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    Default CNC router started breaking end mills after an accident.

    Hi All

    I have a DIY CNC router I built myself, out of aluminum extrudes. I mainly use it for cutting aluminum, in form of 10mm thick plates. Out of those I produce plates and planks of various shapes, containing grooves, slots and holes - the standard 2.5D machining. Using 4mm end mills.

    In order for the workpieces not to become loose while the machine is working, I set the machine to cut all the way down minus 0.1 - 0.2mm so there's something to hold the pieces... Also I use SolidCAMs iMachining feature to cut the plates.

    Another relevant detail is that I set the Z-level manually, and that eventually became the cause of the accident. A few weeks ago, apparently because I had made a small mistake in the Z-level, the machine had cut one of the pieces all the way and it had became loose. Subsequently the loose piece had impacted the spindle several times, since I wasn't there to stop the machine. As a result the spindle became noisier (or changed the sound it makes while spinning) also one of the ball-nut-screw assemblies became noisier too. Luckily there was noting bent. So I set another workpiece and put the machine to work - no problem, the machine worked fine. Also after few workpieces done the noises in the spindle and screw seem to disappear.

    All that continued until the last week, when the machine started breaking end mills like "crazy". Usually one end mill goes through three plates producing tens of parts before it wears out (and it doesn't break, it starts to squeal loudly). That's about 15 hours of non-stop work. And now I need four end mills to finish a single plate. All end up broken.

    My first thought was that something in the machine had became loose and started re-tightening every bolt. I did find loose bolts in a quite crucial spot - holding two of the linear bearings of the X-axis. I tightened them and put some "liquid steel" epoxy glue, to make sure those bolts never become loose again. The glue also binds the bearings to the frame besides the bolts. So I thought the problem was solved, but after three more broken end mills in the span of 2 hours I realized that the problem isn't solved at all. I continued looking for loose bolts and found none in the whole machine. I used an indicator see if there was an actual play in any of the axes, and found "none". I mean whatever play there was, it was well within acceptable tolerances and the way it was before the accident. So no axial play in any of the screws, no play in any of the linear bearing, the frame is nice and tight. the connections between motors and screw are tight and "healthy"

    So now I'm out of options.

    The two things that remain are:
    The screw that had became noisier could have dints made by the bearing balls at the positions where it was at the moments of impact. So the nut-screw assembly is mainly tights except wen it "hits" those specific spots. but that would mean the end mills should break when they reach those few specific spots and they break at random places, and not near the said problematic spots of the screw (since i know at what positions that axis was when the impacts occurred).

    The other weak point that remains is the spindle. There could be damaged bearings, or even slightly bent shaft. The spindle doesn't show any obvious play when I try to move the shaft back and forth. Can damaged bearing be a problem even if their play can't be felt by hand?

    Obviously the spindle is the biggest weak point that could be the cause, but due to my "brilliant" engineering unmounting the spindle is going to be a tough job, so I needed someone to confirm my suspicion before I undergo the procedure... Also as more experienced machinists you might suggest something I haven't thought of. Of course you can't tell for sure since you haven't even seen the machine, but I'm interested in your opinion, as to what might be the most likely cause?

    Thanks a lot in advance!

    Best regards
    Ivan

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    Last edited by Ivan-Ivanov; 05-22-2018 at 10:26 AM.


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    Default Re: CNC router started breaking end mills after an accident.

    If the spindle is all good on my opinion maybe it lacked the RPM the way it used to be, how many RPM is your spindle? I crashed some of the spindles on our units but usually I only get a broken tool bit or a busted fuse from abrupt crashing causing the amperage of the controller to spike and exceed the fuse amp. rating thus blowing it. I also experience broken tool bits by accidentally cranking the speed control knob half way making the spindle turn slower than it used to be while the movement of the XYZ is still the same then voila! I just snap a carbide tool bit...ohh well



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    Default Re: CNC router started breaking end mills after an accident.

    If you have an indicator you can check the spindle for runout....that should tell you if it is the problem.

    Everything in moderation, including moderation.


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    Default Re: CNC router started breaking end mills after an accident.

    Hi

    Both before and after the accident I spin the spindle at 14'000rpm, or that's what the VFD shows. All the other conditions are also the same. The problem is that during the accident, after breaking the tool the out-of-place piece had been caught by the spindles chuck. I could see the deep scratch marks made by the chuck on the offending piece. For all I know the piece got dislodged, was thrown on top of the plate where it got kind of pressed between the spinning chuck and the plate it jumped off of, and apparently there were two or three such collisions since the program continued running. The machine was still executing the program when I arrived. I mean it was bad. All of the other pieces were done, only held to the original plate by 0.2mm leftover on the bottom and they were all moved by the multiple impacts. Since the plate ends up looking like a window frame after all the pieces are cut, in this case one of the beams of that "frame" was bent quite a bit, suggesting a strong impact. Well its aluminum but still.

    It is possible that the VFD got overloaded and is currently not working at 100%. I.e. the vfd doesn't provide enough power to the spindle. Although I can't hear a drop in rpm when the spindle gets actually loaded.



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    Default Re: CNC router started breaking end mills after an accident.

    Quote Originally Posted by eman5oh View Post
    If you have an indicator you can check the spindle for runout....that should tell you if it is the problem.
    Well yes, I had to think of that I'm going to try it...



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    Spindle runout should always be checked after a hard crash. This especially in the case of the smaller spindles with considerable stick out.
    Quote Originally Posted by Ivan-Ivanov View Post
    Hi

    Both before and after the accident I spin the spindle at 14'000rpm, or that's what the VFD shows. All the other conditions are also the same.
    The value of the spindle drive speed readout is unknown and drive dependent. A digital tach is always a good investment.
    The problem is that during the accident, after breaking the tool the out-of-place piece had been caught by the spindles chuck. I could see the deep scratch marks made by the chuck on the offending piece. For all I know the piece got dislodged, was thrown on top of the plate where it got kind of pressed between the spinning chuck and the plate it jumped off of, and apparently there were two or three such collisions since the program continued running. The machine was still executing the program when I arrived. I mean it was bad. All of the other pieces were done, only held to the original plate by 0.2mm leftover on the bottom and they were all moved by the multiple impacts. Since the plate ends up looking like a window frame after all the pieces are cut, in this case one of the beams of that "frame" was bent quite a bit, suggesting a strong impact. Well its aluminum but still.
    If you bent the aluminum there is a goid chance the spindle took a hit.
    It is possible that the VFD got overloaded and is currently not working at 100%. I.e. the vfd doesn't provide enough power to the spindle. Although I can't hear a drop in rpm when the spindle gets actually loaded.
    In most cases you would get a complete VFD failure. If the RPMs are right and you dont hear a significant slow down in speed while cutting most likely the VFD is good.

    What i find intersting here, at least in the way i understand this description of yours, is that the ed mill breakage started up a week or so after the crash. If so i have to ask what else changed?

    Did you change cutters?
    Did you change lube/coolant?
    Did you change material suppliers?
    Did anything else change?

    As for the cutters breaking how exactly are they failing? That is are you seeing aluminum welding to the cutter or does it look like a sudden snap.

    One thing that can most certainly kill a cutter, especially one operated agresively, is bad balls in a ball nut. The failure here would be a ball or balls that catch while running through the nut. If the catch is momentary the drives can end up loading the system such that when the ball rekeases that spring force trusts the axis ahead. The only good way to check a ball screw in my opinion is to pull the screw and run the nut by hand the entire length of the screw several times. The gotcha here is that bad balks will not catch everytime through the nut.

    If you can look back at the failures to seeif they are associated with a common axis. This may help to focus your debug energies. In other words if all breakage is when one axis is moving you likely have localized your problem.

    The trick here is to avoid looking at the big picture with to strong of a bias. Take your diagnostic efforts one section of the machine at a time.

    By the way leadscrew support bearings can have similar issues with bearing catches.

    One other question pops into mind here, are you holding dimensions otherwise? That is if the spindle is good the parts should be coming out to size the same as before the crash.



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    Default Re: CNC router started breaking end mills after an accident.

    I will concur on all of Wizards points except one... I have seen spindles that will cut parts perfect and still have bad bearings in them; granted these were on commercial mills.

    But the basic mechanical principles remain the same. A bearing can be bad in either the radial direction, the axial direction, or both. You could have a bad bearing that only shows up when the forces change direction on the bearing.

    I would be curious to know if the breakage occurs when you are changing directions in any of the axes, X, Y or Z.

    I think Wizard provided an exhaustive list for your troubleshooting. I just wanted to add something I had seen before that made me tear my hair out before I figured it out (of course, I had access to all kinds of test equipment to finally diagnose the problem)



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    Default Re: CNC router started breaking end mills after an accident.

    Quote Originally Posted by Outlawtaz View Post
    I will concur on all of Wizards points except one... I have seen spindles that will cut parts perfect and still have bad bearings in them; granted these were on commercial mills.

    But the basic mechanical principles remain the same. A bearing can be bad in either the radial direction, the axial direction, or both. You could have a bad bearing that only shows up when the forces change direction on the bearing.

    I would be curious to know if the breakage occurs when you are changing directions in any of the axes, X, Y or Z.

    I think Wizard provided an exhaustive list for your troubleshooting. I just wanted to add something I had seen before that made me tear my hair out before I figured it out (of course, I had access to all kinds of test equipment to finally diagnose the problem)
    Ball bearings can certainly throw one curves at times. I can recall times when I completely removed a bearing from a shaft thinking it was bad and felt nothing spinning it in my hands. At least at first. Then all of a sudden it would catch and catch badly enough to be the culprit. Things like this probably explains the bald spot on the top of my head.



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    Default Re: CNC router started breaking end mills after an accident.

    Hi wizard

    Thanks for your detailed response. Thanks to you I was able to discover an unexpected problem that might be the cause... But let me first answer your questions.

    Quote Originally Posted by wizard View Post
    As for the cutters breaking how exactly are they failing? That is are you seeing aluminum welding to the cutter or does it look like a sudden snap.
    It's most definitely a sudden snap. I use 4mm end mills with 6mm tail. the breakage occurs at the spot where the diameter descends from 6 to 4 mm. The machine works normally and then I hear a "tokk" and the mill is broken. I think it's a good idea to explain a little bit what this iMachining feature does: The cutting tool sprals down 5mm and then starts cutting a 7mm wide channel, by taking "bytes" through a semi-circular motions. I'm sure you've seen a lot of those techniques. The tool makes an ark removing some material, then retracts back allowing the tool to cool down and coolant to go between the tool and working area, then it makes another ark this time a little further (0.8mm in my case) and so on it moves forward.

    I use flood cooling by the way.

    Also since the the cutting advances through those arks, the machine actively moves in both X and Y direction (actually simultaneously most of the time) it's difficult to associate the breakage to an axis. However I do suspect the Y axis since it was making rumbling noises after the accident. There are two nuts on that lead-screw with a slight pre-load against each other. I thought in might be useful to mention that.

    Quote Originally Posted by wizard View Post
    What i find intersting here, at least in the way i understand this description of yours, is that the ed mill breakage started up a week or so after the crash. If so i have to ask what else changed?
    Now that you've mentioned it, it also dawned on me that if the problem started a week after the accident, the two might be unrelated or at least, it might be just a contributing factor and not the sole cause...

    Quote Originally Posted by wizard View Post
    One thing that can most certainly kill a cutter, especially one operated aggressively, is bad balls in a ball nut. The failure here would be a ball or balls that catch while running through the nut. If the catch is momentary the drives can end up loading the system such that when the ball releases that spring force trusts the axis ahead. The only good way to check a ball screw in my opinion is to pull the screw and run the nut by hand the entire length of the screw several times. The gotcha here is that bad balks will not catch every time through the nut.
    I haven't yet taken down the lead-screw but I tried pushing on the Y-axis with my hand while jogging it to see if I can feel any "hiccups" during transit. I did this back and forth for a while and couldn't feel any roughness in the motion. Also I think that the rumbling right after the accident was due to all the balls being dinted by the impact, and since all those dints were in sync the rumbling could be heard every time the dints touched the screw, but then due to slippage the dints in the balls got out of sync so the rumbling can't be heard any more... At least that's how I see it.

    Quote Originally Posted by wizard View Post
    One other question pops into mind here, are you holding dimensions otherwise?
    Yes, there is no noticeable difference in dimensions before and after.

    Now to your questions that lead me to my current suspicion of the cause:

    Quote Originally Posted by wizard View Post
    Did you change cutters?
    No, I bought 20 cutters in a box, (only four left now XD) I have no reason to believe there is difference in quality among them.

    Quote Originally Posted by wizard View Post
    Did you change lube/coolant?
    Nope, it's all the same.

    Quote Originally Posted by wizard View Post
    Did you change material suppliers?
    Did anything else change?
    Didn't change the material or supplier, but the previous plates were 10mm exactly so I didn't need to face them to the desired thickness. However I ran out of those and had to buy new ones, that was about a week ago . The new ones were 10.3mm, which meant I had to face them now, so annoying. I use a 6mm end mill for that, and it's important that it's a different end mill since the facing mill is going to endure a lot of wear at the bottom 0.3mm of it's cutting edges. So now I change the mills every time I have to cut a new plate unlike before where I only changed an end mill once it wears out. Now, as the machine is working, and coolant and shaving are flying everywhere, some really tinny shavings get stuck in the gaps of the chucks collet. As I change the end mills those shavings find their way in-between the collet and the cone-hole that house/centers it. Since the collet and that cone "nest" are so designed to automatically center the collet and tool as the chucks nut gets tightened it is very important there is nothing between them .

    I knew that, and I knew shavings get in-between there but since they get squashed as I tighten the chuck I thought It wan't a problem (some NASA thinking again XD). So previously I was cleaning the chuck every once in a while but since I wasn't changing the tool so often it wasn't a problem. But now I change the tool much more often without increasing the frequency of cleaning.

    I discovered that, this morning when I tried to measure the spindles runout (with the indicator). I couldn't see any measurable runout in the spindle but I saw 0.08mm of bias! The tool was off center as if the shaft is bent. Then it dawned on me that I should clean the chuck from shavings. So I did that and the bias became virtually 0.

    Now, that's a strong candidate to be the cause of the problem, but I doubt it's the only thing.

    OK that post got long enough. Thanks a lot wizard, wouldn't have figure that out without your help!

    Tomorrow I'll try to cut again.. With a clean chuck this time XD. I do have my doubts though, if the tool gets broken again I'll focus on the y-axis lead-screw and nuts assembly the screw bearings as you suggested, and I'll replace it if nothing else helps.

    Thanks a lot again

    Best regards
    Ivan



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    Default Re: CNC router started breaking end mills after an accident.

    In my imperial mind 0.08mm sounds like a lot!!! This especially when using advanced machining methods. With that depth of cut the run out could be putting an excessive load on the tool. I wouldn't be surprised if this is the root cause of your problems.

    In any event sounds like you are on the right track.



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    Default Re: CNC router started breaking end mills after an accident.

    Ivan-Ivanov

    As your thoughts are with the cutter run-out of, .08 this is enough to cause some problems, I say because your material is a different lot, it could be a different Grade which could also cause this same problem

    Mactec54


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    Default Re: CNC router started breaking end mills after an accident.

    Hi wizard and mactac54

    Yesterday I tried cutting with my chuck thoroughly cleaned, and I can say things are much improved. With a single end mill the machine was able to finish the last plate (which was like 40% complete), it went through the entire next plate, and the tool broke half way through the next. That means that the machine did cut through about two plates overall without breaking. That's still a problem since I had no tool breakage before, but at least a big part of the problem is now solved. Now I have a poster on the wall saying: "Clean the chuck every time!!!" XD

    I changed the tool this morning and the machine was able to finish the plate.

    I'm starting to think that with so many often tool changes, maybe the chuck sometimes tightens the tool improperly even if clean?! I mean, in order to cut through those three (and two of them weren't even complete) plates yesterday I had to make 6 tool changes, as before I would go through three full plates with no tool changes. I'm interested what you guys think about that, could the chuck still be the culprit?

    At any rate, I'm going to cut several more plates to have more solid sense of the breakage rate and then I'll replace the Y-axis leadscrew and nuts, and the screw bearings too. If the breakage continues I'll replace the two X-axis screws too since they're getting worn as well.

    About the material: It's supposed to be all the same, it's 6082 T650, however as I already mentioned it's thicker (10.3mm instead of 10mm) and the texture of the plates is a bit different. That means the manufacturer is different (btw my supplier is a re-seller they don't make their own stuff), so that means there could be some differences in the material.

    Best regards
    Ivan



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    Default Re: CNC router started breaking end mills after an accident.

    Quote Originally Posted by Ivan-Ivanov View Post
    Hi wizard and mactac54

    Yesterday I tried cutting with my chuck thoroughly cleaned, and I can say things are much improved. With a single end mill the machine was able to finish the last plate (which was like 40% complete), it went through the entire next plate, and the tool broke half way through the next. That means that the machine did cut through about two plates overall without breaking. That's still a problem since I had no tool breakage before, but at least a big part of the problem is now solved. Now I have a poster on the wall saying: "Clean the chuck every time!!!" XD

    I changed the tool this morning and the machine was able to finish the plate.

    I'm starting to think that with so many often tool changes, maybe the chuck sometimes tightens the tool improperly even if clean?! I mean, in order to cut through those three (and two of them weren't even complete) plates yesterday I had to make 6 tool changes, as before I would go through three full plates with no tool changes. I'm interested what you guys think about that, could the chuck still be the culprit?

    At any rate, I'm going to cut several more plates to have more solid sense of the breakage rate and then I'll replace the Y-axis leadscrew and nuts, and the screw bearings too. If the breakage continues I'll replace the two X-axis screws too since they're getting worn as well.

    About the material: It's supposed to be all the same, it's 6082 T650, however as I already mentioned it's thicker (10.3mm instead of 10mm) and the texture of the plates is a bit different. That means the manufacturer is different (btw my supplier is a re-seller they don't make their own stuff), so that means there could be some differences in the material.

    Best regards
    Ivan
    Yes the different manufacturers of the material can change a lot, on the hardness of the grade, it may say it is the same grade, and be quite different, it's how that company do there treatment of the material, and the composition

    For your Collet not seating each time correctly can be a problem, for run-out also, before you tighten it up check the run-out, of the tool up at the collet, some people that have run-out problems, tap the cutter at the top by the collet nut with a piece of copper to get it running true, then retighten to the correct torque

    If you use a torque wrench you will get the collet nut tightened the same every time

    From breaking so many endmills, the collet may also have some damage

    A solid holder for your tools is a good idea also, instead of having a collet you can make a solid holder for each tool, the Endmills you Loctite into the holder, heat up the holder when you want to change the Endmill

    The snip of the solid holder would not be suitable for your use, this was just to show what you could machine for the solid part of the collet end, if you need dimensions they are easy for these ER type holders

    You may have to change the program on the way it is cutting this lot of material, use an extra cut, not going so deep with each pass

    Attached Thumbnails Attached Thumbnails CNC router started breaking end mills after an accident.-torque-er-collet-png   CNC router started breaking end mills after an accident.-er-solid-tool-holder-png  
    Mactec54


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    Default Re: CNC router started breaking end mills after an accident.

    Now that the run out is addressed have you considered spindle speed or feed rate changes. The idea being to change the feed per tooth and thus chip load.

    I wouldnt dismiss an axis problem but right now it seems to be something to put on the back burner.

    Another thing that is always worth inspecting for is poor tooling (cutters). I once had issuesxwith a drill bit chipping for no reason. After sharpening it a couple of times i looked a bit closer and realized the steel had significant inclusions in it. As i ground the tool back new black pits kept popping up on the fresh grind. Kinda pissed me off as it was a domestically produced, brand new drill. Honestly things like this dont happen often at least i havent had that many situations where i can say for certaing the steel was crap. In your case that would be carbide. Just an FYI even if it doesnt fit the data.

    In any event back to the possibility that the material itself is more difficult to machine. This woildnt be a surprise at all and is common when working in structural steels. With steels you can have a significant variance in the hardness of the steel from one lot or vendor to another. You can also find yourself hitting hard spots in the material.

    My work with aluminum is mostly in bar stock which i find much more consitant. That might not apply to aluminum sheet goods especially if you change mills. Anither optio to hell control tool load is adjusting the depth of cut. Im not sure if this project requires multiple passes but some times that is the right answer. It might also help to look at wat the CAM software is applying for a tooth load, it might be excessive.



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    Default Re: CNC router started breaking end mills after an accident.

    It still may be a runout issue as well. If you can find one of the broken bits (the business end that goes flying when it breaks off,) try to inspect it under a microscope or something. Look to see if the wear on all the flutes looks the same. If you've got runout, one flute will be taking much more of the chip load and will likely dull (much) sooner, eventually causing the mill to break.
    Also, how are you measuring runout? In some cases, especially where the issue is at the collet, the runout may be much worse at the cutting tip than near the collet.



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    Default Re: CNC router started breaking end mills after an accident.

    Hi guys, sorry I couldn't write earlier, but I was out of town for most of the weekend.

    Anyway let me answer your questions and comment on your suggestions:

    Quote Originally Posted by mactec54 View Post
    From breaking so many end mills, the collet may also have some damage.
    Yes that's a reasonable suggestion, but In my case I use 4mm end mills I can break by hand . So I don't think that's the case. Also I've been present when some of those breakages occurred, and all that I could hear is a quiet "tok" sound and the end mill is broken. But thanks anyway I can see you guys are really trying to help, I do appreciate that

    Quote Originally Posted by skrubol View Post
    Also, how are you measuring runout?
    I take a broken end mill, they have a longish 6mm (diameter) tail. (It's about 40mm) long. I tighten it on the chuck so that half of it sticks out, and then get the magnetic stand of my indicator and stick it to the work bed of my machine (I bolt a piece of steel first since the machine is all aluminum . I adjust the stand and it's arms (that hold the indicator), so that the indicators pin presses upon the mill-tail. I zero the indicator and rotate the chuck slowly by hand, and so I can see the run-out. So as you can see I measure the run-out where it counts since normally the working part of the end mill sticks out about that much.

    OK let me cut to the chase and tell you guy what happened yesterday morning.I think I found the other cause for the problem.

    But let give some context first: In my machine I use round rails and linear bearings, and the bearing housings have the ability to tighten the bearings, there are tightening bolts. When I started investigating the problem my first job was to tighten those bolts even more (I found some of them were loose actually). I tightened them a lot. So I set the machine to work after I did all the other stuff too and observed the improvement I already mentioned. However yesterday morning I noticed that one of the rails of the x-axis (the suspicious one) is dirty with some black stuff mixed with the grease. I realized that's fine metal powder apparently screaped off of the rail and probably bearing balls. Then I noticed indentations on the rail exactly where the balls are rolling. A tiny channel was formed by the balls, compromising the hardened surface layer of the rail. I thought I've tightened those bolts too much and that rail is a bust. I can't replace it right now, so in order to prevent any further damage, loosened the bolts of that rail only and set the machine to work. The tool broke almost immediately!

    Today when I got back I re-tightened those bolts and set the machine to work again, and it still works as I write, it hasn't broken the tool... Yet... But it's few hours now. However, there is another important detail. This iMachining thing, doesn't let me to set feeds and speeds but there are 8 aggressiveness levels. I used to go with level 3 as it was yesterday. Today however I made a new program (for the same set of parts) using level 2 (to finish the job I started yesterday). I'm really low on tools and I can't risk breaking another one just for the experiment. (ordering new ones takes time, and I haven't even placed an order yet). Unfortunately working at level 2 from now on is not acceptable since it's really slow (level 3 was too slow for my liking)

    Oh and by the way, don't be deceived by the big depth of cut. I've cut plates with 10mm depth of cut (at level 2 though). The idea is that the whole tool is used, the entire length of the cutting edges. It's like big depth but small forward feed, and also done in series arches that gradually increases the thickness of the removed material. It starts from 0, reaches 0.8mm at the center of the arch and then back to 0. Then the next arch is performed a 0.8mm further and so on. And the descend to that depth is also very gradual. It descends in a cylindrical spiral fashion, never taking more than 0.25mm at the time. That mode is actually very forgiving for the tool and increases it's life. I experienced that first hand. When I used conventional gutting I would go 1mm depth of cut and that 1mm of the end mills edges would wear out pretty quickly. After I started using the iMachining my toollife improved dramatically and also the speed of work. Actually the only reason I go with two passes of 5mm and not one pass of 10mm is because the algorithm recommends it. Well now that the machine is damaged I wouldn't go 10mm but you get my point.

    But at any rate, now the rails are a big suspicion for me. Actually I decided I would waste no more time debugging, and I'll proceed with upgrading my machine. I'll not only replace the rails and nut-screw assembly of the offending axis but I'll change the rails with those profile ones that everybody says are much better. I'm in the process of upgrading the machine anyway. I hope that doing that would not only fix the issue but toughen the machine too... So maybe after that I'd be able to go level 4 or higher... The cost, is bearable, the time setback - not so much, but I have no choice since the machine can't work in this state.

    OK that's all for now. Thanks a lot guys you've all helped me a lot. When I finish the upgrade I'll write on this thread to update you on the result. It would take me about a month (since I have a day job too) but in the end I hope I'll end up with a better machine

    Best regards
    Ivan



  17. #17
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    Default Re: CNC router started breaking end mills after an accident.

    High Ivan;

    If your round rails have a significant groove in them the you very likely will have motion issues. This can very easily lead to jerky motion that could easily snap an end mill. You might even see the result of a linear bearing hang up at the point of tool breakage as a sudden increase in depth of cut. The tricky part with these sort of wear out failures is that the bearings can often feel good up until a ball catches.

    If you are waiting on parts some times you can get by with a damaged rail and a new bearing if you can rotate one against the other. In most cases this is only really possible with unsupported rails. However ive done such at work to keep machinery running until i can get new rails cut and machined. Sadly this is pretty hard to do with supported rails, it is only a temporary fix anyways.

    As for your milling technique, i believe it to be tri-chordsl, it is a well acepted technique so it isnt a problem when everything is running well. The problem is when a machine does not run well as improper motion can easily overload the cutters shank. It is easy to see how this might happen when both axis are interpolating a curve. If one axis catches momentarily the other will keep moving, effectively going in a straght line. So all of a sudden instead of trying to remove 0.001" the tool has to cut several thousands worth of material per tooth. That can easily overload a tool when the full cutting length is being used. The other problem bearing catches can cause is a sudden jump in the axis as the stepper loads the mechanics and then that spring energy is suddenky released. That axis suddenly jumps forward and bam destroys your cutter. Either way the bad bearing caused the problem.

    As for cause and effect i reall doubt that the crash you had has anything to do with the shaft wear you are seeing. In fact im reluctant to say that your bearing adjustment caused the problem. Rather i think the problem with respect to this rail is bearing wear out. Given that adjusting these sorts of bearings isnt as easy as some would like to believe and over tightening is extremely easy. I wouldnt attempt to make such adjustments without removing the leadscrews ao that the saddles can be easily moved. Depending upon the axis you may need to remove the gantry / Saddle or whatever from the bearing housing to get the proper feel. It is very easy to get the bearings too tight and yes damage the rails, balls and housings. The reason i dont think your adjustments caused this problem is the fact the axis moved and such damage doesnt happen instatly.

    As for profile rails being better, they are in a sense better due to stiffness for a given size. However profile rails suffer from some of the same issues as round rails. They dont like dust and other materials plugging up the race ways. It is also easy for one bad ball to ruin an entire rail assembly. Frankly bad balls or plugged raceways will cause exactly the same problems on profile rails or round rails. You dont escape the maintenace issues with profile rails but you do gain mechanical advantages.

    Beyond all of that id make sure your machine can handle profile rails before upgrading. Round rails will tolerate more mis alignment than profile rails. As such you may get into a situation where you need to do remedial work on one or more axis. In ither words your upgrade could take longer than expected. It isnt just the rails either ball screw mounts may need rework, it al depends upon the specifics of you machine. Personally id patch up the machine to make new parts for the upgrade.



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CNC router started breaking end mills after an accident.

CNC router started breaking end mills after an accident.