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Aug 28

Waviness on small bore - HMC


Hello Everyone

We have got a HMC , to do boring on two different diameters of connecting rod

We are having waviness issue in small bore

As shown in PPT, there are 3 components loaded on locations A, B, C of fixture, while waviness on A is by and large within limits barring few cases, generally it is out for B and C

Please find couple of actions from our side thus far related to fixture

1. Accumulator charged to 85 bars to ensure adequate clamping pressure of components

2. Rest pads checked and ensured tightness. Blue seating of brass rest pads checked and found ok – although we have no issues with regard twist and bend.

3. Replaced locator for fixture B

4. In the original program, at the end of boring, spindle moves to 180 deg then retracts, blocked this block and retracted in feed till boring bar leaves component

5.Boring bar replaced with new one, Feed rate reduced from 126mm/min to 100 mm/min

6. Dia of small bore is 40.026 for length of 40mm, whereas dia of big bore is 73

7. Coolant pressure, tool clamping pressure found alright

8. Axis Z1 is master having linear scale along with direct motor encoder with it, Axis Z2 is slave with only direct motor encoder


Request your advice to overcome Waviness of small bore


Fixture A,B,Cpptx


Is this a new process - or one that has worked properly in the past, but not now?


It sounds like even the part machined in fixture A is "iffy". If not, I'd suspect the issue to be in the work-holding. If it is, I'd also suspect the cutting tool.


I'm wondering what kind of boring bar is being used (rake angle, cutting material, and carbide shank or steel shank) and how much material is being left in the hole for finishing. Also, what is the length-to-diameter ration for "hang-out" of the boring bar? Ideally it should not exceed 3-1 for a steel shank bar (three inch hangout for a 1 inch diameter bar).


As for work-holding, these can be tough issues to diagnose. If you have a way to measure clamping force within the fixture (not just input pressure for the hydraulic system), you could determine if there are any variations among the fixtures. It's possible that the hydraulic lines are being blocked in some way or the mechanics of the clamping system are not moving freely.

Thanks for your feedback

1. It was doing fine, we have problem since last two months - not a new process

2. L/D ratio should be fine

Shall revert with other details

Aug 30Edited: Aug 30

Then something obviously changed - maybe a crash that changed the centerline position of the spindle (boring bar tip isn't on or slightly above center) - or loosened up the way/turret system? A crash could also through the turret out of position - bending/shearing taper alignment pins, for example. Maybe mechanical components in the clamping system are wearing and not gripping as they used to? Maybe degraded hydraulic fluid in the clamping system in need of change - or air in the lines?

Thanks, we plan to replace spindle and LM blocks of Z axis, shall revert back

After replacing LM blocks, results quite satisfactory and Wc well within limits. Thank you so much for your support

Hello Mike

Please recall issue related to Waviness. Please confirm if there is any way in Siemens 840D SL with Sinamics Servo drive, to find waveform with respect to axis movement so that waveform wud be slightly different in case of any mechanical issue/obstruction as it was due to minor issue in LM Carriage



Sep 24

I'm sorry, but I'm not familiar with this control. But with FANUC CNCs, I do not know of a way to find or modify the waveform of axis movement.

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