Komputer Numerically Kontrolled (KNK) Router

[bigKam]

Section F. on Bill's page points to this link:

http://home.comcast.net/~rodec/woodwork ... myths.html

"...The issue, in a nut shell, is that with a charged conductor the charge is free to move, and thus the charges throughout the conductor can can join forces to make a strong spark. But, because charge on an insulator is not free to move, discharges only occur from a small area, leaving the majority of the charge behind. For this reason discharges from insulators are not as energetic as those from conductors. ..."

"I would also like to point out that you can not truly ground PVC; there is nothing you can do that will guarantee that a static discharge from the duct will not occur. While you can not actually ground PVC, the so called "grounding" seems to help anyway according to many accounts. Typically people add so called "grounding" using one or more of the following: a grounded wire inside the duct work, a grounded wire wrapped around the outside of the duct, or by having grounded screws poking though to the inside of the pipe. Interestingly enough, I've never heard of anyone wrapping grounded aluminum foil around the ducts, which for 4 inch PVC costs about 2.5 cents per lineal foot, or about a dollar to do 40 feet of ducts. Wrapping the ducts in foil will make your attempts at "grounding" much better than simply wrapping in wire. However, as discussed above, backing a PVC duct with a conductor is problematic. At least three effects may play a role in the so called "grounding": leakage currents and shielding for outside wires, and providing a short hop to ground for inside wires or grounded screws through the pipe wall. Leakage currents and the short hop to ground work to reduce charge build up. Shielding works to protect you from a discharge to your body, but does not reduce charge in the pipe. The effectiveness of the attempt at "grounding" will depend on many factors, and may be overwhelmed if the system is pushed too hard. "

And the "Myths" section at the bottom of the page is interesting... for example Myth No. 4:

4. The external ground wire works by reducing the static on the outside of the PVC. There is little or no static on the outside of the PVC unless you are rubbing the outside for some reason; the static is on the inside. The electric field due to the static charge in the pipe can cause a discharge on the outside, but this in no way means there is static on the outside. Indeed, if there were lots of static on the outside, say due to lots of charged dust floating around from poor quality filter bags, the static on the outside will be the opposite charge from the static inside since opposite charges attract each other. Thus the electric fields from the two will tend to cancel each other as discussed above in shielding. Outside static helps protect you! The charge on the external ground wire due to the electric field from the charge in the pipe will also be of the opposite sign. The outside static and the ground wire have the same sign; they repel each other! Static on the pipe is not drawn to the ground wire. There is no reduction in outside charge due to the external ground wire. And, if you believe that not enough charge will go through the pipe to be of help, how is it that the charge will go along the pipe? It is an insulator either way. "

[sammer]

Ya, thanks Kam , I read that after I posted the link.

I also found this;
"If you use PVC the primary issue is to protect yourself from a shock. For this I recommend either a bare grounded wire in the duct, or grounded screws through the pipe spaced every 4 inches. This will reduce the maximum charge build-up by allowing more discharges at lower energies. In a four inch duct, the maximum discharge distance to the bare wire is 4 inches, and the maximum discharge distance to the ground screw is 4.5 inches, so both give approximately equal protection. Because both the wire and screw point have very small radii, they will cause discharges at a much lower charge density than you need for a similar discharge to your finger. The advantage of the screws is that they will not hang up shavings like the wire can. If you are not concerned about receiving shocks, you need not ground the PVC ducts. It is likely that the external ground wire, bare or insulated, or grounded foil wrap is safe in the home shop, but this is problematic if you can generate very large amounts of fine dust."

Well, I'm much smarter now


sam

[G-man]

Well, I'm much smarter now

I'm just basically confused... right smack dab in the middle of my comfort zone.

Thanks sammer and Kam for the additional info. I was too tired last night to follow the links.

G-man

[andi]

hi everybody!
i am new to this forum so please excuse any stupid newbie questions & errors.
not sure if it might make more sense to create a new post for my question? i already got heaps of competent info out of this post, so i thought i might just ask you guys.

i am looking at buying my own cnc, no time to build one myself although i will miss the fun of designing my own.
I will probably get a BZT-PFU2515-PX, look at bzt-cnc.de in their webshop, am not allowed to post the whole link yet.

no my wallet doesn't say big mf on it i will make a small second business out of building custom snowboards and skis and use it for some other projects as well, so rather spend some more and do it right from the start.
last tuesday i drove up to northern germany from switzerland to have a look at the factory and machines. they didn't have my model completed and operable as it's a new model, but i could have a look at the finished frame without engines and ctrls, it all seemed of excellent quality, very well built and designed.
The salesperson then showed me another just finished router, similar to the one above but smaller (1500x100mm ), a little lighter and on the longitudinal axis the servos were not mounted on the nut, driving the nut instead of the screw, but on the ballscrew( 16mm), as in e.g. kam's knk-roter.
I was very surprised that the machine made a substantial amount of friction noise (with router switched of) and the ballscrews were resonating tremendously (amplitudes of their own diameter i guess) at higher speeds.
the salesperson told me the machine had just been finished and wasn't yet perfectly set up and greased. regarding resonance he said by having the servos driving the nut with fixed ballscrews this problem would be mostly solved as it was mainly due to the type of construction.
not sure if I should fully believe him, as I said he was a salesman, not an engineer or service person.

here my question: I haven't yet used a cnc and had considered cnc machines to be very quiet due do to the ball bearings throughout the machine? I thought of a zzzzzii..zzzziiii. sound;)
But it was more like kkrrrr...krrrrr ;(
Or is friction noise something normal?
Or does a new machine need some time to set up properly and be sort of 'broken in' like a new board or pair of skis?

I haven't yet used one so please forgive my greenhorn status.
thanks for your help folks, some quick feedback will be highly appreciated as they have 3 months of delivery time and i need to get going soon.

andi

ps: thanks everybody and especially you guys at skibuilder's for the vast amount of input. without you i wouldn't even have considered my company feasable.
you guys rock!
I will let you know and give some input back when planning is finished and my 'factory' is up and running!