How to control the accuracy is a important technique for CNC metal spinning ,CNC metal spinning form the sheet or tube according to the CNC path ,so the accuracy is better than hand spinning .because metal have spring back and internal stress after metal spinning process ,we can’t achieve the accuracy by CNC theoretical path .Except we need to adjust CNC spinning path and parameter ,we also need to analysis the structure why effect on the internal stress and accuracy ,this article make you control the metal spinning accuracy .

The factor effect metal spinning accuracy

Metal spinning form the blank metal to different shape according to the CNC program path ,there are fourth  main factor effect the CNC metal spinning process accuracy ,the first factor is CNC program of metal spinning ,the second factor is the accuracy of CNC metal spinning machine ,the third factor is the accuracy of gap between roller and mold .the fourth factor is the internal stress can make the metal spinning deform.

1. CNC program

CNC program includes Metal spinning path and parameter ,you need to choose the right CNC program to form the blank metal to different shape . if the first metal spinning product can not achieve the accuracy ,you should adjust the metal spinning path and parameter to achieve the technical requirements of metal spun products. How to make a right CNC metal spinning path ,it needs many years of experience to accumulate.

If the first metal spinning products are made by your CNC program can achieve the accuracy ,you can make 10-20 pieces continuously ,if all the pieces of metal spinning products can achieve the accuracy , it proves the gap and the accuracy of CNC metal spinning machine is good .

2. Metal spinning gap

Gap is another important factor to effect the accuracy of metal spinning product ,you can see  the right of image ,gap value is the Min distance between roller and mold and it is set in CNC program . if the the gap change ,the accuracy of metal spinning product is hard to control .

To control and maintan the gap within a certain precision (about +-0.02mm)  .gap is too big ,the diameter of metal spinning product will increase , the gap is to little ,the diameter of metal Spinning product will reduce .

Three factor effects gap accuracy in the following .

① metal spinning mold  

Mold material : if the mold material is softer than blank metal , the mold diameter will be reduce andrew surface will be scratches after pressing in metal forming .such as we use 45# steel to make metal spinning to spin stainless steel ,the mold will be scratched and deformation.the metal spinning accuracy will be bad with the mold size change .

Solution : Make the mold by Cr12mov with heating treatment

Mold runnout : Mold run out will effect the thickness ,The run out of Mold exterior curve is important for metal spinning accuracy ,it mold run out over 0.05mm, the blank sheet is hard to hold mold tightly and the thickness is not uniform .

Solution : it needs to control within 0.02mm.you can check the mold run out according to the image above .

② Metal Spinning roller

Roller run out : if the roller run out is bigger than 0.02mm, the gap between mold and roller is different ,it will effect the thickness accuracy as well.

Solution : it needs to control within 0.02mm.

Roller quantity : the roller quantity is more ,the spinning force is more balance ,the blank sheet spring back is less .you can see the the image in the following ,when the roller press the blank metal in the mold ,the blank will keep touch the mold in the roller touch side ,but there is max gap in the other side without roller ,so more rollers can spin metal with a good support point ,more rollers metal spin can reduce the spinning back and the accuracy is better .

③ Metal spinning temperature 

Metal spinning is a cold extrusion forming and make temperature rise when blank is pressed and formed, the mold and roller will increase the dimension with the temperature rise .but the CNC metal spinning path is the same ,so the gap will reduce with the roller and mold size increase 0.02mm. gap is litte ,the roller and mold will bear more spinning force and produce more heat .

Solution : 1. We need to put a fan to near the mold and blow it ;2. to use deep drawing oil to reduce the heat except for lubrication ; 3. to put a cooling device inside the mold ; 4.to put the CNC metal spinning machine in thermostatic chamber .

Whatever the ways you choose to control the temperature in metal spinning process ,the key goal is to control the accuracy of gap .this is important to control the accuracy of metal spinning products .

3. CNC Metal Spinning machine

The accuracy of CNC metal spinning : if the accuracy of CNC metal spinning machine is not good , the gap value will change in per metal spinning .The repositioning accuracy of CNC metal spinning machine must be controlled with +-0.01mm.

The spinning force of CNC metal spinning : if the spinning force of CNC metal spinning machine is not enough ,the metal spinning product is hard to hold mold tightly ,the spring back is big when blank metal can't hold mold tightly .

4. Material spring back  

Different metal have different spring back in metal spinning process ,the hardness and tensile strength value is bigger ,the spring back is bigger .such as the spring back value of stainless steel is bigger than aluminum .When the roller move according to the CNC program ,you will find that the final and actual dimension is different with the CNC program value as the metal spring back .

What is spring back : spring back refers to the physical rebound, which refers to the physical deformation of the object under the action of the force, and the physical change of the reduced or nearly reduced state generated when the pressure is released. Spring back will make the dimension of metal spinning product is bigger than the CNC program value .

How to reduce the material spring back 

① Heating can make the metal soft ,the hardness and tensile strength of metal will reduce with the temperature rise, then the spinning back value will reduce .

② Debugging the CNC program and using more back path to make the blank metal hold the mold tightly ＞＞.

③ Necking or tube mold support point needs less gap to reduce the blank deformation in the gap＞＞ .

④ More rollers to forming make the blank more stable and balance .

⑤ To respair the CNC spinning mold according to the spring back value in the following image .

5. Material backflow

When the gap between roller and mold is too little ,the metal will produce backflow and make the bottom of metal spun product have gap between mold . you can reduce the gap when have backflow in metal spinning process .

6. Internal stress  

Metal spun products are thin-wall components ,metal spinning roller make blank metal grain refine in metal spinning process . the gain refining will produce the internal stress in the metal spinning products ,when the strength of the structure of metal spinning products is not enough ,the internal stress will make the shape of metal spinning product deform.

① The disadvantage of internal stress

The existence of internal stress will seriously affect the mechanical properties and performance of the product.‌ Due to the existence and uneven distribution of internal stress, ‌ products will break during use, this reduces its durability and reliability.

② Internal stress effect on accuracy

Internal stress is caused by uneven volume changes in the macro or micro organization inside the material, it can make the metal spinning deform and effect the accuracy ,we can divide the deformation into radial deformation and axial deformation , you can see the two deformation in the following image .

③ How to reduce the effect of internal stress on accuracy

Heating treatment : it can remove the internal stress by vacuum heat treatment ,vacuum heat treatment no only can remove the internal stress and keep the metal spun product keep the original size before heat treatment .Although other heat treatment can remove the internal stress ,but it can make the original size deform a lot .

7. Increase the strength of structure design 

As we know ,metal spun products is a thin -wall component ,internal stress can make the structure deform as the strength of structure is not enough ,if we change the design to improve the strength of structure ,the deformation of  internal stress will reduce .It can improve the metal spinning accuracy.

How to improve the strength of the metal spinning ：You can design rolling , flanging ,necking or expanding to improve the strength of structure like the following image .