Metal work for the Australia Radio Amatuer
some notes of things I have learned in the last 40 years of home construction
by ralph klimek April 2017
I you have the urge to construct your own ham equipment then at some stage you will be required to perform some simple metalworking operations. In the old days this activity was called "chassis bashing" when all things radio were mounted on an aluminium or tin plate steel chassis. Making the large diameter holes for valve sockets or funny shaped holes for inductors was a total pain. These days we have moved on. Tools have improved and the kind of tools once reserved for professional metal shops are readily affordable to the determined hobbyist.
The kinds of metal work I have encountered involve box construction , antenna construction and lately the construction of simple machinery. Simple competence with sheet metal is rapidly and easily acquired with modern tools. Aluminium is the metal of choice due to its high availability, lack of corrosion and easy workability. Brass is the "gold standard" for high power or low loss RF plumbing due to solder-ability, high conductance easy cutting and working compared with steel tinplate. Brass requires a higher level of skill. Leave it till later or untill you have no choice.
Drilling Circular Holes ( how hard can it be....you might be surprised!)
The first and foremost operation that you will perform is drilling holes and forming of large diameter holes in chassis and faceplates. Not all holes are circular the need for rectangular holes will arise from time to time. The first tool to consider purchasing is a hand held drill. Nothing less than a half inch chuck is suitable. Purchase the full set of drill bits up to 3/8s, a 3/8,1/2 and a 5/8 (reduced shank). Purchase a full set of metric drills up to 10mm. metric and imperial drills form a good set of clearance hole drill bits for each other.
Buy a cheap bench drill press. You cannot drill a precision hole with a hand held drill. It is NOT possible. A bench drill is the only way to drill repetitive holes. The hole from a bench press drill is allways perpendicular to the job. This matters when drilling thicker materials especially when trying to mate different objects. A perpendicular hole guarantees that holes will line up and your jobs separate pieces will mate !
Buy some cobalt steel drills. These are costly, some care then is required in their selection. I have 1/8 3/16 and 1/4 for general steel and brass work. I have 2.5mm,3mm,4mm cobalt drills because these seem to be the overwhelming majority of the hole sizes I drill. Cobalt drills through steel with the same ease that high speed steel drills though aluminium. They stay sharp allmost forever. Cobalt drills should not be used for aluminum, they become eroded rapidly due to some chemical interaction. Cobalt is to be used with brass and steel.
A drill requires lubrication. I use a smear of Trefolex (TM) cutting compound on the drill, especially before drilling anything deeper than 3mm aluminium, and all steel. Brass is "self lubricating" or so they say. Still I use the compound, it just works better. The cutting compound mitigates the effect of galling, which is caused by the freshly cut metal cold welding itself to the cutting tool. Holes will be cleaner and better formed and you can drill deeper without risk of the drill binding in the hole and snapping.
Buy a good centre punch and keep it sharp. You cannot drill a precision hole without centre punching the work. The drill bit will wander before it bights the work, and the result is a hole that is offset from where you actually want it be. The small indentation of the centre punch prevents the bit from its initial wandering. This is critical if you are drilling holes for mating pieces. Centre punching everything is a good habit to develop.
Buy a small rat tail file. Despite your best effort, sometime holes just don't match and you have to move the hole along a bit. As your skill increases this will happen less often !
Best advice EVER!
Buy a large diameter stepped cone drill. These have recently come on the to open market and for simple sheet metal work, obviate the necessity of buying drill bits larger than 3/8.
Cone drills are available in metric and imperial step sizes. They are only safe to use with larger 1/2 inch chuck drills any they remove all the drudgery in drilling larger holes for things like SO259 sockets or valve socket hole. You must use a cutting compound. They are safe to use on all sheet materials but steel requires more care.
for the really large diameter holes, say for speaker holes or meter movements, your only real choice are hole saws. They are hard to use on sheet metal. I will provide more detailed advice for their care and use below. They are dangerous in the hands of the unskilled. A nibbling tool is good for this task and maybe your only choice for this duty.
Good welded holesaws are expensive so only buy the size you actually need for a project. Fortunately moving coil meters come in ,mostly, standard sizes.
All good holes require deburring. If the burr is big, this is a sign that you have applied too much force or your drill bit is blunt. The standard deburring bits based on multiple cutting edges gives a woefull result. The best have only one cutting flute and rely on a frictional effect to smooth the hole. These can produce a beautiful chamfered hole that eliminates the need for rubber grommets on holes carrying electrical wiring.
A hole is only usefull if the thing for which the hole is created is large enough to contain the thing. This needs some thought. A 4mm drill bit will drill a hole with an internal diameter of 4mm. A 4mm metric screw will just barely clear such a hole. A 4mm shaft will not ! This is due to interference. The shaft and most screws will require a clearance to be drilled. That drill is the next one up ! Or , for sheet metal use, a tapered reamer will do. A reamer can only make up for a drill missing from your collection, but only for sheet metal use.
Buy a drill stand ! and always return a used drill to it and develop this habit. Nothing is more infuriating than looking for a drill bit that you know you own, but you put it somewhere where you would not forget, and forget it you will. Guaranteed.
Water is the enemy of drills. It will remove the cutting edge of high speed steel long before rust can be seen. Always keep you bits lightly oiled
Sharpening your drill bits.
Your drill bits will wear out. That is life. Cutting compound significantly extends bit life and the larger sizes are costly. Buy a cheap bench grinder. Learn how to sharpen a drill bit. It takes a while to learn this essential metal shop skill. Go to a flea market and buy a handfull of rubbishy second hand bits and practice. There is no other way. Drill bits larger than 1/8 can be hand sharpened. smaller ones take too much skill, sometimes you can fluke it. They are not costly, so do not bother. When bench grinding, periodically dip the drill bit into water. Due not let the drill get red hot, red heat destroys the drill alloy.
Don't apply too much force when drilling. Let the drill bit do the work, you just do the guiding
Do not drill wood with your good metal working bits ! But wood is softer than metal, surely. No it isn't ! Real wood contains silica granules that precipitate there from the sap. Wood can be more abrasive than steel ! Wood will not only dull the cutting edge, but will irreversibly dull the side cutting flutes than are responsible for setting the drill diameter and pumping out the waste from the hole. Use a proper wood bit for wood or use a sacrificial twist drill.
If you are drilling a large hole on unfamiliar material, practice first with a piece of offcut. Do not spoil your good material as you get the feel for a new situation. This is particularly acute for use of hole saws and large diameter ( > 1/2 ) twist drills. For holes any larger than 5/16 or 5mm the work piece must be firmly secured or it will attempt to run away. Work pieces being cut with a hole saw MUST BE FIRMLY SECURED, no ifs, no buts.
such a simple thing, yet so much to say ! A centre punch needs a hammer. Use a small tack hammer as a larger framing hammer is too clumsy and will deflect the punch away from where you want it. The punch must be needle sharp to add to the precision. Keep it sharp on the grinder. The so called automatic centre punches are a useless abomination, do not be tempted.
The work piece must lie on a hard bed when punching or it will uselessly dance about. I have a small steel slab for this purpose. A heavy block of hardwood will also do. All holes, without exception should be centre punched. Metal work is a precision occupation. Precision is its own reward. (also an old school "Prussian Value")
Big Holes in sheet metal.
Welcome to pain. Like pain ? Then you will enjoy cutting large diameter holes !
You will require large hand held drill, hole saw and arbor. First carefully measure the device for which the hole is required. Get this right, measuring is harder than you think. Measure the same thing untill 3 ( yes three !) measurements say the same thing. This is the minimum size of the hole. Now consider the clearance hole. This is at least 0.5 to 1.0mm greater than what you have just measured. Now buy the nearest smaller hole saw. Woe to thee ! Expensive, wasn't it ? Now test the hole saw on some waste. Is the hole size anything like what you require ? The hole it cuts may not be what is advertised ! You will have to make your own arbor. Wot ? The standard arbors are usually based on either a 1/4 or 6mm drill. This is meant to facilitate the cutting of the pilot hole. Then the drill bit just rattles around in the pilot hole , ruining it and destroying the precision of the big hole and causing it the big hole saw to bind. Replace the arbor drill with a 6mm or 1/4 shaft and drill the pilot hole with a standard drill. You will find that the hole saw will now run true and the hole will be clean. The waste chips must came away cleanly or the hole saw will bind to the work piece. To facilitate the chip removal process drill some 1/8 or 3/16 holes along the cut of the big hole saw. The hole saw will now run without binding, chatter or excessive vibration. Use cutting compound on the hole saw and periodically withdraw the saw from the work to clear waste. Now have fun with a circular profile file to enlarge the hole to the actual requirement. Did you use a compass to outline the big hole? Dammn ! You should have done that first! A practical alternative is the nibbler. They are very good for weird shaped holes, can be used on all aluminium, light brass and and mild steel sheet. However you require physical strength and endurance. Nibbled holes should be slightly underdone to permit dressing up with a file.
Cutting metal sheets.
See my remarks about pain. You must have a good tin snips. A word about buying tin snips. Cheap ones will not cut metal! They are worthless and dangerous. The best tin snips in the world were made in Birmingham using traditional British steel and are about 100 years old, purpose built for the tin-smith business. They cut mild steel plate or paper with the same ease. If you visit flea markets you might be very lucky to find one. They are usually colored black and look old. If you find one just buy it. Otherwise, avoid the so called "aviation" snip. The straight snip is the most usefull. A good snip will literally last a lifetime. Keep it lightly oiled. The work piece should be physically secured as you cut, it just makes it easier. Keep the cutting edge sharp by dressing on an oil stone.
Large heavier gauge plates cannot be cut with even a large snip without severe distortion or effort. I have used a jigsaw with a metal cutting blade. The jigsaw should have a speed control. The current speed can only be found by experiment, faster is not better and too slow can actually be dangerous because the jigsaw will bind and jump. The jigsaw must have a linear cutting guide for straight cuts. It is noisy, inconvenient and slow. I do not like jigsaws but they do work and you have little real alternative. They can make precision cuts if you take the kerf of the blade ( the width of the cut line) into account in your layout. The vibration the jig saw induces results in severe scratching of your work piece, so it you can. arrange for a vacuum cleaner to remove swarf as its made. The use of a smear of cutting compound along the path of the cut will ensure and clean fast cut and mitigates the risk of the blade binding. Perspex sheet cannot be cut by jigsaw unless you arrange for liquid cooling. Use plain water with a little detergent and kerosene and arrange a drip feed near the cutting blade. It is now possible to purchase jig cutting for metal, made from a cobalt alloy. Use these for mild steel, tin plate and brass. Use high speed steel for aluminium.
There has recently come into the market the so called counter rotating blade circular saws sold under the name of "Dual Saw" or other. They actually work, are safe and fast to use for cutting non ferrous sheet metal , however I have found them difficult to use for precision cuts. The devices have no simple provision for cutting guides to guarantee a straight edge cut. If they did, I would have no hesitation in recommending one. The beautiful thing about the dual-saw counter-rotating blades is their near zero propensity to bind or kick-back which will happen if you attempt to cut thin sheet with a normal circular saw. They work very well for dry cutting Perspex sheet due to very low frictional loss.
Here is a secret pro tip.
Sheet aluminium, of any thickness, may be safely and quickly cut with a modern hard-point wood saw ! They did not teach that in tech school ! The sheet must be firmly anchored close to the cut line to mitigate chattering. Wipe a smear of grease or cutting compound along the line of the cut. The attack angle of the saw blade must be as shallow as possible, allmost parallel to the work. Begin the cut using a reference edge, a piece of waste wood is good enough. Do not plunge the saw through the work, plunge along the work ! Nothing bad will happen to the saw blade !
You will encounter these threads in new and surplus commercial equipment. Old British RF gear will use the BA series of threads. Americans use both ANC and ANF threads. Modern Japaneses and Asian equipment will generally use metric threads. BA is now thoroughly obsolete. If you have and love radio boat anchors you should carefully save your BA nuts and bolts and use them only for equipment service. New BA hardware , taps and dies are very hard to get. ANC and ANF taps and dies and replacement nuts and bolts are generally not available in Australia. Save your ANC/F loose nuts and bolts for servicing American boat anchors but do not use them in new projects. The only viable options for us is either BSW or metric. BSW taps and dies are widely available in Bunnings et al. However, small nuts and bolts in BSW threads are very hard to get and are of poor quality. The larger construction sizes are widely available but smaller 1/16,1/8,3/32 are now very hard to get. These are the usual sizes for electronic construction. However, the good news is that 3M and 4M nuts and bolts are now readily available in bulk packs from Jaycar and Altronics and many 3M and 4M fasteners can be found in electronic waste in abundance. The other good news is that metric tap and die sets are widely available and replacement 3M and 4M taps are sold at Mitre10 and some Bunnings. I suggest that you buy a good hand tap handle and the following sizes that I find are the most widely used. You must have a 3M , 4M , 5M and 6M. Others are optional and have little application in electronics. You will get little use from dies in practice. These days if I am salvaging something like die-cast boxes I redrill and retap that funny thread to M4. The screws in diecast boxes are a weird thread that I cannot identify, and they are irreplaceable. I never use BSW in new projects, its just a waste of time. It is well worth your while to bulk buy a set of 4M and 3M bolts of both pan head and countersink head.
even the simplest piece of metalwork requires some sort of plan. Marking fold lines, cut lines and hole centres requires the carefull use of rulers and scribers. Get a collection of stainless steel engraved dual metric and imperial rulers. A lot of electronics is naturally laid out on an inches and tenths grid, so ensure that your ruler has inches and tenths scale. A scriber is essential for metal layout. Pencils do not have enough precision and pencil lines allways rub off just when they need to be clearly visible. A scribed line is very fine, allways visible and sensibly permanent so it pays to do your layout on the internal and hidden surfaces. Effective layout is hard to do. Conserve every bit of precision in your measurements because small errors add up and your finished objects and holes will not line up. The hidden difficulty in using a ruler is parallax error. It is not right to lay the ruler flat. The ruler should be perpendicular to the work so that the graduation and work piece are touching. This eliminates parallax errors. If you are marking two points that are eg. 10cm apart, do not use the 0.0 edge. The zero point is not graduated ! Line up the 1cm graduation and the 11cm graduation and scribe a mark away from the ruler. This will ensure that the head of the marks actually are 10cm apart within human ability. If those marks are the centre of a hole then carefully centre punch before you forget what your scribed mark means.
When you are using a circle compass to draw a circle for large device holes, centre punch the centre. This give the compass pivot a concrete place to pivot in. This is the only way to drive a compass. When it is time to cut along a layout line, direct the saw to "split the line" . Don't cut along the line. Why ? a saw blade has finite width. The sawn piece will be too small by at least half the width of the blade !
Did I say always measure ? Then measure it again. If you feel like stopping the tedious business of measurement then this is a sure sign that you need to measure it again ! The eye and brain will play stupid tricks on you. I do not want to even mention the times when I have confused 6 and 9. That can be expensive, some materials are irreplaceable. Count the graduations on the ruler rather than just trying to read them off. Older eyes can cause the graduations to blur into each other and jump about. Measurement is hard and tedious. Get used to it !
Hacksaws and how to cut a bit of metal with a hacksaw.
Its harder then you think. It is easy to merely severe something, it is hard to cut to a particular and exact size. It is hard to make a cut that does not wander and remains perpendicular to the work. You actually do need a selection of hacksaw blades, each metal demands its own blade. Plastic and aluminium require a coarse blade, steel needs a fine blade and brass needs a medium blade. It is possible now to buy cobalt steel hacksaw blades. Use these only on steel and brass. Other materials just wastes the amazing ability of cobalt to cut steel without effort. Always put a fresh smear of cutting compound on the blade or along the cut line. It just works better. Discard worn blades, they are literally a waste of time and effort. The excessive force you put in will just result in spoiled work and a shattered blade. A worn blade will wander and bind in the work. Here is how to cut straight. Do not plunge the blade through the work, but parallel along the cut line. It will straighten and then guide itself ! Why, when cutting along the line, the allready cut portion serves to guide the blade. It does not require more physical effort because the effort required is purely determined by the amount of swarf produced . This is exactly the same amount as generated by a plunging cut. A plunging cut will allways wander but gives the illusion of rapid progress, and, lets face it, hacksawing is hard physical work. Always practice your technique on some softer waste material and learn to do this right, you will not regret the time and effort. A truly square cut in thicker material is harder to do and can only be done consistently with some kind of saw guide as the human skeleton supports meat, not metal ! We are not a milling machine, our skeleton prevents it. Do not forget to include the width of the hacksaw blade in your layout and calculations. Failure to do so will result in your cut pieces always being slightly too small.
Bending sheet metal.
It is easy to do a poor job, its hard to do a good one ! I have tried numerous ways to bend sheet metal. The only truly effective way to do a precision bend is with a panbrake machine. Cold comfort to those of you without such a device, but it is still within the bounds of possibility if you possess a large engineering vice. You will need to manufacture a bending guide. This is nothing more than two sturdy pieces of heavy steel angle stock loosely bolted together that firmly hold the work piece along the folding line. Hold entire assembly in your vice and bend using a larger flat plate and malet. The bend is somewhat curvy and needs to be made good by peening with a hammer. It takes a lot of practice to get this looking good. The metal sheet will not want to bend cleanly, it will be curved and often will not have bent along your desired bending line. It is possible to learn how to do this to get a reasonably nice bend and then clean it up by beating. This is the panel beaters' craft. It is possible to have a curved plate and beat it flat. You will require a serviceable anvil with a good flat surface. Low cost hobby grade panbrakes are available at industrial metal work shops.. Some shopping around is required. Your laying out requires you to take into account the distortion of the metal as a result of bending. The act of bending increases the linear dimension of the metal along the outer surface. It causes an unpredictable amount of shortening on the inside radius of the bend. If you are doing a precision bend you must first do a dry run with an offcut to measure first the distortion in the linear dimension and make the adjustments in your laying out. The distortion depends on the material, the material thickness and your bending technique. Otherwise you will find that your box covers do not line up cleanly with the box. Slightly too large can be made to work, if the folds are too small, then this is a disaster. You cannot unbend metal sheet. If your bend was no good, throw it in your scrap bin and try again. To Which part of your bend do you apply bending force ? The flange or the sheet ? Allways the flange ! (because!) Flanges are allways slightly curved by the act of bending. I have recently become aware that you can now buy bending dies for your vice. Just like having a hydraulic press ! I will be buying a set of dies to see if it is effective in practice. Due to the small working space in a vice, these can really only bend a flange. We shall see how effective this is, Stay tuned.
Joining metal by brazing and welding.
A consumeable electrode arc welder can now be purchased for only $A100 ! Its a good skill to have and not hard to acquire. You need practice. The first skill is to weld a straight flat bead. Find some scrap and keep at it untill your bead is smooth, straight and continuous. If it looks like "cocky poo" you still need to practice. You need to play with the amperage to get it right. Start of with a 2.5MM electrode at about 50 to 80 Amperes. When your flat bead looks good to the eye is when you tackle an actual joining operation. ( Especially structural components on your car !) The bead should have a nice convex surface, no inclusions of slag and should not have burned through the work. The electrode should be close to the work, do not draw it away. Neither should it be too close, for then the arc will blow molten metal away from the work. You have to work with gravity on your side. Skilled welders can weld sideways and even upside down but you cant ! Remember that welding will shorten the work piece and distort with a bend. You will have to take this into account by firmly holding down the work pieces. Start with a "tak" weld that serves merely to hold the work pieces in relation to each other. Then and only then begin the true weld run. Let the metal cool naturally, rapid water cooling will make the bond brittle. Chip of the slag and examine your weld. Is your weld just slag ? Does the bead contain inclusions ? Your work pieces must be completely clean of paint, rust and dirt. Otherwise the burnt remains of surface contamination will cause blowholes and inclusions that will ruin the structural integrity of your weld.
Use welding electrodes appropriate to the work pieces. The instructions on the side of the welding electrode package will detail what they are good for.
No technique is as structurally effective and dependable as a GOOD weld. Nothing maims and kills as effectively as a poor weld. Weld spatter will set fire to allmost anything organic and dry and will bounce about a very long way. Allways weld outdoors away from flammable material to avoid an unpleasent and expensive surprise. Weld fumes can be toxic, weld outdoors !
In the last few years it has become possible for the hobbyist to join metal by brazing. The heat output of a propane torch has only just the energy input to raise small objects and the braze filler rod to melting point. Very small objects can be satisfactorily brazed with propane. Larger objects of high thermal conductance like copper tubing remove heat too rapidly for propane to be effective. You can now buy at moderate cost torches that are designed to burn so called MAPP gas. The retail MAPP gas in reality is propylene not actually MAPP which is a shame. True MAPP gas was the closest thing to actual acetylene chemically. The only maker of MAPP gas on the planet ceased operations some years ago and propylene is the lesser replacement. However, it does burn very hot and is sensibly safe to use by the non professional. It burns with sufficient heat for all large scale silver soldering and can use bronze brazing rods on smaller objects. It will heat steel objects up to dull yellow heat, copper pipe un to dull red heat, more than sufficient for silver solder. This low cost braze is capable of joining stainless steel, brass, copper, iron to each other. The braze filler is compatible with lead-tin standard soldering alloy. The technique , like everything, requires some practice. The filler rods are a ternary alloy of copper, zinc, silver and sometimes other metals. It is not a solder as Radio Hams understand it. The braze cannot be reflowed ! This means you have only one chance to join your workpieces. The reason that the joint cannot be reflowed is because the pressence of silver lowers the melting point of the alloy. Silver is highly soluble in the base metal which depletes the filler material . This raises the melting point of the filler above the temperature above that which made it flow in the first place. To this end, your work pieces must be secured in their final positions before commencing brazing. The brazing operation requires the use of a potassium hydrogen flouride flux. Liberally apply the flux paste to the work and dry it in a gentle flame. Apply the full flame until the dried flux melts into a viscose glass and reaches red heat. Now apply the brazing rod and let the molten metal wick into the joint be capillary action. Keep applying filler rod until the joint if filled and let cool. The hardened flux will soften by absorbing atmospheric water at which point it can be removed cleanly by boiling in water. The silver solder braze is very tough and binds tenaciously to stainless steel, brass and copper . Beware the steel joint. It may look good, however, it is not a weld, and the braze to steel joint can shear off under repeated mechanical stress. This is due to the limited penetration of the braze into the steel. If you are repairing steel objects that carry significant mechanical loads consider welding if possible or true high temperature brazing with brass filler rod which does require acetylene.
The sort of things that I have been brazing for Ham use are antenna fittings, made from stainless steel. This material is compatible with aluminium that is weather exposed. The stainless steel may have a layer of silver braze applied to furnish a tin/lead solderable surface.
A truly remarkable development is the ability to braze aluminium using specialist filler rods. A propane torch will suffice for light gauge material and the MAPP torch for heavier objects. The filler material is sold under the name Durafix of Alumiloy. It is not available from welding supply shops in Australia for mysterious reasons. It is availbe only by mail order from its agents in Australia. I urge you to look at numerous YouTube clips in the application of this braze, it it remarkable and amazing. I have used this to produce fully bonded high performance yagi arrays. The filler metal is compatible with tin/lead solder and thus provides an indirect way of soldering to aluminium.