Accuracy of interpolation vs straight line

jasonrodman · Dec 17, 2019

Hi all -

I need to machine two slots that are 90 degrees to each other. For functional purposes it's required they are within +/- 0.1 degree of each other (89.9 - 90.1). We are consistently falling within +/- 0.2 degrees and are working on how to gain the extra needed precision.

The current part setup has the slots being machined with an interpolated path (x,y moving simultaneously)- would we gain any tolerance in the angle by setting up the part for straight line machining of the two slots? See attached image for reference.

One caveat is the part is a more natural fit for doing the slot with interpolation (stock material shape, setup, other machined features). Also, the part depicted in the image is not the actual part.

p.s. In the pic please disregard the X,Y axis arrows on Option B.

Thanks!
Jason

TeachMePlease · Dec 17, 2019

That depends heavily upon factors including, but not limited to:

What machine are you doing this on?
What condition is the machine in?
What is your fixturing like?
What is your tool?
What is your gage length?
What are your feeds and speeds?
How are you measuring?

Etc...

dandrummerman21 · Dec 17, 2019

In addition to TeachMePlease's questions:

how are you cutting it? Are you slotting it with the same size tool or using a smaller tool and running around the inside like a racetrack?

Assuming racetrack, are you doing a skim pass?

Option b would likely always be more precise, ASSUMING the machine is in good geometric shape (meaning x and y axis are truly perpendicular)

If the slots were 1" long, .2 degrees is about .0035" along that length of the slot. Which seems a lot to me.

If you can, post the code for your slots so we can audit it.

13engines · Dec 17, 2019

If you're consistently creating an error of a known amount and angle, there's no reason you couldn't simply program it out and be down with it.

On another note, when is the last time you had a ball bar test done or did squareness and perpendicularity tests/measurements on your machine? Or leveled it, which can also help. A machine out of square will cut like this, even in example B.

Dave

Finegrain · Dec 17, 2019

Where in WA are you? If not far away, I could stop by and take a look at your process and see if I notice anything weird.

Regards.

Mike

Delw · Dec 17, 2019

jasonrodman said:
Hi all -

I need to machine two slots that are 90 degrees to each other. For functional purposes it's required they are within +/- 0.1 degree of each other (89.9 - 90.1). We are consistently falling within +/- 0.2 degrees and are working on how to gain the extra needed precision.

The current part setup has the slots being machined with an interpolated path (x,y moving simultaneously)- would we gain any tolerance in the angle by setting up the part for straight line machining of the two slots? See attached image for reference.

One caveat is the part is a more natural fit for doing the slot with interpolation (stock material shape, setup, other machined features). Also, the part depicted in the image is not the actual part.

p.s. In the pic please disregard the X,Y axis arrows on Option B.

Thanks!
Jason

you had this same problem in 2018 what did you do to get it fixed before? Same Machine? vf4ss
Holding +/- 0.1 angle between features

gbent · Dec 17, 2019

Everybody else is working on the tooling side. I'll make a couple comments on the mechanical side. How well do your following errors compare between the two axes? How loose or tight are the gibs, and is there any rock in the table? A well used machine with lose gibs and rounded positive ways will do this. And it will get worse the more off center the load.

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