Making a Obsolete Firearm Part
Using CNC Machinery

    This particular part will be a hammer for a Savage model 240, a 410 Over/Under, which was apparently a predecessor to the model 24.  The model 240 uses 2 hammers in the same slot.   It also has 2 triggers.  Shown below are the existing LH and the new replacement RH hammer.

Side view of action showing one original & one replacement hammer	Top view of hammers, with replacement one having squarer corners & the serrations finer

   Since there are no original drawings available (especially old obsolete firearms) from any manufacturer, and they would not supply them if there was, because of product liability now days.  The only way to get a drawing is to use an existing part as a pattern and make your own. 

   In any CNC operation, the initial operation is to CAD draw a part.   CAD is an abbreviation for Computer Aided Drafting.  Here however, we need to reverse engineering using an original one to copy.  In this particular case the LH hammer was there, but no RH one available.  The LH hammer was used as a sample to draw the CAD drawing and was then printed off in a scale of 1 to 1.   From this, the existing LH  hammer could be laid on top of the printed drawing, to see if the drawing needed to be altered to match any of the contours.  You will note that not many parallel lines exist on this part, so guessing & measuring from existing holes become important.   A lot of calculated guessing as to locations went into this initial drawing, as the only known locations were the pivot hole, and angles or dimensions derived from the 2 flat portions.  

    Then using the LH drawing, it was then mirrored 180 degrees to create the basis for the RH hammer.  An illustrated parts picture from an old parts list showing them both was used to somewhat draw in the differences.   Once the first drawing was printed, the I could locate the pivot pin holes, spur location and firing pin pads better.

As with any CNC machining, the CAD drawing comes first

    A prototype RH hammer was then made, put into the gun with the original LH one in place, so a comparison of the 2 could be seen.  Alterations in the CAD drawing were then again made to conform the shape of external spur.  The inner section of the firing pin area was guessed at then, and was then ground by hand and fitted many times, before the proper shape was achieved at the firing pin striking area to achieve proper firing pin protrusion out of the frame.  At the same time give about .015 clearance of the top front of the hammer to the frame at the forward stop position, and have the firing pin retracted to a rebound safe position as was evidenced by the original LH hammer.

    Now this modified prototype was used to redraw sections of the RH CAD drawing to match the new needed functioning dimensions for the final part, and the CNC code was rewritten to match the newer configurations.  

    The actual run time is not anywhere near the total time involved when you consider all the initial CAD time, redoing the CAD drawings, redoing the code to match the final prototype, cutting the steel to length, deburring the saw cut, deburring the sawed off part, cutting the sear notches, serrating the spur, case hardening the part, making the pins and riveting them in, then making the spring.  The cost of the steel also needs to be figured into the equation.  For this part, there was probably more material in scrap and chips than in the finished part.  Plus there were at least 3 days involved before we even got a semi-finished part.  And the cost of the machinery, tooling, fixture making, etc. adds up also.

    As you can see it is not practical to make just one part, and with these particular parts, even making 5 each for possible later resale, a price of $49.50 each is not really a worthwhile project taking into account on how many of these models were made and are still somewhat alive out there.  

    In setting up the CNC program for these parts, the overall dimensions were needed, plus an added .100” to each end for cleanup if needed for an overall material length of 2.60”.   The green box around the part drawing indicates the size of the material needed and where to measure from to locate the machine home, which in this case is X0.0, Y0.0 and which is the center of the larger pivot hole. The thickness of material was decided to be ½”, with the width to be 1 ¼”.  This thickness was decided by calculating the finished part thickness, the width of the saw being used to saw the part off, plus the height of the lips of the vise jaws needed to hold the material securely during the machining operation. 

    1018 mild steel was selected with the thought that since this part was being made on one flat operation, then sawed off to proper thickness, that if a tougher (4140 Annealed) was used, that deep a saw off cut could become a problem in that the material could become work hardened, ruining a saw (at a cost of about $30 each) unless ran at a VERY slow RPM speed and feed rate.  When the parts were final machined, they were then case hardened to give external hardness, yet a softer ductile core.   This is how it was done originally at the factory, so the final case color of the reproduction part matches the original factory part also.

    The CAD drawing was produced in this instance on BobCad version 17.   CNC computer code was written using the BobCad program also, which when in the Numerical Code mode, will when you click onto a line in the drawing, get the cutter path going in a climb mill direction, it will write a line of computer code when you click on each subsequent line.  When you have went around the whole part in your CAD drawing, you now have a computer generated CNC code that the CNC milling machine can read.  This code is in the form of X, Y and Z.    X movement is right and left, and has a plus or minus attached to it depending which side of established Zero you set, here being the center of the pivot hole.  Anything to the right of 0.0 has a plus (+) attached to it, if to the left it will be a minus (-).  For simplicity most codes will not show an actual +, as that being taken for granted if none is listed, however the minus will always have to be listed as a minus (-) preceding the number.  Any movement forward and back is a Y movement, using the same stipulations as previously explained with Y+ to the rear of 0.0.  Z movement is up and down, and as with the others, a set 0.0 is the reference point which is in this instance the top of the material.  You need to set all of the X, Y and Z 0.0 set ups on the machine for the particular part you are making.  Once these are set, this becomes “Home” for that part.

    CNC code writing is like learning another whole language.  CNC code for a straight line is G01, a left hand arc is G02, while a right hand arc is G03.   Feed rate is listed as F5. which equals 5 inches a minute.   Spindle speed of 2000 RPM is shown as S2000.  Code for tool #3 would be T3, H3 would be height for tool #3, and D3 is for the diameter of the cutter again for tool #3. 

    There are codes in letters or combinations of letters for additional things like, turning the spindle on in either RH or LH rotation, spindle start or stop, coolant on and off, rapid moves, cutter compensation plus many more that tell the machine what to do.   If something goes wrong, it is not the machine, it is that you did not write the code right.

    On a CNC vertical milling machine, the machine’s head and subsequent cutter stays stationary, while the table moves in X and Y directions.  It can do a multiple move at an angle or in an arc.  For any up or down move (Z) the normal machine’s spindle will move within the head. 

Jim in front of one of our CNC vertical milling machines

    A water based coolant is used to keep the cutters cool and promote longer cutter life.   Most coolant on smaller machines will usually be a mist type system, which is compressed air siphoning a water based anti-rusting, environmentally friendly, anti-irritating coolant from a small tank, mixing with the air and directed onto the cutter to cool the cutter in a spray form.  Larger enclosed machines will use a higher quantity of pumped coolant called a flood type.

   You will also notice that the code was changed, altering the sear notch location after the parts were run.  This came about when the parts were set up to do the actual notch cutting and it was discovered that the code should be slightly altered for the next run, (if there ever was one).

    In the photos below show usage of a 3/8" roughing cutter being used to hog out excess material, then a 1/4" carbide cutter will be used to finish the periphery of the part to size.  Different size drills are used to put the desired holes in.   A 1/16" X 1" Woodruff saw is used to slot the mainspring plunger pivot and the retracting spring slot. 

3/8" rougher being used to hog off outside	Drilling the pivot hole, note the plunger slot is already done

    Shown below is a 1/8" carbide cutter doing the final sear and retract locations, (which need to be final cut later).    And the 3/32" x 5" saw parting off the part.    Note the coolant nozzle on all these pictures.

Here is the 1/8" carbide being used to get into the corners that the larger cutters could not reach	Sawing off the part, note you will see the mist coolant nozzle on all of these pictures

    The machined part needs to be bandsanded to remove burrs or irregularities from the previous machining, or saw off operations.  Now the part goes into a special fixture in a 6" vise on a Bridgeport type manual vertical milling machine to do the sear and safety notches.  On the left below you will see both RH and LH hammers placed in the fixture (to save time and for more support).  They are placed on a drill bit the same size as used to drill the pivot hole to give a basis for placement of the parts.  The 1/8" pin seen is a stop for the sear notch cut.  There is another pin hole to the right  that is used in cutting the safety notch.

    Here a 3/8" x 24 TPI tap was modified, by removing 3 of the 4 flutes, heated red and bent to the shape of the hammer spur, quenched, then it was re-heat treated.   Now again both hammers were placed in the edge of the same fixture, but using a different locating hole.  In this instance, the part positioning was placed so the bottom rear of the wider section of the spur lays against the LH edge of the fixture jaw.   Both are then clamped in place and the modified tap is ran back and forth in a Y direction to cut the serrations.  This serrating cutter is moved in approx. .005 per pass to prolong cutter life.

Cutting the sear notches	Serrating the hammer spur

    Below are shown the original but reblued hammer on the left, with a sample unfinished setup part next to it.   These setup parts are critical to save for the next person who may make another run of these parts at some later date.   To the right you see both the RH & LH hammers before being heat treated. 

   The case hardened part was then manually flame annealed (softened) at the two pin holes because with them being so thin on the sides, the thought was that when riveting if they were left hard, the side lips may become broken off. 

    In the right hand picture the parts have been case hardened and drawn.   You can see the plunger pivot pin has been added on the rear, along with the rebound spring placed around it's retention pin before riveting of the pins in place.

Original & reproduced unfinished parts	The finished reproduced products

    The CNC milling machines shown and used in these operations are retrofitted old Bridgeport solid ram milling machines.  The electronics of the original machines are long obsolete and were somewhat cumbersome by modern standards.   Some machinery dealers purchase these non running obsolete machines from large shops, strip them down, retrofit with newer modern servo motors and electronics, utilizing a PC computer to run the unit.  Cost for one of these rebuilt machines usually runs between $25,000 and $30,000 not including a $500 vise or any cutter holders which run $100 each (of which 9 were used on this operation).  This particular machine shown and used here was retrofitted using Centroid controls.

   In reassembling these hammers in the frame, the coil mainsprings ride over a long screw that comes in from the rear of the frame and screws into the plunger head that rests in the slot & over the hammer's rear pivot pin.  This firearm is designed to have the retracting hammer (that rebounds by the attached spring), to go back to the safety notch immediately after the gun is fired.  To do this the mainspring plunger needs to be screwed into the plunger head so that the spring is captivated slightly shorter than when fully forward.   This is to pull the hammer away from the firing pin so that the gun will not fire when loaded IF the firing pin is held forward by an improperly adjusted hammer.  The hammer hits the firing pin hard enough by inertia & then bounces away from the firing pin to this safety notch.  Adjustment of this rod needs to be screwed in until there is about 1/16" slack when the hammer is all the way forward.

   On this part, there is a fine line between the hammer rebounding away from the firing pin, allowing it to drop into the safety notch and the trigger sear point.  So you can see that the part has to be made precise and final fitting to be done in a manner that ensures safety.

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Originated 05-11-0206  Last updated 10-31-2020
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