[NicHoza]

Hello everyone,

I just bought a Taig CNC mill, and I am running it with EMC2. Unfortunately, I have run into a problem.

I was facing a block of aluminum with a 3/8" two-flute carbide end mill, and all of a sudden it started sinking into the block. I shut it off. I tried a couple more times and the same thing happened each time. The computer showed no change in Z position, and the drill bit was not slipping in the collet.

The only explanation I can think of is that the bit was pulling the whole Z-axis down into the aluminum. It is only a 280oz-in stepper motor holding it up after all (if I remember that number correctly). I thought this seemed unlikely because the depth of the passes was only about .01", which seemed extremely shallow. So my question: does this seem normal?

Some more questions: If I am right about what's going on, are there some basic guidelines I can find somewhere about depth of cuts vs. holding torque (and probably some other variables such as RPM & cutter diameter)?

Could I fix this problem just by increasing the RPM and keeping everything else the same?

Would the problem be solved if I used a smaller diameter end mill?

In general, what do all of you seasoned CNC experts have to say about this situation?

Thanks!!!
Nic

[NicHoza]

Well, I tried increasing the RPM from 2600 to 4200, but it didn't seem to make much difference. I got through .01", but the next .01" the same old thing happened again. If you look at the picture linked below, you can kind of see (but not really - bad photo) that as the end mill moved to the right, it ramped gradually down into the aluminum (until I pressed the E-stop), which the end mill was NOT supposed to do, and the computer didn't know it was doing. I guess the machine just can't handle a 3/8" end mill making a pass one hundredth of an inch deep in aluminum? That doesn't seem right...

Oh and my feed rate was 15. I haven't double checked the RPM/Feed rate but they seem right.

http://craterfish.org/rrrgh.jpg

Thanks for any comments/help!

[NicHoza]

I've been facing the same part with a 3/16" end mill, and I am not experiencing any problems whatsoever. This is confirming my suspicions that the machine simply can't handle a 3/8" end mill. Comments are still welcome!

[fragger6662000]

I would run a test with the Z locked with the jibs if you can and jog / MDI the machine and see if it is the mill, motor or screw. Are you plunging into the material, if so the machine could be flexing on the plunge and then once side cutting dropping down to the correct level.

I could be just the geomatry of the tool and it is trying to pull it into the material. Greater cutting area means more force, i had similare issues with Z on the X1 machine that i have.

[UUU]

Can you see the feedscrew revolving as the cut plunges? This would support your suggestion that the stepper motor is not holding steady.

[strohkirchw]

I'd interested to hear if it's the screw, motor or stepper. I too noticed my taig doesn't do as well with 3/8" cutters. More than anything I think it's the motor on my end which I'm going to be upgrading soon. I've found 1/4" end mills to be a happy medium between size and speed without constant issues. Even then my 1/4" high helix carbide will pull itself down into the piece randomly.

[sansbury]

That sure looks like the tool pulling out to me, but you say it's not... are you sure?

Do you have ball screws? Reason I ask is that the normal 1/2-20 screws should be pretty hard, if not impossible, to backdrive.

[Isplat]

Hi There,

I have a Taig and I have used a 1/2" 2 flute endmill with a 3/8" shank to face material. You may have to slow things down and use a smaller depth of cut. The Taig can do a lot, it just doesn't do it fast...

Also check that your Z saddle is clampled to the z-ways. The saddle can "appear" tight when there is still considerable slop in it. When it's loose you can get a kind of drop in the z that appears like massive backlash.

The fip side is you can get it so tight that the z axis doesn't move well. After this, check the dovetail that the spindle clamps too. This can be loose too. Either of these could allow the endmill to be drawn down into the work if they aren't tight.

In my experience if either of these are loose, the endmill can end up gouging into the work piece. In the worst case so bad as to stall the motor and really dig into the work piece as everything comes to halt. In this case, I belive the endmill actually gets drawn out or the collet.

Last thoughts would be to make sure the the Z-axis doesn't have excessive backlash. I have a couple mills. More than that and the nut should be adjusted.

The Taig Lathe and Milling Machine has tons of stuff you should check out, especially all the other taig users sites.

Ian

[lsces]

Nic
This is a problem several customers have reported.
Invariably it comes down to something slipping that just needs to be a little tighter since the cutter IS providing a considerable pull down with the bigger cutters. The thing to check is the position on the main z axis block after you find that the cutter it's self is lower than it should be. While you say that the cutter is not slipping, that has been a couple of the identified shifts, but more normally it is the spindle mount against the dovetail.

[rowbare]

Have you checked your gibs? The whole table might be lifting a bit. Or if your fixture plate is loose it could lift a bit too.

bob

[NicHoza]

Isplat, your comment was very useful. I find it quite likely that the spindle was sliding on the dovetails. I do not find any of the other possibilities to be likely, due to the fact that the endmill was sinking so far into the material (on one pass it sank about 1cm into the aluminum).

Once I have a chance to tighten the spindle on the dovetail and face another aluminum part I will post back with results as to whether that solved the problem.

Isplat - what depth of cut were you using for the 1/2" endmill? Were you cutting aluminum? What recommendations do all of you have for depths of cut vs endmill diameter for aluminum (or other materials for that matter). Is there somewhere to find this info or calculate it for a given mill/motor torques? If not, maybe I should make some sort wiki spreadsheet.

[strohkirchw]

Try these slots with a 1/4" carbide and see if you comfortable with the results. Make sure you motor belt is nice and tight.

10600rpm and 30IPM with 1mm DOC
6700rpm and 24IPM with 1mm DOC
4200rpm and 17IPM with 1mm DOC


The other day I tried a 6.75mm DOC and 18% stepover with 4200rpm and 15IPM. It actually did it without stalling though I don't think I'd run something like that without supervision. For the most part I stick near 1mm give or take a little bit. I try to make up for it by running 6700 or 10600 rpm and increasing the feedrate.

[sansbury]

One other dumb thing to check is to make sure the coupling between the stepper and Z axis screw is tight--i.e. that the set screws are secure. If they are loose they might allow the leadscrew to freewheel under the combination of vibration and pull from the tool.

[MechanoMan]

That sure looks like the tool pulling out to me, but you say it's not... are you sure?

Do you have ball screws? Reason I ask is that the normal 1/2-20 screws should be pretty hard, if not impossible, to backdrive.

I agree with the "impossible" part. Even if you powered down the stepper drive and lean on the head, the screw will not turn to drop the head.

It's possible to have it stall on an "up" at some point. Note that any stall on any upward motion moves the head's reference down. You don't need to see a motion plunging below the surface that it didn't lift up from. If, for whatever reason, there's a 0.1" lift after a facing pass, then a 0.1" drop for the next pass, a stall on the lift results in the head being 0.1" too low.

[The Blight]

http://craterfish.org/rrrgh.jpg

Seems like a gradually deeper cut. Would take quite a few Z moves to do that! And on a straight cut? I see no need for it, so I kind of doubt thats the problem.

If it's not the tool slipping, I'm curious about one thing. Is the head and carriage for the Z axis made from one piece or are they bolted together? If they are bolted together, maybe you should check that these bolts are tightened?

[NicHoza]

I tightened the spindle on the dovetails and tried a 0.04" depth of cut (1.016mm), and it made it about halfway and then the belt started sliding so it stopped, but it did NOT get pulled down into the aluminum! So I tightened the belt and tried again, and it worked! Thanks everyone. I'm very new at this.