Feed speed

It is confusing. I'll try!

• What matters is surface speed rather than RPM
• For a given RPM, surface speed increases with diameter making it necessary to reduce RPM on large work
• To maintain a given RPM the motor has to deliver sufficient Torque. Although torque and power are related, they aren't the same thing. It's possible to have high power and low torque or high torque with low power.
• If RPM is controlled by adjusting a belt or gear drive, then torque increases or decreases with the gear ratio. High torque is available at low RPM, which is good.
• If RPM is controlled electronically, torque reduces with RPM, which is not good. There are ways in which the electronics can maintain torque with falling RPM, but torque is limited compared with gears.
• As most modern hobby lathes use electronic speed control rather than backgear, there may not be enough torque available to allow deep cuts in slow turning large diameter work. This is the slow down noticed by Sam. It's equivalent to trying to hill-start a car in 5th gear: might have a 100kW engine, but the engine at low RPM doesn't have enough torque to turn the wheels and she stalls.
• The operator has to balance depth-of-cut and feed-rate so that the braking force applied by the cut doesn't stall the motor. Not a problem with small diameter work, but large diameters need more care.

The amount of abuse a Diamond Cutter can take is limited by what it's made of, which is HSS. The best HSS can't compete with carbide when taking heavy cuts because HSS softens at much lower temperatures. So a brazed tip will outperform HSS provided the lathe is stiff, fast and powerful enough to drive it hard.

Frankly, I find high-speed cutting with carbide downright unpleasant. It creates a spray of smoking hot chips, quite unlike the civilised performance of HSS or carbide inserts cutting at moderate speed. I'm an amateur gentleman machinist, not a piece-rate wage slave in a Victorian sweat shop. I question the desire to remove metal very quickly. Rarely needed in a home workshop, not good for a relatively weedy hobby machine and makes it harder for the operator to get a good finish.

Dave

Sorry to spoil your Christmas but what you see on Youtube can also confuse and get your hopes up. Anyone else spotted it yet

The video Sam linked to shows a guy using a tangental tool holder with a 1/4" sq bit a sit is the 5/8" shank one. This is a screen shot from the video of him about to start a 0.200" depth of cut so one would expect the tool to be overlapping the end of the work by say 75% allowing for the angled grind making the 0.250" edge of the tool look a bit longer.

What he is actually doing is taking a depth of cut of 0.100" or a total of 0.200" off diameter, no wonder his 5HP machine is happy with it. Fair to assume all his other DOC figures are half what he says.

So Sam don't get too excited as he is actually taking off half the amount he says. Nothing wrong with the Diamond tool just understand what it is doing and what you machine can do. What's the saying "don't believe the hype"

Edited By JasonB on 05/11/2021 15:28:15

Edited By JasonB on 05/11/2021 15:29:05

You are making me  think that I should cancel the diamond tool holders & get something for the boat that I understand instead

Edited By Sam Longley 1 on 05/11/2021 17:32:15

A deal of over thinking going on here. Buy the diamond tool, select slowest feed, put on a sensible depth of cut and it will work. Just do it

John

+1

There is bound to be confusion for some in this thread - due to the term 'Diamond tool'. Makers of jewellery, watch cases, and alloy wheels for cars use diamond tools tipped with real diamonds. I suggest that this thread is dealing with an Australian tool holder (which does excellent work) but that is only a trade name, it is made from steel, and holds steel tool bits - no jewels are involved. It is a shame that this sort of muddle is created in the search for whizzy names. No wonder there are folk who just hate marketing.

PS other confusing trade names are available ...

Cheers, Tim

Does the boat need an anchor

As I said there is nothing wrong with the diamond type tools and they certainly make it easy to grind a tool that is almost certain to work but don't think they will mean you can shift metal at production rates using them.

Having now watched to the end he does say 200thou off dia and 100thou infeed, also says on that 100thou cut that the amps he was pulling suggest he was using all of the available 5hp of what looks like a geared head lathe, you will be lucky to have 20% of that from the 250.

I would certainly not dispute that but it demonstrates that the tool does take a lot more than you originally indicated. The grade of steel used has a bearing. The video is working on stainless & that is a bit harder than most of the stuff I turn . However, as suggested by journeyman above , one has to start slowly & build up.

Thanks for the comments re feeds. i need to find the casing for the change wheels. it was (along with the chuck guard) the first thing I discarded when i installed the lathe, On the front should be the plate with change wheel configurations for feeds.

In taking the cover off, I have been able to wire the safety lock to a micro switch on a carriage stop which i find very useful

I will try the paper between gear tooth trick, The gears do rattle against each other. I do not have room for any washers. The whole assembly is poor. I had to obtain a 14mm reamer before I could even get the gears to fit the shafts

I sometimes regret buying this lathe-- as they say " the bitterness of poor quality lingers long after the thrill of low price is forgotten.

Sam I did not say any figure for what the tangental or any HSS too for that matter could remove. What I did say is that the slower surface speed needed for HSS would mean you will need to run about 1/3rd the speed of carbide and therefore it will remove 1/3rd the volume in the same time given similar DOC and feed. If that was carbide in the video it could have been run at 1500-2000rpm on that 1/2" bar . That still holds true and once you fully understand the relationship of surface speed, feed and DOC you will see why.

Don't you have the manual as that also has the chart in it.

As Journeyman says the 250 comes with washers just as my 280 does so you should have them and the room to fit them. they go between the paired gear's stud and the banjo, I'll take a photo tomorrow to show where they go and how they affect the sideways spacing of the gears.

Edited By JasonB on 05/11/2021 20:13:47

various comments.

Jason makes the very good point that it is unrealistic to to expect a small machine, such as 450 watt mini lathe to remove metal at the same feeds and speeds as a larger, heavier and more rigid machine with a more powerful motor.

The manual for my mini lathe errs very much on the side of caution, by advising 0.010" (0.25 mm ) as a roughing cut!

With regard to the Diamond Turning Tool. To most model engineers this would mean a Tangential Turning Tool. Diamond being the trade name applied to one marketed by an Australian Company, and imported into UK, rather than a tool with a mineral diamond tip.. Such tools do exist and are used in industry for high spec, high precision turning on such things as I C Engine pistons

Tangential tools are highly thought of by those whom have experienced them. For many, that has become their general purpose turning.tool. The "Diamond" nomenclature stems for the fact that when sharpened, by grinding the square HSS toolbit at an angle, a diamond (Lozenge ) shaped face results.

By all means find the gear cover and refit it it, if only for the sake of safety and cleanliness. Those are among the reasons why the manufacturer went to the trouble and expense of providing it!

The table will provide details of the gear trains to set for various threads and feed rates. By acquiring extra gears, it will be possible to set up trains for further variants.

(My lathe says that the coarsest pitch that can be cut is 3 mm By replacing the normal 40T input gear to the Norton box with a 30T (supplied with the lathe! ) it was possible to cut a 4 mm pitch thread for a p;articular job..

Similarly, a mini lathe, with a Metric Leadscrew, can cut Imperial threads, (or vice versa ) by including a 63T gear in the train. It is not as precise a using a 127T, but the errors are acceptable for many purposes. .

Setting the gears by with a sheet of paper in each mesh will provide acceptable backlash, to drastically reduce noise and wear. Using a thick gear lubricant will further improve matters, and this is where the gear cover comes into it own, in containing any lubricant that is flung from the gears.

Don't despair, when you become familiar with the machine and how to set it up, you will find that can be a very useful tool.

It may not be perfect, hardly any any machine is. There will always be a job that is not ideal for it or is impossible to do on it. You can't swing a 150 mm job on a non gap bed machine with a 100 mm swing over the bed.

But, you will be amazed at what a skilled operator can do on a fairly modest machines!

At the other extreme, today, I have thinned the head of 10 BA bolts, held in 6 a inch 3 jaw chuck, on a machine capable swinging 12" over the bed! . Looks ridiculous, but the chuck can 10 BA nuts!

Learn how to get the best from your machine before damning it. The Industrial Revolution grew using machines far less sophisticated than your!

Howard (alias Fat Fingers 

Edited By Howard Lewis on 05/11/2021 20:19:15

Image shows how having the correct thicker washer (black) on the lower pair of gear's stud moves them away from the pair above with the smaller washer. gap = difference in washer thickness

I don't know if the lathe came to you second hand without them or they have been missplaced but if you don't have them in place that will be why you have noisy gears

I bought the lathe new

I will check that there is room for the washers & still fit the "circlips" ( for want of a better term) without them flying out

You might want to check how much thread engages when the studs are screwed in place. Mine often unscrewed, because only 3 or 4 threads did anything, and eventually stripped after about 5 years use. I made new ones, better finished and with slightly longer threads, from an aircraft bolt. They actually fit properly and I don't worry about them coming loose part way through a job.

I should make new keyed spacers at some point, as they're the next weak link.

I don't consider these to be real problems, as they only became apparent after several years use.

Sam they won't affect how the gear bush fit son the stud. The washers are 6mmID and only go onto the small M6 threaded spigot therefore moving the whole stud sideway

Not had a problem with mine coming loose and threads still holding after 12-13yrs,

Edited By JasonB on 06/11/2021 07:23:57

Surely any sensible design would have shouldered studs, such that they screw in exactly to the shoulder and tighten exactly at that point? Anything less seems to be yet another cheap short cut, on top of fitting ‘circlips’ to secure the gears/gear-pairs. Should there be a washer between the circlip and gear, too? I would expect that to be necessary to control end-float.

On my ancient lathe, the studs are designed properly - allowing a precise length of hardened, machined stud available for the gears to run on. Proper (thick) washers actually secure the gears in position, the threads on the outer end of the gear stud being female, thus accepting a set screw which is screwed up tight.

As an aside, with this old design, the thick washer is slotted so that it can be removed after only loosening the setscrew, allowing the gears to be replaced without actually complete removal of the fixing. Not too clever for the chinese to copy? No, but more expensive for a proper design (that works).

These are all small, but valid, reasons why ‘old iron’ is better than cheap modern chinese machines - as long as the old machine is not worn out to the point of ‘boat anchor’ status.

Looking at Jason’s pic and Sam’s description, have they cheapened the system in the last decade?

That's what they were aiming for. My new ones do. But the original studs weren't very well made, and eventually failed after several years use.

Most of the hardware is low quality which is a peculiar place to save money, as good quality nuts/bolts/etc are dirt cheap even when bought in small quantities. That said, if the compromise is to replace easy, small parts after a few years use or have to fix the actual machine(which is more than good enough for its intended use) I know which I prefer.

Having separate washers allows you to swap them about depending on the gear train, some trains need the pair to one side some the other,

NDIY if you care to look at my images you will see the studs DO HAVE shoulders so there is a set length for the sintered bush to run on Stud is slightly longer than bush just like yours too..

They are more recessed "C" clip with what would act like an integral washer, so not as poor a design as your blind comment suggests. These washers just like yours can be slid off without loosening or removing anything so gears can be taken off while studs remain in position which sounds as good a design as yours

The spacer washers are thick washers, 1.5mm for the thin and 2.5mm for the thick not just cheap penny washers, not file tested them for hardness.

Few Photos for you Sam, my 280 but your 250 is basically the same.

Shot showing gaps created by the spacer washers between the large gears so they don't touch and ring

Same spacers keep gears away from Banjo

This one shows that with the washer on the 6mm spigot it bears against the studs shoulder so width for bush is constant and you WILL have room to fit the gears with "C clip" in place

Not sure why you had to ream out the 14mm holes, mine are all untouched and the bush a firm push fit so there is no wobble of gear on bush

There should also be a washer on the gearbox input inboard of the fixed keyed bush that has the shear pin through it

The two machined not pressed washers 1.5mm and 2.5mm thick, 25mm OD, 6mm ID

Setting gear positions with a strip of paper

Two images showing that sideways adjustment of banjo on it's pivot can at each extreme have the gears rubbing together or gears rubbing the banjo so set it about mid way

Lastly over riding the interlock this is what my gears run like first at 500rpm, then 1000rpm then with feed engaged. All motor noise to my ears

Jason

I have been out in the workshop, Put the washers in - The rightway round!!!

The gear wheels never fitted the shafts they were so undersized it was a joke. I drilled with a drill bit borrowed from an engineering shop that was very slightly under 14mm, then reamed to get them to fit. I even had to file 2 of the key ways.

I used the paper to set up. When done the drive forced the arm with the change wheels out of mesh & it means I have to push it up very hard & really tighten it beyond what one might expect. But I cannot see where there is any stiffness. The slide is not overly stiff on the bed,

As for noise - Still much noiser than yours, but bearable if I stick below 650 RPM above that & the arm jumps out of gear. Only leave it set up if I intend to use it otherwise I will just disengage.Mine is nearly as noisy as yours without the change wheels engaged.

I did not grease the wheels but used chainsaw oil as it is sticky & does not fly out. I also use it as a cutting fluid & it works OK & is cheap.

So I will see how it goes

But thanks for your help, You have spent too much time on it all ready but it is much appreciated

Sam Longley

They also say 'a bad workman blames his tools'!

I like to break problems down into possible causes. Machining problems are due to the material, or the tool, or the operator, often in combination.

When things go wrong experienced machinists carefully eliminate all possible causes, including the possibility they need to learn new tricks. Is the material sticky, hard, gritty, or liable to work harden? How stiff and powerful is the machine? What balance of depth-of-cut, feed-rate, and speed works best? Is the machine adjusted correctly? If the machine has fundamental shortcomings, can they be circumvented by a workaround or skilled driving? Some problems are complicated, making it all too easy for beginners to get out of their depth and not understand what to do about it. You can guess how I know!

There's no shame in being a Learner. Self teaching is slow and painful compared with having a mentor pointing out simple mistakes as you go. Over-confidence is a sin, so beware jumping to conclusions.

Though YouTube is useful for seeing techniques in action, I find it better to start with books and magazines. A serious problem with YouTube is the huge amount of flawed material posted by unqualified persons: they unwittingly make lots of mistakes, and their advice is often untrustworthy. Another problem is inexperienced viewers drawing wrong conclusions due to not understanding the video's context. Imperfect though it is, this forum does much better because there's dialogue.

Sam's problem may be due to going a step too far. He hopes a diamond/tangential cutter will remove metal quickly before understanding his lathes limitations and the relationship between motor power and surface speed. The house is built on sand.

Actually, even if the lathe were perfect, a tangential cutter removes metal at much the same rate as any other HSS tool. It's because a certain amount of energy is required to cut a material irrespective of the cutter type: rule of thumb for HSS and mild-steel is 1HP will remove about 1 cubic inch of metal per minute. Above that HSS will soften. I suspect Sam needs a bigger lathe, not a 'quality' machine of the same size.

Pinched off Practical Machinist. The formula for Horsepower Required is:

HP = UHP X MRR

UHP = Unit HP (taken off chart)
MRR = Material Removal Rate

Some UHP numbers off chart:

Aluminum .25
Steel 1.4-2.5
Alloys 2.0-3.3

Not so obvious, is what limits removal rates. Motor power, torque, and max rpm; strength and efficiency of the drive train; cutter shape and what it's made of; coolant; desired finish; and rigidity of the machine all effect the rate. In practice, high-speed metal cutting is best left to modern industrial machines. They cost 10 to 30 times more than hobby kit.

For hobby purposes small lathes have fractional HP motors - they aren't metal munchers. General purpose hobby machines are between 400W and 2500W depending on size. It means the hobby operator has to set the machine up correctly, adapt his technique for best results, and not expect easy perfect results. Operated carefully they produce good work slowly. They are what they are.

Dave