Sherline mill off center

Hey, I’ve been playing with a Sherline mill for a while. After cutting some material, the zero of the x and y is shifted to the positive direction. It’s probably somewhere around 0.005” to 0.01” off from where it started. This is really frustrating. For example, I need to cut a cylinder shape and then drill a hole in the middle of it. But after that cylinder shape is cut and I start drilling, it’s noticeably off center a little bit.

I have tried a new, sharp end mill, as well as reducing speeds, so I’m doubting that’s it. I can’t really check the backlash right now. I will have to get a dial indicator so I can look into that. Just wondering, is this normal for a Sherline?

Is this a CNC mill,or is it manual? If it's CNC, have you established a zero point you can return to so you can determine if you lost any steps?

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Originally Posted by awerby

Is this a CNC mill,or is it manual? If it's CNC, have you established a zero point you can return to so you can determine if you lost any steps?

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It’s CNC. I’m not entirely sure what you mean. I set the zero for x and y, which is the center of the cylinder. After shaping the cylinder, I have the machine move back to x0 y0 before I put in a drill bit and drill the hole.

If it's not returning to the center of the cylinder, it has most likely lost steps. That's a fault that happens when external forces overcome the impetus of the stepper motor. To combat this, the first thing to try is lowering the acceleration factor - try cutting it in half to see if that helps, and if it cures the problem but the machine seems sluggish, raise it by increments, but not enough to start losing steps again. The other things to look for are mechanical problems, like lack of lubrication, misadjusted gibs, or simply taking too big a bite of the material at a time.

As awerby said, it may be lost steps, however if it's happening consistently when changing to an op with a twist drill, your problem is most likely that the column is not perpendicular to the x and y axes. I had a the same issue with a larger machine - see my thread linked below for more detail.

https://www.cnczone.com/forums/bench...ml#post2416552

If the column of your mill isn't perpendicular to both the x and y axes, as you change to a longer tool (for example a twist drill) the head of your mill will move up, but it will also move side to side/front to back depending on what angle the column is set at. This gives you the 0.005" to 0.01" offset in hole location. To eliminate this, you'll need some sort of square and a dial indicator - set it up on the table and jog the Z axis up/down along the square and shim the column respectively until there is no more movement in your dial indicator. Do this with the square oriented in both directions (x and y).

-Adam.

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Originally Posted by tkalxx

As awerby said, it may be lost steps, however if it's happening consistently when changing to an op with a twist drill, your problem is most likely that the column is not perpendicular to the x and y axes. I had a the same issue with a larger machine - see my thread linked below for more detail.

https://www.cnczone.com/forums/bench...ml#post2416552

If the column of your mill isn't perpendicular to both the x and y axes, as you change to a longer tool (for example a twist drill) the head of your mill will move up, but it will also move side to side/front to back depending on what angle the column is set at. This gives you the 0.005" to 0.01" offset in hole location. To eliminate this, you'll need some sort of square and a dial indicator - set it up on the table and jog the Z axis up/down along the square and shim the column respectively until there is no more movement in your dial indicator. Do this with the square oriented in both directions (x and y).

-Adam.

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Now that you mention, I had upgraded to a longer column. It might be the cause. Anyway, I should have a dial indicator tomorrow and I will check it out.

Good advice above. Let me add a couple of notes that might help.

1) To check for lost steps I'd suggest, to start, cutting air. Run your part program with no stock in the vice. This eliminates the tool loading so you can look at all the other possibilities first. Lost steps can build up over many moves, run a long program or one program several times.

2) Backlash is also possible. To check this approach at point, like zero, from either direction. If they end up a different points that's backlash.

3) Adam is right about the column needing to be perpendicular to the table. Bringing the vertical Z axis to be perpendicular to the table is called "tramming". Google "tramming the mill" for methods of accomplishing this. Do this before looking for lost steps.

Hope this is of some use. Thanks.

Hugh

Are we sure the hole is off center after milling the cylinder or is it off center the moment it is drilled. Check the tram of the machine. A small error becomes larger at the long end of the drill bit.