Turning small diameters, right/wrong way

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Mr Ron
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Turning small diameters, right/wrong way

Post by Mr Ron »

Here is a shaft I was making. Starting with 1/4" dia 7075 rod. Starting at the left end, I turned, beginning with the largest dia. By the time I got to the small dia's, I couldn't make a cut because the rod would flex away from the cutting tool. This is a lesson learned. I now start turning the smallest dia first, progressing to the larger dia.
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armscor 1
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Re: Turning small diameters, right/wrong way

Post by armscor 1 »

Watch Tom Lipton on youtube making a plastic tooth pick on the lathe.
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Re: Turning small diameters, right/wrong way

Post by whateg0 »

Depends on how you are holding it. Are you holding it in the lathe with the smaller end near the chuck or away?
Mr Ron
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Re: Turning small diameters, right/wrong way

Post by Mr Ron »

armscor 1 wrote: Wed Mar 04, 2020 4:41 pm Watch Tom Lipton on youtube making a plastic tooth pick on the lathe.
I watched the Tom Lipton video and he was doing it the way I stated. He added the steady to the tail stock which I need to incorporate in my projects. Thanks for the link.
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tornitore45
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Re: Turning small diameters, right/wrong way

Post by tornitore45 »

The smallest diameter is 1/3 the length. No problem, held in a collett do that first.
All other feature are "short". No problem there. Push out and turn down. This way you are not cutting to far from the chuck.
The "wrong way" is to weaken the entire length progressively until you are left with a flexible shaft you try to turn the end.
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Re: Turning small diameters, right/wrong way

Post by Wolfgang »

Of course one might support the small dia. outboard end on a male of female centre. The work is less than 4 dia. long thus not really problematic ie. a steady rest or travelling steady are not warranted.

Couple of points though: On small dia. turning it helps if the turning tool is really sharp and honed. And, the cutting tip must be at centre height very accurately. Too low and the work may climb up onto the tool; too high and the tool may simply push the work without really cutting.

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Harold_V
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Re: Turning small diameters, right/wrong way

Post by Harold_V »

I"m not sure I agree that the work doesn't violate the three times the diameter rule (not four). In practice, turns like those indicated can be, and usually are, troublesome in that it's difficult to achieve straight cuts.

I fully agree with using a sharp and well honed tool. I would also suggest that positive rake be incorporated, as it encourages cutting (lowers cutting pressure).

7075 aluminum, especially in a T6 condition, cuts beautifully, but offers some resistance to cutting, so it would be inclined to deflect under cutting pressure.

A trick that works well is to make the part a little extra long, so a center can be used, and later eliminated if it is not desired. In such a case, an 0 center drill will allow one to be created that is limited in depth. Once the part is finished, the center can then be machined away.

It's a good idea to take roughing cuts on all diameters before drilling the center. That allows the material to move if it is so inclined, and it generally does, albeit not significantly. Once roughed, the material should not be moved, as it might be if the lengths of the individual cuts were excessive, so the stock is advanced repeatedly as the turns are roughed. Enough material should remain for the now not concentric cuts to clean up upon final machining.

With the stock in its final position for finish machining, the center is then drilled. It will be true to the centerline of the spindle, assuming it is allowed to freely find center. That way the turns will be concentric when the center is no longer in use. If the center is eccentric, the turns will be concentric until it is removed, at which time the turns are no longer concentric with the material.

Does this make sense to you? If it does not, keep reading until it does. This is critical to achieving an acceptable end result.

H
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ctwo
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Re: Turning small diameters, right/wrong way

Post by ctwo »

How do you machine away the center, because it seems at that point there would be a lot (all) of the part sticking out?
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Harold_V
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Re: Turning small diameters, right/wrong way

Post by Harold_V »

My experience dictates that light passes must be taken, but slight deflection, unlike when turning, doesn't make a significant difference. So long as the depth of cut and feed rate don't cause the part to climb the tool, once roughed, a slight finish pass results in a square end. Also, for small diameters such as we're discussing, spindle speed is important. Throw everything at it your machine can provide.

There's nothing says one can't take off the bulk of the material with a belt sander, either. Leave enough for a light finish pass and you're good to go. That's a good idea, anyway, as it shortens the overall length of the part.

If the turns are long enough, you can always insert the shaft from the rear of a collet and make your cut that way. That's assuming one has access to collets of the proper size.

H
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tornitore45
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Re: Turning small diameters, right/wrong way

Post by tornitore45 »

The smallest diameter is 1/3 the length.
Of its own section
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Re: Turning small diameters, right/wrong way

Post by shootnride »

Joe Pieczynski has a couple of good Youtube videos on turning small diameter and thin wall parts.

https://www.youtube.com/watch?v=vg6ug0FDhos

https://www.youtube.com/watch?v=hXq_HfwG7dA

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Bill Shields
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Re: Turning small diameters, right/wrong way

Post by Bill Shields »

know that you don't have a 'Swiss' lathe in your basement, but just watching a video of how they work will give you a lot of food for thought

https://www.youtube.com/watch?v=-4qYgjzz1jc

The important part for this thread starts about 5:20 into the video..but the rest of it is just a interesting for those 'how do they make something like that' parts you see floating around.
Too many things going on to bother listing them.
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