Messages

901

General Mach Discussion / What is a good probe to use with Mach?

« on: January 19, 2010, 03:06:45 AM »

Now that I have an accurate machine, I am considering taking the plunge to probing/digitizing.

Wondering what people are using and how they like it.

902

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 19, 2010, 02:51:54 AM »

Well, here is something interesting, I measured the voltage at the spindle motor and at full speed it is only50v. I removed the fuses to all drives except the spindle and still got only 50V at the motor at full speed. Of I extrapolate up to 90V from there, the speed is much closer to what I expected.

I have had the PS do this before and then it started putting out the full 72V again for no apparent reason. When I received the PS from Keling, the frame was very badly bent. I straightened it out and used the PS and it seemed to be fine.

My impression was that voltage was determined by the number of windings and is unchangeable. How can a 72V linear PS with a toroidal transformer put out less voltage that it is supposed to?

903

General Mach Discussion / Re: Opto switches behaving badly

« on: January 19, 2010, 02:44:57 AM »

See if there is voltage present at the BOB inputs.

I have all opto and two PP. On one of my BOBs, the input pins have about 1.2v on them all the time. No idea why the voltatge is there,  but it causes unreliable operation. I put resistors fropm the pins to ground to pull them down to zero volts and the problems went away.

The other BOB is a different brand and does not have the same behavior.

904

General Mach Discussion / Re: Would like opinions on servo motor voltage and PS secondary in series

« on: January 18, 2010, 05:47:39 AM »

Thanks for the comments so far. I should have provided more info.

The spindle motor is a servo motor WITH and encoder and is driven by a servo drive, so although I agree with the assessment given relative to PWM vs SCR speed controllers, it does not apply in this case.

The servo drive will limit the amps to whatever I set it at up to 35amp max. The drive is rated to 160V.

Confusion sets in, methinks, when I apply stepper motor thinking to servo motors. If one considers the voltage rating on a stepper vs where they actually run, there is no correlation. My experience with servo motors so far shows me that a servo motor can run much higher voltage than it's rated voltage on an axis where it has plenty of rest time, but a spindle motor has a different environment and my concern is overheating. I can cool the spindle relatively easily, but I have no practical method of cooling a servo motor. It is my understanding that amperage causes demagnetization, not voltage, although I really don't understand that whole phenomena.

Ironically, the idea of running rectifiers in series for double voltage comes from Antek themselves. I should clarify that this is only applicable to a pair of rectifiers running off the same toroidal coil, as is the case with my current PS and the 1,500W models from Antek.

What I am after is to get the full RPM's from the motor that I was expecting. the belt ratios on my new mill were designed around Keling's specs, which I now suspect were exaggerated. The motor does not run at the spec speed, even after deducting 20% as indicated by several tech articles on the subject. I can easily get the speed back with higher voltage and I am going to purchase a separate PS just for the spindle, so that is the basis for the question here. My alternatives are grim; buy another motor or redesign my carefully chosen ratios on my two speed mill head.  I sold my old mill and my two Minarik PWM drives went with it, so as a diagnostic I have purchased another Minaric cheap off eBay to run the spindle motor on 130V 5A to see what speed I get and if the motor gets hot.


 

 

905

General Mach Discussion / Re: Anyone Here Have Experience Milling 6061-T651 Wrought Plate?

« on: January 18, 2010, 05:17:20 AM »

As described, the problem as I see it requires an analysis that is somewhat beyond bobby level, but here it is anyway. For calculating production, you think in terms of the cu.in. of metal removed per minute. Each material has a factor indicating the volume of the material that can be removed per available spindle HP.

Here is an important consideration to keep in mind when deciding on your parameters; SFPM can be all over the map and will effect surface finish and production time, but there is a very wide range that you can use successfully. The same is not true for chip load per tooth. There is a definite 'sweet spot' where the heat goes overboard and the cutter runs smoothly. This is a very narrow range and you will have no success on either side of it. The beauty of CNC is that you don't need to go thru a ton of calculations to arrive at the proper IPM. You just set your feedrate in terms of feed per rev based on the chip load and number of flutes and then run the spindle as fast as your machine will allow (for aluminum). The IPM will automatically be optimized for ANY spindle speed.

Moving on the the analysis; in general terms it goes something like this; lets say you have 'X' spindle power available. You can remove 'X' volume of 6061-T6 per minute.  SFPM of the cutting tool indicates a max RPM of 'X'. With that, you calculate the chip load per tooth required to remove the prescribed volume of metal per time. DOC obviously dictates the VOLUME of the chip at the same per tooth feed, and a 4flute cutter can run at twice IPM of a 2 flute for the same tooth load . . . . BUT, that requires a lot more power and if you don't have the available power (or rigidity), then the solution is not to lower the IPM, which I think most hobbyist's tend to do, which will make the chip too thin, but to use less flutes (for the same DOC), or less DOC so that you can maintain the proper chip load.

Quick word about DOC; the deeper the cut, the more of the tool edge you use and therefor the longer it will last per cu.in. of material removed, but reducing the feedrate to get more depth is not a good trade off if it reduces the chip load beyond the 'sweet spot'.

That being said, it should be simple to combine these factors into 'volume per tooth' thinking. This is the basic idea behind roughing cutters. Obviously a 1/2" cutter is going to make a very much larger volume chip that a 1/4" cutter at the same feed per tooth, the effect of which is what is being observed, methinks. In practical terms, on my little X2 I could not drive a 3/8" cutter at a feedrate (per tooth) that was acceptable, so I went to a roughing cutter in that size.
 
Just my 2 cent's worth . . .

906

General Mach Discussion / Re: Anyone Here Have Experience Milling 6061-T651 Wrought Plate?

« on: January 17, 2010, 01:10:01 PM »

7075 can be cut dry in any hardness and it's similar to mild steel in strength.

2011 (bar) and 2024 also cut nicely. Like 7075, these very nice to cut after figting with 6061 or 5052 (which is even worse), although that joy comes at twice the price of the 'every day' grades.

2 flute cutters definitely help if you have the power and rigidity to make a big chip.  If you look up cutting speed for aluminum in any text older than a few years you will see what is today a very low number. Aluminum is routinely cut many times faster than that today, and one should get their cutting speeds from the toolmaker and also not forget to multiply that by the appropriate number if using cobalt, carbide or coated tools. You will find an indication for 3x to 10x faster speeds. I  run my 5/16" and 3/8" three and four flute cutters at 7,500 RPM . . . . only because that is as fast as my spindle would go.  My new mill is set up for 8,500.  1/4" cutters are the 'crossover point' below which I go to my die grinder spindle with speed set appropriately up to 28,000 for the really small cutters.

Of course, the faster you go, the more heat problem you have and you start to get into water cooled spindles and stuff like that.
   

907

General Mach Discussion / Would like opinions on servo motor voltage and PS secondary in series

« on: January 17, 2010, 06:29:49 AM »

My new mill has a 90V servo motor for the spindle.  My current PS is linear 72V with dual rectifiers. The PS will be overloaded with the new spindle motor and the 4th axis motor, so I am contemplating buying a second PS and I am in something of a quandary of what to buy. The 90V motor runs OK on 72V, but does not achieve the speed I need.

I have axis motors running fine at higher (in one case double) than their rated voltages and I was looking for opinions on where to run a 90V spindle motor and what ramifications if any I would encounter running the motor at 130V

Also I see that dual rectifier PS can have the secondaries wired in series or in parallel for double volts of double amps. I am wondering in this case if it is possible to run one motor by taking the power in series and still be able take the regular voltage off the respective rectifiers. For example, can a dual rectifier 80V SP supply 160V for one motor and 80V for other motors by taking power from a series wiring for the HV and straight into each rectifier for the 80V?

908

General Mach Discussion / Re: USB and Mach

« on: January 17, 2010, 05:50:04 AM »

I am one who needs the swap axis function. If Mach had the ability to run an axis continuously, there would not be a need, and hopefully that may find its way into Mach4.

My objective is to implement some of the capabilities of a machining center and thereby add a great deal of versatility and capability to a standard milling machine. Without the swapaxis function in mach, I know of no other way to get at this functionality.

Selecting a differnet plane is not a trivial task either is there are circular moves involved. I have not had to do this very much and so far I have simply 'messaged' the G-code rather than explore the application of swapaxis to this issue, but it seems like swapaxis would be a good, if not the best solution there if there was a larger program or the need to make a lot of smaller programs run on a different plane than XY. 

I don't understand why smoothstepper would eliminate this important feature of Mach, but that's the reality at the moment. Hopefully Mach4 will implement some more sophisticated interface options so that a single product with buggy firmware and drivers is not the only available choice. How long does a product need to stay in beta and have holes in it's functionality? I keep reading that 99% of SS users are happy campers to the point where it starts to look as if that is justification for the remaining bugs and limitations  . . . .  just my impression.

909

General Mach Discussion / Re: Anyone Here Have Experience Milling 6061-T651 Wrought Plate?

« on: January 17, 2010, 05:05:35 AM »

Kool Mist is not good in my experience. I have tried a couple of lubes, but keep coming back to tapmatic tapping fluid. They now have an aqueous version that is not specd to be diluted, but I cut it about 3:1 with water and put it in a spray bottle with very good results, but for anything heavy, I still have to fall back to the tapping fluid.

Cutting tools . . . think in terms of chip load and not IPM. I have just completed my new mill and have not cut anything with it yet, but with my previous mill, all I could get was .001 per tooth before trouble started.

HSS if fine is the tools are excellent quality and have a smooth bright finish and are razor sharp. My experience with el-cheapo import cutters (Grizzly, etc) is exactly as you describe. Using a brand name cutter makes a huge difference.

There are some cutters that are made especially for aluminum with a higher helix and special coatings that prevent the dreaded build up on the tool. Once the aluminum melts and attached to the cutter you have only seconds before the tool snaps, so you usually wont get the chance to stop and clean the cutter.

Bottom line is you have to have sharp tools and keep them cool. Blow air on the cutter constantly to keep it cool, with or without mist. The MOST important thing is chip load. If you get that right, then the vast majority of the heat will go overboard with the chip. If you get it wrong, the heat goes into the workpiece and the cutter.  

Oh yeah and the two most important words in aluminum cutting: climb mill.

910

General Mach Discussion / Re: Supplier of small quantities of Titanium and machining recommendations

« on: January 17, 2010, 04:44:34 AM »

I'm finishing up on my latest spray etching tank http://www.prototrains.com/etch3/etch3.html where I have a titanium heater and titanium "spider" for holding the workpiece.

Cool project . .  and kudos for the fine craftsmanship. After pondering the potential hazards of messing with strong acids, and condidering the unsophisticated projects that my limited electronics knowledge would allow, I opted to go with routing PC boards. Very simplistic compared to your rig, but it's fun for me.

Here is my old mill cutting a PCB using my die grinder for a spindle; http://www.youtube.com/watch?v=r9Zf_5yHB1I