point of contact of the tool, generally measured in feet per minute. In some cases it's the rate the material moves (lathes, for example) and others it's the rate the cutter runs, mills, saws, etc..

Harold

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lurker wrote in news: snipped-for-privacy@ultrasw.com:

Cutting speed is the rate at which the tool tip passes the material (or the material passes by the tool tip), in either feet per minute (sometimes feet per second) or meters per minute (or meters per second). It has nothing to do with strength of the material, material type, feedrate, or anything else. It is simply a speed measurement.

RPM = (4 * CS)/D

As you can see from the formula, CS (cutting speed) is dependent only on

2 variables, RPM and Diameter (either workpiece or tool, depending on lathe or mill)

Algebraic manupulation of this basic formula allows you to solve for CS, D or RPM.

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Google Scott Logan's recent post on the FAQ, he gives a link to an excellent military manual on milling machines.

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Anthony snipped-for-privacy@hotmail.com

Not the first, the latter.

The answer probably depends on production requirements.

In oneoffs, who cares? FM

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Not true. Depends on the operation at hand. In sawing (or milling, shaping, etc.), it would be the tool tip passing the material. On a lathe, it would be the material passing the tool tip. Both cases are proper.

Anyone that prefers to cut at the "sweet spot", where material behaves much better than at other speeds, and those that prefer to cut at a speed that doesn't destroy the cutting instrument. It also becomes quite critical where grinding wheels are concerned, for they are rated at a given "hardness" based on a given RPM, which quickly changes as you speed up or slow down the grinding wheel. Surface feet, or cutting speed, is important to successful machining.

Harold

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lurker wrote: (clip) However, when asked 'what is cutting speed?', the best I could come up with was it is related to tensile strength of the material, blah, blah, blah. (almost literally I'm afraid) ^^^^^^^^^^^^ So, I ask myself, why would you think that cutting speed has something to do with tensile strength? And, I answer myself, "Becasue it does, but not in the way you think." The method of calculating cutting speed, based on radius and RPM is absolutely correct. But, the question of what is a good cutting speed to use comes from the other direction: what are the properties of the material being cut, and how much punishment will the tool stand. I am sure that tensile strength could be a factor, along with toughness, hardness, etc.

• posted

Cutting speed (I was told) is a value used to figure RPM, given the diameter of the piece/tool.

It varies from material to material, and from tool composition to tool composition. E.g., the cutting speed for aluminum (in surface feet per minute) is different than the cutting speed of Aluminum, if the cutting _tool_ is different. (HSS v Ceramite v Tungsten Carbide, v polycrystalin bonded trainagles, v what ever.)

Also, CS is dependent on "other factors" - tool life being one of them. Again, while the CS for NMS100 is 170, go slow on the roughing cuts, as the scale from forging will eat cutters at that speed. So slow down a little.

>
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"Machinability" is the term you are looking for. Cutting speed is not a physical property of a material since it depends on the type of tool being used.

For example, a HSS tool might cut 6061 aluminum at 300 SFM (surface feet per minute) while a carbide tool could cut the same material at 1000 SFM.

Cutting speed also depends on the length of tool life required from the tool. In other words, cutting faster means resharpening or replacing the tools more often.

Machinability is determined by several factors. One of the most important factors is hardness. A steel which cuts easily when annealed will have to be cut slower when the steel is hardened. Another factor is the alloying elements in the material. Some additives to steel will make it more machinable, like lead, sulfur or phosporus. Some elements added will make steel less machinable, like nickel or chromium.

There are volumes written on the subject, I'm not going to put too much into this. In the real world, published cutting speeds for machining various materials with different kinds of tools are just starting points. Optimum speeds for each job are found by experimentation.

Fred

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Cutting speed in the proper perspective is best applied to planing, shaping, and sawing operations.........

No diameter involved........as of yet...........

To apply to circular cutting operations, simple algebra is applied.

How fast can you pull the tape outa that Stanley tape measure ???

And what if the end inadvertantly gets caught inna lathe chuck ???

Or how many feet of string would get wound up per minute if a piece a string were to get caught up on the drill ???

( Sorry if I come off sounding like some troll, I done trained quite a few in my time, this is the exact parrallel I always used, it usually seemed to work...... )

All good and valid general info, as to "ideal" cutting speed............this IS material and tool specific.

Published numbers should be looked at as guidelines only--recommended starting points....

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Sounds like you've been around. That's my opinion as well.

Harold

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The cutting sfpm numbers, for cutting a given material with a given tool, are pretty well established, and tabulated.

The feed rates however, as a function of things like tooling, material, machine hp and rigidity, are some of the toughest to determine, except by experiment. The late Robert Bastow had an excellent discussion about how this is done, with the essence being: start feeding by hand until the machine complains, then back off a bit. Set the power feed to just about catch up with the previously set hand feed rate.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

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pyotr filipivich wrote in news: snipped-for-privacy@4ax.com:

CS or, more appropriately, Surface Speed, is simply cutting speed, diameter and RPM dependent, it has no relation to material, tool, feed or anything else.

The Ideal CS for a certian machining operation on the other hand, does vary depending on a large package of variables (tool, tool geometry, cutting fluid, material, material condition, machine rigdity, part set- up, etc)

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Unfortunately, the rules don't necessarily apply across the board, something that is likely very true for the home shop types. Those that use antiquated machines are highly unlikely to have proper speeds at their disposal, considering their general high usage of carbide tooling. The older machines were geared to run HSS and lack rigidity, speed and horse power to take advantage of the capabilities of cutting tool technology of modern times.

That isn't my approach, especially not on a lathe. Rarely do I start a cut that isn't near a decent feed rate, but when you've run the same lathe for over 35 years and it's capable of taking respectable cuts, that is likely no surprise. Any of us that have worked in the shop, especially in production, likely do the same thing.

At best, I'd have a small problem with the hand feed rate. Inconsistent hand motion would be a poor indicator of feed, something that would be far better by power, be it too slow or too fast. If one has a quick change, it takes but an instant to change the levers while under cut, which is how I fine tune a cut once under way. I can see, however, that if one prefers to run a machine such that a heavy load can easily stall the machine, my method of working would not be a good idea. I've never stalled my machine because of the cut, although I've had it grunting many times, so I don't risk breaking carbide tooling. An acquaintance chooses to run his lathe with the belts loose enough that the spindle can be stalled, more or less a safety factor for him. Needless to say, one could hardly ever take advantage of the capabilities of a machine under those conditions, so efficient feed rates and depths of cut would be a small consideration.

Harold

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If I recall correctly, Robert was referring specifically to the horizontal mill in that instance.

The horizontal mill likes a manly feed rate, and when you're talking about a

6in dia cutter with a tooth gullet as big as your thumb, horsepower is likely to be the limiting factor.

Paul K. Dickman

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I guess what I am really trying to ask is "How is the rate derived?" How is it determined that xxx sfpm is best for cutting 303 stainless with a high speed steel cutter, for example?

Lurker

Harold & Susan Vordos wrote:

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Oneoffs, who cares? ....... Never mind.....

Lurker

Fdmorris>

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big snip---

That would make sense. The first time I ran a horizontal with a side cutter (back in the 50's) was a real eye opening experience for me. It was almost frightening, the feed rate a cutter like that can handle. On light duty equipment the likes of which I have now (Bridgeport, but I have a right angle attachment for it) I think I'd do exactly as Robert suggested. Rigidity and horse power come into play with side cutters, so you'd likely have to break the rules considerably when running larger cutters on smaller machines. I know for sure I'd not run the cutter according to the formula, highest allowable speed and proper chip load per tooth. Yep, he said a mouth full!

Harold

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That I'm not sure of. I have never given it any thought, but I have certainly relied upon the accepted speeds as a basis for applying cutting instrument throughout my years of machining. There are many variations to the rules, so they should be considered strictly as guidelines, however. Example: Sawing with a band saw. The thickness of the material effects the acceptable speed. Thin material can be sawn much faster than thicker material, because the interval of time the blade makes contact with the material is much longer on thick material than thin (duh!), so the heat generated by the cut does not elevate high enough to burn the edge before it's off the part being cut. Pitch of the blade then becomes a serious consideration, so the chip load doesn't overcome the length of cut so there is no place for swarf to accumulate, but is not so coarse that there aren't at least two teeth in contact with the work at all times. All the rules change when you run the saw above about 1,500 fpm, however, where one can then move into the realm of friction sawing. Don't even need teeth for that function.

I think that the likely answer to the question is one whereby enough history of machining has been recorded that it is well known that particular materials behave in a predictable fashion. It is more than well known that 300 series stainless, for example (excluding the free machining grades), have what could best be described as a high friction coefficient, and require much lower sfpm than mild steel, for example. Run the cutter the least bit too fast and it burns the tip.

It becomes quite apparent that the moment one moves into the alloys, each element added to the base metal changes the characteristics, and I would assume the amount of each element would effect the cutting characteristics, though not necessarily in a liner fashion. Again, I imagine it has been recorded by history more so than a formula one can apply across the board. There's just way too many minor factors that influence the final outcome, including depth of cut, feed rate, whether cutting wet or dry, what type of coolant, etc..

Perhaps someone with a degree in this area could enlighten us.

Harold

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lurker wrote in news: snipped-for-privacy@ultrasw.com:

Starting points are given in Machinists Handbook, but these, as far as I know, are derrived from experience or tooling manufacturers recommendations based on their in-house experiments.

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