Lead screws and nuts for cnc. Do-it-yourself running nut for cnc. Lead screws and nuts

An overview of a specific product: a set of ball screws of the SFU1605-1000 type as elements of a CNC machine tool.
The review will short info about what a ball screw is and how to use it

As a matter of fact, when trying to calculate and build an amateur CNC machine (milling cutter) on my own, I was faced with the fact that we either have expensive components for machines, or not quite what we need. Specifically, there was a problem with the acquisition of a lead screw or ball screw as transmission elements along the axes of the machine.

There are the following types of gears for CNC:

1. belt used together with gears mainly for lasers, since the laser has a light "head"
2. toothed... These are spur or helical gear racks and gears for moving along them.
3. lead screws there are T8 type (mainly used in 3D printers and other small-sized machines), TRR type, for example TRR12-3 with a POM nut (plastic).
4. ball screws- this is a screw and a nut to it. The nut has special bearings that move along a channel inside the nut.

As a rule, they are chosen taking into account the loading (mass of the portal / axis to be moved) and the influence of the backlash. In ball screws, the backlash is less due to bearings, they are considered more accurate and preferable, but at the same time they are quite expensive for homemade products.

Quote from Wiki:

A helical gear is a mechanical gear that converts rotary motion into translational motion, or vice versa. V general case it consists of a screw and a nut…. one of the main types: rolling ball screw (ball screw).

Ball screw(hereinafter referred to as the ball screw) is a more reliable analogue of the lead screw, but instead of a brass nut (or plastic as for screws of the TRR-12-3 type, as in my old project), there is a special nut with balls that mesh with the ball screw, choose all the backlash and at the same time reduce friction. For self assembly a CNC machine or a 3D printer on a ball screw, you will need a ball screw, a nut to it, a coupling to the engine and outboard bearings.

Here's a small render from the internet. You can clearly see how the balls are distributed over the screw. Similar to T8, the ball screw has several threads.

For the CNC machine, two sets of ball screws for 1000 mm were needed for the Y axis, and 600 mm for the X axis.
The ball screw was received by courier mail. This is not an expensive option, given the weight of the package (about 8 kg).

The packaging is a long narrow box, inside the carton there is a synthetic bag type packaging, a very durable material. Unpack carefully. Inside, everyone is familiar with bubble-wrap, that is, a bubble wrap that protects the product from mechanical stress.

We remove the film. There were three sets of ball screws in the package: screw + nut, different sizes... Two sets are designed to move the gantry of the machine along the Y-axis, the third is a short set for the X-axis.

All kits are wrapped in a green inhibitor film that prevents moisture from entering. Plus there is a fair amount of grease on the surface of the product.

In this set, I paid extra for the termination of one set at 600 mm (it turned out to be cheaper). I ordered the machined separately from the same seller (he has such a service in the catalog), it cost 1 dollar for each end of the screw. A good option for those who take screws in a specific size.

This is what the "ending" is. This is turning the screw 16.05 mm to a diameter of 12 mm for installation in the outboard bearing, then the threaded part to fix the screw, then turning up to 10 mm to clamp the end into the elastic coupling of the motor

The package arrived safe and sound, Express delivery this is not Russian post. Applied a ruler to different places to find the curvature. I did not find the ball screws are even. The rest will show installation and usage.

Photo of the threaded part of the screws

Kits appearance

And further. The nuts came already screwed onto the screw ... The balls are packed inside, there is grease. When ordering, ask for spare balls, at least a few.

Next, we begin to check the dimensions of the screws. Short by 600 mm. That is, these 600 include a threaded portion on both sides. The actual travel along the machine axes will be less.
note that in the lot the size is indicated for the ball screw together with the thread and turned ends, that is, the working stroke along the ball screw will be less than its length! Specifically, it is 65 mm less.

Second and third ball screws 1000 mm

Threaded diameters, respectively 1605

seats for bearings BK12 and BF12 10 and 12 mm, respectively.

And on the other side under the bearing. The diameter of the SFU1605 nut itself is 28 mm.

If you remove the plastic plug from the nut, you can service the ball screws, lubricate or change the balls. I check that everything is in stock))))

As a matter of fact, you can remove the nut, wipe it, relubricate it, load the balls back. The plastic cover is fastened with a 2.5 hexagon socket head screw (you can see it at the top).

To install the ball screws in the machine, you will need outboard bearings of the BK12 + BF12 (straight) or FK12 + FF12 (flanged) type, an elastic coupling 6.35 * 10mm for connecting to a NEMA23 motor on one side (6.35mm) and to the end of the ball screw on the other (10 mm) ).

Exterior view of the complete axle set: bearings BK12, BF12, circlip, nut for fixing the screw, nut holder SFU1605, coupling for the motor and the screw itself with nut.

Ball screw sizes for those who are going to purchase or design machine mechanics

And separately for SFU1605

External view of the SFU1605 nut

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 trapezoidal thread on a screw, 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 is based in the machine 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 centrifugal 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.

When choosing milling machine(CNC Router) CNC decide:

1. what material you are going to work with. The requirements for the rigidity of the structure of the milling machine and its type depend on this.

For example, a CNC machine made of plywood will only allow processing wood (including plywood) and plastics (including composite materials - plastic with foil).

On a milling machine made of aluminum, it is already possible to process non-ferrous metal blanks, and the speed of processing wood products will also increase.

Aluminum milling machines are not suitable for steel machining, here already massive machines with a cast iron bed are needed, and the processing of non-ferrous metals on such milling machines will be more efficient.

2.with the size of the workpieces and the size of the working field of the milling machine. This defines the requirements for the mechanics of a CNC machine tool.

When choosing a machine, pay attention to studying the mechanics of the machine, the capabilities of the machine depend on its choice, and it is impossible to replace it without a significant redesign of the structure!

Mechanics CNC milling plywood and aluminum looms are often the same. More details below.

But the larger the size of the working area of ​​the machine, the more rigid and expensive linear guides will be required for its assembly.

When choosing machines for solving the problems of manufacturing tall parts with large differences in heights, there is a common misconception that it is enough to choose a machine with a large working stroke along the Z axis. But even with a large stroke along the Z axis, it is impossible to manufacture a part with steep slopes, if the height of the part is greater than the working length of the cutter, that is, more than 50mm.

Consider the device of a milling machine and options for selection using the example of CNC machines of the Modelist series.

A) Choosing a CNC machine design

There are two options for building CNC machines:

1) constructions with movable table, picture 1.
2) construction with movable portal, Figure 2.

Picture 1Sliding table milling machine

Advantages the design of the machine with a movable table is ease of implementation, high rigidity of the machine due to the fact that the portal is stationary and fixed to the frame (base) of the machine.

Flaw- large dimensions, in comparison with the design with a movable portal, and the impossibility of processing heavy parts due to the fact that the movable table carries the part. This design quite suitable for processing wood and plastics, that is, lightweight materials.

picture 2 Milling machine with a movable portal (gantry machine)

Advantages designs of a milling machine with a movable portal:

Rigid table that can support a large workpiece weight,

Unlimited workpiece length,

Compactness,

Possibility of machine execution without a table (for example, for mounting a rotary axis).

Disadvantages:

Less rigidity of the structure.

The need to use more rigid (and expensive) guides (due to the fact that the portal "hangs" on the guides, and is not fixed on a rigid machine bed, as in a design with a movable table).

B) Choosing the Mechanics of a CNC Milling Machine

The mechanics are presented (see figures in Fig. 1, Fig. 2 and Fig. 3):

3 - guide holders

4 - linear bearings or sleeve bushings

5 - support bearings (for fastening the lead screws)

6 - lead screws

10 - clutch connecting the shaft of the lead screw with the shaft of stepper motors (SM)

12 - running nut

picture 3

Selection of the linear motion system of the milling machine (guides - linear bearings, lead screw - lead nut).

The following can be used as guides:

1) roller guideways, figure 4.5

Figure 4

Figure 5

This type of guide has found its way into the designs of amateur lasers and machine tools from the furniture industry, Figure 6

The disadvantage is low load capacity and low resource, since they were not originally intended for use in machines with big amount displacement and high loads, low strength aluminum profile guides leads to collapse, Figure 5 and, as a consequence, irreparable backlash, which makes further use of the machine unsuitable.

Another version of roller guides, Figure 7, is also not suitable for high loads and therefore is used only in laser machines.

Figure 7

2) round guides, represent a steel shaft made of high-quality wear-resistant bearing steel with a ground surface, surface hardening and hard chrome plating, shown under number 2 in Figure 2.

it optimal solution for amateur constructions, because cylindrical guides have sufficient rigidity for machining soft materials at small size cnc machine at a relatively low cost. Below is a table for selecting the diameter of cylindrical guides, depending on maximum length and the minimum amount of deflection.

Some chinese manufacturers of cheap machine tools install the guides are not of sufficient diameter, which leads to a decrease in accuracy, for example, when using on a machine made of aluminum at a working length of 400 mm, guides with a diameter of 16 mm will lead to a deflection in the center under their own weight by 0.3..0.5 mm (depending on the weight of the portal).

At the right choice the diameter of the shaft, the design of machine tools with their use is quite strong, the large weight of the shafts gives the structure good stability, the overall rigidity of the structure. On machines over a meter in size, the use of round guides requires a significant increase in diameter to maintain minimal deflection, which makes the use of round guides unreasonably expensive and difficult.

Axis length	Plywood machine	Aluminum woodworking machine	Aluminum machine for working on aluminum
200mm	12	12	16	12
300mm	16	16	20	16
400mm	16	20	20	16
600mm	20	25	30	16
900mm	25	30	35	16

3) profile rail guides
Polished shafts are replaced by profile guides on large-sized machines. The use of support along the entire length of the guide allows the use of guides of significantly smaller diameters. But the use of this type of guides imposes high requirements on the rigidity of the machine's supporting frame, since the beds made of sheet duralumin or sheet steel are not rigid in themselves. The small diameter of the rail guides requires the use of thick-walled steel profiled pipes or structural aluminum profiles in the construction of the machine. large section to obtain the required rigidity and bearing capacity machine frame.
The use of a special shape of the profile rail allows for better wear resistance in comparison with other types of guides.

Figure 8

4) Cylindrical guides on the support
Cylindrical guides on the support are a cheaper analogue of profile guides.
As well as profile ones, they do not need to be used in the machine frame. sheet materials, and the profile of a pipe of large cross-section.

Advantages - no deflection and no spring effect. The price is twice as high as for cylindrical guides. Their use is justified when the travel length is over 500mm.

picture 9 Cylindrical guides on the support

Moving can be done both on bushings(sliding friction) - fig. 10 on the left, and using linear bearings(rolling friction)- rice. 10 on the right.

picture 10 Bushings and Linear Bearings

The disadvantage of sliding bushings is the wear of the bushings, leading to the appearance of backlash, and an increased effort to overcome sliding friction, which requires the use of more powerful and expensive stepper motors (SM). Their advantage is their low price.

V recent times the price of linear bearings has come down to such an extent that it is economically feasible to select them even in inexpensive hobby designs. The advantage of linear bearings is a lower coefficient of friction compared to sleeve bushings, and, accordingly, a large part of the power stepper motors goes to useful movements, and not to fight friction, which makes it possible to use motors of lower power.

To convert rotary motion into translational motion on a CNC machine, it is necessary to use a helical gear ( lead screw ). Due to the rotation of the screw, the nut moves translationally. In milling and engraving machines can be used screw gears and rolling screw drives .

The disadvantage of the sliding screw transmission is a rather large friction, which limits its use at high speeds and leads to wear of the nut.

Sliding screw gears:

1) metric screw. The advantage of a metric screw is its low price. Disadvantages - low accuracy, small step and low speed moving. Maximum speed of the propeller (velocity mm`s per min) based on the maximum speed of the stepper motor (600 rpm). The best drivers will retain torque up to 900rpm. At this speed of rotation, linear movement can be obtained:

For M8 screw (thread pitch 1.25mm) - no more than 750mm / min,

For screw M10 (thread pitch 1.5mm) - 900mm / min,

For M12 screw (thread pitch 1.75mm) - 1050mm / min,

For screw М14 (thread pitch 2.00mm) - 1200mm / min.

At maximum speed, the motor will have about 30-40% of its originally specified torque, and this mode is used exclusively for idle movements.

When working at such a low feed, increased consumption for cutters, after a few hours of operation, carbon deposits form on the cutters.

2) trapezoidal screw... In the twentieth century, it occupied a leading position in machine tools for metalworking, before the advent of ball screws. Dignity - high accuracy, large thread pitch, and, consequently, high travel speed. Pay attention to the type of processing than the smoother and Smooth surface screw, the longer the service life of the screw-nut transmission. Rolled screws take precedence over threaded screws. The disadvantages of a trapezoidal screw-nut transmission are a rather high price in comparison with a metric screw, sliding friction requires the use of stepper motors of sufficiently high power. The most common screws are TR10x2 (diameter 10mm, thread pitch 2mm), TR12x3 (diameter 12mm, thread pitch 3mm) and TR16x4 (diameter 16mm, thread pitch 4mm). In machines, the marking of such a transmission is TR10x2, TR12x3, TR12x4, TR16x4

Rolling screw drives:

Ball screw (ball screw). In the Ball Screw, sliding friction is replaced by rolling friction. To achieve this, in the ball screw, the screw and nut are separated by balls that roll in the grooves of the screw thread. The balls are recirculated by means of return channels that run parallel to the screw axis.

Figure 12

The ball screw provides the ability to work under high loads, good smoothness, significantly increased resource (durability) due to reduced friction and lubrication, increased efficiency (up to 90%) due to less friction. It is able to work at high speeds, provides high positioning accuracy, high rigidity and no backlash. That is, machines using ball screws have a significantly longer resource, but they have a higher price. The machines are labeled SFU1605, SFU1610, SFU2005, SFU2010, where SFU is a single nut, DFU is a double nut, the first two digits are the screw diameter, the second two are the thread pitch.

Lead screw the milling machine can be mounted as follows:

1) Design with one support bearing. Fastening is carried out on one side of the screw with a nut to the support bearing. The second side of the screw is attached to the stepper motor shaft through a rigid coupling. Advantages - simplicity of design, disadvantage - increased load on the stepper motor bearing.

2) Design with two support bearings in the spacer. The design uses two support bearings on the inner sides of the portal. The design flaw is a more complex implementation compared to option 1). The advantage is less vibration if the screw is not perfectly flat.

3) Design with two support bearings in an interference fit. The design uses two support bearings on external parties portal. Advantages - the screw is not deformed, in contrast to the second option. The disadvantage is the more complex implementation of the construction, compared to the first and second options.

Travel nuts there are:

Backlash-free bronze. The advantage of such nuts is durability. Disadvantages - difficult to manufacture (as a result - high price) and have a high coefficient of friction in comparison with caprolon nuts.

Caprolon backlash-free. Currently, caprolon is widely used and increasingly replaces metal in professional structures. The running nut made of graphite-filled caprolon has a significantly lower coefficient of friction in comparison with the same bronze.

figure 14 Lead nut made of graphite-filled caprolon

In the ball screw nut (ball screw) sliding friction is replaced by rolling friction. Advantages - low friction, the ability to work on high speeds rotation. The disadvantage is the high price.

Coupling selection

1) connection using a rigid coupling. Advantages: rigid couplings transmit more torque from shaft to shaft, there is no backlash under heavy loads. Disadvantages: require accurate installation, since this coupling does not compensate for misalignment and misalignment of the shafts.

2) connection using a bellows (split) coupling. The advantage of using a bellows coupling is that its use allows you to compensate for misalignment of the installation of the travel shaft and the axis of the stepper motor up to 0.2 mm and misalignment of up to 2.5 degrees, as a result of which there is less load on the stepper motor bearing and a longer service life of the stepper motor. It also dampens vibrations that occur.

3) connection using a cam clutch. Advantages: allows to damp the arising vibrations, transmit more torque from the shaft to the shaft, in comparison with the split one. Disadvantages: less compensation for misalignment, misalignment of the installation of the travel shaft and the axis of the stepper motor up to 0.1 mm and misalignment of up to 1.0 degrees.

C) Choice of electronics

The electronics are shown (see Fig. 1 and 2):

7 - stepper motor controller

8 - power supply unit of the stepper motor controller

11 - stepper motors

There are 4-wire, 6-wire and 8-wire stepper motors ... All of them can be used. Most modern controllers use a four-wire connection. The rest of the conductors are not used.

When choosing a machine, it is important that stepper motor there was enough power to move the working tool without losing steps, that is, without gaps. How more step the screw threads are the more powerful motors will be required. Usually, the higher the motor current, the higher its torque (power).

Many motors have 8 terminals for each half-winding separately - this allows you to connect a motor with series-connected windings or in parallel. With parallel-connected windings, you will need a driver with twice the current than with series-connected windings, but half the voltage will be sufficient.

In the case of a serial, on the contrary - to achieve the rated torque, it will take half the current, but to reach the maximum speed, twice the voltage.

The amount of movement per step is typically 1.8 degrees.

For 1.8, 200 steps are obtained per full revolution. Accordingly, to calculate the value, the number of steps per mm ( Step per mm) we use the formula: number of steps per revolution / screw pitch. For a screw with a pitch of 2mm, we get: 200/2 = 100 steps / mm.

Controller selection

1) DSP controllers. Advantages - the ability to select ports (LPT, USB, Ethernet) and independence of the STEP and DIR signal frequencies from operation operating system... Disadvantages - high price (from 10,000 rubles).

2) Controllers from Chinese manufacturers for amateur machine tools. Advantages - low price (from 2500 rubles). Disadvantage - increased requirements for the stability of the operating system, requires compliance certain rules settings, preferably using a dedicated computer, only LPT versions are available.

3) Amateur designs of discrete controllers. Low price Chinese controllers are supplanting amateur designs.

Chinese controllers are most widely used in amateur machine tool designs.

Power supply selection

Nema17 motors require a power supply unit of at least 150W

Nema23 motors require at least 200W power supply