Internal Threading On The Lathe!

Summary

In this instructional video, Quinn from Blindix demonstrates the art of single point cutting for internal threads, a skill that intimidates many machinists. Quinn dispels the fear by showing that with the right technique and patience, anyone can master it. The video covers the basics of grinding the threading tool, setting up the lathe, and cutting both through and blind internal threads. It also highlights the importance of using the compound for better control and safety, especially when dealing with blind holes. Quinn's step-by-step guidance, from preparing the tool to making test fits, makes this complex process accessible and satisfying to learn.

Takeaways

• 🛠️ The video discusses the process of single point cutting for internal threads, which is considered challenging for many home machinists.
• 🔩 Quinn, the presenter, suggests starting with learning external thread cutting before attempting internal threads due to the increased complexity.
• ⚙️ An internal threading tool is required, and the video demonstrates how to grind one if you don't have access to a pre-made tool.
• 📏 The importance of using a fishtail gauge and proper grinding techniques to achieve the correct angles and clearances for the threading tool is emphasized.
• 👓 Magnification and good lighting are crucial for precision when grinding small threading tools.
• 💨 Respirator use is recommended during grinding operations to protect against dust inhalation.
• 🔩 The video explains how to prepare the stock, including enlarging the hole to the appropriate size for threading.
• ⚙️ The setup of the lathe, including the compound angle and tool height, is crucial for accurate threading.
• 🔧 A scratch pass is performed to ensure the threading gears are set correctly, and the threading depth is accurately measured.
• 🔩 Lighter cuts and more passes are recommended when cutting internal threads due to the less rigid nature of the tools.
• 📏 Measuring internal threads can be tricky, and the video suggests using Machinery's Handbook for dimensions or performing test fits.

Q & A

What is the main difference between cutting internal and external threads?

The main difference is that internal thread cutting involves cutting into a hole, which requires a left-hand tool, and each step has a little twist on it, making the process more challenging and terrifying compared to external thread cutting.

Why does the video suggest starting with external thread cutting before attempting internal threads?

External thread cutting is fundamental and less complex, so mastering it first provides a solid basis and confidence before moving on to the more intricate process of internal thread cutting.

What is a left-hand tool as mentioned in the script?

A left-hand tool is one designed for cutting internal threads, which typically require a left-hand rotation to engage properly, as opposed to right-hand tools used for external threads.

What is the purpose of the clearances underneath and on the ends of the threading tool?

The clearances underneath and on the ends of the threading tool are necessary to avoid interference with the workpiece, allowing the tool to cut close to the bottom of a hole or internal shoulders without rubbing against the material.

Why is it recommended to grind a threading tool from a split blank?

Using a split blank for grinding a threading tool is recommended because it has been pre-split at the ends from the factory, which saves time and effort by reducing the amount of grinding dust and material that needs to be removed.

What is the role of the D-bit grinder in the process described?

The D-bit grinder is used for fine-tuning the threading tool, specifically for plunge grinding the area behind the thread cutting point to clean up rough grinder marks and establish the necessary clearance angles.

Why is it important to use magnification and lots of light when grinding threading tools?

Magnification and ample lighting are crucial for precision during the grinding of threading tools, as they allow the machinist to see the fine lines and grind accurately up to the desired specifications.

What is the significance of the compound angle in internal threading?

The compound angle in internal threading is significant because it determines the direction of the tool's feed, which should align with the cutting edge of the threading tool to ensure proper thread formation and reduce tool pressure.

How does the process of cutting internal threads differ when moving from a through hole to a blind hole?

Cutting internal threads in a blind hole is more challenging because it requires a special left-hand tool and a different approach, often involving starting in the middle and feeding outwards, which reduces the risk of crashing into the chuck or other obstructions.

What is a 'spring pass' in the context of thread cutting?

A 'spring pass' is an additional pass made without adjusting the depth of cut, intended to remove any spring or deformation from the thread cutting tool, resulting in a more accurate and precise thread.

Why is single point internal thread cutting considered valuable despite being nerve-wracking?

Single point internal thread cutting is valuable because it allows for the creation of threads in tight or blind holes that would be difficult or impossible with other methods, such as taps, and it can be done with a simple tool, offering precision and control over the thread formation process.

Outlines

00:00

🔧 Introduction to Left-Hand Internal Threading Tools

Quinn introduces the concept of left-hand threading tools, which are designed for cutting internal threads in a manner opposite to standard right-hand tools. The video aims to demystify the process of single-point cutting for internal threads, a task that can be daunting for home machinists. Quinn emphasizes the importance of understanding external thread cutting before attempting internal threads and mentions using a left-hand tool, which is unusual and will be explained later. The video also touches on the availability of thread cutting inserts as an alternative to grinding your own tool, and the use of a split blank for grinding to save time and material.

05:01

🛠 Grinding the Internal Threading Tool

Quinn demonstrates how to grind an internal threading tool using a carbide blank, detailing the process of roughing out the clearance area on a bench grinder and refining the tool on a D-bit grinder. The importance of using the correct angles for clearance and end cuts is highlighted, with a focus on the precision required for the thread cutting point. Safety measures, such as using magnification, proper lighting, and a respirator, are also discussed. Quinn contrasts the use of a D-bit grinder with alternative methods like a Dremel, emphasizing the need for proper carbide grinding tools to avoid difficulties.

10:02

🔩 Setting Up for Internal Thread Cutting

The paragraph explains the setup process for cutting internal threads, including preparing the stock, setting the compound angle, and adjusting the tool height. Quinn stresses the importance of using the compound to reduce tool pressure and ensure proper alignment of the cutting tool. The use of a fishtail gauge to check the tool's alignment and the process of making a scratch pass to verify the threading gears are correctly set are also covered. The video demonstrates how to perform the initial cuts with precision, taking care to avoid crashing the lathe, especially when the tool is close to the chuck.

15:03

👷‍♂️ Advanced Techniques for Internal Thread Cutting

Quinn covers advanced techniques for cutting internal threads, such as measuring the thread dimensions using Machinery's Handbook, performing test fits, and using a spring pass to remove any thread flexibility. The video also explores the challenges of cutting threads in a blind hole, where the risk of crashing is higher, but the precision of single-point cutting is particularly beneficial. Quinn introduces the use of an offhand threading tool for this purpose and explains the process of setting up and cutting threads in reverse. The video concludes with a demonstration of cutting threads in a blind hole, emphasizing the safety and control of this method.

Keywords

💡Single Point Thread Cutting

Single Point Thread Cutting is a machining process where a single cutting edge is used to create threads. In the video, the host demonstrates how to cut internal threads using this method, which is foundational for understanding the video's theme of machining. The process is shown to be both precise and versatile, allowing for the creation of threads that are not easily achievable with other methods.

💡Internal Threads

Internal threads are threads that are cut into the inside surface of a hole. The video focuses on the technique of cutting these threads using a single point tool, which is a critical skill for machinists. The host explains that internal threading is more challenging than external threading due to the tool's reduced rigidity and the need for careful measurement and control.

💡Left-hand Tool

A left-hand tool is one that is designed to cut in a counterclockwise direction. In the script, the host mentions having a left-hand threading tool, which is significant because it allows for cutting threads in different directions. This is important for machinists who need to work on parts that require non-standard threading directions.

💡Carbide

Carbide is a hard material used for making cutting tools due to its durability and resistance to wear. The host starts with a 'split blank' of carbide, which is a pre-shaped piece that simplifies the grinding process. Carbide's properties are crucial for the longevity and effectiveness of the threading tools being demonstrated.

💡D Bit Grinder

A D Bit Grinder is a specialized tool used for sharpening and grinding cutting tools. The host uses a D Bit Grinder to refine the threading tool, showcasing the importance of proper tool preparation in achieving accurate threads. The grinder's precision is highlighted as a key factor in the successful creation of the tool's cutting edge.

💡Compound Angle

The compound angle is the angle at which the tool is set relative to the workpiece, which affects the direction of the cut. In the video, the host adjusts the compound angle to ensure the tool feeds in the correct direction for internal threading. This is a nuanced aspect of the process that significantly impacts the quality of the threads produced.

💡Threading Gears

Threading gears are mechanical components used to generate the lead or pitch of the thread during the cutting process. The host mentions setting the threading gears correctly to ensure the thread's accuracy. Understanding how to use and adjust these gears is essential for any machinist looking to create precise internal threads.

💡Scratch Pass

A scratch pass is a trial cut made to verify the settings of the threading process. The host performs a scratch pass to check the threading gears and ensure the thread's accuracy before making the final cut. This step is crucial for avoiding errors and ensuring the thread meets the required specifications.

💡Blind Hole

A blind hole is a hole that does not pass completely through a piece of material. The video discusses cutting internal threads in a blind hole, which is a more advanced technique. The host demonstrates a method for safely and accurately cutting threads in such holes, which is valuable for machinists working with limited access to the inside of a part.

💡Machinery's Handbook

The Machinery's Handbook is a comprehensive reference guide for mechanical engineers and machinists. The host refers to this handbook for thread dimensions and tolerances, illustrating its importance as a resource for precise machining. The handbook is a trusted source for the technical data needed to create accurate threads.

Highlights

Introduction to single point cutting internal threads, a technique that can be intimidating for home machinists.

The importance of learning external thread cutting before attempting internal threads.

The necessity of having an internal threading tool, with a demonstration of left-hand and right-hand tools.

The option to use thread cutting inserts for larger holes as an alternative to grinding.

Demonstration of grinding a threading tool from a split blank for precision.

Use of a bench grinder for rough shaping and a D-bit grinder for fine grinding of the threading tool.

Techniques for grinding the clearance area and the thread cutting point at a 10-degree angle.

The importance of using magnification and proper lighting for precision grinding.

Safety measures like wearing a respirator during grinding operations.

Preparation of the stock material by enlarging the hole with a boring bar.

Setting the compound angle for internal threading and the use of a protractor for accuracy.

The significance of using the compound for internal threading due to the tool's reduced rigidity.

Technique for flattening the top rake of the threading tool for neutral angle cutting.

Procedure for setting the tool height and ensuring the tool's alignment with the work.

Performing a scratch pass to verify the threading gears and transmission settings.

Method for cutting internal threads with multiple passes and lighter cuts for precision.

Measuring internal threads using Machinery's Handbook and an indicator on the tool post.

The satisfaction of achieving a perfect mating thread after cutting it manually.

Transitioning to cutting threads in a blind hole, a more challenging form of internal threading.

Use of a left-hand threading tool for cutting threads from the middle outwards in a blind hole.

Technique for starting the thread in a blind hole using the threading tool itself.

Procedure for cutting internal threads in reverse on a lathe, reducing the risk of crashing.

Cutting metric threads with an imperial lead screw using the single point internal thread cutting method.

Encouragement for viewers to try single point internal thread cutting as a valuable skill.

Transcripts

you see i would call it a left hand tool

because normal tools are right hand but

it's facing left which would make it a

left-hand tool maybe one's canadian

or maybe one of them is metric

so which hmm

hello internet my name is quinn and this

is blindix

single point cutting internal threads

that's a phrase that strikes fear into

the heart of many a home shot machinist

but i'm going to show you that you can

do it

well not you you've got a bad attitude

but everyone else can do it

the fundamentals of single point thread

cutting internal threads is the same as

for external threads but each step has a

little twist on it and the whole process

is 37 percent more terrifying to that

end you definitely want to learn

external thread cutting first i'll link

to my video on that topic below for this

video i'm going to assume you're

familiar with the basics of that process

to start with we're going to need an

internal threading tool i've got a left

hand one and a right hand one i actually

don't know which is which but this one

is sort of the normal one and then this

one is an unusual one that i'll explain

at the end here and you can see it's got

clearances underneath and on the ends

now of course thread cutting inserts are

also a thing so if your hole is large

enough to accommodate that that's an

easy way to skip some of these next

steps

now you can buy these of course but i'll

show you how to grind one just in case

you want to do that so i've started with

a split blank this is a piece of carbide

that's been split at the ends from the

factory saves you a lot of grinding dust

so i'll put some layout blue on there

and just lay out the tool that i want to

grind for this i usually just use my

fishtail gauge and trace it and these

lines are just a template for roughing

it in the lines just let you get close

to your goal with the grinding

i've got a bunch of material to remove

in the clearance area behind the

threading tool so i'll start by roughing

that in on the bench grinder the bench

grinder is a very coarse tool but it

will get us kind of in the ballpark here

and save us a bunch of time for the next

step so i'm just kind of hollowing out

the main

shank area there and kind of getting the

very vague idea of the point there you

can see this is all very rough but you

can still see my lines on top and we're

close enough now that we can bring in

the big guns and that's the d bit

grinder now i'm going to do this on the

debit grinder because i have one and i'm

fancy last like that but you can also do

this with a dremel with a diamond wheel

on it or those little grinding stones

you can get for dremels the point is you

don't have to have a d bit grinder to do

this now if you are going to use one of

those other methods i would suggest

using high speed steel and not carbide

grinding carbide without proper carbide

grinding tools is no fun at all i start

by plunge grinding the area behind the

thread cutting point there just to clean

up the rough grinder marks there and

even that all out i'm also adding a

little bit of clearance there so the

tool is at a 10 degree angle

so i'm cutting the clearance on the

underside of the flat area

then i switch to the outside edge of the

actual thread cutting point and i'm

cutting that also with the clearance

angle underneath at the same time

next i move to the end clearance make

sure i've got again a 10 degree angle

under the end of the cutter there so

that we can get in close to inside

shoulders and things inside of a part

you'll note that i'm using magnification

and lots and lots of light that's really

key for these tiny grinding jobs with

magnification you really can see those

lines and grind right up to them very

precisely i also wear a respirator for

all grinding operations a lot more dust

is coming off of this than you think

there is and that's going to sit in your

lungs forever

then the inside of the thread cutting

point is the one angle i can't do with

the work head on the d bit grinder

there's no way to contort it into the

angle that i need to do this it will go

to negative 30 degrees but not negative

60 as i need here i just do this by hand

and again just take your time go slow

this is a 600 grit diamond wheel so you

have lots of time to check your work

against the fishtail gauge as you go

i ground the other one basically the

same way but d-bit grinder work heads

are not really set up for left-hand

cutting tools but pro tip you can do

them upside down in the workhead and get

most of your angles that way

let's prep the stock next i happen to

have this piece with a hole in it

already that's a good size for this

demonstration but i'm going to open it

up a little bit here with my boring bar

you would normally start with a drill

and or boring you want to get this to

the tap drill size for the thread if it

was going to be tapped with a hand tap

if you don't know the tap drill size you

can also look in machinery's handbook

and it will give you the tolerance range

for the minor diameter of the internal

version of this thread and that will

tell you what dimension you need to hit

if you're wondering why my lathe sounds

like a goat trapped in a cement mixer

full of loose change it's because i

already have the threading gear set on

the change gears there i'm doing a bunch

of threading today and i want to keep

changing them back and forth so all the

feeding you see here is being done by

hand

for your first internal thread you

definitely want to do a through thread

it's much much less stressful than a

blind hole so make sure your stock is

thin enough to be able to do that with

the amount of reach that you ground on

your threading tool

next set your compound angle and you

want to swing it around such that you're

feeding in the direction of the cutting

edge of the threading tool just like

with external threading now for internal

threading that means swinging it around

this way however on this lathe that

makes it very difficult to crank so i'm

going to swing it this way instead which

gives me the same angle just feeding the

other way however the markings on my

compound don't show me the angles on

this side so i'm using the protractor

there to set it up

in my external threading video i noted

that the compound is not really

necessary for small threads and soft

materials and it really isn't but on

internal threading i do always use it

and that's because internal threading

tools are much much less rigid they are

the boring bars of the threading world

and so you really want every advantage

you can get including using the compound

which lowers tool pressure so you want

to angle the compound such that the

forward edge of the tool is doing the

cutting as you feed in now truth be told

if you feed the other way on the

compound and the tools cutting on the

back edge that's frankly fine too but

technically you're supposed to feed

forward now because this tool is ground

from round stock i need to get the top

of it flat so that the top rake is

neutral as you want with threading for

that i'm using an adjustable square here

and i just line that up and press it

down onto the top of the tool and that

flattens it out and gets that thing nice

and level

and it's facing backwards while i do

this because i also need to set the tool

height if this is your first time using

this tool or this tool in this tool

holder then you need to swing it around

point it at your tail stock center and

line it up so that the top surface of

that tool is right on center

just like with external threading you

have to set the alignment of the cutting

tool so that the point is square with

the work so that your threads have the

correct angle on them now the fishtail

gauge often won't fit though for small

internal threading tools like this so

you can either cut that end off or turn

it sideways like this and just use one

edge it's not quite as good but in a

pinch gets it done

as a reminder the purpose of this step

is in case the grind on your tool is not

perfectly square to the shank of the

tool so squaring up your tool post

wouldn't work in that case because the

point of the thread that you're cutting

wouldn't be square to the work so you

want to use the fishtail gauge to get

the point square regardless of how good

your grind is

next up is our scratch pass to make sure

that the thread cutting gears and or

transmission are set correctly so i'll

get some sharpie in the hole there and

then all set up to do the scratch pass

so i'm going to feed my tool in here and

i'm going to touch off on the surface

that we want to put the thread in just

like with external thread cutting and

then i can zero my cross slide right

there and put an indicator on my tool

post just like with external threading

and i'm going to zero that as well so

this is our zero point this is the peak

of our threads

now i can spin up the lathe and watch my

threading dial wait for it to hit the

right number

and engage the half nut and let it ride

now the point is basically right on the

surface here so this is our scratch pass

when it gets inside i unlock the half

nut i push the cross slide in clear of

the thread wind it back out and there's

our scratch pass so now i can get my

thread cutting gauge in there to check

it now if the thread is too small for

you to get the gauge in there you can

also do a little scratch pass on an

external part first before you do this

just to make sure that your thread

cutting gears are set correctly

that looks good so now i can pull the

cross slide back out to my zero and if i

did my job right the indicator will be

back on zero as well and now i can use

the compound to feed in some depth of

cut i'm gonna go five thousandths for my

first pass just like with an external

thread cut and you'll note that i'm

pulling the cross slide towards me there

because of course we're cutting on the

near side of the part

and once again wait for my number on the

threading dial engage it

when it gets through the part to the

other side disengage

push the cross slide in wind the

carriage out

reset to zero dial in more depth of cut

rinse and repeat now you'll note that

the lathe is running nice and slow here

only 100 rpm or so and that's fine

especially for your first cut that is

too slow really for a great surface

finish but it does lower the danger and

anxiety of this operation quite a bit

because of course we are feeding towards

the chuck stopping quite close to it

with the half nut engaged which if you

crash the lathe in that state that's

basically the worst kind of crash

crashing with the carriage is one thing

there's a clutch but with the half nut

the whole machine is physically

connected together and you really don't

want to crash it in that state

otherwise this is the same as external

threading at this point i'm just

engaging the half nut at the given point

on my threading dial this being an even

numbered thread on my lead screw i can

engage on any whole number or on any

half although the halves on my threading

dial aren't marked so it doesn't help me

much

the only difference here is on internal

threading i take lighter cuts and do

more passes because again internal

threading tools are inherently less

rigid so more patience is required i'll

do a couple of 5 passes in brass at the

start when there's almost no engagement

of the tool but then very quickly i go

to two thou and then one thou passes

internal threads can be tricky to

measure you can't use thread wires like

you can with an external thread so

typically i'll just look up what the

major and minor diameters of the

internal thread are in machinery's

handbook and that gives you the distance

or the depth of the thread that you need

to cut which you can measure on the

indicator on your tool post typically

you're trying to match a male thread

that you already have and so when i'm

getting close to where machinery's

handbook says it should be i just start

doing test fits

let's see how we're doing here that

looks close should be close

and that doesn't quite want to go it's

thinking about possibly forming a

committee to start but it won't go in so

i'll do one more cut and i also did a

spring pass where you do another pass

without adjusting the depth of cut just

to take any spring out of the thread

cutting tool hence spring pass

and let's try one more time and see

where we're at

and that

i think has got it yep that threads

right in just beautifully boy there's

really nothing quite so satisfying as a

mating thread that you cut yourself i

really strongly encourage you to try

this if you've never single point cut

threads on your lathe there are a few

operations that are as satisfying and as

sophisticated yet done with such a

simple tool

once you've got some confidence with a

through bore thread then it's time to

move on to a blind hole and this is a

more challenging form of internal

threading obviously because there's high

chance of crashing into things but this

is also where single point internal

thread cutting really shines because you

can do very shallow threads like this

where you could never hope to even get a

tap to start in there you might get one

thread in there with a tap but with

single point thread cutting we can get

lots of threads in there sufficient for

whatever we need

to aid this i'm going to bring in the

other thread cutting tool that we ground

the offhand or left hand right hand i

don't know the other the odd one that we

made and for this we're going to start

in the middle and feed outwards i showed

the same technique for external threads

in my other threading video but you can

do the same technique with internal

threads except that you do have to grind

a special tool for it because you have

to have one facing the other way you can

also buy offhand thread cutting tools

but they are very very expensive as

before we need the tool level and check

the height on it in this case it's going

to be the same because both these tools

were ground from the same stock

and now i'm going to feed in all the way

to the back and then pull away a little

bit for clearance and then i'm going to

set a zero right there on my carriage

indicator and that's going to tell us

where to start each pass and then it's

time to put the lathe in reverse this

entire operation is done in reverse so i

don't recommend doing this if you have a

threaded spindle and your chuck is

threaded on because it might thread

itself off doing this

as with the traditional method i'm going

to start by simply touching off on the

surface of the work there and then i'm

going to zero my cross slide hand wheel

and put an indicator on the tool post

and zero that this is our starting point

this is the minor diameter of the

internal thread and now i'm going to

move a little clear and i'm going to go

all the way down to the bottom of my

hole the zero that i set on the carriage

and we're gonna make a little starting

groove in there just using the threading

tool itself

if you wanted to be super proper you use

an internal grooving tool for this but

yeah we can cheat a little bit and just

use the threading tool so just feed

straight in a little past where the

major diameter of the thread should be

and you can see that we've got a nice

little starting groove in there now now

if you want to get it closer to the

bottom of the hole you can see that i've

wasted some space there then you have to

grind a smaller point on the threading

tool which limits the depth of threads

that you can cut with that tool

for each pass we feed into zero on the

carriage where that groove is then we

can feed in depth of cut with the

compound and the tool is just feeding

into that open groove so it's not

touching the work yet and then we engage

the half nut as before this saves you

ever having to plunge the threading tool

into the work while you're cutting the

thread which makes for a nicer thread

because the tool is always entering from

outside the work which is ideal

after each pass we wind clear of the

work and then we crank the carriage back

to the zero on the carriage indicator so

that we know the tool is back in that

groove

and then we can dial in the next bit of

depth of cut be aware that as you feed

in depth of cut with the compound the

tool is also moving horizontally

slightly so after a while you're going

to start losing your groove and you'll

need to feed a little past zero on the

carriage to stay in that groove before

each cut but you'll be able to feel that

when it happens it's not a big deal

otherwise it's the same procedure as

before but you can see that there's no

anxiety here because we're feeding away

from the blind hole in the chuck so

there's no risk of crashing into

anything when we're feeding in towards

the blind hole we're not under power we

don't have the half nut engaged and it's

really very low risk and this method is

nice for through bores as well you don't

have to only use it on blind holes it's

just generally a very nice way to cut

internal threads if your lathe is safe

to run in reverse

so now we have basically the same thread

we cut before but with a blind hole and

threads almost all the way to the bottom

of a blind hole this is again something

that's pretty difficult to do any other

way besides single point internal thread

cutting this method also really shines

if you need to cut metric threads with

an imperial lead screw or vice versa

because in that case you're keeping the

half nut engaged for the entire process

and so this is a safe way to get into

the bottom of a blind hole this is a

really valuable skill a little bit

nerve-wracking but definitely something

you should try once you've gotten the

hang of external single point thread

cutting i hope this video was helpful

for setting you down the road of

learning this valuable skill

thank you very much for watching if you

can swing it throw me a little love

there on patreon it really means the

world to me it helps me make these

videos every week and i will see you

next time

you