THK KR20 linear actuator  

In photomacrography, especially in the studio/laboratory, it is essential to have some means to focus with a precision substantially higher than that provided by photographic bellows and focusing racks designed for use with a camera. To be really useful in photomacrography, a focusing rack must be able to carry the weight of a normal camera, lens and bellows without detectable sagging and vibration. It must focus smoothly without seizing and creeping, even when the camera is mounted vertically. It must allow a reliable focusing with a precision that varies, depending on magnification, from a few tens of μm to a few μm.

Since focus stacking is now commonly used in photomacrography, the focusing mechanism must be able to provide this precision repeatably, while being able to move the camera by a total travel of a few mm to about ten cm (the longer, the better). Since the length of the optics and extension tubes or bellows used in photomacrography is quite variable depending on magnification, focal length of the objective, and the use of a tube lens as well as its focal length, the amount of travel allowed by the focusing mechanism should be enough to accommodate this variable length.

In the magnification range between roughly 1x and 20x, an industrial linear stage can provide a sufficient travel length and precision. At higher magnifications, a microscope stand and focuser with coarse and fine focus controls is usually better, although work may still be possible up to about 50x with the best industrial stages.

Figure 1. THK KR20 linear actuator with NEMA17 stepper motor.

The THK KR20 actuator shown in Figure 1 provides a total travel of 140 mm. The rail is 205 mm long and the platform moves along the rail on a double set of recirculating steel bearing balls on each side. Both rail and platform are made from solid blocks of forged and machined steel. This gives very low amounts of twist and tilt of the moving platform. THK is a Japanese company making a variety of precision mechanical equipment.

Figure 2. Platform from another THK KR20 linear actuator. The arrows indicate guides for ball bearings.

Before acquiring the actuator shown in Figure 1, I happened to have another THK KR20, albeit without motor and motor cage. It proved too difficult to find a separate motor cage, so in the end I gave up and purchased a complete actuator. Nonetheless, my first KR20 proved to be instructive when disassembled.

One word of caution: The ball bearings of both platform and nut are simply too difficult to reassemble without specialized equipment. In particular, the recirculating balls of a nut with 1 mm thread pitch are extremely small and slippery. Therefore, disassembling these parts is essentially an irreversible process. You can learn a lot through the disassembly of this and similar equipment, including that disassembling equipment that you still intend to use in the future may be a costly experience.

The screw that moves the platform has a thread pitch of 1 mm per turn and uses dozens of recirculating sub-millimeter sized steel balls between the fixed helical groove of the platform and the rotating screw. In other words, there are no metal-against-metal sliding contact surfaces between rails and platform, and between screw and nut. All surfaces between platform and screw, and platform and rail, are rolling on ball bearings. The screw likewise turns on ball bearings at both ends of the rail. The moving platform has a brass nipple for re-lubricating with grease. Rubber skirts around the platform give some protection from contamination of the ball bearings located between platform and rail.

The profile of the grooves that guide and hold the ball bearings on the sides of the platform and rail is not a simple semicircle, but slightly trapezoidal. This minimizes the contact surface between balls and grooves, reduces friction, and allows grease to flow around the balls when the latter roll within the grooves. This is also true of the profile of the helical thread on both nut and advancing screw. They are not simple trapezoidal threads and they do not directly slide against each other, unlike the threads on simpler screw-operated linear actuators.

Precision-molded plastic parts receive the balls at either end of the platform and forward them to the respective recirculating guides drilled within the platform. A single grooved plastic part shuffles the ball bearings from one end to the other of the multi-turn helical groove between platform and screw.

The THK KR20 linear actuator is sold in a variety of configurations. The most common option is a choice between 1 mm and 10 mm screw pitch. For focusing in photomacrography, 1 mm is the obvious choice, while 10 mm is probably a bit too coarse for magnifications above roughly 5x-10x. A higher screw pitch makes travel of the platform along the rail faster, less precise, and more likely to be accompanied by vibration introduced by the stepper motor.

Another option is the type of motor (servo vs. stepper, or no motor). In general, stepper motors are easier to control with general-purpose microprocessor boards and simple motor controllers.

Yet another option is whether the rail has one or two moving platforms. Two platforms reduce the amount of twist and sag of equipment attached to the platform(s), but also reduce the maximum amount of actuator travel along the rail. Using one or more additional rails placed some distance from the actuator and multiple sliders on the additional rail(s) provides an even better reduction of twisting, sagging and play, but requires all rails to be perfectly aligned parallel to each other.

The KR20 is usually equipped with a NEMA17 stepper motor with 200 steps per revolution (1.8° rotation per step). I replaced this motor with one providing 400 steps per revolution (0.9° per step). This gives a theoretical travel of 2.5μm per full step of the motor. Microstepping can give a finer travel resolution, but the platform should not reverse in direction during travel, if backlash is to be avoided. In addition, with simple stepper drivers the size of individual microsteps varies non-linearly according to where the microstep is taken with respect to the boundaries of full steps, and changing the loading of the platform changes the exact location of both full steps and microsteps along the rail.

Further options are the length of the rail (between roughly 10 cm and over 1 m), the width of the rail (1 inch in the KR20), the stepper motor size (usually, NEMA 17 for the KR20), and whether the screw is protected from direct hand contact by an aluminium cover (the SKR20 series, which, however, is not dust- and dirt-proof). Second-hand KR20 actuators salvaged from equipment may or may not include a flexible coupler between motor and screw, a cage for the motor and its coupling, and a damper (roughly working as a flywheel) to reduce the vibration of the stepper motor. One or two limit switches (usually optical) may also be present at one or both ends of the rail.

The KR20 rail can be attached to a support with a number of M3 screws (six in the specimen shown in Figure 1). The moving platform has threaded sockets for four M3 screws and two M2.5 screws. Only the M3 sockets are generally used.

There are a number of "tricks of the trade" that make it easier to work with linear actuators, stepper motors and their drivers. For example, a simple but useful "trick" to identify which pair of wires is connected to the same winding (or "phase") of an unknown stepper motor is shorting a pair of wires while turning by hand the motor axle. When you short the two wires of the same "phase", the force necessary to turn the axle suddenly increases.

Two-phase stepper motors have two independent windings (usually with four connecting wires, although sometimes three if the two windings use a common earth). These motors are the simplest to interface to a driver and to control in software. Multi-phase stepper motors can have a smoother rotation, but require more complex drivers and are usually reserved for higher torque applications.

Plenty of useful information can be obtained on forums that discuss the construction and maintenance of 3D printers and CNC mills (which also use linear actuators).

Other linear actuators

Before the THK KR20, I used several other sliding linear rails and actuators, albeit models meant primarily for manual rotation of the micrometer screw. For example, the iShoot Macro Focusing Rail 150 proved disappointing, while the Newport UMR 8.25A is extremely well made. The Manfrotto 454 is a very basic focusing rail, with a manufacturing quality more similar to something made decades ago in the USSR rather than in present-day Western Europe.

I also used, and was not satisfied with, a Velmex Unislide linear rail. Although I had heard good things about the Velmex rail, I discovered that it has multiple things I do not like: thin nylon sliding inserts between platform and rail that collect dust, no way to adjust the tolerances between platform and rail once these inserts wear down, advancing screw running within a slightly elastic, self-lubricating plastic nut, and fiddly adjustment of the anti-backlash mechanism of the nut. Comparing the THK KR20 with a Velmex Unislide is like comparing a classical Mercedes with a Soviet-era Lada.

Relatively cheap, China-made industrial linear stages operated by screw micrometers have been flooding the market for several years. Surprisingly, I found that they are usually quite adequate as focusers and subject positioners in photomacrography, and certainly better than virtually all the photographic focusing racks and subject positioners I have tried.

Further examples of focusing racks I used in the past can be seen here.

There are other brands and models of industrial motor-operated linear actuators on the market. I have little experience with any of them, and cannot say much about alternatives to the THK KR20.

There are also extremely precise (on the nanometer scale) motorized linear actuators occasionally available on the second-hand market. They are so delicate that they should never be operated by hand, never lifted by holding their actuator part, and preferably not touched at all. Considering the rough treatment typical of surplus industrial equipment, I would use extreme caution when purchasing any of them in used condition. Piezoelectric linear actuators have been the subject of interest by photographers lately, and are probably more tolerant of rough treatment (albeit their total travel is quite small). They can also be operated by closed-loop electronic systems that can largely correct the non-linearities of the piezoelectric elements and their mechanical flexing frames.

My judgement of different models of focusing rails may seem excessively severe. However, when compared with the best industrial actuators, microscope focusing blocks and microscope subject positioners, the extreme difference in quality, user experience and photographic results is really plain to see and feel.

External resources

Since the THK KR20 is primarily meant to be operated by a motor, the following resources can be useful:

  • machinedesign.com introductory-level discussion of choices when choosing a stepper motor driver and operating mode.
  • photomacrography.net thread on using the THK KR20 with 400 steps/revolution stepper motor, and links to other threads discussing microstepping drivers.

Summary

The THK KR20 linear activator is at present one of the best choices for a screw-operated linear actuator for use as a motorized focuser for photomacrography within a reasonable magnification range (i.e., at lower magnifications and longer total amount of travel than provided by microscope stands and focusing blocks).



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