Fine point bits break when Z-zeroing

[phil_o]

I'm having a problem when I use the Z-zero plate. When I zero bits that have a very fine point such as a pointed roundover bit or a 60 degree V-bit the fine point breaks off. This doesn't happen every time but often enough to be very annoying. Any suggestions on how to keep this from happening would be greatly appreciated.

[Paul Amos (Unregistered Guest)]

Hi Phil

What Z plunge speeds are you using

[paco]

Must quite frustrating...! Paul is suggesting the right parameter to look at; you can adjust this speed to very low if need... and/or create a "special" Z-zero routine and start it very close to the Z-plate for this kind of brittle bits... thought I did'nt had this happen on my side with the V-60 and a 1/32" 7 deg. tapered bit... make sure that the tips and the plate are clean and "condutive enought"...

[gerald_d]

Two things to look at:

1. reduce the approach speed as the other guys suggested.

2. make the plate "spongy" by putting rubber/sponge under it to absorb the shock.

Both of these approaches mean that you would have to adjust some values in the z-zero program (.sbp file).

[arundo]

What about the foil foam that is used for home siding. Take an alligator clip on the end of a second wire tied in to the same spot as your zero plate. Or contact cement aluminum foil on to a piece of foam. Mic it and reset the z-zero setting.

[phil_o]

I'm using the Z-zeroing routine accessed by entering C (cut), 2. I don't know how to change the plunge speed.

[Paul Amos (Unregistered Guest)]

Phil

You can change the Z feed two ways, either enter the speeds on the Shopbot control screen or edit the C2 file. The C2 custom file calls up
FP, c:\SbParts\zzero.sbp you can edit zzero.sbp for the required feed speed.

[phil_o]

c:\SbParts\zzero.sbp has a plunge JZ -.5. I guess if I change JZ to MZ the plunge would be much slower.
Thanks.

[paco]

In the Sbparts folder of your hard drive, you will find S_zzero.sbp (which is the routine call by C2) which you will open in any text editor including SBedit and go to about line 32 and will read " VS, , .25,,,, .65 '... and assign slower speeds"... .25 is the Z move speed and .65 is the Z jog speed. I suggest you try to set the jog one to the same speed setting as the move one... COPY THE ORIGINAL FILE (S_zzero.sbp) on your desktop BEFORE DOING ANY CHANGES!!! Change only thoses number and NOTHING ELSE if you are not experienced with this... After this done, save your changes and you may test this in the air (holding your Z-plate in hand about an inch from solid surface so there will be no risk of touching something "hard"... though you may want to keep PANIC switche handy!!!). You will then observed the speed change. The first plunge (the approach plunge) is a jog plunge and the second plunge (the "near to" slow approach), the slower, is a move plunge... This line is use by the routine to set it's Z speed values... You may set thoses even slower if you wish...
Keep us informed of your findings Phil...

[phil_o]

I'll have to wait til tomorrow when I'm at work. Thanks for the help.

[dingwall]

There's one other thing. On my Porter Cable 7518 router, I've had to quit using the Z-zero plate because the bit and collet are somehow isolated from ground. Maybe the bearings heated up and turned the grease into varnish who knows, but it takes a lot of pressure to make contact - making the Z-zero plate un-useable.

[Brady Watson]

Sheldon,
If that is the case, you can simply place an alligator-type clip (large one) on the bit, and the other end on a bolt or other metal part on the Bot & use the Z_Zero again.

Test for continuity so that you know you are getting a good ground.

-Brady

[paco]

Here what I done about this...

5030
5031
...much easier and reliable since it's begining!

[billp]

Phil,
You could also make up a "composite" zeroing plate . I have made them using the back plate of a standard electrical socket box, with a piece of copper circuit board soldered to the face of it. This way the bit comes down and hits the copper face which is softer, and also conducts the electrical current very well.....

[paco]

Is'nt aluminium already quite soft Bill?... are you using a steel Z-plate Phil?!?

[gerald_d]

Sheldon, your "varnished grease" problem is a good illustration of why it is necessary to hard wire grounding leads to each of the moving carriages. We cannot rely on the V-rollers to make proper electrical contact.

[jerryk]

I had a similar problem with the router not making contact with the zzero plate. Cleaning the bearings in the router rectified the problem.

[dingwall]

I should clarify that it's the router's bearings that are at fault not the V-rollers, but still a good point Gerald.

[phil_o]

I'm using the Shopbot aluminum plate.

[phil_o]

I changed the Jog (JZ) to Move (MZ) int the Z Zero routine. It worked the first time. I haven't been brave enough to try it several times. Bits are too expensive to experiment but I think I'm OK. Thanks for the help.
Also, I measured the thickness of my zero plate, it is .121" thick. I edited that line in the file.

[paco]

GREAT Phil! You've just introduce yourself to SB code programming!... if you want, you can slowdown even more the plunges; though you should'nt need it!?!?!

[Brady Watson]

Phil,
It is also helpful to chage the move speed by inserting a MS,1,.1 before the 2nd plunge...and change the JZ -5 to MZ -5 if you are using small bits. The initial JZ pluge kills small bits. Make sure you change your MS after running the file, or insert another line after the last plunge to change the MS to something more reasonable for moving the tool around/cutting.

The slower you go on the Z move speed, the closer the zzero value will be to true zero. Since the Z doesn't instantly stop when it hits the plate (we're talking ms here), there are less steps when the motor turns at .1 IPS than at 1 IPS...so the overshoot will be less at the lower speed...since nothing really stops 'instantly'...

-Brady

[bleeth]

Hey guys-you just solved one of my pet peeves. The slower plunge speed resulting in closer accuracy not only saves the bit tip but also means that for bit changeovers 0 Z is more equivilant when going from a roughing to a finish bit. I really like accuracy and repeatability and hate having a 1/8" finish bit dig a ditch below my 1/2" end mill clean out or my other previous option of using a scrap on the side to manually adjust my finish bit to the same 0 as the last bit.

Dave

[Brady Watson]

Dave,
It sometimes takes me 5 times to get the Z zeroed out to my liking...particularly in situations like you just described. I run the zeroing routine, and then set the z-height at my plate thickness, .121...I then slide the plate under. If it is too tight (won't go under) I run it again...too loose and I run it again. It frustrates me to no end when you do a tool change from say, a 1/2" bit to 1/4 or 1/8 to get the areas that were missed...and there is a step in z-height.

-Brady

[bleeth]

Okay-So slowing the rate of drop improves bit changeover accuracy but doesn't final it. Darn. Back to trial and error and scrap sheets. Whenever I have to do that it just seems to go against the whole concept of the cnc. Since more sophisticated equipment mills to accuracies far beyond the bot the question is how can we improve tool changeover process within our tool and software limits? Stop collars on bits for precise length control? Increased number of contacts in the z-zero routine for better averaging? An improvement in this area would save a bundle of project time for everyone.

Dave

[fleinbach]

I'm not sure but could your problem be something I noticed a few months ago. While zeroing I noticed at times if you watched closely the zero plate would move very slightly. There are several reasons for this. The one that seems to be most prevalent is that the spring cord will take a different path with each use. So it is stretched differently each time. This puts enough tension to cause the plate to be slightly floating over the material.

Another way to get a zeroing error is the material itself. Although MDF seems to be the flattest sheet material all sheet material has some warp to it. After one particular cut job I noticed I wasn’t cutting all the way through a sheet. I looked at my file to make sure I had the correct depth and there was no problem there. Next I noticed that even though I had a hold down screw just 6 inches from the corner where I zeroed, I could push down and detect that the corner was slightly raised. Naturally this gave me a false “0” Z height above the rest of the sheet.

To remedy these false Z heights since these discoveries I now hold pressure on my zero plate every time I zero. I haven’t had any problems Z zeroing since using this procedure.

[paco]

I did too have some trouble to get two bits VERY WELL calibrate at first... but since I ALWAYS use the same spot, I can't notice a steps from between two different bits pass. When I say SAME SPOT, I mean THE SAME SPOT... and preferably one that is VERY WELL SIT; near a clamp. I'm quite suprised that you need to do thoses steps to get your Z accurate... I must add that my spoilboard is'nt much rigid (plywood support with 1" MDF glued and screwed); I have to surface it often and that my Zeroing routine is VERY FAST; fast because there are no pauses and I re-plunge from 0.25"... I have used it with 1/32" tappered bit and 1/16" WITHOUT any breakage. As Frank, I apply a GOOD pressure close to the bit touching area on the Z-plate... and I believe too that material can be problematic about this if you zero at different locations each time.

[rjguinn@optonline.net]

I agree with Frank 100%. I've been using a .231" precision ground steel plate which measures 4.0"x6.0" for years & haven't experienced any of the problems noted here. I use an MS speed of .2 throughout the entire routine & no jog speeds. The weight & precision grind of the plate, plus the slow MS gives me accurate repeatability with no broken or dulled bits. I've never understood the need to have a fast Z zero routine at the expense of accuracy.

[ckurak]

I agree with Frank that holding the zero plate down helps. I've also noticed that the aluminum zero plate can get bowed. And, parts of the dust collection boot want to move the zero plate around, sometimes tipping it up if I am near the edge of the table. Both of these problems seemed to be resolved by applying some pressure.

[paco]

I go fast because I like it fast!... and I did'nt loose any accuracy. If slow is good... then slow is OK! One step that got me more accurate too is to throughtly clean the calibration area AND the Z-plate from dust...

[gerald_d]

Phil hasn't solved his bit breaking problem. He has now started a new thread over here (http://www.talkshopbot.com/forum/messages/29/6229.html).