Need Help!- Plunging chatter and other Z-axis issues

[cnczoner]

My CNC mill came with the quill CNC'ed for the Z-axis. I get a LOT of chatter when plunging into aluminum, and have to slow down the plunge rate to SUPER-SLOW (0.1 IPM at times with a 0.5"-dia relatively new HSS 2-flute center-cutting endmill at 1500rpm). Sometimes the chatter throws off the machine quite a bit (sideways). But with Z-fixed, the machine cuts quite nicely.

Lately however, I also noticed that while profiling (Z is fixed), the Z was going lower (cutting deeper into the aluminum). I can't understand how that happens, but something has to be slipping inside the head. I haven't paid attention to the steppers to see if it's that, or if the steppers stay fixed and something else is slipping.

Anyone ever seen this before and have any clues as to how to tackle this? I want to avoid tearing into the head if I don't have to, as I understand it can be tricky to get the gears all re-aligned etc.

BTW, the mill is a CNC'ed this... http://www.harborfreight.com/cpi/cta...emnumber=42827

Any clues appreciated.

Cheers,
-Neil.

[cadmonkey]

I've seen the Z done 2 different ways on that machine - judging by your wording I take it someone else converted it and you bought it used? I've seen either the stepper connected to the quill feed handle, or through a ballscrew/ballnut mounted to a clamp attached to the spindle forcing the quill up and down that way. Which way is yours done?

Since you are getting chatter when plunging and noticing digging in I'd first be inclined to say the two issues are connected. Knowing how your Z is driven will help troubleshoot. A pic of your machine would help too.

[cnczoner]

Exactly -- it was converted by someone else, though I've been improving it lately -- finally converted by Y-axis to ballscrew, and X-axis is next. I've been using it for a lot of PCB routing (only separating boards on panels made by another company), lots of acrylic routing, and some UHMW parts I use in my products.

I'm mentor a high-school robotics team and was trying to make an aluminum part and this is plaguing me, so I ended up doing it with UHMW just now. UHMW machines nicely on this mill, btw. Since we're in the middle of build season, I'm avoiding tearing into the mill now.

Attached pic is from a cellphone, but clear enough...

[ataxy]

the chater might come from the quill, did you check the quill to see that it had no play

[HimyKabibble]

Chatter indicates something is moving that shouldn't be. That can be either slop in the quill, or the table moving around. You can elminate the table by locking it in place. If the chatter goes away, then you have backlash in the axes you need to get out. If it's the quill, that's a lot harder to fix, as it means the bushings in the quill are gone, or the spindle bearings are gone.

The Z dropping while cutting can be either the head moving, or the tool pulling out due to the cutting forces. How are you holding the tool? If collets, perhaps they're crappy collets, or you've got an out of spec taper in the spindle. Same thing if you're using endmill holders held in by a collet. You can elminate the head dropping as a cause by, again, locking it securely before making a long cut. If the cut depth doesn't increase, then the head is moving. If it's still there, then look at the tool. Try marking where it is in the collet or holder, and see if it's still there at the end of the cut.

Regards,
Ray L.

[cadmonkey]

What they said - also since you are driving the axis through all the gearing of the fine feed (which I meant to say instead of quill drive) you are going through all the gearing in that assembly which isn't really how that was intended to be used - it's getting a bit of a workout that it wasn't 'designed' to perform. Once you get out of season I'd say you need to investigate the bearings in the spindle, and the gears and bearings in the fine feed.

Mind you I don't have one of these machines, nor have I taken one apart, but I did look at an exploded parts diagram when I was considering getting that model instead of my X3 and there's a lot of places for backlash, slop and play to be introduced in that mechanism. It's handy and easy to use it for the CNC conversion but what's easy isn't always what's best as I'm sure you've already learned

Go through the eliminations that were suggested by the previous posters and try to narrow down the cause. I hope it's something a simple as needing a quality set of collets - that would make life a whole lot easier Be methodical and only try to eliminate one variable at a time if at all possible.

What kind of DOC and feed rate are you getting tool diving at? Conventional or climb milling?

[escott76]

What kind of backlash does that "fine feed" mechanism have? I can't imagine it's all that great.
Perhaps clamp something in the spindle you can grab a hold of, like just a piece of shafting or similar. With power to the steppers, see if you can move it by pulling up and down. Put a DTI on it, you'll find out what kind of lash you have. If that isn't it, perhaps as others have said it could be the tool slipping in it's holder. Make sure your collets or end mill holders are clean and the correct size.

[cnczoner]

Good ideas. I just got back home, so I'll check some of these tomorrow.

The quill definitely has some play (backlash), but I haven't measured it. The X-axis has 0.004" backlash, and the Y-axis has zero.

I'm using collets, but I don't think the tool is pulling out -- I usually set it so the end of the flute is just at the edge of the collet and when taking it out it later I don't remember seeing anything odd. I would've probably noticed. I will try to reproduce the chatter to see. When it was pulling into the material, I was slotting 6061-T6 with a 0.5"-dia 2-flute HSS endmill, 1500 rpm, 0.1" DOC, and it pulled down another 0.1" or more.

I've never used the fine feed (this mill was converted when I got it, and I've never used a mill before this), so I'm not sure how it works. But I'm looking at the crappy multiple-time photocopied exploded parts diagram now and I can see the many gears. I do plan to go into the head sometime soon, as I want to see if the bearings are easily replaceable with higher-speed types, as I want to double the motor speed. Perhaps I should look into changing the way the quill is CNC'ed.

I'll be back when I run the tests.

Thanks,
-Neil.

[ataxy]

if the quill as lash and you can definitly feel it by hand then you have a problem so just move in order tool, quill, z axis, table ounce you illiminated the lash in all the machine then the only thing left is your feed or doc that can cause the machine to jump around.

[HimyKabibble]

Originally Posted by cnczoner Good ideas. I just got back home, so I'll check some of these tomorrow. The quill definitely has some play (backlash), but I haven't measured it. The X-axis has 0.004" backlash, and the Y-axis has zero. I'm using collets, but I don't think the tool is pulling out -- I usually set it so the end of the flute is just at the edge of the collet and when taking it out it later I don't remember seeing anything odd. I would've probably noticed. I will try to reproduce the chatter to see. When it was pulling into the material, I was slotting 6061-T6 with a 0.5"-dia 2-flute HSS endmill, 1500 rpm, 0.1" DOC, and it pulled down another 0.1" or more. I've never used the fine feed (this mill was converted when I got it, and I've never used a mill before this), so I'm not sure how it works. But I'm looking at the crappy multiple-time photocopied exploded parts diagram now and I can see the many gears. I do plan to go into the head sometime soon, as I want to see if the bearings are easily replaceable with higher-speed types, as I want to double the motor speed. Perhaps I should look into changing the way the quill is CNC'ed. I'll be back when I run the tests. Thanks, -Neil.

1500 RPM is mightly slow for a 1/2" cutter in 6061. You should be more like twice that.

If your Z drive is going through gears, you've got a problem. When you plunge, you'll be taking up any backlash in one direction. Once you start cutting, the tool will be trying to pull the tool down further, which will take up the backlash in the opposite direction, so you'll have a very hard time getting Z accuracy and repeatability to any better then the amount of backlash. 0.1" of slop in a quill drive is a lot, but certainly not unheard of - it was simply never intended to be used that way.

Also, be sure you're not putting the endmill *too* deep into the collet - that can reduce clamping force, depending on the fit.

Regards,
Ray L.

[evo4wrx]

I have the same problem with mine. Its a Hm45 and I use the quil for Z.

I changed the motor on top to a 2HP 3 phase capable of running upto 3800 RPM. I even tried some really expensive 3 flute tooling and index cutters in it.

Ive cut 7075 and 6065 at different speeds and cut rates but dont seem to have a lot of luck. The surface finish is very poor when feeding or when plunging. The finish when fly cutting is perfect....

I couldnt really messure any major backlash coming from the bed but the fact that the fly cutting was perfect but vibrating everywhere else leads me to believe that the quil is slopping about. Ive replaced all the bearings in it and put rubber mounts under the motor. Ive checked the quil for runout and its 0.01 +-

[ataxy]

a quill is not intended for z axis work its meant for drilling a bridgeport type machine might deal with it due to its size and fit but the smaller machine are not meant for that