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Sep 6

More cut along Z axis in HMC - intermittent issue

12 comments

Hello Sir

We have HMC, which is used for making 24 holes ( it is a combination of 4X6 holes) of same size on face of a casting component by using standard combination boring bar - in which initially a small hole, concentric with one of the big 4 holes, is made and then feed changes to make big hole. .

Cycle time is close to 30 minutes

Component is rested against 6 rest pads on Fixture, there is air seat check to ensure positive butting all along between job and fixture

Say 1 st component is alright and may be next component, depth of hole is found to be increased by upto 1mm scrapping the job

We have checked relevant CNC parameters for In-position, acceptable tolerance between commanded and actual position and compared with other similar machines for similar components - found same

Tool clamping force checked and found alright

Z axis repeatability in Auto cycle checked and found OK as per readings on Dial

Compared program with another similar machine - found no change

Tooling and cutting parameters are same between this and other similar machines

Now we have decided to check "Distance to Go" along Z axis and only then allow feed to start, to see if "Distance to go" itself is appearing more than required, which is unlikely in a semi closed loop CNC system

Request your advice

Sep 6Edited: Sep 6

It sounds like you've covered pretty much everything I'd suggest. Again, is this a new process or one that has been working in the past but now is not?

 

Assuming it has been working previously, again, something has obviously changed. If the machine has not been crashed, I'd suspect cutting tool or workholding devices. Also, you say the depth of the bore varies, but what machines the surface into which the bore is machined? Could it be varying? 1.0-mm is a big variance. Are parts coming off the process varying with this from part to part - or do several parts come out okay and then a bad one shows up?

 

I know you said the cutting tool has been clamped properly, but if it is a straight shank tool held in a collet, I'd check the cutting tool after a variance to see if its length has changed (though you'd think depths would come out shallower - not deeper).

 

Thanks for your response. Sorry for not having informed you that it is a pretty old machine as well as process. In fact most of the machines as well as processes are pretty old and tailor made for such operations. This machine was not having proper levelling since inception ie for about 15 years and as a result machine itself used to drift a little on the floor and as a result relationship between Gantry and fixture used to get disturbed but out of 10 other stations, process parameters used to be the best out of this station. We have done proper levelling of the machine but this again, in our opinion, can not have this sort of issues Mike

There is no crash on the machine

What machines the surface into which the bore is machined? - basically we get semi finished components from supplier and we do finish operations. Again exactly similar semi finished components being finished in other similar machines as well hence we can not doubt the precondition here.

I think out of about 20 jobs, 6 to 7 had variance upto 1 mm

Regards

Sep 11Edited: Sep 11

I'm pretty good helping with application issues but not so much with service and maintenance issues. You've pretty much eliminated the issues I've come up with from a usage perspective.

 

Based upon the machine's age and seemingly poor preventive maintenance practices over the years, I think the issue could be more of a machine problem, in which case I'd suggest that you get a service technician from the machine builder to help.

 

The problem could, for instance, be a malfunctioning encoder for the drilling/depth-controlling axis (the Z axis, I assume). In this case, I believe that the machine would display that everything is moving properly on the display screen (like distance to go and final absolute positions), but the axis would not physically move to the correct location. You would need a service technician to determine if this is the case, and to correct the issue if it is.

 

Thanks Mike your responses do help me a lot. Yes we are in touch with OEM as well, as u rightly mentioned encoder might well be reason, we would replace and revert. R u on Linked in, if yes please share your coordinates as well as contact details, if u r okay to share as I want to connect with u

Hello Mike, although I know u support immensely on CNC application related issues, was wondering if u can guide on Application related issues of Honing machines as well, please comment

I'm sorry, but I don't have any experience with honing - if they happen to be CNC honing machines, maybe I could help, but my experience is more with metal-cutting machines.

Thank you. We have a PLC based Honing Machine. Slide movement to bring down honing cutter close to component, movement of honing tool inside bore and expansion of tool for finish honing, all these three functions are hydraulic, position feedback thro Servo. Bore length is 200mm, sometimes, we find black marks/rings ( 2 or 3) on top of bore but this mark not effecting surface finish Ra, flow of cutting oil adequate. Unable to understand reason for black mark. After replacing tool, no mark found but we are unable to conclude reason for such mark

Sorry, but I cannot think of a potential cause for the rings.

Thanks for your response, just in case if u know about a tech forum on Honing?

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  • 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 Regards
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