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Electronic Leadscrew for the New Lathe
PekkaNF:
I find the two leadscrew or one leadcref for threading and one splined shaft for feeds approach a little cumbersome.
I have been thinkking one leascrew and simple gearbox (reduced set of threads) or standard change gear arragement at spindle end and athe tail stock end coupling, gear and electric motor only for feeds. Bit like Hardinge, but much simplified.
http://www.lathes.co.uk/hardinge/index.html
I have one little French made junk that has permananetly connected leadscrew on apron. No split nut. It is a little annoying to use but most of the time not as much as you might think. It got me thinkking if I could get away with rotating nut on the apron. I.E:
* threading per changegear (it has plastic change gears)
* Manual feed at the end of the leadscrew....there is a dog coupling to disengage threading/feed.
* electric motor to rotate the nut on the apron for variable speed feed.
Only downer is that to move the saddle from one end of the machine to the other would take a lot of crankking.
Myford 10 has very interesting arragement
http://www.lathes.co.uk/myfordml10/
There is very basic changegear arragement for threading/feeds that is straight connected to leadcrew.
On apron there is no halfnut, but a helical gear meshing with leadscrew. The gear can be turned with handwheel. Downside is that handwheel turns when leadcrew turns and saddle is stationary
But here is the idea: What about having a coupling (or gear) that will alternatively mesh with handwheel or electric motor?
The big idea:
1) Leadcrew is rotated only for threading and it has whatever gear set you need or forgot there. To thread you need to mesh gears on banjo and gear on apron is stationary (worm gear on electric motor or halfnuts).
2) Leadscrew is moved out of mesh, stays stationary and is used as a gear rack. For all other uses than threading. Myford 10 has crank at the end of the leadcrew for hand cranking (inching, halfnuts coupled), but you have to drive all selected change gear cluster.
3) Automatic feed with an electric motor, at the end of the leadcrew(=need coupling) or at the apron, needs coupling too.
4) Manual feeds with granking on apron, halfnuts disconnected and possibly.
Then again maching small gear rack and pinion is not too expensive here, but one more arragement to lined up an to be fitted.
Feeds, even spindle speed coupled is very easy to make with electric motor.
Threading looks very simple on ELS-principle, but there are very narow margins, unless 1 or 2 axis CNC approach is fully accepted. Then it is not anymore very cheap and simple.
I am electrical engineer (control systems) and my choice for threading would be between change gears and/or greatly simplified crew cutting gearbox. And electric feeds.
Sounds like VT:s choice is between CNC and thread cardride/follower. Two really different animals! I'm really intered to see which way is taken. I think that follower would be very beautiful and it screams cast parts. CNC approaches often seems to come out untidy. Not pretty.
Pekka
S. Heslop:
I'm no lathe expert but looking at them it seems that it's quite common for there to be a leadscrew with a spline in it, that works as a leadscrew and an autofeed thing. You could probably do some sort of worm gear thing with that, maybe all attached to some sort of pivoting block that rotates a gear meshing with the pinion down and out of the way. I was looking at those popular youtube videos of people hobbing worm gears on the lathe with regular old taps and it seemed pretty viable to me (but probably has alot of backlash). It's something I want to try out for making guitar tuners at some point.
vtsteam:
Norman, Phil, Russell, Pekka, thank you for your thoughts here -- it really makes this interesting and helps greatly in weighing the options. :thumbup:
STEPPER:
Phil I wonder how big a threading stepper and reduction on the spindle I'd need to cut, as a practical maximum let's say an 8 tpi Acme thread on say 1" dia mild steel rod.
Would a 270 oz-in stepper motor handle it with say a 4 to 1 reduction? (The spindle is roller bearing supported, and the leadscrew would be separately driven, so, not a load in this case.)
CHASE THREADING:
I see lathes UK calls the master screw and followers "chase threading". Must have evolved from the hand chaser and Tee rest method.
I'm still kind of attracted to this one because of its wonderful simplicity. I would definitely drive the apron in front, though, and run a pushrod parallel to the leadscrew, unlike any of those I've seen so far.
I would likely put the master screw on its own idler spindle with timing pulley -- close to the pushrod bearing, on the gear side of the headstock -- and run a belt to the spindle. This would reduce the length of the pickup arm and allow a change of simple pulley ratio between spindle and master screw. That last would reduce the number of master screws and follower nuts needed for a good range of threads. Pratt and Whitney did this, I think with master screws of 5, 5.5, 6, 6.5, 7, 8, etc.
It would also utilize the carriage and ways guidance for the cutter, rather than requiring the heavy rod on the rear of the lathe as a guide. The front mounted apron pushrod would no longer need to be of heavy proportions of the old style chase threader attachment. To me, the old style needed beef because they were acting effectively as an auxiliary cylindrical ways, supporting their own toolpost. It might be argued that the rear toolpost was a benefit, but that can also be achived on the conventional carriage -- as is done for an auxiliary parting toolpost these days. I was already planning on that anyway.
CHANGEWHEELS/GEARBOX:
Still a possibility. I think the things I hate most about the Craftsman could maybe be tamed somewhat in a new lathe. The blackened oily gears, heavy floppy three slot banjo, indecipherable gear chart and awkward insert system with conventional bolts could probably be better thought out.
I guess one benefit of plastic gears is no oil needed. Maybe timing belts could replace some of the gearing. Or the gears run for threading only without oil somehow. Simple things like an internal light and custom made gearing wall chart would make sense -- and probably everyone else is smart enough to already have that. What about a quickly removable gear frame, so you could lay most of the train out on top of the workbench while changing gears around?
I could use the Craftsman gears on the new lathe, and with these changes make the operation more tolerable.
Also, feeds could be handled the same way many of us add feeds to a mill -- a DC motor and speed controller -- detach changing feeds from the need for gear changes in threading. That could be done through two leadscrews, or one with a simple dog clutch and engagement lever for the motor (as done on mill tables to allow manual use).
Okay so if I did something like an adjustable DC motor drive for feeds, and a removavble "frame of gears" for threading. The frame could be stored out of the lathe normally, and just popped in when threading was needed. Easy to work on, easy to keep clean, and set already to the last thread cut -- no need to revert to a feed ratio. That doesn't sound too bad....
philf:
--- Quote from: vtsteam on March 29, 2015, 09:32:33 AM ---
STEPPER:
Phil I wonder how big a threading stepper and reduction on the spindle I'd need to cut, as a practical maximum let's say an 8 tpi Acme thread on say 1" dia mild steel rod.
Would a 270 oz-in stepper motor handle it with say a 4 to 1 reduction? (The spindle is roller bearing supported, and the leadscrew would be separately driven, so, not a load in this case.)
--- End quote ---
Steve,
As has already been pointed out 270 oz-in will be the holding torque and as soon as you try to bring speed into the equation the torque will fall off. The reduction between stepper and spindle will increase the torque proportionately but at the expense of max speed. Speed doesn't seem to be an issue for screwcutting as many users advocate hand cranking the spindle which probaly means you'd be 60 rpm maximum. If I were doing this I'd try a more powerful stepper (my CNC mill uses Nema 23 3.1 N-m (440 oz-in) motors which are cheap enough) and a higher reduction.
What's the biggest diameter and coarsest pitch you're likely to need? A 3" diameter 8 tpi thread will obviously require much more torque than a 1/2" 32 tpi.
Something else to think about is what will happen if the spindle stepper stalls - The control probably wouldn't know and the tool would try to plough a groove along the workpiece.
There's a lot of discussion on various forums about using stepper powered spindles but not many seem to have got beyond the talking stage. There are kits available for the tiny Sherline lathes.
Phil.
Fergus OMore:
Steve,
By now you will have copies of what Martin Cleeve did to the part Myford ML7 that he purchased.
Obviously, it is all 60 year old stuff but it sort of collates a lot of original thinking.
Me, I was mulling over all this excellent information and began to realise that I had a spare Myford rack( actually 2) and wonder why no one has gone along propelling machine tool movement( model wise) with hydraulics. I was having a bit of a down with the death/funeral of an old marine engineer mate/engineering college mate/boat builder mate and lots more. I sort of think that he would have had similar thoughts .
Hope my downloads are of interest
Norman
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