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Posted on July 6, 2012:
Hello again folks!! It’s time to get back to the cylinder head. This particular piece has a lot of work to be done on it so I spent some time figuring out the “order of operations” I will be using. Machining in the right sequence is important because the removal of metal makes future operations easier or harder depending on what you are doing. For example: if you are drilling into brass with a stepped hole you want to drill the larger “top” hole first and then chase it with the smaller drill bit.
If you drill the smaller hole first the larger drill bit can “catch” the brass and most likely destroy the part or the drill bit :0P This is why it is good to plan out every step including how you are going to hold the workpiece and what you are going to use to cut the metal. Luckily I have a good imagination which helps me “see” the part as it progresses through the steps much like a simulation. I can avoid a lot of mistakes before they happen using this method (of course I still make mistakes ;0).
The first step in my plan was to make a reference line on the “top” of the head that indicates vertical plumb. This line will be used to orient the head to vertical when it is machined on the mill. I placed the casting over the prints and “eyeballed” the best possible orientation for this alignment.
Note: It is always a good idea to reference the drawing frequently especially if they are 1:1 scale drawings. You can compare the part to the drawings as you progress to see if the part resembles the drawings as it is supposed to. It sounds simple but you can avoid a lot of mistakes by comparing the part to the drawing. An example would be to drill a hole on the wrong side of a part because the sides look similar :0P
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I set up the head in a three-jaw chuck on my rotary table and “zeroed” out the table with the head perfectly vertical. I used a granite surface plate to ensure accuracy as seen below...
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Using my machinists square I aligned the rotary table to the mill table as seen below. This should orient the head perfectly with the table’s travel.
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Using my wiggler tool I located center of the head and zeroed out the hand wheels for a starting reference (below). Note: Most Chinese mills have trapezoidal screws for the table travel and there is inevitably backlash that comes with wear of the lead nut. You can adjust the backlash of the lead screw to some degree although there will always be some play to account for when taking measurements.
I have grown accustomed to compensating for backlash on my old mill by rocking the hand wheels so I can “feel” how much backlash is present. I then make a mental note of this amount and add it to my travel whenever I move the table. This can also be done with a dial indicator which can tell you when the table actually moves between a change in direction. Once I know what to compensate for I can center the wheel between the lash (example of .004” total, centered between each side of .002”) and know that I have to add .002” to every move in one direction and .004” when I change directions.
It might sound complicated but it is quite easy to get used to and makes a better machinist IMO. Although a good set of Digital ReadOuts or DROs would eliminate the need to compensate for lash. For me they are too expensive so I am happy with my method right now ;0)
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Using the graduated dial of the “X” axis I moved the table over 0.875” for the head bolt pattern radius. This will be the diametrical location for the four bolt holes.
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Using the rotary table I turned the head 18.73° counterclockwise for the first hole which was converted to 18°-43’-48” which could then be implemented on the vernier wheel of my rotary table. It’s probably not that critical although I wanted to learn how to use minutes and seconds of arc-measurement for learning purposes.
Once the head was positioned for the first hole I used a #2 center drill to “spot” the hole before drilling it with the twist drill. A center drill is much more rigid than a regular drill bit so it will stay straight when it contacts the metal. A regular drill will sometimes deflect and drill at an angle messing up your axial alignment.
Note: In most cases I will use a center drill to “spot” critical holes to ensure accuracy even if it is not shown in the photographs. I will be omitting several of these photographs to reduce the size of these articles (including a majority of the center drilling shots)...
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Using a #21 drill bit I drilled almost all the way through the head making sure not to drill into the jaw of the chuck (the hole will be finished later).
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I rotated the head at 90° increments and repeated the process to finish the four bolt holes taking care to not drill into the chuck jaws that were directly below the holes.
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At this point I loosened the chuck and spun the head so the remaining unfinished holes could be finished without hitting the chuck jaws (not shown).
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Now it was time to lay out the center of the head on the inside for the valve holes. I aligned the vertical outside line with one of the inside lines to properly orient the head on the mill.
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I used the wiggler to center the head on the table and also align the vertical line with the table...
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I positioned the intake valve using the X & Y coordinates on the prints and spotted the hole with a #2 center drill (not shown). I then drilled through the head with a 9/64” bit...
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Without moving the table I set up the mill with a 11/32” drill as seen below. Using my digital depth gauge on the mill I was able to accurately drill the hole 1” deep at about 200 RPM (not shown).
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I could now use a 5/32” chucking reamer to ream out the valve stem hole (at about 150 RPM).
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The next step was to countersink the valve seat with a 45° (90°) countersink bit. I carefully plunged the bit to achieve a .531” OD on the top edge of the hole.
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The process was repeated in mirror to the other hole below.
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At this point I positioned the head so that the rocker arm bracket bolt hole could be drilled. I used a #38 drill to make a .313” deep hole at the required location.
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Without moving the table I placed a 5-40 tap in the chuck and hand fed the tap into the casting...
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The next hole is used for the compression relief valve. Although the compression relief feature on this model is not necessary it adds a nice scale look to the engine. I decided to make my valve functional so I drilled all the way through the head with a 1/8” drill bit at the required location. I then drilled a #22 hole at .275” deep for the valve threads (prints call for a 9/64” drill).
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A 3/16-40 MTP tap was used to thread the hole as seen below. Note: I use the milling chuck as a tap guide while I hand feed the tap to prevent the tap from breaking.
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For the next step I remounted my rotary table vertically so the port bosses could be machined flat.
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I realigned the head vertically with a square (not shown) while the table was at the zero mark. This will allow me to surface the exhaust port for a nice flat surface. I repeated the process for the intake port after rotating the table 90° (not shown). Note: I won’t be machining the valve push rod boss until later when I can attach the head to the engine frame.
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I located the center of the boss with my calipers and positioned the spindle over the mark with the wiggler tool.
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Using a #2 center drill I spotted the exhaust port hole.
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I used a 5/16” drill to bore a 1.125” deep primary hole followed by a 11/32” at .5” deep (prints call for a “R” size drill).
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I can now tap the hole with a 1/8” NPT tap using the power tapping feature of the mill. I used a little cutting oil to speed up the process. I rotated the head 90° and drilled out the intake port using the same process with the exception of a 1.625” deep primary hole (not shown). This completes the majority of the head work...
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I used some “medium” CyanoAcrilate (CA) glue to temporarily attach the head to the engine frame so the #21 head bolt holes could be transferred to the frame.
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I used a square to align the head (using the vertical line scribed earlier) and fixed the head into place.
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A cordless drill was used to drill the four head bolt holes into the frame using the head as a drill guide. I used a marker to mark the drill bit (hard to see) to achieve .5” deep holes...
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To remove the head I gave it a little tap with a nylon faced hammer. If the CA glue won’t release a little heat will melt the glue which can be cleaned off with acetone.
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The next step was to drill out the head bolt holes with a #11 drill to clear the 10-32 head bolts.
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I can now tap the holes in the frame with a 10-32 hand tap as seen below.
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I installed the hardware kit’s 10-32 x 1.625” studs in place for a fit check.
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A perfect fit ;0)...
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So far so good! I am having a lot of fun with this kit and there will surely be a lot to learn using my new equipment. Please be sure to join me again for the next episode and until then...
Stay inquisitive my friends!!!
Don R. Giandomenico
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