Month: April 2006

Precision Machining

Posted by on April 4, 2006

I offer this for those who like to work with lathes, and who make an effort to produce accurate work. A couple of years ago I made a small quantity of arrow heads, target heads, not hunting broadheads, for a guy in the UK. He was a BA crew member. They stay at the Lord Charles Hotel in Somerset West on their Cape Town stopovers. The guy rocked into Allied Arms on the off chance that we would have them or could make them. Apparently they are expensive to buy and expensive to have made in the UK.The following is the text of the letter we enclosed with the heads when we posted them. As you will see, I found it very difficult to achieve exact repetition, but I think I did quite well. The big lesson, however, was that such repetition within close limits is not easy, and I now understand why machine shops in the UK charge so much for this kind of work.I never got a reply to my question as to how the heads performed, which I thought a little unkind considering the amount of work that went into them for a peanut price.Dear Mr —-,We enclose herewith your arrow piles, with our further apologies for the delay.Whilst the difficulties we encountered are not of direct interest to you, we believe certain aspects are worth mentioning in the interest of completeness.QualityAlthough we had only one sample, we got the impression that these piles are made to close tolerances on CNC machinery. The sort of consistency obtainable on such equipment is rather more difficult to achieve on a manually operated lathe. Consequently, we cannot claim to have achieved precise dimensional consistency from one pile to the next, and you will notice in particular slight differences in the position of the annular groove just behind the conical point. That’s because there are slight differences in the length of the piles, and the position of the groove is measured from the base not from the point.Nonetheless, we believe we achieved sufficient consistency that the differences will be too small to show in their performance.All were measured and weighed. The shortest and longest are 30.90mm and 31.00mm, which is a variance of 0.10mm or four thousandths of an inch. The smallest and largest diameters are 7.015mm and 7.025mm, which is a variance of 0.01mm or four tenths of one thousandth of an inch. Minimum and maximum weight are 67.70 grains and 69.00 grainsThe sample measures 30.70mm long, 6.99mm diameter, and weighs 68.80 grains. Close inspection showed that the nose had been slightly flattened, presumably from impact on something harder than a target. We concluded therefore that the true length was slightly greater, hence our estimate of 31.00mm. In the event, the shanks turned out fractionally longer than the sample.The diameters were machined 7.02mm simply because we made our collet tool slightly oversize, and the shank of the pile must be a close fit in the collet to ensure concentricity of the conical nose and the tapered tail. 7.02 is only one thousandth of an inch bigger than 6.99mm and we considered that close enough not to necessitate remaking the collet.The weight of the sample is within the 1.30 grain variance of the thirteen piles produced.Just for the sake of interest, there are thirteen because fifteen were made in case some were lost because of production errors, and in fact two were.Finally, there is slight variance in the surface finish, but in general it is equal to the sample.We are confident that it would be possible, for the manufacture of larger quantities, to halve these tolerances or even better.MaterialAt first we thought the sample was aluminium or aluminium alloy, because that’s what it looked like on visual inspection, and because it could not be attracted by a magnet. Being such a small and light item, it was not heavy enough to discern what it was made of just by feel.Consequently the first copy was made of aluminium. We were surprised to say the least when it weighed only 40% of the weight of the sample. The weight difference indicated that the sample was steel, which seems to be borne out by the weight of the finished piles. We are somewhat perplexed therefore, that while our steel piles are attracted by a magnet, the sample is not. So what is it made of? It can’t contain any aluminium, or it would not weigh the same as our steel piles.On a slightly different angle, we think silver steel might be a better material from the manufacturing point of view. We made the piles from free cutting steel which is the easiest steel to machine. But we found that the most time consuming operation was turning to the exact diameter required. Silver steel is precision ground which would obviate that operation.It is however, otherwise difficult to machine to a good finish, and also has an affinity for rust so it would have to be blued or plated. At any rate it would need further investigation.Rust PreventionAs noted we made the piles from steel because that’s what we thought the sample was made from and because the finished piles weighed the same as the sample. The fact is however that unprotected steel rusts especially in an outdoor environment. We have coated them with light oil to prevent them rusting in transit, but you will have to ensure rust prevention thereafter. We suggest cleaning them and oiling them for storage after each use. That is regarded as standard practice by those of us who use firearms.The holes which accept the shafts should be cleaned of oil before the shafts are fitted. In particular, if the shafts are glued in, the oil will prevent adhesion if it is not removed. Dip the folded end of one of the pipe cleaners enclosed with the piles in paint thinner and swab out the holes with it. Use the other pipe cleaner to dry them out, not that they won’t dry anyway, as thinner evaporates quickly.We could have blued them, which would have provided some protection. However, it would probably wear off quickly from the friction of target penetration and withdrawal. A better method would be nickel plating which is very durable, but it could raise problems with the fit of the shaft into the pile, as it would be necessary to match plating thickness to drill diameter, which would require a consistency of plating thickness that would be difficult to maintain.With the advantage of hindsight, we would suggest that these piles would be better made of brass. It’s much easier to machine and of course is totally rust resistant. The machined finish of brass is matt rather than bright, but that shouldn’t be important.It’s also about 15% heavier than steel, but the same weight could be achieved by drilling a smaller diameter hole forward of the shaft hole, towards the point. That would be possible because the shaft hole is quite short, leaving a lot of solid material in the nose.ConclusionIt has been an interesting exercise. Considerably more difficult than expected, let it be said. But we have learned much, one of the main lessons being that precision machining of such things as gun parts and jigs and tools on a one off basis is very different from producing a number of identical items to the same precise dimensions.It’s not that one is more difficult than the other. It’s the relative importance of time. One can afford to be lenient about time when making a single component or a tool that may see repeated use. But series manufacture must be rapid to be economic, and we found it impossible to achieve the necessary production speed.Consequently we now understand why the machine shops in the UK charge so much for such work. The reality is that only substantial quantities produced with special tooling are viable.We would not be averse to producing more of these but only at a higher price and at least 100 pieces per batch. Otherwise it’s not worth setting up the lathe to make them.Finally, we would very much like to know how our piles perform, whether good, bad, or indifferently.Yours faithfully,Dick Boothroyd.[Originally posted to SATalkGuns -- Admin]

Re: Lynx rings needed!

Posted by on April 3, 2006

Sometimes I lose track of things or don’t reply because of too much traffic to deal with. So this thread is a bit old. Chris B and Alex H’ comments about scopes came as a surprise. I knew some guys like big scopes but I thought it was mostly because that’s what’s mostly available plus the “bigger must be better” syndrome. Now I know that some actually like them, for what seem to be good reasons.We all know that big objectives see a bit better in some conditions. But I’m surprised that a 2 – 7 x 28 Leupold was not clear at 200 yards. Admittedly I don’t hunt these days so I can’t say too much except that I found that a 4 x 32 fixed would take care of most things up to 200m and further.It does also depend on size, age and physical condition. Alex H is not small. I’m smaller but still nearly six foot and I find the magnum action Brno rifles too big and heavy to carry in the field. But I’m 62 and might think differently if I were 25.I would still choose the most compact rifle and scope combo that circumstances allow, and of the two, a bulky scope is a bigger liability than a big rifle. Aside from anything else, scopes are fragile and big scopes are more liable to damage from knocks. But that’s my opinion and obviously others feel differently.Fitting is something else. I’ve lost count of how many badly fitted “professionally fitted” scopes I’ve seen. Slightly loosening the screws that secure the rings to the bases and using the scope to get them aligned, Christo, is of course the right thing to do, provided the opportunity is taken to bore sight the scope at the same time by lateral adjustment of the rings. The idea is to get the scope more or less right laterally without having to adjust it optically to the left or right extreme, ie leaving it without any further room for adjustment. If that seems obvious I’ve seen a couple in exactly that condition. Professionally fitted.Same goes for tightening the top caps. It should be obvious that progressive tightening of alternate screws is the right way to do it, but these are simple details of correct engineering practice that apply to a lot of things besides scopes. None of this fixes the problem that afflicts all rings, namely that they are never in alignment because of tolerances in rifles, bases and rings.Nine times out of ten you will have no trouble, but they don’t grip the scope as securely as they should, and the worst cases can and do scrape the finish off the scope as it shifts from recoil, and I’ve seen scopes that were actually dented.So, like I said, there’s more to scope fitting than just being careful in tightening the screws. But this seems not to be understood except by those who have experienced the negative consequences mentioned above.[Originally posted to SATalkGuns -- Admin]

Actions, calibres and cast bullets

Posted by on April 3, 2006

As cast bullets are one of my interests I think about matters related to them. I’ve often wondered what would be a good rifle for range use only. Something like a rimfire target but which would incorporate the challenge of loading. That would also be suitable for shorter ranges like the 50m range at my local club.Cases that are full of powder are mose accurate than those only partly full. With pistol powders therefore a samall case is better. But how small can we get without going to exotic calibres like the 308 x 1.50 inch. Or maybe the 7.62 x 39 necked to 308. Otherwise the 30-30 is better than 308 Win for 30 cal cast bullets. Also has the material advantage of a long neck.Rim diameter is a bit less than 13mm. Could easily be turned down to 12mm for use in rifles with the standard 06 bolt head. Should work nicely in actions like my Sako medium. Wouldn’t feed through the magazine because the lips would be wrong but a sheet metal box mag could be made, but in any case such a rifle would be a single shot anyway.Yeah, I know, something like an RSA action would be better, but I was thinking more of any action. For that matter, building a nice informal target rifle on a SMLE action would be an interesting project, especially on a one piece laminated stock with a custom made roll over trigger. Yeah, I know, not in this life, but one can dream about impractical things, right?[Originally posted to SATalkGuns -- Admin]

Short barrels and fast powders

Posted by on April 1, 2006

Much of what I post to this forum is theoretical “what if ” stuff that probably has no practical usefulness. But one never knows and it is all intended to provoke further discussion.I have been thinking about light loads, ie pistol powders and cast bullets in rifles. We all know that there is a relationship between burning rate, pressure curve, bullet weight and barrel length. That is why MS200 will produce more velocity with a light bullet in a two inch barrel than MP200, but the opposite is true for a heavier bullet in a six inch barrel.This phenomenon is much more apparent in rifles because of their much longer barrels. I shoot a 180 grain cast bullet from my Sako in 308 Win. I load with 11 grains of MP200. While I haven’t pushed the limit, I reckon 12 grains is the maximum. That’s because the fast burning rate and resultant pressure rise will burst the barrel long before the bullet reaches the muzzle if the powder charge is not kept within limits. It also means that maximum velocity is reached in the first part of the barrel, probably within the first ten inches. Velocity in my rifle with 11 grains of MP200 is about 1600FPS.So far so good. More velocity needs a slower burning powder with a slower pressure rise, to accelerate the bullet for the full length of the barrel. In the 308 Win that used to be the medium burning MR200. These days it is S335. But a powder with a burning rate between those extremes will produce a MV also somewhere in the middle, right? Right. Whilst I’m not familiar with all Somchem’s powders, there are some, maybe S265, that are slower than MP200 and will thus produce higher velocity say around 2000FPS with about 20 grains of powder, but are still too fast to produce maximum velocity for calibre.There are two threads here. The first is that the faster powders are more economical at the cost of velocity. This would be relevant in a survival situation in which powder supply is limited. There is a lot of game that can be taken with a 180 grain bullet at 1600FPS, and not much soft skinned game that can’t be taken at 2000FPS. 11 Grains of MP200 is 700 shots per can of powder, 20 grains of S265 is 385 shots, and 42 grains of S335 is only 183 shots.Smaller powder charges and lower muzzle velocity is less muzzle report. My 308 cast load is a lot louder than 22 rimfire but very much less than a full service load, and therefore much easier to suppress if necessary.The second thread is to look at it the other way round. That is, if max MV of 1600FPS with MP200 is reached in say ten inches, why not fire it from a ten inch barrel? I have first hand knowledge of a silhouette pistol in 308 Win that delivers 2600FPS with a 150 grain bullet from a 15 inch barrel. That’s only 200FPS less than a full length rifle barrel.This makes possible practical ballistics in short barrels. So the logical answer for those who might need a light, portable and easily concealable weapon would be a silhouette type pistol. But it also makes for a very compact rifle. The clue lies in a report I read some time ago which described the result of cutting a 22RF barrel to ten inches. The rifle was naturally then out of balance. The solution was to extend the barrel with a ten inch tube not much bigger in diameter than the barrel. There was no reduction in accuracy but muzzle report was reduced because the tube was in effect a tunnel.What has that to do with compact rifles? A fifteen inch barrel with a detachable tube or possibly a suppressor and a folding stock would be a very portable and compact package, with a MV capability from 1600 to 2600FPS depending on the powder used. Might also be a handy combat piece in some circumstances.This concept is not that much different from the De Lisle carbine we discussed a while ago, but the ballistics are much superior.[Originally posted to SATalkGuns -- Admin]