The statistical analysis method is used if there is a requirement that the slot must be .500” wide with a +/- .003” tolerance, but there is no need for the radii (.125”) and the flat (.250”) to be exact as long as they fit within the slot. In this example, we have 3 bilateral tolerances with their standard deviations already available. Since they are bilateral, the standard deviation from the mean would simply be whatever the + or – tolerance value is. For the outside radii, this would be .001” and for the middle flat region this would be .002”.

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Great question Graham! There are many factors that go into choosing which method is best for you. Please send an email to [email protected] with all your information and they will be able to help you out as soon as possible.

Think of it like a cup of coffee being made with 3 different sized beans. In order to make a delicious cup of joe, you must first grind down all of the beans to the same size so they can be added to the coffee filter. In this case, the beans are the standard deviations, the grinder is the tolerance distribution factor, and the coffee filter is the root sum squared equation. This is necessary because some tolerances may have different distribution factors based on the tightness of the tolerance range.

Statistical tolerance analysis should only be used for assemblies with greater than 4 toleranced parts. A lot of factors were unaccounted for in this simple analysis. This example was for 3 bilateral dimensions whose tolerances were representative of their standard deviations from their means. In standard statistical tolerance analysis, other variables come into play such as angles, runout, and parallelism, which require correction factors.

A 60 year old worn out manual Bridgeport is all I have to work with. (It is in better shape than my 65 year old worn out body.) I rarely climb mill anything, specially not steel. But if the cut is really light, and I want a good finish, I climb mill. , I apply some drag with the table lock screw, and that seems to eliminate the chatter.

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Added uses for conventional cutting: Never climb cut across the end of an upstanding thin rib in aluminum or plastic (You will rip it off). Use reduced feed and conventional cut it or use multiple small depth cuts. Commonly made cutting to length T or L extrusions. You’ll only make this mistake once. It helps to conventional cut torched or burnt out steel plate rough profiles first, then switch to climb cut after you mill through the slag. Same principal as case hardened material. Corn-Cob or serrated cutters work nice here too.

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Understanding tolerance stacking is crucial for three key reasons: preventing manufacturing failures, optimizing production costs, and ensuring consistent quality across high-volume production runs. This post provides comprehensive analysis methods, practical examples, and best practices for managing tolerances.

This is one of those left hand cutters! How about turning the tool, and cutter the correct direction. G41 climb cutting on the right side. G42 or conventional cutting on the left. Sorry I couldn’t help myself.

There are two distinct ways to cut materials when CNC milling: Conventional Milling (Up) and Climb Milling (Down). The difference between these two techniques is the relationship of the rotation of the cutter to the direction of feed. In Conventional Milling, the cutter rotates against the direction of the feed. During Climb Milling, the cutter rotates with the feed.

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As machinists are always trying to find ways to increase efficiency and tool life, climb milling has gotten a lot of recent traction in the space. Less heat is generated within the tool, and friction is more easily mitigated. These two alone lead to longer tool life, allowing for more parts completed per tool, lowering a shops bottom line. Also, climb milling can lead to a better surface finish due to how the chips are formed at the cutting edge.

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Worst case analysis can also be used when choosing the appropriate cutting tool for your job, as the tool’s tolerance can be added to the parts tolerance for a worst case scenario. Once this scenario is identified, the machinist or engineer can make the appropriate adjustments to keep the part within the dimensions specified on the print.

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Thanks for this very clear and informative explanation. It has been decades since I worked in a factory. Back then the Bridgeport milling machines had terrible backlash. They would chatter or jump when using climb milling. Plus, if one was approaching the end of a cut, one wouldn’t know if the tool would grab at that point and pull the work past the past the desired end point. I almost exclusively used conventional milling and couldn’t understand why many people on YT now talk about using climb milling.

The standard assumption is that a part tolerance represents a +/- 3  normal distribution. Therefore, the distribution factor will be 3. Using equation 1 on the left section of figure 1, we find that its corrected standard deviation equates to:

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In addition, conventional milling should also be utilized on casting, forgings or when the part is case hardened. This is due to the cut beginning under the surface of the material, where it will gradually build a chip. Climb milling into these materials will see maximum chip thickness on engagement, which could lead to premature failure of the cutting edge due to the forces generated. print

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A part or assembly can be subject to inaccuracies when its tolerances are stacked up incorrectly. Therefore, proper manufacturing tolerance calculation is paramount in winning at the spindle. Starting with a tolerance stack-up analysis prior to machining ensures parts are achievable with current setups, reducing scrapped parts. Continue reading to explore the importance of proper tolerance stacking, common pitfalls, and equations to ensure success.

Hey there, first of all thank you so much for this post and honestly I was searching for the same information from last few days. Keep posting and keep sharing..

Conventional Milling is the traditional approach when cutting because the backlash, or the play between the lead screw and the nut in the machine table, is eliminated as seen in Figure 1 below. Recently, however, Climb Milling has been recognized as the preferred way to approach a workpiece since most machines today compensate for backlash or have a backlash eliminator.

Tolerances directly influence the cost and performance of a product. The tighter the tolerance, the more precise a finished part becomes. Tighter tolerances also make a machined part more difficult to manufacture and therefore often more expensive. With this in mind, it is important to find a balance between manufacturability of the part, its functionality, and its cost.

Statistical analysis looks at the likelihood that all three tolerances would be below or above the dimensioned slot width, based on a standard deviation. This probability is represented by a normal probability density function, which can be seen in figure 2 below. By combining all the probabilities of the different parts and dimensions in a design, we can determine the probability that a part will have a problem, or fail altogether, based on the dimensions and tolerance of the parts. Generally this method of analysis is only used for assemblies with four or more tolerances.

Keep up the good work with these articles! Maybe I’m a nerd, but I’ve found a good percentage of these very interesting and practically helpful. Thanks!

Additionally, unnecessarily small tolerances will lead to longer manufacturing times, as more work goes in to ensure that the part meets strict criteria during machining, and after machining in the inspection process.

I like what you said about chip width working. I need a milling machine for some steel. I’ll have to get a dye-cutter that is discounted.

By using these conversions, you can easily convert speed measurements from Millimeter per second (mm/s) to Inch per minute (ipm) or vice versa.

After a tolerance is identified on the dimension of a part, it is important to test whether that tolerance would work with the chosen tool’s tolerances: either the upper end or lower end. These tool tolerances are often called out within tool dimension charts. An example can be seen on a Harvey Tool Miniature Square End Mill where the tolerance of +.00005″ and -.0005″ is shown at the Cutter Diameter dimension.

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With more modern machines now compensating for backlash or utilize backlash eliminators, it has become a much easier strategy to adopt within shops. While we went over some reasons why climb milling is not an effective strategy above, here are some reasons why a machinist may want to explore climb milling:

For this example, let’s find the standard deviation (σ) of each section using equation 1. In this equation represents the standard deviation.

For example, a corner radius end mill with a right and left corner radii might have a tolerance of +/- .001”, and the flat between them has a .002” tolerance. In this case, the tolerance window for the cutter diameter would be +/- .004”, but is oftentimes miscalculated during part dimensioning. Further, placing a tolerance on this callout would cause it to be over dimensioned, and thus the reference dimension “REF” must be left to take the tolerance’s place.

I feel like something was skipped.. Where did the 0.00122″ value suddenly come from and how were the 68, 95 & 99 percentage values arrived at? If you could help clarify that it would be awesome.

After arriving at these standard deviations, we input the results into equation 2 to find the standard deviation of the tolerance zone. Equation 2 is known as the root sum squared equation.

Have seen this article several times. One consideration is roughing, my observation is that the load against the cutter when cutting in the conventional direction is lower and reduces the risk of tool breakage. Another is where the tool-paths leave “posts” in corners and so-fourth when hogging out parts, conventional milling will not grab into a post and break the cutter. Another application is when making long thin flats in rod shaped parts either on a indexing head or on a swiss type automatic lathe through a guide bushing, the conventional path will produce less taper and more parallel surfaces. Just a few thoughts.

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I have a Warco 16B milling machine. This is a medium size manual hobby mill. I am confused as people recommend Conventional and Climb milling in about even numbers, this goes for YouTube too. What would you recommend on this type of machine? Your help would be appreciated as I last worked in industry in 1979 so I am extremely out of date. I generally mill Conventionally.

As stated above, tighter tolerances lead to a higher manufacturing cost as the part is more difficult to make. This higher cost is often due to the increased amount of scrapped parts that can occur when dimensions are found to be out of tolerance. The cost of high quality tool holders and tooling with tighter tolerances can also be an added expense.

Great explanation of the differences between climb milling and conventional milling! I appreciate how you highlighted the advantages of climb milling, especially in reducing tool wear. It’s very informative for someone looking to refine their machining techniques. Thanks for sharing!

Worst case analysis is the practice of adding up all the tolerances of a part to find the total part tolerance. When performing this type of analysis, each tolerance is set to its largest or smallest limit in its respective range. This total tolerance can then be compared to the performance limits of the part to make sure the assembly is designed properly. This is typically used for only 1 dimension (Only 1 plane, therefore no angles involved) and for assemblies with a small number of parts.

I think you left out force vectors during cutting. This can influence tool defection and taper on the side wall on the part. The force vectors are different magnitudes between climb and convectional cutting, so this impacts work holding and this parts or this walls.

That is not to say there aren’t benefits to climb milling. For example, this strategy offers a machinist more control and less vibration than its climb milling counterpart. Similarly, for materials that traditionally chatter or tear, conventional milling would be the proper strategy to choose. On the other hand, here are some reasons why it might be most beneficial to adopt a climb milling strategy:

These confidence windows are standard for a normal distributed set of data points. A standard normal distribution can be seen in Figure 2 above.

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Therefore the worst case scenario of this slot is .500” +/- .008”. Therefore the worst case scenario of this slot is .500” +/- _.004”_. (.504/.496)

“Conventional Milling should be… utilized on casting, forgings” In my head, I logically organize castings as least processed, forgings as most processed, and everything else (hot rolled, cold rolled, extruded) somewhere in between. So to me, that part seems to say ‘use conventional milling for everything’, which is obviously not right. Could you show me where I went wrong,? More specific examples, like case hardening, why a particular direction of cut is preferred for a chunk of metal with unknown provenance.

Before starting a statistical tolerance analysis, you must calculate or choose a tolerance distribution factor. The standard distribution is 3 . This means that most of the data (or in this case tolerances) will be within 3 standard deviations of the mean. The standard deviations of all the tolerances must be divided by this tolerance distribution factor to normalize them from a distribution of 3  to a distribution of 1 . Once this has been done, the root sum squared can be taken to find the standard deviation of the assembly.

Add all of these together to the lower specification limit: Add all of these together to the _upper_ specification limit:

However, though Climb Milling is often the current preferred way to machine parts, there are times when Conventional Milling is the necessary milling style. One such example is if your machine does not counteract backlash. In this case, Conventional Milling should be implemented. Without accounting for backlash, breakage can occur due to the forces within the machine during tool engagement.

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At this point, it means that 68% of the slots will be within a +/- .0008” tolerance. Multiplying this tolerance by 2 will result in a 95% confidence window, where multiplying it by 3 will result in a 99% confidence window.

You’re correct with the .00122” tolerance. This should have said .0008”. Thanks for catching that! In terms of the 68, 95, and 99 percentage values, this just applies to the 68-95-99.7 normal distribution rule, in which 68% of the data is within 1 standard deviation of the mean, 95% of the data is within 2 standard deviations of the mean, and so forth.

This is great info. I have always conventional milled with face mill to remove the scale off of titanium. Tool life is increased and getting under that scale to machine it off instead of slamming the insert into the scale each time. Once scale has been removed, go back to climb cutting.

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It depends on your perspective. Are they showing a view looking down at the work piece or looking up from the workpiece.

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As previously stated, traditionally conventional milling has been the common choice for most machinists. This is where the cutting edge of the tool is actually rotating away from the direction of the feed. An example of this is seen in Figure 2 below. Until recently, this has been the common choice due to backlash however, the rise of climb milling has caused machinists or machines to adapt and compensate for this issue.

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Climb Milling is generally the best way to machine parts today since it reduces the load from the cutting edge, leaves a better surface finish, and improves tool life. During Conventional Milling, the cutter tends to dig into the workpiece and may cause the part to be cut out of tolerance.

Thank you for the question Dustin! We would suggest conventional milling when your material has a rough surface, such as cast iron, or is anodized because when conventional milling your cut is scooping underneath the surface to remove your material making it easier on your tool. Also, you want to conventional mill when using a dovetail cutter that has a weak neck diameter because this will help relieve the pressure on the neck of your tool.

Great article. I’ve had to use conventional milling when for example, I’d have my thin unsupported part sticking out of work holding with the tool path contouring around the part (think milling end while cutting a part in a lathe) with the material flexing would cause snapping while climbing because it wants to take a large bite as opposed to ramping the cut in. But yeah, 95% or more is climbing.

There can only be one way to interpret the cut because machines only turn in one direction. In the examples the view has to be from under the workpiece looking up at the tool

Tolerance stacking, also known as tolerance stack-up, is a critical step in manufacturing that determines how individual part tolerances combine to affect overall accuracy. In precision machining, this analysis is crucial for ensuring tools maintain their functional requirements in the cut while still remaining cost-effective to produce. In essence, tolerances are combined or “stacked-up” as a cumulative equation in machining to ensure feasibility.

It should be noted that the worst case scenario rarely ever occurs in actual production. While these analyses can be expensive for manufacturing, it provides peace of mind to machinists by guaranteeing that all assemblies will function properly. Often this method requires tight tolerances because the total stack up at maximum conditions is the primary feature used in design. Tighter tolerances intensify manufacturing costs due to the increased amount of scrapping, production time for inspection, and cost of tooling used on these parts.