A few questions relating to my mill

[RSG]

I've been working on getting the DRO installed and along the way have run into a few challenges so to speak.

One being the centrifugal cap start clutch was acting up, It wasn't set correctly at the factory so I fixed that....

Then last night I went to machine a bracket and stupidly turned on the machine when the endmill was up close to the edge of a block of aluminum which tore it right out of the vise. I'm hoping this won't be an issue moving forward as I could have sworn I could hear the bearings rumble in the head for a bit after but seemed to go away after a half hour or so, perhaps it wasn't the bearings after all???

My main question pertains to feeds and speed. The Little Machine Shop has a calculator to figure out RPM for milling and when I enter the info for a 3/4" dia endmill it spec's it at 840 RPM but the machine seems to perform and react better when I set the RPM around 2400. Can anyone explain why this is?

Thanks
RSG

[John Evans]

What material ? what depth of cut? what cutter step over [width of cut]? 2-4 flute mill ? mill HSS or carbide? Coolant? All those figgure into RPM and feeds,aluminum likes lots of speed compaired to steel. Doubt if you did any damage from your crash ,you are always hyper sensitive to "strange" noises etc. afterwards.

[RSG]

Thanks John, the endmill was HSS, 3/4" dia, 2 flute. The charts showed an RPM of 870 but that was way to slow for aluminum.

[John Hasler]

That calculator gives me 1020 rpm, 7 ipm for a HSS 3/4" dia 2 flute endmill cutting aluminum. It has no inputs for depth of cut and stepover, though, and gives 200 fpm surface speed which seems low.

Other sources suggest up to 3000 rpm depending on surface speed.

[GlennW]

I think a lot of those charts and calculators are made by "home shoppers" for "home shoppers".

I use some pretty decent software for CNC and it shows 2250 rpm at 18 ipm (.004 chip load) with a .550" depth of cut for full width slotting and requires 2.25 hp in 6061-T6.

It bases that on the cutter type and length maintaining a reasonable amount of deflection.

[RSG]

2250 rpm at 18 ipm (.004 chip load) with a .550" depth of cut for full width slotting and requires 2.25 hp in 6061-T6.

Well I'd never consider that depth of cut before but I also didn't have a 3 hp machine then either. Based on my experiences I think the RPM you suggest is appropriate or at least it feels like it is when using the machine. I was doing the cutting the other night at 2300 RPM and it felt smooth and did not feel like it was laboring the machine, that was with passes at approx. .200" deep.

So you feel those charts are bogus then Glenn? Or perhaps I should look to them more for other materials like steel?

On another not I think you (John Evans) were right about being hyper sensitive to noises after the crash incident. Last night while running the machine it was nice and quite.....LOL

[GlennW]

I had to make channel from 2" x 3" 2024-T3 bar.

Used the recommended settings, which was .470" doc with a 3/4 two flute and the old dinosaur blazed right through it.

It's a 3hp machine that I hardly ever use, but it has a lot of X travel and I needed every bit of it.

[RSG]

Cool, I like that two vise set up.....I could use a second one for long stuff like that!

[Mr Ron]

I machine aluminum, 6061, 2024 and 7075 almost exclusively on my mill and I use the highest speed I have available. Seems to work fine for whatever aluminum I'm using and with any size end mill. If I'm using a fly cutter, I usually reduce the speed a bit. Instead of adjusting the speed, I adjust the feed. Saves changing speeds, my mill has 9 speeds selected by moving the belt from pulley to pulley, time consuming.