Tony K
Elder Statesman
Posts: 1,573
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Post by Tony K on Aug 11, 2007 8:14:59 GMT
Al, I have a Boxford 4.5" lathe with the belt fully enclosed but it does have an inverter drive so I can make it inch round very slowly, although it had never occurred to me that the torque would be high enough for tapping at such low speeds...... David Not that it will change the situation, but just for completeness, the torque is fairly constant over the whole speed range - not quite, because we are dealing with sine waves and not square waves. This is one of the great virtues of inverter speed control. I could produce the formulae, but it would not add much. Sounds like you are nearly there! Best of luck. Regards, Tony.
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Post by houstonceng on Aug 11, 2007 9:21:25 GMT
Hi Al, I use a myford with an inverter drive and use it for all tapping and threading with a die (dieing???) I wouldn't go back to single phase for all the tea in China. The worst of single phase motors is having to wait for so long before you can reverse them, while they spin down. Chris. But worse than that, they :- 1. Don't like too many starts/stops per hour (such as MEs usually give them on lathes) - often leading to premature failure of the start cap or start winding. 2. They "cog" rather than rotate smoothly - causing vibration and sometimes paterning on the workpiece (unless very well "insulated" - like ribber mounted - from the lathe). 3. They are often more expensive than equivalent 3-phase power for power.
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Post by houstonceng on Aug 11, 2007 10:14:26 GMT
Dave, SNIP I understand a 3 phase inverter alows you to inch the spindle, does anybody with one of these know if it can be used for tapping? Al Depends on the inverter make. Some have "JOG" control - usually using a "momentary push button". That may wind the motor up to it's set speed if held or give a fixed slower speed. Suck it and see !
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Post by houstonceng on Aug 11, 2007 10:23:20 GMT
Al, SNOP Again, I think reversing the direction should be possible, but I may need to add a switch. I can certainly do it by programming the inverter. Ah, I need to go and look at it to see if there is already a button for direction control. David It would be an unusual Inverter if it didn't have provision for Reverse running. However :- a. Some makes require an input to be held Hi/Lo to assert direction and a separate input is required to assert Start/Stop b. Some makes have two inputs SFWD (Start forward) and SREV only one if which should be asserted. c. Some makes have a latching mechanism and require a STOP signal. d. Some makes have to be programmed to allow reverse running as they have a parameter(s) which effectively bar it inless re-set. Easy when you know what you're doing. Dangerous if you don't.
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Post by jeffhills on Aug 12, 2007 18:50:38 GMT
I have an old Boxford with an inverter and always use tailstock dieholder under power, just pushing the whole tailstock along to reduce drag and undersize treads. I have the controls over the headstock with speed control giving 200-3200 rpm (yes it runs fine at that speed) and a 3 postion rotary switch for reverse - off - forward, that way it is easy to stop and back out when the correct depth is reached. Its also great for threadcutting., I just leave the leadscrew engaged at all times, when the the required length is reached just snatch the crosslide out and reverse the motor (on long threads even cutting fast and slowing at the end. Regards Jeff
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David Thompson
Active Member
Building 'Marquess', 3 1/2" gauge.
Posts: 46
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Post by David Thompson on Aug 13, 2007 13:11:26 GMT
Chris,
Thanks. I just had a momentary memory glitch. Of course I can drive it both ways, it's just some time since I set it up and I haven't found much use for it. What I actually did was to wire all the low voltage control signals through the original Boxford switchgear, so I can select on and off and forward and back using all the original levers and interlocks I was quite proud of myself when I had got it working, but I usually just use the mechanical reverse to wind backwards when screwcutting. Now I shall have to try power threading with the diestock. There seems to be plenty of torque at the low speeds.
And I absolutely agree about the convenience of using the inverter. If you have a two speed motor that can be swapped between star and delta wiring, it can be quite a cheap device. Mine is an Optidrive from Invertek Drives and cost about £120 a few years ago.
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JohnP
Hi-poster
Posts: 186
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Post by JohnP on Aug 13, 2007 20:40:26 GMT
Alright, I'm lazy. I admit it. This is how I get a thread up a piece of bar in the lathe 1) Put die into ordinary die holder. 2) Put drill chuck into tailstock 3)Adjust (open/close) jaws of drill chuck so that the front of the jaws will bear upon the back face of the die. 4)Slide everything up to the work. Start to cut the thread whilst keeping the drill chuck jaws in contact with the rear face of the die, advancing as necessary. The flat front face of the jaws keeps the die square with the job.
It takes longer to describe than to do it!
If it's not clear (and anyone is interested), I'll try to post a picture.
JohnP
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Post by Boadicea on Aug 13, 2007 21:10:31 GMT
I use the same method as JohnP, except I simply use the tailstock without a chuck to keep the die square whilst advancing behind. I rotate the chuck manually but you could power it - the die holder is prevented from rotating by the saddle. If you rotate the chuck manually, mind your hand on the tool - better to remove it if you have the time - classic for cut hands!
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