Mill Boring Out of Round?

lethal375 · Apr 17, 2020

My mill is boring .oo15 out of round in almost perfect alignment with X and Y. Long in X narrow in Y
The mill is CNC, but I am talking about using the spindle with boring head.
I put tenth indicators on the table to ensure no table movement X or Y
Machine Head "Z" is also rock solid.
This problem is repeatable with different materials, setups, boring bars and feeds/speeds.
No CRASH
I have never had a mill do this before.
Any ideas?

Gordon Heaton · Apr 17, 2020

Spindle bearings? If not the business end, perhaps the top maybe causing a wobble?

lethal375 · Apr 17, 2020

I have checked the spindle and it has a tenth or so radial and end play

gbent · Apr 17, 2020

I presume this is a vertical mill. Your z gib is loose or barreled in the x direction. Use an angle plate and indicator, and align the angle plate with Y. Run the spindle down with an indicator against the angle plate. When you reverse the z axis you should see the indicator jump. Make sure the indicator is similar length to your boring tool.

For a work around until you can fix it, or hire it fixed, stop the spindle at the bottom of the bore, index the spindle, and retract with the spindle off.

Phil in Montana · Apr 17, 2020

If the head is not trammed to the axis you will bore a egg shaped hole every time....Phil

lethal375 · Apr 17, 2020

Thanks guys
This is a vertical mill with box ways
I will double check the tram
I have no way to adjust the Y on the head only the X
The hole is larger in the X so the tram does make sense

John Garner · Apr 17, 2020

Spindle "tram" error manifests itself differently when boring as a function of how the feeding is performed, using the quill or using the knee. If quill feed is used, the bore will be generally circular but tilted; if knee feed is used, the bore will be SLIGHTLY elliptical but not tilted.

Unless your dial gage checking of machine stability reaches from the workpiece to the quill, you probably would be wise to do more testing.

lethal375 · Apr 18, 2020

John Garner said:
Spindle "tram" error manifests itself differently when boring as a function of how the feeding is performed, using the quill or using the knee. If quill feed is used, the bore will be generally circular but tilted; if knee feed is used, the bore will be SLIGHTLY elliptical but not tilted.

Unless your dial gage checking of machine stability reaches from the workpiece to the quill, you probably would be wise to do more testing.

I am feeding with the entire head
I will be rechecking the tram later but I know I have a .001 or so out in the Y. This machine has no way to adjust tram in the Y.
Any thoughts on how to adjust for Y in this situation?

If my thinking is correct, what ever direction the tram is out should be the narrow/small dimension of the elliptical?

So if the head is out of tram in the Y axis then this would account for my exact problem. Oval, Small in the Y and large in the X?

I should mention that the bore I was having a problem with is 1.25 ID by 1.25 deep with .0015 out or round

DMF_TomB · Apr 19, 2020

lethal375 said:
My mill is boring .oo15 out of round in almost perfect alignment with X and Y. Long in X narrow in Y
The mill is CNC, but I am talking about using the spindle with boring head.
I put tenth indicators on the table to ensure no table movement X or Y
Machine Head "Z" is also rock solid.
This problem is repeatable with different materials, setups, boring bars and feeds/speeds.
No CRASH
I have never had a mill do this before.
Any ideas?

.
some cnc have servo oscillation problem and also backlash. (ball screw might need replacing or something loosened up)
.
temperature control or changes can easily cause problems too
.
once saw a cnc with encoder wheel in contact with side a machine slide to register actual movement. if dirt or chips got between encoder wheel and it wouldnt turn and machine would see no movement, it would command a move till it saw movement. i have seen 0.1" jumps snap end mill off, obviously closed loop feedback when position readout malfunctions it can be a problem, some encoders also get dirty or oily and have trouble reading electronic scale
.
if you take a test cut and part moves to side to more easily measure, it might not go back to position exactly when you finish the bore

Gordon Heaton · Apr 19, 2020

lethal375 said:
. . .This machine has no way to adjust tram in the Y.
Any thoughts on how to adjust for Y in this situation?. . .

If you can adjust in X, there is an interface at the head/ram mounting point. You can usually affect Y/nod by adjusting the tension (torque) of the mounting bolts, the same ones you'd loosen to adjust tilt. It doesn't sound like it would work, but it does.

boslab · Apr 19, 2020

If your boring an ellipse the axis of the cutter isn’t perpendicular to the table or the vise
Mark

gbent · Apr 19, 2020

To bore an ellipse of 1.2485 x 1.25 by using a spindle centerline nonparallel to the feed axis would require the spindle centerline to be offset by 2.8 degrees.

Bobw · Apr 19, 2020

I have questions.

1) Are you boring in and then boring out? If so, and already suggested,
bore down, stop the spindle and then pull out and see what happens.

2) How are you measuring the hole?

3) Do you have a cranky X motor and its oscillating?

I'm voting for slop in the Z axis ways.

moldcore · Apr 19, 2020

My guess is that the machine is not level so the head is out of square with the table. Leveling the machine will effect the tram on each axis. I have a shop where the concrete slab moves from time to time and we have to relevel the machine about twice a year. But I have to admit I've never seen a boring head make an elliptical hole.

DMF_TomB · Apr 20, 2020

part warpage

also some parts warp considerably when unchucked and have had time to think about it.
.
I have seen bore go out of round that is days later and off the machine, sometimes if part is chucked too tight loosening a vise you can immediately measure a bore go out of round. other parts you might measure out of roundness, hours or days later.
.
just thought I would mention chucking or clamping pressure effects and parts warping after some time has gone by.

Bobw · Apr 20, 2020

DMF_TomB said:
also some parts warp considerably when unchucked and have had time to think about it.
.
I have seen bore go out of round that is days later and off the machine, sometimes if part is chucked too tight loosening a vise you can immediately measure a bore go out of round. other parts you might measure out of roundness, hours or days later.
.
just thought I would mention chucking or clamping pressure effects and parts warping after some time has gone by.

Had some castings a while back, delivered the first couple immediately after machining and the rest sat
around while the customer got their shit together. Thin wall, really complicated, large (for me, not Tom).

Every fricken hole went out of round and/or shrunk sitting around for a year. Bunch of linear
dimensions from one side to the other moved. Really wasn't too bad, the bearing bores came back
round when a bearing went in there, and the smaller holes for bushings and what not, I just re-reamed them.
It all went together just fine.

I couldn't believe how much those castings moved sitting for a year, it was an eye opening experience.

gbent · Apr 20, 2020

Bobw said:
I couldn't believe how much those castings moved sitting for a year, it was an eye opening experience.

Just for curiosity's sake, what material were the castings?

Bobw · Apr 20, 2020

gbent said:
Just for curiosity's sake, what material were the castings?

Mil-A-21188 A356 class 11(eleven) temper T6.

13engines · Apr 20, 2020

What about something simple like distortion from work holding? Is the work being held in a vise? Is the roundness error being measured while the work piece is still on the machine or when taken off? Or is it measuring the same regardless?

DMF_TomB · Apr 21, 2020

even a simple mill vise can squeeze or compress a part and when unchucked part bores often go out of round
.
this is often seen with lathe parts too. a 3 jaw chuck when part unchucked bore is often rounded triangle a bit, just saying bore can distort more than simple oval, depends on how it was chucked
.
also mill parts, if bore off center often many materials distort or metal might be harder near skin where rolling forces work hardened part material more. this is usually why cold rolled steel milled more on one side will bend / distort when unchucked
.
castings often the center of thicker sections cooled slower when casting poured compared to casting skin which metal cools quicker, thus often there are different material properties in the same part (even if cast or rolled), this often why (a opinion) machined castings distort
.
obviously if you open up a off center pilot bore alot than it will machine not round due to varying deflection from varying loads. for example 1.5" dia hole and 0.1" off position trying to open bore to 2.000" in one pass, longer length boring bars can deflect considerably under load.
amount you are boring can effect bore roundness

Mill Boring Out of Round?

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