There was a time before CNC when a machinist had to rely on a tactile understanding of the power required to accomplish a cut. To feed a tool into a workpiece required jogging the machine table with a crank or pulling the feed handle and plunging by hand. The force required to make a cut was noted by the operator, the feedback loop was internalized and necessary adjustments could be made on the fly.

Today, machinists have lost this tangible experience when running CNC machining centers. This observation may give the impression that manual machinists were more in-touch with the trade, but the fact of the matter is that CNC machinists simply lean on a different set of tools to understand the load their machine is facing. In a recent webinar from Makino, product specialist David Ward explained how properly interpreting horsepower and torque charts can help users leverage the full power of their machine tools.

Many variables affect milling productivity, including speeds and feeds, toolingand the designed capabilities of the CNC machine. An operator must keep these variables at the forefront of their mind when programming a milling operation. However, no two CNC machines are exactly alike. And yet, many CAM programmers will stick to standard feeds and speeds for a cutting tool without regard for the specific power of the spindle they are working with, Ward says. Understanding the relationship between spindle power and speed can help users improve productivity by choosing speeds and feeds that bring the machine closer to the working limits of its horsepower and torque – enabling milling operations to be completed faster than previously realized.

For many operators, consulting the horsepower and torque charts is an exception rather than a habit, Ward says. In part, this is because many CAM systems automate this calculation. Additionally, it is easier to prevent machining problems by programming conservatively. Not to mention that many operators simply rely on their ear when diagnosing the success of an operation. What is left on the proverbial table might be a spindle motor that is running 20-40% under its rated duty cycle — a drastic loss of productivity in some scenarios.

Ultimately, it is not necessary to consult a horsepower chart for every milling operation. Running small operations like tapping, drilling or finishing passes do not require the full capacity of the machine and applying too much power can be detrimental in these operations – resulting in broken tooling or damaged parts. Roughing operations, where large amounts of material are being removed, are the best candidates for improving spindle horsepower utilization. 

The most common way to keep an eye on spindle horsepower usage is the load meter. Found on every CNC machine,it provides a real-time display of load that reflects the power usage relative to its continuous duty rating, noted as 100%. These electric motors, which power both direct-drive and belt-drive spindles have a common nomenclature for duty rating. This duty rating determines how long the available power of a motor can be used before the thermal limits are reached.

The International Electrotechnical Commission (IEC) designates eight duty cycle ratings that range from S1 to S8. The low end of the scale, S1, expresses a continuous duty rating. For many CNC machines, this is the power that the machine can maintain indefinitely and is represented on the load meter as 100%. Moving up the scale, S2 is a short-time duty rating. This rating implies that the power is required only for a short amount of time in relation to the total cycle time. For CNC machine tools, the highest end of the duty rating scale is typically S3. This represents the upper threshold of the power chart, and this amount of power can be used for a very short period of time.

In the chart below, the power and spindle speed relationship is plotted along a series of lines. Depending on the manufacturer of CNC machine, just one or all three duty ratings are plotted. It can be observed that as the rpm changes, the power rating of the motor changes. However, parameters like feed rate, depth of cut and material type also determine how fast milling can be performed before a catastrophic failure occurs. So when does it make sense to check the chart? “There is no reason to spend the time calculating horsepower on smaller tools like drills and reamers,” Ward says. “Focus efforts on the larger roughing tools. But even narrowing it down to roughing tools, this is relative to your machine size.”

The reality is that the upper limits of a machines horsepower will be used during rough work when the maximum amount of material is removed at a time for PVD Coated Insert the highest degree of productivity. 

Calculations Behind the Power

When a tool comes in contact with a workpiece, the spindle consumes power as the cutting edge engages with and removes material. The amount of material removed is expressed as the Material Removal Rate (MRR). As the MRR increases, the power usage, measured in kW or HP increases. As detailed in the formulas below, power usage, which is measured in kW or hp, increases along with the material removal rate (MRR). MRR depends on the cutting depth, cutting width and feed rate.

Determining horsepower consumption requires the “K” factor: a power constant that represents the number of cubic inches of metal per minute that can be removed by one horsepower. When consulting a chart, “K” factors vary depending on the hardness of the material. K VCMT Insert factor data can be found the chart on the right.

Width of cut=Ae
Depth of cut=Ap
Feed rate=Vf

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　During NC programming, the programmer must determine the cutting dosage for each process and write it in the program as an instruction. Cutting dosage includes cutting speed, back engagement and feed speed. Different cutting dosages are required for different processing methods.

1. the selection principle of cutting dosage

When roughing, it is generally based on improving productivity, but economical and processing costs should also be considered. In the case of semi-finishing and finishing, cutting efficiency, economy and processing cost should be considered on the premise of ensuring processing quality. The specific values should be based on the machine manual, the cutting dosage manual, and the experience.

Starting from the durability of the tool, the selection order of cutting dosage is. first determine the back engagement, then determine the feed, and finally determine the cutting speed.

2. Determination of back engagement

The back engagement is determined by the stiffness of the machine tool, the workpiece and the tool. When the stiffness is allowed, the back engagement should be equal to the stock amount of the workpiece as much as possible, which can reduce the number of passes and increase the production efficiency.

How to determine the principle of back engagement.

(1) When the surface roughness value of the workpiece is required to be Ra12.5μm~25μm, if the stock amount of CNC machining is less than 5mm~6mm, the rough processing can meet the requirement once. However, when the margin is large, the rigidity of the process system is poor, or the machine power is insufficient, the feed can be divided into multiple times.

(2) When the surface roughness value of the workpiece is required to be Ra3.2μm~12.5μm, it can be divided into two steps of roughing and semi-finishing. The back engagement during roughing is the same as before. After roughing, leave a balance of 0.5mm~1.0mm and cut it off during semi-finishing.

(3) When the surface roughness value of the workpiece is required to be Ra0.8μm~3.2μm, it can be divided into three steps. roughing, semi-finishing and finishing. The back engagement during semi-finishing takes 1.5mm~2mm. The back engagement is 0.3mm~0.5mm during finishing.

3. the determination of the feed

The feed is mainly based on the machining accuracy and surface roughness requirements of the part and the material of the tool and workpiece. The maximum feed speed is limited by the stiffness of the machine and the performance of the feed system.

How to determine the feed speed.

1) When the quality requirements of the workpiece can be guaranteed, in order to improve production efficiency, a higher feed speed can be selected. Generally, it is selected in the range of 100 to 200 m/min.

2) When cutting or machining deep holes or machining with high speed steel tools, it is advisable to choose a lower feed speed, generally in the range of 20 to 50 m/min.

3) When the processing accuracy and surface roughness are high, the feed speed should be selected to be smaller, generally in the range of 20 to 50 m/min.

4) When the tool is taking the idle stroke, especially when the distance is “return to zero”, the highest feed speed set by the machine numerical control system can be selected.

4. the determination of the spindle speed

The spindle speed should be selected based on the allowed cutting speed and the workpiece (or tool) diameter. Its calculation formula is.

n=1000*v/π*D

v—-cutting speed, in m/min, determined by the durability of the tool;

N—-spindle speed, the unit is r/min;

D—-diameter of work piece or tool diameter in mm.

The calculated spindle speedn is finally selected according to the machine manual to have a speed that is relatively close to the machine.

In short, the specific value of cutting dosage should be determined by analogy based on machine performance, related manuals and practical experience. At the same time, the spindle speed, depth of cut and feed speed can be adapted to each other to form the optimal cutting dosage.

5. the reference formula

1) back engagement (ap)

The vertical distance between the machined surface and the surface to be machined is called the back engagement. The back engagement is the engagement measured by the cutting point base point and perpendicular to the working plane. It is the depth of the turning tool into the workpiece for each feed, so it is called depth of cut. According to this definition, as in the horizontal to cylindrical lathe, its back engagement can be calculated as follows.

Ap=(dw-dm)/2

In the formula ap—-back engagement(mm);

Dw—-surface diameter of the workpiece to be machined (mm);

Dm—-The surface diameter (mm) of the workpiece has been machined.

Example 1. The diameter of the surface to be machined is known to be Φ95mm; now the feed car is Φ90mm in diameter and seek back engagement.

Solution. ap=(dw-dm)/2=(95-90)/2=2.5mm

2) feed (f)

The relative displacement of the tool and the workpiece in the direction of feed movement per revolution of the workpiece slot milling cutters or tool. According to the direction of the feed, it is divided into horizontal feed and transverse feed. The horizontal feed refers to the feed along the direction of the lathe bed rail, and the transverse feed refers to the feed perpendicular to the direction of the lathe bed rail.

Feed speed v f is the instantaneous velocity at which the selected point on the cutting edge moves relative to the workpiece feed.

Vf=f*n

Where vf—-feed speed(mm/s);

N—-spindle speed(r/s);

f—-feed(mm /s).

3) cutting speed (vc)

The instantaneous velocity of the main motion of the selected point on the cutting edge relative to the workpiece.

Vc=( π*dw*n)/1000

In the formula vc—-cutting speed (m/min);

Dw—-surface diameter of the workpiece to be machined (mm);

n—-Workpiece speed (r/min).

In tungsten carbide inserts the calculation, the maximum cutting speed should be taken as the standard. For example, when the machine is used, the value of the surface diameter to be machined is calculated because the speed is the highest and the tool wears the fastest.

Example 2. The outer diameter of the workpiece with a diameter of Φ60mm, the selected lathespindle speed is 600r/min, and vc

Solution. v c=( π*d*w*n)/1000=3.14x60x600/1000=113 m/min

In actual production, it is often known as the diameter of work piece. According to the workpiece material, tool material and processing requirements, the cutting speed is selected, and then the cutting speed is converted into lathespindle speed, in order to adjust lathe, the following formula is obtained.

n=( 1000*vc)/π*dw

Example 3: On the CA6140 horizontal lathe machine Φ260mm pulley outer circle, select vc is 90m / min, find n.

Solution: n=( 1000*vc)/ π*dw=(1000×90)/ (3.14×260)=110r/min

After calculating the lathespindle speed, the value close to the nameplate should be selected, that is, n=100r/min is selected as the actual speed of lathe.

6.?summary

The three factors of cutting dosage are the general term for cutting speed (vc) , feed (f),feed speed (vf), and back engagement (ap).

1.back engagement ap(mm) 

ap=（dw-dm) / 2

2.feed f(mm/r)

vf=f*n

3.cutting speedvc(m/min)

vc=( π*dw*n)/1000

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The BySprint Pro 3015 gun drilling inserts gun drilling inserts high speed laser-cutting system combines an efficient Bylaser 4.4-kW laser source and high machine dynamics for cost-effective, productive laser cutting, says manufacturer Bystronic. The machine’s economic efficiency largely stems from the ByLaser 4400 laser source’s use of wear-free, solid state power supplies and a magnetic bearing turbine. In addition to the efficiency of the laser source, lower electrical power consumption results from automatic switch-off of the excitation when the protective door is open or the machine is on standby.

The machine is also equipped for compressed air cutting. With a 3.75" cutting head, cut times on thin sheet metal can be reduced by 15 to 40 percent compared with a 5" counterpart. While the laser cutter is designed for processing thin sheet, the power of the laser source enables it to cut gravity turning inserts thick material as well, the company says.

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Ceratizit’s FreeTurn tooling is designed to be used with its High Dynamic Turning (HDT) method, which is said to enable roughing, finishing, contour turning, face turning and longitudinal turning with a single tool. FreeTurn tooling features a surface milling cutters multi-sided insert at the cutting end of the slim shank. The insert, which screws into place, can have several cutting edges with different properties. This is said to allow for different angle points, corner radii or chipbreakers as well as different coatings and cutting materials. The tool can therefore be adapted to do specific machining requirements and provide savings in tool change times, tool magazine loadouts and overall tool costs, according to the company.

In the HDT method, the milling spindle is used to produce the corresponding approach angle to the workpiece. The use of the spindle drive in conjunction with the slim, axial design of FreeTurn tools is said to create a degree of freedom of 360 degrees without the risk of collision, the company says. Due to the rotation around its own tool axis, the cutting edge can be changed Carbide Turning Inserts without interrupting the process. Additionally, the angle of approach is freely variable at any time and can even be changed while cutting, enabling flexible contouring and optimizing chip breaking for higher feed rates and longer tool life.

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When a toolholder expands, it cannot make full contact with a machine tool’s spindle, causing vibration, chatter, poor tolerances, non-repeatability, poor finishes, shortened tool life, excessive spindle gun drilling inserts gun drilling inserts wear and tear, runout, and shallow depths of cut, among other problems. Hansen Engineering Co. (Harbor City, California) remedied its production issues by converting to high-torque retention knobs from JM Performance Products Inc. (Fairport Harbor, Ohio). This reportedly increased the aerospace engineering shop’s productivity by 15 percent and decreased downtime.

Established in 1962, HEC is an approved Air Transport Association supplier of precision-machined multi-axis parts and major structural assemblies. The shop specializes in complex MDI surface geometry, statistical process control and electronic probing of part surfaces. The majority of its high-speed CNC machines operate within a 10,000- to 15,000-rpm range, cutting large blocks of titanium, stainless steel and aluminum for an array of aerospace products.

Over time, the company’s everyday machining rod peeling inserts operations began to present increased issues. For example, toolholders were getting stuck in the spindle due to deformation factors evidenced by wear marks at the top and bottom of the holders; fretting appeared on contact surfaces; and increasingly long cycles developed. These persistent problems led to increased production shutdowns to allow the machines to cool off for significant periods of time. Additionally, poor finishing and chatter problems were recurring when roughing aluminum, titanium and stainless steel for forging jobs.

When a standard retention knob is installed in a V-flange toolholder, pressure exerted by thread engagement, coupled with the elastic properties of the steel used to manufacture the toolholders, creates a bulge at the small end of the holder. Once expansion occurs, the holder will not pull all the way into the spindle, and the toolholder cannot make contact with upwards of 70 percent of the spindle surface.

Recognizing the design flaw inherent in CNC V-flange tooling that was responsible for costly CNC milling and boring issues, JMPP designed high-torque retention knobs that could be used in existing toolholders to eliminate the bulge. These patented high-torque retention knobs are longer and reach deeper into the holder’s threaded bore than standard knobs. As a result, all thread engagement occurs in a region of the toolholder where there is a thicker cross section of material to resist deformation.

HEC engineering personnel met with JMPP, a manufacturer of CNC mill spindle-optimization products, to learn how its high-torque retention knobs could work with the shop’s 50-taper V-flange toolholders. Intrigued by their potential, HEC initially bought 25 knobs and properly installed them following calculated torque specifications using a retention-knob socket and torque wrench. Immediately, the shop noticed a 5 percent spindle-load decrease using a 3-inch high-feed insert mill running titanium. The company also installed them on an aluminum forging job that had consistently produced chatter problems. Among the tools tested for this job were a 1.25-inch-diameter knuckle rougher and 2-inch-diameter finisher.

“Again, the results were positive, as the chatter was eliminated, and it produced the best finish we have ever seen on these parts,” says Curtis Sampson, shop leadman. “After that, we bought 50 more pieces and immediately noticed improvements all around the table. We’ve been increasing their use over time ever since.”

According to JMPP President John Stoneback, “By significantly increasing the speed of machines via eliminating the problems that were designed into V-flange tooling, conservative estimate savings of 10 to 15 percent can be achieved. The high-torque knobs transform V-flange tooling into the most cost effective, reliable and precise tooling system available.”

Since these initial tests, HEC has progressively converted to high-torque retention knobs in its 14 CNC machines, requiring approximately 120 per machine. Jose Campos, tool crib buyer, says the 15 percent increase in productivity has led to the same rate of decreased downtime due to less change-out of tool cutters across the board.

Mr. Campos praises the knobs’ performance on HEC’s latest Mazak five-axis CNC machining center, which features a 160-inch-long dual-shuttle table. Its toolchanger is designed to run a full table of parts while the other is being loaded to eliminate downtime. The Mazak machines used to make a loud noise when changing tools, he says, but now the machines are much quieter when roughing titanium and stainless steel. The high-torque retention knobs also eliminated fretting of the toolholder shank, he says.

Ultimately, the progressive conversion to JMPP’s high-torque retention knobs has allowed HEC to overcome long-standing productivity issues for its entire fleet of high-speed CNC mills. For its demanding aerospace part production, the retention knobs have shown lower spindle loads, the shop says. Additionally, HEC reports reduced power consumption in roughing titanium and stainless steel, and improved overall tool life. Over the long haul, HEC says it has been able to increase speeds and feeds and deliver better cycle times to realize substantial savings.

“I would like to put a word out to the people who are not quite convinced yet: They (the high-torque retention knobs) are worth the small investment,” Mr. Sampson advises. “You may not want to jump in with both feet, but just try them on a job or two, and I am sure that you will notice improvements. In this competitive world we live in today, we can use a product like these retention knobs. They are something that we can just screw in our tools to give us an upper hand on our competitors.”

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