Author Topic: Mill Creep  (Read 12068 times)

Offline Mosey

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Mill Creep
« on: January 04, 2015, 03:16:22 PM »
Needed to mill a large slot, 1" x 5/8" deep x 3" long in aluminum bar, so I put a 1/2" roughing mill in a collet in my mill, and carefully set the depth to allow a little cleanup.
I had oiled the spindle this week and noticed a seepage out of the spindle nose which indicated that the oil had reached the spindle nose.
As I was milling along the piece I noticed under the huge pile of chips that the depth of the slot had grown! It was now well into my finished workpiece.
Apparently the roughing mill has so much bite that it walked out of the collet, especially one that was freshly oiled. The collet is a well-used one, so it probably doesn't hold as well as it should. I will check its inside dimension for wear and wipe it dry.

I have never had this problem with regular end mills. Also, I prefer ER collets to W-20's becasue I think they hold better, but they are not readily available in my spindle size.
So Dave, here is another place to use those metric change gears...to make an ER x W-20 collet holder
Mosey   :'( :'(

Online b.lindsey

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Re: Mill Creep
« Reply #1 on: January 04, 2015, 03:20:01 PM »
Mosey, did you notice the problem before the end mil depth spoiled the workpiece?  I suspect the oil is what allowed it to creep in conjunction with the helical milling forces pulling down on the end mill. I hope you may have caught it in time.

Bill

Online Jo

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Re: Mill Creep
« Reply #2 on: January 04, 2015, 03:36:07 PM »
Why were you using an end mill to cut a slot  :headscratch:

End mills are designed to cut on the side not the face. If you try cutting a slot with them you get pressure on the two side and the front teeth, pushing the cutter backward and causing an over width slot. I would suspect the backward push is what also encouraged the cutter out of the collet  :-\. Did you damage the collet as well  :(

W20 collets are very good and like all collets they should be used dry  :) Schaublin do both ER and the W series collets, they decided that that W series were better for use in their mills so I will  :NotWorthy: to their greater knowledge on this matter.

Jo
Usus est optimum magister

Offline Mosey

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Re: Mill Creep
« Reply #3 on: January 04, 2015, 03:46:12 PM »
Mosey, did you notice the problem before the end mil depth spoiled the workpiece?  I suspect the oil is what allowed it to creep in conjunction with the helical milling forces pulling down on the end mill. I hope you may have caught it in time.

Bill
Bill, The problem became visible just as the mill spoiled the work, hidden by the large pile of swarf. I will clean and dry everything and make a test cut before proceeding. It is possible to cut most of the one-sided recess ( I should not have described as a slot) away on the table saw, so I may see about that to eliminate the large milling event.
In my experience ER collets hold better than W type, but are not as accurate in centering of short pieces. They are much better at releasing the mill or other piece. I think they each have their advantages in certain circumstances, not a universal one type better than the other situation.
Mosey

Offline cheepo45

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Re: Mill Creep
« Reply #4 on: January 04, 2015, 04:00:38 PM »
This is a well known issue in production machine shops where the depth of cut and the feed are much more aggressive than in home shops.
I prefer to use end mill holders with set screws to avoid this. This is also the cause of the tapering gouges found on many used milling machine tables!
I hope your part can be saved or you have more material. The rest of us can learn a lesson from this. Thanks for posting.
            cheepo45

Offline gbritnell

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Re: Mill Creep
« Reply #5 on: January 04, 2015, 04:10:55 PM »
Jo,
I'm a little confused about you statement. If not using an end mill to cut a slot what would one then use? I use end mills to cut slots all the time but having learned the things that could possibly go wrong while doing so I make adjustments, stay well away from side and bottom surfaces when roughing, make sure that the chips are continually cleared from the slot, (air) and make sure the cutter is fully held by as much of the shank as possible. Roughing end mills are generally quite forgiving while taking heavy cuts as they break up the chips into smaller bits so as to lessen the load on the cutter.
gbritnell
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Online Jo

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Re: Mill Creep
« Reply #6 on: January 04, 2015, 04:47:01 PM »
Jo,
I'm a little confused about you statement. If not using an end mill to cut a slot what would one then use?gbritnell

You use a slot drill to cut a slot ;)

Jo
Usus est optimum magister

Offline DavidF

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Re: Mill Creep
« Reply #7 on: January 04, 2015, 05:07:58 PM »
I dont know if my technique would be considered proper, but I use a center cutting endmill to plunge cut the majority first. The chips priral up and out of the hole just like drilling and get the hole opened up so there is somewher for the chips to go once you start milling out the sides.

Online Jasonb

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Re: Mill Creep
« Reply #8 on: January 04, 2015, 05:10:39 PM »
Jo as Moseys slot was 1" wide and he was using a 1/2" cutter the problem of an over width slot does not come into it. I suspect he did one cut down the middle and would then use the SIDE of his cutter to take the slot to width.

It is also possible to buy roughing end mills that are suitable for slotting and even ones that are suitable for cutting keyways to the same tollerance as slot drills ;)

What worries me more Mosey is were you trying to take a full 5/8" depth cut in one pass with the 1/2" ripper or did you go down in steps and it moved on teh last one. A full 5/8" depth in one go would be a bit much in my book.

J
« Last Edit: January 04, 2015, 05:30:15 PM by Jasonb »

Online Jo

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Re: Mill Creep
« Reply #9 on: January 04, 2015, 05:40:34 PM »
We seem to have gone over slot drills and end mills a number of times  :facepalm:. This is how Tubal Cain explains it in the Model Engineers handbook, P77: 

"End mills are designed for profiling, and are at their best in the situation shown in the sketch below. The maximum width of cut is one quarter of the diameter of the cutter, and the depth of cut is up to one diameter deep. This depth should not be less than about 0.006"; the width of cut should not be greater that that which engages two teeth at once. A better result will be obtained, with probably longer cutter life, if several cuts D/4 wide are made rather than fewer cuts the full width of the tool.

If an endmill is used as a facing cutter, with a very light depth of cut, then the corner may break down and rubbing will result. It is preferable to stone a 45 degree bevel or chamfer on the sharp corner, ensuring that clearance or relief is maintained, and to emply a depth of cut not less that 0.006" even in finishing; more is better. The bevel need not exceed 1/32" even on a larger cutter.

End mills are not suitable for cutting slots equal to the cutter diameter. For this duty a slot drill should be used; this has the advantage that, unlike the endmill, it may be fed axially down into the work so that slots with blind ends may be cut. "


Jo
Usus est optimum magister

Online Jasonb

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Re: Mill Creep
« Reply #10 on: January 04, 2015, 05:51:26 PM »


End mills are not suitable for cutting slots equal to the cutter diameter. For this duty a slot drill should be used; this has the advantage that, unlike the endmill, it may be fed axially down into the work so that slots with blind ends may be cut. "[/i]

Jo

Like I said Mosey was not cutting a slot equal to teh cutter dia, he used one half the size of his slot so after a basic pass where the width is not critical would have used the side of teh cutter to profile the slot to the required width.

Also the old slotdrill/endmill differences have become somewhat irrelevant with modern cutter technology, you can have centre cutting endmills and as I said above endmills and roughers that will cut a keyway to P9 tollerance the same as a slot drill would.

J

Offline Mosey

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Re: Mill Creep
« Reply #11 on: January 04, 2015, 06:03:58 PM »
"What worries me more Mosey is were you trying to take a full 5/8" depth cut in one pass with the 1/2" ripper or did you go down in steps and it moved on teh last one. A full 5/8" depth in one go would be a bit much in my book.
J"
[/quote]
Jason,
Yes, of course, that would be foolish.
I chose to take a 1/2" depth cut, with passes about 1/8" wide. I considered taking shallower depth cuts and wider passes also. My feed was quite slow to allow the cutter to cut freely, as chips are quite abundant with such a large cut as this. Is a wider shallow cut better? Honestly, my table moves much more conveniently in the X axis than Y, hence the choice of deeper narrower feed yielding less passes. Naturally I intended to leave sufficient meat to finish with a couple of cleanup passes of .005", final pass of .001".
Keep in mind this is an open-sided cut.
Perhaps, an high-helix 2 flute end-cutting mill for aluminum with a dash of Tapmagic (shaken, not stirred) would be best? I don't know what a Slot Drill is.
At no time did I attempt to cut a blind slot the full width of the mill, so I think that is a red herring. This thread is about mills being pulled out of their holders.
I am learning here and thank you all for your great inputs.
Mosey

Mosey  :headscratch:

Online Jasonb

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Re: Mill Creep
« Reply #12 on: January 04, 2015, 06:14:54 PM »
1/2" deep x 1/8" width should be OK.

Not quite sure how you were able to take 1/8" cut though? did you feed in 1/8" then move 1/2" across, then another 1/8" if so this does put more cutter edge in contact with the work, maybe a picture would help?

A slot drill is a term more often used in the UK for a two flute centre cutting milling cutter

Offline crankshafter

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Re: Mill Creep
« Reply #13 on: January 04, 2015, 06:43:45 PM »
Hi Mosey.
is it possible you can put up a picture of the W20-ER collet-holder that you have. My be you have a drawing too :naughty:
CS
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Offline Mosey

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Re: Mill Creep
« Reply #14 on: January 04, 2015, 07:32:20 PM »
Hi Mosey.
is it possible you can put up a picture of the W20-ER collet-holder that you have. My be you have a drawing too :naughty:
CS
Sorry CS, if I mislead you, but I don't have an W-20 ER collet holder, I want to make one. My new metric change gears might make it possible.
Mosey