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Lately I've been watching some videos on machining and hydraulics as it always interested me.

I was toying with the idea of building a budget CNC mill with threaded rod for the axes, but the issue of backlash was bugging me.

Then I thought about using hydraulic pistons instead. If used in conjunction with a positive displacement pump and using a large hydraulic transmission ratio the issue of precision could be sorted out, and the incompressibility of the fluid would help to keep backlash to a minimum.

I searched but found little info on hydraulic actuators for precision positioning, so I believe there's something I must be missing

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  • $\begingroup$ YouTube creator This Old Tony released today a video regarding a German milling machine that used a single motor driving a hydraulic pump, which provided the forces to the various axes via hydraulic motors. There was minimal information in the video, but a reference to the model and manufacturer may be of value to you. As I lack complete information, this is a comment, not an answer. youtube.com/watch?v=Ogwp7zD59og $\endgroup$ – fred_dot_u Oct 9 at 23:21
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It would certainly be possible to make a machine like you are proposing, but there are good reasons why hydraulics aren't usually the optimal choice. There are exceptions to this, for example CNC press brakes for metals or CNC mandrel benders often have hydraulic actuators. The distinction is that usually the down sides of hydraulic positioning are only worth it if you need a lot of force to complete the operation. In your case with a hobby-scale CNC mill all of your travel axes can operate on very little force.

Here are some of the down sides to designing for hydraulic actuation:

  • Speed: even smallish hydraulic cylinders require a significant volume of oil, and there are practical limits on the flow speed inside of hoses, so moving enough oil to get a hydraulic cylinder moving as fast as a lead screw or rack and pinion can move would require a large pump, large hoses, and large ports. All of this is possible, but very expensive as compared to a simple stepper or servo motor.
  • Leakage: By there nature, pretty much all hydraulic valves involve some amount of leakage. Part of designing hydraulic systems is characterizing the amount of leakage and taking measures to compensate for it where the consequences are unacceptable. Again, it is possible to overcome these challenges, but buying high-precision valves, detailing counterbalance circuits, and maintaining the seals are all unnecessary costs.
  • Controls: Hydraulic valves have moving parts that take time and power to actuate. They don't have perfectly linear responses with respect to time, voltage, or pressure differentials. As a result, achieving precise position control requires more careful control tuning and feedback. By contrast, for a hobbyist level machine using stepper motors, you can pretty much treat this entire challenge as a solved problem. Even at a higher level, configuring a VFD and controlling its setpoints is much easier than controlling a hydraulic valve with feedback.
  • Cost: Aside from the ancillary challenges of integrating hydraulic actuators into machine design, the actuators themselves are simply more expensive. Hydraulic components are not produced in the same volumes that electric motors are, so there's not nearly as much of an economy of scale. In general hydraulic parts require more precise machining, more raw materials, and more detailed QC than motors, all of which translates into cost.
  • Routing: Whereas an electric motor can be driven by one cable with power and control, hydraulic actuators need two hoses at a minimum for precise control in two directions. The hoses are much larger and stiffer than corresponding cables, and failures are more of a mess. When an actuator is being moved by another axis, the logistics of routing the hoses is a fairly big pain.
  • Serviceability: Most users are better equipped to service electrical equipment than hydraulic equipment. A simple multimeter can allow a wide range of diagnostics on electrical equipment, whereas one needs a fair number of gauges and fittings to really find a problem in a hydraulic circuit. Even more than that, hydraulics are harder to disassemble since pretty much everything is a pressure component and can create a big mess.
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  • $\begingroup$ Great answer! You brought to light many issues which I hadn't even considered or knew about., thank you. $\endgroup$ – jjpprr Nov 10 at 20:20
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Hydraulics can be precise, down to ~ 100µm. However:

hydraulic pistons [...] used in conjunction with a positive displacement pump and using a large hydraulic transmission ratio the issue of precision could be sorted out, and the incompressibility of the fluid would help to keep backlash to a minimum.

This is not how this high precision is achieved in practice. AFAIK (I'm not in the field) high precision hydraulics are pre-packaged units of piston, valves and position sensor and control the piston for position.

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  • $\begingroup$ Hmm I see, so kind of like a servo with a rotary encoder. Although that would defeat the purpose of going with my idea since it was primarily to have a simple solution. $\endgroup$ – jjpprr Oct 10 at 22:46
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Tempersonic cylinders will monitor the position very accurately but are very pricey.

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    $\begingroup$ Welcome to Engineering! Please edit your answer to add more details. What are tempersonic cylinders? How accurate are they compared to hydraulic actuators as mentioned in the OP? $\endgroup$ – Wasabi Oct 10 at 14:31

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