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CNC Lathe turns OD's well, but does not repeat when boring

texasmci

Plastic
Joined
Oct 2, 2021
I have a Doosan Lynx 220A that has a repeatability issue.
We machine smaller parts, usually less than .5" in diameter with bores less than .25" regularly.
The machine will turn OD's straight and consistant all day long, but if you are boring an ID it doesnt repeat. I get a large amount of inconsistent taper in the bore, even when i feed up to the bore diameter and across.
it wont make 2 parts the same. Machine has 4400 hours of cut time on it although it is an older machine.
I'm not sure why but I think it may have something to do with turret clamping, although I wasnt able to get more than a couple tenths (.0004") of movement wiggling the turret by hand.

Any Ideas?
 

chad883

Cast Iron
Joined
Jan 12, 2012
Location
indiana, usa
First guess would be tool deflection when boring. Hard to say without knowing the material, and depth. Also depth of cut on bore.
 

texasmci

Plastic
Joined
Oct 2, 2021
solid carbide coated bore bar removing .002" depth of cut, 1500 rpm .0007" feed. bore tapers bigger in the back.
Doesnt matter what the material is. Others have complained about this machine, but I figured it was something they were doing wrong. Apparently it has an issue on every job.
 

pcasanova

Hot Rolled
Joined
Mar 14, 2008
Location
vacaville ca
with that small of a cut wouldn't think this would matter but is one pushing the turret down and the other up? hydraulic pressure good? try swapping tool pockets and holder to see if problems follows or goes away
 

latheman78

Hot Rolled
Joined
May 28, 2022
Location
Southern Ca Mtns.
solid carbide coated bore bar removing .002" depth of cut, 1500 rpm .0007" feed. bore tapers bigger in the back.
Doesnt matter what the material is. Others have complained about this machine, but I figured it was something they were doing wrong. Apparently it has an issue on every job.
Can you program the taper out? If so do that and be done with it.
 

texasmci

Plastic
Joined
Oct 2, 2021
Can you program the taper out? If so do that and be done with it.
it doesnt repeat, which is the problem. im trying to hold .0004" tolerance on the bore, it varies way more than that part to part.
The issue is that it only has this issue while boring, not turning. The other machines have no issues, its only this machine and its an issue on every job.

I have someone coming out to look at it, but if they cant figure out what is going on I will have to sell it and get something else.
 

colton_m

Aluminum
Joined
Apr 16, 2015
I had the same problem with a .335" bore 1" deep (4140 HT) on the same machine.
We put in three of the same boring bar and did rough, semi-finish, finish.
After that it would repeat, then we could program out the taper and hold +/- .0002" that we required.
 

mmurray70

Hot Rolled
Joined
Jan 11, 2003
that machine doesnt care if its doing inside or outside work. If theres no taper on OD tools its probably fine for ID tools.

Your bore is less then 0.25"?? Almost certainly an issue with tooling or process. Add more passes, multiple tools, spring passes whatever to get rid of deflection. The last finish tool should be razor sharp and remove very little to get rid of all deflection.

Better still, can you bore to get on center and then ream to finish size??
 

latheman78

Hot Rolled
Joined
May 28, 2022
Location
Southern Ca Mtns.
I always program that the tool approach will take up any backlash and feed to a starting point whether on x,y,or z. If I am boring at x1.0 I am not going to rapid a boring bar there and start cutting.
 

jonnyrocket56

Plastic
Joined
May 21, 2013
Location
California
So I have a bunch of doosans and more then once the locating system inside the turret can get dirty and lead to inconsistent sizes. The best way to test it is to put a indicator of the station in question and zero the machine. Move the machine up in the x and index to a different tool. Then index back to station in question and go back to the zero location and see if the station is repeating. I typically write a program and repeat it while watching the indicator. If it does not repeat then something is wrong.
 

gmc1724

Aluminum
Joined
Oct 24, 2013
Location
United Kingdom
I have a Doosan Lynx 220A that has a repeatability issue.
We machine smaller parts, usually less than .5" in diameter with bores less than .25" regularly.
The machine will turn OD's straight and consistant all day long, but if you are boring an ID it doesnt repeat. I get a large amount of inconsistent taper in the bore, even when i feed up to the bore diameter and across.
it wont make 2 parts the same. Machine has 4400 hours of cut time on it although it is an older machine.
I'm not sure why but I think it may have something to do with turret clamping, although I wasnt able to get more than a couple tenths (.0004") of movement wiggling the turret by hand.

Any Ideas?
You say the material doesn't matter but let's say for example you are cutting SS316, at .25" dia you would need a rpm of around 5000ish so 1500 rpm seems awfully slow.
 

cnctoolcat

Diamond
Joined
Sep 18, 2006
Location
Abingdon, VA
If your machine repeats for OD tools, it will repeat for ID tools---if you have a good process and setup.
What you are seeing is a combo of part and/or boring bar deflection, along with chips causing issues down in the hole.
Make sure the boring bar isn't picking up some kind of double or erroneous tool-nose radius compensation. Your X value should stay the same on your screen as it's boring, programmed to a straight-line bore.
At .25" diameter, possibly bore undersize and finish ream if you have to.

ToolCat
 

Ox

Diamond
Joined
Aug 27, 2002
Location
West Unity, Ohio
Are your small boring tools cutting on center? The smaller the bore, the more important that alignment is.

Indexing the tool into a proper position was my first thought as well.
Are you using flats on the tools to locate?
Or just locating in a collet?

I use small bars in collets all day long, but I don't normally get as small as you are if I can help it.
So I eyeball my bars all the time, and like was said above, the smaller the bore, the more critical the indexing.
I try to keep my bars "below" center.
More neg tip angle, but the heel doesn't drag.

Another option:
Flip your bar around to X- where you can see it better, and run your program in X-.
Your diam adjustments are backwards, but ...
Be carefull with your pull-out line.
G0 U-.01 Z1 won't give good results here. ;)


---------------------

Think Snow Eh!
Ox
 

Eric U

Hot Rolled
Joined
Feb 26, 2003
Location
Eastern AL
I’ve made about 14,000 bushings out of 4140HT in the last 18 months. OD is 1/2”, ID between .220” and .375”, 3/8” deep. Holding bore tolerances of +/- .0001”. Doosan Lynx 220.

It took a while to really dial it in. I had issues with chip welding and would scrap between 20-30% of my parts on ID size. Switching to cermets from coated carbide knocked that scrap rate to under 10%.

Using boring bar holders helped to get the edge of my 4.5mm and 5mm boring bar inserts on center. Trying to get a tiny bar lined up using its flat in a collet is an exercise in futility. Quality Japanese boring bars really helped too. The pocket in the Japanese bars could last 4000 parts. The cheap Chinese bars would last less than 1000 before the pocket got too loose to hold size consistently.
 
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