Kerf Explanation

[platonas1]

Hello all,
I have a yoda waterjet (abrasive) machine.
Just need an explanations for kerf in waterjet. I use pronest and in settings I give
1,2 mm kerf. And when I cut square part I measure the exact dimensions that I want. All good.

But what settings or procedure I must take to cut several parts into other parts to make one part without gaps and fit perfectly together.
I must cut a marble part like picture but I have problems with kerf.
Please give me some detailed info about this.

Thank you!

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[underthetire]

Kerf is basically the diameter of the water "stream" or jet. Cut a single slot and measure the width, that is your kerf.

Sent from my A3-A20FHD using Tapatalk

[KilrB]

Kerf is the width of the slot cut by the jet.

What you really want to know about is "offset" which determines which side of the tool-path the jet cuts on, the left or the right, and how far off the tool path it is "offset".

Picture the jet cutting away from you and offset to the left or right of the tool-path become clear.

This will also depend on which direction you are cutting, clockwise (cw) or counter-clockwise (ccw).

This is critical when cutting mating parts as you are.

So:

If you are cutting a "hole" counter-clockwise (G03) you would want your jet offset to the left (G41) so that your kerf is inside the ID you want to cut.

If you cut the same hole with either offset to the right (G42), or clockwise (G02) with offset to the left (G41), your hole would be over-size by twice your kerf width because you jet is cutting on the wrong side of the tool-path in those instances.

If you are cutting the mating piece that is supposed to fit in that "hole" you would want to use either counter-clockwise (G03) with offset to the right (G42), or clockwise (G02) with offset to the left (G41).

When cutting mating pieces like this on a waterjet, if you do not have draft compensation, it also helps to cut every other piece upside down so that the angles match.

Just don't forget to mirror the ones you cut upside down ...

[platonas1]

Thank you for your reply,

I don't undestand left and right can you please explain? does the leads define this? for a hole if leads are in the circle this means is right and if the leads are outside of the hole left?

Thank you

[KilrB]

While your lead-in line generally determines which side of the profile/toolpath your kerf will be on, whether it is "to the left" or "to the right" is dependent on which direction you are cutting.

"Left" or "right" are as viewed with the tool (your jet) cutting away from you.

So:

If you are cutting a hole, and you are cutting cw, your kerf should be "to the right".

If you are cutting a hole, and you are cutting ccw, your kerf should be "to the left".

But:

If you are cutting a disc, and you are cutting cw, your kerf should be "to the left".

If you are cutting a disc, and you are cutting ccw, your kerf should be "to the right".

This is why on many of the newer water-jets and lasers that use .dxf files instead of g-code, the programmers/operators are taught to cut everything with the same offset and swap directions from ID to OD.

Whereas I was taught to always cut the ID and OD the same direction and swap the offset.

[platonas1]

Thank you for explain this, all is clear now.
Just final question please.
All the time I have to move abrasive nozzle before starting to cut, I am confused where is the correct start position on the part.
Using pronest to program I have starting point lower left (picture START.JPG) .
For example recent work I had to cut was the part in picture STOCK.JPG.
Initial stock was a square of 800 x 800 mm, I never accomplish
to have the dimension 570 mm correct.
What position is correct in picture? (position.jpg)
I’ve tried to cut without kerf and I can say is more closely to 570 mm .
I also tried to cut from lead 2 with kerf left and CCW.

Thanks once more

[KilrB]

(Sorry for the delay, Friday was very busy and I was not well Saturday.)

OK, I'm not familiar at all with Pronest but it looks very similar to the Flow software.

If that is the case either the blue or green start point should work.

If your part is closer to size with no kerf allowed I can only suspect that you are getting your kerf on the wrong side when using offset.

You should be able to select a lead-in/out length and extend both the beginning and end of your profile,

If cutting cw put your kerf "to the left".

If cutting ccw put your kerf "to the right".

Not sure about Pronest but Flow will then put the start where it deems necessary.

Now, let's talk about the dimensions on your profile.

That 570mm dimension to a razor sharp corner is a no-no.

That dimension should be calling out either the center-point of the radius or the height to the mid-point of the arc.

Because:

With either water-jet or wire-edm you are always going to loose more material on the narrow section than you think you are.

Depending on the tolerance for the location of your radius, it is doubtful you will be able to achieve the 570mm dimension.

Putting a twizzle on it will help but not eliminate this problem.

EDIT:

This is all assuming you are cutting the entire profile of the part ...

But if you already have the 800mm dimension on your blank why would you do that?

You should be able to cut only the radius.

Again you would choose a lead-in/out length and extend the beginning and end of the profile (arc) and cut only it.

You will come much closer to maintaining both your radius location and 570mm dimension with this method as you will not be cutting both sides of the narrow portion of the profile.

[platonas1]

Thank you for the detalied explanation,
Is there any deifference cutting cw with kerf left and cutting ccw also with kerf left?
In pronest you can choose all combinations of left right cw, ccw.

Thanks

[KilrB]

Thank you for the detalied explanation,
Is there any deifference cutting cw with kerf left and cutting ccw also with kerf left?
In pronest you can choose all combinations of left right cw, ccw.

Thanks

Absolutely, if you are cutting the same profile one combination will make your part to the correct dimensions and the other will make it under/over size by twice your kerf width.

Just remember to keep your kerf on the "scrap" side of your profile no matter which direction you are cutting and you'll be ok.

[plasmabeard]

Another thing to consider is taper. The faster you cut, the more taper (cut angle) you will have on your parts unless your machine has built-in taper compensation. This may make it harder to fit your mating pieces together.

[agroni01]

platonas1,

Can you please tell me where does Pronest software has this option to set the kerf side (left or right). I'm new at pronest i can't find this option

[woodworker2]

We use Pronest to run our high definition plasma machines at work. It is made by Hypertherm. They have excellent tech support. I would call them and tell them what is going on. You could have an incorrect setting in the machine setting files on the software. Go to their website https://www.hypertherm.com/en-US/glo...ing-systems%2f select your country and contact tech service.