If manufacturers do not modify their existing conventional lathes or lathes that have been in use for a long time, and instead simply purchase new CNC lathes, it will significantly increase their equipment costs. Therefore, retrofitting conventional lathes and lathes that have been in use for a long time with CNC technology is an inevitable path for manufacturers.
The process of CNC retrofitting is complex and chaotic for manufacturers, but how to control the quality of the retrofitted CNC machine tools is a question we have been exploring. Here we will discuss how to control the quality of retrofitted CNC lathes.
CNC retrofitting of conventional lathes is divided into new machine retrofitting and old machine retrofitting. New machine retrofitting involves the user purchasing a conventional lathe or a basic lathe (referring to a lathe with only a headstock and longitudinal and transverse guideways), and the retrofitting company performing CNC retrofitting according to the user's requirements. Old machine retrofitting refers to the user refurbishing and CNC retrofitting of an already used conventional or CNC lathe. Old machine retrofitting includes overhauling lathes and modifying user-owned machine parts. This article briefly discusses the mechanical quality control methods, key control points, and inspection process for CNC lathe retrofitting.
1. Both new machine retrofitting and old machine overhauling lathe retrofitting must undergo the following identical modifications.
(1) Replace the X-axis and Z-axis lead screws, bearings, and motors.
(2) Add an electric tool post and a spindle encoder.
(3) Add an axial motor drive unit, a limit switch to limit overtravel, a frequency converter (as required by the customer), and electrical components required for processing and safety.
(4) Scraping the bearing surfaces at both ends of the lead screws of the X and Z axes, scraping the ball screw pair bracket and the saddle, and scraping the bed and the saddle guide rail pair.
(5) Add protective facilities as needed, such as protective covers for lead screws, safety doors, and protective devices for limit switches.
2. Differences between new machine modification and old machine overhaul lathe modification
(1) The spindle and tailstock of the new machine have not been modified and no further modification is required.
(2) Due to long-term use, the guide rails of the old machine overhaul lathe are worn. In order to ensure that it can continue to be used for a long time without deformation after the overhaul, it must undergo a quenching process and then grind the guide rails. After grinding the guide rails, the hardness of the guide rails must be ≥ HRC47.
(3) When overhauling an old lathe, the spindle and tailstock parts should be modified and adjusted according to the customer's needs.
3. Precision inspection of new machine modifications and overhauls is an important inspection item.
Accuracy inspection shall be performed in accordance with JB/T8324.1-1996 "Accuracy of Simplified CNC Horizontal Lathe".
4. The precision and quality control of the new lathe modification is as follows:
(1) Scraping Inspection. The new lathe was modified by scraping the support surfaces at both ends of the lead screws of the X and Z axes, scraping the ball screw pair bracket and the saddle, and scraping the bed and the saddle guideway pair. The spindle and tailstock of the lathe were not disassembled. The inspection method is as follows: the mating surfaces are colored, the mating surfaces are joined together and rubbed against each other, and then the scraping points are checked. The degree of joint is checked with a feeler gauge. The scraping points should not be less than 6 points/25*25mm. A 0.03mm feeler gauge should not be inserted into the joint.
(2) Parallelism inspection of lead screw and guide rail: When assembling the lead screw, the parallelism between the lead screw and the guide rail must be ≤0.02mm.
(3) In the accuracy inspection item G1, the straightness of the guide rail in the vertical plane (only convexity is allowed) should be guaranteed by the ordinary lathe manufacturer and is not a key inspection item.
(4) The accuracy of the spindle part in the accuracy inspection, items G4, G5 and G6, should also be guaranteed by the ordinary lathe manufacturer and are not considered as key inspection items.
(5) The equal height of the headstock and tailstock centers in item G11 is guaranteed by the ordinary lathe manufacturer and is not a key item for quality control by the modification manufacturer.
5. Precision inspection of user-modified overhaul lathes
Because the guide rails have been ground, the reference surface has changed. Therefore, all items in the accuracy inspection must be inspected and strictly controlled to ensure the performance after the modification.
6 Other key quality control points for overhauling lathes and modifying new machines
(1) Rust inspection: all horizontal and vertical guide surfaces, main shaft, main shaft flange, and tailstock hollow sleeve.
(2) All exposed non-painted surfaces must be protected against rust, such as cleaning them and then checking for rust prevention with grease. The scraped surfaces, lead screws and bearings must be cleaned before assembly and must not contain red lead powder, iron filings or other dirt. The inside of the electrical box and the inside of the protective cover must be free of dust and dirt.
(3) Leakage inspection: The spindle bearings and gears of the overhauled lathe must be kept lubricated. The axial lead screws and bearings of the overhauled lathe and the new lathe must be lubricated and have a cooling device. There must be no leakage at the joints of the above lubrication and cooling, oil tanks, etc.
(4) Machine tool noise, temperature rise, speed, and no-load test:
① The spindle is continuously idled for 4 minutes at various speeds, with the highest speed running time not less than 2 hours. The total idle running time of the machine is ≥16 hours. Simulated idle running tests are conducted on cyclic turning of arcs, threads, outer diameters, end faces, etc.
② After the spindle bearing temperature stabilizes, measure the bearing temperature and temperature rise. Rolling bearing: temperature ≤70℃, temperature rise ≤40℃; sliding bearing: temperature ≤60℃, temperature rise ≤30℃.
③ The machine tool noise sound pressure level under no-load conditions should be ≤83dB(A), and there should be no abnormal screeching or impact sounds. The feed motion in each axis direction should be smooth, without obvious vibration, chattering, or crawling.
④ The machine tool was tested during continuous no-load operation. It operated reliably and stably without any faults within the specified continuous no-load operation time.
(5) Modification of user-replaced parts (including machine tool repair): Since there are many modification projects for replacing parts of lathes, the main ones are replacing spindle bearings, axial lead screws, axial motors, axial bearings and systems.
① Replace the spindle bearing: Since replacing the spindle bearing is to ensure the accuracy of machining the outer circle and end face, the noise of the spindle must be checked first after replacing the bearing. Under normal circumstances, the noise level of the whole machine must not exceed 83dB(A). Then, the machining accuracy is checked, and the surface roughness of the machined workpiece is also checked.
② Replacement of Axial Lead Screw Inspection: Inspect the positional accuracy in all directions to ensure it is within the specified range, and that the running machine operates without abnormal impact noise or disturbances during axial movement. Replacement of Axial Motor: Since no other modifications were made, the inspection only checks the noise level of the running machine during operation, ensuring there are no abnormal impact noises or disturbances during axial movement. Inspect its axial backlash to prevent assembly issues that could cause the backlash to fail to meet requirements.
③ Replace the axial bearing: When replacing the axial bearing, it is necessary to ensure that the axial reverse difference meets the requirements and check for any abnormal noise.
④ System replacement test: If the system is replaced, only the system function is tested, whether there are any alarm phenomena, and the thread test is also tested to see if it is normal (for lathes with encoders).
