SMLE bolthead manufacturing process

[spentprimer]

I have been around machining and manufacturing for most of my life. Sometimes in my work, sometimes as a source of recreation. With my recent bolt opening on being fired problem with my No.1 Mk.3* and having read many of Peter Laidler's excellent articles on these rifle, my curiousity is killing me.

Can anyone fill me in on the machining process in the manufacture the boltheads? With the required indexing on the bolt body, the volume of boltheads that did not meet the index specification first time had to be enormous. With variations in the manufacturing process and associated parts there must have been a large fall-out of spec. parts? Any insight on the process and percentage of non-compliance parts would be appreciated.

Information

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[Homer]

I would have thought numbers of rejected parts were minimal.

[Bruce_in_Oz]

None of the drawings carry any manufacturing process information. The great mystery is : what happened to the "process" books?

If you don't mind making a lot of swarf, you could try:

Start with a bar of steel of about 2" diameter.

Lathe turn the step for the threaded "tail".

Cut the thread, bore a basic striker hole and part off the workpiece at the approximate final length. .

Now, wind that threaded lump into a master jig and mill a couple of datum surfaces; the top and front faces of the lug would be a good start.

Once you gave a couple of these datum surfaces, the part can be transferred from jig to jig in a succession of carefully set-up machines until you arrive at a component ready to be polished and heat treated.

I’m not saying that this is how it was actually done; it’s just a quick thought.

The "Go-No Go" metrology after each separate machining process is potential nightmare, requiring a sizeable tray of precision gauges.

These precision gauges, in turn, had to be regularly inspected and re-certified by the metrology staff.

In one of Ian Skennerton's books there is a picture of the production gauging for a SMLE. It shows a substantial pile of intricate gizmos.

There was a small herd of "tool setters" whose job it was to "set" each machine and its jigs and cutting / drilling etc. tools. If a drill, reamer, etc broke, got tired or was due for scheduled replacement, these characters would bring their specialised gauges and jigs around and do the job. The machine operator had different gauges to assess the task that they, in turn, were doing. And then there were the inspectors.

These days, many CNC machining centres have integral CMM (Co-ordinate Measuring Machine) systems. After a LOT of precise set-up and tool setting, these wonderful devices not only automatically feed in the raw metal and go to work with multiple cutting tools, the CMM probes also MEASURE all critical cuts on the fly, between stages. It's not that the machines are inaccurate, but that a CNC machine so equipped, can either ADJUST for tool wear (up to a point), or trigger an alarm to tell the "operator" that a tool needs replacing / resetting. "Operators" need good programming skills.

For half a million dollars and six months training, you too could be making your own bolt heads!

Oh, and then there is heat-treating and surface finishing..........

[spentprimer]

The threads appear to be 20 tpi, given that, there is one thread every 0.050". If my memory serves me from one of Peter's articles the No.1s indexed + or - 16 degrees and the No.4s were + or - 3 degrees. So assuming the length of the area to be threaded is exactly the same length on each part, in the case of the No.4 the threads must start cutting the thread + or - 0.000414" from the correct starting point, so they end in the correct place. In the case of the No.1 that dimension is 0.002208". This is quite difficult to do on a recurring basis, making me think the percentage of failed parts on the first try were quite high. This starting point for the threads to index correctly assumes the bolt body threads start in the same correct location everytime. Even if the bolthead were made in two pieces and not one, the problem would still exist. And perhaps the bolthead is made in two pieces, I do not know.

It would seem to me that there was a trmendous amount of hand fitting to these rifles and with the volume that were made the fitting crew must have been quite large.

[Bruce_in_Oz]

Beware! The thread form on a No1 bolthead is another "Enfield special" and very odd, even for them. It is, indeed, 20TPI. BUT, it conforms to "Gauge No. 675". It certainly isn't Whitworth or any "normal" thread.

You also need a dinky slotting machine to cut the "window" for the extractor; and it isn't just a simple rectangular slot, either.

And then there's the extractor screw thread; "42 TPI "Enfield Standard"".

[spentprimer]

You also need a dinky slotting machine to cut the "window" for the extractor; and it isn't just a simple rectangular slot, either.

Bruce, I must confess, that little slot has had me perplexed as to how it was done. My first thought was perhaps a double broaching operation? I would have loved to see the entire step by step machining of these rifles. If made in the same way, I wonder what one would cost to build in today's monetary values?

As I have stated earlier, I am quite new to Enfield Rifles, but, the more I look at them and try to guess what the manufacturing process might have been, the more I marvel at them. There was a lot of work put into each one. I would think that very few, if any, were actually assembled and not fitted.

[ssj]

Having watched the price of size 3 no4 bolt heads get to silly money I have to wonder why no one has started making them. What would they cost t make say a few thousand on a cnc machine? $150USD doesnt seem an outragious sale price. Even making a size 4 (and yes Peter I know it means the reciever is worn out).

Someone mentioned to me he was looking at getting old ones "welded up" to do the job, that worries me. Im no metalurgist but welding an 80 year old hardened high carbon? steel bolthead...oh boy.......

http://www.esabna.com/EUWeb/SA_handbook/585sa3_1.htm

"In welding high carbon or alloy steels, however, there is a danger that the weld deposit and heat-affected zone will contain a high percentage of martensite, a hard, brittle form of steel. Such welds have extreme hardness and low ductility, and may even crack while cooling. "

bottom cheeks flutter....

regards

---------- Post added at 01:12 PM ---------- Previous post was at 01:10 PM ----------

8><--- the more I marvel at them. There was a lot of work put into each one. I would think that very few, if any, were actually assembled and not fitted.

Yes indeed....looking at a no4 receiver v a modern bolt action and the mind just boggles at the number of machining processes on the old stuff....

[jmoore]

There was method to old time production madness. A major feature was the creation of dedicated machinery that did one job. That's it. Cutting threads in one pass with consistant radial positioning Cutting a slot. Maybe even just cutting a portion of a slot. Then the piece part was moved to the next station which completed the next task. Things a CNC still can't do well, if at all, in some cases.

Another example of a product not easily milled from scratch is the M1/M14 receiver, even with multi-axis CNC machines. Special tooling sometimes is the only way to go.

Consider also that many barrels also have timed threads.

[ssj]

yeah they had "castle" turrets on the lathes....each side of 6? or 8? held a one tool job...

[jmoore]

yeah they had "castle" turrets on the lathes....each side of 6? or 8? held a one tool job...

That sort of thing is the father of CNC machinery. A general purpose piece of equipment which can be relatively easily retooled for different jobs.

I'm thinking of machinery that is built expressly to do one operation on a specific part, or maybe a few operations at once. But not good for much of anything else except for the one part that it was built to process. When production is done, then the machine is usually scrapped or rendered down for components. The dedicated tooling/machinery could be fairly small and ganged with other tools in a small area. So what would now be normally done on one machine with multiple operations and "flexible tooling" (i.e. quick change), was done at a series of stations, each performing only a single task or two.

Of course, there would be the usual lathes, turret lathes, various sorts of milling machines, drill presses, etc. which weren't special built, but did have assigned tooling for the production line.

Somewhat in between the two types of equipment would be such things as rifling machines. Good for only one task, but capable of being converted to different bores and twists fairly easily and without permanent modification.

Not quite on topic, but during the bulk of L-E production the machinery was usually remotely powered via overhead belting. So less cost there when changing out machines. Way higher maintenance requirements, though!