Metal Lathe Tutorial 21 : Speeds & Feeds

Summary

In this informative episode of 'Lathe Skills', Quinn delves into the complexities of 'speeds and feeds' in machining, debunking common misconceptions and guiding hobbyists through the process of determining optimal cutting parameters. He emphasizes the importance of surface speed, explains the limitations of generic charts, and provides a step-by-step method to calculate spindle RPM and feed rates tailored to individual machines. Quinn also demonstrates the impact of various speeds and feeds on surface finish, encouraging viewers to experiment within safe limits for better machining results.

Takeaways

• 📚 Speeds and feeds is a complex topic with nuances that can be overwhelming for beginners, often leading to a search for simplified guides or charts.
• 🔍 The script emphasizes the importance of understanding where speeds and feeds numbers come from, rather than just using them without context.
• 📐 Surface speed is a fundamental property in machining, and while units may vary, the goal is to determine the appropriate spindle RPM for the task.
• 📊 Common charts for surface speed are often based on unspoken assumptions and may not be accurate for all machining setups, especially for hobbyist machines.
• 🛠️ The Machinery's Handbook provides detailed tables for calculating speeds and feeds, but they are calibrated for professional settings with more aggressive cutting parameters.
• 🔨 There are two main approaches to calculating speeds and feeds: the correct, detailed method involving depth of cut, feed rate, and surface speed calculations, and the more common shortcut method often used by hobbyists.
• ⚙️ Depth of cut is a critical factor influenced by machine rigidity, setup, and urgency of the task, with different materials like aluminum, brass, and steel requiring different approaches.
• 🔄 Feed rate is determined by the machine's capabilities and is often limited by the number of change gear options available on hobbyist lathes.
• 🌀 The relationship between spindle RPM and feed rate is crucial as they both contribute to the final surface speed, affecting the quality of the cut.
• 🛑 The script advises against using aggressive cutting parameters from professional guides on hobbyist machines, as they can lead to machine damage.
• 🔬 The process of determining optimal speeds and feeds involves a balance of factors including material removal rate, machine power, and desired surface finish quality.
• 🔄 Experimenting with different speeds, feeds, and tool nose radii can yield a range of surface finishes, from rough to mirror-like, depending on the setup and material.

Q & A

What is the main topic of the video 'Blondie Axe - Lathe Skills' episode 21?

The main topic of the video is 'Speeds and Feeds,' which is a critical aspect of machining that the host, Quinn, explains in detail.

Why does the host mention that speeds and feeds is a complicated topic?

The host mentions that speeds and feeds is complicated because it involves many variables and nuances, which is why there aren't many quick summaries or straightforward guides available.

What is the fundamental property that the host is focusing on in the context of machining?

The fundamental property the host focuses on is surface speed, which is measured in surface feet per minute (SFM) in the Imperial system.

Why does the host say that the units for surface speed don't matter in the end?

The host says the units don't matter because the ultimate goal is to determine the spindle RPM, which can be derived from various units of surface speed regardless of what they are.

What is the common mistake the host points out regarding the use of charts for surface speed?

The common mistake is that these charts often have unspoken assumptions about cutting conditions, and they are usually calibrated for professional settings with more aggressive numbers that may not be suitable for hobbyist machines.

How does the host describe the correct way to calculate speeds and feeds?

The host describes a three-step process: 1) Select the depth of cut, 2) Figure out the feed rate, and 3) Convert the derived surface speed into spindle RPM.

What is the significance of the nose radius of the cutting tool in relation to the feed rate?

The nose radius of the cutting tool is significant because it affects the surface finish. The host suggests that the feed rate should be less than half the radius on the nose of the tool to ensure the best possible surface finish.

What does the host mean by 'the correct way' and 'the way that everybody, especially hobbyists, tend to actually do it' when referring to calculating speeds and feeds?

The 'correct way' refers to the detailed three-step process involving depth of cut, feed rate, and spindle RPM calculation. The 'way that everybody tends to do it' likely refers to using simplified shortcuts or charts without considering the nuances of their own specific machining setup.

How does the host demonstrate the effect of different speeds and feeds on the machined surface?

The host demonstrates this by machining parts at various speeds and feeds, showing the resulting surface finishes and explaining the relationship between the chosen parameters and the outcome.

What is the importance of the surface speed in tool wear according to the host?

According to the host, surface speed is an extremely important factor in tool wear, especially in a production CNC shop, and it has a significant impact on tool life, which is why it is extensively covered in the Machinery's Handbook.

How does the host suggest hobbyists approach calculating feed speed on a mill, given the limitations of their equipment?

The host suggests that hobbyists map the feed speed to the power feed control knob by zeroing out the hand wheel, accounting for backlash, and timing how far the power feed moves the workpiece over a fixed amount of time.

What is the host's advice for hobbyists who want to understand their machine's capabilities better?

The host advises hobbyists to go through the calibration process he described to determine the upper bounds of speed and feed for their particular machine and then adjust based on their comfort level.

Outlines

00:00

🔧 Introduction to Machining Speeds and Feeds

In this introductory segment, Quinn, the host of 'lathe skills', embarks on a comprehensive exploration of 'speeds and feeds' in machining, a topic of significant complexity. He acknowledges the common desire for a quick summary of this subject but emphasizes the importance of understanding the derivation of the numbers used in machining. Quinn clarifies that while surface speed is the fundamental property sought, the unit of measurement is irrelevant as the ultimate goal is to determine spindle RPM. He critiques existing charts for their inaccuracies due to unspoken assumptions and promises to guide viewers on creating personalized charts based on first principles, taking into account the nuances from the Machinery's Handbook.

05:01

📚 Calculating Speeds and Feeds from First Principles

Quinn delves into the detailed process of calculating speeds and feeds from first principles, starting with the selection of depth of cut, which is influenced by machine rigidity, setup, and urgency. He discusses the common materials used by hobbyists, such as aluminum, brass, and steel, and their respective depth of cut ranges. The second step involves determining the feed rate, which is often limited by the machine's capabilities and is crucial for calculating surface speed. Quinn explains the relationship between spindle RPM, feed rate, and surface speed, highlighting the importance of these factors in achieving the desired surface finish. He also touches on the impact of the cutting tool's nose radius on the feed rate selection for optimal surface finish.

10:03

🔄 Translating Surface Speed into Spindle RPM

This paragraph focuses on the conversion of calculated surface speed into spindle RPM, a critical step in the machining process. Quinn provides a method to adjust the baseline surface speed using scaling factors based on feed rate and depth of cut, derived from Machinery's Handbook. He emphasizes the importance of calibrating one's lathe using this data and provides specific RPM ranges for different materials like steel, aluminum, and brass, considering both roughing and finishing passes. The segment concludes with a demonstration of the impact of aggressive cutting on the machine and the material, illustrating the need for careful selection of cutting parameters.

15:05

🛠 Practical Demonstration of Speeds and Feeds

Quinn presents a practical demonstration of the effects of different speeds and feeds on the machining process. He shows the results of roughing and finishing passes using calculated parameters for steel, brass, and aluminum. The demonstration includes visual and auditory cues to convey the intensity of the machining process and the importance of proper setup and support. The segment highlights the consequences of pushing the limits, such as loss of grip and induced taper due to inadequate support. Quinn also showcases the potential for achieving a mirror-like finish with the right combination of speeds and feeds, emphasizing the importance of experimentation within the calculated bounds.

20:07

📘 Understanding the Nuances of Milling Speeds and Feeds

In the final paragraph, Quinn addresses the complexities of calculating speeds and feeds for milling operations. He explains the concept of 'inches per tooth' and how it is influenced by the power feed rate, spindle RPM, and the number of flutes on the cutter. Quinn acknowledges the challenges faced by hobbyists in controlling these variables, especially with manual mills, and suggests methods for approximating feed speed. He also touches on the importance of being aware of nuances such as milling orientation (climb vs. conventional) and slot cutting, which can affect the cutting forces and, consequently, the selection of speeds and feeds. The segment concludes with a reminder of the value of understanding these principles for hobbyist work, despite the availability of simplified formulas and rules of thumb.

Keywords

💡Speeds and Feeds

Speeds and feeds refer to two critical parameters in machining: the speed at which the cutting tool rotates (spindle speed) and the rate at which the tool moves through the material (feed rate). In the context of the video, these terms are central to achieving efficient and accurate machining. The script discusses how to calculate and adjust speeds and feeds for optimal cutting performance, emphasizing the importance of these settings in determining the success of a machining operation.

💡Surface Speed

Surface speed is the linear speed at which the cutting edge of a tool moves across the work surface, typically measured in surface feet per minute (SFM) in imperial units or meters per minute (M/min) in metric units. In the video, surface speed is highlighted as a fundamental property for achieving the desired material removal rate and surface finish. The script explains how to calculate surface speed and its significance in relation to the machine's spindle RPM and the chosen feed rate.

💡Machinery's Handbook

The Machinery's Handbook is a comprehensive reference work used in the field of machinery and machining. It contains a wealth of data, including tables for speeds and feeds for various materials and cutting conditions. In the script, the handbook is mentioned as a source of information for determining appropriate speeds and feeds, but the video also points out that its recommendations are calibrated for professional settings with more aggressive cutting parameters than typically used by hobbyists.

💡Depth of Cut

Depth of cut is the distance that the cutting tool moves into the material in one pass. The script discusses how the depth of cut is a crucial factor in determining the machine's rigidity requirements and the amount of material that can be safely removed in a single pass without causing tool deflection or machine instability. The video provides guidance on selecting an appropriate depth of cut based on the material being machined and the capabilities of the machine.

💡Feed Rate

Feed rate is the rate at which the workpiece is moved relative to the cutting tool, which directly affects the efficiency of material removal and the quality of the machined surface. The video script explains how to determine feed rate based on the machine's capabilities, the material being cut, and the desired surface finish. It also discusses the relationship between feed rate and surface speed, emphasizing their combined impact on the cutting process.

💡Surface Finish

Surface finish, also known as surface quality, refers to the condition of the surface of a workpiece after machining, including characteristics such as roughness, waviness, and lay. In the video, surface finish is discussed in relation to the nose radius of the cutting tool and the chosen feed rate. The script suggests that a finer feed rate can help achieve a better surface finish by ensuring that the tool's path overlaps and smooths out the marks made by previous passes.

💡Nose Radius

Nose radius is the radius of the rounded tip of a cutting tool, which can affect the quality of the surface finish by 'smearing' the tool marks made during machining. The script explains the importance of considering the nose radius when selecting a feed rate, recommending that the feed rate should be less than half the nose radius to ensure a good surface finish without visible tool marks.

💡Spindle RPM

Spindle RPM (Revolutions Per Minute) is the rotational speed of the spindle, which is directly related to the surface speed of the cutting tool. The video script discusses how to calculate spindle RPM based on the determined surface speed and the diameter of the workpiece or cutter. It is a key parameter in achieving the desired cutting conditions and is adjusted to accommodate different materials and cutting depths.

💡Cutting Conditions

Cutting conditions encompass the various factors that influence the machining process, including the material being cut, the type of cutting tool used, the depth of cut, feed rate, and spindle speed. The script emphasizes the importance of understanding and adjusting these conditions to optimize the machining process, achieve the desired surface finish, and prevent tool breakage or machine damage.

💡Hobbyist Machining

Hobbyist machining refers to the practice of machining as a hobby, often using smaller, less powerful machines compared to those used in professional or industrial settings. The video script addresses the specific challenges and considerations for hobbyists, such as the limitations of benchtop machines, the need for more conservative cutting parameters, and the importance of understanding basic machining principles to achieve success in a home workshop environment.

Highlights

Introduction to the complexity of speeds and feeds in machining.

The importance of surface speed in machining, measured in surface feet per minute.

Critique of existing charts for surface speed due to unspoken assumptions about cutting conditions.

Explanation of the correct way to calculate speeds and feeds in a three-step process.

The significance of depth of cut in determining machining parameters.

How to select feed rate based on machine capabilities and material properties.

The relationship between feed rate and surface speed in achieving desired surface finish.

Technical explanation of how nose radius of the cutting tool affects surface finish.

Demonstration of calculating spindle RPM based on surface speed and material diameter.

Calibrating a lathe based on data from charts and the Machinery's Handbook.

Practical examples of speeds and feeds for different materials like steel, aluminum, and brass.

Visual demonstration of aggressive cutting passes and their effects on the machine and material.

The impact of cutting conditions on tool wear and machine longevity.

Macro lens comparison of surface finishes from different cutting passes.

Discussion on the biggest contributor to tool wear: surface speed.

Common shortcuts for calculating speeds and feeds and their limitations.

The nuances of calculating speeds and feeds for milling operations.

Advice on adjusting feed rates on manual mills with variable power feed settings.

Conclusion emphasizing the value of understanding speeds and feeds for optimal machining.

Transcripts

hello Internet my name is Quinn and this

is blondie axe

this is lathe skills a series of quick

videos on getting started in machining

this is episode 21 speeds and feeds yes

the one everybody's been waiting for

this is a meaty topic so let's go speeds

and feeds is a very complicated topic

and I think that's why you don't see a

lot of really great videos or kind of

quick summaries of this topic everybody

always wants like a quick summary of

speeds and feeds just tell me what to do

right okay fine

here's the numbers you should use got it

no don't go back and try to freeze frame

your YouTube player because I'm going to

show you where those numbers come from

and how to get those numbers for you

you know Machinery's handbook devotes a

hundred pages to this topic and there's

a reason why there's a ton of nuance to

it however there are some kind of rules

of thumb and there are some shortcuts

that I think hobbyists can take and I'm

going to show you you know what happens

when things go right yeah and we're

gonna push things to fail you're on my

little machine so that's gonna get

exciting stay tuned for that the

fundamental property that we're after

here is surface speed in the Imperial

machine shop we measure that in surface

feet per minute but that's a really

stupid unit the thing is though the

unit's don't matter we're ultimately

after a spindle RPM so it doesn't matter

what we use to get there it could be

centimeters per deck a second or King

George's thumbs per fortnight doesn't

matter if like most beginners you

frantically googled around looking for

some kind of guidance on speeds and

feeds mostly what you'll find are these

charts of surface speed that people have

produced and you know they'll

confidently state things like steel

should be cut at 120 surface feet per

minute it'll have a formula so you can

convert your the diameter of your part

or your cutter into surface feet per

minute and that's all you need to know

the problem is that these charts are all

varying degrees of wrong they all have a

set of unspoken assumptions about your

cutting conditions so what I'm going to

show you here is how to work forward

from first principles to basically make

your own chart for your particular

machine now most of those homemade

charts are getting their information

from this section of Machinery's

handbook and so there's a series of

tables in here for different materials

so here we're looking at 10:18 mild

steel and do you know the Brinell

hardness of your mild steel I bet you

don't but

most people are assuming you know

somewhere in this range here ninety 220

surface feet per minute for high speed

steel so you know this is where those

numbers come from that you find in all

those charts however what nobody ever

seems to mention is that the tables in

Machinery's handbook are calibrated for

a hundred and twenty five thousand of

cut and a twelve thousand feed rate now

for the hobbyist those are extremely

aggressive numbers those are very

unlikely to be numbers that your average

hobbyist machines can achieve remember

Machinery's handbook is a professional

tool for professionals who have big

machines and are trying to maximize

production so if you try and push your

bench top hobbyist machine to 1/8 inch

depth of cut that's a quarter of an inch

off the diameter on a direct read hand

wheel a quarter inch diameter reduction

at 12,000 you're gonna blow that machine

up so I hope you got the warranty I

think I can summarize this in that there

are two basic ways that you can

calculate speeds and fees there's the

correct way and then there's the way

that everybody especially hobbyists tend

to actually do it the correct way is a

three step process step one is to select

your depth of cut now this is generally

dictated by three things it's dictated

by the rigidity of your machine the

rigidity of your setup and how much of a

hurry that you're in if you've got two

hundredths out of a move off a dimension

obviously you want to do that as quickly

as possible so you're not standing

around all day for this exercise we'll

assume that your setup is as rigid as it

can be for your machine so you've got

good tail support you've got a good grip

with your Chuck you know all those sorts

of things so from there it's really down

to the material that you're cutting you

know the most common hobbyist materials

are going to be aluminum brass and steel

for aluminum and brass they're very easy

to cut so 60 70 80 thousand that cut all

you know all within the realm of

possibility now a steel we're much more

in the four teeth out of cut range kind

of at the upper limit I don't typically

get that aggressive with steel and my

little machines I tend to stick around

you know 30,000 of cut or you know sixty

sixty to eighty thousand wheel for a

direct read hand wheel so step one

establish your depth of cut step two is

figuring out your feed rate now with

larger machines with complicated

transmission

or electronic leadscrews or things like

that you may have a lot of choices on

feed rate however with hobbyist benchtop

machines like this you're probably only

going to have one two or three choices

here so this is typical of these

benchtop machines you're going to have

change gears in here that are set up for

specific feed rates so there's on this

particular machine there are two sets of

change gear options for power feeding so

there's a course set and a fine set the

course set is incredibly fast and I

don't know why you would ever use them

so you're going to start with the fine

set and the fine set is what's installed

on the machine in this case from the

factory and so this tells you what your

actual feed rates are for both turning

and facing because this machine also has

power cross feed now on these tables

here for a given gear set there are

three options C a and B and those

correspond to the quick change gearbox

so for a given set of change gears

you've got three feed speeds and this

chart tells you precisely what those

feed rates are now these units here are

in thousandths of an inch per rotation

of the spindle which is a very logical

way to measure feed rate and here's why

we measure feed rate in distance

traveled per rotation of the spindle

because as you recall from the surface

finish video the lathe is always cutting

a helix and like I said at the top of

the show ultimately what we care about

here is the surface speed of the cutting

tool over the material however you know

people think of rpm as being the only

thing that contributes to surface speed

that in diameter but in fact the feed

rate also contributes to that and as you

remember from your junior high math

classes you know we've got two vectors

to get the final velocity you add them

together so in this case we've got one

vector here coming from the spindle rpm

that's you know contributing to our

velocity and then you've got a second

vector here that's coming from your

power feed rate contributing to your

velocity and so this the sum of these

two vectors is ultimately going to be

your service feet per minute and this is

what we care about so part of the reason

that we start with power feed rate and

in this calculation is because we have

less control over that rpm we have lots

and lots of choices but power feed rate

is much more limited but we do have a

couple of choices you know on this

machine that I'm Dennis

straining we have three choices for

power feed rate so how do we choose

between them well there are three things

that go into our decision for feed rate

first is once again how big a hurry

we're in if we have a lot of material to

remove we want to be feeding as fast as

possible to is the power of the machine

the the feed rate is limited by the

horsepower of the machine and how

aggressively you know it can pull that

cutter through the material and the

third main factor is surface finish now

recall from the surface finish video

that one of the main contributors to

surface finish is the radius on the nose

of your cutting tool what I said in that

video is that the affect of the nose

radius is basically to smear the thread

cutting lines that would otherwise exist

if the tool was sharp nosed again since

the lathe is cutting helix a nose radius

kind of blurs the lines well now that

we're trying to choose feed rate it's

time to get technical about this or best

possible surface finish the rule of

thumb is that you want your feed rate to

be less than half the radius on the nose

of your tool and that ensures that each

lap around that helix

some portion of this curve is gonna be

there to smear out the the tool marks

from the previous pass so guesstimating

this guy with the calipers this

particular tool has a nose radius

between 10 and 15 thou so I'd be looking

for a feed rate somewhere like 500 bucks

mmm surface finish so if I'm setting up

to do a finishing pass with that tool I

just showed we know that we want a power

feed rate somewhere 5,000 feet gear

range on this machine because I'm not

insane and so on the turnin section here

and I've got three options I've got

10,000 or five or two and a half

thousand the a or C range five or two

and a half though but for finishing I'm

always going to go with the C range

because that's gonna ensure that each

lap around that material is going to be

within my nose radius once again

grounding tools by hand it can be tricky

you know exactly what your nose radius

is so just go as fine as you can for

finishing

but once again for this exercise we're

trying to determine the upper bounds how

aggressive we can be with all the

variables for speeds and speeds so we're

going to resume our calculations using

this 10,000 will use steel so I've

established my depth of cut the maximum

that I can do is 40,000

most I'm comfortable with and my feed

rate the maximum on that machine is

going to be 10,000 so I go to my

materials chart here and Machinery's

handbook and let's take this 120 surface

feet-per-minute number as my baseline

and then I multiply that base speed by

this scaling factor for my feed rate

which is 10,000 tells me that I can

increase that speed by 1.08 and this

reflects the difference between the

12,000 the tenth our eight that I'm

going to be using and then we look up my

depth of cut and we find it there's an

additional scale factor of 1.15 because

this 47th out depth of cut is going to

be quite a bit less than the 125 that

the original table was calibrated for so

with our depth of cut and our feed rate

established the final step is convert

that surface speed that we derived into

a spindle rpm and this is done based on

the diameter using this chart here so I

like to do this for a one-inch diameter

so that's going to be the diameter of

the stock for the lathe or the cutter

for the mill and one-inch is kind of a

nice average of things you're going to

be working with in a hobby shop and then

you can mentally fudge these numbers up

or down for smaller or larger stock or

cutters so for steel for example we're

typically going to end up in this

surface speed range so we're gonna end

up somewhere between like 450 and 800

rpm so a great exercise is to calibrate

your lathe based on this data so

starting with the surface

feet-per-minute baselines on those other

charts which again are for an eighth

inch depth of cut and a twelfth AO feed

rate and then modify those numbers using

this chart for you know the maximum

depth of cut that your lathe can handle

you need to those materials and then

modify it again for whatever your

fastest feed rate is that you're going

to use and that's going to give you the

upper bounds of RPM and feed rate for

each of your materials and then you can

kind of go down from there now that's a

lot of fooling around so I've done that

math for you for a typical bench top you

know hobbyist lathe and the nine or ten

inch swing range and if you have larger

machines these numbers will still work

they'll be you know more conservative

than you need or steel your base speed

is 120 for a roughing pass of four teeth

out of cut and

10500 RPM a finishing pass of 10th our

less that's a two and a half thousand

rate 800 RPM for aluminum your base

speed is 500 for a roughing pass of

60,000 cut that's a ten thousand two

thousand rpm finishing pass of tenth our

lists you want a two and a half thousand

as much RPM as you can muster for brass

your base speed is 350 for a roughing

pass of sixty thousand of cut and a ten

thousand hundred RPM for a finishing

pass of tenth our less you want a two

and a half thousand as much RPM as you

can muster okay so let's see what those

numbers actually look like on the

machine so this is the roughing pass

that I just calculated for you in steel

and this is definitely what I would

consider the most aggressive cut that I

would ever do on this machine and steel

it looks pretty calm on video but in

real life there's a lot of noise a lot

of smoke and overall quite a lot of

drama you can see when I wind the cutter

back how it drags a fair bit on the

surface and that spring pass tells you

how much load there was on that cutter

because it sprung back quite a bit once

that cutting force was removed and here

you can see that the chip action there

is okay it's not breaking as soon as it

should the grind on my tool might not be

perfect might need honing but it goes to

show why I don't typically run the

machine this hard now let's see if we

can clean up that finish using the

finishing pass numbers that I just

showed you and that's pretty good but I

think we can do better so just to show

you the affected speeds and feeds have

on your surface finish here's a little

higher rpm and a finer cut this is a

5000 cut and that's a very fine finish

indeed for mild steel anyway and here's

a roughing pass with those brass numbers

now you can tell when you're really

pushing brass because it's wheels like a

banshee

now let's see what happens if we push it

too hard so this is that same tenth

outfit but I've put some aluminum shims

in there so the Chuck doesn't have as

good a grip you can see that the force

of the feed rate is actually pushing the

part into the Chuck and has caused it to

lose its tail support and well

everything's gone pear-shaped quite

literally you can see in addition to the

dreadful finish that there's a taper in

there that's been introduced because the

part got pushed out of the tail stock

just to show you how much wiggle room

there can be in these numbers here's

that same 60,000 if cut very rigid set

up sharp nose tool mind you but a very

fine feed the fineness this machine will

do and look at how nice that finishes

that's actually quite a good final

finish for this part so as you can see

play with the numbers and you'll find

that there's maybe more possibilities

than you thought quick sidebar note that

that wasn't actually 1,400 rpm as I said

in the numbers that was actually a

thousand rpm because I'm my lathe that's

as high as I can go without moving the

drive belts so it just goes to show that

there's a fair amount of wiggle room in

these numbers especially at the higher

rpm ranges now let's look at the other

extreme here's a tool with a huge nose

radius and I'm doing a very light pass

extremely fine feed an extremely high

rpm and that finish is like a mirror

this is probably the best finish that

you're going to get right off the

machine without any kind of polishing so

play with the extremes of the numbers

just to see what's possible you might be

surprised just to drive this point home

let's take a look at these parts under

the macro lens so here's the roughing

pass that we did on the brass and you

can really see the tool marks there very

clearly from that sharp nose tool and

the aggressive feed is also going to

make this effect much worse but now

let's look at that finishing pass that

we did with the exaggerated round nose

tool and this finish is so good that the

camera the macro lens on the camera

actually has trouble focusing it can't

quite resolve the surface on it now over

here on the steel this is the finishing

pass and it's definitely good for mild

steel but it's not gonna win any mirror

Awards

pop quiz what do you think is the

biggest contributor to tool where is a

depth of cut no it is surface speed this

is an extremely important topic in a

production CNC shop so much so that the

big book devotes an entire section to

calculating it full of lots of more

tables

the hobby shop that doesn't matter a

whole lot but be aware that the tables

in the big book are calibrated for 15

minutes of tool life so if you want your

tools to last longer than that yeah

things down a bit

so that's the complete process of

determining speeds and feeds for your

particular cutting setup from first

principles now why doesn't anybody

actually do it that way at least not on

YouTube well first of all it's a lot of

fooling around it's a lot of math it's a

lot of looking up tables in this very

expensive book so people have shortcuts

the most common shortcut that you'll see

is this one you take your surface

feet-per-minute for the material from

the big book here or from a chart that

was most likely copied from the big book

and you just simply take that speed

multiply it by 4 divide it by the

diameter of your stock or your cutter

and this mostly works and it's mostly

fine why does that mostly work because

it's obfuscating all of these very

important cutting conditions that we

talked about like depth of cut and feed

rate and surface finish and so on well

because people are taking those numbers

from these charts and Machinery's

handbook again which are calibrated for

1/8 inch depth of cut and 1/12 our

revolution feed rate but ignoring that

calibration and using lower depths of

cut and lower feed rates in general at

least in the hobby shop and the end

result is that if you use that shortcut

formula you're running slower than

necessary and that's mostly fine you

know running slower to a point is always

okay and can be more pleasant for the

hobby shop you know it's sort of less

dramatic there's less heat less smoke

less you know noise from the machine

less wear and tear on the machine so you

know running slower is okay and these

formulas work for most people because of

that because they are under estimating

the ultimate speeds and feeds that you

know you can use on your particular

cutting setup so feel free to use those

shortcuts but I think it's helpful to

understand where they come from and I do

think it's a useful exercise to go

through this calibration that I

described for your particular machine

and kind of know what that upper bound

is and kind of work down from there to

wherever you feel comfortable on speed

and feed for your particular machines

we've been focusing mainly on the lathe

here because it's easier to understand

but speeds and speeds on the mill has a

lot more nuance in modern milling

operations the unit that we use to

measure speeds and feeds is a little

different here we talked about

inches per two inches per tooth refers

to the amount of material that's scooped

out by each tooth as it comes around and

goes through the work and this is a

combination of three variables first of

all the power feed rate how fast the

stock is being pushed through the cutter

by the power feed and then it's also a

function of the RPM which is controlling

how quickly these teeth are coming

around relative to that table feed and

this is also affected by the number of

flutes on the cutter because of course

the more teeth you have coming around

the less each tooth gets to take for a

given table feed and RPM interest per

tooth is a very valuable and important

metric on modern CNC machining where you

can tightly control all these variables

for optimum performance on the manual

mill and especially the hobbyist manual

mill don't really have the luxury of all

that data because while we can control a

lot of these variables like depth of cut

and number of flutes and rpm the table

feed rate is a lot more tricky because

either you're feeding manually in which

case your feed rate is you know however

good your arm is feeling that day or

you're using some sort of power feed now

on some larger mills if the power feed

is driven via transmission you might

have known gearing and there may be

markings on your power feed controls

that tell you you know say an inch per

minute rate for feed but on a lot of

mills you've got an electric power feed

like this almost certainly and it's got

these utterly decorative numbers on the

dial here that don't mean anything so

how you calculate feed speed so this

process is simply a matter of zeroing

out your hand wheel make sure you take

out the backlash in that guy as well

zero that guy out and then you need to

know how many thousands are on one full

revolution so this particular mill has a

hundredth out on one revolution and

you're just simply gonna run your power

feed for a fixed amount of time say 10

seconds and just see how far it went

so for the current setting on that power

feed knob it went 380,000 10 seconds

which is two and a quarter inches per

minute to give or take so now you can

kind of map that speed to this knob so I

don't know let's say the 4 is up so you

might call for 2 and a quarter inches

and you know that you know smaller

numbers are gonna be less than that and

larger numbers are gonna be more than

that but you don't know that the scale

is linear it may not be and there's no

kind of markings or anything on here so

yeah honestly this whole exercise is a

little fruitless on these crappy hobby

power feeds so what I would suggest is

just set it low however the good news is

you can pretty much gloss over that

nuance for the most part and use the

same rules of thumb same formulas same

numbers just using the diameter of your

cutter instead of the diameter of the

stock however it is worth being aware of

some of this nuance so that you can

fudge the numbers up and down a little

bit as needed for example if you're face

milling it's clearly going to make a

difference if your n mill is fully

engaged or you know is only engaged on a

little bit of the material you know all

this is going to affect to a pressure

and thus you know how hard you can push

your surface speed and it matters if

you're a climb milling or conventional

milling but I'll be covering some of

those details in the mill skills series

but then if you're doing things like

slot cutting it gets even more

complicated because you've got one side

conventional milling on one side climb

milling so the differential forces on

the cutter complicate all these

variables and if you really want to get

into it Machinery's handbook does have

tables for all of this including you

know adjustment factors to feeds and

speeds based on slot cutting or

different levels of engagement of the

face mill but you really don't need to

do that for Hobby work so I hope you

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