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RueBear

Plastic
Joined
Apr 14, 2019
I have ran lathes before in my younger years at a machine shop. I recently purchased a 1700lbs lathe and have had great success other than a few minor issues. One being that I can not for the life of me get all of the chatter out of my single point threading. I have the correct set up, my brother-in-law is a machinist...a good one, and he can’t figure it out either. So I’m guessing it is in my tooling at this point. Any advice is welcome, it is a stock lathe that bolted to the floor and leveled. Runout on both 3 and 4 jaw chucks are less than .0003. Stock tool post, Shars tool holders and tooling. I have adjusted spindle bearings, backlash and the gibs. I have a slight vibration in the lathe which I believe is coming from the fairly light base the machine comes with. Thought about bolting lathe to the wall with all thread for stability. Please advise.
 
1700 lb seems should not be chattering.
Tool bit side rake angle perhaps 5* and very sharp for mild steel.
pick up on spindle with a 2x2 pry to see less than .003 slop under an indicator.
check for pulleys and things being lose or out of balance at high speeds.
A bad gear or chips in gears.
Too much part hang out with not using you tail.
Compound at angle and set too far to the left side of saddle.
loose gibbs.

Threading going straight in (not at 29- 30*or so)causes both sides of the tool bit to try to cut. the right side of the tool bit may be having a negative rake angle so very much stress in the cut.

Using insert, some are not that sharp so perhaps use an aluminum designated insert.Yes for mild steel..but going straight in makes both side of the insert try to cut and that may be a big bite.

https://www.amazon.com/carbide-inse...?k=carbide+inserts+for+aluminum&rh=n:16310091
 
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I have experimented with that, using dycam on threads and picking them up and and at varying speeds
 
it's not a great picture but I think I'm with michiganbuck. I would say use a more positive insert, like inserts intended for aluminum, or a nice sharp piece of hss. Also if you haven't already, set your compound at 29 degrees (or 61 depending on how your compound is oriented) and feed in with your compound so you are only cutting on one flank of the thread.
 
Another good check is to just run a normal turning of a mild steel part...that is just a very fine thread going toward the tail. If it is good then no reason a turning toward the head stock should not be just as good if the cutting tool and set-up are correct.
 
This is the tool holder I’m using. Do I need to go to a positive insert holder.View attachment 260863

I like a little positive rake attitude..with a HSS bit I put on 5* side cutting edge rake. (I know How to run a lathe seems to say 5* back rake for mild steel if I remember right.)

So for my choice/liking I would bring that insert up to the part at center and look at how the cutting edge looks to the part..wishing to see some clearance below the touching and some positive rake attitude at the top of the insert... yes positive would touch the part and then angle down about 5 degrees going away from where it touches.
I would have my compound swung 29 -29 1/2 or 30* going to the right off of being straight. and in-feed with the compound dial that started out zero and intended to go to .005 short of my thread depth target...
You don't want to go all the way home on the first part because off center whip in the part or error in the point flat may make your cut/depth too deep/or short.

With watching the thread top to just make to or near a point getting near my dial size. Coming to a a point before my dial target I would stop in-feeding and check my numbers. With compound about 30* the cut should be only on the left edge of the insert.
 
I can't tell from the photo if the problem is chatter or tearing. If it is tearing, use a high sulfur cutting oil.
 
Also, I’ve ground my own Hss tool and still get chatter with that as well.

Have the compound off, then.

Shim a tool to height, top bridge bar, two bolts, one each side to keep it there. Prolly borrow them out of you mil'ls clamp set.

Place the tip of the tool right about over where the compound's pivot center was. Support extends further toward centreline than the tool-tip above it. "negative" hang-out or overhang, IOW.

Set the stock well-back in your collet.

Advance straight-in, each pass, clamping the carriage cross top-slide to the saddle for the duration of the pass.

Age-old tradeoff, light lathes with wear.

Pain in the ass setup || chatter.

Pick any ONE.

:)
 
How far out from the chuck are you cutting the threads? Could it be the party chattering rather than the tool?

I just got a well worn lathe and the carriage could rock back and forth on in the bed. Easy test is to tap the four corners of the carriage and listen for a "hollow" sound. That means one or more corners isn't touching the bed.

I think someone back at the beginning mentioned checking the spindle for play as well.
 
Backstory: My primary use now of a manual 3 jaw is to grab round things in the mill. Its a POS cheap chinese chuck. And she's a bit sprung after a decade of use.. I found this out the hard way, with inconsistent sizes and chatter..

CNC lathe... Same damn thing. Too much or not enough tapered bored into the soft jaws. CHATTER..

Solution. TAPE.. High temp masking tape. Its thick (for tape) and one wrap cures a lot of ills.

Why does tape cure the problem.. A sprung chuck isn't grabbing on 3 lines, its grabbing on 3 points, allowing your material to bounce around all kinds of way, and that mean CHATTER..


This may not be your problem, but its a quick check. Wrap the part of the part that is grabbed in the chuck in tape. Masking tape works.. If its really bad, you can pick up this problem with a feeler gage..


I'll also ask. Material? Thread? Diamter? Stickout? DEPTH of cut??

In your really bad picture, I can see BAD chatter.. I just did a left hand 3/4-16 on 3/4" 17-4 sticking out 4.5" yesterday. there was a bit of choppy-ness in the thread. But it was acceptable...

Long story short.. Unless you are doing something incredibly stupid (massive L/D ratio).. Something is moving, you just need to figure out what.. And then you need to figure out why (not always easy) and fix it.
 
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yes, what Bob said...something is moving that shouldn't.

Stick mag base and indicator on things while running to isolate and narrow it down....just like you would look for a short in wiring....work your way through logically.
 
Use a tail center to steady the part, if not tried already.

A vibration pattern will self replicate to quite some extent. So you want to avoid having it even begin to chatter.

Chatter is often due to too much cutting edge engaged, and too light of a cut. I am not a compound rest 'set to 29°' type of machinist; I have it set at 90° :D So I cut with the tool straight in, cutting both sides at equal cuts. When the depth gets so great that chatter begins, then I tweak the compound ahead a little bit so the tool cuts on only the front edge. Still take a fairly heavy cut to prevent chatter from occurring. I might on a subsequent cut move the compound backwards and cut with the rear edge of the tool.

If you get chatter starting to happen, it can be beneficial to speed the spindle up a gear for one pass. Then drop back a gear. Anything to defeat the harmonic vibration.

Make sure the gib in the compound is set tight. It should be difficult to turn its crank.

I quite like the laydown threading inserts. Should have the one with the slightly raised edge chipformer as it will really roll the chips out nicely, on both sides of the cut at once.
 








 
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