I deleted the first version of the process of making the LC rotary bolt. I discovered A. That the process was not suitable for the particular lathes I currently have access to. It would work fine on a Hardinge, Pratt and Whitney, American etc. toolmakers lathes but not on the less rigid ones that I currently run. These are much more typical of what the home gunsmith is likely to have., and B. The actual part as I began to machine and recheck it varied too much from the original in some places. I will need to revise the drawing and post again soon. I have another bolt well underway that is checking much better, but the order of operations is all different. Thanx
Back in the Saddle
Here in picture #1 we see the raw stock for the rotary bolt. This is a small piece of Accraloy 4140 polished rod 3/4 diameter. One of a run of pins for an agricultural job that came up a tad short and was scrapped. Pictured with the original bolt drawing (so far). The original set up--First the outside of the new bolt is turned to .6875 finish diameter, and Picture #2 a new 5/16 end mill is clamped in a v grooved tool holder, then indicated into the exact center of the lathe spindle using the coaxial indicator. Also indicated longitudinally along it's shank and any misalignment corrected. The bolt is chucked back up in the independent 4 jaw chuck Picture #5 and the jaws drifted to indicate zero on two opposing sides, then indicated to plus .0468 and minus .0468 on the other opposing sides. Now just a simple matter of setting up an indicator on the longitudinal travel Picture #3, and gently plunging the end mill straight into the bolt .5625 deep, which is running off center .0468. Simple and done, right? Not quite. Our rather old and rickity lathe lacks rigidity and the tool holder allows some flexing during the cut. The result is a rough and over sized hole, Picture #4, not to print. And as it turns out our off center dimension for the hole was incorrect all along. Well, we still have the other end of the rod stock to get it right.
Next up--Another way to skin the cat
First two cuts on the LC Smith rotary bolt-post #1
I first moved the operation over to our larger lathe to gain a bit of rigidity in the set up. Unhappily, I was now limited to the 3 jaw self centering chuck because the 4 jaw independent chuck for this machine is very large and would not chuck down on this small diameter. The mistake that I had made in the amount of offset needed from center for the inside pocket was caused by the fact that the next upcoming cut, the 5/16" slot cut 9/16 deep through the pocket, is cut in from the side past center by about .045 thousandths, which I failed to catch.
As I continued to study the geometry I finally noticed the very thin outline left of the pocket in the bottom of the slot. See picture #1. I measured the web here and got an accurate reading of .110 thousandths. From there simple addition and subtraction from the known dimensions gives us the correct offset, .093 or 3/32nds inch. The print is revised accordingly and we begin again.
Four tools are required to form our pocket in this new approach. Working from left to right, we will start with a center drill to get our drills started nice and straight, we will then drill in to a mark 9/16 deep with a .3593 (29/64) drill. Next we will grind a flat version of the same drill and clean the metal left by the drill point out of the pocket to a depth of 9/16, then we will run a .375 (3/8) chucking reamer in full depth to size the pocket. See picture #2
This is close up of our flat bottomed drill. These are not over difficult to grind if you have some experience with offhand sharpening regular drill points. The procedure is the same, only the normal 118 deg angle is left off. Care must be taken to preserve the separate cutting tips in order for the drill to properly cut to center though. See picture #3
We get our offset close to the required .093 thousandths by placing a shim under one of the chuck jaws to move the 3/4 in stock over. In this case a short piece of coarse emery paper doubled over on itself was enough to get us very close. See picture #4.
The stock was then carefully spotted with the center drill shown chucked up in our jacobs holder in the tailstock, see Picture #5
. . . continued in following post #2
First two cuts on the LC Smith rotary bolt-post #2
We have run the regular 23/64 drill in to the mark, followed by the flat bottomed version of the same drill. On the flat one however, we run it in only till we feel the resistance of the complete bottom cut. We err a tad bit long on our pocket, we will put it to spec later, as this is much easier to fix than if we were to go to short. By a tad, I mean we need go no more than 1/32 deeper, that is plenty.
Lastly we run our reamer, the rule being to ream about 1/64 inch (.015) on a small hole this length, and to ream at half the drill speed and twice the drill feed. We never want to pull more chips than the reamer flutes have room to handle, otherwise the chips will scrub and heat, the reamer can be jammed etc. etc. A good cutting oil is a must, and chips are kept cleared. By far the most common mistake when running a chucking reamer is to chuck it too tightly in the Jacobs at the get go. All Jacobs chucks run out some, it is only a question of how much. I always chuck only 3/8 or so of the shank in the tips of the Jacob chuck jaws, and tighten just enough to hold from spinning and pick up the hole. A small lead-in chamfer put in the front of the hole with the center drill is a necessity. Once started I back out, cinch the Jacobs down a bit more and complete the reaming.
I got so wordy there you might think simply reaming a hole is something of a science. It is. See picture #1
Our pocket is now complete except for putting our depth into spec. This is done by taking a measurement, and facing off whatever material we need to lose in order to get the correct depth. I use the ever present indicator on the longitudinal feed to put the depth in on the thousandth. See picture #2. A quick polish of the surface edges inside and out finishes the pocket.
We now pull our bolt out and return it to the small lathe and chuck it in the independent chuck with the offset side lined up closely with one jaw. We use a small "finger" type indicator with a small ball tip and adjust the independent jaws till the pocket runs exactly .093 out of center. See picture #3
Then we set up a 1" travel indicator to track travel on our crosslide-picture #4, and set up a 2" travel indicator on our carriage/bed to track longitudinal travel, see picture #5.
. . . continued on next post #3
First two cuts on the LC Smith rotary bolt-final post #3
With our pocket indicated in off center and travel indicators tracking our cuts on the outside diameter in both diameter and length, we begin to turn the outside of the bolt to our correct dimension of .6875 (11/16). See Picture 1.
Touching off the face of the bolt we set the longitudinal indicator to zero, picture #2, and turn .875 (7/8) in, picture #3, somewhat longer than the finished bolt.
.75 (3/4) stock turned to .6875 (11\16) means three cuts of approximately .020 each. We are working in radius rather than diameter on our cross slide indicator though so one grad increment gets us two grads actual cut. We move the tool in .01 twice and make the cuts, then take a measurement of the part and compensate our last cut to finish at .69 leaving 2 1/2 thousandths for polishing. Our cross slide indicator allows us these very accurate and predictable cuts.
.6875 (11/16) dead nuts as they say. See picture #4 The 2 1/2 thousands carefully filed and emery smoothed leaves us a nice finish and a part that checks as it should. See picture #5. The extra stock is left on for now to serve as a handle for the many upcoming cuts, it will be removed only toward the end of the job when we begin the work on the solid end of the bolt. The first two cuts are successfully completed. Thanks all for your patience.
Next up--Milling the 5/16 slot and the aligning face clearance cut for the trip
Making the LC Smith rotary bolt--milling cuts one and two
Monday, and back to work. Coffee and lunchbox in one hand, and the LC Smith project box in the other.
I took picture #1 before departing from home. This is the old and new bolts with the two lathe cuts completed on the new one.
I arrived an hour early and got the new bolt clamped in the milling machine vise. Pictures #2 and #3 show it clamped with the extra length outside the vice, and oriented so the thin side of the web between the inside counterbore and the outside diameter is pointed upwards.
Picture #4--We have milled a small flat on the extra length. This will serve as a flat side to hold the part against the vise jaw in the correct position when we next stand it up to make the two cuts.
Picture #5--The new bolt is stood up on parallels and the first cut is made. This is the trip relief cut, that part of the face of the bolt cut down to clear the trip as it rotates. We can only cut the right hand side of the clearance, because the left hand side is on a different axis. We will need to rotate the bolt 30 degrees clockwise on our next set up to cut and blend the left side.
. . . continued next post
Making the LC Smith rotary bolt--milling cuts one and two part 2
Here in picture #1 the .3125 (5/16) slot is cut in to full .562 (9/16) depth. I found the slot location, touched off, zeroed the dial and then moved the cutter in .010 increments and plunged the end mill to depth. Less chance of moving it in the jaws that way, taking light vertical cuts. I cut past center .0468 per the dimension on our print. A final sweep full depth into the pocket and back out with our 5/16 on size endmill smoothed up the cut and brought it to size.
I pulled the bolt out, gave it a light debur and put it in my LC box and got ready to start my day of work.
When I got home this evening I made picture #2 of the old and new bolts with 4 cuts complete. They are beginning to look and measure more alike now.
Picture #3 is my LC box. I get these at the local dollar store. They hold the receiver, parts, small tools and I can stuff in the digital camera and the rolled up prints too. They are labeled to the receiver. These greatly increase the odds of keeping the various parts together during transit.
Next up---finish the trip relief cut on the bolt face.
LC Smith Project Finishing the Bolt Face Trip Clearance Cut
I had a pretty easy morning this time. Only one small cut to get out before work, with plenty of time to get it done.
Picture #1 was taken once again before I left for work with the project.
The first order of operations was to establish a new axis to mill the second side of the bolt face trip clearance cut, this time at 30 degrees out from the x axis of the mill, for a total of 120 degrees between the side cuts of the trip clearance.
Once again, I used the "drop" ie. the extra material still attached to the new bolt to my advantage, namely to get a flat to orient the bolt for the 30 degree cut.
By clamping the bolt flat in the vise again with the extra material extended, it was fairly simple to set up, using a a protractor head for the combination square set to 30 degrees. The first flat was then picked up and the protractor head set level to clamp the part in the vise again, but rotated out 30 degrees. See Picture #2.
A second flat was then milled alongside the first flat with 30 degrees between the planes See Picture #3
By placing our new flat against the vise jaws we can make the cut, along the x axis of the mill this time' .0625 deep to blend. The small step on the lower left shoulder of the notch measures .203 measured along the notch on the original bolt, so I matched the new step to that dimension. See Picture #4
The last picture #5, was taken when I arrived home showing the new and the original bolt, with the first three milling cuts complete
. . . .next up side milling the trip detent notch and its accompanying angle
Making the LC Smith rotary bolt--milling the trip detent notch
Here once again in picture #1 the photo was taken before I departed for work showing the original bolt with the cut for the trip detent, and the new bolt that needs the cut for the trip detent. The original machining cuts gives us some clues as to how the original cut was put in at the factory. We can tell with a 1/2 inch radius gauge that the cutter used was a side milling cutter 1 " in diameter. See picture #2. It was (9/64) .1406 wide on the face with a straight cutting surface on the bottom, and an angled cutting surface on the top, probably 15 degrees, to produce a notch with a flat side, and an angled side in one pass. The lead in angle allows the trip to engage the detent smoothly when the bolt is rotated and the notch comes around. The trip has an angle matching this one to fit snugly when it snaps up and engages the detent. More on this later when we build the new trip.
The depth of cut .4375 (7/16) from the face, .375 (3/8) from the recess surface, is marked using a black Sharpy and then scribed very lightly with our caliper points. We will use a .125 wide cutter, 3/4 in diameter, see picture #4, and cut the .1406 width by moving the cutter up in the cut by 1/64, then get our angle by indexing the bolt 15 degrees and picking up the upper side of the slot at this angle and cut again. This is a cutter we have, and can be used to make a correct dimension slot, but requires a couple of extra steps. The slight difference in the cutter diameter is of no matter since the radius end of the slot is "in the wind" and does nothing. See picture 5.
I was not "firmed up" on the 15 degree angled side of the notch, I had guessed 20 degrees, but that looked a bit steep once I got it set up. it was a tough dimension to measure since the original trip notch is washed out and peened a bit from many years of use before Rusti was stripped for parts. I wanted to have a look at the relationship of the sides of the notch once the first cut was made., so I stopped there with the notch at .125 wide and cut full depth at .125 (1/8)
. . . next up finish the width and angled side of the trip notch
Making the LC Smith rotary bolt--milling the trip detent notch-part 2
Picture #1 shows the initial cut where we left off last post, namely with our slot cut in full depth at .125 wide and full length.
Our next step is a repeat of the setup we used to make the second cut on the face of the bolt to provide relief for the trip. This time, however; we need a flat cut at 15 degrees to orient the second cut for our trip notch. We use the combination square with the protractor head set at 15 degrees, and pick up the original flat we made on the extra material "drop" last time. Then tighten the part in the vise on it's side. See picture #2
Then we mill the flat 15 degrees out Picture #3
By chucking the part with our new flat on a vise jaw we now have the bolt caught by the extra material, with the bolt extending out of the vise, and the trip notch tipped downward at 15 degrees See picture #4
We pick up the top side of the notch and move up and in to the notch at the new angle with our same cutter til we get a cut that widens our slot to .140 at the bottom and cuts the top side to 15 degrees simultaneously. Due to the angle this is a go slow and make small adjustments kind of cut with the part well blacked with marker til we get the exact shape we are looking for. See picture #5.
. . .continued next post
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