[Grant Nicholson]

To determine feed rate, if our goal is a chipload between .015" - .020" (thickness of chip), the formula says chipload = feedrate divided by (# of flutes x RPM).

What's missing from this equation is the relationship of RPM to bit diameter.

Can anyone help me complete the formula? I'm trying to maximize my tool life. Most of my wear and tear occurs cutting wood products.

[awerby]

Any bit has a maximum size of chip it can make. A bigger bit won't necessarily make a thicker chip, but it's capable of doing that. Yes, the flutes of a bigger tool will be traveling at a slightly higher speed in relationship to the material than those of a smaller one, but that doesn't affect the chip thickness. I think the formula has everything you need, if your tool is capable of cutting that thick a chip.

Andrew Werby
www.computersculpture.com

[Grant Nicholson]

Thanks Andrew - I create a lot of artwork/sculpted items in my business, which leads to a range of diameters from 1/16" diameter to 1" diameter tooling. The speed at the outside of a half inch bit must be significantly more than a 1/16" bit.
That said, I just did a test using the formula as-is on a half" bit and produced .017" thick chips, so it certainly worked out well after all! Nicholson Design - The Fine Art of Computer Carving

[ger21]

For any specific style of bit, the chip load will be different for each diameter.
If you have two of the same model bit, but one is 1/8" and one is 1/2" diameter, the 1/2" bit will have a much higher recommended chipload.

If you want to maximize tool life, you need to cut as fast as you can, without cut quality suffering.

[Grant Nicholson]

Ger, I have been gauging my feedrates by looking at my chip size (and finish quality) during the past 4 years. Almost all of the work I've been doing has been one-offs. At this point I'm getting into production work, where tooling wear becomes a much bigger issue. When you note "recommended" chip load, does this suggest that tooling companies will have chip load ratings for all tools, or is there a table that machinists refer to?

[ger21]

When you note "recommended" chip load, does this suggest that tooling companies will have chip load ratings for all tools,

Yes, they should, but not all do. Onsrud has the most comprehensive list I've ever seen. https://www.onsrud.com/plusdocs/Doc/...ode=FeedSpeeds

[BanduraMaker]

In the actual chip load calculation, bit diameter doesn't come into play. e.g. Whether you've got a 1/2" tool or a 1/16" tool (let's say two fluted), if you move .001" in one tool revolution, you will cut off .001" of material with a chip load of .0005" irrespective of the diameter of the tool. This is not to say that tool diameter doesn't matter, it just doesn't enter into the calculation of chip load.

Bit diameter is a factor in the calculation of "surface feet / min" or SFM. This is the speed at which the edge of the tool travels with respect to the material you're cutting. Different materials and bit coatings can dictate different optimal SFM.

[nigjoe]

Chipload Calculator » Royce//Ayr

[BanduraMaker]

I like this one as it has all the parameters and it will solve for anything: Milling Speed and Feed Calculator

[jweinhold]

Most chip load charts or calculators are based off of ½”tool diameter cutting ¾” thick material.

Soft Plastic: 0.004-0.012
(very narrow chip load range for good finish)
Incorrect chip load will often be indicated by visible knife cuts or meltback of plastic.
Proper cutting will produce a curled chip

Hard Plastic: 0.003-0.009
(very narrow chip load range for good finish)
Incorrect chip load will be indicated by visible knife marks, cratering or meltback.
Proper cutting will produce a block or lightly curled chip

Wood/Wood composites: 0.004-0.040
(very broad chip load range) recommend: .015-.030 range nominal
Incorrect chipload will appear in quality of the cut.

Solid Surface: 0.002-0.015
Aluminum: 0.002-0.015
Phenolic: 0.003-0.005



Tools less than ½” diameter reduce chip load 20% per every 1/8” of diameter
Tools greater than ½” diameter increase chip load 20% per 1/8” of diameter.

Chip load is reduced as depth increases (by 25% each time the depth is doubled)
EX. Increasing from 3/4" to 1-1/2" DOC - decrease chip load by 25%

[areinike]

I'm just starting to figure this stuff out. I am cutting circular pockets in 3/4" poplar wood. The circular pockets range from 3/8" to 1 1/8" and go 0.625" deep. I have to double check, but I have been using a 2 flute, upward spiral HSS bit at 12,000 RPM (max of spindle) at 25 inches per minute. My ramp speed is about 0.125 per pass. So I figure it should cut easy but I manage to get vibrations and cuts are not 100% (of course, it is poplar).

From calculating ... I should be traveling either 144 inches per minutes at 12,000 RPM or 25 IPM at 3600 RPM? Assuming the chip load goal is about .006. Sorry, just confused. I NEVER tried cutting pockets in wood that slow of RPM and can't imagine it working, then again, I'm a newb.

[Spk64]

Sorry did not read post correctly.
The SFM for wood is generally pretty high so 3600 rpm is going to be a slow SFM unless it is a large cutter.

[areinike]

Oh, duh. Yeah, the bit is 0.25" I double checked and I have the spindle at 12,000 RPM with feedrate at (SFM) at 25 in/min. Too slow?

[Spk64]

If your machine can handle it yes crank up the feed. I run 60-100 ipm and 16,000 rpm.