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Member postings for SillyOldDuffer

Here is a list of all the postings SillyOldDuffer has made in our forums. Click on a thread name to jump to the thread.

Thread: cutting spur gears on a mill
10/10/2021 10:55:19
Posted by Nigel Graham 2 on 10/10/2021 00:01:52:

...

I must admit I have never thought the headstock gears and the change-wheels on my ML7 either "crude" or "square"; whatever those were meant to mean. If my screw-cutting comes out ragged, I blame the operator not the machine!

...

Myford and most other lathe gears are 'good enough' rather than well made. The main disadvantage of their relative crudeness and imperfect curves is noise. Vintage car gearboxes use basic gears and the whine is all too obvious, but lathe gears don't work that hard. Backlash is also evident, but doesn't matter when screw-cutting or anything else lathe related.

Putting the best possible gearing into a lathe is a waste of money unless the machine needs to be as quiet as possible or is exceptionally powerful. Myford lathes, bless 'em, aren't powerful!

Dave

Thread: Paint stripper
10/10/2021 10:06:07

A couple of posts mention Brick Acid or Patio Cleaner (ie Hydrochloric Acid) as a way of neutralising Caustic Soda. Although cheap and readily available I wouldn't bother: Caustic Soda is very water soluble, so just wash well in water. Dilute rather than neutralise. Less fuss and it saves money.

Another good reason to avoid. Neutralising stripped steel with Hydrochloric Acid promotes rust. Chloride ions penetrate deep into the pores of iron and steel and are difficult to remove. Over time they cause rust under paintwork. Neutralising Caustic Soda with Hydrochloric Acid is particularly bad because it produces salt-water which is notorious for rusting metal.

A big advantage of commercial paint strippers is they come with instructions. Following them is a good way of avoiding unexpected side-effects!

Dave

Thread: What Did You Do Today 2021
09/10/2021 15:49:35

Wot I did was drill holes, put up shelves, fix curtains, swap a broken mirror (seven years bad luck), and see my first UKCA mark in the wild!

img_1918.jpg

Is this a first? Is there a prize?

Only fly in the ointment : the mark is on a Smart Meter controller.

Dave

 

Edited By SillyOldDuffer on 09/10/2021 15:49:46

Thread: Antikythera Mechanism
09/10/2021 10:18:41
Posted by Michael Gilligan on 08/10/2021 21:51:37:

As promised; here is my updated spreadsheet :

...

The results are, I think, pretty clear :
  1. Based on my measurements ; the most likely count for the complete ring would be 352
  2. The counts predicted from small groups of holes vary enormously !
  3. [because] The predicted count changes dramatically with a tiny change in Angle per step

...

MichaelG.

Many sources of error in this ancient mechanism make it difficult to be certain about anything, but I think Michael has shown conclusively that there aren't 365 holes. Therefore the dial can't be indexing earth days. It is however a good match to the moon. And although we today are used to a solar calendar, the ancients went lunar. The Islamic Calendar is still lunar.

So although Michael's results don't prove the ring is lunar, they are consistent with the suggestion. The lunar hypothesis is strengthened rather than weakened by Michael's analysis, but nothing is solid yet.

I enjoy watching 'The Curse of Oak Island' on TV (Blaze channel on Freeview). In it a large team use high-tech methods to search for treasure mysteriously buried and elaborately booby trapped by the Vikings, Knights Templar, Spanish, Pirates, or the British army. See Money Pit. The fun is in deciding which 'evidence' is meaningful and what's utter rubbish. So far I've seen no evidence whatever of treasure on the island. Instead absurd coincidences are taken as gospel. I feel it's unlikely that the garden layout of the Palace of Versailles is a minora deliberately aligned with a marker in Nova Scotia by someone hiding treasure! Archaeology on acid. The whole programme is based on proving a negative - there is no treasure - when science knows it's impossible to prove a negative.

The Antikythera Mechanism is far more predictable than Oak Island, but still tempts false conclusions. Proceed with care!

Never mind drilling the holes, I don't know how they were laid out. Assuming the number is correct, how would you draw 352 equally spaced dots on a 6" diameter with a pen?

Dave

Edited By SillyOldDuffer on 09/10/2021 10:20:52

Thread: Paint stripper
08/10/2021 12:21:27

I don't think there's a Universal Paint Stripper that works on everything, and you always have to be careful about damaging the surface underneath.

  • Caustic Soda works well on Oil Based Paints, but don't use it on Aluminium or Zinc plate. It can be aggressive on wood and it attack glue: beware dipping pine doors and kitchen chairs - they might fall apart.
  • Car and some other spray paints are cellulose based; Cellulose Thinners works on them. Hydraulic Fluid works by accident, but isn't ideal because it leaves a difficult to clean off oily residue which can spoil repainting.
  • Most paints come off reasonably well with Methlylene Chloride, which is, or was, the active ingredient in most commercial paint strippers. As Methylene Chloride is harmful, it's being phased out in favour of various alternatives.

In general, commercial products are safer and work better than home-brew alternatives, plus they come with instructions, including what they're compatible with!

Dave

Thread: What was the purpose of these tools
07/10/2021 17:23:53

My guess is the spirit-level and wooden template are used together to help fit gutters and downpipes. The gutter fits into the notch and is lifted by hand until the slope is correct, then the brackets are marked:

The 45° angle is used to match to pipe junctions.

gutter.jpg

Dave

Edited By SillyOldDuffer on 07/10/2021 17:24:05

Thread: Mill power feed using stepper motor
07/10/2021 10:33:14

Posted by Speedy Builder5 on 06/10/2021 16:29:36:

...

power feed 3.jpg...

Basically I have a pair of Arduinos, one which controls the stepper motor and the other for the display. The advice I was given was that a single processor didn't have the speed to doo both tasks.

...

Though it complicates the wiring, using a second Arduino to drive the display is a good idea.

The problem is the display rather than the microcontroller, because the Arduino 'blocks', ie stops and waits until the display gets its act together and does it's stuff. LCD displays in I2C mode are slowest, 4-wire much better, and 8-wire parallel the quickest but they're all slow compared with writing to USB or a serial comms connection. It makes sense to have one Arduino blast data to another, and have the second Arduino sort out the display.

A while ago Duncan Webster and I collaborated on a Dynamometer project and looked into improving Arduino performance in some depth. On the dynamometer, display delays limit the maximum RPM that can be measured and/or accuracy. (Be nice to measure RPM with more than one pulse per revolution.)

Our cure was to stream the data to a PC over USB for off-line analysis, and to send edited highlights only to the onboard display. This approach isn't OK for all applications.

USB/Serial can cause trouble too. If USB speed is important, it pays to choose between the various Arduino boards. For example Uno USB is relatively slow and primitive, whilst a Mega is smart and faster.

I've recently been experimenting with the new Raspberry Pi Pico, a £4 board with two fast CPU's on the same chip. Each is about ten times faster than an Arduino Nano. In theory, I ought to be able maximise throughput by collecting data on one CPU whilst the other manages the slow display. In practice, using two CPUs is slower than one! Ho hum, something is wrong... At least Speedy's circuit works!

Dave

Thread: gr 8.8 ht bolt steel which carbide tip
06/10/2021 11:45:30
Posted by brian jones 11 on 06/10/2021 09:52:44:

...

I have formed the simple idea that using carbide

a) the tips are not sharp as per HSS, they are deliberately rounded slightly becasue a sharp edge in carbide would quickly chip and fail, so rounding gives better strenght

...

I can only assume that the tool tip with pressure reaches a high surface temp that softens the skin and lets mtl be rubbed off below - what an appalling thought

...

Maureen wasnt happy with even a mere 5 thou DOC

I did check the bolt and could use a file on it so not fully hardened

A puzzle?

Brian

A puzzle? Yeah, but no, but yeah.

Carbine is harder and more heat resistant than HSS, and it's at its best used far more brutally. The bluntness improves edge strength but also suits the rate at which carbide works best. It's counter-intuitive compared with HSS in that improving finish with carbide usually requires higher speed, feed-rate and greater depth-of-cut. HSS tends to improve finish by taking lighter slower cuts.

Problem with Miss Maureen Myford is she's too slow, weak, and bendy to get the best out of carbide, partly because her electric motor is called Weedy Walter.

Carbide produces good results on slower machines but it's a tad harder to get good finish with it: experiment.

The bolt not being fully hardened is a misunderstanding too. 8.8 bolts are tough rather than hard so they can be cut with a hacksaw or filed. However, there's no reason why a steel bolt should machine well. Ordinary mild-steel is a bit soft and smeary, Boron Steels are tough and smeary - an unfriendly material. Pot luck,

Dave

Thread: (old) Proxxon BFW 36/E vs. (new) Proxxon BFW 40/E
06/10/2021 10:39:54

Posted by John Smith 47 on 05/10/2021 22:22:03:

...

The more research I do, the clearer it becomes ...

Not from where I'm sat! In software jargon, I fear you've become entangled in anti-patterns. Candidates are: Analysis Paralysis, Bicycle Shed, Over-engineering, Scope Creep, Accidental Complexity, Golden Hammer, Silver Bullet, and Wishful Thinking.

wishful_thinking.jpg

I short I see John has requirement conflicts that cannot be satisfied.

  • Small Size
  • Low Cost
  • High Capability
  • General Purpose
  • Hobby versus Commercial

I suggest the problem isn't selecting tools, it's sorting out the requirement conflict. Requirements are important. Most obviously in this example, there isn't a small, cheap, high capability milling machine available. High-capability machines tend to hold their prices unless worn-out or damaged, in which case they aren't high-capability. Small, affordable mills are aimed squarely at genteel modelling - HO railways and so forth.

I suggest the most suspicious requirement is 'high-capability'. Stop looking for a Golden Hammer! Dump that, and a number of small machines become acceptable. Chinese, Proxxon, whatever, they will all do the job provided they're operated within their shortcomings. The user gets the job done by applying skill and patience rather than relying on the machine.

Compromise is almost universal on the forum. I don't know of any hobbyist rich enough to buy the best possible equipment: our budgets limit us to Far Eastern hobby machines and older second-hand gear, mostly manual. Even though they do all we need and much more, no hobbyist owns an 11-axis machine centre.

The Hobby versus Commercial issue also needs attention. If the goal is a commercial product, it's worth investing in. Development is a team game: lone inventors rarely succeed. Take advice. Pay someone else to make prototype parts: you do the design, they make it. Or raise the money to buy or hire the equipment and space needed. If the goal is commercial, different rules apply. Time is of the essence because someone else may be working on the same idea: first one to patent it wins. (Have you checked the idea isn't already patented?) Sell the car, mortgage the house, sell shares in the City, find a partner, put daughters on the street etc.

And once the item is in production marketing is even more a team sport than manufacture. James Watt found this: not enough to be a technical genius - his success only came after partnering with Matthew Boulton who knew how to organise, finance, and sell stuff.

Is the object to buy the best possible small mill at the lowest price for future fun, or is it to make a prototype? If the latter, compromise on the requirements or raise the cash. Be ruthless if the goal is a commercial product. Strict intellectual honesty is necessary because emotion badly fogs the issues. Worth writing a Business Case if not done already: working out how to make stuff is only one factor, and it's often the least difficult. Forcing oneself to think through all the risks and issues is always valuable, especially when setting priorities.

Metalwork for business is much harder than metalwork for fun. Don't assume they're the same!

Dave

Thread: gr 8.8 ht bolt steel which carbide tip
05/10/2021 21:57:22
Posted by DC31k on 05/10/2021 19:47:59:

I wonder if it is worth noting that 'Grade 8.8 steel' as such does not exist.

...

It is worth noting. The Grade defines the strength of the fastener, which could be made of any suitable high-tensile steel. The cutting properties of any given bolt depend on the alloy it was made from, so how well it machines is in the lap of the gods.

Whatever high-tensile steel meets the fastener spec will do. One source suggests most modern 8.8 bolts are made from Boron Steel - basically mild-steel with a dash of Boron in it. The resulting alloy is cheap and strong but it doesn't machine well, and it welds badly.

If a Grade 8.8 bolt cuts OK with HSS, it's not made of Boron Steel. When HSS fails, try carbide!

Bolts being made of different steels probably explains why people are reporting different results. My bolt gave carbide such a hard time I don't think HSS would cut it, but I'm prepared to believe HSS would be OK on other 8.8 bolts. I've had such bad experiences machining unknown alloys I avoid scrap, instead buying in metal I know is good to machine.

Dave

05/10/2021 17:23:09

Never tried to machine an 8.8 bolt before, so in the interests of science:

dsc06529.jpg

I skimmed the top lip with a square insert, an interrupted cut bouncing off the hex head. Tough test on tough stuff, but carbide chewed into it reluctantly at 1900rpm without breaking. The motor is 1100W, but it wasn't working hard. Didn't remove much metal because I bought the bolt for a proper job and didn't want to spoil it.

8.8 bolts are now on my 'difficult but possible' list. I'd rather work with machinable steel!

Dave

Edited By SillyOldDuffer on 05/10/2021 17:23:32

Thread: Saving the Planet or is it ?
05/10/2021 16:14:56
Posted by duncan webster on 05/10/2021 12:50:34:

Going back to SOD's latest (03/10), if I just take 3000 as the expansion ratio of a coal fired set, that is 3 stages of 14.4 ratio, so the IP entry is 208 psi. I do know this is a bit crude, but it shows that somewhere along the HP set the pressure has dropped from 3000 psi to 1000, and probably not very far along. It should therefore be possible to change the blading in the HP or even replace it complete to work on 'nuclear' steam. I will take some convincing that a single SMR boils more water than a big coal fired set, so the condensers and cooling towers should be re-useable.

Yes the power output of my conversion would be lower, and replacing the whole setup might be a bit more efficient, but the depreciation on the existing turbines, alternators, cooling towers etc is over, and they are not cheap.

My reference is 'Turbine Steam Path Damage: Theory and Practice', published by the Electric Power Research Institute in 1999. Volume 2 is about the nasty things that happen to turbines, Volume 1 covers Thermodynamics and Turbine Design.

Unfortunately Volume 1 doesn't answer Duncan's point directly - engineering books rarely do, and I don't have the intellectual Horse Power needed to understand this one properly! However I suggest these two schematics pinched from it illustrate significant architectural differences between fossil fuel and nuclear turbine chains. Both are about 800MW.

First FOSSIL:

dsc06527.jpg

Second, NUCLEAR:

dsc06528.jpg

Lots of noise on the diagrams due to the re-heater and other details but big difference:

  • Fossil has a High Pressure Turbine, linked to an Intermediate Pressure Turbine, linked to a back-to-back 2-flow Low-Pressure turbine, whereas
  • Nuclear has no IP Turbine. Instead the HP turbine links direct to a 3-flow back-to-back LP turbine.

The reheat plumbing and design of the blades is somewhat different too. 'Design of HP turbine blades in nuclear units manifest similar design features as those in fossil units, for example using the same root attachment designs. HP blades in nuclear units are longer to handle the volumetric flows. Same comment made later about longer blades in nuclear LP turbines.

I suspect analysing the through-life cost of all the components in an ageing power station would soon dismiss ideas of plugging in new reactors to fossil generating sets, or vice-versa. Maintaining a high availability power station without breaking the bank isn't like keeping an old banger going for fun.

Dave

Thread: Antikythera Mechanism
05/10/2021 12:11:00

Posted by Neil Wyatt on 04/10/2021 21:48:01:

...

On the face of it 360 or 365 seem right, but measurements of the hole spacings suggests 354 holes. Errors of ~0.1mm in hole placement in radius would be sufficient to account for this error (because a randomly wandering sequence of holes around the circumference would always appear to be longer distance).

...

As the Lunar Year is 354 days possibly the mechanism is based on, or predicts, phases of the moon rather than earth years and days. In which case the hole placement and accuracy looks reasonable for hand methods.

I like Ady's method, but would expect the reference holes to be more accurately set than intervening holes and don't see much sign of that - they're all a bit random. If I was making it, I'd try marking out a much larger circle, and using a long rod to scale down to the instrument's circle. The outer end of the rod could be moved a fixed distance from the previous point to step out the angle, or moved between pre-made marks.

Not sure how I'd mark out the outer circle because the lowest factor of 354 is a prime number (59) which can't be made easily by dividing by two. However, as 60 is easily divided by two, maybe it was done by marking out 60 divisions on the big outer ring and then losing one by eye.

A long rod being a little bendy coupled with a 60/59 approximation might explain why some of the holes are noticeably off, see the wide gap between 55 and 56.

The mathematics of the day were fairly advanced, making the theory available to the builder. What's surprising is the use of gears in an instrument this ancient. Most impressive.

Bow driven drill seems likely, but not obvious to me how very a largish number of fine drill heads could be made to the same diameter. They weren't ordered off the internet!

Dave

Thread: Workshop lighting / energy costs
05/10/2021 10:51:00
Posted by Brian Wood on 05/10/2021 10:13:20:

I too am careful about switching things off after use and have some interesting stand-by loading figures to add to the discussion. A domestic shredder uses about 15 watts when not in use, the extractor unit over the kitchen hob gobbles 45 watts on stand-by...

+1 for investigating what's causing the background load.

First, I unplugged everything and checked the meter: it was still turning! This was traced to my daughter's electric blanket, powered by an inaccessible socket under her bed. Anyone in your family leave stuff running all the time?   People who don't pay bills are remarkably careless!

Then I went round my house in the same way as Brian a few years ago with a plug in watt-meter and measured everything. Worst result was from a HiFi unit that used 40W on standby and only 15W when ON but idling. A number of devices, like a clock radio, turned out to be surprisingly greedy and worth turning off at the mains. Others, like my TV, use so little power on standby it's hard to measure it. Older gear tends to be much worse than modern, I think because the Energy Rating system requiring products to be labelled encourages energy saving design. Unlikely to find filament bulbs in modern kit! Anyway, I ended up with a list of devices worth turning off or replacing.

A friend found an unnecessary 80W Black Heater in his airing cupboard and that his wife* had left a 60W bulb on in the loft. *Wife got the blame: I reckon it was him!

Many domestic appliances use kilowatts in short bursts that don't add up to much in total. The devices that do the damage are those that sit quietly consuming tens of watts continuously rather than apparently obvious big hitters like washing machines. In my workshop the fluorescent lights use far more power than the machines and will be replaced with LEDs in due course.

Well worth checking power consumption around the house - I ought to do it again!

Dave

 

Edited By SillyOldDuffer on 05/10/2021 10:51:54

Thread: Recommend a grade of steel
05/10/2021 10:15:08

From the comfort of my armchair, I think Silver Steel was the right choice. Some yield strengths from the Model Engineer's Handbook:

  • Aluminium, 5 tons per square inch
  • 60:40 Brass, 8 tons per square inch
  • Malleable Iron, 8 to 10 tons per square inch
  • Dural, 15 to 19 tons per square inch
  • Free Cutting Mild Steel, 20 tons per square inch
  • Mild Steel, 24 tons per square inch
  • Stainless Steel, 36 tons per square inch
  • High Tensile Steel, 40 tons per square inch
  • Silver Steel, 35 to 50 tons per square inch

Bearing pins are worth hardening if they are subject to high wear AND are difficult to replace.

Unhardened Silver Steel is stronger than most common metals and could be used as a pin without heat treating it. Not the best possible, but pretty good compared with common alternatives.

The performance of Silver Steel can be improved by heat treating it, and this is probably where Robert's pin went wrong. Working with small parts and a blowlamp, it's quite easy to botch the heat-treatment. For best results it's necessary to heat the part to the correct temperature, (cherry red in dim light whatever that is), hold that temperature steady for a time related to the weight of the part, quench the part without allowing the it to cool en-route, and then remove the part at about 200°C and put it straight into a pre-heated oven to slowly anneal it.

Plenty of opportunities to get it wrong:

  • Steel not hot enough or too hot for the incorrect time. (Too long or too short)
  • Steel cools on the way to the quench
  • Insufficient quench liquid (bucket rather than cupful)
  • Failing to stir the part if the coolant is water
  • Cooling too quickly in the quench causes Silver Steel to become brittle and - in the worst case - to crack. Problem here is microcracks, invisible to the eye, which severely weaken the part, especially if it's repeatedly stressed as a pump pin would be.

My inexpert experience of silver steel suggests it's fairly forgiving of mistakes. However, early on I had a high failure rate - either not hard, or super-brittle. Better now. I think the main difference is not flapping about, instead: smoothly heating up to about the right temperature, holding it roughly for the right length of time, not too hot, and then putting the part straight into the quench and stirring vigorously.

How the part is held can be problematic too. Pliers act as a heat-sink, likely to make one end of a short pin much cooler than the other on the way to the quench. Various ways of avoiding that, for example making the pin much longer than required and grinding it to length at the un-held end.

If the part comes out very hard, it needs to be annealed. Small parts are fairly easy - they can held at 200°C for half an hour in a domestic oven. Might be necessary to get a divorce before this is allowed...

My advice, persist with Silver Steel.

Dave

 

Edited By SillyOldDuffer on 05/10/2021 10:16:38

Thread: Old rule divisions twelfs etc
04/10/2021 14:58:37
Posted by Bazyle on 03/10/2021 17:57:30:

Anyone remember the film Saturday Night and Sunday Morning (1960). The lead character was a lathe operator on piece work and said that he didn't measure the parts because he could get to size without and it would slow him down.
I wonder when that job (bicycle axles) went CNC. It was a bit odd that it wasn't an Automatic but may have been poetic licence by the scriptwriter to establish his devil-may-care attitude but based on some research of real machinists comments.

I think that very possible. The film script was developed from his own novel by author Alan Sillitoe, who in 1942 started work aged 14 in a Raleigh Bicycle Factory.

Dave

Thread: Unwanted Taper
04/10/2021 14:16:10
Posted by Michael Gilligan on 04/10/2021 11:39:44:

Posted by Steve355 on 02/10/2021 22:06:57:

[...]

here is a 0.004 feeler gauge under one end. I would imagine that makes it sensitive enough to be useful?

59cb1427-9990-4c48-9967-d35e36ece22c.jpeg

 

.

Q.E.D.

MichaelG.

The case for the defence. I think I show my Machine Level to be considerably more sensitive than Steve's Rabone.

The apparatus is a 670x30x20mm bright mild steel bar laid on my milling table, with a Level in the middle, shimmed at one end so the bubble is nearly level. It's a bridge:

dsc06522.jpg

The beam isn't quite level: nearly 3 graduations right, 2 left, a slope of 0.02mm per metre, about 0.001°. It takes the bubble just over 10 seconds to settle in this position after hands-off.

dsc06523.jpg

Unlike the Rabone which was tested with a 4 thou feeler, a 3 thou feeler under the right hand side of my level moves the bubble well outside the graduations - the resulting slope is bigger than the level can measure:

dsc06526.jpg

Inserting a 0.15mm shim at extreme right (370mm away from the level, about 0.023° ), also moves the bubble off-scale. The graduations can't be trusted.

dsc06524.jpg

My thinnest feeler is 1.5thou (0.059mm). Inserting it 370mm from the level moves the bubble off-scale, but it can be guestimated at about 4 divisions. To really test the level by shifting the bubble a graduation or two, I need a shim about 0.02mm thick at the far end. Can't think of anything 0.02mm thick to use: cigarette paper maybe but I expect it's too soft to take the weight without deforming.

Happy to have the methodology criticised, for example that 20x30mm steel bar must be bending slightly! But I hope m'learned friend and the jury accept the Rabone isn't anything like as sensitive as my genuine Machine Level. Whether the Rabone is good enough to level a lathe is another question: I think not, but I'm often wrong...

For most other purposes the Rabone is a far more practical instrument than my Machine Level. It's a pain in the proverbial in action: far too sensitive, nothing but fiddle, fuss and bother. Not one of my best buys! But it does do accurate levelling when that's really needed, way better than a Wixey or an ordinary Spirit Level. Ordinary Spirit levels get far more use in my workshop though, simply because most practical levelling is done better with a sensibly insensitive instrument! As tiny amounts of twist cause a taper, lathe beds need a sensitive level. Or consider Rollie's Dad as a alternative!

Dave

Pesky smileys!

Edited By SillyOldDuffer on 04/10/2021 14:18:04

04/10/2021 10:40:29
Posted by Steve355 on 04/10/2021 08:47:31:
Posted by SillyOldDuffer on 03/10/2021 10:16:07:.

...

As an alternative, Rollie's Dad's Method ...

Dave

...

My little rabone level is obviously an engineers level of sorts, it is adjustable, with a ground bed and far more sensitive that a regular spirit level. Whilst it may not of give me absolutely precise measurements, what it did do was give me a “direction of travel“ and magnitude for the amount of twist that there might be. When I started shimming up 1 foot of the bed, it moved substantially, and was useful in that it told me the direction of twist and that it was quite considerable, and if I continued shimming, it would probably damage the lathe.

...

Your Rabone might be part of the problem. Thing is, hitting the target is downright difficult if the level isn't sensitive enough for this job. It may be necessary to remove or add so much shim to move an insensitive bubble that the bed twist is always over or under-corrected. For this reason have a look at Rollie's Dad's Method which only needs a rod and DTI. Although the instructions look complicated at first, the Rollie's is actually quite simple. Don't fixate on the Rabone just because you have one!

An indication of sensitivity is how fast the bubble moves. Engineering levels are poor for putting up shelves because tilting the shelf half a millimetre causes the bubble to shoot across the scale and disappear. Conversely, when measuring a fine level correctly, an engineering bubble moves very slowly across the scale maybe requiring a minute or two to settle before the measurement is taken. Another difference, Machine Levels are usually graduated, whilst ordinary levels aren't.

It's possible to estimate the Rabone's sensitivity by laying it on a stiff beam, say 1 metre of 2 by 4" or similar, and packing one end of the beam so the Rabone bubble is centred. Flipping the Rabone around should put the bubble back in exactly the same position. Assuming it passes that test, use a strip of aluminium cut from a drinks can to lift one end of the beam. (About 0.15mm). If the Rabone really is a Machine Level, the bubble should move to indicate the tiny change of angle caused by inserting the strip. Actually, a Machine Level should be sensitive enough detect even smaller changes.

Given it's unclear the Rabone is fit for purpose, don't rush to twist the bed based on it - cast-iron breaks easily if overstressed in tension.

Stick with it - realigning a lathe from scratch may be time consuming but it's highly educational! As a beginner wanting to get on with metalwork rather than fixing machines, I bought a new Chinese Lathe to avoid this sort of shenanigans. The problem with second-hand is condition is uncertain, varying from 'as-new' to 'completely knackered' via 'maladjusted' and the other circles of lathe hell. The good news is 'maladjusted' can be fixed, but depending on what's wrong it may take a fair amount of effort, especially if skills and tools are limited. For example, in the factory the headstock of an inexpensive early lathe would have been aligned in a few minutes by a trained man using a jig. Newbie in a shed 70 years later has to work from first principles, which is much harder.

Dave

Dave

Thread: Spindle Bearing Wear Check
04/10/2021 09:21:31

The trouble with pushing a rod held in the chuck is it's unclear what's being measured. It might show the bearings are loose, but maybe not. It's quite easy to be misled, causing one to break the golden rule: 'if it ain't bust, don't fix it!'

I'm in favour of starting with 'does the lathe cut metal OK' before moving on to DTI's and so forth! Are there any signs the lathe isn't cutting correctly? If so what are they? The symptoms may suggest causes other than the bearings. And if the cause isn't obvious, it may be necessary to pin it down by carefully eliminating possibilities one by one.

Dave

Thread: cutting spur gears on a mill
03/10/2021 21:18:26
Posted by John Haine on 03/10/2021 20:02:47:

But surely if the cutter is triangular it should cut an involute tooth?

I don't think it's possible to predict the form because the shape depends on how much the blank slips during the cut. And because it slips, I think the form will be different on each side of the same tooth. For the same reason all the teeth will be different. The problem is fixed by indexing the blank, but that's not Brian's way.

How good free-formed teeth will be in terms of meshing with other gears is difficult to foretell. Be interesting to see some close-up macro photographs of several examples. Jason's simulations illustrate a likely problem: gears that have to be backed off to turn without jambing at the price of excessive backlash.

Dave

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