The Essential Role of Accurate Cycle Time Calculation in Your Machine Shop
The ability to compute CNC turning cycle time is essential for successful machine shop operations. It does not only concern figures. It is also about quotes, production planning, and efficiency. A timely estimate on the job supports your competitive pricing. Additionally, it guarantees that you can deliver your work on time.
This instruction manual will be the guide for everything you will require. Firstly, we will introduce the basic formulas for calculating machining time. Subsequently, we will identify the hidden factors that many people overlook. Through precise steps and practical examples, you will acquire the skill of giving accurate estimates for your turning jobs. This ability is extremely important for individuals engaged in manufacturing today.
Building Blocks: Fundamental Formulas for CNC Turning Time Calculations
At its most basic level, calculating machining time is not a complex topic. The key principle revolves around the movement of the tool: dividing the tool path by the speed of the tool. For processes like turning, there is a predetermined equation for the specific process. This is our initial reference point for calculations.
The General Cutting Time Equation
The predominant formula for finding the time taken to execute one cut is:
This formula gives you total cutting time in minutes for one cutting pass. In the event of multiple cutting passes, you shall compute the time for every single pass and then add them up.
Each Variable Explained
Each variable of the formula denotes a critical aspect of the cutting process. To obtain the correct answer, it is recommended to use the appropriate measurement units.
| Variabel | Beschreibung | Metric Unit | Imperial Unit |
|---|---|---|---|
| T | Machining Time | minutes (min) | minutes (min) |
| L | Total Cutting Length | millimeters (mm) | inches (in) |
| f | Feed Rate | mm per revolution (mm/rev) | inches per revolution (inch/rev) |
| N | Spindeldrehzahl | revolutions per minute (RPM) | revolutions per minute (RPM) |
The Total Cutting Length (L) does not only refer to the length of the workpiece but the length of the path taken by the tool, which includes the approach distance and the over-cut amount. This length is known as over-travel.
How to Calculate Spindle Speed (N)
Spindle Speed (N) refers to the rotation speed of the part in the lathe. The speed must not be arbitrarily chosen. It is derived from the tool and material that are utilized. The main determinant is the Cutting Speed.
This can be done through the formulas to find Spindle Speed (RPM):
- Metric:
- Imperial:
Cutting Speed (Vc or SFM) is the tool speed along the surface of the material where the tool hits. Harder materials like stainless steel demand that cutting speed is slower. Aluminum, as a soft material, can handle a faster cutting speed. These recommendations are given by the manufacturers of the tooling. This is a vital component of the general equation for multiple processes.
Restricting Formulas to Certain Turning Tasks
The basic equation applies well to only horizontal cuts. Nonetheless, the market parts mostly demand the execution of various types of turning operations. The ability to adjust your estimation for such is essential. It would then give you the precise CNC turning cycle time.
Straight/Longitudinal Turning
This is the most common operation. The tool moves along the length of the part to reduce its diameter. The main formula applies directly here. Remember to calculate this for each pass if multiple cuts are needed.
Facing Operations
During a facing operation, the tool moves from the outside diameter toward the center of the part. Here, the diameter is constantly changing. This presents a challenge for our calculation.
- Simplified Method: For a quick estimate, you can use the average diameter of the face cut in your RPM calculation. This gives a good estimate, though not perfect.
- Constant Surface Speed (CSS) Mode: Most modern CNC lathes use a G96 command for facing. This mode automatically increases the RPM as the tool moves toward the center. It keeps the surface speed constant. Calculating this exactly is complex. A good estimate involves finding the time it takes to travel the radius of the part.
Grooving and Parting-Off
For operations where the tool plunges straight into the part, the feed rate is often given differently. Instead of feed per revolution (mm/rev), it might be in feed per minute (mm/min or IPM).
The formula changes to:
This is due to the fact that the tool moves in a flat line. It takes time based on the feed rate per minute, not the spindle’s revolution.
Taper and Contour Turning
Performing the taper or the contour is not limited to only horizontal tool movement. Again, the length (L) in your formula must be the actual length of the tool’s path along that taper or curve. You can’t use a simple horizontal distance of the feature. You can find this length from your part drawing or CAM system.
Putting It Together: An Example of Step-by-Step Calculation
Let’s go through a real-life example. This will demonstrate how to figure out the CNC turning cycle time for a simple part. The example shows the use of formulas to find the length for the real job.
The Case Study
Consider we have to make a simple pin from 4140 alloy steel.
- Part: A pin with a starting diameter of 50mm and a final diameter of 45mm. The length of the cut is 100mm.
- Parameters:
- Roughing: We will use a CNMG insert. Recommended cutting speed is 180 m/min. Feed rate is 0.25 mm/rev. Depth of cut (DOC) is 2mm.
- Finishing: We will use a different insert for a smooth finish. Recommended cutting speed is 220 m/min. Feed rate is 0.1 mm/rev.
Step 1: Calculate Roughing Time
We have to remove the bulk of the material first. The total material to remove from the diameter is 50mm – 45mm = 5mm. This means we need to remove 2.5mm from the radius.
- Determine Number of Passes: With a 2mm depth of cut, we need two passes. The first pass will remove 2mm. The second pass will remove the final 0.5mm of rough material.
- Calculate RPM for First Pass: We use the starting diameter of 50mm.
- Calculate Time for First Pass: Our cutting length (L) is 100mm. We’ll add 5mm for approach and over-travel. This makes L = 105mm.
- Calculate RPM and Time for Second Pass: The new diameter is 50mm – (2mm * 2) = 46mm.
- Total Roughing Time:
Step 2: Calculate Finishing Time
Now we just have one last light pass to get to the exact size and good surface finish. The new diameter is 45.5mm (with the second rough pass). We leave 0.5mm for the finish pass.
- Calculate RPM for Finish Pass: We use the finishing cutting speed and the current diameter.
- Calculate Time for Finish Pass: We use the finishing feed rate of 0.1 mm/rev.
Step 3: Total Cutting Time
Finally, we need to add the roughing and finishing times together.
The computation of 1.378 minutes (close to 83 seconds) is solely the tool cutting time. It is naturally one of the elements in the overall cycle time but not the only one.
The Horror of Missing Out Unseen Factors: Accounting Non-Cutting Time
One of the common errors ascertain in estimating is the failure to account for the non-cutting time. In essence, it is the time the machine is operating but the tool does not cut the workpiece. This “silent productivity killer” can sometimes account for a major part of the complete cycle time.
What is Non-Cutting Time?
Non-cutting time consists of every step not involving cutting. If you only take into account the cutting time, your estimate will always be shorter than the real time. The knowledge on how to calculate CNC turning cycle time should also include these concerns.
Key Contributors of Non-Cutting Time
These are the most common elements that add to your cycle time:
- Tool Changes: The time it takes for the turret to index from one tool to the next. This can be anywhere from 3 to 10 seconds per tool change. It depends on the machine.
- Rapid Movements: The time the tool spends moving quickly from its home position to the start of the cut (approach) and back again (retraction).
- Spindle Acceleration/Deceleration: The spindle needs time to speed up to the target RPM. It also needs time to slow down before a tool change. This can add a few seconds to each operation.
- Part Loading/Unloading: This can be a huge factor. Manually loading a heavy part can take minutes. An automated bar feeder might only take a few seconds.
- Programmed Dwells or Stops: A program might include a deliberate pause (G04 command) to help break chips. It might also include a full stop (M00) for an operator to perform a check.
How to Control and Reduce Non-Cutting Time
The most effective technique to determine non-cutting time is past experience combined with knowing your equipment. Utilizing a stop clock, you can time specific actions carried out on your lathe.
As a common principle, you can expect to add anywhere between 5 to 8 seconds for every tool change. Furthermore, you should expect to wrap up anywhere between 5 to 10 seconds for rapid moves on every tool. For example, in our part, we had just one tool change. We added 8 seconds for the change and 10 seconds for all rapid moves, which results in 18 seconds added to our 83-second cutting time. The total is now 101 seconds. This is a notable rise.
Reducing this time can be done by optimizing your program. You can rearrange the tools in the turret to minimize the travel distance. Rapid moves should be programmed to be as short and direct as possible. For professional solutions, you may need expert CNC-Drehmaschinen-Dienstleistungen.
Pencil and Paper to Software: Tools for Cycle Time Estimation
So, there are many options to calculate how to compute CNC turning cycle time. They range from simple math to sophisticated software.
Manual Calculation
For a beginner, the best way to grasp the concepts is to manually calculate as we did in our example. It is best for basic parts and quick quotes. Nevertheless, it can be tedious and prone to errors for complicated jobs.
CAM Software Simulation
For complicated parts with multiple tools and profiles, the industry standard is to use CAM (Computer-Aided Manufacturing) software. Systems like Mastercam or Fusion 360 generate the toolpaths. They also offer very accurate cycle time estimates. The software simulates every move, including cutting and non-cutting time.
Online Calculators
For a quick and efficient calculation, on-line calculators are the most reliable option. A specific CNC Turning Cycle Time Calculator can greatly help you to finish process optimization. It is an accurate way without the full CAM set up complexity. Such tools are handy for estimators as well as engineers who require instant and reliable numbers.
From Theory to Profit: Applying the Exact Calculations
The total cycle time for a part is simply the sum of all cutting times and all non-cutting times. The process of learning how to calculate CNC turning cycle time encompasses much more than a theoretical exercise. It becomes the most effective instrument for the company to run a more productive and profitable operation.
This expertise makes you more competent at estimating. This indeed leads to more jobs being secured. It allows for a more efficient production schedule thereby saving downtime and raising output. For businesses aiming at optimizing every process, the partnership with professionals can be a game-changer. Explore advanced manufacturing solutions with Mekalit.
Frequently Asked Questions (FAQ)
What’s the real difference between cutting time and cycle time?
Cutting time is strictly the time the tool is removing material. Cycle time is the total time from the start of a program to its end for one part. It includes all cutting time and all non-cutting time. This includes tool changes and rapid movements. For accurate quoting, you must use the total cycle time.
How does the workpiece material affect the turning time?
Material is a primary factor. Harder materials, like Inconel or Titanium, require lower cutting speeds. They sometimes need lower feed rates to manage heat and tool wear. Softer materials, like Aluminum or brass, allow for much higher cutting speeds. This directly impacts the RPM (N) in your formula. It significantly changes the final calculation.
My actual cycle time is always longer than my calculation. Why?
This is a common issue. It is almost always due to underestimating or completely ignoring non-cutting time. Factors like tool changes, rapid moves, spindle ramp-up/down, and any programmed dwells (pauses) add up quickly. For the most accurate estimate, you must add these “hidden” times to your calculated cutting time.
Can I use the same basic formula to calculate CNC milling time?
The principle of is the same. But the variables and formula are different for milling. In milling, you calculate time based on the tool path length divided by the feed rate in inches per minute (IPM) or mm/min. This feed rate is calculated as .
What are the quickest ways to reduce my CNC turning cycle time?
The top two areas are optimizing cutting parameters and minimizing non-cutting time. Safely increasing feed rates and cutting speeds is the most direct way. Stay within tool and material limits. For non-cutting time, ensure your program minimizes rapid travel distances. Use the most efficient tool order to reduce turret indexing time.
