Diagnosing a Milling Machine problem

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KellyJones
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Diagnosing a Milling Machine problem

Post by KellyJones » Sun Dec 09, 2018 1:27 pm

Hi folks. Been a while since i posted. I could use some advice.
I have a Grizzly G3617 milling machine that's giving me some problems, and i'm not sure if it's operator error, or a problem with the machine.

I'm trying to mill a part with a 4 flute 5/8" diameter end mill mounted in an R8 collet. The end mill keeps walking out of the collet. There is little, if any, vibration during the feed. The material is mild steel, and the depth of cut is about 0.050". The draw bar is about as tight as i can get it.

Any thoughts?

thanks in advance.
Last edited by KellyJones on Sun Dec 09, 2018 4:12 pm, edited 1 time in total.
Kelly Jones, PE
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SteveHGraham
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Re: Diagnosing a Milling Machie problem

Post by SteveHGraham » Sun Dec 09, 2018 1:41 pm

The question got me wondering, and I found some advice from Tormach.

https://www.tormach.com/blog/best-pract ... llet-slip/
Every hard-fried egg began life sunny-side up.

choprboy
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Re: Diagnosing a Milling Machie problem

Post by choprboy » Sun Dec 09, 2018 1:45 pm

1) Chip stuck in the collet gap?
2) Actually a 15mm endmill, or a 16/17mm collet?

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Dave_C
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Re: Diagnosing a Milling Machie problem

Post by Dave_C » Sun Dec 09, 2018 1:49 pm

I'm trying to mill a part with a 4 flute 5/8" diameter end mill mounted in an R8 collet.
That seems a bit odd given the size of the end mill. I'd start with cleaning the inside of the collet that holds the end mill and also the top portion of the end mill as well.

I've pulled a few .375" end mills out while machining hard castings but tool pull is generally no much of an issue with R-8 collets. (at least in my experience)

I don't see .050" as an issue, but what about cut width and feed rate?

Dave C.
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KellyJones
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Re: Diagnosing a Milling Machie problem

Post by KellyJones » Sun Dec 09, 2018 2:04 pm

Don;t think i have any metric tools. At least they are all marked in imperial units. I will check though, just to be sure. I also checked for chips in the slots. Everything looks clean.
I am cutting with the side of the mill. The cut is about .050" into the side of the cutter and .525 along the length of the cutter. The feed rate is slow - about 4 inches per minute, if i recall correctly ( i measured it with a wall clock and the DRO a few weeks ago - it's about as slow as the power feed will go without stalling.)
I did notice that the anti-rotation pin inside the spindle is nearly non-existent. I was careful to align the groove in the collet with it, thinking if they were not matched, the collet would not clamp correctly. Maybe i missed it or is slipped?
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SteveHGraham
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Re: Diagnosing a Milling Machie problem

Post by SteveHGraham » Sun Dec 09, 2018 3:05 pm

Tormach says you want the end mill and the inside of the collet to be very dry, but you want the inside of the taper and the outside of the collet to be well-lubricated.

I doubt I've ever removed the oil from an end mill before putting it in a collet.
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KellyJones
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Re: Diagnosing a Milling Machie problem

Post by KellyJones » Sun Dec 09, 2018 3:18 pm

Took the setup down and cleaned an measured everything. No debris, tools are the size marked.
Found a small dent in the end mill. Looks like it came that way as it appears to have TiN coating in it. Found no burrs in the collet or spindle. Lubricated the draw bar and the threaded end of the collet. Put it all back together and after the first pass the collet walked about .030”.
Stopping for the day or until I figure this out.
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jcfx
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Re: Diagnosing a Milling Machie problem

Post by jcfx » Sun Dec 09, 2018 4:29 pm

My guess is that your gripping problems stem from anti rotation pin is missing, the anti rotation pin would keep the
collet from spinning insuring that the draw bar will draw the collet tight around the shank of your endmill.
Is that pin accessible from the outside of the spindle ? on both my 5C collet chuck and collet blocks it's an adjustable dog point screw.
You may want to check if it walked back into the spindle or replace it if it's missing and try a cut again.

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Re: Diagnosing a Milling Machine problem

Post by KellyJones » Sun Dec 09, 2018 4:56 pm

Given what I’m seeing, that has the ring of truth to it. Can’t get to it from outside. Need to figure out how to get to it.
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Richard_W
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Re: Diagnosing a Milling Machie problem

Post by Richard_W » Sun Dec 09, 2018 7:05 pm

jcfx wrote:
Sun Dec 09, 2018 4:29 pm
My guess is that your gripping problems stem from anti rotation pin is missing, the anti rotation pin would keep the
collet from spinning insuring that the draw bar will draw the collet tight around the shank of your endmill.
Is that pin accessible from the outside of the spindle ? on both my 5C collet chuck and collet blocks it's an adjustable dog point screw.
You may want to check if it walked back into the spindle or replace it if it's missing and try a cut again.
I don't think that this is the problem. If it did slip on the pin it would only grip tighter, unless the spindle is run in reverse. The pin is only designed to assist in the removal of the collet in the event the threads are tight and not meant to be a drive key. By either damaged threads or trash material in the threads. Most shops don't have the pin installed or haven't replace it when it was sheared off and have no problem.

My thought would be the no name collet being to soft to get a good grip on the end mill shank.

Richard W.

KellyJones
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Re: Diagnosing a Milling Machine problem

Post by KellyJones » Sun Dec 09, 2018 7:19 pm

Too soft? I’m still trying to figure that one out. Maybe. I can see if it was too hard it wouldn’t flex enough... There are witness marks in the collet and on the OD of the cutter, but not vertical. There is evidence the tool has spun a little, but not much. I also notice that this cutter in this particular collet seems to be out of round. In other words, when I spin the tool in the collet (not in the machine) I feel alternating tightness and looseness between the two. This different than all the other collets and cutters. Maybe one is out of round enough that it can’t grip?
Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
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choprboy
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Re: Diagnosing a Milling Machine problem

Post by choprboy » Sun Dec 09, 2018 8:27 pm

KellyJones wrote:
Sun Dec 09, 2018 7:19 pm
... I also notice that this cutter in this particular collet seems to be out of round. In other words, when I spin the tool in the collet (not in the machine) I feel alternating tightness and looseness between the two. This different than all the other collets and cutters. Maybe one is out of round enough that it can’t grip?
That is weird... sounds like maybe a collet segment could be bent or the collet was manufactured significantly out of round and/or non-parallel axis between the collet taper and endmill hole.

As for the non-rotation pin... are you absolutely sure the collet is not riding up on the pin when you are tightening the collet? My R8 spindle does not have an alignment pin and I have never had problems with a collet spinning , in fact I have to knock the collet out with a sharp rap on the drawbar when removing. Perhaps try removing the alignment pin? If the collet were riding up on the pin, it would bind (prematurely tighten against the drawbar), move the neck out of axis, and not seat correctly in the spindle taper.

Do you have a piece of 5/8" ground rod (or maybe a drill wit a long shank)? Put the rod in the collet, in the spindle, and measure the runout with an indicator at the collet and then again a couple inches out from the spindle. Repeat with another rod/collet size that works well. Ideally, there should be <0.001" runout. If the 5/8" has large and relatively equal runout at the collet and a few inches out, the collet out non-concentric. If it has little runout at the collet and lots a few inches out, the collet is not holding the tool in the same axis, the collet is not holding the tool straight, or the collet is bent.

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