Having Trouble cutting with machine cutting correctly

[ztimbo]

Hey Guys,

I'm trying to machine some small rectangular shaped pieces. They are a bronze alloy and are 1.370"L x .583"W x .500"H with a 9/64 radius. I'm using a Hyundai Kia VX-500 and a .500" 3 Flute Kennametal Endmill. My finish pass removes .030 at the full depth of .500" with 2 spring passes. I'm having trouble with the fitment of the radii of the parts. It's cutting the 1.370 and .583 to size correctly. If this makes sense, across diagonally of the part isn't the same. I'm measuring 1.407 across one side and 1.411 across the other side.

I've also had this issue with cutting an octagon. In gibbs cam everything is symmetrical and the machine is cutting a non symmetrical part that varies anywhere from .003 to .010.

Does anyone know what this could be caused by?

[Delw]

Have you checked backlash?

[ztimbo]

Have you checked backlash?

Nope..Haven't thought about it with this machine only be 2 years old and very few hours. How can I check it?

[RICHARD ZASTROW]

Back in the old days we called that axis reversal error.LOL

Dick Z

[txcncman]

Nope..Haven't thought about it with this machine only be 2 years old and very few hours. How can I check it?

Do you know what backlash is?

[txcncman]

Maybe this will help:

What is backlash? Detail of CNC machine tool backlash in ballscrews, bearings, gibs, and servo backlash.

Or:

Milling: Operations - Backlash

[ztimbo]

Do you know what backlash is?

yes, i know what backlash is. I didn't think it was a problem with CNC's nowadays.

[txcncman]

yes, i know what backlash is. I didn't think it was a problem with CNC's nowadays.

Ok. Yes, backlash is always a problem in mechanical movement. If you know what backlash is, then you should know how to check it. You mount a dial indicator on the spindle so that it touches a solid point on the table. Then you move the axis over to load the indicator and move to zero. Then move the axis back the opposite direction at the smallest resolution and count the detents to find the amount of backlash. Then you can go into the parameters and adjust the backlash for that axis. Then repeat for the other axis.

[ztimbo]

Ok. Yes, backlash is always a problem in mechanical movement. If you know what backlash is, then you should know how to check it. You mount a dial indicator on the spindle so that it touches a solid point on the table. Then you move the axis over to load the indicator and move to zero. Then move the axis back the opposite direction at the smallest resolution and count the detents to find the amount of backlash. Then you can go into the parameters and adjust the backlash for that axis. Then repeat for the other axis.

hmmm, sounds simple enough...I've only been using a cnc for the last 2 years so im kinda new to all this. All ive ever done has been conventional mill and lathe stuff. I was just under the impression that backlash didn't occur with cnc's due to the ball screws.

I also understand that backlash occurs in all mechanical movement if you get right down to it, but I'm talking about a substantial amount here, around .010". I just didn't think cnc's got that kinda of backlash

[Delw]

hmmm, sounds simple enough...I've only been using a cnc for the last 2 years so im kinda new to all this. All ive ever done has been conventional mill and lathe stuff. I was just under the impression that backlash didn't occur with cnc's due to the ball screws.

I also understand that backlash occurs in all mechanical movement if you get right down to it, but I'm talking about a substantial amount here, around .010". I just didn't think cnc's got that kinda of backlash

acording to you first post its only .004 which is .002 perside thats a far cry from .010
how are you checking the part calipers? Mic? toss that calipers in the garbage if you are and use a more accurate way of checking the part.
also look for taper on the flats, burrs etc.

[ztimbo]

acording to you first post its only .004 which is .002 perside thats a far cry from .010
how are you checking the part calipers? Mic? toss that calipers in the garbage if you are and use a more accurate way of checking the part.
also look for taper on the flats, burrs etc.

I will not toss my calipers in the garbage. I can measure within .001 all day everyday with them.

I'm using a mic to check this tho, and please re-read my first post and you will see where I was talking about cutting the octagon.

[Delw]

can you show a picture of the part?
BTW anyone can measure .001 with calipers, all day but its not correct trust me.
I saw the part about an octagon does it measure the same across each flat on the same part?
if not more than likely backlash or drives need to be tuned