The peculiarity of its design is that the lead screw along the X axis is fixed motionless (does not rotate). A static screw requires a special spindle nut. In CNC machines not big size usually the travel nut is rigidly fixed and the screw rotates to move the carriage. For me, the opposite is true - the running nut rotates around the screw, driven by a stepper motor. Well, it is obvious that a large-size CNC running nut must be made by hand, because such a nut is simply not sold anywhere!
Why do we need to rotate the lead nut instead of the lead screw on a large size CNC machine?
- An industrial ball screw with a length of 2 meters or more costs just insane money (in comparison with a construction hairpin). It must be pretty large diameter- from 20 mm and thicker, which costs even more crazy money. Plus, not every shag can crank such a colossus, and you need to install a servo, which costs even more crazy money (compared to a shag). And, generally speaking, 2 lead screws (one on each side) are usually put on a large CNC machine. It's double budget madness.
- An extremely budgetary and good option is a construction hairpin (see), but if we try to rotate it with a length of 2 meters, it will start jumping like a jump rope and eventually fall off.
- On a long bed of 2-3 meters with a fixed screw along the X axis, you can put not one, but as many as two or even three independent Y axes, each of which will individually work on its own order. Those. on one bed, as it were, 2 independent CNC machines with one mechanically common X-axis will be installed. Obviously, independent carriages will not work with a rotating screw, but only cloning of the axis will turn out.
A do-it-yourself running nut for CNC is made quite simply: we take a piece of caprolon of the required length and just cut it internal thread under the construction hairpin. Caprolon is quite soft and the thread can be cut even with the construction hairpin itself, having previously made a tap from it by cutting grooves with a grinder. I made an internal thread on my home lathe, and then I made a pass with such a homemade tap from a hairpin for a more accurate and tight fit of the thread. To do this, on the turner, you need to specially not cut the thread in order to leave it under the passage with the hairpin itself. Then the running nut will move tightly and without backlash. Backlash is also removed by increasing the length of the travel nut. Already at a length of 35-40 mm, the backlashes completely disappear. On the Internet, you can find many designs with a double adjustable running nut, which can also remove backlash, but its drawback is that it significantly complicates the design. If you use your CNC machine for a hobby, then an ordinary caprolon running nut will serve you for a very, very long time - a few years for sure! They are still alive with me, although I even saw aluminum on them 
The running nut for my large CNC machine will rotate itself around a fixed screw, so we support it on both sides with bearings and clamp it quite tightly between two aluminum plates. Bearing seats are milled in these plates. It doesn't matter if the seats are a little crooked. Aluminum is very soft, so the bearing can then be pressed tightly with a vice through plywood spacers. And this is even better, because we need to completely eliminate the longitudinal movement of the nut in the gap between these two plates. For rigid fixation of the plates to each other, as well as for transferring the translational movement of the nut to the carriage of the machine, we apply sheet metal 4-5 mm thick (there she is - a flaky rusty piece of iron in the photo). The photo lacks a similar bunch of plates in the horizontal plane (right under the nut) - I will finish this later.
It remains only to transfer the rotation from the stepper motor to the nut. I plan to do this with a toothed belt. But the catch is that I will have to make my own custom gear, which I have never done before.
To make my own gear, I had to pant a little. And I had to puff at the computer. I wrote my own program for calculating pulleys with given parameters, because I could not find anything efficient and free. The basis was taken open file on Thingiverse in OpenSCAD, which I rewrote in Python and exported to DXF. I made the gear from caprolon - this is a durable constructional and easily processed plastic. In addition to the gear itself, the toothed belt also needs a belt tensioner (aka a damper). I also made it from caprolon, but I inserted a bearing inside.
After installing the rotating nut on the machine, I suffered a little with the motor pulleys, which kept moving out due to the very high rotation speed and strong tension. I even had to drill small grooves in the shafts of the stepper motors and fix the pulleys to the shafts with Allen set screws. But in the end, the result was pleasing: along the entire length of the lead screw, the nut went smoothly swaying and the screw did not flutter a bit.
The reduction of the running nut was 30:12 (30 teeth on the nut, 12 teeth on the engine pulley), i.e. the reducer increases the engine torque by 2.5 times. The resolution of the machine on a hairpin with a step of 2 mm / rev is 0.004 mm (2mm / rev ÷ (200 steps / rev * 2.5)).
The lead screw is important detail which is used as a motion transducer. It changes rotary motion into linear motion. For this, it is supplied with a special nut. In addition, it provides movement with a predetermined accuracy.
Screw quality indicators
The screw, as a very important part, has to meet many requirements. In order to be able to use it, for example, in table vise, it must be suitable in terms of such parameters as: diametral size, profile accuracy and thread pitch accuracy, the ratio of the screw thread to its bearing journals, wear resistance, thread thickness. It is also important to note that depending on the degree of movement accuracy provided by the screws, they can be divided into several accuracy classes from 0 to 4. For example, lead screws of metal-cutting machines must correspond to accuracy classes from 0 to 3.4 accuracy class is not suitable for use in such equipment.
Lead screw blank material
As a blank for the production of a screw, a regular bar is used, which is cut off from the high-quality metal. However, it is important to note here that some requirements are imposed on the material that serves as a workpiece. The metal should have good wear resistance, good machinability, and a stable equilibrium state under internal stress conditions that occur after machining. This is very important, since this property will help to avoid deformation of the lead screw during its further use.
For the production of this part with an average accuracy class (2nd or 3rd), which will not be subject to increased temperature resistance requirements, A40G steel is used, which is a medium-carbon steel, with the addition of sulfur and steel 45 with the addition of lead. This alloy improves the machinability of the screw and also reduces the surface roughness of the material.
Screw profile
There are three screw profiles that are used in the production of the lead screw of a lathe or any other. The profile can be trapezoidal, rectangular or triangular. The most common type is trapezoidal thread. Its advantages include the fact that it is higher in accuracy than rectangular. In addition, using a split nut, it is possible to adjust the axial clearances with a trapezoidal screw, which arise due to equipment wear.
It is also important to note here that cutting, like grinding a trapezoidal thread onto a screw, is much easier than a rectangular one. But at the same time, you need to understand that the accuracy characteristics of rectangular threads are higher than those of trapezoidal ones. This means that if the task is to create a screw with the best adjustment for accuracy, then you still have to cut a rectangular thread. Trapezoidal screws are not suitable for very precise operations.
Screw processing
The main parts on which the screw in the machine is based are the bearing journals and collars. The screw thread is considered to be the actuating surface. The greatest accuracy in a table vise and any other machine tools with such a screw must be ensured between the working surface of the part, as well as the main reference surface. For this reason, in order to avoid deformation, the processing of all these surfaces is carried out using the application of this part determines the specifics of the processing of the lead screw.
It is also important to note here that the screw with by different class accuracy, processed to various values. Parts that will belong to the 0.1 and 2 class of accuracy are processed up to the 5th quality. The screws belonging to the 3rd class of accuracy are processed up to the 6th quality. Screws belonging to the 4th category are also processed up to the 6th grade, but they have a tolerance field for the outer diameter.
Centering and tapping
In order to obtain an acceptable quality screw, it is necessary to carry out several more operations. One of them was the centering of the part, which passes on lathe... The lead screw, or rather, the workpiece for this part is centered on the specified equipment and here the ends are cut to it. In addition, an operation is carried out to grind the workpiece. To do this, use centerless or cylindrical grinding machines in the centers. It is important to add here that grinding in the centers is carried out only for screws of 0.1 and 2 accuracy classes.
Further, before proceeding with threading, the workpiece must be straightened. It should be noted here that only screws with the 3rd and 4th accuracy class are subjected to this operation. After that, their surface is additionally polished. As equipment for threading on the lead screw, a screw-cutting lathe is used.
Screw nut description
The lead screw nut is designed to provide precise positioning movements. In some rare cases, they can be produced from a material such as antifriction cast iron. This element must ensure constant engagement with the turns of the screw, and also act as a compensating part. You will have to compensate for the gap that will inevitably arise when the screw is worn. For example, nuts for lead screws used in lathes are made in double. This is necessary in order to remove the gap that can arise either as a result of the manufacture and assembly of the machine, or as a result of wear of its parts.
The peculiarity of a screw with a double nut is that it has a fixed and movable part. The movable part, which is on the right, can move along the axis of the stationary part. It is this movement that will compensate for the gap. Nut production is carried out only for screws of zero, 1st and 2nd class of accuracy. Tin bronze is used for their manufacture.
What are nuts made of and their wear?
The most common materials for the production of this type of parts are aluminum-ferrous bronzes, according to the MT 31-2 machine tool standards. In addition to this material, antifriction cast iron can also be used as a substitute for irresponsible
It is important to add here that the nut wears out much faster than the lead screw itself. There are several reasons for this:
- the thread of the nut is poorly protected from any kind of contamination, and it is also quite difficult to clean it of these unnecessary elements;
- it often happens that this element is initially poorly lubricated and this greatly affects the service life;
- when the nut meshes with the screw, it turns out that for the second element all the turns work simultaneously, but for the screw only those that are coupled with the nut.
For these reasons, screws with a nut must be checked more often, since the nut will wear out rather quickly.
In a CNC axis drive, a gear is used to convert the rotary motion of the motor shaft into translational motion along the axis. In order to make it easier for you to choose a gear for CNC, below are the most commonly used types of gears in CNC machines. Gears exotic to the DIY sector, such as the linear servo motor and the linear stepper motor, will remain outside the scope of this article for practical reasons, and the most common ones will be discussed.
Transfer screw-nut
A screw-nut transmission is a pair of steel screws with a trapezoidal or metric thread and a nut. This type of transmission is a transmission with sliding friction and in practice, in turn, has several varieties.
- Construction stud and nut. Most a budget option... The construction hairpin is generally not intended for use in machine tool building, the technical process of its manufacture is aimed at use in the construction industry, as a result of which given view transmission has the most complete set of disadvantages - high error, low straightness, low load characteristics, low wear resistance, high friction, etc. However, it is still used in DIY-machines, made for educational purposes, due to the low cost. If you decide by all means to save on gear and put a construction pin, be sure to consider replacing it with a trapezoidal screw or ball screw! Most likely, the machine on a construction hairpin will not meet your expectations.
- Drive screw with trapezoidal or rectangular thread. A trapezoidal screw is the most common type of transmission in machine tools in the last century and up to the present. Trapezoidal screws are produced by their different types structural carbon steels by threading or rolling a steel bar. Knurled screws have significantly better performance than threaded screws. The widespread use of trapezoidal screws is due to their wide range, the availability on the market of screws of different accuracy classes, from C10 to C3. The screw nut is made of wear-resistant materials such as polyamides (caprolon, nylon), teflon, and bronze. Correctly designed and manufactured trapezoidal gears are characterized by high wear resistance, because friction occurs with low pressure (due to the relatively large friction surface). On many still working Soviet-made machines, pairs have been standing since the release of the machine, and have not changed for 30-40 years. It is also possible to use split nuts on such lead screws, which allows you to adjust the tightness by compressing the nut and choose the backlash that appears over time. Of the minuses, it is worth noting, oddly enough, the simplicity of making the screw, which automatically means the presence of many manufacturers, with a very wide range of quality indicators. Low-cost series of screws are made from # 45 steel without surface hardening, which can lead to a violation of the straightness of the screw (in other words, small-diameter screws are soft and often bend during transportation). The pluses and minuses at the same time include high friction in the transmission. On the one hand, this reduces efficiency; a more powerful motor is required to rotate the propeller. On the other hand, friction somewhat dampens the rotational vibrations of the screw, which can be useful in the case of using stepper motors (see resonance of stepper motors). This effect, however, it is manifested rather weakly, and other methods are needed to deal with the resonance. Summing up, we can say that the trapezoidal screw has not yet lost its significance as a transmission of a CNC machine tool and is successfully used in machine tools of all classes.
- Ball screw() Ball screw, or ball screw (also called "ball screw"), is currently the de facto standard in the construction of CNC machines. Induction hardened and then ground steel treadmill screw and specially fitted nut with balls circulating inside. As the screw rotates, the nuts roll on the treadmills, transmitting force to the nut body. This transfer is different high precision, high efficiency (80, 90% and more) and resource. Ball screws are more often used in CNC machines, since its use allows the use of motors of lower power (such significant breakaway forces are not required as in the case of a screw-nut transmission). Ball screws are supplied as a complete pair, does not require adjusting the nut and often does not require machining the ends for installation in the supports - this is done by the manufacturer, i.e. Ball screws are often plug and play, while in the case of trapezoidal screws, nuts and screws are often made in different places and may require careful adjustment, without which gaps, backlash, increased friction, wear, etc. can occur. The ball screw tolerates sawdust, dust and lack of lubrication worse than the screw-nut transmission; adjacent balls in the channel rotate in opposite direction... Often required additional protection screw using corrugated materials. Ball screws, as well as a trapezoidal screw, have length restrictions - a screw that is too long sags under its own weight and when the screw rotates (the speed of rotation of the screw with a pitch of 5 mm in gantry machines reaches 10-15 rps and higher) behaves like a jump rope, from which the machine vibrates, and the units that fix the screw experience shock loads, their resource is rapidly decreasing, in seats gaps appear, which in turn increases the vibration of the machine and reduces the quality of the products. Experience shows that the ratio of the ball screw diameter to its length should not be less than 0.022, and it is also not recommended to exceed the screw length of 2000 mm. To eliminate the "skipping rope" effect, structures with a fixed screw and a rotating nut are used, but such assemblies, as a rule, are much more expensive and more difficult to manufacture, and also require space, which is not always possible to implement on compact portals. If you plan to sometimes turn off the drive motors and work on the machine in manual mode, then it is better not to use a ball screw - a transmission without self-braking can give you a lot of trouble. For the types of ball screws and their features, see the main article.
Gear
Gear transmission used in CNC machines are of 2 types 
How to choose a gear for a CNC machine
To select gear for CNC machine, the choice should be based on those characteristics that are most critical for your machine. Screw-nut transmissions are used where there are no high requirements for accuracy and speed of movements, if self-braking is required from the transmission, as well as in case of severe budget constraints. Ball screws have the widest range of applications, you can buy ball screws with the required accuracy class, pitch, the ability to create a preload and without it. The only case when the ball screw cannot be used is if self-braking is required from the transmission, but if we are talking about braking the transmission for safety purposes (holding the spindle head), then the issue is solved by using an electromagnetic brake on the motor, counterweight, etc. Rack and belt are used in machines with a large working field - from 1.5 square meters and more - primarily to achieve high cutting speed and idle movements. On machines of this size, the goal is not to achieve an accuracy of tens of microns, 0.2-0.3 mm in most cases is more than enough, therefore, the elongation of the belt and the accuracy of the rack and pinion are not an obstacle to their use.
So, if you have a large cutting machine, you should choose a rack or belt drive. If you have a desktop milling and engraving machine for educational or hobby purposes, a screw-nut gear is suitable for you. If you are building a medium format machine for business, for manufacturing, the best choice there will be ball screws. After choosing the type, you should decide on the specific transmission parameters.
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Ball screw- a "screw-nut" transmission that converts the rotational movement of the screw, transmitted to it by the shaft of a stepping motor or servo drive, into the translational movement of a nut fixed on / in a table or spindle box. Initially intended for use in high-precision equipment, but in fact it serves as the basis for constructing kinematic diagrams of controlled axes in 90% of CNC machines created today, regardless of accuracy requirements.
The advantages of ball screws over other types of gears:
- high accuracy of linear movements;
- The efficiency reaches 98%;
- long service life;
- in ball screws, in contrast to gear pairs, a preload is created according to the required class;
- the possibility of using motors of lower power due to the fact that the ball screw does not require the application of increased force to transfer the table or spindle box from a state of rest to a state of motion.
Disadvantages: they are afraid of dirt and dust, length restrictions (due to the danger of the screw sagging, which leads to deformation of the attachment points and accelerated wear of the nut), increased sensitivity to vibrations.
Ball screw classification
Ball screws are classified according to several criteria.
Lead screw manufacturing technology. On rolled screws, the groove is applied by cold rolling. This method is cheaper, but it is only suitable for products of the middle class of accuracy. On ground screws, a groove is cut before heat treatment, and then grinded. It turns out more expensive, but more accurate.
Nut type. There are flanged and round, inside each type they are divided into single and double.
Ball return mechanism type. External recirculation - balls return to working area through a tube located outside the nut body. The return cycle is from 1.5 to 5.5 turns of the screw. Internal recirculation - Ball adapters are carved into the inner profile of the nut at each turn. The return cycle is one revolution. End return system - the ball travels a full path along all the turns inside the nut. Used in coarse pitch gears.
The pitch of the screw is the basic criterion for choosing a gear for solving specific problems. Ball screws with a small pitch are used in low-speed machines, they are characterized by a high resource and high load capacity. Increasing the step leads to a decrease in the ability to take high loads, but increases the speed of movement.