Hi everyone!
Here is Paul from Poland. I have not found discuss about mold milling, so i start to discuss.
I am beginner in this part of cnc machining so i am glad for any advice.
This is my story.
Our customer gave me stp model and material. It was Tolox44 (44HRC). First i wanted to mill on HAAS VM3, but after little espresso my brain decided to mill that baby on UMC750 using indexation of B and C axis because of places impossible to reach on 3 axis and i don’t have EDM to burn it.
Smallest radius in this shape was 0.5mm (0,019").
**The problem was - UMC750 is no precise during indexation, 0.05mm (0.002") difference. **
Second problem was: use bigger ball mill 6mm and then rest mill with ball mill 1mm? Or do everything with smallest ball 1mm? What do you think is better? What is your experience? I wanted the same Surface on each walls. Using different tools is visible on surfaces. More hand polishing.
I decided to do all Surface with smallest ball mill 1mm (0.0032"). It was not economical, but it was good way to success. 
I made precise rest milling with end mills 3mm and 2 mm (imachining 3d from SolidCAM) with 0.05mm stairs and offset 0.2mm because i wanted this small ball mill to take the same layer everywhere (can crash in full material). After that i made milling by ball mill all sufraces with offset 0.1mm (0.0039") because of machine precise. It tooks 3 hours.
After that quality manager took measuring arm and told me the difference on each walls.
All walls was made in separate oprerations on Solidcam. So - i could precise change offsets on each operation to reach ZERO.
And once again 3 hours of kissing that baby and cathing micrones 
Effect: lot of time, but mold was PERFECT. Difference was near 5 micrones (0,00004")
Customer was happy Without measurming arm i could do nothing. This is key to quality and success. You will fail your job on DMG Mori without measurment equipment.
My advices - keep temperature in your shop on the same level. (as possible)
Do not start this kind of work in the morning when shop is cold after night and machine are cold.
Working on Haas you will have suprise in Z axis after few hours of milling
-after roughing cool down mold. Give this baby one hour to get a rest after next kissing
Never finish hot mold alfer hi speed roughing, you will avoid suprise with dimensions after.
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