Cutting tools: What is insert grade, and insert grade selection
Insert grade selection is important because there is no one ‘carbide’. There are carbides and carbides and carbides and…
Metal cutting is merely a controlled scratching of one material by another. The harder material scratches the softer one. Scratch hardness is defined on the Mohs’ scale of hardness (developed by Frederich Mohs, a German geologist, in 1822), on which Talc is 1 and Diamond is 10. Mild steel is 5 and glass is 7. Tungsten Carbide is 8.
The three key qualities of a tool material are Hardness, Toughness and Wear resistance. Toughness is the ability of the material to withstand interrupted cuts. A tool requires higher hardness to cut a harder work piece material, but requires higher toughness to withstand interrupted cuts. Unfortunately, some of the materials with the highest Mohs hardness are also the ones with least toughness. Tungsten Carbide, Ceramic and Diamond are all very hard but have very low toughness.
A tungsten carbide insert actually consists of hard tungsten carbide particles bound together by a soft cobalt binder. Want more toughness ? Increase the binder and reduce the carbide. Want more hardness ? Decrease the binder and increase the carbide. Insert material design is actually a balancing act between hardness and toughness, and a cutting tool manufacturer has a different grade of carbide for each application. An application is defined by a combination of these:
1. Work piece material
2. Type of operation – face milling / turning / threading, etc.
3. Amount of material removed – roughing / medium machining / finishing
4. Rigidity of workpiece holding – overhang, vibrations, etc.
5. Extent of interrupted cutting
An insert’s material, or grade, is designed to suit a particular machining application, and even though two inserts may look the same, the base material and the coating (if there’s one) can be very different. The tendency on a lot of shop floors is however to think of ‘carbide’ as a single tool material, use whatever insert is on hand in the shop, and then blame the tool manufacturer for poor tool life and high tool cost.
Action point
Study the tool manufacturer’s catalog and do insert grade selection seriously, based on the application. Doing this can make a big difference to your shop’s profitability in CNC turning and CNC milling.
Text source: CADEM NCyclopedia multimedia CNC training software.
Here’s the explanation
The cutting load is proportional to the cross sectional area of material being removed. If the depths of cut are constant, the load increases with each cut. The load in cut 2 is twice that in cut 1, in cut 3 it is 4 times more than in cut 1, in cut 4 it is 6 times more, etc. Disastrous for the tool and the part.
To prevent this, controllers have a constant area cutting logic in the threading cycle. The depth of each successive cut is reduced to keep the cutting area and hence cutting load constant. Unfortunately for the programmer, the G76 Fanuc threading cycle (as well as on Haas and Mitsubishi) requires that you specify the first depth of cut in the threading cycle command. They calculate the remaining depths of cut from this. This involves a small calculation, and most programmers do not do this, ending up getting the thread right after some trial and error that involves rejecting a few parts.
This is how you calculate the first depth of cut – just 3 steps:
1. Determine the number of cuts based on the workpiece material, type of thread (Metric, UNI, etc.) and the pitch, from the tool manufacturer’s catalog.
2. Use this formula to determine the first depth of cut.
E.g., if the thread depth is 1.28 and the number of cuts is 8, the depth of the first cut is 0.45.
3. Use this value in the threading cycle. E.g. on a Fanuc controller you would program this as Q450 in the second line of the G76 cycle (the value is programmed in microns in Fanuc).
Action point
Ensure that this simple method is used for calculating the first thread depth to program threading.
OR
Get a software like CADEM CAPSturn CNC lathe programming software that automatically determines the number of cuts based on the type and size of thread, and outputs the correct value in the threading cycle in the program.
