Cost per cutting edge – why it is lower for a negative insert
A negative insert, like the VNMG insert in the picture, has a zero relief angle. Both faces of the insert are therefore the same, and you get cutting edges on both.
The cost of a positive and negative insert is almost the same, but if you get twice the number of cutting edges you are halving the cost per cutting edge. So use a negative insert whenever possible. Read this post for a detailed explanation on the difference between positive and negative inserts, and their applications (the ‘whenever possible’).
Another thing, what we do in Vodenicharov and son is to use inserts with this geometry in this priority W->C->T->D->V (if possible, because the geometry of the part may not allow this). Keep in mind that different geometries of turning inserts can perform work with different loads (see the following figure for a description).
That will decrease your budget and make your parts cheeper and your customers will have smaller prices. That make you more smart and competitive.
Effects of Cutting Conditions for Turning and Insert Life:
Ideal conditions for cutting are short cutting time, long tool life, and high cutting accuracy. In order to obtain these conditions, selection of efficient cutting conditions and tools, based on work material, hardness, shape and machine capability is necessary.
This will save you a lot of money and time when replacing worn out lathe inserts that were selected incorrectly or with the wrong settings. Here I have described everything about insert wear.
1.CUTTING SPEED
Cutting speed effects tool life greatly. Increasing cutting speed increases cutting temperature and results in shortening tool life. Cutting speed varies depending on the type and hardness of the work material. Selecting a tool grade suitable for the cutting speed is necessary.
Effects of Cutting Speed:
1. Increasing cutting speed by 20% decreases tool life by 50%. Increasing cutting speed by 50% decreases tool life by 80%.
2. Cutting at low cutting speed (20–40m/min) tends to cause chattering. Thus, tool life is shortened.
3. Therefore, increasing the speed wears out the lathe insert, which will increase your costs. You will also have downtime to replace worn inserts.
2. CUTTING FEED
When cutting with a general type holder, feed is the distance a holder moves per workpiece revolution. When milling, feed is the distance a machine table moves per cutter revolution divided by the number of inserts. Thus, it is indicated as feed per tooth. Feed rate relates to finished surface roughness.
Effects of Feed:
1. Decreasing feed rate results in flank wear and shortens tool life.
2. Increasing feed rate increases cutting temperature and flank wear. However, effects on the tool life is minimal compared to cutting speed.
3. Increasing feed rate improves machining efficiency, which means you will have less costs for purchasing new turning inserts.
3.DEPTH OF CUT
Depth of cut is determined according to the required stock removal, shape of workpiece, power and rigidity of the machine and tool rigidity.
Effects of Depth of Cut:
1. Changing depth of cut doesn’t effect tool life greatly, therefore you will safe money and machining time.
2. Small depths of cut result in friction when cutting the hardened layer of a workpiece. This tool life is shortened.
3. When cutting uncut surfaces or cast iron surfaces, the depth of cut needs to be increased as much as the machine power allows in order to avoid cutting impure hard layers with the tip of cutting edge to prevent chipping and abnormal wear.
The following figure shows the maximum cutting depths of different turning inserts. Keep in mind the limitations of each different geometry.
Conclusions
From this information we understand that when trying to reduce the cost of turning tools we should increase the cutting parameters in the following order:
DEPTH OF CUT->CUTTING FEED->CUTTING SPEED
*Thanks to Sandvik Coromant and Mitsubishi Materials for the photos
