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The Four Jaw Chuck

Posted by on September 30, 2011

I have published an e book entitled “The Four Jaw Chuck – the hidden secret of precision lathe work.”  

The three jaw chuck is a convenience tool.   It is quick to use because all three jaws move in unison, so that round stock can be chucked quickly, more or less to centre.   I say “more or less” because there is a built in ”inaccuracy” in three jaw chucks.   It is not really inaccuracy, it is that, because of  inevitable tolerances in the scroll, they seldom close dead to centre, usually one or two thousandths out ( 25 – 50 microns).   Also, the error is different at different diameters, and often, if  a piece of work is removed from the chuck and later returned, it will not be exactly on centre.  Three jaw chucks are thus quick and convenient for repetitive round work at one setting, but not for much else.    

The four jaw chuck is much more accurate and versatile.   That’s why most books recommend that, if only one chuck is to be purchased, it should be a four jaw.  Most mention that it can hold rectangular work, and can do eccentric turning work, but the full extent of its versatility is seldom explained.

On its own, its only advantage is that it can hold rectanguler work.  But if the work can be set in the chuck to exact measurements, it opens up a whole world of versatility and precision that can’t otherwise be imagined.   That measurement is provided by the dial test indicator.   The four jaw chuck and DTI should therefore be considered a two-piece combination tool, each as important as the other.

It is the most important lathe accessory, more versatile and useful than all others combined.  It is indispensable for serious work, and makes possible a variety and complexity of work, and a degree of precision that cannot be achieved otherwise.

My book is not big at 52 pages, but with 13500 words and 77 illustrations, it explains in detail the advantages of the four jaw, with several working examples.   It is not armchair theory – the four jaw chuck was the vital tool for the many parts, jigs and tools I made in my gunsmithing work. 

It is a E book, available direct from me, boothroyd@polka.co.za, and will be sent by e mail.   I will gladly send sample pages to anyone who wants to see the quality of the content before buying.

Payment will be credit card via www.gunownerssa.org.   Price is $20.   Payment instructions will be sent with the book.

Posted by on September 16, 2011

My buddy, Richard Bowman, and I have often discussed making bullet moulds, so that we can make experimental moulds and save some bucks as well.  But its not easy, and needs some special equipment when you get past the first rough attempts.  Last week, Richard showed me a mould he had made.  He’d be the first to admit that it’s a bit rough, but it was made only to test the method, with the better build quality coming later.  It works very well.  The blocks are aluminium, dimensioned to fit RCBS handles.  The alignment pins have matching bushings in the soft aluminium blocks.  It was made for a very big bullet, for a 4 bore rifle.  Bullet diameter is almost 19mm (3/4″).   It is too big to weigh on my scale, but measurements indicate about 800 grains.  Big cavities are easier to cut, which is why Richard chose a big calibre for his first effort.

The cavity was drilled out with a series of ordinary twist drill bits of increasing diameter, and finished with a twist drill bit that Richard had ground to desired diameter and shape.   Again, a bit rough, but creditably good for experimental use.  The driving bands were cut in the four jaw chuck with a suitably ground boring tool. 

It casts  creditably good, perfectly shootable bullets, considering its experimental nature.  Making moulds as good as factory moulds will need a lot more work, but this is a good start and proves that it can be done.  Professional mould makers naturally don’t publish their trade secrets, so intending mould makers have to figure it out the hard way.

Bullet Casting for Beginners

Posted by on September 8, 2011

I have published an e book entitled “Bullet Casting for Beginners.”  There are many books and articles about cast bullets, but most say little about the actual casting, and how to do it efficiently and well.  One book says that a high temperature furnace is needed for melting antimony, and that multi cavity moulds are no faster than single or double cavity.  And how often have you read that tin and antimony will separate from the lead if not fluxed and stirred ?  All are untrue.

Then there are the things they just don’t tell you.  Like the fact that bevel base bullets can’t be lubricated in RCBS, Lyman and Saeco lubricators.  Well, they can, but so slowly and with such difficulty that it’s not worth the struggle.  How many guys have bought moulds they can’t use, but can’t return because they’ve used them once ?

I am a commercial bullet caster.  I have hand-cast handgun and rifle bullets for quality conscious customers for many years.  In doing so, I have found what works and what doesn’t, and have encountered and sorted out every imaginable hitch and glitch. 

My book is not big at 55 pages, but with 21500 words and 75 illustrations, it is packed with the sort of “how to” detail you need to cast quality bullets.  It covers equipment selection and use, alloy blending, hardness testing, casting methods, temperature control, sizing & lubricating, gas check seating, and much more.

It is an e book, available direct from me, boothroyd@polka.co.za, and will be sent by e mail.  I will gladly send sample pages to anyone who wants to see the quality of content before buying.  As far as I know, it is the first bullet casting book written by a commercial bullets caster, with the wealth of experience that implies.

Price is $20.  Payment will be credit card via www.gunownerssa.org.   Payment instructions will be sent with the book.

Sighting In

Posted by on July 18, 2011

When I asked y’all to cut me some slack I didn’t expect I’d need it so soon.   The following article is an extract from a book I’m writing about scopes.  It should contain four diagrams, but my blog won’t import in CAD format.  Then it screwed them up when converted to jpeg.  Obviously I have a lot to learn about blogging.  I have posted the article without the illustrations because I think it tells the story clearly enough, but I will gladly send it complete with diagrams to anyone who wants it.   Just contact me at boothroyd@polka.co.za.

A recent question about sighting in on a firearms related forum, reminded me that such questions are a recurring feature.   But not much is published, and what is published tends to be sketchy.   So it seemed like a good topic.     

Sighting in is so simple that it seems hardly necessary to explain it, but I have observed difficulty with it often enough to justify an explanation, and as it is the final step in scope fitting it must be included for completeness. 

It is not possible to zero a rifle dead on in the workshop, either by optical bore sighter or the expedient method.  It is possible to get quite close but not close enough for use in the field.  The final adjustments must therefore be done on the range.

The basics are simple enough.  It gets more complicated at long range because of the exponential increase in bullet drop as the range increases.  But let’s start with basic zeroing.   Because of the unavoidable approximations in workshop zeroing, the first shot might miss a small target at 100 metres completely.  It is then impossible to know whether the bullet went high or low or either side, or by how much.  The target should therefore be set up at 25 metres and the rifle fired from a solid dead rest.  Unless the workshop zeroing has been done very badly the point of impact should at least be on the target.

A good target is a sheet of white A4 paper divided into 25mm squares with solid black lines that are easily visible through the scope ( Fig 24 ).  Make the middle square solid black.  The importance of these squares is that 25mm at 25 metres is roughly four minutes of angle which is 16 clicks of the drum on most scopes.  Keep the original as a master and make as many photocopies as you need.  Fix one to a sheet of cardboard about 400 x 600mm and attach it to the target frame.  There’s no need to waste ammo in the early stages of the job.  Aim the first shot carefully and note the position of the bullet hole.  If it is two squares ( 50mm ) left of centre adjust the windage drum of the scope 32 clicks to the right.  Be careful to turn it in the right direction, but don’t worry about it too much, the position of the next bullet hole will tell you whether you’ve got it right.  If you haven’t, reverse it.

If the first shot is also high or low, as is likely, the same principle applies.  Suppose it’s one square high.  Turn the elevation drum sixteen clicks down.  If you’ve got both the adjustments right or almost so, the second bullet hole should be close to centre.  Another two or three shots accompanied by fine adjustments of the scope should be enough to get as near to dead centre as makes no difference.  There’s no need to fine tune it to dead centre, just close enough to ensure that the shots will be on the target at 100 metres.

Patch the holes or replace the A4 target with a fresh one, move the target to 100 metres and carefully fire a shot.  It will almost certainly be off centre laterally or vertically or both.  Adjust the scope in the same manner as was done at 25 metres, but remember that only one quarter as many clicks will be needed to shift point of impact one square than was needed at 25 metres, because one minute of angle covers four times the spread at 100 metres than at 25 metres.  One click ( 1/4 MOA ) that shifts point of impact just under 2mm at 25m will therefore shift it 7mm at 100m.  Note that some target scopes shift 1/8 MOA per click not 1/4 MOA.

It will take a few more shots to get dead on at 100m than it took to get roughly on at 25m, but 15 or 20 should be enough.  When you are satisfied that the rifle is shooting to point of aim, shoot a few five shot groups to confirm the rifle’s grouping ability.  For this purpose a similar A4 target with 25mm squares works well, except that there should be four black squares in different places on the target ( Fig 26 ), so that four groups can be fired without the need for walking out to the target to patch or replace it.

This book is about fitting and zeroing scopes, not about accuracy per se, but a few words in that direction will not be out of place.  Careful zeroing is academic if the rifle isn’t respectably accurate in the first place.  I would define accurate as 1 MOA for a hunting rifle, and 1½ MOA is acceptably good for most hunting purposes.  2½ MOA is OK for ranges up to 150m.  A rifle that won’t group better then 2½ MOA  ( 70mm at 100m ) needs attention, and can usually be improved with a little work.

            Fig 24 25m Target      Fig 25  100m Target

                        Missing test target diagrams

                       

Long Range Zeroing

The previous section on basic sighting in is sufficient for rifles that will be used up to 150m.  A typical medium calibre sporter in the 30-06 class zeroed at 100m will print more or less to point of aim at all ranges under 100m and about 50mm below point of aim at 150m.  All hunting up to 150m can therefore be done without the need to allow for bullet drop. 

Longer ranges are another matter.  As an example let’s look at a 308 Cal 165 Grain spitzer fired at 2600 FPS .  Zeroed at 100 yards, point of impact will be 4.7″ below point of aim at 200 yards and 17.2″ below at 300 yards ( Fig 26 ).  Of course anyone hunting at those ranges should know the trajectory of his ammunition and hold over accordingly.  The problem comes with range estimation.  Between 100 and 200 yards there’s no problem, but if the range is estimated at 300 yards but is actually 250 yards, bullet drop will be about 10″ not 17″ as expected.  If the rifle itself doesn’t group better than 2½ MOA and shooter error is factored into the equation, that could be enough to miss an animal or worse, wound it.  It’s even worse if the range is estimated at 300 yards but is actually 350.  Bullet drop is then 27″ rather than the expected 17″.

At longer ranges the problem is exponentially greater because of much greater bullet drop and difficulty of accurate range estimation.  With the same ammunition, bullet drop ( 100 yard zero ) is 17″ at 300 yards, 39″ at 400 yards  ( Fig 26 ), and 72″ at 500 yards.  It is obvious that significant errors in range estimation at these ranges will result in hopeless accuracy. 

A partial solution is to zero the rifle at longer range.  This ammunition zeroed at 300 yards will print 6″ above point of aim at 100 yards and  7″ above at 200 yards  ( Fig 27 ).  That’s a lot easier to handle in the field than the first example above.  Note however that impact will be above point of aim at all ranges under 300 yards and will require hold under rather than hold over.  But, as long as the amount of hold under is more or less right, significant errors in range estimation will not make enough difference to the point of impact to matter.

A better method however, is to zero the rifle at a range somewhere in the middle of the maximum and minimum ranges likely to be used.  If it is known, for example, that shots under 150 yards will be unlikely and shots over 300 yards not desirable because of the high risk of missing or wounding, the best scope setting will be somewhere in the middle.  More precisely, it should be zeroed at that distance at which the hold under at 150 yards and the hold over at 300 are the same.  With this particular ammunition that would be 260 yards.  Hold under at 150 and hold over at 300 will be the same at about 5″, less at ranges nearer 250 yards, and the effect of errors in range estimation will be much less ( Fig 28 )

Missing trajectory chart 

Comparison Between Calibres

It is a common belief that some “flat shooting” calibres have trajectories so much flatter as to significantly reduce errors in range estimation.  While there is some truth in that, it has often resulted in poor calibre choice.  So let’s take a closer look at two calibres, one of which is considered as fairly pedestrian and the other quite hot in the velocity stakes.  To make a fair comparison, let’s assume “standard” bullet weights and typical velocities for calibre.  As flat base bullets are also more used than boat tails, let’s use those too.

So let’s compare the 165 Grain spire point at 2600FPS in the 308 Win with the 130 Grain spire point at 3000FPS in the 270 Win. 

We have already shown in Fig 28 that the 308 equalises maximum hold over and hold under at 5” each from the muzzle to 300 yards when zeroed at 260 yards.  Fig 29 shows that the 270 equalises hold over and hold under at the same 5” to a range of 340 yards when zeroed at 290 yards.  It thus extends the range by only 40 yards at the same 5” hold over.

Figs 29 & 30            Equalising hold over & hold under

Missing trajectory chart

If zeroed at the same 260 yards as the 308, the 270 would reduce the maximum hold over and hold under to 3.25” from muzzle to 300 yards.

While the trajectory advantage of the 270 over the 308 is thus apparent, it is debatable whether it makes a material difference in the field.  The difference in remaining velocity at 300 yards is about 300FPS but the difference in energy is only 50 foot pounds.

To add a bit more perspective, the trajectory of a 30 calibre 180 grain spire point fired from a 30-06 at 2700FPS is about half way between the 308 and the 270 but the velocity at 300 yards is only 170FPS behind the 270 and the energy is considerably greater.

The 175 Grain spire point fired from a 7mm Rem Mag at 2700FPS is less than 100FPS faster than the 180 grain 30-06 bullet at 300 yards, and the trajectory is only half an inch flatter at all ranges out to 400 yards.

This shows that a great deal of nonsense is talked about “flat shooting” and that the advantages of the hot high velocity calibres are more imaginary than real.  On top of that, most game is taken at ranges under 200 yards, at which the hot calibres destroy more meat than the more modest velocity cartridges without noticeable improvement in killing power.

Point Blank Range

Above we discussed zeroing to a maximum hunting range and establishing what the high and low points of the trajectory will be at that range.  Point blank range is the same principle but the emphasis is reversed.  That is, we first take the size of the vital area of an animal, then calculate the maximum range at which a shot will hit within that vital area without allowing for bullet trajectory. 

To make that clearer, let’s use the same 308 load used in Figs 28 and 30 and assume that the vital area of the animal is 6”.

The method is similar to that described on pages 25 & 26 but is done in reverse, that is, the maximum hold over and hold under are first decided, which will be 3” above and below the line of sight in our example (Fig).

The trajectory diagram establishes that the point blank range of the example is 255 yards for a 6” target area.  Obviously it will be longer for a bigger target area and shorter for a smaller target, and will be different for each and every calibre and velocity.  It can easily be calculated with a simple diagram constructed from the ballistic tables in manuals such as the Hornady Handbook.

Fig 31   Point blank range

            308 Win, 165 grain, 2600FPS

Missing trajectory chart

Theory versus Practice

The above explanations of the min/max range and point blank principles are of course theoretical, because there is no other way to explain them.  But the reader will no doubt realise that a point blank circle of  6” diameter only holds good if the rifle will shoot one hole groups and if a good dead rest is available in the field.

If the rifle groups in 1.50” at 100 yards it will group in 3.75” at 250 yards.  When other factors like nerves, fatigue and lack of a convenient improvised dead rest are added to the equation, the 6” circle can be reliably hit only if the hunter is himself capable of shooting into one MOA or less in hunting conditions.  I am willing to bet good money that no hunter alive can do that.  The net effect, therefore, is to reduce the maximum range to the distance at which the 6” vital area can be hit reliably.  If that happens to be 150 yards it makes that the point blank range for that type of animal no matter what the theoretical range might be.

Therefore, while the point blank principle is a useful aid for scope zeroing, shooter ability (or lack thereof ) can render it theoretical and to be used with due caution.

Fresh Start

Posted by on July 16, 2011

This blog was set up for me by my good friend and IT guru Wouter De Waal, because he figured, rightly, that I needed one.   He started it with a bunch of postings I had posted to an e mail based firearm related forum in South Africa.   Because of my lack of computer literacy, I never learned how to drive the blog, so it has remained unchanged and unserviced since then.   That is about to change, but please cut me some slack as I learn how to use it properly.     Later, I will post something about my personal history, but the first post after this will be about sighting in rifles, as there seems to be a lot of confusion about it.

Lathe cutting tools

Posted by on September 21, 2008

It should be obvious that good work can’t be expected of a lathe whose bearings or slideways are well worn, or that is not set up straight in all respects. “Straight” means particularly no twist in the bed and tailstock dead on centre. Once those factors are taken care of the other vital factor is properly ground and sharpened tools. A blunt or improperly ground or sharpened tool cannot cut smoothly and accurately. I mention this because of the boast I hear from tool makers and fitters and turners that they are able to grind tools accurately by hand. These are the guys that claim to be able to grind drill bits by hand. Well, it is not impossible to grind a drill bit by hand sufficiently to make it cut. Making it cut accurately and without wandering is another matter. Fact is, it is not possible to grind the angle exactly the same on both sides of the point or get the point dead to centre by hand and anyone who says he can is lying.Lathe bits can also be ground by hand sufficiently to cut, but having tried it often I can tell you that each time you present the bit to the grinder the angle will be slightly different and you end up with a dozen different facets on the ground surface. Why anyone would want to do that when you can get it perfect with a simple jig beats me. Note that while you can get by hand grinding most tools the exception is threading tools. There are various thread included angles. BA is 47.5 deg, Whitworth 55, metric and US 60. If the tool is not accurately ground the thread it cuts will not be a proper fit to the female. Next time somebody boasts that he can grind 60 deg exactly ask him to draw a 60 deg angle on a sheet of paper freehand. The guy has yet to be born who can look at an angle whether on a sheet of paper or a lathe cutting tool and tell you for sure whether it is 59, 60 or 61 degrees.I have just ground a 60 degree threading tool with a simple jig I made. I can’t guarantee that it is dead on with zero error if measured in a lab, but I am satisfied that is within a quarter degree each way because of the way I set it up. How you set things up is the secret of accurate work. For example, use templates. When I set the compound slide to cut a metric thread I set it with a template to feed 29.50 degrees in accordance with standard thread cutting practice. But I also set the tool in the toolpost to exactly 90 degrees to centre line with a template or engineer’s square, having first ensured that the point has been ground exactly 30 degrees each side not 29 and 31.None of this is rocket science, in fact it couldn’t be simpler. Most amateur lathe users are like me, not into complex work like model steam locomotives. Most of us need to do simpler things like tools for gunsmithing or something of the sort, but still need to do accurate work. Why accept inaccurately ground tools out of sheer bloodymindedness when you can make them accurate with simple methods ?Another simple thing to fix is those awful English and American tool posts that come with most lathes, but I’ll make that the topic of another posting.

via email :Every time I try to reply it cuts me off. haha. At any rate here is the source for finding the book by Cleeve._http://www.bookfinder.com/search/?ac=sl&st=sl&qi=7Gk5T.k80bwS,4aAcXbPqsdAmsk_4812493540_1:38:1546&bq=author%3Dmartin%2520cleeve%26title%3Dscrew%2Dcutting%2520in%2520the%2520lathe%2520workshop%2520practice_(http://www.bookfinder.com/search/?ac=sl&st=sl&qi=7Gk5T.k80bwS,4aAcXbPqsdAmsk_4812493540_1:38:1546&bq=author=martin%20cleeve&title=screw-cutting%20in%20the%20lathe%20workshop%20practice)I just ordered mine and it is on the way.Humpy

You found one quickly, eh ! It is a British publication, one of the “Workshop Practice” series of books by Argus Books. It is detailed in the extreme and one could almost call it academic. Much of the content will not be of use to the average hobbyist. Cleeve earned his living making parts including lots of screws. It was making special screws in large batches that forced him to develop sophisticated techniques to speed and simplify the work. One of those was an automatic clutch that disengaged the lead screw at a predetermined position. That allowed cutting threads at 500RPM without running the tool into the shoulder. That is of no value to most of us but it saved him days of work when making a batch of two or three hundred screws six inches long. The real value of his book to we hobbyists is the detailed explanation of exactly how to best cut threads, an explanation I have seen nowhere else.Perhaps the most interesting point is his condemnation of gearboxes. He says there are simpler ways of building rapid feed and threading settings into a lathe without the limitations imposed by the gearbox. Gearboxes are great, seductively so, for quick feed changes, as anyone who has battled with change wheels will know. But they actually limit the variety of threads that can be cut. Furthermore most (but not all) will cut threads only in the language of the leadscrew, ie either inch or metric but not both unless an expensive translation gear set is purchased. A simple change wheel set up if properly designed permits almost unlimited choice. My old Myford will cut every imperial thread pitch plus all the BA pitches, and practically every metric pitch to an error seldom worse than 1 in 1000 and mostly a lot better. For all practical purposes I can cut any thread.Anyhow, it is a fabulous little book and essential reading for anyone who wants to cut threads properly. The other books in the series are all pretty good. I will list them if anyone is interested. One that is particularly good is “Drills Taps & Dies” by Tubal Cain. His “Work Holding in the Lathe” and Hardening Tempering & Heat Treatment” are also well worth having.