half nut wear vs lead screw wear

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epanzella
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Location: Higganum, Ct

Re: half nut wear vs lead screw wear

Post by epanzella »

What is the tolerance on center height? Is it sufficient to set tool height by eyeballing a 60 deg center or do you need a gizmo?
Ed P
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ctwo
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Re: half nut wear vs lead screw wear

Post by ctwo »

I usually pinch a rule between the cutter tip and fresh turned part. The rule will stand straight vertical when the tool height is correct. Otherwise, the rule will lean toward (too low) or away (too high) from you.
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Harold_V
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Re: half nut wear vs lead screw wear

Post by Harold_V »

Do not trust eyeball for setting tool height. For small work, being above center by even a couple thou can be fatal. It is all the more critical if you're using carbide.

While most cutting tools will function adequately when set below center, you risk pulling off the tool tip, and you encourage the part to climb the tool, which isn't all that unusual when operating light duty machines. The best advice I can offer is to ensure that your tool is set correctly (on center). The one exception that comes to mind is in boring. If you are not facing, setting the bar a few thou above center can have benefits in that when the bar springs, it moves the cutting tool away from the cut instead of going deeper.

Pinching a ruler is a fast and easy way to check center height. Even better is to take a facing cut, assuming that's possible.

Harold
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epanzella
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Re: half nut wear vs lead screw wear

Post by epanzella »

ctwo wrote:I usually pinch a rule between the cutter tip and fresh turned part. The rule will stand straight vertical when the tool height is correct. Otherwise, the rule will lean toward (too low) or away (too high) from you.
Yeah I forgot to mention I use both the ruler as well as the 60 degree center. I just don't know if that's accurate enough although I hope it is. Most alum & brass I buy new but the steel is usually whoknowsium from the scrapyard. Some times it threads beautifully and sometimes I get some tearing. I read on this forum that mild steel has a tendency to tear but I want to make sure I'm not blaming the steel for my mistakes.
PS
Harold, I didn't see your post before I posted the above. If the ruler trick is accurate enough I'm all set. Thank you.
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Re: half nut wear vs lead screw wear

Post by spro »

In all cases it is important to set the point of the cutting tool or insert, on center. Center requires a base and that base requires two points of reference. A simple piece of right angle aluminum "flashing" material can be the two angle base for the center gage. It can be scribed across a sharp pointed tail center, for center between inside flats. It can be scribed across the center of the height when on the cross slide or compound. These marks can be cut to points which are the centers of any tooling height. It's just a light gage for that lathe but useful for later setups.
tfluthy
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Re: half nut wear vs lead screw wear

Post by tfluthy »

I know that this is delayed, but since people like to know how things work out, here is what happened.

I found my threading issues were related to two specific items.

1. The halfnuts were worn badly and were very dirty. They had a step groove in the wear, so if there was incomplete lockup, then your thread would be offset.
Solution? replace the halfnuts. Worked like a charm.

2. when looking at gear train, the safety gear part number was not right for the safety gear in the manual. I suppose that when they were getting the lathe ready for auction, they just slapped one in they had on hand. The gear was 72 tooth, while the gear train requires a 60 tooth. Replace the gears, and everything worked out okay.

And so it goes, but I learned alot.
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hammermill
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Re: half nut wear vs lead screw wear

Post by hammermill »

thanks for the post action report. that 60 tooth may have been left over from a different pick threading set up. it keeps life interesting for sure
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