Mill slots not cutting parallel , why?

Ozwelder

Ozwelder

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Hi guys,
My students are milling a slot in a piece of 1" cold rolled square section about 100mm (4") long.
Some have made a real good job of it and a couple of others have managed to cut a slot that tapers off to 1 edge- not parallel- the edges of the cut slot are sort of rolled?.

The slot is 16mm (5/8") wide and 4mm (maybe 3/16 deep) and cuts have been 1/2mm ( .020") each pass.

Being a relative newby to milling operations - all I can come up with as a reason is they have too much feed.

I mentioned to the individual concerned that the feed seemed too fast - the machine seemed to labour, to my ears.
This 16 year old has done work experience with a local engineering company and has maybe 80 hours of experience, so he thinks he knows it all now.

The mill vice was checked for tightness and ridigity as they often don't bother to clean between the mill jaws before setting up a work piece.


The only other variables I an come up with are :
Rotation of cutter is incorrect
Cutter too far extended out of spindle
Spindle quill (if that s the term) is extended too far out of the barrel.

Can any kind soul point out any more reasons that may cause this problem.?


Thanks


Ozwelder
 
That's a pretty light cut, so it shouldn't be the machine flexing, but you listed the most common causes. What was the spindle speed, and what kind of end mill? 2 ot 4 flute? Sharp or dull.

I'm not sure I understand what you mean by the edges being rolled. If you mean not perpendicular to the bottom of the slot, I'd suspect a dulled or possibly chipped corner of the end mill.

One thing to remember is that end mills like to have "support" on both sides when cutting a slot. It would tield more accurate results if the outside dimensions, in your case the 5/8" was established first, then the material in the middle removed. If you do that first, the pushoff will be less, but you still can have a mill "pull over" into the work and make the slot wide. If the machine can handle it, go to full depth less 0.010, and the final width position in -Y- less 0.010, then get the middle. That should leave 0.010 all around for finishing. Up the spindle speed and go to full width and depth.

To be sure, too much feed will cause the pulling of the end mill, but on such a light cut it would have to be added to by the other things you mentioned. Too much length or quill extension makes things easy to flex.
 
When I first started machining I often tried to cut a slot with an end mill the diameter of the cutter. Example .500 slot just grab a .500 end mill right. This never works you get slots that are not straight or the right size because of the tool walking around. The right way to do it is use a smaller cutter to hog out the center of the slot then go back and clean the edges up to size. Here is a video on youtube that shows the steps that I think Tony is talking about. This is the way I would do it. The guy in the video is climb milling which is fine for plastic or if you know what your doing and know your mill but for students I would do the same steps but make sure they are doing conventional milling on the cleanup passes.

[video=youtube;lqMNWy5OM2A] http://www.youtube.com/watch?v=lqMNWy5OM2A
[/video]
Here is the same basic process but on a CNC mill.

[video=youtube;9Zv1eHLn6is] http://www.youtube.com/watch?NR=1&v=9Zv1eHLn6is [/video]
Only difference here is the cleanup is done at each level instead of at the end at full depth.

Hope this helps
Jeff
 
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THANKS YOU BLOKES

Excellent responses. I can apply this stuff as I did suggest the 16 mm cut .OK! We shall go Back to a 12mm cutter.

I checked again today and thought I would check the table lock downs and sure enuff they were not tightened.The same thing for the head of the mill which extends out on a horizontal dovetail.


A 16mm ring spanner was applied to the appropriate places and now we have no more movement.The cutter was a 2 flute so it seems pretty obvious that the feed plus the lack of rigidity from the loose locks.

Even worse he has broken 3 carbide inserts on the the 3" face cutting tool and was about to attempt to unscrew the broken ones using an allen key when I had already told him they are torx drives.
Some blokes just have not got it. The lights are on but no one is home. 1 hour before we discussed how carbide tool breaks or shatters when subjected to intermittent cuts. So our genius machinist places his job that has been welded in to the mill vice and attacked it with the cutter running too fast. He is now hand filing his job to make him think about being kinder to machinery.

I think my error was not placing the required info on his bloody Iphone and downloading it to what serves as his memory.

Are some kids on your side of the pond this slack? Maybe I'm getting too old for this ?

I suppose what makes up for it is one of my girls, who at first go ground her own HSS steel and machined an amazing finish on a piece of 316 SS. She listens and is one of the best hands on students in the shop.

Thanks again for your help and listening .

Oz
 
I teach a bunch of designers in an intro to machining class at the college level. I get some that have never drilled a hole and some that have worked in machine shops. Its a great class to teach but you are dead on about some of the students. Some of the time the lead screw is turning but the nut just isn't engaged. I would be interested in hearing more about your class and if there are any others on the group that teach metal working.


Jeff
 
We have a TV program up here called " Canada's Worst Handyman" I suspect there may be the equivalent in the US. It's like watching a train wreck - it's horrible, but you can't look away. Many of us take certain skills and abilities for granted. It comes as a shock to realize that we may be the weird ones.

Let's crush the cell phones, turn off the computers and games, and teach the next generation that they CAN do something.
 
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