[tef9]

My machine is up and running - making several parts (its great!) However it has several problems that need your help.

1) My x ball screw is bent...sadly! It does not have a drastic effect on accuracy (0.022 backlash!) however when the mill moves at 300mm/m + it starts to grind and groan and I think it is due to the ball screw being bent. Can this be straightened if so how?
2) I am getting tremendous vibration when I cut ... I can feel it through the floor The depth of cut is 1mm (I dare not go higher) and I have used a speeds and feeds calculator.
3) The y axis “looks” like it moves when it changes direction however it measures backlash of 0.02 so it’s not bad. I have not used Longrats spacer idea..Yet.
I really don’t want to spend hours waiting for a part that should take minutes. I also would like to cut more than 1mm off at a time (for roughing) at least.
I dare not “plunge” into the material it makes some awful sounds when I do – I tend to drill a whole first and let the mill go from there.
If you can help I would appreciate it as I am really confused as to what I can do! Yes everything is bolted down tight...can’t think of anything else.
Thanks,
Andy

[rcazwillis]

What size end mill and what material are you cutting?

[tef9]

10mm Diam in Aluminum.

Usually going at 150mm/m with the spindle at 2250.

DOC=1mm

Thanks,

Andy

[pete from TN]

I would really take that screw out and at least try to make it straight or better yet replace it. The screws are usually not too expensive it is the nuts that really get pricey most of the time. I am sure it is not helping you and if it is bent enough it may be inputting harmonics or tiny vibrations into the cut as the machine moves. You say that your machine is tight otherwise but honestly if that is the machine I think it is you should be able to take a DOC at least that deep pretty easily. You might try taking a piece of scrap and making cuts along each axis individually to try to isolate where the problems are coming from. Also you say that you cannot plunge. I am going to assume here you are using a center cutting endmill and the appropriate speeds and feeds for plunging. That would make me REALLY suspect something is loose. How is your Z gibs adjustment? Sometimes just because the adjustment screws seem tight that does not necessarily mean your gib is. My machine had a problem with the depth of the Z gib adjusting screw threads. It FELT like it was adjusted tight but in actuality it was not.

A lot of times problems like this can be isolated by running just one axis and perhaps locking down the others to SEE what is going on. Good luck... peace

Pete

[rcazwillis]

10mm Diam in Aluminum.

Usually going at 150mm/m with the spindle at 2250.

DOC=1mm

Thanks,

Andy

Assuming a 2 flute, an end mill that size and at that speed, the depth could easily be three times as deep. On my 704, I run a 3/8 end mill at 8 ipm at 1800 rpm. I use a light blast of continuous air to move chips. It cuts at this rate all day. If I push faster, the finish gets bad.

Instead if plunging, try ramping down. I use center cutting end mills but find ramping much easier on the machine.

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[fragger6662000]

are you sure that the gibs are properly seated.

Is it a centre cutting tool?

[LongRat]

Pete's advice is what I would look at. Isolate the problem to 1 axis, the chances are you only have a problem in one area. I'd look at the Z first myself, this is the most problematic area. To give you a clue, I could cut 3mm deep at 150mm/min with a 10mm 2-flute carbide endmill in 6061. Try cutting along either X or Y with the Z axis locked up solid.

[tef9]

Thanks for the replys - I will give the advice ago tommorrow night.

Will report back soon!

Andy

[tef9]

Thanks for all the advice so far!

OK the gib strips seem fine and all locked in place. I checked the strips themselves and all seems well I cant see any problems, however I dont have a great amount of experiance.

I will do some test cuts this weekend, I have too much Open University Home work to do before then.

Will report back soon!

[photomankc]

What kind of floor is it sitting on? Is it on a concrete floor or on some type of wood floor. The latter could be acting like a drum with transmitted vibration and making it sound worse. My sheet-metal stand can really get to rattling around too at just the right 'note'.

Bending the screw back is kind of a shaky deal as I have read it. you can end up making it into a real mess too. I'd agree with Pete that for the cost of the screw it might be easier to simply get a new one.

I have the smaller machine and I can easily cut aluminum 3 to 4 times that deep with a 3/8" endmill (~10mm) and so long as I keep it buried it will just hum right through.

[nomedia="http://www.youtube.com/watch?v=6MVQFPNCdkg"]YouTube - First CNC Part[/nomedia] - Skip to about the 4:00 mark.

How does that compare with what you are getting?

[tef9]

Hi Photomankc,

Ok so the floor is concrete and was really hard to drill through, we had to borrow and industrial drill from a friend (it leaked oil while drilling so it was not easy). It took about 30 mins per hole, and they were drilled at 18mm dia for the expanding screws to fit inside. They have been torqued, leveled and shimmed (torque wrench to about 25Nm). So the stand is really solid. However I checked them tonight just to make sure they were not loose for some reson, and they a still all really tight.

The problem with buying a new screw is that I will have to grind the X nut again...dont want to go through all that again. So straightening is probably the better option unless someone has any good ideas of getting over clearance issues with that nut!

Your machine is great, have you used rails and/or servos as everything looks so smooth on that link. Well done!

Thanks,

Andy

[photomankc]

If you got the same type of screw, why would you need to regrind a nut? Just transfer the existing nut onto the new screw... assuming the same manufacturer.

Thanks for the comments.... Mine is not yet complete really. Z is running on 3/4" ball screw and is real nice, about 0.0015 backlash. The X and Y are running the stock screws and are around 0.004-0.007 backlash (ugh). All are steppers. They look smooth because the X/Y are slow. About 40 IPM max. Backlash is compensated out but still can show up with aggressive cuts.

I would break out the indicators and start looking for slop you cant feel but might still be there. My X and Y have to run a little loose or they bind at the ends and that means that I can get a couple of thous of movement by grabbing the table and yanking. Get enough of that and it can start to vibrate at the right note.

Can you get a video of whats going on by chance? Are the end mills new, and quality? I have a few China mills from a set that wouldn't cut butter very well much less metal and that makes it a moaning, angry sounding machine. My good end mills just hum right along.