N00b broke a couple bits - advice?

So I have an XCarve, one of those hobby-class CNC routers based on a DeWalt trim router spindle.

I have started doing some light aluminum work, mostly cutting bolt holes and component cutouts into thin plate (3/16" - ish)

I have been using a carbide, coated, 3-flute flat end mill for this work. At 19,000 RPM, 30 in/sec, .020 DOC, no coolant this has worked well for adaptive clearing operations. I spiral-ramp the cutter into the work, and off it goes. The shallow DOC means it takes a little while, but the ability to precisely locate holes in X/Y and cut weird component fitting shapes makes it totally worthwhile.

Yesterday though I tried a parting operation where I attempted to cut the plate to size once all the pockets were cut. The program was a straight profile starting at one end of the plate and running straight down the X axis to the other end. Cut, retract, return, cut.

At .020 DOC, 30 in/sec the cutter made it in about 1/4" then snapped.

Ok... so set DOC to .010 in (almost set it to 0.10 in but caught the mistake on the visualizer before I ran it). This ran fine until the last pass. I have the cutter extend through the work 0.003 in to make sure I fully cut through the part, this made it in about 2" (on a 7" total cut length) and *snap*.

Given that the adaptive clearing operations ran fine, there must be something different about slot cutting that places more load on the bits.

So then - what is your advice to better run these sorts of operation? Slow down, less DOC, thicker bit?

Spray some wd40 on the path to cut... Was there aluminum built up in the broken flutes?

I don't think so - easy enough to check later on.

reccedg said:

So I have an XCarve, one of those hobby-class CNC routers based on a DeWalt trim router spindle.

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You would be better served over at cnc zone dot com

I prefer to learn from people who know what they are doing.

30in/sec feed rate with a 3 flute tool at 19000 rpm is a chipload of about .031" per flute. That is pretty aggressive. Perhaps cut the feed by a factor of 10 and try. Lubricant on the cut line may help keep the tool from plugging up.

reccedg said:

I prefer to learn from people who know what they are doing.

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https://www.practicalmachinist.com/vb/general/machinery-discussion-guidelines-137724/

reccedg said:

I prefer to learn from people who know what they are doing.

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Good answer.

I also question 30 inches a SECOND.. Are you sure you don't mean the standard feed
unit of inches a MINUTE? IPM?

> I also question 30 inches a SECOND.. Are you sure you don't mean the standard feed
unit of inches a MINUTE? IPM?

Ah, yes - you are quite right. 30 IPM.

reccedg said:

I prefer to learn from people who know what they are doing.

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That may not work so well here when you ask questions about application issues on this hobby-grade machine. Few of us here have or use that type machine so not much real-world experience. Other than a couple basic questions like galling or something, I don't know what all might be wrong.

A forum where folks use similar type machines would be far more likely to provide answers or examples of what they have to do in similar situations.

reccedg said:

I prefer to learn from people who know what they are doing.

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Okay.

You are using the wrong tool for the job. Try using this one.

did you ever call out the size of the cutter? You usually want to use the biggest cutter possible. Is the cutter center-cutting?

are you plunging in Z or ramping in Z as you're cutting around?

yes, cut dynamics are strange when you're engaging 100% of tool diameter; when you're doing adaptive clearing or whatever, usually those operations default at 50% stepover since that keeps the tool loaded properly.

Also, something to think about is that a dewalt spindle isn't anywhere near as true running as a real cnc spindle. a thou or two of runout could cause tool breakage that you wouldn't see on something with higher precision bearings.

Yeah, let us know what size tool you're using and maybe we can get you a workable recipe. How's your fixturing done? Rigid or ringing like a bell?
Got an air compressor? Rig up mist coolant, or at least an air blast. It'll help you a TON. Chip evacuation is likely your biggest problem.

Can't do that? Get a 1 (Sometimes called an "O") flute endmill for maximum chip clearance, or at least a good 2fl designed for aluminum. If all else fails, a spritz of WD40 will work in a pinch. Get stub length tools and keep your stickout as short as possible.

you're never going to make gorgeous parts on the xcarve, there's simply not enough lead in their butt to counteract cutting forces.

carbide tools like to climb cut, but heavy climb cutting requires rigidity like mass and ballscrews / servos. your belt and stepper setup, plus the ~60 lb weight of your machine, is nowhere near stout enough to get these tools to perform the way you would like

agree with npolanosky. single flute HSS cutters (read: router bit) with some kind of chip evacuation would be your best bet.

coyoinu said:

did you ever call out the size of the cutter? You usually want to use the biggest cutter possible. Is the cutter center-cutting?

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AH, you're right - I skipped that.

0.0625, centre cutting,

are you plunging in Z or ramping in Z as you're cutting around?

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For the cuts that worked with no issue, helically ramping in. For the problem cut, I'm leading in off the edge of the part.

yes, cut dynamics are strange when you're engaging 100% of tool diameter; when you're doing adaptive clearing or whatever, usually those operations default at 50% stepover since that keeps the tool loaded properly.

Also, something to think about is that a dewalt spindle isn't anywhere near as true running as a real cnc spindle. a thou or two of runout could cause tool breakage that you wouldn't see on something with higher precision bearings.

Click to expand...

I'm using a high-precision collet to help with that... but you are right that there might be runout in the bearings. On the list for me to check.

That's a fairly small endmill. Try lowering the feedrate.

Yikes! 1/16", 3fl? What's the flute length? If you can profile with a larger tool you'll have way better results. Anything below 1/8" starts getting really fragile, especially if you have a flexible machine with potentially sloppy bearings. I'd be nervous about cutting full slots with a VMC with flood coolant, nevermind an Xcarve dry. Try a 3/16" or 1/4" endmill instead. A tool with twice the diameter is 16x more rigid, and likewise a tool with half the stickout is 8x more rigid. Stack the deck in your favor as much as possible.