Turning: the problems of bar machining/1
IN THE WORLD OF BAR TURNING, THE MOST COMMON PRODUCTION SYSTEM IS MACHINING FROM BAR. WHY? BECAUSE IT IS THE MOST LOGICAL.
In the world of bar turning, the most common production system is machining from bar. Why? Because it is the most logical. It is no coincidence that practically all single and multi-spindle machines use it: it is by far the most common system for most turned parts. This is a fact.
But above all, do we know that the bar has limitations? There are some aspects that are worth focusing on and reflecting on, even if, as an old adage goes, 'it has always been done this way'. In reality, working from bar in the traditional way is a real generator of hidden costs, which have to do with the very nature of this production process. But let's take a look at how things work by analysing the characteristics of this type of production.
Basically - reducing the necessary steps to the bone - I load the bar onto the feeder connected to the lathe and get the finished product.
Simple, no? It is understandable that everyone adapted thinking that this was the most convenient process... but is it really so? What are the main problems faced by users? Let's look at them one by one.
Vibration › The bar feeder pushes the bars into the machine's (or lathe's) multi-spindles. The bars are then clamped and put into rotation with the spindles themselves. At this point, the entire bar (or bars), as it cannot be perfectly balanced, inevitably creates vibrations that are transmitted to the spindle and also to the workpiece. In order to reduce the negative effects of the rotating bar, it is therefore necessary to:
- purchase raw material, i.e. bar, of superior dimensional tolerances and quality (e.g. drawn bar, much more precise and more expensive);
- equip the machine with more sophisticated and expensive bar feeders to reduce the negative effects of induced vibrations.
Noise › The issue of noise is closely linked to the problem of vibration and is a far from negligible problem. For this problem too, bar loaders must be equipped with sound-absorbing hoods and noise containment means. The result? Additional costs and not a complete resolution of the issue, indeed! In this regard, the case of multi-spindles is emblematic: each machine must be equipped with a bar feeder (6 or 8 in number) that are rotated simultaneously. It is easy to imagine, therefore, how the noise problem will increase in proportion to the number of machines installed.
Instability on inertia › As the bar is machined, its length is progressively reduced. This leads to a continuous variation in the inertia of the rotating mass, which makes it impossible to find an effective dynamic balance to eliminate induced vibrations.
Quality consequences › Induced vibrations in a machine of any kind, and for whatever reason, are always an extremely negative factor with more or less obvious consequences on the quality of the products machined, the life of the machine components and the life of the tools. Here are some of the main factors affecting quality:
- the negative effects on the machined workpieces are obviously due to the vibrations caused by the rotation of the bar, given that the bar is directly connected to the workpiece;
- as a result of vibrations, tools are also subjected to abnormal oscillating stresses and wear their cutting edge more quickly. Consequently, the frequency with which machine downtime is required to replace tools increases, with an increase in direct costs (new tools) and indirect costs (machine downtime for replacement).
Repercussions on the machine › Vibration is not just a noise problem that affects the working environment. Vibrations are in fact forces that are discharged in an oscillating manner throughout the machine and especially on the spindle bearings. The direct consequence is a negative effect on the expected life of the bearings and the most sensitive components of the entire machine. Here too, the result is an increase in maintenance costs and a reduction in efficiency due to downtime.
Additional costs › In order to find a solution to these problems, the user will be forced to buy a drawn bar, which is much more precise and, consequently, very expensive.
The limits are not over... shortly the effects on time/cycle and waste of material and time!