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Simple Milling Tips

 

When attempting to perform Milling operations there are some simple guidelines to follow, always keep in mind you should consult your local tooling representative for advise on specific tooling and cutting parameters they have supplied to you. 

Each manufacturer has styles of inserts and grades they recommend for the optimal performance. Quickmill also recommends you call our Application & Tooling Department for advice on your particular model for some starting conditions based on your machine configuration. 

   Cutter Diameter / Positioning 

Cutter diameter to the part width of cut ratio should be approximately 70% to 80%.  For example if you have a 4.0” diameter face mill you should offset the cutter so when in the cut the over-hang of the cutter should be approximately 1.0”,  in other words the cutter is only removing a width of 3.0” of material.   

Cutter positioning is also an important thing to consider, for example if you have a 3.0” flat bar that you want to mill the face and you plan on using a 4.0” cutter, do not cut directly down the center of the work piece.

When cutting, the cutting forces are constantly changing as the inserts move through the cut. You can re-direct the cutting forces by shifting the cutter to one side of the work piece, this will stabilize the cutting forces and make it a much more stable cut.

 Cutter Pitch 

The Pitch of a cutter refers to the total number of inserts in the cutter body diameter.  There are three main classified pitch types. They are coarse, medium, and fine. During manufacturing of the cutter the pitch is determined by how much material that particular cutter is meant to remove in both depth of cut and feed per tooth. 

Coarse Pitch 

A coarse pitch cutter is designed to take more square inches of material while in the cut. A course pitch allows maximum chip clearance in the body and allows chips to pass though the cutter without restricting its formation this style is best suited for general purpose milling where adequate horsepower is available and where maximum depth is required. 

Medium Pitch

 A medium pitch cutter has slightly more inserts in the cutter diameter then the coarse pitch and it has slightly less chip clearance then the course pitch this style is best suited where more moderate feed per tooth is required and less of a milling depth.   Having an extra tooth in the cutter diameter can also help to reduce cutting pressure and maintain a higher feed rate. 

Fine Pitch 

A fine pitch cutter has again more inserts in the cutter diameter then the medium pitch and it has again has less chip clearance then the medium pitch. This style is best suited for milling a severely interrupted cutting surface. Fine pitch cutters can maintain a high feed rate per min do to the number of inserts and the feed per tooth recommendations, but can experience high cutting forces due to the number of inserts engaged in the cut at one time.    

Cutter Lead Angles/Chip Thinning/Cutting Forces 

The cutter lead angle refers to the angle at which the insert lays in the cutter body with reference to outer cutting edge of the insert perpendicular to the bottom of the tool.

Generally the more the lead angle the more of a feed per minute can be achieved, this is due to what’s called Chip Thinning.  The more lead angle of a face mill, causes the chip being formed to be thinner than the Inches per tooth programmed into the control.

Therefore if a 90deg face mill is selected versus a 45deg face mill at the same chip load, the average chip thickness will be much less on the 45deg face mill. In order to maintain the average chip thickness, for example  (.008”) when using a 45deg face mill, the feed would have to increase to accommodate for chip thinning. 

There are some calculations to determine the proper feed per min regarding the insert lead angle and the desired chip load per tooth. One advantage of chip thinning is that the horsepower draw is less with a 45deg face mill versus the 90deg face mill running at the same speeds & feeds. 

As a direct result of these two advantages a 45deg lead face mill generally speaking can feed faster than a 90deg face mill.  When ever possible try to use a milling cutter with a lead angle, this equates to faster metal removal rates, problem free milling, and will result in lower production costs.

The formulas for determining the actual chip load on lead angled cutters are given below.

Lead Angle
(Degrees)

Inch Per Tooth
(Chip Load)

Actual
Chip Load

0 deg

X

X

15 deg

X

.96 (X)

25 deg

X

.94 (X)

30 deg

X

.86 (X)

45 deg

X

.707(X)

 

Proper Tool Selection 

Selecting the right tool for the job is imperative to an efficient and profitable machining process. Researching the tooling is a must when selecting the proper tool. There are many manufacturers out on the market that offer tooling.

Being faithful to one tool manufacturer has its advantages but it also has its disadvantages. It’s sort of like the saying “don’t put all your eggs in one basket”.

Each tool manufacturer generally shines in one area better than his competitor based on specific applications, material type, and or set-up. You may often find that certain tooling manufactures have a much better milling cutter for removing harder material and so on.

This could be based on tool design, diameter, length, or insert grade. By doing your research and homework when selecting a tool you will then be able to begin to optimize your machining process.  

Work Piece Support 

Work piece support is one of the key components of a successful machining operation. If the part is not set up properly this limits your ability to machine with the maximum potential of the tool.  As much thought and knowledge must go into a part set up as choosing the right process and tooling.

A lot of the time in part set up, fixturing, and clamping becomes an over site. Carbide tooling imposes great forces when being used on both the machines axis’s and the actual part due to relatively higher speeds and feeds. There is no harm in over supporting your parts when it comes to carbide tooling, the more support the better.

Carbide is very hard but brittle and when vibration occurs it can chip or crack, this will damage, or drastically shortening the life of the tool. Unlike high speed steel which is relatively soft and can flex and be a little more forgiving in a less rigid set up.  

Always keep in mind the amount of time you spend setting up a part on a machine tool should always be gained back and most often exceeded when using proper speeds and feeds. Many times this is overlooked and said to be wasted time. 

 

Chason Thompson
Tooling & Application Specialist

 

 



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