Am I cutting too slow plz?

[ImanCarrot]

I know nothing about Conventional CNC milling, but found a 3 axis thingy in the machine shop- so I drew something up on AutoCad, imported it to Dolphin and exported it as stuff the machine would understand- it amazed me by actualy working!

However, I think I'm machining too slow. I've looked all over the web and there seem like so many variables I can't get my head round it. I've got a Presto Engineering Hand book and even that fails to make it clear. It seems to depend on number of flutes, material, depth, RPM, material removal constant and the phase of the moon (perhaps).

Can anyone please tell me a rough cutting speed in mm/min? here's my setup-

Milling Aluminium with a 2mm dia End Mill (2 flutes), Depth of cut 0.1mm, Feedrate 200mm per min, spindle speed 2000 rpm.

I'm sure there's something wrong because I just guessed them all.

Many thanks in advance!

[Geof]

Originally Posted by ImanCarrot ...Milling Aluminium with a 2mm dia End Mill (2 flutes), Depth of cut 0.1mm, Feedrate 200mm per min, spindle speed 2000 rpm. I'm sure there's something wrong because I just guessed them all. Many thanks in advance!

It would be nice if you used units that an old guy can understand. Assuming carbide tooling aluminum can be cut at high surface speeds, 2000 to 3000 fpm, (roughly 1000m/min). Probably on the machine you have you cannot go anywhere near this speed; around 160,000 rpm, if I have not dropped a decimal. Feedrate depends on the tooth loading and on your teeny end mill I would be very conservative at 0.0002" for 0.004 per revolution or 4 inches per minute for each 1000 rpm. So I guess your feed rate is right there. Depth of cut you could maybe take up to around 40 or 50% of the tool diameter but this depends whether you have the full width of the tool engaged. Possibly at 100% width you might drop to 20% depth or 60% at 40%, 20% at 100%.

You also need coolant for best results.

[ImanCarrot]

Thanks Geof! as always, good advice.

I forgot about the coolant... which dun seem to be working so I'll spray it with WD40. Gonna up the cut depth to a conservative 0.4mm and see if anything breaks

*searches for safety glasses*

Haven't had this much fun in years!

Oh! the edges around the milled part (outer edges and Island edges) all seem a bit rough... is this normal? Do I just file them, or take a skim of the surface please?

[ImanCarrot]

Oh dear...

My first ever driving accident (sure it won't be the last lol).

The thingy broke.. so I stuck a new one in and that broke too- still at least I know how to change the things now

What are they called anyway? are they "bits" or "end mills" or what?

[higgrobot]

Ive never had great results with WD40 or KERO, i use either a premix cutting fluid or even tap magic. But, wd40 is better than nothing at all, someone suggested crisco!? never tried. Anyway, like geof said, 2000rpm is way too slow. For a high speed steel end mill you should be running it about 8000.
This will help with spindle speeds and stuff: http://littlemachineshop.com/Referen...tingSpeeds.php

You could try a bigger end mill, it will be stronger and the speed will be closer, try 1/4". I guess you'd have to balance between tool strength, cutter speed and machine rigidity. Obviously depends on what you are cutting, you may not be able to do it with 1/4"?

[ImanCarrot]

Thanks for the advice, I'll need to get the coolant sorted I guess.

I can't go to a larger end mill due to the gaps between the islands being about 3mm.

That site looks good! the calculator thing is v good! but it works out that I should be cutting at 8000 RPM (my machine only goes up to 4200 RPM) and a feedrate of 165fpm which equals 50295mm/min!! the thing would take off and end up at Manchester Airport lol.

At least I know now what speeds break the little thingies so I'll just potter about below this speed

[higgrobot]

Nah, the feed rate would be 8000(rpm of cutter ) x 0.0002" (depth of cut per tooth per revolution per minute) x 2 (number of teeth on your cutter) = 3.2"/min (feed rate 81.28mm/min)

The cutter's surface speed is 165fpm (surface speed of cutter on it's outside diameter feet per minute) = you can convert that to meters/min by dividing it with 3.28
=50.3 (M/min surface speed, not a feedrate)

Then 50.2 x 314 (that's pii x 100)
= 15762
now divide by your cutter size 2mm
= 15762 /2
=7881rpm spindle speed - rpm

if you can only spin 4000 then the feed rate should be adjusted..like so
4000 x 0.0002" x 2 = 1.6"/min (feed rate)
x 25.4mm (convert to non prehistoric values)
=40.64mm/min feed rate


so spin in flat out and try 40.64mm/min feed rate

[higgrobot]

Umm, i'd actually feed it higher than 0.0002, i'd consider 0.001" and see how it cuts, on larger cutters 0.006" would be great but these smaller things self destruct.

4000 x 0.001 x 2 = 203 M/min, lol you had that to start with, just spin it faster

[ImanCarrot]

Hey! I guessed right! this is my first CNC Milled part ever (apart from one engraving attempt, which dun count). I feel like a Jedi lol I must have used the "force"

I shall try them calculations again... I must have gotten something wrong since I got a totaly innacurate result!

After it's milled, I'm gonna face it off (cos it's slightly wedged) with a big thing that's about 3" in diameter with 4 square things that look like cutting tools in it. I spent an hour finding bolts and stuff to do this next step to hold it on the table, I dun think my double sided tape will suffice on this cos the cutting forces might be a bit high (I think?).

Then I'm gonna bead blast it to get rid of the milling marks.

Then, after that I'm gonna drill a hole in the back, M6 tap it and hacksaw a bolt in two so I can bolt the part to my Diamond Turning lathe's spindle and skim the face.. should give a mirror finish.

God I can see how this could get addictive! I was actualy considering coming in the weekend (unpaid) to mess about!

Still don't know what the little thingies are called that do the cutting though...
are they "bits" or "flat drills" or "flat end mills" or "end bits" cos I'm sure I shall need to order more at some point- wrecked 2 today already!

[higgrobot]

Please take a pic so we can see what it is you are playing with, i think you have either an end mill or a slot drill.

[ImanCarrot]

Thanks for the interest! I certainly will take a pic, but my camera's at home so it will need to be Monday when I bring it in

I know I probably sound silly, but it's amazing the stuff this Mill can do. I want to try everything! the more I read on it the more ideas I got!

Sorry to go on.. but it's about the most excited I've felt in ages (sad I know lol). Is there no help for me?

[Geof]

Originally Posted by ImanCarrot ...Sorry to go on.. but it's about the most excited I've felt in ages (sad I know lol). Is there no help for me?

Young people always get excited with new toys.

Old people too for that matter, I should show you my latest new toy.