[Cosha]

I am a total noob here to be honest!

I hand built my CNC machine and it works OK!

i have been using 1mm cut x 1mm shaft bits from world of cnc. i have been happy cutting 9mm MDF using these bits for the last year.

i just come to cut some 3mm MDF and i have broken 6 cutters in one sitting!

what am i doing wrong?

im using a Kress 800 milling machine

here is some pictures of my machine




my settings are feedrate 450ipm and plunge 300ipm and the kress set on full

i expect thats quite high

what am i doing wrong?

[Cosha]

here is the cutters im using

http://www.worldofcnc.com/products.asp?recnumber=161

have i got the spindle speed too high?

i have brought down the plunge rate and the movment to less than 100ipm and still makes no difference

[Donkey Hotey]

my settings are feedrate 450ipm and plunge 300ipm and the kress set on full
i expect thats quite high

How did you decide to use those feedrates? How many RPM is your spindle. Yeah, I'd say you're feeding wayyyy too fast.

With little endmills like that, I can't imagine feeding faster than 0.002 inch per tooth and that's in a super soft material.

• At 0.002" X 2 flutes, that's 0.004" per revolution.
• 0.004" per revolution X 20K RPM = 80 IPM.

That assumes that the spindle doesn't bog down. If it bogs at all, the chip load goes up and that just exacerbates the problem. For you to be feeding at 450 IPM, you'd need a 112K RPM spindle.

If your spindle is a 20K RPM, I'd start with 60 IPM feed and 30 IPM plunge.

[Cosha]

Thank-you for the reply!

The kress i have at full power idles at 30,000 RPM

i always thought it was running very fast, i was told to use this by someone at work that had been into cnc machines for a long long time!

i wasn't to know any different as i used the settings he told me to use and they have been cutting fine!

He also told me that each pass should be 1.5 - 2mm deep!

lucky i have only been cutting 0.5 per pass!

im such an idiot then, or have been made to look like one! i should of just asked the questions here first, get real advice for real people!

i guess he just hates me

[Cosha]

i just found his notes he game me over a year ago, def says 450 and 300 ipm!

i think from what your saying he should of told me 45 x 30 IPM because that would sound more like it!

[Donkey Hotey]

Whenever you're picking feeds and speeds, you have to consider the strength of the cutter, but more importantly, you need to know where the chips are going to go during that cut.

With aggressive feed, you have thick chips. If you are fully burying the cutter (slotting--180 degrees of engagement), the thick chip has to have room to curl up in that flute and feed out the top and out of the cut-zone.

Now with a really deep cut (more than 1 times the diameter of the cutter) you have a really tall, really thick chip, that is also really long (because it's cutting 180 degrees of its rotation). Chips that large can't flow and will just bind up in the flute. Once chip evacuation stops, it's no longer a cutter, it's just a dowel waiting to be snapped off as the machine continues to feed.

[kn6za]

Cosha,


300ipm like Greg says is super fast. Your machine does not look like it could even rapid that fast, are we talking mm/min or in/min.

[dertsap]

your probably better off using a 2 flt for that small of a diameter in mdf , its probably going to be plugging up with mdf dust using the three flt and burning out the tool , plus your feed is much too aggressive

[Cosha]

mm Sir,

OK i understand now thank-you

ill slow her way down...

just strange how i have been using the settings for a year or more and only snapped 2 blades?

puzzled!

she can motor along quite fast actually on all axis, its made from 1inch marine ply and sturdy?

[Cosha]

on another thought, i have now got a few spare shafts, can i use a needled file to put a D shape profile on the end of a broken one and re temper it and use it as a cutter?

what feed/Plunge rates should i be using in MM?

The software i use is sheetcam

[stevespo]

450mm for the XY feedrate sounds about right to me. That's roughly 18 IPM.

Honestly, with a 1 or 2mm endmill (.04" or .08") I would actually start running them even more slowly (maybe 10-12 IPM) and work my way up. I'd also start with a depth of cut of 1/2 the diameter of the mill, and then perhaps get more aggressive over time. I'm pretty conservative about these feedrates, so YMMV.

Are you cutting pockets with these mills or doing profiling operations? You should really be using the largest bit that the job will allow. For cutting MDF, I'll use a .125" (~3mm) bit if I absolutely have to, but my preference is to use a .250" (~6mm) minimum. That is going to give you a lot more speed in terms of feedrate and depth of cut.

You can grind/reshape HSS end mills, but what you have is carbide. Perhaps it could be done with diamond stones, but I don't have any experience trying to sharpen or reshape those.

Steve

[kn6za]

450 mm/min is more like it! You are about right with the feed rate you have been using.

in/min is a much different scenario.

[Cosha]

im doing a few pockets but mainly cutting out sheet material shapes

im actually making cockpits for my rc aircraft like this:



this was my actual first cut using 3mm lite ply again cutting with 1mm at 500mm feed x 300mm plunge cutting 1mm each pass

i think i was just lucky they never broke before then

[stevespo]

It could have been luck in the past, but perhaps something else has changed with the machine, material or setup. Those small bits are going to be sensitive to runout on the spindle. Vibration on the work will cause problems. The work has to be held very firmly.

Have you changed anything else in your process, other than the thickness of the material? Definitely consider using a 6mm bit for cutouts, and you can easily run this 4-5x as fast as the smaller bits.

Steve

[Cosha]

all that i have changed is the cutting material - thats 9mm MDF - 3mm MDF

i think i will look over the whole machine again to make sure nothing is slipping/binding or anythink like that! and run at 45 x 30 mm / min

i have one blade left!