A bit of math saves time, money in high-speed machining

High-speed machining can reduce cycle time for molds and dies, produce better surface finishes, reduce polishing time, decrease lead time to market for the customer, and enhance productivity. These gains however do not come automatically with the deployment of high speed machining technology.

Proper speeds and feeds in the high speed machining process play a vital role in helping operators to achieve the result expected from this advanced technique.

In this article I will discuss the importance of using mathematical formulas. We need to rely on these as a means of determining a range of speeds and feeds for the correct tool operation.

What kind of difference can the correct speeds and feeds of a cutting tool make? Conventional machining allows some forgiveness if an operator uses the incorrect values. With high-speed machining the use of incorrect speeds and feeds can result in burned out tooling, broken tooling, damaged work pieces and equipment, and even injury. Using a high-speed machine in a conventional manner is a waste of the investment. In order for industry to remain competitive in a global environment proper use of this technology is a must.

So where do I start? Let’s look at a couple of sample calculations to see how mathematics can help us use the proper machine settings. These are the formulas we will need to use:

RPM = SFM x 3.82

Tool Diameter

FEED RATE (IPM) = RPM x Number of Teeth x Chipload

SFM = RPM x Tool Diameter

3.82

Chip Load = IPM (feed rate)

RPM x Number of Teeth

RPM — revolution per minute (speed)

SFM — surface feet per minute (feed)

IPM — inches per minute (feed)

For the purpose of this article I will use graphite as the material for the electrode. We will examine both roughing and finishing applications.

*Chip load for graphite: Roughing – 0.003 – 0.005 feed per tooth

Finishing – 0.001 – 0.003 feed per tooth

**Cutting Tool: 0.375 in. diameter; 4 fluted, TiALN coated Solid Carbide cutter

Surface feet per minute for graphite: 400 to 3000 SFM

Example #1

Let’s say we want to cut at 1000 SFM. What RPM and feed rate do we need?

RPM = SFM x 3.82

Tool Diameter

RPM = 1000 x 3.82

0.375

RPM = 10,186 RPM

Roughing Application: 0.004 chip load

FEED RATE (IPM) = RPM x number of flutes (cutting teeth) x chipload = 10,186 x 4 x 0.004 = 163 IPM (in./min.)

So machining graphite in a roughing operation at 1000 surface feet per minute with a 3/8 in. tool would require you to run the machine at a feed rate of 163 IPM.

Finishing Application: 0.002 chip load

FEED RATE (IPM) = RPM x number of teeth x chipload = 10,186 x 4 x .002 = 81 IPM (in./min.)

So machining graphite in a finishing operation at 1000 surface feet per minute with a 3/8 in. tool would require you to run the machine at a feed rate of 81 IPM. By continuing these calculations for different tool sizes speeds you can build a chart, a portion of which is shown below (Table A).

Table A: Speed and feeds chart

Material: Graphite Chip load: Roughing: 0.004

Tooling: 4 flutedFinishing: 0.002

Use SFM range recommended for cutting tool material

Guidelines and hints:

1) cutters over 3/4 in. diameter should be indexable carbides

2) TiALN coated tooling for graphite is a good buy

3) Rigid setup, vacuum system for dust removal is critical

4) Avoid tool without corner radius where possible

5) Use positive rake tools

* Obtained from manufacturing specifications for a given material

** For graphite application I would recommend TiALN or Diamond coatings. Diamond coating costs about 8 to 10 times the cost of TiALN coated cutters. Diamond coated cutters have a longer life but operators must be aware that they require special care in handling.

Mark is the chair of manufacturing and transportation at St.Clair College in Windsor, Ont.

Cutting tool materialRecommended cutting SFM

High Speed Steel end mills400 – 600

Carbide end mills600 – 1200

Coated Carbide end mills1000 – 3000

Inserted Indexable carbide cutters1000 – 3000

Tool Dia.SpeedSFMOperationFeed Rate

RPMIPM

0.12512,000 to400 toRough192 to 1440

(1/8)90,0003000Finish96 to 720

0.18758,100 to400 toRough130 to 978

(3/16)61,0003000Finish65 to 489

0.3754,000 to400 toRough64 to 488

(3/8)30,5003000Finish32 to 368