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Old 06-29-2011, 01:01 AM   #1
jtrout13
 
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Default Turning small long shafts on big lathe?

I recently built Elmer's Fancy engine and while it performed well, I was unhappy with the piston. I am having trouble turning the connecting rod for this piston (it is a 0.25 inch OD rod, with a section turned down to 0.125 inch diameter, 1.25 inches long).

My lathe is large Romi 13-10. I keep having trouble with one end of the turned-down section being a larger diameter than the other end, and I can't seem to get a constant OD all the way down the turned section. I have tried doing it between centers (difficult) and also tried turning it in small steps of length (only neck down about 0.25 inch at a time) to reduce the cutting force deflecting my workpiece. However, I am still having no luck. (have tried using both HSS and indexable tooling).

Would anyone be so kind as to provide some advice?



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Old 06-29-2011, 01:05 AM   #2
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Default Re: Turning small long shafts on big lathe?

What is occuring? Taper?, chatter?

Have you leveled your lathe?

Dave



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Old 06-29-2011, 01:09 AM   #3
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Default Re: Turning small long shafts on big lathe?

Also, what's the material? Aluminum, steel?

Have you tried taking more, shallower cuts and gradually approaching the target diameter? I have heard on steel that swelling from heat can cause this issue.

- Ryan

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Old 06-29-2011, 02:02 AM   #4
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Default Re: Turning small long shafts on big lathe?

Sorry for the poor description. I am using drill rod O-1, and the problem I am having is taper. Chatter isn't a problem, and surface finish is great, but I'm getting the taper and having problems zeroing in on the final dimension.

Also, it's tough to take small cuts on this lathe due to deflection of the workpiece (the tool just bends the shaft instead of cutting into it), so I am doing a minimum of 0.030in depth of cut (0.060 reduction in diameter).

I have always had great accuracy and never had taper occur with this lathe, but I usually don't turn any diameters smaller than 1/4 inch. This is the first part I've had this problem on, so I'm totally confused.

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Old 06-29-2011, 02:45 AM   #5
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Default Re: Turning small long shafts on big lathe?

You need to fashion a follower rest, it will come in handy more often than you'd expect, especially doing model engine parts.

Frank Ford has built a nice version here:

http://www.frets.com/HomeShopTech/Tooling/MiniFollower/minifollower.html

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Old 06-29-2011, 06:08 AM   #6
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Default Re: Turning small long shafts on big lathe?

Quote:
Originally Posted by jtrout13
Sorry for the poor description. I am using drill rod O-1, and the problem I am having is taper. Chatter isn't a problem, and surface finish is great, but I'm getting the taper and having problems zeroing in on the final dimension.

Also, it's tough to take small cuts on this lathe due to deflection of the workpiece (the tool just bends the shaft instead of cutting into it), so I am doing a minimum of 0.030in depth of cut (0.060 reduction in diameter).

I have always had great accuracy and never had taper occur with this lathe, but I usually don't turn any diameters smaller than 1/4 inch. This is the first part I've had this problem on, so I'm totally confused.
A follower rest is a good idea, but I would still look into the possibility of heat expansion. It's possible you're still cutting true, but as the stock heats it expands, so you're cutting deeper relative to the stock. When it cools and shrinks it shows as a taper. Smaller stock has less mass to soak up the heat and less surface area to shed it, so it gets hotter, quicker.

The follower rest will prevent deflection, allowing you to make lighter cuts, which will both cause less friction so less heat, and give the stock time to cool between cuts.

Of course, your mileage may vary, and i'm approaching it all from the theory angle .... I could be way off base

- Ryan

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Old 06-29-2011, 06:38 AM   #7
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Default Re: Turning small long shafts on big lathe?

I believe what you need is a box tool.This feeds the rod to be cut through a sized bush with a cutter being placed after the bush.This allows thin rod to be reduced in size without bending the rod.Of course,you need a bush for every size rod.F.i.,if you want to cut a 1/4" rod to 3/16",you will need a 1/4",and possibly a 11/16" bush.
There have been various designs mentioned on forum boards,but goshdurnit,I can't find a single one at the moment .
I seem to remember Tel had a nice simple one you could copy.

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Old 06-29-2011, 07:52 AM   #8
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Default Re: Turning small long shafts on big lathe?

I think this is the tool bentprop is talking about.

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Old 06-29-2011, 08:05 AM   #9
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Default Re: Turning small long shafts on big lathe?

Quote:
I seem to remember Tel had a nice simple one you could copy.
Indeed I dew!





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Old 06-29-2011, 08:39 AM   #10
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Default Re: Turning small long shafts on big lathe?

Hi All
.030" is not a small finishing cut! when not supported especially with HSS.
You may need to try several "spring cuts" to try and stop deflection.
Other than that all the metods described are worth a try.
Graham



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