Why do CNC precision parts require repeated rework when quoting based on drawings?

Shenzhen Huiwen Zhizao has been deeply involved in precision machining of robot components for many years. Last week, an engineer who works on robot joints came over  said with a hint of anger, "Can you stop quoting  then adding prices,  reworking samples like the previous companies did? The project timeline has already been delayed for three weeks

We have been working in the field of robot component processing for many years,  the answer is  complicated: the quotation is repeatedly reworked, the finished product does  match expectations,  it is likely that you laid a thunderbolt  the moment you sent the first picture. Many engineers save time by sending a 3D model via WeChat to urge for a quotation,  once the dimensional tolerances are marked, they feel that the drawings are complete  lively. Being able to process CNC parts is  a guessing game -3D models can only show the shape,  what precision, material,  surface treatment you need. The processing plant can only estimate the number according to the most conservative plan. When it really starts, it will be discovered that this one requires extra money  that one cannot be done. The quotation will be reworked  the parts will be reworked,  it will take two  three weeks to go back  forth. The urgency lies with yourself. Today's article will make it clear: when sending drawings to CNC machining plants to make precision parts, in addition to 3D models  dimensional tolerances, what other things need to be clearly written in order to quote the price  make the right parts at once.

1、 Why do quotes  finished products always deviate  expectations when only 3D models are sent

Let's talk about the most common scenario first. A customer who works on robot structural components received a step file on WeChat, saying 'Can you help me CNC process this part  give me a price?' We opened the model  found that the shape was  complicated, it was made of aluminum alloy, but the problem arose - which side of the part was the assembly reference? What is the required positional accuracy of the hole position? Should the surface be anodized  natural? Is the material 6061  7075? Is the batch size 10 pieces  500 pieces? There is  a single word on the 3D model of this information. Can you guess how the processing factory quotes? Either quote higher according to the most expensive plan,  you'll be scared off by the price; Either report downwards according to the cheapest plan,  wait until halfway through until it is discovered that surface treatment  heating treatment is required, then start to increase the price. No matter what, in the end, it's all about quoting for rework  going back  forth to argue. This is the first pitfall of only sending 3D models for CNC machining parts: incomplete information, quoting is just a headache. The processing factory did  intentionally quote a high price, but the information you provided was only enough for him to estimate roughly. If he really wants to make precision parts, one technical requirement is missing,  the cost may differ by 30%.

2、 The dimensional tolerance has been written, but if these technical requirements are missed, they will still be reworked

Many customers actually have 2D drawings with dimensions  tolerances marked, which look very formal. But when it comes to the CNC machining process, problems still arise - because the drawings only indicate "form  position tolerances" without writing "process requirements". After reviewing thousands of machining part drawings, we found that the following items are the most likely to be missed by customers:

1. Surface roughness (Ra value)

The same CNC machined part requires Ra1.6 for the mating surface  Ra3.2 for the non mating surface, with completely different process routes. The surface of Ra1.6 needs to undergo precision milling  grinding, with a tighter tolerance band; Ra3.2 conventional precision milling can achieve it. You don't distinguish on the drawing,  the processing factory defaults to using Ra3.2 for all parts. However, when the assembly line discovers that the bearing mating surface is too thick  the clearance exceeds the standard after pressing, the entire piece will be reworked. Key sliding surfaces, sealing surfaces,  mating surfaces must be separately marked with Ra values on the drawings,  cannot be written as "smooth surface" in a general way.

2. Material grade  heat treatment status

Both aluminum alloys, 6061-T6  7075-T651, have a yield strength difference of nearly twice  a raw material price difference of over 30%. Choosing the wrong material state for the load-bearing components such as robot joint shells  arm rods resulted in deformation after three months of installation  operation. A more common pitfall is that only the three words "aluminum alloy" are written -6061, 5052, 7075 cannot be distinguished  the appearance,  the processing plant only relies on what materials are available in the warehouse on the day of purchase. The correct way to write it is to label the brand  status together, such as "6061-T6"  "7075-T651",   just "aluminum parts".

3. Marking standards for geometric tolerances

Marking the dimensional tolerance does  mean everything is fine. The positional tolerance, coaxiality, flatness,  perpendicularity must be specified with a reference (A/B/C reference plane),  the positional tolerance without a reference is equivalent to no marking. We have seen too many drawings with a positional tolerance of ± 0.05, but the benchmark is empty - we cannot determine which surface to start measuring  during processing, so this tolerance cannot be executed. For precision parts, the reference system (GD&T) is more important than simple dimensional tolerances.

4. Surface treatment  film thickness requirements

Anodizing, hard anodizing, conductive oxidation, sandblasting, wire drawing, blackening - these post-treatment CNC processing plants generally do  do them themselves  need to be outsourced, with a cycle of 3 to 7 days. Simply writing 'anodic oxidation' is  enough. The hard anodic oxide film is 25-50 μ m thick  the ordinary anodic oxide film is 8-12 μ m thick, with several times worse wear resistance  different prices. Without specifying the film thickness grade, the wear resistance of the finished product cannot meet the requirements for friction parts such as robot joints.

5. Deburring, blunting,  testing requirements

The easiest one to miss. If the burrs on the edges of precision parts are  treated, scraping hands  sealing rings during assembly,  serious metal shavings falling  the reducer is considered an accident. The statement 'All sharp edges should be blunted by C0.3-C0.5,  burrs should be removed' on the drawing is assumed to be ignored. In addition, key dimensions should indicate whether they are fully inspected  randomly inspected - hole position  coaxiality, which affect the assembly dimensions. It is required to attach a three coordinate inspection report with the goods, rather than just providing a certificate of conformity.

3、 Repeated rework of quotation? The problem often lies in the unclear writing in these areas of the drawing

By reversing the above points, you will understand why the quotation always requires rework. We do CNC machining,  the first thing we do after receiving the drawings is  scheduling, but DFM review. A qualified precision part drawing is sent over - with material grade  status, dimensional tolerance zone benchmark, roughness marked on the mating surface, film thickness grade written on the surface treatment,  clear batch quantity - all of this information is complete,  engineers can provide itemized quotations within 24 hours: how much rough milling costs, how much fine milling costs, how much heat treatment  surface treatment outsourcing costs, how much coordinate testing costs, with each process listed separately  the delivery time clearly stated. On the other hand, when a drawing with incomplete information comes over, we can only list the questions first: "Is the material 6061  7075? What is the roughness of this surface? Do you want hard anodes? How much is the batch size?" You answer one round,  we will check the price for another round; After changing the plan, you can confirm it again. It's another round. It takes two  three days to come back  forth,  when it comes to urgent sample making, it takes half a week just to wait for the quotation. Find a CNC machining factory to process parts, spend half an hour writing down the technical requirements of the drawings in the early stage,  save at least two weeks of rework time in the later stage. This account includes both the process  procurement personnel.

4、 What will a legitimate CNC processing factory do after receiving the drawings

Speaking of which, by the way, let me mention our process here. You have a comparison in your mind. You can send the 2D engineering drawing (PDF  CAD)  3D model (STEP/IGS format) together. The first thing we do is  to rush to quote, but to conduct DFM manufacturability review - which thin-walled features of this part are prone to tool deformation during machining? Which tolerance of ± 0.01 is within the existing equipment capability boundary? Is it possible to add a process protrusion to make the clamping more stable? These opinions will be directly fed back to you,  after modifying the drawings, we will proceed to the quotation process. The advantage of doing this is that the quotation is in place once,  there will be no temporary price increase due to design issues in the middle; The pass rate of the first sample is high,  there will be no need to change the drawing again if the sample is found to be unqualified. In the batch stage, each batch of precision parts is accompanied by a German Zeiss three coordinate inspection report, with key dimensions listed point by point  form  position tolerances also reflected in the report. You don't have to rely on your hand to judge whether it is qualified  . We are equipped with Demage five axis  four axis CNC machining centers for the processing end, with batch accuracy stable in the range of 0.005-0.01mm. The workshop operates in two shifts for 24 hours, with a delivery rate of 99.5%. If there is a quality problem due to processing reasons, we will provide free repair,  we will bear the shipping cost incurred by the repair - this is written in the quality agreement,  a verbal commitment.

In conclusion

In the field of CNC machining parts, ultimately it's  about who offers the lowest price, but about who can fully understand the technical requirements of the drawings, align the process route,  control key dimensions on the first attempt. Before you send the picture, spend ten minutes writing down the material status, mating surface roughness, form  position tolerance benchmarks,  surface treatment film thickness. The saved trial mold costs, rework costs,  delay costs later on are much better than redoing them all over again. Welcome to provide drawing consultation. We can provide 2D drawings, 3D models,  technical requirements to obtain DFM process evaluation  itemized quotations.