"Not" round holes

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DustyGeek
Posts: 6
Joined: Fri Aug 21, 2015 2:39 pm

Re: "Not" round holes

Postby DustyGeek » Sun Oct 04, 2015 10:51 am

Dumb question, but are you sure your spindle is square to the table both side-to-side and front-to-back? While it seems odd that it would be off enough from 90° to cause oblong holes like you're describing, it can cause that effect if it is.

A quick, cheap and simple way to do that is to find a piece of 1/8" rigid wire about 6" long. Bend about 1" at each end to 90°, one up and one down. Then mount it in a 1/8" collet. The wire itself doesn't have to be accurately bent or anything, but it does need to be rigid. Manually lower Z until it touches the table. Manually rotate the spindle 360°. If it touches the table exactly the same all the way around, you're square to the table. An example of this on a drill press is here: http://www.bladeforums.com/forums/showt ... ress-Table

A second thought that comes to mind is Z axis speed that's too fast. On my X90, I found through trial and error that if my drilling and plunge are faster than about 5-10in/min, I get an effect similar to what you're seeing - oblong holes. Any faster than that and there's enough backpressure from the plunge flex the spindle up slightly due to the fact the machine isn't extraordinarily rigid. It obviously gets more pronounced with end mills nearing the end of their usable life, and a machine that might need some bearings replaced due to wear, introducing more flex than desired.

KLWestfall
Posts: 93
Joined: Fri Aug 28, 2015 10:17 am
Location: Red Deer, Alberta

Re: "Not" round holes

Postby KLWestfall » Sun Oct 04, 2015 11:39 pm

Thanks, I'll check that again. I just replaced my spoil board and surfaced it with a 2" bit and it surfaced near perfectly. I would think if I was off (tramming) I wouldn't get a good surface.

Len suggest different plunge speeds and feed rates, which I did a bit, but I get the same result in any kind of wood - that doesn't seem to matter.

I will slow down the Z cut and see what happens.

Appreciate the time everyone has taken to help.

klw
Probotix Meteor - :D 8-)
Lots of Wood - 8-)

Funny, I don't remember being absentminded...

User avatar
HANNAH024
Posts: 289
Joined: Fri Aug 21, 2015 9:57 am

Re: "Not" round holes

Postby HANNAH024 » Wed Oct 07, 2015 11:17 pm

klw
The G64 P0.001 code tells the cnc system to use "continuous path control" with a tolerance of 0.001.
With this code in effect, the cut path will not vary from the programmed path more than 0.001. The system
will slow down the movement so it can maintain the specified tolerance.
If you look at the machine code that you are sending to the machine (the ngc file), you should find the G64 code followed by a P-value (tolerance value) in
one of the first few lines of code. It will be prior to any of the commands that start with the code G01.
You can use "notepad" to open the machine code file or you can look at the code listed near the bottom left of the display when the program is opened in LinuxCNC.
For most wood projects like signs, plaques and ornaments a tolerance of 0.025 gives good results. But when you are pocketing holes or cutting mateing parts, you need a tolerance more in the order of 0.001.

When I first got my Comet, I ran a rectangle cut at 100 ipm. The only square corner that I cut was the one where the cut started. All three of the other corners were rounded and they were rounded different amounts depending upon the speed that the cut was being made. The short sides had a smaller radius, but the corner following the long side of the rectangle was a large radius. I inserted the G64 P0.001 code into the ngc file and ran the program a second time and all of the corners were nice and square (very tiny radius), but it took more time for the program to run since it slowed down to go around the corners.
It is what it is!
Have a great day.
HANNAH

4DThinker
Posts: 254
Joined: Wed Oct 07, 2015 10:57 am
Contact:

Re: "Not" round holes

Postby 4DThinker » Thu Oct 08, 2015 11:51 am

There is some potential for flex in the gantry of the Nebula/Asteroid and to a smaller degree in the shorter gantry of the Meteor and Comet.
The MDF bed will also flex down if a bit plunges faster than it can remove the material below it.
The drill toolpath is a straight plunge.
If your endmill doesn't plunge well then it will put upward pressure on the gantry and downward force against the MDF bed. You'll get more flex toward the center of the gantry/center of the table.
If anything flexes a round bit will make an oblong hole.

Slow down the plunge rate and see if that improves your results.
Look for a different bit tip design, or even consider using a drill bit instead of a router bit.

4D

KLWestfall
Posts: 93
Joined: Fri Aug 28, 2015 10:17 am
Location: Red Deer, Alberta

Re: "Not" round holes

Postby KLWestfall » Thu Oct 08, 2015 11:41 pm

Thanks, I haven't had a chance to set up some more trials yet, but I do want to try the slower plunge and maybe a drill bit, since I'm doing .125 holes.
Probotix Meteor - :D 8-)
Lots of Wood - 8-)

Funny, I don't remember being absentminded...

KLWestfall
Posts: 93
Joined: Fri Aug 28, 2015 10:17 am
Location: Red Deer, Alberta

Re: "Not" round holes

Postby KLWestfall » Mon Dec 07, 2015 12:06 am

Well, it took a while, but I found the solution to my problem! Quite by accident, and almost ashamed to own up to it...

I was drilling some more holes in a crib board and just happened to notice that my dust shoe, which is made out of drafting brush bristles, were stiff enough to cause the whole unit to flex as it pushed down on the bristles.

When I took the shoe off, it drilled perfect holes!!

I need to make a different set up for drilling the holes to collect the dust and chips, and I will be good to go.

Not sure why I didn't notice it before, guess I was not looking at the right angle to notice.

Maybe it will help someone else...

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Probotix Meteor - :D 8-)
Lots of Wood - 8-)

Funny, I don't remember being absentminded...


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