Interpreting Cutting Speed Tables

[goldstar31]

Goldstar31, a required topic in Mechanical Engineering school (Manufacturing Processes class to be specific) is learning how to calculate the temperature rise of metal chips as they exit the cutter-head of a lathe (a function of the material's yield strength, tensile strength, Young's modulus, Poisson's ratio, shear-angle of the cutter, and obviously the volumetric removal rate). It's calculus based equations, rooted in both Mechanics-of-Materials and Thermodynamics.

The purpose of the lesson was to show that it's mathematically possible to accurately predict the actual cutting-power requirements for any particular machining operation, and the expected temperature rise of the cutter (which will temper/anneal if it gets too hot), and also the expected temperature rise of waste chips - because some metals will make for very exciting shop-fires with inadequate cooling . . .

Welcome to reality. . . . .

Reality is the ability to retire comfortably at 55 years( far to late in life but who cares now at 88)
Reality is to realise that
'one man's meet is is another man's poisson' and of course for the Francophile -- Poisson D'Abril.'

But for those who can really do maths, having a machine shop or whatever you call it and whatever I would call it, is a fine way not only to alienate neighbours but to ruin a fairly substantial investment of 2/3rds of a million in sterling and that , if you are still with me, is a lot of scrap lathes.

Verb sap

Norm

 

[goldstar31]

I thought that a bit of the old Statistical Analysis would surprise.

I have two Myfords. One of which ranks as a Rolls Royce of nodel engineering lathes- by some.
Mine is ancient but arguably more accurate than the day it came out new but the speeds for a 1440 rom motor are as follows
Ungeared. Geared
2105. 135
1480. 95
1050 77
600. 54
420. 39
300. 27
210

the ML10 which was eventually the basis of CNC only gives
840
490
280
145
85
48

Even at that the Bull wheel has to to disconnected half way through,

Little wonder that one modification on the ML7 was - fast and loose pulleys but utilised a full 1 HP motor and a 1/3rd HP. That was Martin Cleeve's modification

Little wonder that the Sieg C4 is half the price of an as new Super 7- and that I paid 1/10th of a Myford price. for mine.

Something to think about- if one thinks engineering and cost accountancy together

N

 

[Entropy455]

For those living in a town where peasants own million dollar homes, and everyone retires at 55, it's no surprise to find someone shocked & intimidated at the thought of a green building the woods.

You want to talk outrage? Here's a picture of my neighbor's house. . . .

And just for chit-chat - below is my large pickup truck. It will haul 20,000 pounds in the bed, while pulling 30,000 pounds on a trailer. It's also green - so as not to offend my neighbors (wink wink). . . . And it's environmentally friendly - an electric-start hybrid vehicle, that gets an impressive 2 to 3 mpg. I'm not sure if they have any of these North of Hadrian's Wall or not? They probably should - they're excellent shopping vehicles, and do well in the snow too.

 

[mfrick]

So I taught my students for years the following CS X 4 divided by diameter of material to be turned equaled RPM. This is a quick and easy method for them to use and it works well. CS (cutting speed) Example 80 x 4 = 320 divided by 2" diameter = 160 rpm. Simple great starting point then adjust RPM up and or down depending on how well your tool performs.

 

[XD351]

Thank you mfrick for bringing this discussion back on topic ! Nice simple formula that and well worth scribbling it on the lathe somewhere or a toolbox lid .

 

[john_reese]

If you are cutting with HSS and the chips are coming off a straw color you have a good surface speed. If the chips are much darker you shorten tool life so lower the surface speed. If you are cutting with carbide the chips should be coming off blue. If not you are running too slow a surface speed. Of course this only applies in steel. Other metals do not show temperature related color change.

 

[DJP]

I'm happiest when I see long continuous curls of steel coming off the tip of the cutter. I don't mess with it when the lathe is cutting so nicely.

I do like the quick math check and I'll try to confirm a correlation with long curly steel chips.

 

[Entropy455]

So I taught my students for years the following CS X 4 divided by diameter of material to be turned equaled RPM.

Simple equation, yet effective enough.

Using the times-4 approximation introduces a 4.5% error. If you multiplied by 3.8 instead of 4, the error drops from 4.5% to 0.5%. But then people would have to remember the number 3.8 in lieu of just the number 4. If you have a calculator that will do pi, the most precise value is (12/pi). But 4 will get you close enough to make chips nonetheless (and prevent others from questioning their reality).

 

[kuhncw]

I agree with TonyM. For hobby work you can tell pretty quick whether the manual mill or lathe like your speed and feed.

However, I do pay attention to speed and feed when using my CNC Taig mill in order to minimize cutter bending, etc. Bob Warfield's GWizard Machinists Calculator works well for me. There are other similar calculators.

Chuck

 

[Nick Hulme]

and the expected temperature rise of the cutter (which will temper/anneal if it gets too hot).

I have CBN

Welcome to reality...............

 

[goldstar31]

I have CBN

Welcome to reality...............

I tend to agree but note that with the reduction of diameter, speeds can be upped.
The poster actually has one of these 7 x x lathes which comes with a nob on which one can start operations at zero and progressively( or aggressively) turn the revs up-- without having to go to a college or other house of correction to understand.

Probably the poor soul has chickened out with all the maths

Who would blame him?

N

 

[Jasonb]

It's a German machine, the table will make sense when you use METERS not feet for surface speed.

If you use the calculations on the previous page (again in meters) with the speeds from the chart you will get the right results.

Try it with HSS on aluminium which is given as 40-100M/min so say 50m/min on a 25mm dia workpiece

(50 x 1000) / (25 x 3.142) = 513rpm

Simples

 

[Entropy455]

It's a German machine, the table will make sense when you use METERS not feet for surface speed.

In that case multiply the equation by 12.5 instead of 4

Example: 100 meters per minute (the target feed-rate), times 12.5 then divided by 2" (diameter part in inches), will equal 625 rpm spindle speed (required chuck speed).

 

[bazmak]

I have been in engineering all my life,sufficient to judge cutting speeds and feeds.I have never/ever calculated them
since my apprentice school days

 

[Entropy455]

I have been in engineering all my life,sufficient to judge cutting speeds and feeds.I have never/ever calculated them
since my apprentice school days

What do you normally machine? Aluminum, steel, cast-iron, stainless?

What if you were handed a very high-dollar piece of Inconel, K-Monel, or Titanium, and you needed to make first-time quality cuts? Would it be worth your time to look up the recommended tooling & feed rates?

 

[bazmak]

Yes it would,but I have yet to pick up a piece of Inconel or titanium for machining in my model engineering workshop

 

[Entropy455]

It's nice to know that you've personally never needed to calculate feed rates. That doesn't really help Bob Gibson - who's trying to learn how to.

 

[goldstar31]

It's nice to know that you've personally never needed to calculate feed rates. That doesn't really help Bob Gibson - who's trying to learn how to.

Not with your 'tales of derring do' which have no relevance to model engineering problems

 

[Entropy455]

Not with your 'tales of derring do' which have no relevance to model engineering problems

Norm, it's an honor to receive your passive-aggressive & condescending jabs. . . .

 

[Bob Gibson]

OK guys thanks for the replies.
Jasonb solved the puzzle for me. Yes its a German designed machine and the cutting speed table is METRIC. So it all makes sense to me now.
One final NEWBE question.
Am I correct is assuming HSS, P10, P20, P20, K10, HC P40, HC K15, and HC M15/K10 in the table are all identifiers for the cutting tool type?