Questions on using a moving gantry style mini-mill...

[Davek0974]

Hmm, back to school, two issues tonight.

Offsets, seems i have not grasped it yet or I am in the wrong work sequence...

Yesterday I saved the work offsets for my production job - G55, I also added G55 to the start of the code. I then loaded up the material, changed the tool & set my top-of-material Z zero. Pressed cycle-start and got Soft-Limits warning on Z, looking at the numbers on the DRO's I could see something was wrong.

I deleted the the G55 from the code, switched to G54 Offset, reset my tool zero just in case, set work X&Y zero as normal, and ran the job as before.

So, not sure what i am doing wrong but clearly offsets do not work totally how I envisaged them to work. Do you have to be in the correct offset BEFORE setting the tool and Z zero?? If yes then that was likely mistake No1 as I was in default G54 when I set the tool. What I expected was for the offset to only affect X & Y not Z (i think)

Z Zero, again.

yesterday the job cut perfectly, onion skin just about held it together and a nice part came out. Tonight, same file, onion skin failed as the tool chewed the bed by about 0.1mm or a bit less - this meant the part got 90% of the way and failed on the finish pass, luckily the cutter survived but the part was toast and now resides in the scrap bin.

So, I have two issues here, top-of-material setting and work-holding.

Here is the part before it went flying to the bin in a red-mist


Now, after cooling down, I miked the metal up and it seems the thickness is not very reliable, the 7 bits i have left ranging from 5.05mm to 5.25mm depending where you measure it - I think this accounts for the failed onion-skinning and subsequent junk output.

My first resolution to this is to mark a spot with a sharpie, measure it, note it, CAM the part specifically for that thickness, set the tool zero exactly on the sharpie mark and try again - I do not think this is a machine error, more a material error.

Work-holding, these parts must be 100% flat and clean, if I tried adding some carpet tape etc under it, would I be able to get the part off the bed when cut?

[lucan07]

If this is a regular job I cannot see why you do not create a jig waste board to hold material in a recess with appropriate hold downs permenantly in place so 4 nuts or so to lock jig onto bed, material in recess and locking tabs clamps (cheap chinese 120kg hold downs or similar you could have a couple to swing in after finishing pass goes by if needed and cut clean threough each time) to hold material off you go, no more biting into the bed, cutting through not going to matter occasionally one out quick vacuum next one one in, seems to me a lot of wheels being reinvented.

[Davek0974]

If this is a regular job I cannot see why you do not create a jig waste board to hold material in a recess with appropriate hold downs permenantly in place so 4 nuts or so to lock jig onto bed, material in recess and locking tabs clamps (cheap chinese 120kg hold downs or similar you could have a couple to swing in after finishing pass goes by if needed and cut clean threough each time) to hold material off you go, no more biting into the bed, cutting through not going to matter occasionally one out quick vacuum next one one in, seems to me a lot of wheels being reinvented.

Ok, but if the part is finished all round that would mean diving in to fix clamps while the finish pass is running ? Or pressing feed-hold mid-cut? Neither sound good to me personally. I'm not deliberately trying to reinvent wheels, I just do what makes sense to me at the time i need to do it. The plate is fixed with the three bolts down the centre, these go through the scrap part, the outside clamps don't really do too much.

I just can't 'see' a way to clamp the parts as they are being cut.

As for spoil board then yes, if the bed was not already damaged I would fit one, this one sacrificed itself in the name of education;)

[CharlieRam]

I just can't 'see' a way to clamp the parts as they are being cut.

Use a M00 at a convenient point, then press cycle start to continue, spindle stop might also be good at this point :)

[Davek0974]

Use a M00 at a convenient point, then press cycle start to continue, spindle stop might also be good at this point :)

Yes but the finish cut is one continuous cut, you don't want an M00 in the middle of a continuous cut because you will not get another lead-out and lead-in so a tool mark is going to show, let alone stopping and starting the tool mid-cut will probably break it.

Fitting clamps will mean working in the short space of time while the cutter is up the other end of the job, these parts are small so time is short and this is therefore dangerous. The Z axis mount is wider than the job so all clamps have to be low-profile <=10mm high or it means using a longer stick-out on the tool which again is not good on a 2mm cutter.

These are difficult parts to make well but that does not mean they cannot be made ;)

[routercnc]

Hi Dave,

I tend to use the tab method but this does leave a small witness mark (although the parts are just for me so that's ok). For customer parts I can understand why you are trying not to.

So I think if I were to try the onion skin method I would skim the bed first then zero the end of the tool to the BED. Then MDI jog the Z axis up to 5.1mm (if the part is nominally 4.9-5.1 say) and then ZERO the DRO again.

In your CAM you would than ask for a depth of 5.0mm. This would leave exactly 0.1mm of onion skin. This method is independent of the variation of part thickness.

[Davek0974]

Thanks, that sounds a very similar method to Gerry's - using his "material height" DRO. I might have a go at skimming the bed and try that out.

[lucan07]

When I use the M00 I simply extend cut into scrap section past end of cut or put in a small loop and restart on a corner with change of direction and no signs of me doing so. think about it like cutting it on your manual mill overshoot on external long straightish edge to fascilitate later clamping, pause in scrap material on overshoot, add clamps to suit which could be pre mounted to swing over and clamp and carry on.

[CharlieRam]

Looking at your part, there are quite a few holes to screw into a spoil board so I would have the M00 after the holes were drilled to insert some screws to clamp the piece down, then just cycle start for the profile

[Ger21]

On my website, there are some modified macros that will zero the Z axis for all of the G54-G59 offsets at once, regardless of which one you are currently in.

As for material thickness, there's another option in the 2010 Screenset called "Material Offset". When checked, you zero the Z axis to the table, and Z zero is set at the material thickness value above the table. It's works basically the same as setting Z zero to the bottom of your material. If you are doing pockets, note that this will affect the pocket depth if the material thickness varies.