First Time Milling Steel - Snapped a cutter - HELP!

[UKRobotics]

Today I tried milling a simple part from some 1/4" steel on my recently finished bridgeport interact retrofit. This was the first time I've tried milling anything harder than wood or plastic. I ran the program cutting air first and everything looked fine so I tried again a second time round so it would actually cut something and within a second or two of touching the metal the cutter snapped.

That was the only one I had so decided to call it a day and got some more on order for tomorrow. However even the cheapy end mills are not that cheap so I was hoping to glean some advice from the experts on here to save myself from snapping too many more cutters.

My suspicion is that the breakage was caused either due to a feedrate/spindle speed/cutter size mismatch or due to the way the material was clamped to the machine .. or both. or maybe something else entirely that i didnt even think of.

My setup was with a small piece of 1/4" plate approx 3" x 5" clamped in a vice with about 1/3 of the piece sticking out the side of the vice cause i was cutting from the edge and wanted a bit of clearence from the jaws incase the machine decided to have a funny moment. I'm wondering if maybe even though the vice was done up really tight, that the bit sticking out was enough to cause some deflection which then broke the cutter ?

The cutter itslef was a 0.16" diameter slot drill, spindle speed was 2000 rpm and horisontal feeds were 11ipm. Does this sound about right or was I way off the mark ?

Thinking out loud here, should I have been climb or conventional cutting and if i'd been using the wrong one is that likley to have contributed to the breakage? I'm using a new cam package and I still dont fully understand what i'm doing with it.

Appologies for all the questions and i'm sure I sound like a total noob, but this is a new machine and all my cnc experience up until now has been with softer materials which are a lot more forgiving to the blundering hobbiest. Now trying to cut a bit of mild steel and all of a sudden it seems like I have to be a lot more diligent.

[kling8]

alright with what you wrote, I think your spindle speed is to slow and your feed is to fast. I would conventional mill the part. Are you using a high speed steel cutter or a carbide, if carbide your spindle is for sure to slow. Depending on how you clamped if your part is vibrating that can break the tool. So I think you are on the right track, one thought is to call the tool rep you are ordering the tooling from and ask them for recommended speeds and feeds. Personally I would use carbide maybe around 3500-5000
and feedrate between 10-20, and what is your depth of cut?
Hope this helps good luck
kling8

[keebler303]

How deep were you cutting? You generally want to use the biggest cutter possible as they are more durable and generally more efficient.

If you are side milling the end of the piece, You could go full depth with maybe .01-.02" cutter engagement. If you are cutting full width, you probably want to use a depth of .01-.02".

Matt

[UKRobotics]

I'll try again tomorrow with higer spindle speed, lower feed rate and the part centred in the vice. Should be able to get away with a slightly larger diameter cutter too.

[Sea_en_see]

The tool maker should be able to tell you some "starting point" parameters for your cutter. Then based off of your machine, rigidity, RPM, HP, ect. you can make adjustments to the feeds and speeds. I do agree with what has been stated, too little RPM, too high feedrate. Give some more details about the cutter, and your machine.

[UKRobotics]

Machine is a 2.5ton bridgeport series II interact 4 . Tool was a 0.16" 2 flute slot drill

[rowbare]

Check out Bob Warfields GWizard. I includes an excellent feed and speed calculator.

http://www.cnccookbook.com/CCGWizard.html

a different bob

[UKRobotics]

Thanks for the link.

Tried again today, used a slightly larger slot drill, pushed it further into the collet, jacked up the table so the quil didnt have to come down so far, increased the spindle speed and decreased the feeds. Not so much a methodical approach as try everything at once and hope for the best. Did the trick though. Machined out 4 identical parts and not snapped anything yet.