# Still a mystery, why it undercuts



## flyforever (Jan 4, 2013)

If I cut a rectangular shape with a .250 carbide bit, and then add some pockets and holes, the numbers are not consistent, and I cannot necessarily attribute the error on backlash.

First, when I check for backlash using a dial indicator, the numbers are right on.

After the cutting, the large rectangular shape is right on, but the holes (.525) and the pockets( .750 x2") are always between .020( the pocket) and .040(the hole) off.

If the one of the axis were off, the hole would not be round, that's why I ruling out backlash. It appears to happen when the movements are rapid and small, but I cannot pinpoint the possible cause. 

I am using Mastercam in computer tool offset mode. For cutting the holes, I am using a contour toolpath. The cuts look uniform, except for the error. 

I am cutting only at 25ipm, so this should not be an issue. I am cutting .375 mdf in three passes to make sure that there's plenty of torque. 

The circles are perfectly round, which proves that backlash is not an issue.
What's the mystery? Why can't I find out what's wrong?

Can the router bushing be the cause? How does one check for that? By hand, it feels tight.

tony


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## Chris Curl (Feb 13, 2012)

might it be a result of play in the movement/positioning system?


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## fixtureman (Jul 5, 2012)

Next time when cutting the circles cut them in the opposite dirction. and see if that will help


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## Quillman (Aug 16, 2010)

"Can the router bushing be the cause? How does one check for that? By hand, it feels tight."
**********************************************
Classical phenomena with skinny anything supported only on one end.
Entry is almost always larger than the cutter.
Once the drill or cutter gets >1 or 2 diameters into the stock, it is effectively now supported on both ends. 
As such it will hold a diameter as it cuts after that.
And the main reason I rarely cut but one diameter in blind excavations.
Try this: say, create a 1/2" x 2" cut with a 1/4" cutter. Start in the middle of the cut, let the cutter scream. But program the rest of the ride towards the dimensions taking maybe .010 or so as the cutter hits the last bit of stock at the walls. Routs x hand.


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## bobj3 (Jan 17, 2006)

????? say what ???

=




Quillman said:


> "Can the router bushing be the cause? How does one check for that? By hand, it feels tight."
> **********************************************
> Classical phenomena with skinny anything supported only on one end.
> Entry is almost always larger than the cutter.
> ...


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## Dmeadows (Jun 28, 2011)

Chris Curl said:


> might it be a result of play in the movement/positioning system?


Think that's called backlash!


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## rwl7532 (Sep 8, 2011)

Quillman said:


> "Can the router bushing be the cause? How does one check for that? By hand, it feels tight."
> **********************************************
> Classical phenomena with skinny anything supported only on one end.
> Entry is almost always larger than the cutter.
> ...


I understood all of that. Which means I'm making progress.:yes4:


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## flyforever (Jan 4, 2013)

Well, I finally solved the mystery. Apparantly, Mastercam has a "remachining tolerance", which is automatically set and , so far, I cannot figure out how to disable it unless I add a finish operation.
You can change the tolerance to a small number, but you cannot bring it to zero.
This tolerance, as it had been set, corresponds exactly to my undercut pockets.
Mystery solved. Thanks for all the suggestions.
tony


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## fixtureman (Jul 5, 2012)

You could trick it and put in a larger size bit. Say your bit is .25 tell it it is .26


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## laurent_parti (May 15, 2013)

*loosing steps maybe*

It can comes from loosing steps on motor.

1-Try to reduce speed and check the result.

2-Make 10 run and back on 1 axis and check the position before and after moves.

Hope it can help


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## flyforever (Jan 4, 2013)

I decided to simply turn on the machine and , with a dial indicator, see how much the dial moved when I pushed or pulled on the various axises.
The needle moves a good .030" on both axis. 
While the backlash with the steppers doing a measured travel back and forth is zero, I am finding that these same steppers or the entire machine flexes.
I am not pushing hard when I do the tests, but can this be the cause of undercutting?
This needle movement occurs when I place the dial indicator on the table and then push on either axis. If I look closely at the slide bearings riding on the shaft I can see the movement.
Can this be that the steppers are not holding well? Is it normal to see this kind of movement when doing the test as I just described?
All I know is that this mystery is still hounding me, and my lack of experience keeps me chasing something I am not sure of.
tony


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## laurent_parti (May 15, 2013)

Fly,

which kind of linear guide do you use, is there some possible set-up ?

On mine, there is a screw to adjust more or less the play between the carrier and the linear guide.

Maybe you can check it with your indicator ?


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