Subscribe to Precision Engine
Sign up for our E-newsletterCNC-MACHINING AN ENGINE BLOCK
A step-by-step overview of a
specific block being machined
on a CNC from start to finish.
by Mike Mavrigian
all photos by author
CNC machines vary in terms of footprint size and axis capabilities.
Thinking about taking the plunge and buying a CNC (computer numeric control) machine? Or have you wondered what’s involved in machining a block on a CNC? Do you think that operating a CNC machine is too complex and requires too steep a learning curve? In order to address these concerns, I visited Gressman Powersports in Fremont, Ohio, to follow the CNC machining of a single engine block, from start to finish.
Why purchase a CNC machine? The answer involves two criteria: reducing job time and obtaining dimensional accuracy. Utilizing a CNC machine means that the job can be performed faster, as opposed to moving the block from machine to machine and taking the required setup time. If speed is not an issue, and if worktime is the only concern, then you don’t need a CNC. However, if you want to obtain accuracy without the need for specialty accurizing fixtures, then a CNC will fulfill that need.
One aspect of owning a versatile CNC machine is the fact that you can take advantage of it’s capabilities beyond block or head work. Given the machine’s programmability, clamping fixtures and tooling, you can extend your work (and profitability) by fabricating specialty pieces from scratch (making your own valve covers from billet stock, milling custom engine accessory brackets, making engine mounting plates for race cars, making spacer plates, bushings, etc.). Having this capability expands the services that you offer to your customers. Plus, you can pursue fabrication jobs from local industries beyond the automotive scene. If this is what you have in mind, carefully research the machine(s) that you’re considering to make sure it can handle all of the work that you currently have, plus any additional work that you may become involved with down the road.
Scott Gressman owns an RMC V-40 CNC, so the examples you see here are based on using that particular machine. However, our intent is not to favor any single brand. Rather, we’re simply using this machine as an illustrative example.
In this work-up example, the sample block is a Dart smallblock Chevy iron unit. The customer requested specific dimensions in terms of cylinder bore size, lifter bore size and deck height.
AXIS PRIMER
Whenever you discuss CNC-machining, you need to think in terms of the axis of approach that the tooling will follow. Depending on the model, a CNC milling machine may operate on one to seven axes:
• X-axis: The tooling moves left-to-right (horizontal travel)
• Y-axis: The tooling moves in-out (fore/aft)
• Z-axis: The tooling moves up-down (vertical travel)
• A-axis: A fourth axis. The axis of rotation is parallel to the X-axis. In our example, this allows the block to rotate in relation to the tooling head.
• B-axis: The workpiece would rotate perpendicular to the A-axis (if viewed overhead, the workpiece would rotate clockwise/counterclockwise)
OPERATING THE MACHINE TO MILL A BLOCK
Start the machine and let it warm up, allowing the computer to fully boot. During this warmup, everything moves to a “known” plane.
Before installing the block into the machine, the main bore must be at its final diameter and alignment, so perform align-honing first if this is planned.
Depending on what you plan to cut, install the appropriate tooling head and cutter tip. Some machines offer a tool changer that stores all of the tooling you plan to use and will automatically change tools at the touch of a button.
The machine must know the block’s crank centerline. A fixture/locating bar is inserted through the main bore (this requires using precision spacers to locate the bar based on your main bore diameter). Using a probe, touch the probe off of the top of the fixture (centerline of the crankshaft). Zero the machine. The machine now knows precisely where the crank centerline is located.
Install a cam bore centering fixture into the cam tunnel. This bar features adjustable fingers that self-center the bar within the tunnel. The bar protrudes out of the front of the block to provide a reference surface for the probe.
At this point, we need to let the machine know where the block itself is located. Using the machine’s auto setup program, the probe contacts the cam bar,
referencing the X-, Y- and A-axes. This provides coordinates that tell the machine exactly where the block is positioned.
The machine-control display allows you to select a block from a menu of block designs (various styles of Chevy, Ford, etc.). Programs are already loaded for the block designs listed in the menu. You can now allow the machine to reference OEM blueprint dimensions or, if the machine provides a “conversational” feature, you can override any known dimension to change the desired deck height, bore diameter, etc. The machine automatically assumes blueprint data unless you input your own custom data.
A milling head is installed, fitted with either a CBN cutter (for cast iron) or a carbide cutter (for aluminum), and cutter height is adjusted.
The first operation involves cutting the block decks. This will allow you to
create the desired deck height and will ensure that both decks are “square”
(parallel to the crank centerline, the correct bank angle, and identical bank-to-bank).
Zero-out the X-axis for crank centerline. The probe touches the main fixture bar to locate crank centerline. Next, the probe touches the cam centering bar using the A-axis to place the crank and cam tunnels in a vertical plane (the machine performs a bunch of complex math a this point, but you don’t need to be involved in that).
Next, the probe touches the front of the block (the timing cover mating deck). The machine now knows where the X-axis travel will start.
The machine already knows to machine to blueprint specifications based on a stock GM block (GM bases its dimensions from the dowel pin hole in the right front corner). However, don’t assume that your specific block is correct. Let the probe find the existing cylinder bore centerlines (the probe touches at two points in the X-axis and two points in the Y-axis, and automatically knows the existing bore centerline).
The probe then touches six points on the deck to determine existing deck height. In this particular example, Scott Gressman wanted a deck height of 9.010″. At the touch of a button, the program is called up and the display will ask you to input the desired deck height. Enter the desired deck height, then enter the existing deck height and tell the machine how many passes you want to run to accomplish this. While the machine head is cutting the deck, the display constantly shows what’s taking place in terms of material being removed, cutter location on the deck, which pass is currently running, etc.
With the iron block and cutter that Gressman installed, you’re able to take as much as a 0.008″ cut per pass.
Once the first deck has been cut, the machine automatically rotates the block and cuts the opposite deck. Once the second deck has been cut, the milling head automatically pulls away from the block and the machine stops. The result is perfect deck relationship-both decks are perfectly parallel to the mains, in this case at a perfect 45 degree angle relative to the centerline, and both decks are perfectly parallel to each other.
Now it’s time to bore the cylinders. On a remote tabletop fixture set up the CBN cutter on the cylinder boring fixture to the desired bore diameter. Look at the computer display screen. As a result of the previous probe-check of the bores, the machine knows how far off from centerline each bore is located. For example, on our sample block, cylinder No. 2 was off about 0.004″ in the X-axis and about 0.002″ off in the Y-axis. At this point you need to make a decision: do you want to follow blueprint specs or are you going to follow the existing bore centerline? Gressman decided to follow print specs and to make the cut using two passes.
Now the machine needs to know where the existing deck height is located in order for the machine to know when to begin the cut, and it wants to know the bore depth so that it knows when to stop cutting. Here, you have an option-let it run down to the bottom of the bore or let it offset cut at the bottom to gain web clearance. No special programming skills are needed. The display asks and you follow the question/answer format. With the boring head that Gressman chose, the machine can cut up to 0.040″on a single pass. A beefier bar (option) can whack as much as 0.250″ in a single pass.
When boring, you should bore one cylinder and then check to see if any cutter depth adjustment is needed. In our example shown here, total boring time for all eight cylinders was a mere 4 minutes and 40 seconds.
Once all holes have been bored, change to a chamfering cutter to create a perfect (and identical) chamfer at the top of each cylinder. The program brings the fixture to bore center. You then enter manual mode on the controller and punch in the desired chamfer.
In our example, all eight bores were chamfered in a total time of 1 minute and 9 seconds.
For lifter bore correction (if needed), the computer is already programmed for lifter centerline. Adjust cutter diameter and let her run. Machining 16 lifter bores takes about 10-12 minutes, depending on depth of your lifter bores.
MACHINE TIME
(TYPICAL, SMALLBLOCK CHEVY)
Auto program setup…..1 min., 20 sec.
Cutting block decks (2 decks)…..4 min, 30 sec.
Probe 8 cylinder bores to determine bore centerline…..3 min., 50 sec.
Cylinder boring (8 cyls)…..4 min., 40 sec.
Cylinder bore edge chamfer (8 cyls)…..1 min., 9 sec.
Bore lifter bores (16)…..10-12 minutes
Tags: BLOCK, BLOCK CORRECTIONS, BLUEPRINTING, CNC, GRESSMAN POWERSPORTS, RMC
