Z axis rebound

[Bank Walker]

Ok, I have been cutting some knotty pine. My problem is that the z axis will rebound about .125 when a v bit or a endmill hits a knot. Not every time but often. 12,000rpm, 80 inches per minute. I was running 100 and slowed it down last week. Because I thought it would help with this issue. Today I noticed the bit would spin in the collet with the collet tight. So, I am ordering a new collet but was wondering if there is some sort of mechanical tension adjustment on the z axis of a Shopbot? Any advice you can give would be appreciated.

" Life is hard when your dumb" John Wayne

[coryatjohn]

Is the Z Axis actually moving or is the bit being driven deeper into the collet?

It is almost impossible to overtighten an ER25 collet using the wrenches supplied by Shopbot (unless you are an ironworker or a WWE professional).

You should replace your collets when they cannot hold a bit properly.

[Bank Walker]

No the bit stays the same depth. The z axis assemble rebounds. Like if you lower the z axis too low.
I have ordered a new collet. I have a Porter Cable router.
Just wanted to know if there is a tension adjustment I might not be aware of.

[coryatjohn]

It's always best to precisely describe your equipment, including model, year, software version, feeds, speeds, anything pertinent.

[tlempicke]

A lot of vendors will send out 6MM and 1/4 inch bits interchangeably. There is a difference and that might be what is causing your problem. Mike your bit and see just what it is. Keep in mind that collets and nuts are wear items and need to be replaced when worn.

[jerry_stanek]

I have bought .25 inch bits that said .24 on them and that is what they were.

[Bank Walker]

I have bought .25 inch bits that said .24 on them and that is what they were.

Well, I ran two different programs this afternoon, I changed the vbit. Tightened is as much as possible. Holding the router from turning the bit did not slip as before. No issues with running either program today. Very possible that the bit was just enough undersized to allow it to slip.

[Bank Walker]

It's always best to precisely describe your equipment, including model, year, software version, feeds, speeds, anything pertinent.

Shopbot, Alpha, PRT 4G, Mach 3, V Carve Pro 9.5, Porter Cable 7518, 13,000 rpm, 80 inches per minute.

It is a older used machine that has several items that are not original to the machine. Still trying to figure out if the rebound that i am getting is normal for these machines. I did improve the cutting today with the bit change. But still leaves me not knowing if i have a secondary issue or the z axis should be well from now on.

[Brady Watson]

Shopbot, Alpha, PRT 4G, Mach 3

FYI - There's no such animal...the machine is either an PRT Alpha or it is a PRT Standard 4th Generation (4G). An Alpha machine will have violet/black motor label, whereas a Standard will have a red/black motor label.

In the case where your machine is an Alpha - the early PRT Alphas used direct drive AS911 motors. These had the tendency to be 'springy or spongy' when the control box is on and you attempt to move an axis by hand with enough force to overpower the motor. It will very much act like a spring - and spring back as the internal encoder and driver attempt to resolve the original position. If this is what you are referring to, it is considered 'normal' for that vintage machine. Plunge speeds should be carefully set so you don't get that deviation in position while the motor is in 'alpha mode' trying to correct itself. It is simply a matter of asking more from the motor than it is capable of delivering.

If running a Standard, the Z will simply lose position and never correct itself. Since you didn't mention position loss, I would guess you have an Alpha.

The length of time that the motor is allowed to self-correct is factory set at 4 seconds. After 4 seconds, if it was not able to return to the commanded position, the driver will alarm out and power to the motor is shut down for safety - just in case your arm is in there.

From a machining standpoint, the AS911 motors in direct drive format suck. However, if you were to buy a 5:1 planetary gearbox for each, this would result in incredible performance/torque that rivals even the newest Alpha machine with none of that 'spring' in any axis.

[Bank Walker]

FYI - There's no such animal...the machine is either an PRT Alpha or it is a PRT Standard 4th Generation (4G). An Alpha machine will have violet/black motor label, whereas a Standard will have a red/black motor label.

In the case where your machine is an Alpha - the early PRT Alphas used direct drive AS911 motors. These had the tendency to be 'springy or spongy' when the control box is on and you attempt to move an axis by hand with enough force to overpower the motor. It will very much act like a spring - and spring back as the internal encoder and driver attempt to resolve the original position. If this is what you are referring to, it is considered 'normal' for that vintage machine. Plunge speeds should be carefully set so you don't get that deviation in position while the motor is in 'alpha mode' trying to correct itself. It is simply a matter of asking more from the motor than it is capable of delivering.

If running a Standard, the Z will simply lose position and never correct itself. Since you didn't mention position loss, I would guess you have an Alpha.

The length of time that the motor is allowed to self-correct is factory set at 4 seconds. After 4 seconds, if it was not able to return to the commanded position, the driver will alarm out and power to the motor is shut down for safety - just in case your arm is in there.

From a machining standpoint, the AS911 motors in direct drive format suck. However, if you were to buy a 5:1 planetary gearbox for each, this would result in incredible performance/torque that rivals even the newest Alpha machine with none of that 'spring' in any axis.

Ok, thanks for the help.The machine has had so many modifications. Lord only knows the true original model or year it was made. The man I got it from bought it in SC years ago. He said it was as is then and he has moved 8 or 9 times himself. I only said it was an Alpha based on size. I think you called it PRT 4G based on the pictures I posted last year. I honestly do not know enough to be trying to describe it to professional cnc people. Might be why i was kinda hesitating on posting machine info. Lack of knowledge. I did recieve a thumb drive with it that was supposed to have the machines info but when it tried to view it the drive was not formatted.

Great to hear the rebound I am getting is normal. I do have a new spare z axis assembly that came with the unit. Still wrapped in original plastic. If needed i could swap it out but it appears there are newer upgrades that can be made that would get me better performance.

When the rebound occurs, say up .125, it maintains that new level as being correct. I do loose the original depth. That being the case the machine is probably a PRT 4G like you thought it was last fall when I bought it.

[Bank Walker]

32144
Here is a picture of how it stepped up.

[Bank Walker]

32145

This motor might be the best way to identify the model.

[Bank Walker]

I had figured that router speed and machine speeds and depth of cut all need to be fine tuned based on the material I am using. Still learning.

[srwtlc]

You stated that you are using Mach3. You may need to tune the accel/decel/velocity settings in the config for the Z axis. If too aggressive, you could be losing steps during the quick up/down/in/out motion while vcarving. Along with that, knots being hard, can add to the stress factor on the motor and cause lost steps.

[Bank Walker]

Thanks, I will check my settings to see what I have now and try to figure out if I need to change anything. I will have to ask for help with that.

[Bank Walker]

Hello again, I know this is a year old post but I had the same issue today of the z axis jumping up while in operation. No knots this time on poplar wood. Brady you mentioned that i need to look at 5:1 planetary gearbox for direct drive motors. I have 2 motors on my cnc that are direct drive. The y and z axis. Both are getting hot compared to the two Vexta motors. The first time I noticed the z axis move up was when I was changing a bit. Since then it has moved several times causing me to have to stop the program and reset the bits position as close as possible to the original and restart the program. My direct drive motors do not have any markings but they both appear to be Nema 34. Which gearboxes would you suggest buying and should I replace the motors also?

[Bank Walker]

https://www.ebay.com/itm/ORIENTAL-MOTOR-VEXTA-GEARHEAD-4GK5K-D9-RATIO-5-1-NEMA34-FRAME-MOUNT/261709724290?hash=item3cef1d7a82:g:E8IAAOSwiCRUmDm X

[Brady Watson]

No,
You don't want that gearbox. Check your PM. You'll ultimately need the gearboxes, motor swing plates and compatible pinions.

[Bank Walker]

Thanks Brady. I felt like I was stumbling around in the dark.

[Brady Watson]

Mailbox empty now...

[Bank Walker]

Update: Last week I had already ordered two 960 oz in stepper motors. Yesterday I replaced my 600 oz in motors, z and y axis with these new motors. Tested my systems new operating limits, I raised the y axis as high as 300 in per min and my z axis to 100. I ran two programs and had no issues with my z axis rebounding or the y axis overheating. The new motors highest temp during 3 hours of testing was 60C.

[Brady Watson]

You're really missing out on what your machine is capable of by not running some kind of gear/belt reduction to gain mechanical resolution. By replacing the closed loop nature of the Alpha motors with generic 960 Oz steppers, you're going backwards...not to mention killing any resale value. Kudos for the effort of interfacing those motors to the control, but it isn't what you ultimately want in the long run.

Yes...the gearboxes are not cheap. However, they are practically a bolt on affair and will transform a direct drive Alpha into an absolute workhorse with gobs of torque and an unbelievable increase in all aspects of cutting, particularly V-carving and 3D work. Plastic and metal edge quality is excellent on a tight machine. Say goodbye to chatter. With gearboxes, it would rival anything to ever come out of SB...

A word to the wise should suffice...this is one area where you really don't want to skimp. It's the heart of the machine.

[Bank Walker]

Brady, I agree 100%. My machine is more of a Frankenstein cnc than a Shopbot. Long ago someone built this huge oversized y axis setup that is unlike anything I have ever seen before. This motor just allows me to use raw power for the issue. X# of ft pounds per degree of rotation. It is so big compared to normal pinion rack and pinion setups. That is my next step, buy a new rack and pinion for the y axis. Buying a motor 60% larger was my first step. Even with its huge size. My testing shows it has the capability of moving at very small steps as low as .001. This is actually the first money I have spent on the machine since purchase. All the vcarve programs I have designed are setup for 100 inch per minute. I am going to maintain that setting until i get the new y axis and gear boxes. Then I will plan to program a few projects at incremental speeds to determine what my new limiting factor will be. More than likely it will end up being what the Porter Cable router can actually cut. As far as resale goes I am not really worried about ever selling it. I will probably gift it to one of my son's or grandchildren. It is only a hobby deal I use once ever other month or more.
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