Opening: A "tuition fee" worth 170,000
In March last year, I was in the workshop, looking at the batch of 6061 aluminum parts that had just been abandoned, and I felt a chill in my heart. There are 300 parts, all of which are out of tolerance. What is the reason? It's because there was an error in the selection of equipment. We forced a job that should have been processed on the lathe to the milling machine. This 'tuition' worth 170,000 made me realize a truth: If you don't understand the essential difference between CNC Lathe and CNC Mill, you are wasting money.
Part One: Two Worlds, One Logic
The core difference between lathes and milling machines is one thing: who is turning .
The world of lathes follows a simple model: the workpiece rotates and the tool remains stationary. Try to imagine this situation, you are holding a knife and cutting a spinning apple. There are various types of rotary parts, such as shafts, discs, sleeves, and threads, all of which are included in its processing scope.
In a milling machine, it's exactly the opposite: the tool is rotating, while the workpiece either remains stationary or moves slowly. This situation is like holding an electric drill to carve a square piece of wood. Objects that are square, have slots, have cavities, and have complex curved surfaces are all handled by it.
Let’s look at a set of hard data:
Material removal rate :
There is a lathe that is used for rough turning of the outer circle. Its typical value is in the range of 500 to 800 cm³ per minute. Here, 45 steel is used as an example.
Typical values for milling machines, in this case face milling, are in the range of 200 to 400 cubic centimeters per minute, and that's for the same material.
Conclusion: Lathes are "bulldozers" and milling machines are "excavators" .
Cost per piece (batch of 10,000 pieces) :
Turned parts (such as pins): about ¥3.5-5.0/piece
Milling parts (such as brackets): about ¥8.0-12.0/piece
Conclusion: If you can make a rotary body, don’t use a milling machine.
Accuracy level :
Ordinary lathe: IT7-IT8
Ordinary milling machine: IT7-IT9
Top turning and milling compound: IT5
Five-axis milling: IT6
Conclusion: The accuracy is about the same, but the lathe is easier to maintain batch consistency .
Part 2: A cruel truth – 90% of people choose the wrong device
I have been in this industry for fifteen years and have witnessed many examples. It is like killing a chicken with the sharpest blade of a cow. It is not that the quality of the equipment involved is poor, but that there are huge errors in the logic of selecting the equipment.
List of common misunderstandings:
Misunderstanding 1: Having a machining center is everything
There is such a real case. There is a factory that deals in auto parts. It uses equipment like Lijia to make a batch of threaded shafts with specifications of M20×1.5. The processing time of each single piece is 14 minutes.
Change to CNC lathe: 2 minutes and 30 seconds .
Gap: 5.6 times . Not counting the cost of the lathe insert, it is only 1/5 of the milling cutter disc.
Misunderstanding 2: Lathes are cheap and can be used for many purposes
There is a real case where a hydraulic valve body with three vertical holes is to be made. If you choose to use a lathe for processing, that won't work, because you can't even solve the clamping problem. Therefore, a milling machine must be used, and it must be a milling machine with a fourth axis.
There are some shapes that the lathe simply cannot "bite".
Misunderstanding 3: Only look at the price of the equipment
A domestic 850 machining center costs ¥190,000.
A domestic 6132 CNC lathe costs ¥80,000.
However, let’s calculate the unit price per square meter. The comprehensive cost of the machining center line per hour is about RMB 120 to RMB 150, while the comprehensive cost of the lathe per hour is about RMB 60 to RMB 80.
The lathe is the "assembly line" and the milling machine is the "studio" . The former is about volume, the latter is about shape.
Natural integration of keywords: equipment selection
There is such an iron law when it comes to equipment selection. What is it? That is, the maximum diameter of the part plays a role in determining the lathe, and the smallest feature plays a role in determining the milling machine. Lathes should be selected based on the diameter of the rotary body, and milling machines should be selected based on the minimum slot width and depth-to-diameter ratio. When there is a conflict between the two, the milling machine has a higher priority. Why is this? It's because the lathe cannot complete the milling work, but the milling machine can barely do the work of the lathe, but the efficiency is extremely low.
Part 3: The pitfalls I’ve stepped on – the hidden costs that no one tells you
Do you think that’s it after buying the equipment? Too naive.
Fixture cost :
Lathe: A pair of soft claws, ¥150-300. Just drive by yourself.
For milling machines, vise is a standard accessory. However, for mass production, hydraulic clamps start at 5,000 yuan, and pneumatic clamps are even more expensive. As for non-standard workwear, it is quite common for the price to be between 8,000 and 20,000 yuan.
Programming time :
There is a program for a lathe, which contains an outer circle part, a threaded part and a part that can perform cutting functions. It can be completed in ten minutes through manual programming.
Regarding the milling machine program, for parts of the same complexity, it takes two hours for a novice to complete the drawing in the CAM software, select the tool, and set the tool path, while it only takes 40 minutes for an experienced user, but you still have to make adjustments later.
Tool consumption :
Turning tool: One blade has 8 edges. Break one off, turn it around and continue using it.
Milling cutters, solid carbide end mills, will be scrapped once one edge breaks. If the diameter is less than 6mm, the cutter will break like a pacgo.
A set of real loss data (8 hours of continuous production):
The situation presented by the milling machine is "saving tools and waste electricity", while the lathe belongs to the type of "saving tools and waste electricity". There is no situation where one is absolutely good, only which one is more suitable.
Part 4: Q/A – your top 5 questions
Q1: Which is more suitable for making disc parts, lathe or milling machine?
A: For discs, the lathe is the first choice. It can make the rotary body have high efficiency and good precision, while the milling machine will only be treated as a supplementary process when there are holes in the side wall or special-shaped grooves.
Q2: When does a piece of equipment need to be equipped with five-axis milling?
If it is the kind of complex curved surface mentioned by A that cannot be achieved by three-axis milling through one clamping, or if it is a deep cavity undercut, then five-axis machining must be used. Otherwise, the cost will double.
Q3: Can the lathe mill flat positions?
A: It can be equipped with a C-axis powered turret. However, its milling efficiency and surface quality are far lower than those of regular milling machines.
Q4: The material has high hardness, how to choose between lathe and milling machine?
A: Choose a lathe because the arc length of tool contact will be shorter and the cutting force will be more stable. However, when a milling machine encounters materials such as HRC45+, the risk of vibration and edge chipping will increase dramatically.
Q5: For small batches and multiple varieties, what equipment should I buy first?
A: First buy a milling machine. A milling machine has high flexibility in clamping and can cover a wider range of parts. However, a lathe only makes rotary bodies, and its scope of application is much narrower than that of a milling machine.
Natural integration of keywords: material removal rate
The material removal rate is a key hard indicator used to measure the competitiveness of machine tools. In the roughing stage, the removal rate of lathes is generally three to five times higher than that of milling machines, which directly determines the time it takes to process a single piece and the minimum limit of the quotation. A machine tool with a higher removal rate means a lower unit cost and a stronger ability to participate in bidding competition.
Part 5: Answer these three questions before making a decision
I recommend to every decision-maker that before signing on to purchase equipment, find a quiet place to answer these three questions:
Question 1: What is the diameter to length ratio of my part?
The ratio is greater than 3:1 (slender shaft type) → choose a lathe with your eyes closed
Ratio less than 1:2 (disc or block type) → tend to milling machine
In between → Continue to the next question
Question 2: What is the minimum inner corner radius of my part?
Larger than R3mm → Lathe can barely handle it
Less than R1mm → Milling machine is the only option
An industry secret tip for you: the smaller the R angle, the more likely it is to be used on a milling machine because there are physical limits to the turning tool tip arc.
Question 3: In what range is my annual output?
Less than 500 pieces → Milling machine (flexibility preferred)
500-5000 pieces→ Calculate the cost carefully
More than 10,000 pieces → Most likely require lathe + automation
Natural integration of keywords: precision control
For the debate between turning and milling, precision control is a very critical trump parameter. Lathes have advantages from the beginning in controlling the cylindricity and coaxiality of long-axis parts. Why? That's because it can complete all cylindrical processing in one clamping. However, when a milling machine processes a slender shaft, it needs to be clamped multiple times. As a result, the accumulated error is unavoidable. Are you really willing to bet on this probability?
Part Six: My Action Suggestions—Three-Step Selection Method
Stop shooting based on feeling. Follow this process:
Step 1: Forced classification (completed within 24 hours)
Take out your parts atlas and sort all the parts into three piles:
Pure rotary body (shaft, sleeve, spacer) → lathe
Pure prism (squares, plates, brackets) → milling machine
Mixed body (there are grooves, holes, and flat surfaces on the rotary body) → Go to the second step
Step 2: Cost simulation (completed within three days)
Find three different processing plants and quote the same hybrid part using lathes and milling machines.
Compare: working hours per piece, tool cost, number of clampings.
Remember this formula: the comprehensive cost is equal to the hourly wage of the equipment multiplied by the working hours of a single piece, plus the value allocated to each piece of the tool and the value allocated to the fixture.
When making samples, if the quantity is less than 30 pieces, the differences between the equipment need to be ignored. In mass production, when the quantity exceeds 500 pieces, optimal equipment must be used.
Step 3: Equipment matching (direction determined within a week)
If more than 70% of the parts are rotary bodies, then a CNC machine must be installed and equipped with a small vertical machine for supplementation.
If more than 70% of the parts are prisms, send them to a machining center, and install a second one if necessary.
If the proportion of both is half → think carefully about turning and milling. But keep in mind: the price of a turn-milling machine ≈ the sum of the prices of a vertical machine and a slant bed CNC lathe.
Final conclusion: There is no best machine tool, only the least wasteful machine tool
I once witnessed someone using five-axis equipment worth 200,000 yuan to make M3 screws. It's not that it can't be done, but that the method is so wasteful that it makes people laugh. I have also seen someone using an old-fashioned hand-operated lathe to produce 100,000 pins per month. It is not that the equipment is so advanced, but that it has reached the pinnacle of matching in all aspects.
Regarding the topic of "cnc lathe vs cnc mill", the essence is not a technical comparison, but a trade-off in terms of cost efficiency.
A lathe is like a scalpel and a milling machine is like a Swiss Army knife. It is not necessary to carry a Swiss Army Knife in every situation, but when the need arises, the scalpel can cause problems.
Now, back to that lesson worth $170,000:
The aluminum parts that were judged to be scrapped were a type of flange with three axial grooves. At that time, I forcibly placed it on the milling machine for processing, but ignored that it was still essentially a rotary body. If I had first used a lathe to properly process the outer circle and end face, and then placed it on a milling machine to machine only the three slots, the final result would have been completely different.
Got it?
It is not a lathe rolling over a milling machine, nor is a milling machine replacing a lathe.
It’s about being in the right place, using the right equipment, doing the right thing .
Spread out the drawings used for drawing, clearly calculate the work used to calculate the output, and calculate the accounts involving costs clearly and thoroughly. Then, make a decision on which device to press the startup button.
Action list:
[ ] Before leaving get off work today, classify all parts into rotational bodies/prisms/hybrid bodies.
[ ] Within this week, find two suppliers with different processes to measure and quote.
[ ] Keeping this principle in mind, the milling machine deals with the question of "can it be done", while the lathe deals with the question of "can it be done with profit".
[ ] Before signing to buy the equipment, read the Q/A section of this article again
