Precission issue when milling

alexp · May 1, 2019, 6:15am

Hi,

I have a strange precission problem when milling.

I’m milling a square of 30.0mm width as pocket with a 10mm island in the center.

I’m using 6mm tool, with reasonable settings (12k rpm, Fxy 10mm/s 2.5mm depth per pass, total 5mm depth) and a peeling strategy leaving enough space to the edges, followed by a first pass along the edges (outer and inner) with finishing allowance 0.4mm and a finishing pass.

I see /hear that the finishing pass really cuts some material - so the over-dimension could not come from tool vibrations during peeling.

The result is as shown here:

[attachment file=98863]

On the larger/outer square, I have about +/- 0.5mm deviation. As it is + (in Y) and - (in x) it could not be a tool diamteer missconfig (btw: the tool is brand new and the cutted surface looks reasonable).

Spindle is aligned perpendicular to the surface. I also tested planing the workpiece (milling away 1mm on top), before doing the squares. Same effect.

I have these results reproducible in different wood and also in foam XPS (which however is a bit hard to measure).

So my first idea was that the axis position accuracy might be bad.
I inserted a needle as tool and placed a tape measure along y.

1st test: position accuracy exactly where I milled the squares with a step of 30mm:

[attachment file=98864]
[attachment file=98865]
With this approach, one can easily ‘see’ deviations of about 0.1mm or even better. We would expect 0.4 - 0.5 mm, so this should be obvious. However, position accuracy is good.

Then I tested along the complete Y axis

([attachment file=98866]
[attachment file=98867]
127 - 697 mm = 570mm which is exactly my Y width.

From the second image one could estimate that it might be about 0.1mm off (shorter), which is good i think.

So now I’m helpless. Where could the miss-dimensions when milling come from?

Any help appreciated

Alex

vicious1 · May 1, 2019, 8:36am

The axis that is making a larger island and smaller hole (looks to be your X? 29.6mm), My first guess is your belts on that axis could be a tiny bit loose. Or can you show a picture of your zipties.

Another test is resize that same part to double and see if the error is the same or doubled. How I see it is slightly oversized consistently could be explained by run out and other “standard” spindle/collet/tool issues. Undersized ID oversized ID is a little different suggesting a little flex.

alexp · May 1, 2019, 8:49am

The axis that is making a larger island and smaller hole (looks to be your X? 29.6mm),

Yes, it's X. Would that only apply when milling or also in the needle test?

vicious1 · May 1, 2019, 8:51am

Only when loaded, if the cable ties are too large they create a stiff spring, or just the belt being too loose in general. You can either make them smaller or zip tie it…hard to explain. I looked I could have sworn I had a picture of this. I can not find it. I will take another and add it to the instructions.

vicious1 · May 1, 2019, 9:01am

Well, a poor picture.

The best thing is to have your cable ties as short as possible. No matter what one side should be all the way tight, the other end as short as possible. If you messed up and have a short belt and need a long cable tie on one side do like I did in this picture.

https://www.v1engineering.com/forum/reply/29634/img_20170316_075636/

The left most front cable tie is tied to keep it from springing.

billsey · May 1, 2019, 10:08am

You might also double check the actual dimensions of your bit. A 6mm bit that’s actually cutting 5.85mm would put you into a bit small for both a pocket and island.

alexp · May 1, 2019, 10:55am

Hi Bill,

this was my 1st idea, but its to small in x and to large in y, so this cant be due to but diameter wrong - not?

alexp · May 1, 2019, 11:04am

Thanks Ryan for the image regarding cable ties.

The needle test shows about 1mm less actual travel on 800mm.

Yes, one side is long, but I replaced them by metal wires. The X was really a bit loose, so I added some tension. Needle test not much better.

I then milled another test and a double sized test. Here are the results:

[attachment file=98918]
same sized square: numbers in green: effect worse - I would not have expected that.
Double sized: in X error gets a bit larger, but much less than doubled. Y: for the outer square I see a huge (triple) error.

I’m clueless

vicious1 · May 1, 2019, 11:50am

What drivers and board are you using?

Some import 8825 drivers do not work correctly.

guffy · May 1, 2019, 12:21pm

Try to fix the stock as high as possible

alexp · May 1, 2019, 1:08pm

Its a RAMPS 1.4 with DRV8825.

Could it be the drivers when the needle test (no milling / no force) is almost exact?
@Guffy: Do you mean Z as high as possible?

guffy · May 1, 2019, 2:13pm

Yep, close to gantry

vicious1 · May 1, 2019, 2:39pm

There is nothing consistent about your numbers, nothing that can be fixed until those stabilize, at double the size some of your tolerances got better. I can only suspect you have a large build or your gcode is not correct.

Again, pictures would really speed up this process, guessing take forever.

alexp · May 1, 2019, 9:50pm

I was also thinking it could be nothing consistent in the mechanics as the effects are opposed (in x/y and outer inner).

I will (tonight)

have a look at the G code
position the machine when milling with individual G code from repetier as I do with the needle test (which is fine) to see if this makes a difference
test at Z up
provide some pictures (my builds is this one, however the picture still shows the first setup with the long plastic cable ties)

vicious1 · May 2, 2019, 7:15am

Well you have a very large/heavy spindle, and a gantry cable chain on a very long lever arm connected above the Z axis, and it is a pretty large/tall build. That lever arm is working against the axis you are having the most trouble with and would explain non consistent error on only one axis.

I think some of these numbers can get a bit better but a shorter build will help for sure.

alexp · May 3, 2019, 10:27am

I made some further experiments (later more).

First you ‘main point’:

you have a gantry cable chain on a very long lever arm connected above the Z axis

I was following the advices regarding not to connect the cable chain etc. on the Z axis. In fact it is NOT as far as I understand the situation. Here is a more detailed image:[attachment file=99150] The drag chain rail is connected to the x-steppers, the aluminum harm is connected to the middle part; by this, thecables leaving the cable chain and going to the z axis (stepper and spindle) are 'static' viewed from the z axis (the cable chain movesand the middle part connected aluminum arm hold them in place).

Is this still problematic? I can not think of a better solution.

large/heavy spindle

Yes, you are right. I partly regret this (for the weight) but it was chosen to allow for ER-16 inserts (10mm) which I will need.

I clearly see the effect of the heavy spindle when makin 1mm spets, the elasticity of the belts/the machine leads to some ocillating (could be reduced but I’m not sure if its worth it).

Regarding the cable ties:

I found it beneficial to replace them by metal wires as shown here:

[attachment file=99151]
This reduces the elasticity in this region to almost 0. I made experiments and found the parallel wires best (due to beeing metal, they do not flex in this shape).

Next will come some results of new experiments wich look much better (i need to get the numbers in excel first)

Alex

vicious1 · May 3, 2019, 10:32am

Yes, it is above your Z axis, it is almost as bad, in this case it is so tall I would easily say it is worse. Torque multiplier.

alexp · May 3, 2019, 12:18pm

Yes, it is above your Z axis,

I don't understand this. The arm is connected to the middle part, not to the z axis. In fact I have nothing connected to the z axis except the necessary cables which however are not moving when the machine is moving and they do not induce any force on the z axis (the force enters - via the arm - into the middle part)

How else could one get the wires force-less to the z axis?

vicious1 · May 3, 2019, 12:29pm

Just my opinion, I use wires from the Z to the Middle, then the middle to the roller in a loom, not cable chain.

Most people do not care about 0.3mm in wood, you do so I am trying to point out all the possibilities. A bar that long is acting as a torque multiplier, any drag induced is multiplied linearly to your gantry. Is you issue happening on that axis? No pictures to reference with your cut so I can only guess. If that is not it, try cutting your entire machine down to 3" Z, or each axis shorter in general. Like I said you have a large machine, that is tall, with a giant spindle, using cable chains, all of which can be working against you with tolerances you are wanting.

alexp · May 3, 2019, 12:48pm

OK, no offence meant, I appreciate your help! It was just that I did not get it.

You say: the cable chain has drag and the arm sclaes up that drag against the mid part when moving (in this case the Y axis should be affected - right?)