People ask how easy is it to screwcut with our CNC lathe? Hmm.. I have no idea about other peoples machines, however I screwcut on a Harrison Teach CNC, so I can tell you about it.
This particular machine is a 400T so a bit of a beast, it is a conventional format machine as opposed to slant bed (Google it!) It can take around 1200,00mm between centres and turn a piece of material 400,0mm in diameter. It is equipped with 3 Jaw, 4 jaw and 5C Collet chucks on a D1-6 system and has Fanuc controls.
So reasonably large…
I can operate it in a few ways.
Firstly as a manual machine, the only difference to a more conventional lathe is this is ‘Fly by wire’ so when I operate a hand wheel, instead of turning a lead screw I am effectively turning a potentiometer (Think of a volume knob) that sends a signal to a motor. This is quite an accurate way of operating and I can control the movement to 0.001mm/1 micron (Not a lot) A human hair is around 70 microns.
Secondly, I can draw what I want to turn in a plan elevation with CAD, convert it to a .DXF file and export. Then import it into something else which allows me to chose feed speeds and depth of cut and the tool to use, which in turn is converted to a language the lathe understands. The resulting code is then sent to the lathe and all I have to do is tell it where to start and off it goes (OK, this is the simplified version to save those at the back falling asleep)
Thirdly, I can use the teach CNC side of things at the machine control panel. As an example I can tell it to machine internal and external tapers (Not so easy on a conventional lathe) and screw threads! (yes, I can also program threads via CAD however today it is local to the machine)
So what does a screw thread comprise of? Well a major diameter, a minor diameter and the pitch, or number of threads per inch is about it, here is a picture of what I see on the touch screen.
X1 the major diameter
X2 the minor diameter
T is the tool I am using (I always use 31 for external threads)
P is the pitch, this is a metric thread so 1 is 1mm
A is for angle, 60 degree for metric
L is the length of thread
Z is the overall length of cut. The minus symbol means go towards the chuck.
NS is the number of starts, just one as this is a conventional thread
NP is the number of passes to get to the finished diameter. I usually choose 15.2 with the .2 denoting two ‘spring’ passes where the machine makes a couple of finish diameter passes to allow for any deflection in the work piece.
Once completed I test the fit either with a gauge or the part to be fitted and I work to a 6h tolerance (Quite precise) If the part does not fit I remove 0.1mm from X2 and run the machine again to creep up on the final fit. I would normally expect to be at 16.000mm major and 14.917mm minor diameters.
Yes there is a bit more to it however this gives a good idea of the approach and it certainly takes less to do than write.
Cutting speed? It depends on the material however 600-1000 rev/min is not out of the norm.
Towards the end of last year I was staring at an absolutely original No32 Mk1 telescopic sight that was sitting on my desk. I was constantly moving it anywhere but back in the cupboard which meant I had to find a suitable project for it. The problem was what, so I mentioned it on the Full-Bore forum and GeeRam mentioned a No32 Mk1 had been fitted to a Pattern 14 rifle. A rifle I own and one which I really enjoy shooting.
So, I hunted down the post, contacted the person who had built it and was told that the source of the drawing was secret and that the bracket build he had commissioned had been a nightmare to do and probably best avoided.
Use the people you know I thought, so I spoke to Paul Whitelam of Northern Shooting Show fame and he waved a magic wand (OK, he spoke to the Leeds Armoury people) and a drawing was supplied, I think it was a fiver and I still owe him for it!
With the drawing to hand first job was to draw it in CAD, it is always easier to work with a 3D model and it soon became apparent that what I had modelled and what had been built previously were really quite different. So I started again and paid extra attention to the job in case anything has been over looked and yup, what I was looking at was spot on to drawing, even down to a couple of small errors on radius paths, but way different to the example that had been built recently. My goal was to build something that was totally 100% accurate.
I could certainly build a fair amount of the project, however it was also apparent that I would need a 5 axis machine, something I do not possess, so I was going to need a grown up on board, certainly if I was going to build a handful of the things.
As luck has it, Robert Chombart of CG fame and I worked with another company when we were building rifle actions a few years ago and they have some fairly hefty CNC kit, so we sat down and talked, in fact we talked a lot and a plan agreed. Time to do some more research, I reckon I put well over 50 hours (I stopped counting last December) into 3D modelling, researching material specification and modern equivalents and this was before the emails, conversations, drawing mark ups and annotations. The master folder alone is over 240 MB and the pile of drawings in the folder is a couple of inches thick. Eventually the project was ready to be kicked off and set tool to material.
The original drawing calls for malleable cast iron and that is certainly an idea for the future however right now my plan was to build a dimensionally perfect replication of the drawing with modern and robust materials which we have done.
Final job now is to tumble the parts and black them. Yes them, we have built half a dozen.
Interestingly, the drawing called for 4BA coned head screws or D.D.(E)2441/5 Screw, Holder and that drawing just happens to be for the 4(T) bracket (Another fiver I owe Paul!) Which makes me wonder why they chose to design a bracket for the Rifle, No3 Mk1 (P14) when the H&H modified No4 Mk(T) was already in place. My personal view is the P14 is an inherently accurate rifle and the one I have would easily out shoot the 4(T) I used to own. The downside was always spares for the P14, plus the ‘limited’ magazine capacity and a whole host of other things. What I do like about this bracket is it is robust, it used 5/16” thumb screws instead of the 1/4” used on the 4(T) and it also dovetails into the rear left wing that protects the rear sight so it has a very solid recoil system, unlike the 4(T)
Anyway, here it is, call it a bit of a world premier. A bracket to hold a No32 Mk1 rifle telescope to a No3 Mk1 rifle, this was originally designed and built in 1940, this is not a fantasy build, this is just a late build.
More to follow, in fact I could probably write a book on it!
The project continues apace with more turned parts. Again, these will need to be heat treated however at least I get to work with a fairly standard 5/16″ BSF (British Standard Fine) I cannot say I never work with imperial threads as all of the last weeks screwcutting was imperial and the time consuming part is always the clocking in as opposed to the screw cutting itself. Oddly, I do tend to work in metric for some imperial threads and as an example the 20×1/2″UNF thread so often seen on the end of barrels and used to attach a sound moderator starts off as a 12,70mm diameter to me and finishes at around 11,00mm for a class 3A thread (External) I say around as the ultimate diameter is down to the part I am attaching to the end so you could argue it is not really a class A thread.
I am sure some of you have shouted ‘Gotcha!’ confident now that you know what I am making. Good idea in principle however I think you will find this is the wrong thread.
All will be revealed over the next couple of weeks.
We also flute barrels occasionally…
Fluting is one of those jobs that you just have to get on with. I would love to say we are currently fluting on a three axis CNC mill however we are not. Instead, we use a Bridgeport manual and either a dividing head or a rotary table, we have a variety of different ‘Turny-roundy-indexy’ sort of things in the workshop, however it is often down to what is on the machine.
On this occasion I used what was on the machine as it was already set up. This particular rotary table can be moved in two axis and currently wears a 4 jaw chuck with a collet block in it, so it was just a matter of set it to zero degrees and pop a collet in it.Incidentally, the writing on the table are notes from a previous job.
At the other end is a tail stock and with the barrel in place, the penultimate job is to check everything is correctly setup across all axis and fit the support.
The support (Not shown) either supports the centre of the barrel from the back or bottom and is either an angle plate or a jacking plate dependant on if I am fluting from above with a carbide bull nosed cutter or to the side with a rotary cutter.
At this point it is a matter of setting the start point, feed speed and depth of cut and kicking the job off. The downside is it a messy job on a manual and very time consuming and this stainless barrel was fluted over an evening and the following morning.
I have fluted many barrels and handled many more and my pet hate is sharp edges on the flutes so I always take care to de-burr them, ideally they are then blasted and Cerakoted however this barrel was stainless and the customer preferred a self colour finish.
So there you are a heavy stainless barrel fluted.
The technical bit:
8×6.0mm equispaced flutes cut to 3,0mm deep.
Length of cuts 330,0mm
Total weight saving 295 grams