Screwcutting for beginners

[Kody]

G'day Geoff
These tools are used for general turning of diameters. The shape allows them to machine in both directions along the bed. The 80 degree tool will do the same but might be more suited for forming a bevel on the edge of the work. To grind the 63 degree tool to suit screwcutting would be a real pain. The problem is getting the exact thread angle, forming the proper side clearance and getting a suitable surface finish on the cutting edges. They would be very good IMO for roughing brass castings and may work on cast iron. Carbide tools used for brass, bronze / steel, cast iron are not usually interchangable. Each carbide tip is designed for a specific purpose and type of metal. There are "multi-use general purpose" tips but their performance is only average. This looks to be what you have. Some of the high production machinists here might be able to give you more info.

Kody

[snowyskiesau]

Thanks Kody,

It was only a cheap set to get me going while I learn to grind HSS.
I could identify the uses for most of the other tools but assumed those ones were for screwcutting, just as well I measured them!

Can you identify what the yellow tools in the attached picture would be for? They have a flat end to them. I can;t find them in any of the books I have.

[Kody]

Hi Geoff
Thats quite an arrangement of tools you have. I'll offer my thoughts on what they might be used for.

The Top row
The Green ones are general purpose, Left to Right and Right to Left machining, I dont know what metal they are suited to.
The Yellow ones would most likely be used for facing off. Angle them so they dont drag on the left side of the tool. I would set them to about 5 to 10 degrees. The sharp corner should be altered to a radius of about 0.2 to 0.3mm
The Brown (?) ones would be ideal for cutting bevels and similar use like the green ones

The Middle row.
The Orange tools, normal R to L machining. They appear to have a chipbreaker (?) on the left side. These might be used for heavier cutting.
Blue (?) tools. L to R facing, AKA Left Hand knife tool

Bottom row
Red tools, heavy cutting from L to R.
Greeen (?) tools Right Hand knife tool, facing and finishing.

The tools suggested as suitable for heavy cutting, means that they will possibly take a 50% increase in depth of cut. This "deep" cut might only be 0.100" to 0.150" deep. Everything is relevant to the size and robustness of the lathe. All carbide tools require heaps of power to achieve good results coupled with cutting speeds that are at least twice or much greater (3X) than the normal cutting speed of HSS. Vibration and chatter usually "kill" these tools, especially the cheap ones and if you stall the lathe when cutting with a carbide tool, it will always shatter..
To shape these tools, you need a Silicon Carbide grinding wheel (the green wheels) of 60 grit to rough out and 100 / 150 grit to finish grind. The cutting edges should be honed with a diamond hone for best results. Use only the Fine to Very Fine hones..Do not try and shape or clear away the "normal" steel used for the shank with the silicon carbide wheel. Do this with the standard Aluminium Oxide wheels. The green wheels dont cut normal carbon steels very well at all.

Have fun, it may be a great learning curve for you (and others),

Kody

[snowyskiesau]

Kody,

Thanks for the response and explanation.
I haven't actually measured any angles on the tools, with the exception of the green and brown tools in my previous post but will do so tomorrow to see if they are usable without further grinding.
My lathe (a Sieg C6, H&F AL-60) is certainly in the lower range when it comes to rigidity.
I got these tools as a result of discussions on various yahoo groups where the opinion seems to be that the brazed carbide are ok on this size lathe but tools with carbide inserts are a waste of money.
As they only cost $46 for the lot, it's not a great financial loss if I break most of them while practicing - as long as I know WHY they break, then I'll have learned something.

[radish]

I do think it might have been wiser, to have spent the 40 bucks on tool steel.

The el-cheapo stuff in that box is rather poor quality, go get a magnifying glass and have a good look at the cutting edges to see just how sharp that stuff really is.

You will have to give them a grind before you try to use them, as they might not have any clearance on the cutting edge at all, some of that stuff straight out of the box just rubs, not cut. The chinese reckon it costs money to have somebody sharpening them.

Do as Kody has said and go get the correct wheel to sharpen that stuff with, so you better add a bit more expense to the price of the cutters.

I bet you got a grinder on the bench right now, that will handle tool steel and will put a bloody good cutting edge onto it too.

With sharpening tool steel, the angles that you see mentioned , are the absolute best you could grind to, not very often does anybody actually achieve this perfection either.

But, tool steel will still cut, even if you haven't ground it to perfection, so just use these 'angles' as a guide. If you have ground it and it wont cut, then just have another go at it, there's nothing like practice, that's the best way to learn.

Remember that near enough is normally good enough when sharpening tool steel, it's a very forgiving product, oh, it's cheap too.

regards radish

[Rocky5]

Screw cutting for Beginners

Screw cutting can be an enjoyable task and it can also be the most exasperating task of all. I will endeavour to show how it can be fun to do and evade all the pitfalls. The key to understanding screw cutting is to understand the relationship between the lead screw, the job being done and that horrid little clock dial gizmo that is attached to the leadscrew. The units, metric and imperial will be mixed thru the discussion but don’t get too excited about that. For the discussion, I am using an Imperial lathe, the Leadscrew has 8 TPI.

The first part we will look at is how to set the tool up in the tool post. All the tooling used in this discussion is HSS, the tools being ground by hand using a thread gauge to set the angle. The thread we will cut is a standard UNC – 16 TPI form. Nearly all threads cut at home on the hobby lathe are usually never less than 10 TPI but increase to 32 TPI or even finer if making parts for a steam locomotive or steam engine. I like to shape the threading tool so the flank / edge of the cutting tool is never longer than about 3mm. or 4mm at the most. Take a stick of your favourite HSS and grind the required shape on the end. Keep trying the shape in the gauge and shape the “V” as accurately as possible. The sides of the tools must be straight, no facets are allowed whatsoever. When the tool is finished, it will have a sharp point at the end. This point must be removed and shaped to a small flat. But how much do we remove? Because of my experience, I use the TLAR technique (That Looks About Right) but a beginner will need an actual dimension. This can be determined from the formula Width of Flat = 0.25 x Pitch. See “Machinery’s Handbook” for more details.

The shape of the “V” cut in the bar is also affected (greatly) by how you set the tool. Accurate form and V angle can only be achieved by having the top face of the tool, flat and level and exactly on center. You cannot grind the tool to a perfect shape and then place it in a tool holder that sets the tool up on an angle, i.e., so the tool has any top rake. The angle of the “V” so cut will be formed to a wider angle and a poor thread is the result. Always pack the tool to its correct center height before starting any thread cutting. You can grind the top rake off the tool but then the thread angle will need to be adjusted, it’s best to have the tool clamped horizontally in the tool post from beginning the job.

There are two ways to set the Top Slide for screw cutting, it can be set at 90 degs. to the Cross Slide or at half the thread angle which in this case will be 30 degs. We will go the half thread angle route for this exercise. There are pros and cons for each technique which I will explain later.
Set the shaft to be threaded in the lathe. It is essential that the shaft run perfectly true. This is best done by machining the shaft to the finished size and then commence the threading in situ.
Loosen the Top Slide and swing the slide to the zero point. This is where the Centerline of the T/S is parallel with the center line of the Cross Slide. Now swing the T/S to the right to 30 degs. Did I say 30 degs? No, move it to 29 degs. and clamp it firmly. Now using the Thread Angle Gauge (also called Thread Gauge, Fish Tail, Fish Tail Gauge) set the tool to be perfectly square to the job. I can hear you calling “Hey Joe! What’s with the 29 degs?”
By setting the T/S to 29 degs, the right hand side of the tool bit will scrape a minute shaving off the thread flank each time the tool is advanced. This shaving-cut helps to produce a beautiful finish on the thread flank and also helps to stabilize the tool as it cuts. The finished thread form is still a perfect 60 degs. Included angle. Try it out and if you don’t like it, change the angle to 30 degs.
Position the tool somewhere along the job. Wind the Cross Slide handle so the tool almost touches the job. Now wind the cross slide out one full turn. Without the tool hitting the job, wind the cross slide in and stop when the handle that sticks out from the cross slide hand-wheel, is located at the 3:00 o’clock position. Set the cross slide dial to read Zero. This is essential and must be done as described. Do not touch the cross slide dial after this is set. Start the lathe and slowly and carefully, wind the top slide in until the tool just touches the job. Set the top slide dial to zero at this point and stop the lathe. Wind the tool away using the Cross Slide and position the tool about 1 inch past the end of the job. All tool advancement is now done with the Top Slide.

Did you notice that to bring the tool away from the job, the cross slide handle was lifted up to start it moving (or rotating). By setting the handle in this position, any panic attack to suddenly get the tool away from disaster should result in rotating the handle in the correct direction. Think of it this way. When the half-nuts are disengaged, the handle is lifted upwards. When you take a tool off the bench, you lift it upwards. When you take the threading tool away from the job, you lift the handle upwards and continue winding anticlockwise. Only when you have screwcut at least eleventeen hundred threads will you be able to rotate the C/S handle the correct way from any other position. This is why the above technique is best for beginners.
If you set the T/S at 90 degs. and screwcut, you must be able to wind the tool out from any position of the handle. Both of the above cutting techniques can be made easier by cutting a groove at the end of the thread to form clearance for the tool to slide into. Such a groove is not always possible or practicable to cut. For a Newbie with no experience, I recommend machining the .625 dia bar as shown to practise on. The tool can proceed into the relieved section with total safety and will help you develop your reflexes without tears.
And now for the next exciting part, ……..

The Horrible Clock thing on the Leadscrew
Actually, this is your very best friend, he’s a bit simple but you can’t do without him/it. Take a good look at it and count the number of longer and more prominent lines on it. Most “clocks” have four heavy and well defined lines and are also numbered. Wind the carriage up or down until one of these lines is aligned to the datum mark displayed near the edge of the “clock”. At this point, the halfnuts should be able to engage fully into the leadscrew, so push the handle all the way down. Get a pencil and draw a line on the lathe bed at the front of the carriage. Lift the handle up (disengage the halfnuts) and wind the carriage towards the Tailstock until the next heavy line on the “clock” aligns with the datum once more. Engage the halfnuts fully once more. Measure the distance from the pencil mark to the front of the carriage. It should be 1 inch. Are there any smaller lines between the heavy 1” marks? There may be one mark between the heavy lines and these are the “1/2 inch” distances. Depending on the diameter of the “clock”, there may be an extra line displayed to represent “1/4 inch” distances.
The thread we are cutting has 16 tpi. The Leadscrew has 8 tpi. Note that the thread we are cutting (16) is a multiple of 8.
Look at the drawing of the thread with the ruler above it.
A thread of 16 tpi has 8 whole threads at the 1/2” position. This means we can engage the halfnuts at this point and the cutting tool will follow exactly in the groove (thread) being cut. At the 1/4” position there are 4 whole threads and again we can engage the halfnuts. At 1/8” there are 2 whole threads so you can engage here also. The 1/8” position also corresponds with the pitch of the Leadscrew and this means that you can therefore engage the Leadscrew at any place displayed on the “clock” or anywhere the halfnuts will “drop in”. The drawing shown in the article will display this feature. Look at the thread on the RHS of the drawing. This thread to be cut has 10 tpi. Note there are only two places where you can engage the halfnuts. They can be engaged at any 1” mark and any 1/2” mark. One inch equals 10 whole threads; 1/2" equals 5 whole threads. At the 1/4" position (2 ½) threads, shown by the RED toolbit, the tool would cut into the thread you are trying to make and totally destroy it.

Now the fun part
Cutting the thread
With the tool about an inch from the end of the job, wind the T/S in 2 divisions. Start the lathe with the speed set to the slowest it can go and engage the halfnuts on any line shown on the “clock”. Run the tool to the end of the thread length required and in one motion, lift the handle of the halfnuts and the C/S handle to lift the tool away from the job. Stop the lathe and start to breathe again. Get a Thread Pitch Gauge and check that you are cutting the correct TPI that you wanted. Wind the carriage back to the original start position and wind the C/S in to the original zero point. This point must be set very precisely every time. Wind the T/S in another 3 divisions and start the process over again. Watch the video on Screwcutting for more details. If you get stuck, you can ask me here for any info you need. Unfortunately, I was unable to get the drawing to display within the thread as I wanted. Sorry for the awful long article, it’s hard to write this any shorter. I hope all the Newbie’s can understand the above and that the experienced can glean a few tips.

Kody

[Rocky5]

Kody

Great article on thread cutting... Im starting out as well. I was given this big old lathe and have been trying to cut threads. Its a Mascon WW2 Ministry of Munitions Plated, used to make gun shells or something like that... ... Has a big mechanical box 3 speeds and a lever for setting long travel speed. All imperial of course no gauges but a big down travel handle with indication marks...

Im not sure but I can not get it too work, keeps checwing out on the second pass.. ... could I have trouble with backlash considering its age.. How do I check this ...

Randal

[Kody]

Hi Randal
Thank you for your compliments about my thread cutting article. You certainly have a large machine there. If it keeps ripping up the thread on the 2nd pass, check the gear train on the end of the lathe. Check the gears (or lever positions) required to cut the thread you want. There may be a notation on the quick change gear box refering to what gears are needed in the gear train at the end of the headstock. Many lathes have two gears set side by side for use in this gear train. It's very easy to have one of the gears set back-to-front. Check this out and you may find it may be set up incorrectly.
Also check out if the lever that engages the half-nuts is going all the way down. If the lever is not seated at its full depth for each pass, cutting a thread will be impossible. Have you checked the tpi of the thread after the first pass? Use a thread pitch gauge for this to be sure the tpi is correct.
Good luck, contact me if you still have problems. Let me know what the tpi of the thread is that you are trying to cut. Also, how many TPI is the thread on the leadscrew, I'm guessing its 4TPI. Lastly, how many divisions and numbers are there on the thread dial indicator?
Kody

[tanii51]

.i am about to start my first thread cutting exercise using kodys method ie 29 degree ofset with t/s
how does this relate to the graduations on the t/s screw
does it mean 1 division equals .ooo33 instead of .001
for example a .625 maj diam needs the screw thread depth taken down to .5152
thread depth=.1098
if the topslide was at 90 degrees this would = 109.8 divisions(1 full turn+.98)
does that mean if the t/s is at 29 deg(30 for simplicity) we need to multiply by 3
hope this makes sense

[Kody]

G'day Tanii
I think you have your numbers a little bit out.
The figures you have quoted would refer to a 5/8 -11 tpi - UNC thread. (yes?). So I will start with a little bit of theory first. When refering to the depth of a thread, the measurement is made from the top of the thread down to the root of the thread. There are three basic dimensions of any thread that are crucial to having a thread that will fit (ie, fitting either the bolt or the nut). These dimensions are, the Pitch diameter, the width of the crest of the thread at the top and the width of the thread at the root diameter. The most important of the dimensions is the Pitch diameter. If the Pitch diameter is correct, the crest will be at the correct width. The root of the thread is where some people stumble a bit. If the tool is too sharply pointed, the root diameter section will be undercut and it may break thru lack of core strength. If the point of the tool is too wide, the minor diameter will be too large and the nut (when you'r'e making a bolt) wont fit. It is at this part of screwcutting that many beginners make the big mistake of cutting just a little deeper. The end result is the Pitch diameter is reduced to the point that the thread is useless. The first indication of impending trouble (disaster) is shown by the width of the flat on the crest of the thread. The width of the crest for Metric and all UN threads is , Width of Crest = 0.125P. These figures are from "Machinerys Handbook". Don't rely on this dimension as being correct if you are making a bolt to fit a specific nut.
In answer to your question then, don't worry too much about how far the tool moves on the TS. Your attention should be on the width of the flat on the crest. The figures you have given don't compute properly. At an angle of 30 degrees, 1 unit moved on the TS equals 0.866 units on the CS. Therefore, if you move the TS in 0.010", the tool has moved inwards, measured on the CS, a distance of 0.00866"
Measuring the width of the crest can be very difficult but measuring the Pitch Diameter is relatively easy. To measure the PD, you need some small diameter wires or use the shank of small drills. The diameter of the wires/drills is very important and you need a chart to ckeck the diameter measured. This all sounds far to complicated but it is the accepted way to do it in industry. The chart can be found in Machinerys Handbook or probably on the Web.
After you have gained enough experience, you can cut the thread and know by simply looking at it if it is at the right size or not. After eleventeen thousand bolts later it starts to become fun.
All that to simply say that 10 thou in on the TS equals 8 1/2 thou on the CS
Kody

[Rocky5]

Kodi & Tanii

Hi you're light years ahead of me. I'll just sit and watch from the side line. Im looking thru the Machinery's Handbook which has some great pictures of what you are going thu...

Kodi - sad news - "she who must be obeyed" wants shed man (that' me) to clean up my act and sell some lathes and equipment because there is no room. The big Mascon girl (refer to my listing on the forum - free to a good home) is on the market. So far no takers.. Ive sold one other and have an offer on the other. Aim is to pool the cash and have one good lathe. I plan to get back into thread making after that.

regards, Randal
aka - "My Shed Don't Stink"

[electrosteam]

A small comment on cutting threads:
- use the published tables, do not measure a "sample" thread, especially a worn component.

I have metric measurement gear and I had to cut a 3/8 inch Whitworth thread for the lathe.
The sample bolt I measured (in metric) was over diameter, probably because of tooth deflection.
I cut the new thread depth correctly, but the root diameter was too big.

A lot of head scratching and examination of the sample and the work with magnifying glasses.
That's when I looked up the tables to get the correct OD, and convert it to metric.
Arghhh !!!

Another attempt today.

Happy machining,
John.

[.RC.]

Just came across this thread for a threading tool which makes the job a lot easier Swing threading tool - The Home Shop Machinist & Machinist's Workshop BBS

[j.bain87]

Wonderfull design with that tool, it kind of makes a mockery out of all the advice that has been written on the subject of ---- "How To Do It".

Got to go make one of them tools now.

John

[pipeclay]

The tool seems to be a feasable idea,would be good to see how it performs with a finer pitch thread.
Curious also to know if or how you remove any swarf from under the tool,or maybe they stop and clean after every pass.
Appears that you would still require to withdraw the tool if not threading to an undercut.