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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: Britannia Lathe
05/07/2020 14:54:27

Good news - you know what the steel is! Here's what's said about EN3B's machinability, my bold:

EN3B can be machined at reasonably high speed with moderate feeds; although machinability is relatively easy it should be noted that surface ripping is a common problem unless an adequate lubricant is used during the machining process.

And:

Therefore, it is the weldability of EN3B that defines its use – if good weldability is not a consideration and greater machinability is required then a free cutting steel such as EN1A should be considered as an appropriate alternative.

So, EN3B isn't a good steel for beginners to start with on an unknown lathe. It may well be the guilty party, which is good news.

An adequate cutting fluid might be:

  • Water (avoid due to rust)
  • Lard, or Milk (best not, milk stinks and both cause infected cuts)
  • Cutting Oil, as used for screw-tapping CT90/Rocol/etc or one sold specifically for machining steel.
  • 3-in-1 or a clean plain motor oil (not the usual car type full of additives)
  • Cutting oil made into an emulsion with water 'SUDS' The water evaporates leaving a film of oil behind and it doesn't cause corrosion like water.

For cleanliness, I use either CT90 (pricey), or neat cutting oil, or plain Lawnmower oil, or similar. Although I bought a flood cooling system, it's rarely used. A mug full of liquid and keeping wet with a brush is good enough. Be careful not to splash coolant on to a carbide cutter because it tends to crack due to thermal shock. Either brush the work away from the cut or flood the whole lot continuously.

Although Lard, Sunflower Oil and additive packed motor oils are best avoided in the long run, not much harm will be done trying them provided they're cleaned up carefully. Paint the job with what you have available and try again.  3-in-1 works OK but it's smelly.

Dave

 

 

Edited By SillyOldDuffer on 05/07/2020 14:59:20

Thread: We need Pi
05/07/2020 14:28:30
Posted by pgk pgk on 05/07/2020 14:14:34:

Immaterial but we always rounded up to 3.142

pgk

Very sensible, it reduces the error to 0.013%. I may not have been paying attention on that day...

embarrassed

Dave

05/07/2020 13:52:06
Posted by Mike Poole on 05/07/2020 11:32:18:

355/113 is a bit more accurate, good enough for most workshop stuff.

Mike

For pi to 30 places (3.141592653589793238462643383276), here's a list of approximations, poor to good, and their error as a percentage to 3 places:

pi=3 Error 4.507%
pi=3.1 Error 1.324%
pi=3.14 Error 0.507%
pi=22/7 Error -0.040%
pi=3.141 Error 0.019%
pi=3.1415 Error = 0.003%
pi=355/133 Error = 0.000085%
pi=3.14159 Error = 0.000084%
pi=3.141592 Error = 0.000020%

pi to 30 places of decimals compared with pi to 100 places is a really tiny error, a little over: -0.0000000000000000000000000001%

For workshop tasks 22/7 is comfy for pencil and paper arithmetic and the result is plenty good enough for most practical purposes. Remembering pi ≈ 3.141 and doing the sum in decimal halves the error caused by 22/7, not that it matters much for ordinary work.

Now we have calculators, I don't rate 355/113 in practice. Multiplying by 355 and dividing by 113 with paper and pencil is hard work! Better to memorise 3.14159, which is slightly more accurate. Best of all is the value of pi stored in a calculator, which is more accurate again, and it doesn't have to be remembered at all. Why risk making mistakes with clumsy fractions?

Some reasons why pi needs to be accurate

  • the object is very large, like the radius of our universe,
  • a high level of accuracy is demanded, as in the Global Positioning System,
  • the object is very small, like an atomic particle;
  • some iterative calculations lose accuracy due to rounding errors.
  • number theory, mathematical exploration and computer improvements

In practice, GPS only uses 16 digits. Atomic science seems the main application for very high accuracy but even they only use 32 digit pi in calculations.

My computer can easily do pi to a 1000 digits, but it's a novelty. In the workshop I normally use 3.141 and a calculator. At school most calculations were done with a slide-rule only roughly representing 3.14, and I don't remember it ever not being 'good enough'. I know a more accurate pi was in a book of Mathematical Tables but don't recall ever using it. (Could be because I was a lazy student and bored by maths!)

The different ways of calculating pi are fascinating and researching them opened many high-technology doors. Everything from weather forecasting to Light Emitting Diodes and the Internet.

Dave


Thread: Britannia Lathe
05/07/2020 10:47:01

+1 to Hopper's "poor finish could also be a result of poor toolbit, worn bearings, worn bed, job sticking out too far from fhe chuck, worn chuck, or even a duff bit of material. If you are using Chinese brazed carbide toolbits, they will need sharpening before use."

I'd eliminate the duff material and carbide as possibilities first, not least because my last bit of turning produced almost the same spiral torn finish.

I was running a vibration test, not making anything, and ran a big blunt carbide cutter at slow rpm down a length of scrap mild-steel pipe at fine feed speed. The result is a poor cut like yours.

Not unexpected in my case!

  • I know that bit of scrap pipe doesn't machine well - many metals don't! So before assuming it's your lathe, buy some EN1A or (even better) EN1A-Pb. They're mild-steels formulated to be machinable, much better than ordinary structural mild-steel. As a beginner I had a thoroughly bad start when my entire collection of scrap turned out to be nasty - very misleading. Since then I prefer metals of known specification. I do work with scrap but it's much easier spot and compensate for difficult material problems now I'm more experienced.
  • Carbide performs best when run much faster and deep than HSS. Carbide is at it's best with powerful, fast, rigid machines. The Britannia pre-dates modern Carbide and was designed to work with HSS tooling, which in many ways is a first class option. Carbide performs reasonably well at slow speeds, but it's relatively blunt edges tend to tear. One trick is to use sharp carbide inserts intended for cutting light alloys like Aluminium - they work well at HSS speeds on steel.

I can get a good finish on my nasty steel pipe from the same carbide insert simply by doubling the rpm, feed-rate and depth of cut. Probably not an option on a Britannia, nor is high-power machining how I like to work! A nice sharp HSS or carbide insert might well fix the problem.

If a sharp cutter fails to get a reasonable finish on EN1a-Pb after investigating various rpm and depths of cut, time to look at Hopper's other suggestions. I'd start with easy problems like adjusting loose gibs, but don't be disheartened by inconsistent results because an old lathe might be worn in several places. Take it step by step. The forum is helpful, especially if you can provide photos.

Maybe the banjo gear ratio could be improved, though it looks reasonable to me (any Britannia owners about?)

Dave

Thread: What Did You Do Today 2020
04/07/2020 16:31:31

Thanks Sam. I guess we were looking for different things in different contexts. I mostly recruited for the long term with an expectation that development would be needed. The main exception was contractors, who were expected to know their stuff.

Depends on the job, but when interviewing I didn't weight basic practical skills that highly. Most people can pick them up with training and experience; for example tape measures and photocopiers are both easily learned on the job. A Quantity Surveyor not remembering everything needed to survey a shed off the cuff wouldn't necessarily phase me either. Evidence people know how to find out and make progress is usually more important in thinking jobs than memory tests.

I'm amazed an £80k Management candidate was expected to order a Fire Door. That's emphatically not a management skill! Good managers rarely need practical skills, their job is to coordinate and organise the do-ers, not to sink into technical details themselves. Management involves a lot of guess work and accepting risk, which your chap demonstrated. Better to ask managers about Priorities, Resource Management, Strategy, Effectiveness and Organisation etc. Same is true in reverse: it's not a good idea to recruit Welders by asking them to explain a Discounted Cash Flow!

Leaders are another case again. Being a Sociopath is an asset! The bigger the lie the more people fall for it!

By the by, after a long career in IT, I'd have to think about what a Macro is, and would probably take a couple of pages to define them thoroughly. Beware of experts: I might find your answer in the "That is when you press some keys & things happen" class too. Is Excel VBA Turing Complete? Discuss. (Interesting question that might be asked in an IT Job, but mostly irrelevant, even to Excel Power Users!)

Glad I'm retired

smiley

Dave

Thread: Old School Drawing Exercises and 2D CAD
04/07/2020 15:12:48
Posted by JasonB on 04/07/2020 13:21:00:

I did it a bit differently.

...

I like it, and there's a parallel between creating a new plane and my book's tilt a semicircle method.

As expected I had trouble using the book approach with FreeCAD's Part Design Sketcher. The lack of Layers to hide construction detail makes it clumsy, and the need to Constrain everything is time-consuming. It can be done, but I made an early mistake in this model that takes a lot of backtracking to fix. The top dimension has floated to 0.751" because the sketcher honours an incorrect constraint, and it can't be forced to 0.750" without undoing several steps. Lots of constraint clutter in red already with many lines still to be added:

freecadmoulding.jpg

FreeCAD's sketcher makes a poor general purpose 2D drwaing aid. Of course it's not meant to be used as one! It has an easy to use Spline Tool. The draughtsman just adds a spline curve and shapes it to look pretty. The whole moulding can be drawn in a few minutes provided Old School exactitude isn't demanded.

freecadspline.jpg

and seconds later:

freecadmouldingext.jpg

One more small step and my 3D printer could be churning a moulding out while I enjoy a glass of wine!

Dave

Edited By SillyOldDuffer on 04/07/2020 15:13:31

04/07/2020 11:52:57
Posted by JasonB on 03/07/2020 19:54:05:

How did I do? one line produced from a 1.5R semicircle

daves latest.jpg

 

Fair effort, the curve is fuller than mine, I give it a B+, same mark I give my attempt at scaling and aligning our two drawings!

jasondave.jpg

Here's how my book does it:

mouldingsoln.jpg

  1. Find the origin of the blue 1.5" radius outer semicircle by drawing two 1.5R arcs from points A and B
  2. Draw the blue semicircle and then several perpendiculars to it from the 60° line AB
  3. Then draw connected horizontal lines of the same length, the ends of which define the required curve.

A CAD package should include a spline curve tool to span all the construction ends and draw a clean curve. The easiest way to join them manually is with a French curve tweaked to blend nicely by eye. Not as easy as it sounds to draw really smooth curves by hand, my attempts usually have obvious joins! And see Georgineer's post about being marked down for blending his curves too well in a City & Guilds exam!

Comment on the book method - unlike bending a spline, or picking a random French curve out of the box, it generates accurate reproducible dimensions for the maker. Whether or not the result more artistic than, say, Jason's fuller example is a matter of opinion. Might just be fashion, but some curves, like a Spitfire's wing, definitely look better  to most people. I've no idea why!

Dave

 

Edited By SillyOldDuffer on 04/07/2020 11:59:29

Thread: What Did You Do Today 2020
04/07/2020 10:14:27
Posted by Sam Longley 1 on 04/07/2020 08:59:50:

...

One question I sometimes posed to degeree students was:-

I have to dig a hole of volume 1 million cubic millimetres. My lorries carry 8 cubic metres of earth. Allowing for earth bulking 25% when dug, how many lorry loads will I need?

...

A very odd question to ask either Quantity Surveyors or £80k Managers. What was your purpose in asking it?

Look at it from the point of view of the poor old candidate. He has to decide if the question is:

  1. testing mental arithmetic with an artificial problem using wildly dissimilar metric units, or
  2. deeply subtle needing careful analysis, or
  3. looking for a robust response to daft questions, as might be needed to rein in a bumptious apprentice, or
  4. seeking a polite response, as when dealing with a naive customer, or
  5. testing his negotiating skills.

Did getting the answer right or wrong make any difference to getting the job, and if so why?

I'm genuinely interested because selecting candidates is remarkably difficult and error prone: most methods don't work well. Fortunately, most people most of the time are adequately competent however they got the job!

The worst performers are untrained interviewers with no criteria - they look for Old School Ties, firm handshakes, and warm feelings. Assessment Centres are most effective. At them candidates are carefully put through several scenarios by a team over a few days, and assessed continually - even at lunch. Assessment Centres are very expensive, and although better on average they're far from perfect. They still miss exceptional talent and select men of straw for top jobs!

Dave

04/07/2020 09:18:57
Posted by Michael Gilligan on 04/07/2020 08:59:53:

Where’s the poetry in that, Nick ?

MichaelG.

No such thing as Imperial poetry - it's always been written in metres, ho ho...

Dave

Thread: Lathe dogs
03/07/2020 17:52:29

Yet to use a faceplate on my big lathe or turn between centres myself ! I've not had the need to use either straight or bent dogs because if a job will fit in a chuck I'd much rather use that. In the good old days dogs were much used because chucks were expensive. Less call for dogs now, though they're still useful for some types of work, which might be your thing of course. However, if unsure about straight and bent dogs, maybe they aren't needed at all? Yet! When one is needed, you'll know exactly what to buy!

As Bo'sun explains the bent ones are meant to engage in a hole in the faceplate, which is fine and dandy if the two fit together. (Old lathes had a specially slotted plate for bent drive dogs, but general purpose faceplates are far more common nowadays.) A straight dog might be easier to fit if it has to be driven via a bolt or some other arrangement. Both types of dog might need to be fiddled into position.

Made some different dogs once for the one and only faceplate job I ever did on my mini-lathe. Clamps fouled the job so I had to shape some metal specially to suit. Never needed them again...

Dave

Thread: Old School Drawing Exercises and 2D CAD
03/07/2020 16:06:10
Posted by Neil Wyatt on 03/07/2020 11:57:51:
Posted by Michael Gilligan on 03/07/2020 11:33:39:

What remains of my brain is at risk of frying ... So can anyone help me, please ?

Taking Dave’s triangle as an example:

We know that

  1. the centre of the inscribed circle is located at the intersection of the ‘bisectors’ of the three angles
  2. the centre of the circumscribed circle is located at the intersection the ‘perpendicular bisectors’ of the three sides

So ... There must be some elegant relationship between those two facts

But what is it ?

A geometric demonstration would be appreciated

MichaelG.

As a rider...

There are actually THREE points defined solely by the triangle itself that all lie in a straight line.

Michael has mentioned TWO, what is the THIRD?

And what are their correct names (I had to look these up)?

Neil

Michael and Neil have caused a complete meltdown here. Now I know I know nowt about triangles!

A guess; Neil means the centroid, incircle and orthocentre, all of which I had to look up and read four times. And there must be a million other triangle facts out there - my brain hurts.

Anyway, noticed this coincidence not mentioned by Wikipedia's Triangle entry. The inner circle is located by bisecting the angles (point I), whereas the outer circle (point O) is located by dropping verticals from the centre of each side. The coincidence is the white bisectors and green mid-verticals meet together on the perimeter of the outer circle when they're carried on outside the triangle (Points D, E and F).

outerinner.jpg

More! The orthocentre is found by dropping the red verticals at a right angle into the triangle's opposite corner. They cross at the orthocentre (Point H). If the red and green lines are also carried across the circle, they too meet on the perimeter.

So all three centres I, H and O are all related to each other, and Point H is also the triangle's centre of gravity. There's deep logic in this I don't understand. Arggh!

Dave

03/07/2020 14:15:11
Posted by Gary Wooding on 03/07/2020 13:45:41:

How about this?

 

mecadprob2.jpg

Pretty close, your curve goes slightly deeper than mine:

garymatch.jpg

I'm not complaining!   Fusion did better than I thought it would!

Dave

Edited By SillyOldDuffer on 03/07/2020 14:16:39

03/07/2020 13:55:13
Posted by Neil Wyatt on 03/07/2020 12:56:48:

Posted by SillyOldDuffer on 03/07/2020 12:23:07:

...

With CAD the curve is best done as a simple bezier with a control point at each end. Trivially simple.

Manually just as easy using French curves, the hard bit being finding mine if they turn out not to be where I put them years ago...

Neil

CAD may make drawing Bezier curves easy thank goodness, but are they trivially simple?

bezier.jpg

Doing this manually, I'd use a French curve at different angles to fair the curve, but it would be lucky to find an exact match. One of mine comes close, but not at both ends:

dsc06286.jpg

two or moredsc06285.jpg

And someone designed that French curve on a drawing board in the first place! How was it done?

Re Duncan's comment, I'm afraid the curve isn't drawn with arcs. (Maybe it could be?)

Once the approach is explained, easy enough to do manually, with Qcad, or with any other competent 2D CAD package. Not sure about FreeCAD & Fusion360's sketch tools for this particular job. They might not have the 2D primitives needed to construct the curve. (I haven't checked.) They can both do spline curves, but in this example it might be easier to construct the outline in 2D CAD and import the answer rather than sketch it.

Dave

03/07/2020 12:23:07

While I ponder Michael's problem and worry from his video if Thales is pronounced Thay Lees or Talez, here's another hard one. Well I think it's hard!

Replicate the curve of this moulding by whatever method. Clue - it's generated from a 1.5" radius semicircle, but the semi-circle's origin isn't on the 60° line. Whilst a CAD package can draw the spline curve given relatively few points to follow, producing a 'fair curve' manually needs several.

moulding.jpg

Nigel complained about hard-to-make drawings and this might be a good example! But shapes like this are common in the real-world. Streamlining and decorative work demand tricky curves, and made cheaply too! My stair bannister is an example, and dining room coving is another. No sympathy for the poor old tool-maker - the curve has to look right!

Dave

Thread: My new lathe a Warco 918
03/07/2020 09:33:59
Posted by Martin Connelly on 02/07/2020 15:16:20:

... It does require start up current to flow through the diodes though so heavy duty diodes would be required. I wonder what would be specified. Don't think my version needs manual resets. I've drawn the switches open in the operated position when they should be closed (nc contact) which may be why you think it needs resetting.

Martin C

Big current diodes aren't my thing but many automotive types look good, this MBR10 from Farnell does 10A (surge 150A) up to 100V for under a quid. I doubt Ron needs a heatsink for this application because the diodes only have to work until the switch takes over again - milliseconds, unless there's a jamb.

I thought your circuit needed resetting because I didn't think it through! Slapdash, sorry...

Dave

Thread: Old School Drawing Exercises and 2D CAD
03/07/2020 08:56:29


Posted by Michael Gilligan on 03/07/2020 00:07:14:
Posted by SillyOldDuffer on 02/07/2020 12:33:00:
.
Steps:
1. Drop the vertical line AD
2. Draw an arc from the corner ABC that crosses AD (in Blue on Diagram)
3. Draw two circles from the each end of the blue arc where it meets AB and BC (Yellow on Diagram)
4. Draw a line from B to the intersection of the yellow circles. (Light Blue on diagram) This line bisects the angle ABC, and where it crosses AD is the centre of the biggest circle that will fit inside the triangle.

.
With respect, Dave ... Your Step 1 is cheating, and disqualifies you.
But it’s simply a matter of adding the ‘mirror equivalent‘ of Steps 2&3 to locate the centre point.
... This also provides a legitimate way of placing that vertical, by construction.
MichaelG.


Posted by Spurry on 02/07/2020 16:30:38:
Ah, but that needs a set square, ruler and pencil in addition to the compasses. Is that allowed now?
Pete

To Pete, anything goes in my workshop. All's fair in Love, War and Model Engineering. Triple points for innovative cheating in my book, but don't try it in an Exam!

To Michael, no need for the 'with respect' - like Ian's comment on dividing, you are completely orthogonal...

smiley

Dave

02/07/2020 15:42:53
Posted by IanT on 02/07/2020 14:46:02:

That seems a bit complicated to me SoD.

I'd just draw two circles the same (but overlapping) size and then draw a line between their intersection points. That would give me the exact mid-point I'm sure.

IanT

...

You're right Ian, easier to do AND it would be more accurate! blush

Meanwhile, following Duncan's divide by 5 instructions:

fifths.jpg

You chaps are all too good at this...

Dave

02/07/2020 14:17:00
Posted by Spurry on 02/07/2020 12:54:05:

That's sneaky, stating that a ruler could be used, after posing the question. wink The answer was simple in Turbocad, but I could not work out how to measure a 14.42 diameter circle with a compass.

Pete

Yeah, sorry about that! My bad entirely - I'm in the dog-house again!

Going back to first principles again, a compass can determine linear distance by using it as a divider or a doubler. Quite easy to halve a line by setting a compass until the intersections it draws match exactly, and dividing can be repeated until the human eye can't cope.

dividing.jpg

The two red circles are obviously too small, and the two yellow circles equally obviously overlap. The two green circles are either spot on or pretty close.

Good eyesight can divide an inch down to 1/128ths, and dividing from another line at an angle to the target amplifies results allowing even finer graduations. Most struggle to read a 1/64" scale in practice. These days 1/32" is as good as this poor old duffer can do without a magnifying glass.

Dividing scales by factors of two is useful, but decimal scales are even better. Another drawing challenge is how to divide by 5 with a compass to make a decimal scale, and then a vernier? At this rate, engraving micrometers tomorrow!

Dave

Thread: My new lathe a Warco 918
02/07/2020 13:38:11
Posted by Martin Connelly on 02/07/2020 12:44:07:

The diodes will be for spark suppression,

...

I thought that at first, but looking again they have a smart purpose.

If the limit switches are hit they open and stop the motor. Now the clever bit! When the current is reversed with the DPDT, the diode bypasses the open switch and allows the motor to back away from the limit. The motor can't drive past the limit but it can always reverse away after being stopped.

Ron's first circuit is wunderbar because it recovers automatically from a crash. The diodes are needed and they're shown the right way round. Martin's circuit is OK but it requires manual resets after bumps.

Dave

Thread: Old School Drawing Exercises and 2D CAD
02/07/2020 12:33:00

Apologies for complicating the circle in a triangle question by allowing folk to assume they had to draw the triangle itself with only a compass! No, it's the radius of the circle I was after - 7.207592

Gold star to Gary for solving it with trigonometry, but I have to mention he's used maths tools provided by someone else! Was it a calculator or a set of trig tables that delivered 71.565° ?

Some lessons learned:

  1. The key to solving the problem is bisecting an angle, with luck this is remembered from school!
  2. Need to know how to bisect an angle with a compass, or
  3. Understanding a 2D CAD package will almost certainly have a tool for bisecting angles, in which case there's no need to remember points 1 & 2 above. CAD wins because it's quick and doesn't make mistakes, but the operator has to know which button to press! Beware of easy to use arty drawing software - it may not do techy stuff like this.

Can the entire problem be done with just a compass? Someone did in the distant past. Imagine a flat damp sand beach, a few sticks, a home-made compass and a length of string. The corners of the triangle can be found with the compass and fixed with sticks. A straight line can be marked on the sand by pulling the string tight between two sticks and twanging it on to the ground. Builders use much the same trick with chalky string to this day. No need for a hinged pair of compasses either - the 'compass' can be two sticks with a length of string held taut between them.

Here's bisecting done with a compass:

tricircbisect.jpg

Steps:

  1. Drop the vertical line AD
  2. Draw an arc from the corner ABC that crosses AD (in Blue on Diagram)
  3. Draw two circles from the each end of the blue arc where it meets AB and BC (Yellow on Diagram)
  4. Draw a line from B to the intersection of the yellow circles. (Light Blue on diagram) This line bisects the angle ABC, and where it crosses AD is the centre of the biggest circle that will fit inside the triangle.

Comment - problem looks easy, but has hidden depths! More to technical drawing than CAD buttons or pencil and paper, but CAD hides a lot of underlying complexity.

Dave

 

Edited By SillyOldDuffer on 02/07/2020 12:33:57

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