Jim hello I have worked with a lot of taiwanese hole drills ( hopops for short )
On 02/11/17 04:59, Jim Craig wrote: > I am working on a project at work where we are designing a new CNC > machine for a particular application. Up to this point I have been > planning on using a standard machine spindle. > > The more I think about the actual process and needs of the machine I am > thinking that a small hole EDM drilling head would be better for this > application. > > The application required drilling small 0.050" diameter holes through > .135" thick stainless steel. The back side of these holes are in an > annular space that cannot be easily reached for deburring. ok scale & magnitude in this conversation... you say 'small hole' and... .050 is almost optimum size ( 1mm dia is pretty the optimum for the technology) 2mm is difficult, .006 is difficult ( for std cheapo (<20k$) hopops ) > The reason I am contemplating the small hole EDM process is three fold. > First it is difficult to drill these tiny holes in stainless and the > drill bits frequently break. Secondly the back side of the hole needs to > be burr free but is difficult to get to with conventional tools. scale again... 'burr free' well edm produces a burr, its small a few thou to 10 thou depending on energy at breakthru and the system ability to control position well ( hammering thru with bang bang control has larger burr ) to get almost no burr, use a sacrificial plate and burn thru the SS and into the sacrifice > Third > the machine could be lighter and simpler if I use EDM vs a standard > milling head. the time to burn .050 hole thru 0.1" SS should be near 45seconds go to a shop to verify, or take a test pc to a sales house > > So my questions are as follows. What type of power supply would I need > to use for doing small hole EDM drilling? Can I use a tig welder as the > power supply and control pulsing with LinuxCNC driving an external IGBT? igbt's are not popular/common in commerical units igbt's have not been driven on and off as clean as individual fets, esp the irf series (its the clean off drop that marks a low wear unit) irf 540s and up are very very common ( theres a high-v spike due to overall system wiring, things like wire wrapped resistors, cable length etc add capacitance and inductance) the power should be , in _very_ general description... 100V open volts, 0-32 amperes supply in short circuit the polarity will be negative to tool and the tool will be brass tubing the on time will be 10 to 50uS and duty cycle 1-50% since linuxcnc's ability to do stuff quick is limited by thread speed you wont get better than your fastest thread as a time quantum for spark timing ( a 1ms thread period gets you 1ms time divisions, or 2ms with any jitter overages ) so plan on a pic ( very common ) or an arduino (fairly easy to hack) the medium will be tap water, deionizing is not a _large_ benefit submersing the part will aid in breakthru and less burrs oh, a 'soft start' at lower current for a couple seconds will reduce entry burr. the tool will be a tube with a .01ish hole thru it a special feed thru chuck with a rubber seal is used the rubber seal is pressed by water pressure onto a flange in the albrecht like chuck the water pressure is 900-1500psi but delivered by a simple air driven pump the pump has a large bore (4-8") and the piston is on a rod that goes thru both ends of chamber this piston cycles back and forth thru the pressure chamber, driven by shop air (8bar+) the cycle is controlled with 2 prox switches sensing a ferrous slug on the piston. the ENDs of this shaft enter small bores at each end of the large bore and the ENDs are pistons themselves! the small ends are sealed for the small bore and in the small bore is water! this size ratio is the pressure amplification, 100psi in, 1500 psi out, stupid and simple VERY low volume though, we only have a .01 hole to pass thru. the system is controlled by the 2 prox swxs, so it the piston just cycles back and forth, pumping tiny spoonfuls of hipressure water thru the tiny hole. a leading air pressure unit (drying and pressure regulator) will controll the maximum value, a gauge on output leg can show operator the value. it goes chug chug chug all day long. After this high pressure water makes it thru all the plumbing restrictions it isn't strong enuf to do more than dimple your fingertip. any line break will immediately reduce to water at less that shop air pressure. oh the thru hole water is neccesary, and as soon as you break thru, its LOST. so the back up plate helps a lot, the submerging helps to a lesser degree Could I use the Mesa THC to control gap of the electrode to the workpiece? i got a mesa thc but used a hal component and a window comparator myself for sink edm i did not build a hopop supply yet. got lotsa parts tho the windo comp used a preliminary voltage divider to reduce 100V to 5V the windo comp had 2 setpoints ( pot adjusted) the output was Hi MId Low if the gap voltage was MId, the hal componet and subsequent pid did NOTHING if HI the subsequent pid had 1 unit of measure added to the target position if LO it had 2 units of measure subtracted ( creep in, but run away ) its bang bang and run on a 1ms thread but worked ok for me ymmv > It appears that spindles don't need to rotate very fast. What type of > rpm do the spindles typically run at? spindle rpm is 0-100rpm, this is kinda like grinding with a small wheel in one way... the edm is a spark and rotating the tube lets the spark occur at different places 'distributed' ( as in car rotary distributor) around the tube's leading edge. if you spin _too_ fast you will disturb the edm process by making the 'apparent' distance too large, so slow (relative terms again) is good. the tool will wear, and wear fast compared to other edm plan on 1 length per unit of work, and another for loss ( 2mm workpc? 4mm tool ate up!) the tool will 'bullet nose' also, and a better supply and control system will be flatter ended. an intermediate cut a a piece of nickel will flatten the end again ( like dressing a wheel ) and a flat end will cut better. usually the top of the spindle is inside a pressure chamber and a carbide slug rubs on the surface, this way the electic power is supplied to the rotating device. the hi-pressure water feed enters this chamber and flows thru the rubber seal, tube and chuck > That is all for now. I am sure I will have more to come. > > Thanks, > > Jim sorry i gets carried away with edm stuff tomp tjtr33 ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
