GWizard Strangeness

I was wondering, why does RPM change when you change depth of cut? If you have width of cut fixed, chipload fixed, SFM fixed and just vary the depth of cut, the RPM changes???

Select constant rpm if change in rpm is not desired.

Shazam said:

I was wondering, why does RPM change when you change depth of cut? If you have width of cut fixed, chipload fixed, SFM fixed and just vary the depth of cut, the RPM changes???

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if the sfpm/rpm did not slow down with increasing Depth and width of cut its not worth much.
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normally you increase sfpm/rpm when taking lighter cuts cause you usually can unless you are getting vibration problems but with less lighter cuts usually vibration is less anyway.

DMF_TomB said:

normally you increase sfpm/rpm when taking lighter cuts cause you usually can

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What's the reason behind this? I don't understand the intuition behind this.

I've had nothing but bad experiences with g wizard...the feeds and speeds are usually way off the mark.

Shazam said:

What's the reason behind this? I don't understand the intuition behind this.

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say end mill has recommended range of 150 - 450 sfpm
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say 1" dia end mill if you go 0.5" depth of cut at full width the severe vibration at 450 sfpm will usually damage it relatively fast not to mention a bad finish left on part and unstable control and it will cut bigger than 1.0000 width as it vibrates
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at 150 sfpm you have a better chance taking a 0.5" DOC and if not vibrating too much. at 0.250 DOC you might find you can go to 250 sfpm. at 0.100 DOC you might be able to go 300 sfpm without excess vibration and at 0.020 DOC you can often go 400 sfpm as long as finish is exceptable
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the longer the length to diameter ratio the more you are limited. a 1" dia end mill 2" sticking out of tool holder can take 8 times more than one sticking out 4". when you double length sticking out your limit is 0.5x0.5x0.5 = 0.125 its the cube of the length change
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1018 steel machinability is roughly 0.8-1.2 cubic inches per minute per hp, so you can calculate hp required and the cutting forces which determine cutter deflection. as you increase sfpm maintaining ipt feed and cut depth and width the hp required goes up. so often slowing sfpm and maintaining ipt feed the vibration chatter is lowered for example going 50% rpm and 50% feed if its vibrating too much at 100% rpm and 100% feed since you are creating 50% of chips per minute you are using 50% of hp compared to going 100%
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most tooling has a type of hp limit
width/ depth of cut the most effect,
then sfpm effect has less effect
and feed has a even lower effect usually
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by using the depth and width of cut you can better calculate maximum tool limits. it is often better to take less depth and width of cut and increase sfpm and increase ipt feed. tooling often lasts longer. many a tool the feed can increase from .002 ipt to .010 ipt feed by lowering depth and width of cut. if i lower to 50% of depth but increase feed 300% its .5x3.0= 1.5 or 150% more material removed per minute. often you can remove more per minute by lowering depth and width of cut and increasing sfpm and ipt feed. when ipt feed increases it uses less hp per minute per cubic inches. if i shear a 1" plate into 2 pieces it uses a large sudden force to make one big chip (plate 1/2 sheared off) if i mill the same plate in to .001 thick chips i will use more hp to convert it all into .001 chips. calculations can become complex as it takes into account many factors. obviously if tool breaks suddenly overloading it thats not good. better calculators are trying to calculate overload limits based on many factors
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feel free to test the limits of each tool in tool holder setup to precise lengths. by recording when you have vibration problems or sudden tool failure problems you can get a ideal what the repeatable limits are. but keep in mind the part and machine vibrate too. many a big tool can take 20 hp as long as part is rigid and machine is rigid with the hp available. same facemill might run 60 ipm feed on rigid part and give problems at 20 ipm feed on a non rigid part
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better feed and speed calculators try to take into account a estimated vibration or chatter limits based on many things like tool length to diameter ratio, how many cubic inches per minute being turned into chips, etc. say i got a 0.250" dia drill bit and if sticking out 2" out of tool holder i can have rpm and feed relatively high. if i got a 0.250 drill bit that 18" long normally you have to reduce rpm and feed if you dont want it to bend and break. as you slow rpm/feed it bends less and you got less chance of it breaking. i record all sudden tool failures on standardize precise tool setups in tool holders. it dont take much to find tooling limits in a type of chart and what has the most effect on shortening tool life

I would assume the reason is HEAT (or lack thereof)

Machining is just controlling heat build up by friction. As you take a larger cut, more heat is created.
Endmills, carbide or HSS, coated or uncoated, die because of heat. Too much heat kills the tool.

So the larger the cut, the higher the friction, the more heat created.......
Following so far?

The smaller the cut, the less friction.

Now, apply the same mentality to DEPTH of cut.
The deeper the cut, the more heat.

heat is part of it but vibration is more of a limit
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limiting factors in order on importance
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1) tool stickout length and tool holder length double length has 1/8 capacity or 1/2x1/2x1/2=1/8 this effects whole tool vibrating
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2) depth and width of cut this effect tool vibrating
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3) sfpm which is rpm this effects heat limit and vibration
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4) ipt feed or chip thickness this effects vibration and too much corners break off a cutter
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basically complex formula with estimates on how powerful each effect things like sudden tool failure or corners breaking off cutting tool
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drill bit is a classic example as a 1/4" dia drill goes from 3" length stickout of tool holder to 18" length sticking out of tool holder and how deep it is drilling into part like 1" deep or 18" deep effects max feeds and speeds