Hi Andy, Yes and no. A turning operation is actually just a very fine pitch threading operation although ideally you don't actually want the tool bit to enter the work at exactly the same point. A tad bit of randomnous on each start is nice to shave the crests from the previous pass.
In either case though, at the 3000 RPM or even 6000 RPM that a small Unimat DB-200 or Sherline can turn, you want to know the RPM and you need to track the turns per inch; which granted may only 0.001 for a nice smooth finish but is essentially still threading. Just the tip of the tool is so round for smooth turning that there is no root or crest cut. For example my Machinist toolbox calculates for turning 1/8" brass: MATERIAL= Brass SURFACE SPEED= 91 MPM CUTTING TOOL MATERIAL= HSS NOMINAL STOCK DIAMETER= 0.125 inches 3.175 mm DEPTH OF PASS= 0.1 mm SPINDLE SPEED= 9419 RPM FEEDRATE= 32.44734 IPM 0.00344 IPR 824.1625 mmPM 0.0875 mmPR MATERIAL REMOVAL RATE= 0.04859 in³/min MATERIAL POWER CONSTANT= 0.5 REQUIRED HORSEPOWER= 0.02858 You still have to know when to stop turning so you are tracking 9419 RPM (157 RPS). Assume a small lathe has a directly coupled stepper with 1600 steps per rev and 20 TPI lead screw (0.050/per turn). Divided by steps/turn we find we need 108.8 steps per spindle rev to create 0.00344 IPR. Doesn't matter how fast the spindle is turning yet. The spindle is turning 157 RPS so to keep up we turn the stepper at 17,081 steps/second. My ELS would then throttle that max stepper speed of say 10,000 as would MachineKit based on the number in the ini file. But wait, the lathe can't do 9419. Max RPM on this theoretical Unimat is 6000 RPM or 100 RPS so now steps per second is 10880 which is within range more or less of the torque capabilities of the stepper. However, turn only a 0.25" length for that miniature model part and the stepper may not even get up to speed before it has to decelerate to the shoulder you are turning. We went through all sorts of those types of scenarios on the E-Leadscrew group during the development of the ELS. John > -----Original Message----- > From: andy pugh [mailto:bodge...@gmail.com] > Sent: October-16-17 5:29 AM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] MachineKit on the BeagleBone Black > > On 15 October 2017 at 18:58, John Dammeyer <jo...@autoartisans.com> > wrote: > > > Anyway, back to the Beagle and the QEP. A 1000 line encoder on the spindle > > and 1000 line encoder on the lead screw motor results in 4000 PPR for each. > > To turn a 1:1 thread they both have to be turning the same rate. Say you > > are doing high speed threading of leadscrew pitch (8TPI) at 2400 RPM. > > That's 40 RPS or 25mS per turn. If the Beagle PRU cycle time is 10uS then > > it's there to read the QEP counter 2500 times per revolution. (25mS/10uS = > > 0.144 degrees of spindle rotation) . > > > It's worth bearing in mind that you typically wouldn't thread 8tpi at 2400rpm. > Even if your counter can't manage maximum spindle speed you may still > be able to use the lathe perfectly happily. > > -- > atp > "A motorcycle is a bicycle with a pandemonium attachment and is > designed for the especial use of mechanical geniuses, daredevils and > lunatics." > � George Fitch, Atlanta Constitution Newspaper, 1916 > > ------------------------------------------------------------------------------ > 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 ------------------------------------------------------------------------------ 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
