CNC Machining Eccentric Shafts: Process and Machines
An eccentric shaft looks like a drafting error at first glance. One journal sits off the centre line, a counterweight web hangs opposite it, and the part seems to be built around the wrong axis. That offset is the point of the design - and it is also what makes the part awkward to machine. The moment you clamp a shaft whose mass does not sit on its axis of rotation, the work starts to fight the setup. It deflects, it vibrates, and the offset journals have to come out in the correct phase with each other and with the keyways that drive them. This article covers how Handemo CNC machines eccentric shafts: the shapes we see, the materials they are made from, the machines we put the work on, and the process order we follow when the centre of the part is not the centre of the cut.
What an Eccentric Shaft Is, and Where It Goes
An eccentric shaft is a shaft with at least one journal whose axis is deliberately offset from the main axis. Turn the shaft and that journal orbits the centre line, converting rotation into a reciprocating or oscillating motion. The idea is old and it is everywhere in heavy equipment. Jaw crushers use a large eccentric shaft to swing the moving jaw. Vibrating screens, vibratory compactors and shaker drives use eccentric weights and offset journals to generate the stroke. Eccentric presses and punch presses convert motor rotation into a controlled downward stroke through an eccentric shaft or crank. Hydraulic motors, variable-displacement pumps and eccentric-drive pumps use shorter eccentric shafts to move a piston or a swash plate, and many indexing and clamp mechanisms use a small eccentric cam shaft to lock or release parts.
In every one of those applications the shaft carries the same family of features. There are the main journals that sit in the bearings and define the axis of rotation. There is the offset journal, which is often the working surface of the part. There are the webs or lobes between them, frequently shaped as counterweights so the shaft does not shake itself apart at running speed. And there are the driven features - keyways, splines, flats, cross holes, oil passages and sometimes a threaded end. A crusher eccentric shaft is a large, heavy, long part. A pump eccentric shaft is short and compact. Both are the same problem in different sizes: the offset has to be established, held, and phased correctly against everything else.
Materials We See on Eccentric Shaft Work
Most eccentric shafts we machine are alloy steel. 42CrMo and 40Cr are the workhorses for medium and large shafts - tough enough to carry a heavy offset load and responsive to hardening and tempering, which is what lets a crusher or press eccentric survive years of cycling. Where the surface has to resist wear under sliding contact, as on a cam lobe or an eccentric journal that runs against a bushing, 20CrMnTi and similar case-hardening grades are carburised and hardened after machining, so the shaft arrives at heat treatment with a small allowance left on the working surfaces. Simple eccentric drives are often made from 45 steel or its equivalents. Larger crusher eccentrics are sometimes cast, and ductile iron appears in lower-load positions where a casting is cheaper than a forging.
Two material habits matter on this work. First, alloy shaft stock moves when the outside skin is removed - a long bar or forging carries internal stress, and roughing a deep eccentric offset releases it unevenly. Second, the case-hardening grades cut differently before and after heat treatment, so the sequence has to leave the surfaces that will be ground or lapped for the state they will be in after hardening, not before.
Why Eccentric Shafts Are Hard to Machine
The imbalance is built into the part
A round shaft spins in balance around its own axis. An eccentric shaft with a counterweight does not: even when the design is balanced as a finished part, the intermediate shape during machining is not. An unfinished counterweight web, a half-cut lobe or an offset journal that has been roughed on one side but not the other all pull the part around. At low roughing speeds this shows up as an uneven cut and poor tool life. At higher speeds it turns into vibration that marks the journals and pushes the part away from the tool. The answer is not to slow everything down - it is to plan the setup so the unbalanced mass is supported, and to keep the roughing passes on both sides of the axis balanced against each other.
The offset has to be set up, not just dialled in
On a normal shaft, turning between centres is simple: the centre holes define the axis and every diameter follows from it. On an eccentric shaft, the offset journal has an axis of its own, and getting the tool to cut around that axis is a setup decision. We use eccentric setups - offset centre holes that are established from the main axis, sleeve or dog fixtures that hold the part on the offset centre while the main journals stay as datums, or a four-jaw or fixture arrangement on larger work where the part is indicated to the offset rather than centred. Whichever route is used, the offset has to be created from a datum that will still exist after the part is finished, because that is how the rest of the shaft is checked later.
The phase angle between features
An eccentric shaft is rarely a single offset. A crusher eccentric, a press eccentric or a pump shaft can carry more than one offset, plus keyways, splines and oil holes, and what matters is the phase relationship between them - which direction each offset faces, and where each driven feature sits relative to it. Get the offset right but the phase wrong, and the part will not time with the mechanism it drives. Phase is controlled by referencing every feature back to a common datum, by index marks that are cut into the shaft and left visible through the process, and by cutting the features in an order that does not destroy the reference.
Long heavy shafts move on their own
Large eccentric shafts are long and heavy, and they are usually machined between centres or on a steady rest. Deflection from the cutting force is one problem; the other is what the part does after roughing, when the material it removed was holding the shaft in a stressed shape. A rough eccentric can come back visibly bent. We deal with it the way heavy shaft work is always dealt with: rough everything with allowance, let the part settle - including a stress-relief step where the material and the customer's requirement call for one - then finish on the settled part rather than chasing a shape that keeps moving.
Interrupted cuts, keyways and cross holes
Eccentric shafts are full of interrupted features. Keyways cut across a journal, splines interrupt the surface, and cross holes break through the side of the part. Each one interrupts the cut and each one is a chance for the tool to be pushed off line. Cross holes that meet an internal oil passage are the classic trap: they are drilled after the outside surfaces are established so their position is set from the finished diameters rather than from the raw bar.
Recommended Machines for Eccentric Shafts
Because eccentric shafts run from pump-sized parts to crusher-sized ones, the machine choice follows the size and the weight of the offset.
For large, heavy eccentric shafts - the crusher and press family where the shaft is long and the counterweight is substantial - the starting point is the TCK6350B heavy-duty CNC slant bed lathe. Its rigidity is what keeps a long offset part from being pushed out of shape by the cut, and the slant bed sheds the heavy chip load that comes with roughing an offset down from a large forging or bar.
Medium eccentric shafts - pump drives, eccentric cam shafts and the shorter press parts - run comfortably on the TCK6050 CNC slant bed lathe, which handles the mix of roughing passes and finishing passes these parts need without a long setup change between them. Where the journal surfaces carry the eccentric's working load, the finishing work is done on the high-precision lathe models rather than on the roughing machine, because the surface has to survive sliding contact rather than just hold a dimension. The full range is listed on the CNC slant bed lathe page.
Many eccentric shafts are not turning jobs alone. When the same part carries a keyway, a spline, a flat or a cross hole that has to sit in phase with the offset, the case for a machine that turns and mills in one setup is strong: the CNC mill-turn machines cut the turned journals and the driven features without the part being re-clamped, which removes the risk of losing the phase when the shaft moves to a second machine. Large eccentric shafts with major milled webs, flange faces or drilled patterns are better suited to a horizontal machine such as the HMC630-DT horizontal machining center, where the part is presented to the spindle from several sides and the offset can be referenced from one datum. Smaller eccentric components that are mostly milled - a cam plate, an eccentric hub, a locking eccentric - are run on a vertical machining center on a purpose-made fixture.
Process Rules We Follow on Eccentric Shafts
Turn the main journals first
The main journals define the axis of the part. We cut them first and treat them as the datum for everything that follows - the offset, the keyways, the cross holes and the final inspection. When the main journals are established early, the eccentric setup can be indicated against a surface that will still be there when the shaft is finished, instead of against raw stock that is about to be removed.
Balance the setup, not just the part
Eccentric work is planned so the unbalanced mass is supported in the fixture and so roughing passes stay balanced against each other. On a part with a counterweight web, both sides of the web are roughed in the same stage rather than one side being finished while the other is still raw. Where the offset is large, the setup itself - chuck, fixture, steady rest - is arranged so the part is not left hanging on the heavy side.
Establish the offset from a real datum
The offset is created from the main axis using offset centre holes, sleeve fixtures or indicating, and its direction is marked on the part. That mark is not cosmetic - it is what the next operation reads. We also confirm the offset on the machine before the finishing cuts, so a setup that has moved is caught before the working surfaces are cut rather than after.
Rough, settle, finish
Heavy offset work is taken in stages: rough with allowance, allow the released stress to show itself, stress-relieve where the material and application require it, then finish on the part as it now sits. Chasing the final surfaces immediately after a heavy roughing cut is how a large eccentric ends up with a bend that has to be straightened, and straightening a finished eccentric shaft is far more expensive than leaving it to settle.
Cut the driven features on the same centreline
Keyways, splines, flats and cross holes are positioned from the main journals and phased from the mark that was cut earlier. Where those features are produced on a second machine, the datum is transferred and checked rather than assumed. On mill-turn machines they are cut in the same setup as the journals, which is the cleanest way to keep the offset and the driven features in agreement.
Verify the phase before the part leaves
The last check on an eccentric shaft is not a diameter - it is the relationship between features: which way the offset faces, where the keyway sits against it, and whether each journal runs true to the axis it belongs to. That check is done with the part supported the way it will be supported in service.
Quality and Consistency
An eccentric shaft is rejected for a phase angle that does not match the mechanism, an offset that was set from the wrong datum, journals that do not run true to the axis they belong to, a shaft that has bent after roughing, or a keyway that does not line up with the offset it works against. None of those faults are about the finish of a single surface. They are about the relationships between surfaces, and relationships are set by the setup and the sequence - which datum was used, when the part was allowed to settle, and whether the phase was carried forward through every operation.
Handemo has been building machine tools in Tengzhou since 2008, across a plant of 220,000 square meters with more than 660 staff, and eccentric shaft programs here start the way any shaft program does: prove the setup and the sequence on a trial part, then run the batch on the setup that was proven. If you are planning an eccentric shaft, the drawing that tells us the most is the one that shows the offset direction, the phase of the driven features and the journals that have to stay true to each other - those are the three things that decide whether the part behaves in the machine.
Frequently Asked Questions
What is an eccentric shaft used for?
It converts rotation into a reciprocating or oscillating motion. Jaw crushers, vibrating screens, vibratory compactors, eccentric and punch presses, eccentric-drive pumps and hydraulic motors all use a shaft with an offset journal. The offset journal orbits the main axis and drives whatever mechanism the machine needs to move back and forth.
How do you machine an offset journal on a CNC lathe?
By setting up the offset as a separate axis of its own. We establish the main journals first as the datum, then hold or indicate the part on the offset centre using offset centre holes, sleeve fixtures or a four-jaw arrangement, so the tool cuts around the offset axis. The direction of the offset is marked on the part and used as the reference for every feature that has to be phased against it.
Why do eccentric shafts vibrate during machining?
Because the mass is not on the axis of rotation. A counterweight web or an offset journal that is roughed on one side only pulls the part around, and the imbalance grows with speed. The fix is a setup that carries the unbalanced mass, roughing passes balanced on both sides of the axis, and support from a steady rest or tailstock on long shafts.
What materials are eccentric shafts made from?
Alloy steels such as 42CrMo and 40Cr for medium and large shafts, case-hardening grades such as 20CrMnTi where a working surface has to resist sliding wear, 45 steel for simpler eccentric drives, and ductile iron for some lower-load cast eccentrics. The choice decides the hardening and stress-relief steps that sit between roughing and finishing.
Can you machine an eccentric shaft with keyways and cross holes in one setup?
Yes, that is exactly what a mill-turn machine is for. Turning the journals and milling the keyways, flats or cross holes in the same setup keeps the driven features phased with the offset without the risk of losing the reference when the part is re-clamped elsewhere. Very large eccentric shafts with heavy milled webs are usually run on a horizontal machining center instead.
Get an Eccentric Shaft Machining Plan
Send us the shaft - the offset and its direction, the journals that have to run true to each other, the phase of the keyways, splines or cross holes, the material and the batch size - and Handemo will recommend the right lathe, mill-turn or horizontal machine and the process sequence to go with it. Contact Handemo CNC | Email: sales1@handemocnc.com | WhatsApp: +86 135 6320 3038
Related reading: CNC Machining of Transmission Shafts and CNC Machining Stainless Steel Shafts cover straight shaft work, and CNC Lathe vs Mill-Turn explains when driven features justify a single-setup machine.
