Which is Best - Deeper Plunge/Faster Feed

[bill south]

This weekend I was milling parts for my bridgeport conversion using .25" 6061 and a 1/8" carbide cutter. I had the speed set at 5 ipm and a plunge of .06" on my lathemaster benchtop. Well, broke 2 mill tools (ouch) and then slowed down the feed to 3 ipm and everything was fine. BTW, using cool tool misting for coolant.
Afterwards, a question came to mind. Which is better A lighter plunge (Maybe .005) and a faster table speed or am I on the right track with a heavy plunge and slow speed?
Can you tell I'm new to machining? Help from an expert will be appreciated.
Bill from Spartanburg.

[pastera]

I think it is more a question of cutter RPM and chip load - at 1920 RPM max you are not even coming close to the recommended cutting speed for aluminum (500-600 SFM for HSS; 2820 carbide).

Take a look at the broken cutters - Is there buildup on the cutting edges?

Aaron

[RICHARD ZASTROW]

When you plunge straight in, be sure you are using an endmill that is center cutting and has adequate clearance angle to avoid rubbing behind the cutting edge. Remember, the cutters surface speed decreases as the diameter decreases at the same RPM to "0" at dead center.

[DareBee]

5 IPM is fine as long as you are running maximum RPM (1920) and using a 2 flute cutter.
.06 DOC is good but you MUST be blasting the chips out.
This should work really well.

You don't have enough RPM to make up the time doing light cuts.

[bill south]

Thanks guys. That's what I kinda thought. Guess it's time to look at the motor upgrade for more rpm. Always gotta tinker with something!
Bill

[mrk]

I am also still very new to CNC so this topic is of interest to me since I am doing a good bit of work with 6061.

I was once told that on machines with limited spindle speed it is best to stick with HSS and avoid carbide end mills?

Yesterday I was cutting a .400" deep slot with a .250" EM using .1" DOC and flood coolant. On the first cut, I did not turn on the coolant until a few seconds into the cut I also did not have it on full blast. The result was the EM clogged up right away (I hit the E-stop before anything bad happened). After that I changed my coolant spray system to use a full pressure stream so the chips would get cleared away and was able to cut 10 IPM with .100 DOC at a speed of 3500 RPM.

I actually am trying to adjust my thinking to use surface speed (i.e. fpm) and chip load rather then thinking in rpm and feed rate.

In general I use a cutting speed of 260 fpm for my 6061 calculations. Since my CNC has a max spindle speed of 3500 RPM I adjust the surface speed rates and chip load for the calculations when working with small tools. I also find that flood coolant makes a big difference and it is important to have enough pressure to clear the chips away from the cutting zone.

With a .125" EM I would use a cutting speed of 115fpm and a chip load of .001". The calculations I use for RPM come from the WIKI page. I also try to use about 50% of the tool size as the maximum DOC. I am still simply guessing at chip load. With a small EM I usually get nervous and reduce the chip load so in the case of a .125" at 115fpm I might use .001" chip load for a feed of 7ipm.

With bigger tools I use .005" chip load and a surface speed of 260fpm since my machine can turn them fast enough. Based on some of the comments I see on the forum wonder if I should use a larger cutting speed value for my calculations.



p.s. Soaking EMs in drano is a good way to clean them off if they are clogged with aluminum since the drano will melt the aluminum. Just be careful to treat the drano with respect.

[DareBee]

MRK

Yes and no to HSS in small diameter.

NO you don't need carbide for tooth wear issues.
YES you DO need carbide because small dia HSS endmills flex like a SOB

My rule of thumb is never use HSS for cutters smaller than 1/4"
Example pricing for 2 FL 1/4 dia
"China" brand HSS $6
"China" brand HSS TiN $6.50
Niagara Cutter HSS $14
"NN USA" carbide $14
"NN USA" carbide TiN $17
Niagara Elite carbide TiCN $32

As you can see accept for the China cutter that the carbide is so close in price it might as well be used anyway.

IMO for aluminum cutting, a cutter such as the Elite will cut machining time by 30-50% and outlast an uncoated carbide 3 - 1. How do you beat those numbers?

[mrk]

Thanks!

I will put carbide on my shopping list.