Unwanted overcutting advice please

[grim_d]

Hello everyone! I'm new around these parts and almost a year ago bought a 1300x1300 machine via one of the major machine importers. As the end of my warranty looms I have overall been very disappointed with the machine and the service provided from the seller and I am considering rejecting the machine.

I wont go too far into it but when the machine arrived it had many defects which I have been trying to have rectified over the course of the year to no avail.

However one of my major issues is that the Y axis consistently overcuts on internal corners, particularly when doing pockets and I would like to establish if its a problem with the machine or an operator issue. I have attached a photo of the issue.

When I first mentioned this to one of their visiting engineers I was told it was due to backlash and we spent some time tightening everything up, which included taking a die grinder to one of the mounting holes for the drive pulley so it actually aligned with the hole...high quality! I also discovered the linear bearing blocks on the axis had been left loose from the factory, all of this did improve things but my issues returned in short order and no amount of tightening and adjusting appears to solve it, I managed to get the free play on the axis to around 0.15mm in each direction, which largely appears to be flex in the machine.

I've also tried playing around with speeds and feeds in case it was a tool deflection issue but I was unsuccessful

Id be grateful if someone could identify the reason behind my issue so I could investigate further, sadly the supplier is largely uninterested in my issues.

Thank you in advance!

Graham.

[Lee Roberts]

Hi Graham, C'mon then who is it, who is major machine importer, what brand of machine is it, what machine is it?

Hard to really give advice with out seeing the machine, can you post lots of pics for us to look at, try and cover all aspects of the machine, including the control/electronics cabinet.

If free play is 0.15mm and because of flex like you suggest that may not be that bad if its some rubbish made out of Meccano and elastic bands...could it be your trying to push the machine to hard ? The corners look consistently wrong so that's a plus I guess.

Also, what CAD/CAM software are you using, how are you drawing these corners, are they a known radius that is large enough for the cutter to move in to and do its job?

Like I say without more info we cant really help you much...

[grim_d]

Hi Lee,

It's a Spartan 1313 "from" Mantech, ive attached a generic image as I'm not with the machine right now.

I don't push the machine hard, cutters and material are expensive and I'm not trying to get cycle times as short as possible, certainly not in the case of my example photo as far as I know. It was only a 2mm cutter so I was taking it easy, I'll need to check what my feeds and speeds were.

CAD/CAM is Vectric V-carve 11.5.

The corners are sharp on the vectors but V-carve (should) automatically adjust the toolpath based on the cutter diameter to cut a radius in a corner the same as the cutter radius. (Though I'm sure you know that!).

I could do with running some more tests to see if I can establish if it's the machine or the operator but I'm so busy and time is running out I was hoping it might have been a common issue with a relatively straightforward cause. (Is it ever!)

[Lee Roberts]

The corners are sharp on the vectors but V-carve (should) automatically adjust the toolpath based on the cutter diameter to cut a radius in a corner the same as the cutter radius. (Though I'm sure you know that!).

Ok, try the same settings again but this time actually make the internal corners radiused (filleted) for the 2mm cutter, a 1mm radius should be great. I would then say its a hardware issue if the problem persists.

[m_c]

You really don't want to be machining radiuses that match the cutter radius.
Even on extremely rigid machines, it's bad practise, as you're likely to run into chip clearance and resonance issues.

As for the machine, anything with rack and pinion, is never going to be the most precise of machines.

[Lee Roberts]

Not sure if that's what he really means/understands m_c, presumably Vectric would at least shoot for half the OD on the cutter maximum when generating the tool path? or do you know something we don't about the software?

[grim_d]

Ok, try the same settings again but this time actually make the internal corners radiused (filleted) for the 2mm cutter, a 1mm radius should be great. I would then say its a hardware issue if the problem persists.

Ok I'll give this a try. To be completely honest I think it may have been tool deflection in my example as I cut something yesterday with a shorter cutter in a smaller diameter which worked out absolutely fine.

I'm used to using bigger cutters on metal so deflection isn't quite as much of an issues, carbide has far more flex in it than I expected.

I do plan on doing some more experiments but getting the time is extremely difficult.

You really don't want to be machining radiuses that match the cutter radius.
Even on extremely rigid machines, it's bad practise, as you're likely to run into chip clearance and resonance issues.

As for the machine, anything with rack and pinion, is never going to be the most precise of machines.

Noted regarding the radiuses, thank you for the information. Shouldn't V-carve handle that though?

Regarding the helical rack and pinion, what's the other option for larger machines? (Not necessarily mine), as I understand it ballscrews run into vibration and whipping issues at longer lengths?

Just curious for if I get a bigger machine in future.

I understand it's not a high end machine, but similarly it's not a wet noodle from AliExpress, it certainly seems like it should be capable of holding the tolerances I desire....operator error aside.

The next league up of machine is well into the tens of thousands, unjustifiable currently.

[m_c]

Ok I'll give this a try. To be completely honest I think it may have been tool deflection in my example as I cut something yesterday with a shorter cutter in a smaller diameter which worked out absolutely fine.

I'm used to using bigger cutters on metal so deflection isn't quite as much of an issues, carbide has far more flex in it than I expected.

I do plan on doing some more experiments but getting the time is extremely difficult.



Noted regarding the radiuses, thank you for the information. Shouldn't V-carve handle that though?

I've never personally used V-carve so can't comment, but I'd normally design the parts with suitable radiuses or select the cutter to avoid radii matching.
I can't remember the last time I had to machine an internal sharp corner, so I can't even think how my usual choice of F360 handles it, but I'm sure there will be a setting somewhere.

Regarding the helical rack and pinion, what's the other option for larger machines? (Not necessarily mine), as I understand it ballscrews run into vibration and whipping issues at longer lengths?

Just curious for if I get a bigger machine in future.

I understand it's not a high end machine, but similarly it's not a wet noodle from AliExpress, it certainly seems like it should be capable of holding the tolerances I desire....operator error aside.

The next league up of machine is well into the tens of thousands, unjustifiable currently.

R&P can work if designed well, and you're not expecting micron tolerances.
Other options are rotating ball nuts, supported ball screws, high pitch ball screws.
There's not really an ideal solution, but as with most things, it can be done if you select the correct components.

But I hate to tell you, you can probably buy your machine on Aliexpress, if not, Alibaba.
I looked at a 4x4 machine, and there are some compelling options, but they're all built to a price.

It's a shame Jazz isn't about, as he's the one who really knows about Routers (I'm more metal working, than wood bothering!)

[grim_d]

R&P can work if designed well, and you're not expecting micron tolerances.
Other options are rotating ball nuts, supported ball screws, high pitch ball screws.
There's not really an ideal solution, but as with most things, it can be done if you select the correct components.

But I hate to tell you, you can probably buy your machine on Aliexpress, if not, Alibaba.
I looked at a 4x4 machine, and there are some compelling options, but they're all built to a price.

It's a shame Jazz isn't about, as he's the one who really knows about Routers (I'm more metal working, than wood bothering!)

I'd hope nobody with a router is expecting micron tolerances!

A machine of this type can definitely be bought on there, hell, you can buy them direct from the manufacturer that makes them for mantech. I'm not naive enough to think this is anything other than a green painted generic Chinese machine, it's the UK sales and "support" you are paying the extra for...not that mantech have been any good.

I too am a metal worker though I'm still on manual machines, no CNC metalworking here but should be adding a surface grinder to the fleet soon!