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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: What Did You Do Today 2019
26/05/2019 13:00:17
Posted by Neil Wyatt on 26/05/2019 12:27:16:
Posted by Nicholas Farr on 26/05/2019 08:34:46:

Hi Neil, nice piece of kit. What size are the units in the combo and power rating?

Regards Nick.

Combo is the classic 4x10" but the original celestions have been replaced with Eminence Alpha 10As rated 100W each. The cab on top is 2x12" Mackenzies (not sure but probably 100W each).

That's the original Trace Elliot GP11 Mk V combo so it's an apparently feeble 150 watts, but they have a reputation for being very loud and I am sure it's as loud or louder than an 500W ashdown amp and head I played with recently.

Trace gave very conservative ratings and headroom is more important than RMS for bass. The power supply is easily sufficient to put around 400 watts into 4 ohms. Today that would probably mean an amp with a sticker saying "1000 watts"...


Edited By Neil Wyatt on 26/05/2019 12:30:30

This table from Hyperphysics lists the power outputs of various instruments and rates a large orchestra at under 70W. I'm always a bit surprised how weedy musical instruments are. Are big amplifiers only needed because loudspeaker efficiencies are so low, or is it because rock and roll sounds best with the Volume at 11?

Power Output (watts)
Large orchestra
Bass drum
Snare drum
Double bass
French horn

As all true music fans know even the best amps and speakers are garbage unless connected with Industrial Gauge Oxygen Free Gold Plated Silver Litz Wire Tri-filar wound on a helical PTFE core, and then fitted with bespoke insulating jackets hand-knitted in special-mix Kevlar, Carbon Fibre, and Vucana by nubile maidens. Nothing is too much trouble for proper audio pleasure...


Thread: Turning long slender arbors
26/05/2019 10:06:55

As the job has caused trouble on a Cowells and an ML10, both of which are up to cutting Silver Steel, I wonder if the problem is entirely due to choice of knives? Old-timers ground a huge variety of shapes and sizes to get the job done, and carbide inserts are even more diverse. I still get into trouble through not changing cutter to suit the job. (Perhaps I should order a quick change tool-post). Bother getting big tools into tight spaces, not getting enough reach from a small tool, trying to finish with a roughing tool, and wasting time while a baby insert eats it's way through a lot of metal. Etc.

Grinding HSS is an acquired skill, and it seems not everyone is good at it, blush! Partly for that reason I use carbide most of the time. To get the necessary range of cutting options I have holders in 6, 10 and 12mm for round, triangular, square and sharp inserts. (Not in every size!) Although I own a good selection, I still occasionally find myself grinding HSS.

Jason pictured two cutters shaped to tackle the problem. My preference would be the sharp carbide insert because I have one and it's easy. Second choice would be to grind HSS to shape. Although lots of chaps will tell you grinding is easy, it would take cack-handed me a while to get a satisfactory cutting profile on Jason's HSS example. Try it and see. You might be a natural, but don't despair if sharpening HSS takes more practice than you were expecting. Sparey's 'The Amateur's Lathe' is helpful.

Unless a clockmaker knows different I wouldn't put collets in a tail-stock and then make centres to fit them. I'd buy a set of live centres to fit the ML10. If necessary centres can be made to fit that.


Thread: Restoring a Myford ML1
25/05/2019 18:27:01

Posted by Peter Beeby on 25/05/2019 14:27:59:


Myford ML1/ML2? ... doesn't have enough change wheels ...pulleys have been replaced ... cracked head stock. ...weld either failed or cracked again. Tail stock little broken... needs a motor ... 1/4 hp how high can I safely push this? ... VFD. missing the rack...

I really appreciate any advice offered and hopefully this is the start of a wonderful hobby!

Hi Peter,

That's an unpleasantly long list of faults already on a small, slow, old-fashioned basic lathe in suspect condition.

Some kind of plan on how to deal with its issues is very sensible but I think you're going to have to strip it down to find out all the bad news. Likely there is more sorrow to come, for example, I associate cracked head-stocks on that design with someone trying to compensate for a completely worn-out bearing by overtightening the headstock. May be necessary to add a new bearing to the repair bill.

As the lathe may have been thrashed and abused, I would check everything. How worn and/or damaged are the bed and cross-slide, screws, half-nut, nut and gibs? Is the tail-stock in decent order or is it's spindle bent or worn out in addition to the other damage. Lots of photos please!

The missing rack suggests an old-timer decided the lathe wasn't worth repairing and cannibalised it for parts. More may be missing or is so bad it wasn't worth stripping.

Quite likely the lathe could be fixed, but it's going to take lots of time, commitment, and - unless you can do all the work yourself and/or get lucky - a money pit, hundreds of pounds and hours. A frame will be needed to hold the motor. Personally I would walk away - I want a lathe so I can make things. But if you like restoring old machines, or enjoy owning one, it could be your new hobby. The disadvantage is it might take several months before you get it working well enough to learn how to use it. (Though you will be expert at restoring lathes!)

As a lathe, is it worth having? Maybe. Pre-War they were one of the better examples of the simple low-end lathes then available. Certainly capable of doing precision work on small jobs. But notably unwanted as soon as Myford introduced the ML7, a much more capable lathe, in 1946.

Massively increasing the power isn't on though a 1/3 or 1/2hp motor would give it more zip. Bear in mind the machine was designed at a time when treadles were more common on small lathes than motors.

Brand new these machines weren't as capable as a Mini-lathe, and the cost of restoring it could be similar.


Thread: Steam Engine Number One
25/05/2019 11:00:25

Angle iron to lift the work so the flame can heat underneath as well! Such an obvious good idea : why don't I think of them?

Thanks again,


Thread: Interesting??
24/05/2019 18:43:01
Posted by Phil Whitley on 24/05/2019 18:16:34:

I would recomend everybody to do some research, especially on Edison, a brilliant self publicist with a very active worldwide network of offices that fed information back to him and his team about what was in development throughout the world, so he could get first US patent. A very smooth operator with a very musky odour!

We can all agree about that! Another example would be Alexander Graham Bell, whose claim to have invented the telephone depends entirely on dishonest jostling at the US Patent Office.

The world is littered with misleading claims. Samuel Morse didn't invent the Morse Code or the electric telegraph. Edison didn't invent the light bulb. Radar was invented by Hulsmeyer, not Watson-Watt. Marconi didn't invent Radio, Watt didn't invent the steam engine, Rocket wasn't the first steam locomotive, and Colt didn't invent the revolver etc etc. Not that these chaps were complete fakes, if nothing else they delivered when others failed, even if their methods were dubious.


Thread: Steam Engine Number One
24/05/2019 16:55:52

I thought all Patio Cleaners were based on Hydrochloric Acid. Apparently times have changed! It now seems most Patio cleaners don't contain acid of any type and - like the Swarfega example - would be useless as a pickle.

The right stuff is still available. Look for Acid Brick, Mortar or Patio Cleaner like this example on Amazon.

Though it's fast acting I'm not entirely happy with Hydrochloric Acid as a pickle though. I've read it leaves chloride ions trapped in the pores of steel that cause rust later. Even under electro-plate or good paint. If used and corrosion is a concern, give the steel an extra thorough cleaning.

By the way, no need to pickle freshly machined steel. It's plenty clean enough to make life easy for the flux. Pickling might even make things worse, especially if the wrong chemical is used.


Thread: HSS or CS taps and dies
24/05/2019 15:28:33
Posted by AdrianR on 24/05/2019 14:50:59:


I also discovered that my old place was 1 mile from Cromwell tools. It must be the last bastion of storemen. Dont get me wrong they were totally polite and helpful. Just I got that feeling that I was only allowed in if I did not touch anything.


My local metal vendor is like that too! I got on much better once it was established I knew what I wanted and was going to spend a decent amount of cash.

Having watched other customers while waiting for metal to be cut, I started to sympathise with the staff's off-putting attitude. Rather too many of the public arrive seeking free advice on all things metal related, or die of shock when told the cost, or otherwise dither and waste time. Much better to be the kind of customer who knows what he wants, can discuss alternatives, and then pays up cheerfully.

Last time I was in an elderly chap (ie a gent slightly older than me) was most concerned to explain why all new steel is, I quote, "crap". He lectured the assistant for about 10 minutes until he realised the queue behind was annoyed at which point he left without buying anything! I wish I'd thought to recommend the forum, what he said was misplaced in a busy shop but would have made an interesting post - his views were based on professional experience.


Thread: Steam Engine Number One
24/05/2019 15:02:47

Posted by Iain Downs on 24/05/2019 13:42:19:


I'd thickened the flux up, but I was still expecting something smooth (like the so often described cream), but it remains a powder suspended in water - lots of lumps. Is this right?

I originally bought firebricks (from a domestic fire) but for some reason thought that they were absorbers of heat not reflectors so got these bricks instead. Looks like I was wrong.


I'd also spread my research wider and found I could 'pickle' the assembly. I'd bought 1kg of citrix acid which I dissolved in about 4 - 5 litres of hot water. I was expecting it to become saturated, but think I need even more.

It was in the bucket for about 2.5 days before I got bored and took it out and rinsed. The scale has largely gone, but the metal rusted very quickly after being rinsed. Should I have left the pickle on? Should I have left it in for longer? What's a sensible amount of citric acid for a bucket (3.5 litres of water?). Can I filter the nasty bits out or do I need to throw the lot away?


One of the challenges with coming into this sport is that you don't really know what to expect. For example, I had to heat this assembly for 4 - 5 minutes before the solder ran. I had the impression that this would happen much quicker. And I don't really know at which point the solder should run - what colour should the metal be? And if I want to use Low and High temperature solders to assemble a complex piece, How do I know which colour to aim for aim all cases?

I suppose the practical answer is clear - and I'm already applying the technique - trial and error! Try often enough and instinct starts to work ... sad


Gritty flux rather than Dairy Double Cream is what I get.

Using the wrong firebrick will cause trouble if your torch is on the small side. Insulating firebricks are usually light and soft, absorbing firebricks are heavy and hard. The heavy absorbing type are NOT what you need. Don't guess, get the right type. (Or a much more powerful torch!)

Pickling: Dilute Sulphuric Acid if you can get it is best because it is fast and cheap. Minutes rather than hours. Citric Acid is a lot slower (overnight). How fast scale is removed depends on the concentration: your mix was weak, try for about 1:1 by weight, ie 1kg of Powder to 1 litre of water. Heating the water will allow a higher concentration and speed the pickling reaction up. Once by inspection the acid has cleaned the scale off, mild-steel reacts immediately with air. The trick is to clean the pickle off get soldering the metal as soon as you can. Keep using the acid until it stops cleaning metal or goes off (Citric Acid is biodegradable!) No particular reason to filter it.

I think you're doing the right things, but likely too slowly. How to fail: buy an ordinary DIY blow-lamp with a low heat output. Build a leaky heath out of heat absorbing brick. Then take two hefty lumps of rusty steel, apply a thin watery layer of flux, and poke at the job with a wobbly flame for far too long before applying the solder. The small torch is hot enough to activate the flux, but not powerful enough to get the metal up to temperature quickly, especially if the operator has L Plates. By the time the metal is hot enough to melt solder the flux has failed, scale reforms, and you get a dud joint.

Trial and error teaches you to get everything right quickly, it's experience not instinct. A beginner would probably do better starting with a big torch because getting a small one to do the same job needs more skill. I'm too mean to buy a big torch, and my results are intermittent. I'm much more likely to succeed soldering smallish lumps than big ones, and I believe the reason is the extra time bigger jobs take to get hot.


Thread: Threading plastics
24/05/2019 12:51:22
Posted by Chris Suddell on 24/05/2019 06:35:02:


I need to make a thread to set a copper wire around a plastic or GRP tube. My preferance is GRP.

Just wondered if anyone had any tips fo doing this? I have read GRP is nasty and my other option white drainage pipe will heat up.

Any advice will be appreciated.

Bit more detail would help Chris - presumably this is a transmitting HF whip antenna? If so the frequency and power matters, white drainage pipe probably fine with 50W at 3.5MHz, and dodgy with 150W at 28MHz. Well worth applying Joe's microwave test - I've found some PVC drainpipe to be much better than others, probably because they use different fillers.

Does the coil have to be part of the structure? The other possibility is to air-wind a large diameter coil and run the smaller whip through the axis, the bigger the gap the better. This example kept in shape by gluing wire to four thin black plastic strips.



Thread: Interesting??
24/05/2019 12:00:55
Posted by Phil Whitley on 23/05/2019 21:06:20:


But some observations. Edison invented nothing apart from his telegraph device...

Tesla, Heavyside, Steinmetz and Maxwell all denied the existence of the electron, and their calculations are no less correct for it.

I won't address everything in Phil's post between Edison and the electron because anyone who is interested can check the facts for themselves on the internet or - even better - at a library.

But to illustrate why I think the post is generally off-target, let's take a close look at Phil's thoughts on the electron.

That something like an electron might explain physical phenomena was first suggested in 1838 when Maxwell was 7 years old. I doubt he had an opinion at that stage of the game.

Scientists debated the unknown nature of the electron throughout the 19th century. Several hypotheses were investigated. Maxwell had been dead for 11 years before understanding was solid enough to coin the word 'electron' in 1891. Maxwell had been dead for 17 years before Thompson successfully measured electrons at the Cavendish Lab in 1897. Given the time-lines it's not surprising Maxwell's equations do not depend on the electron, and it's very naughty to claim he denied electrons in a way that makes sense today.

Heaviside took both sides of the real-or-not electron debate, and although Phil's right he wasn't keen on the idea electrons actually exist (rather than being a useful model), he was the first scientist to suggest what an electron's mass would be.

It's dangerous to draw firm conclusions from limited data and inappropriate examples. Tesla and Steinmetz were interested in electric power where the existence or not of electrons is almost irrelevant to the sums. But they're not the whole story. Electron theory matters in electronics, high magnification microscopes, and lasers - all technologies developed long after Tesla and Steinmetz were dead.


Thread: Childhood diseases
23/05/2019 19:47:22
Posted by Samsaranda on 23/05/2019 19:17:33:

I see that according to Silly Old Dufffer the state should have stamped on me many years ago for my stand on my children’s well-being, sorry to disappoint still alive and well, well I was the last time I checked and my three children made it through life successfully. 😇.

Dave W

Which is why you have to beware of individuals with quick easy answers to difficult questions. They could be idiots! I was never in your position and of course you did the right thing.


Time to change my policy slightly. Parents shall only be stamped on in the absence of contra-indications.

As the greatest leader Britain has never had, it's possible I was poorly advised. Not a problem though, the orphans of any parents I'd already had executed would have been sent a bunch of flowers...


Thread: DraftSight no longer free
23/05/2019 19:09:09

Hi Andy,

I (and a fair few others) use QCAD which is LibreCAD's older brother. QCAD also has a free community edition, plus a commercial version (about £30), that adds some worthwhile features. All three of them are competent 2D engineering drawing packages. QCAD is actively maintained. LibreCAD looks stalled at the moment: I couldn't download the latest stable version and the PPA site says it hasn't been updated for 166 weeks. That said, version 2.1.2 in the Ubuntu Repository looks in good shape. QCAD is my go-to tool for drawing single engineering objects, scale plans, templates etc - anything from back of an envelope to proper projection drawings with layers, dimensions, hatching and a version number! Simpler and easier to learn than Autosketch, which has too many unrelated bells and whistles for me.

I also use Fusion 360 which is a much more powerful 3D CAD tool. It comes into it's own for complex objects and especially Assemblies. For example, Fusion lets you model the individual parts of a machine and then model them together with approprate joints such that the machine can be animated. You can see if moving parts are going to collide without making a real one. Fusion can also produce rotatable photo realistic images, do stress analysis, produce 2D drawings, and output CAM instructions.

Although I find Fusion fairly intuitive, 3D CAD can be a steep learning curve, and the package has multiple capabilities. Getting to grips with it is a hefty investment of time and energy and you might have better things to do. MEW is running a course on Alibre at the moment; also very capable and well worth a look, but Alibre will eventually cost money.

Of concern is that Autodesk might change their minds about the free licence currently on offer. Draftsight was free for a long time before users got that nasty message. A pessimist might expect Autodesk to pull the same trick - get you hooked, and then demand money. For that reason I also keep up-to-date with FreeCAD, it's more obviously in development and whilst not in the same league as Fusion & Co functionally, it's genuinely free. An optimist might think Fusion a good bet because Autodesk are implementing a clever long term strategy: they don't intend ever charging hobbyists and students, the plan is to put loads of self-trained Fusion people on the job market, such that employers switch to Fusion to reduce their eye-watering training costs.

To keep life simple, it might make sense to choose the minimum the part-cutter will put up with. If he only needs properly scaled 2D plan drawings, LibreCAD/QCAD is easier to learn than Fusion.

Producing good engineering drawings can be quite hard; there are do's and don'ts. A basic book might help get you started.








Edited By SillyOldDuffer on 23/05/2019 19:11:21

Thread: Beginners question (sorry) - why I am breaking my small centre drills?
23/05/2019 17:57:28

A few more possibilities:

  • Small centre drills are rather weak and you're heavy handed. (I break them fairly often.)
  • The centre drill is too cheap...
  • The tailstock is off-centre. Drilling off centre causing it to bore, cutting on one side rather than drilling. Usually fairly obvious when the tip first makes contact, but the other give away is the drill cutting an over-sized hole.
  • It's one of those days. (I bored a cylinder exactly 0.1mm too big this afternoon.)


Thread: Childhood diseases
23/05/2019 17:44:26
Posted by Neil Wyatt on 23/05/2019 16:58:17:


It's the aversion to dirt and obsessive cleaning that ... is possibly behind the increase in auto-immune conditions like asthma, eczema, coeliac disease, nut and many other allergies.


Phew! I'm safe then. Been compared to a goat down a coal mine I have...

Thread: Fire bricks
23/05/2019 16:13:21

Firebrick is one of those words with two contradictory meanings. There's the type that absorbs heat, and the type that doesn't. What fun the confusion has caused in this thread!

Can anyone think of other examples? For example, bollocks is bad, but the dog's bollocks are good. And I might say of a too hot to touch loco belching steam and smoke, 'gosh that's cool'.

Is there a linguistic name for words that contradict themselves?


Thread: Childhood diseases
23/05/2019 13:12:05

The problem with achieving 'natural immunity' is developing it requires you to catch the disease. Measles, mumps, and chick-pox are mostly mildish illnesses, but they can all turn nasty. Killers. Not unusual for measles to cause blindness or even death. Grown men fear mumps for good reason - it causes agonising swelling of the testes and sterilisation.

The advantage of artificially immunising against disease is that vaccines are much less aggressive than wild bugs. As soon as they became available, most countries used them and real outbreaks all but disappeared. Denied of hosts Smallpox has been eliminated entirely, but not the others. Sadly, because vaccines are not completely safe, and one has been falsely linked to autism, many parents have chosen not to vaccinate their children, and as a result, the real diseases are back. (In my opinion objecting to vaccination is so stupid the state should stamp on the parents. Selfishly refusing vaccination endangers the population as a whole - people risking their own kids is one thing, but in my view it's not acceptable for other children to suffer because of misplaced parental concerns. )

Before 1950 contagious diseases were common, and they were rampant before 1930. Measles killed 1,145 children in 1941 Britain, between the years 2000 - 2016 only 1, 2 or zero per year. I'm lucky to be here because my mum, aged 4, only just survived Diphtheria.

Immunity provided by vaccination is at least as good as naturally acquired immunity, applied consistently it protects everybody, and it's far less dangerous.





Edited By SillyOldDuffer on 23/05/2019 13:12:33

Edited By SillyOldDuffer on 23/05/2019 13:15:09

Thread: Interesting??
23/05/2019 12:09:31
Posted by Phil Whitley on 23/05/2019 09:13:00:
Posted by SillyOldDuffer on 20/05/2019 21:07:05:
Posted by Phil Whitley on 20/05/2019 20:07:54:

... Science and scientists have a very poor record of invention of anything in common use today. ...

A very unfair criticism of science because the goal of science is understanding, not inventing! It's other people, like engineers, who exploit scientific understanding by inventing new things or improving old ones.


I don't see it as unfair, pointedly accurate perhaps, science is often understanding for the personal satisfaction of the curiosity of an elite, who never put that understanding into practical use. ...

Our entire electrical system was brought to the point it is in today by a small group of brilliant engineers, Tesla, Heavyside, Maxwell and Steinmetz are the main ones, and their work has been advanced very little in the years since their passing, and also no one ever heard of them, except perhaps Tesla. Einstien on the other hand is universally known, and much of his work is theoretical and obscure (and possibly wrong, they are only theories!), he got his Nobel prize for a paper on the Photoelectric effect, which he neither discovered or put to use, he merely "quantified" it.

Gosh Phil, we must agree to disagree. That seems a warped view of science and scientists to me! It's a dangerous view too, because our economy can't exist without scientific method. It's fire the goalkeeper because he never scores and he lets a few in.

Einstein is a good example. At the end of the 19th century most scientists thought a complete understanding of Physics was in sight. It appeared that Mechanics, Thermodynamics, Magnetism and Electricity were nearly complete with nothing else to study. Only a few anomalies like black body radiation, why the sun is hot, and the photoelectric effect needed to tidied up. As it turned out, these are doorways to new vistas of investigation and studying them made 'High Technology'. Semi-conductors rather than Steam Hammers.

I suppose Einstein's paper on photoelectricity could be written off as elitist, but the work ignited Quantum Mechanics, without which - for instance - GPS wouldn't work. Another theory - Relativity - led to an entirely different insight without which GPS wouldn't work either, and to the potential of Nuclear Weapons. He discovered new worlds of thought and the extent of his genius is 100 years later most people, including me, are incapable of grasping all the concepts.

Einstein got the Nobel Prize for Photoelectricity because his theory of 1905 was confirmed experimentally in 1914. (Experimental confirmation of theory was essential before a Nobel Prize would be avoided.) His other work is much harder to prove experimentally, and although parts have been confirmed in the real-world, it remains a Theory. Einstein was not satisfied the theory is complete or necessarily correct, but so far, with modifications, it's holding up. But, like 19th Century Physics, there are a few embarrassing anomalies to be explained... Failure to find an end is the nature of science, the goal is disciplined enquiry, analysis, and understanding, not making better mousetraps.

Tesla and others made Electrical Distribution practical, and it is the system we have today. But their achievement wasn't the end of the story by far; they did not deliver the internet!

Science, Mathematics, and Engineering are close relatives. When Bessemer invented his world-changing converter, joy turned to misery when customers bought expensive blast furnaces and found they made brittle crap. A job for the chemists, who discovered that the problem lay in high levels of Sulphur & Phosphorous found in some ores. (By chance Bessemer had tested with uncontaminated ore.) Once the cause was understood, chemistry quickly provided the answer by recommending a flux based on science, not guesswork.

Interestingly, not the end of quality issues with Bessemer steel. The modern process blows Oxygen rather than Air because chemists eventually found Nitrogen, normally inert, can react in tiny quantities to make mild-steel brittle. The problem was subtle, and no way could an engineer or furnace-man have fixed it.

Edison gets the credit for being the first to methodically organise scientists, technicians and mathematicians into teams working on sophisticated goals. His approach blurs the distinction between specialisations. Engineers and scientists both make extensive use of advanced maths. Engineers use scientific method and scientific facts to solve practical problems, and experimental scientists have to be good engineers.

The lone inventor is all but extinct. Individuals still have good ideas, but most easy to make inventions have already been done. (Unlikely I shall get Artificial Intelligence working on my dining table.) Instead, most R&D is done by collaborating specialists working in teams, including accountants! And when they've done the R&D, they will almost certainly need a production engineer and practical men to make it work.


Thread: sieg mill: normal chuck or collet chuck?
23/05/2019 09:40:40

Like as not you will use both. Most of the time I mill and drill with a metric ER32 collet chuck. Three main exceptions:

  1. Too often for comfort, Imperial drills aren't a natural fit to metric collets. Although Imperial collets will adjust to hold a metric drill and vice versa, it can be a faff.
  2. If the job requires many holes to be drilled in different sizes, drills can be changed faster in a drill chuck because you don't have to swap collets.
  3. The smallest ER32 collet can't grip less than 2mm, and many drill chucks can hold smaller. For very small drills, I grip a miniature drill chuck in the collet chuck.


With care it's possible to mill with a drilling chuck, but in my experience it's poo!


Thread: HSS or CS taps and dies
22/05/2019 10:14:36
Posted by AdrianR on 21/05/2019 19:37:30:

Hmm tapping compound, when I was at school we had a pot of tallow, and happily my father had a similar card board pot too. I have used lard in the past, but since my move all I have is neat oil.

I have seen people mention Trefolex and Rocal any other suggestions?

Regarding tapping drill size. For M6 I have two books, one says 5.6mm the other 5.3mm. I am tapping a 19mm deep hole so chose the 5.6mm to reduce the thread percentage and make tapping easier. For relatively deep holes what percentage thread would you use?

Tapping compound - always a good idea to use something! I use CT-90 because it's what my local place keeps on the shelf. The modern mixtures have somewhat better heat, pressure and lubricating properties than old-school organic recipes. They are also unlikely to be a biohazard. In the old days lard etc would rot and cause painful infections via a scratch.

For tapping drill sizes, how strong do you need the connection to be? The average bolt isn't heavily loaded, and when more strength is needed it might be easier to use a bigger one than a close fitting thread. Production tends to avoid tight threads because making them wears out tools and slows down assembly.

Most of the time I use over-size tapping holes. However, rarely - when strength or safety matters - I use the recommended tight drill. Not often because I'm not bolting wings on airliners!

I don't think the depth of the hole makes much difference to choice of tapping drill diameter. Surprisingly few threads are needed to achieve full-strength. Perhaps 4. After that, additional threads in a deep hole don't add more strength. So, it's the same decision: if maximum strength is needed, drill for tight threads and accept the tap will wear faster and is more likely to break, otherwise drill larger holes to favour the tap.


Thread: Vickers Bl 8 inch Howitzer cannon of 1917
22/05/2019 09:23:30

Seriously good work Mal. I'm admiring it in stunned silence!


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