When to zero off the table rather whan workpiece.

[Hoytbasses]

Greetings botters:

we're 6+ months into the use of our bots.We've cut a lot of projects alreay and I'm feeling pretty confident using the Bot.......We're getting ready to cut our first electric guitar blanks. These are 1.625-1.750 thick.

I loaded the first body blank (recycled bleachers) into the bot. Zero'ed off the top of the workpiece, double checked the job setup, zeroed the x,y and loaded the cut file. the cutter dove about .375 into the top of the bleachers body blank. as it's 'starting point'. I flipped it over, rechecked everything, and it did the same thing.

so I made up a .75 MDF body blank, reset the job dimensions to .75 and adjusted the depth of the pockets to be cut, zeroed off the top, it ran fine.

I loaded another full size body blank, set up the job dimensions accurately , but this time zeroed off the bed. The part cut perfectly.

I don't get it: I double and triple checked all the toolpaths, thickness of the materials etc when I zeroed off the top of the thicker body blanks and twice it dove into the part as it's starting point , but when I referenced the Z off the bed, it was fine.....

what gives, Oh experienced botters?

thanks as always for your patient help.

Karl Hoyt

[jhedlund58]

look at your part file.... that is where u make this decision.... in the material setup

[paul_z]

Do you have enough vertical travel for the 1+ inches plus the safe Z height? If not, you may have hit the top vertical stop, lost steps and then plunged into your piece.

Paul Z

[myxpykalix]

Paul hit it on the head there. try this...set you file and bit and Z up like normal using the top of the material like before.

Then take the bit out, remove your material, and run the file without turning on the router so you can hear what is happening when it is moving and i bet you will find that it is "topping out" because you have a safe Z set too high with the bit height and material height. But you can't hear it because your router is running.:eek:

[Hoytbasses]

you are probably correct: I was using a long bit (1/2" 4 flute endmill with 2" length of cut:) the workpiece was 1.604, it was sitting on a .75 spoilboard. The ruter was nearly at the top of it's travel just to clear the piece!

I'm going to run the same bleacher blank, only with a much shorter cutter and see what happens.

thanks for the feedback, all.

Karl Hoyt

[Hoytbasses]

we got through most of the electric guitar bodies....will finish them up this week. I zero'ed off the table for each one. Getting a little cocky I put another 1.5" 'dummy ' blank on the machine and zeroed off the top of the piece. It dive-bombed into the piece when I tried to run the part.

The Z travel on the desktop is 4.....so with a spoilboard it's only 3.25 ish. I think what you suggested about hitting the top of the travel is in fact the case as the tool tries to clear the piece.......

so......it seems that anything thicker than 1" on the desktop should be zeroed off the spoilboard , not the top of the piece. Never had a problem with zeroing off the material when it was .75 or so.

so , for this rookie's question of the week: What is YOUR rule of thumb for determining when to zero off the material vs off the bed?

You folks are so patient and helpful... Thanks so much!

Karl

[adrianm]

I zero off the bed probably 99% of the time.

I only ever zero to the material if I'm doing a batch run with material that varies slightly in thickness and it's a toolpath that could be affected by that such as VCarve or 3D.

I added another proximity switch to the top of my Z (PRS Alpha) so I don't have the issue of hitting the stop unexpectedly.

[Roy Harding]

I zero off the material surface when making depth critical cuts (mortises for inlays and so on).

For almost everything else I zero off the table.

[Bob Eustace]

Adrian - what does your machine do when you hit the Z proximity switch please?

[adrianm]

It doesn't actually prevent the machine from hitting the upper limit if you move it manually etc.

What I did was to modify the C2 routines so that after setting the Z zero it then moves upward until the the Z switch is triggered. This then sets the safe pullup and the top of the Z.

I've modified the Post Processors so that rather than using a fixed value for the Home Z (set in PartWorks/VCarve) they use the value set in the C2 routine.

So with all of that the machine always knows how far it can go up without triggering a problem.

I got the idea from posts by Dana. He did the same mod a long time ago.

[scottp55]

It doesn't actually prevent the machine from hitting the upper limit if you move it manually etc.

What I did was to modify the C2 routines so that after setting the Z zero it then moves upward until the the Z switch is triggered. This then sets the safe pullup and the top of the Z.

I've modified the Post Processors so that rather than using a fixed value for the Home Z (set in PartWorks/VCarve) they use the value set in the C2 routine.

So with all of that the machine always knows how far it can go up without triggering a problem.

I got the idea from posts by Dana. He did the same mod a long time ago.

Looking for Dana's C2 mod. Can't seem to find it. Working off spoilboard surface for the first time with 5/4 stock, so set VC to 1.5" safe height and then surfaced piece. then opened different file measured from top of material and promptly "Topped out" on upper prox switch after C2. I can easily fix by changing VC safe Z back to .2", but was wondering if Dana and Adrians C2 routine would eliminate me having to do this every time? Not looking to modify PP. I'm dangerous enough as it is:) Mainly using this surfacing routine to get uniform thickness on blanks without having to measure each piece of rough.