Why can't I make round holes?

[Acmeaviator]

Morning Dana,

I will post some pics and cut files tonight when I get home. I have a BT48 so I am guessing that it could be a problem with my powerstick being out of adjustment or with the Y axis on my Z car. My gears themselves are cutting ok - especially after taking the advice to hand trace as opposed to just using the auto-fit to scanned .bmp's. The holes that are causing problems are the arbor (shaft) holes through the center of the pieces. The long axis of the "ellipse" is on the X axis of the hole if that makes any sense - like the table is moving too far on the X axis and the right amount on the Y axis.

[paul_z]

I cut lots of small holes and they measure almost dead on cicular. (I have a PRT alpha with the 7.2:1 upgrade.)

If the "out of round" is slight, it could be that your spindle/router is not perpendicular to the table. If that is the case, The front and back of the holes should look different with the back looking better.

Paul Z

[CNYDWW]

Best way i've found to determine at what speed you can cut with small bits and minimal deflection is by doing a simple profile on a square corner. Set it up single pass at depth and make several copies. Each profile either set the feed rate higher or lower in progression. Run it in scrap material then use a square to see what speed there is minimal deflection. When the bit makes a hard 90deg corner, you'll see the deflection as a slight curve after the corner is made. Once the direction changes the direction of the deflection changes. Another way to try drilling the holes is doing a raster pocket with the grain. Slightly offset inward with a profile pass after. Lets say .0325 offset at the feed and speed with the acceptable amount of deflection.

Best of luck
Randy

[Acmeaviator]

Here are a few pics to help with the problem:
Y axis is left to right, X axis is top to bottom in each picture






This is some deep pocket scroll work I cut that includes some small triangular cutouts - all the cuts are spot on and used the same cutting tool as the one I used for the gears:

[bleeth]

Brian:

Looks to me like you have either slop in your pinion gears or a backlash issue. Since it is a buddy the slop could be in the table.

[Gary Campbell]

Dave is right on. machines motion adjustments need attention.

[Brady Watson]

Make sure your powerstick disengage lever is allowing the pinion to fully mesh with the rack. Adjust as needed - although it should have been factory set.

-B

[Acmeaviator]

Thanks for the replies gentlemen It seems like the problem is on the X axis which would be the powerstick - I did pull the disengage lever when I first set up the machine to install my spoilboard. I'll check that and make sure the lever is not partially disengaged.

[dstacey]

Dana,
I have had a similar problem with circles that seems to have just shown up lately. I have read and tried your suggestions. Everything seems tight, but I do notice that it seems like I can feel just a little movement in the y motor. Its just a little movement, but it's definate. If I stand in front of the X car with the bot turned on and lifted off the table, I can feel just a little movement right or left along the X car rail. I can also see the y motor rotate just a tiny bit (maybe 1/2 of a tooth or less). I checked the y motor mounting screws, and they were tight against the base, but I loosened them and tried to push the wheel up and engage the rail tighter, but it didn't seem to make any difference. Any Ideas?
Dave

[dana_swift]

If your machine gets 0.010" of backlash in one axis, the circles will end up 0.020" out of round. Tight is required!

Since you have just noticed the problem, it is probably something you did that caused the change.

The good news is that its almost certainly a fixable problem, once you figure out just which problem you are having. Fixing the wrong problem can make a problem where there wasn't one. Once you have found the right problem its just a short bit of work to fix it.

Use a dial indicator and measure the play in each axis. Borrow a dial indicator if you don't have one. The more setup you do on your machine the better at it you get.

Good luck, and keep us posted-

D