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Parts are not correct size!
« on: April 05, 2009, 02:39:11 AM »
Hello All,

I've got a slight issue at the moment...

I'll use one of my parts as an example but I get the same results from all toolpaths.

Basically everything is machining beautifully with the exception that nothing is 'spot on' for size ... sounds like a bit of a contradiction I know - but what I mean is that the same part can be made over and over again and each time the job completes without issue .. no missed steps, no chattering, beautifull straight cuts with a mirror like finish, internal circular pockets are also mirror like and shaped correctly (and a I said above this is the same for multiple toolpaths).

I can make the same part 3 times over out of one piece of stock ... my toolpaths use offsets - so I just re-home and apply new offsets then pick up the sub file and begin the next part.

PC is a dedicated dual core AMD, 2gb RAM, etc etc - it is a dedicated PC with NOTHING except XP and CAM software on it.

The issue is that all outer edges (ie profiles) are approx 0.3mm too big and circular pockets are approx 0.3mm too small - so a part that has an external dimension of 70mm comes off the machine as 70.3mm ... the 26.25mm dia pocket comes out as 25.95mm.

Toolpaths are all fine - I've checked the positional data and allowing for tool diameter the toolpaths are spot on. I should mention that the same toolpaths run under TurboCNC3 cut the part exactly as it should be (ie correct sizing).

Tools are high quality TiCN and Zirconium bits (approx $50 & $70 each!!)

I have Mach3 set to Exact Stop mode. There are no tool offsets in the system at all - I never use them. Every toolpath has the same header info in it:

G61           (Set exact stop mode)
G49           (Cancel tool offsets)
G17           (Set X-Y plane)
G40           (Cancel cutter radius compensation)
G21           (Set units to metric)
G80           (Cancel motion and canned cycle mode)
G90           (Absolute mode)
G52 X0 Y0     (Remove any offsets applied to X & Y)

Backlash compensation is used but the system has very little backlash anyway (.04mm on Y and .03mm on X ... so lets say 1 thou" on each). Having backlash compensation on or off makes no difference to this issue anyway.

With digital calipers fixed to the table and head the measured movement is spot on ... tell it to move 100mm and it moves to 100.00 on the calipers - so motor setup/ratios are correct.

Have tried various cutting speeds, acceleration, top speed (rapids) etc - again no effect on the outcome.

I've had the CRO's on both step and direction outputs for each axis card - again they are all good.

It seems to me that Mach3 is applying a 0.15mm cutter compenstion on every toolpath or is somehow rounding off all movements.

I don't know if I'm missing something - or if there's a bug in 3.042.020.

Sorry if the answer to this is elsewhere on the forum ... searching for it came up with either no or unrelated results (everybody terms things differently).

Can anyone shed any light ?? ???


Thanks


Marc
PROMiCA - CNCKits
Melbourne Australia

Offline Graham Waterworth

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Re: Parts are not correct size!
« Reply #1 on: April 05, 2009, 04:36:55 AM »
Hi,

is the error proportional to the size of the part, i.e. is it getting larger on a bigger part?

Have you checked to see if the sides of the part are square not tapered.

Are the scaling DRO's set to 1

Have you a stray G41/42 in your code and a offset in the tool table.

If you milled a hexagon or octagon would all sides be to big?

Graham
Without engineers the world stops
Re: Parts are not correct size!
« Reply #2 on: April 05, 2009, 05:52:43 AM »
Thanks for the reply!

The error does not appear to be proportional at all .. as mentioned an 'outer' profile that should be 70mm is actually 70.24mm (rounded off in earlier post for simplicity) ... on the same job a circluar pocket that should have been 26.25mm is actually 26.00. On a completely different job a circular pocket that should be 22.0mm is 21.73, and outer edge that should be 47.0 is 47.18mm.

Outer edges are always slightly oversize and inner are always slightly undersize.

Sides are square and parallel. The machine is also trammed correclty.

There are no G41/42's in the toolpaths at all - but as listed in my first post there is a G40 in each header.

A tool offset was the first htis I suspected - but as I've never used them I was pretty confident that there was nothing set in the table - and there isn't (scrolled right through the table to double check). I do use work offsets on pretty much the bulk of my jobs but that should have no bearing on this issue.

I haven't tried a hexagon or octagon but going by all the parts I have made I would virtually guarantee that they will be equally to big on all sides - or if done as a pocket they would be all too small.

I should also mention too that I use 'climb' milling on all toolpaths ... so outer profiles are machined clockwise around the part and all pockets are machine anti-clockwise. Again that is what makes the issue look like a tool offset - but all are 0. These parts are also cut with a 10mm bit ... even on a crappy Chinese bit there's not going to be THAT much flex! - but the tools I use are very high quality - they aren't going to be the cause of this issue...

Scaling on XY & Z are all 1.0000.


I've just rechecked all axis with the digitial caliper attached and all of them move the correct amount using MDI commands - all are spot on ....from X0 a G0X50 will move to read 50.00 on the calipers - resollution is only to two decimal places however that is fine (basically within 1/4 thou).

I'm hoping it will be one of those 'slap myself on the forehead' events .... but so far I think I've checked pretty much all I can as far as the physical hardware and electronics go.

Regards

Marc

(PS - might try a hexagon tomorrow if I get the chance ... would be interesting to see what comes out!!)


PROMiCA - CNCKits
Melbourne Australia

BClemens

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Re: Parts are not correct size!
« Reply #3 on: April 05, 2009, 07:03:14 AM »
Just a thought.... but have you certified your tooling for size? For close work we usually run the tool through a piece of similar material, then measure the slot. Maybe the offsets are not dead-on?

Again, just a thought,

Bill C.
Re: Parts are not correct size!
« Reply #4 on: April 05, 2009, 08:36:10 AM »
Thanks for the idea ..but yes I have checked the tool bits ... all the jobs I've run with 2 different bits and both are as close to 10.00mm as I can measure - certainly they aren't anything like 0.1mm (0.004") out that would be needed to make this magnitude of error.

All the toolpaths have a roughing sequence that cuts to 0.5mm and then do a light finishing cut.

The work offsets that I use would simply perform the job in the wrong position if they weren't right ...not make the part an incorrect size.

I have also checked the tool offsets yet again - there are absolutely no offsets entered into Mach3 for tooling.

Marc
PROMiCA - CNCKits
Melbourne Australia
Re: Parts are not correct size!
« Reply #5 on: April 05, 2009, 12:46:53 PM »
I don't think you said what machine you're running this on.  It could well be flex in the machine itself.  If you repeat the finish cut several times, does the dimension improve?

Regards,
Ray L.
Regards,
Ray L.
Re: Parts are not correct size!
« Reply #6 on: April 05, 2009, 06:39:10 PM »
This is running on one of my X3 kits (see http://www.cnckits.com.au).

It's not machine flex ... the X3 is definately sturdy enough plus as mentioned in the first post the same toolpath will cut correctly if run under TurboCNC3.


Marc
PROMiCA - CNCKits
Melbourne Australia

vmax549

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Re: Parts are not correct size!
« Reply #7 on: April 05, 2009, 09:57:53 PM »
HOw was the machine configured Imperial(") or metric.   

I just finished some bearing pockets here and a 1.374" hole came out at 1.374" 

(;-) TP
« Last Edit: April 05, 2009, 10:00:12 PM by vmax549 »
Re: Parts are not correct size!
« Reply #8 on: April 06, 2009, 02:15:06 AM »
Hi

Machine is configured metric and all part/toolpaths are metric.
PROMiCA - CNCKits
Melbourne Australia

Offline Graham Waterworth

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Re: Parts are not correct size!
« Reply #9 on: April 06, 2009, 09:47:54 AM »
can you post the code that is giving the size error?

Graham
Without engineers the world stops