Job undersized

Hi, folks.
I'm using a Zen 7x7, Mach 3 & VcarvePro. The finished job is undersized.
I've-
1) Calibrated the router carefully, it's as good as my vernier can measure.
2) Entered the actual tool cutting diameter/width in Vcarve, and ensured it's in Mach3, too (3.175mm endmill)
3) Looked at the produced G-code, & it's OK.
4) Ensured the tool positions are the same, in Vcarve & Mach3

I have tried a work-around by fudging the tool dia, but that's not my preferred solution, I was wondering if there was anything I was missing? Apart from a life...:livid:
Any ideas welcomed...

TIA

How much is it undersize (and is it the same on both axes), and can you also say what DOC and feeds you are using?

Hi, Neale. I have just cut some components, and can get some numbers. Whilst cutting an outside profile (around 43mm sq), it's under by 0.3 in X, and 0.5 in Y. An inside profile (hole) of 4mm got me an actual of 3.7mm. Bugger.Feed is around 200mm/min, which with my spindle at 3300rpm seems fine through 3mm ply, cutting 1mm at a time. DOC?
I can live with it, but that's not the way to do it, I like everything to be spot on...I think I must be a bit anal...Pass the anorak :glee:
Cheers,

Quote:

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Originally Posted by Skysurfer

Hi, folks.
I'm using a Zen 7x7, Mach 3 & VcarvePro. The finished job is undersized.
I've-
1) Calibrated the router carefully, it's as good as my vernier can measure.
2) Entered the actual tool cutting diameter/width in Vcarve, and ensured it's in Mach3, too (3.175mm endmill)
3) Looked at the produced G-code, & it's OK.
4) Ensured the tool positions are the same, in Vcarve & Mach3

I have tried a work-around by fudging the tool dia, but that's not my preferred solution, I was wondering if there was anything I was missing? Apart from a life...:livid:
Any ideas welcomed...

TIA

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Hi It would help if you could submit info regarding the setup of the machine ie Ball screws or lead screws etc A pic could help as well. It might be backlash or missing steps, is the error consistent etc.
When you command the machine to go say 100mm in X does it go 100m same in Y. ..Clive

I had (still have) a similar problem with broadly similar numbers. However, in my case it was always undercutting - external sizes were about 0.3mm oversize and holes about 0.3mm undersize. After a bit of discussion on the forum, I concluded that it was machine weakness - bend, bearing slop, backlash, or whatever - and for the parts I was cutting I found that defining the 3mm cutter to be 2.3mm instead and generating gcode accordingly gave nicely fitting parts. Over the weekend I was cutting some birch ply instead of the 3mm MDF I had been working with, using a 6mm cutter instead of the 3mm cutter I had used, but the error was about the same. Cutter speed around 12K, 500mm/min speed, 2-3mm DOC (=depth of cut). Missing steps would probably show up quite clearly with the size of error as the cuts would not line up at the end of cutting out a component.
Given that you are seeing everything coming out slightly undersize (both holes and outside profiles) it does sound like a calibration problem.
I know that it's not a fix and a really crude workaround, but there's a useful box in the toolpath generation window in VCarve labelled "allowance". That lets you specify an amount to overcut/undercut a given part (although really it's intended for something like cutting male and female parts to the same nominal dimension but allowing you to create a bit of clearance so that they fit nicely). Saves faking the cutter diameter, which was my first workaround!

Well, thanks to you both for your input. The machine seems to be solid enough, as it's so tiny it doesn't take so much to keep it fairly solid. It's using M8 threaded bar as leadscrews, but the anti-backlash device Zen had added seems to be working well as far as I can tell. I was just making sure that it wasn't my noobie knowledge of the software, primarily. I'll keep battering away, and keep an eye on any mechanical problems. I've decided that using Neale's workaround of Vectric's allowance box (0.2 for outside & -0.1 for inside seems to do the trick).
Thanks to all, I'll be posting if I get any new info.
Of course, all my mates now have personalised 3mm ply keyfobs...:friendly_wink:
Cheers, all.