Problems with repeatablity on CK - Homing switch accuracy?

[rcferguson]

First of all, thanks for sending a new anti-backlash nut... that solved my Z axis problem.

I've discovered a new problem now that I'm trying to produce some mold blanks:

First, I put a blank piece of aluminum stock in a vise and bore 6 holes, some for tooling pins and others for mounting screws. Accuracy of these holes is pretty good... I can stack several pieces using tooling pins and the holes all line up within a couple thousandths, so I don't think I'm loosing any steps in the X and Y axes.

 

After boring the holes, and lining up several pieces, the edges of the pieces don't match. This isn't a problem at this stage; I can attribute my sloppy vise to any discrepancies, and the edges will be cleaned up anyway at the next operation, where I remove quite a bit of stock round the edges of the piece. I use one of the tooling pin holes as my work zero, using a dial indicator to find the center, so all operations from this point on are based on a consistent point. I do 5 operations on this blank, all using a 1/4" flat end mill... rough surface, rough contour, finish contour, pocket, and finally a finish surface. The machine goes home (I have homing switches), I remove the blank and replace it with another, then repeat the operation.

The problem is that after I do several parts, if I stack them using tooling pins, the edges are wildly inconsistent with each other. The worst discrepancy is in the Y axis, where out of 4 pieces they are all off by at least 10 thousandths, and one is out by as much as 50 thousandths,  but the X is also off.

  

Assuming I'm not losing any steps when traveling to the part, the only thing that could be causing this discrepancy is the homing switches. I've made sure they are tight and not moving, but I'm worried that the make/break point in the switch is not consistent. Could I be missing some other possibility?  I can live with some slop in the positioning, maybe 5 -7 thousandths, but the results I'm getting are not good enough to make a reliable mold for a part. It's bad enough that the positioning is off that much after going home only once, since I don't have any tool changes, but when I start actually cutting the molds I'll be using several different cutting tools, which will involve going home several times.

I'd appreciate any ideas.

Thanks

Rob

[riotonthebay]

You should be able to test your theory about the homing switches by setting up a dial test indicator to be zeroed in the home position, jogging around randomly, and then going back home. This should also let you independently test the X and Y to figure out which switch is acting up.

[rcferguson]

Yes, spent the last couple hours testing this out... placed an indicator at the Y axis home and alternated between home and zero about 30 times. It was never more than 1 thousandth off. Moved the indicator to the Y axis zero and again, after 30 reps it was never off by more than a thousandth. Didn't check the X, but did discover one leg of the capacitor was not soldered on. I re-soldered it.

     

I still don't think I'm losing steps, as the overall dimensions of my work pieces are correct. One thing that occurs to me... this thing generates a LOT of chips, and they go everywhere. The Y axis switch bracket is a little bigger than the body of the switch itself, and sticks out a little. I'm thinking chips may be piling up on that little resultant ledge and interfering with the movement of the actuator. I could grind down the bracket a little so that it doesn't stick out, but it still leaves the switch terminals exposed, and I'm concerned that enough chips could short something out. I'm going to 3D print a cover for the switch on a flexible support that will cover the entire switch when the machine is in operation, and will be pushed out of the way when the machine homes.

[Derek the Admin]

Hi Rob. This one is pretty puzzling.

Is the hole spacing entirely symmetrical? Can you verify in your design that one hole isn't closer to the edge than the other, causing the orientation to matter? Are you profiling your part from a larger piece of stock, or are you setting it up with the rectangle already cut to size?

[rcferguson]

Yes, I've gone back and double and triple-checked the location of the holes in my design, they are all where they should be. I can stack 4 blanks on top of each other and all the holes line up perfectly, even when I rotate or flip the parts, it is just the relation of the holes to the outsides of the part that vary.

To reiterate:

Starting with pieces of 3 inch wide stock that I have cut to 4 inch lengths.

The finishing operation will remove stock from all 4 sides of the part so the resultant part is 2.75" x 3.75"

I put a blank piece of stock in the vise, find the edges, compensate for size of tool, set X, Y, Z Zero point, run my code which bores 6 holes in the blank.

Remove the blank from the vise.

Insert another piece of stock into the vise.

Run the program again to bore 6 holes.

Rinse, repeat 2 more times so I have 4 blanks with all holes bored.

Stack the blanks on top of each other and all holes line up perfectly (or at least within a couple thousandths... enough so that I can run a tooling pin through any hole on all 4 blanks).

Next operation:

use locating pin in my fixture to find X, Y, Z with a dial indicator and set this as my machine Zero.

Place a blank in the fixture, run my finishing operations (surface, contour, finish contour, pocket, finish surface) all with one tool so no tool changes.

Job finishes, router goes Home.

Remove finished part, place another part in fixture.

Run program again.

Rinse, repeat 2 more times.

Stack the 4 finished parts, holes still line up (of course!) but edges vary from 10 to 50 thousandths in the Y and in one case 5-10 thousandths in the X.

Everything is solid, nothing is moving around...head, fixture, etc.

As I mentioned in the last posting, I suspect chips interfering with homing switch in Y, and unsoldered capacitor may have caused problem with X.

I printed a cover for the Y axis homing switch, tomorrow I'll cut some more blanks and we'll see if it makes a difference.

       

[stever]

Have you tried doing all of these operations without homing your machine each time? You really don’t need to return to home between operations as long as you’re not disconnecting your machine or you’re not changing your zero location. That might let you know if it’s the homing switches

[Derek the Admin]

Also consider the possibility of taper. If your router had a nod in Y forward or back then depending on the flipped orientation of the part you would get a different edge to hole measurement.

[rcferguson]

Feb 7, 2019 16:58:23 GMT Derek the Admin said:

Also consider the possibility of taper. If your router had a nod in Y forward or back then depending on the flipped orientation of the part you would get a different edge to hole measurement.

I don't think the holes would line up in that case.

I'm cutting some more blanks this morning... ran out of 7075 so am using 6061. Very different characteristics! Much gummier, and causes the tools to wear a lot faster. I'm going back to 7075 when I use up this stock. Costs about 4 times more, but worth it when it comes to finish and tool wear. I'll et you know when I get a few done.

[rcferguson]

Still not perfect alignment, but within 10 thousandths now. Part of my problem is that I'm not getting a good right angle on my cuts. I'll increment the left side of my Y axis to see if I can get it straighter, then realign my fixture. Problem is that as soon as I power it off the X axis will be off again. Anyone have a good solution for keeping the X and Y axes at a perfect 90 degrees?

[Derek the Admin]

Shouldn’t be the case that powering up moves a motor. It could be expected that they love to the next pile at power up, but I wouldn’t expect what you are experiencing. What happens if you swap where the motors plug in? Is it then off in the opposite direction ?