End mill won't sit square in collet

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James Temple
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End mill won't sit square in collet

Post by James Temple »

Hello all! I've been using a Proxxon MF-70 for the past several months to rebuild a 1:32 scale model train, and I've run into an intransigent problem. When I mount an end mill in the collet and check it using a dial micrometer, there's always about .005-.007" of variation when I rotate it manually. I presume if it was running square there'd be none (or maybe a thou or two). I've swapped collets, end mills, and cleaned/checked everything for chips or other interference and nothing seems to make any difference.

Searching on the Web (including this forum) so far hasn't produced a solution. Do any of the minds here have ideas? Does this mean the collet set is bad? Or is this just the best I can expect from the MF-70 (admittedly, something a budget model, though one reputed to be reasonable quality)?

Thanks to anyone who can help --
John Hasler
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Re: End mill won't sit square in collet

Post by John Hasler »

Exactly how and where are you making that measurement?
James Temple
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Re: End mill won't sit square in collet

Post by James Temple »

IMG_6055.JPG
Here's a picture of the setup. The end mill in that shot has a 3mm shank. I determined the variation by seeing what range the needle moves in while I turn the spindle manually.

The variation increases the further down the bit I measure. At that location, it's .005"; about 7mm further down, it's .007".

The bit is inserted as far as the collet will allow. Even without the micrometer, when I bring the cutter near a surface, I can see the distance vary (at first I thought the cutter was bent, but as noted, this happens with any cutter, any collet).
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GlennW
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Re: End mill won't sit square in collet

Post by GlennW »

If that is a double end mill, there is no way it will run true as the collet is only gripping it by a very short length.

If so, you might try cutting the mill in half so that most of the shank will be in the collet.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
James Temple
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Re: End mill won't sit square in collet

Post by James Temple »

All my end mills are single end. That particular one is a Niagara, 39mm total length, with about 20mm in the collet. It's very close in overall dimensions to the cutters Proxxon makes specifically for this mill (also 3mm shank, ~38mm long).

That said, I'm going to keep that in mind if I ever acquire any double-enders.
John Hasler
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Re: End mill won't sit square in collet

Post by John Hasler »

Check the runout on the inside of the spindle taper. I think that you will find that the problem is the collets: that design is simply not very precise. There is reportedly an ER11 upgrade available. I suggest that you consider that.
pete
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Re: End mill won't sit square in collet

Post by pete »

I'd very much agree with John about checking the spindle taper the collet fit's into. But you'll need a different indicator type. You really need a finger type or what's known as a dial test indicator. .005"-.007" is way too high so you need to eliminate each surface until you find the one or a combination of issues for what's causing that much run out. As is it's just about unusable with small end mills.
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Harold_V
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Re: End mill won't sit square in collet

Post by Harold_V »

pete wrote: Tue Mar 30, 2021 1:50 am As is it's just about unusable with small end mills.
The caution on small end mills is valid, as they will be much easier to break as compared to larger end mills. Cut, in this instance, will be intermittent, with one side of the end mill will doing all the cutting.

Fact is, size has little to do with its ability to be used properly. Runout at the spindle is not acceptable, especially when it isn't parallel to the spindle, as yours is (the farther away you get from the spindle nose, the greater (or lesser) the error). The resulting cut won't be parallel to the spindle centerline. T

Because the readings you reported are not consistent, getting worse as you move away from the spindle nose, Indications are that it's possible that the spindle itself is not ground concentric or perpendicular to the centerline. That's most likely true if you've tried various collets and you get similar results, all in the same place. However, if the error is limited to one collet size, then it's the collet that is defective.

You should be able to determine concentricity of the spindle at the nose with the indicator you have at your disposal, although a DTI would be a little easier to use. Simply place the indicator shaft at a slight angle, so the tip can contact the interior surface of the tapered surface on which the collet bears. The amount of error you're looking for will be quite evident if it's the spindle. If you get no movement of the indicator, pretty good chance the collet is the problem. If you get slight movement or more, then it's important that you be able to run an indicator on the deepest portion of the taper. Readings, what ever they may be, should be identical. If they are not, then the grind is not parallel to the spindle centerline. Take note where your high readings are by making a mark with a felt pen or grease pencil at the high points.

Your machine is not a jig borer, but it isn't unreasonable for you to expect a level of concentricity/perpendicularity that is not greater than a half thou TIR. Ideally, there will be no movement at all, but that's a pretty tall order for most machines. A tenth or two is quite common.

H
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R.W. Parker
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Re: End mill won't sit square in collet

Post by R.W. Parker »

How is the accuracy of your cuts? Presuming that inaccuracy of the machined feature led you to make this measurement, I would agree that a test indicator should be used. I'd also measure over something larger, say a .500 plug gage held in a 1/2-inch collet.

Richie
R.W. Parker
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Re: End mill won't sit square in collet

Post by R.W. Parker »

Just noticed this:

<The variation increases the further down the bit I measure. At that location, it's .005"; about 7mm further down, it's .007">

Have you trammed the head of your machine? Sounds like it's not perpendicular to the table.

Richie
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NP317
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Re: End mill won't sit square in collet

Post by NP317 »

Off topic:
Are you the same James Temple who attended University of Washington?
Photo attached from 2004.
RussN
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rklopp
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Re: End mill won't sit square in collet

Post by rklopp »

What is that collet setup in the first pic? It does not look like an appropriate device for holding an endmill. Can you show us the pieces from the endmill to the machine’s spindle taper?

I checked on the Proxxon site, and I am afraid the runout you are measuring resides in the collet or machine's spindle taper. It looks like barely a step up from a Dremel collet, which is a poor tool for holding an endmill. How closely does the collet fit the endmill before tightening? The fit should be close so that you don't have to squeeze the collet much. Is the interior of the collet nut in good shape where it meets the collet?


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Last edited by rklopp on Tue Mar 30, 2021 1:50 pm, edited 1 time in total.
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