How to make a self reversing screw?
I am interested in learing how a self reversing screw is made. I have not found much info on how these are made. Is this thread cut on a lathe? How does it return on itself? How is the nut block designed? If you have any info on how or where to look for cutting this thread I would appreciate it.
Here is a link of what I am talking about:
Self reversing screws : Linear Products : ABSSAC Power Transmission
This screw is also used on baitcaster fishing reels.
The thread profile looks to be square, so there is no reason it couldn't be cut on a lathe, provided that your lathe could cope with the high lead angle and cutting both left and right hand threads.
Rather than a conventional nut, these work with effectively a blade running in the groove and reverse by turning the 'nut' through an an angle (double the helix angle). For experimental purposes you could probably get away with a pair of dowel pins. I've never seen one of these that reverses mid stroke - the kick over is aways at the end.
Stanley tools had a screwdriver that worked with a similar screw. If you could find one that might give some more clues.
I tried to cut this on my Pratt Whitney lathe. The only problem is how do you get the thread to return and not cut a groove when the tool stops in linear motion and the spindle is still turning? Here is an example of what it looks like.
The Wedin brochure is cool. Never saw a 30 ft long, 6 inch diameter ball screw!
I think you can to in forward and reverse fairly easily by changing the angle of attack on the thread engager.
Neutral would involve pulling the part that engages the thread out I guess. Would want a brake on the system at that point.
personaly i would guess that its milled. Would be easy on a cnc mill with 4 axis. simply rotate part, feed along, then feed back. Just keep part rotating the same direction all the time. Only thing is that reversal point at the ends probaly a lot more advanced profile than a slot.
On a 4-axis mill, they are also referred to as diamond threads. Need to do it on the mill if you wish for it to self reverse when it gets to the end. As if you do it on the lathe you'll end up needing a under cut at both sides of the thread. But you can always machine a couple rings, cut it in half and weld them in place, if you did want to do it on the lathe.
Here is my attempt to cut this thread. This was cut on my PW lathe. The only problem is how to get it to return. I tried to chisel out the area so it will return but no luck. Also does the nut have to swivel in the thread? How were these made back before CNC? Any one have any pictures of any they have made?
Cory, How about solenoid driven pins in place of your bolts? You will probably need at least two, maybe three. WWQ
Well if I see your pictures correctly you still need to cap off the ends.
With 2 half rings, on both sides, shown is just the one piece of the half ring for clarity.
P+W or Lees Bradner thread miller
This was cut on a Pratt Whitney Model C lathe. I see that I need to cap off the ends per your drawing. But how does the nut work? How is the nut built?
Dimitri: What would the nut look like that follows your drawing?
The same as the nut in your link. The nut is allowed to "float" in a slot as it moves so it can twist and turn as it goes forward and back. As far as I know the screw needs to "hit" the end to get it to go the other way, its "self reversing" not controlled reversing. The round piece, with the C looking feature on the protrusion is the nut itself. The nut is then in cased in a assembly. However not sure how that looks like. I've dealt with the screws but never actually seen a nut assembly.
Check this one the original patent ....
Edit, I just realized I modeled the screw wrong. Woops sorry all the screws we made were always milled, I was told about the "caps" for lathe work. But the caps need to be there so there is no undercut and produce the diamond like end for the pattern to self reverse. Look at the Patent it shows the end profile better then my drawing. Again sorry for the confusion.
I have never cut one or seen one cut, but some thoughts:
1. Pins will not work as they can go either way at the corssovers.
2. You need a fairly steep helix angle to minimize the size of the crossovers. This translates to a low TPI.
3. If you are cutting it on a lathe, a hand crank would be helpful to allow stopping at a precise point.
4. Cutting on a lathe would require different setups for the left and right hand threads. At the very least, you will need to stop and reverse the tumbler gears when you reach the ends.
5. The turnaround at the ends could be done by making a non helical slot for a partial revolution between the left and right hand threads. Turn right hand to end, disengage tumbler and turn non helical slot for a bit (60, 90, 180 degrees?), then reverse tumbler and turn left hand thread. Opposite procedure at other end. Repeat as needed. Some hand work may be needed to make this straight section a bit curved.
The non-helical slot at the ends would allow the follower to reverse more easily. It will need to be some fraction of a turn and that fraction will need to appear on the threading dial as a division that will not engage the original thread. You will need some kind of angular scale on the chuck to tell you where to start and stop each segment.
The screwdrivers use cast "threads".
I am sure CNC would do a bang up job of this, but it had to be done before CNC, didn't it?
It's also used extensively on pretty much every decent winch on every research ship I've ever worked on. Neat devices.
Originally Posted by Cory
As for how it's made, if I was making one, I'd mill it using a powered dividing head geared to the table travel. In fact I've given a bit of thought to making one to have a level wind anchor winch on my sailboat, as a bragging piece.
I *think* the nut basically has a single pin that follows the square thread form.
Oh yes, after you make the screw you need to think carefully about gearing, cable diameter and drum width because these things all interact and you can end up with some truly lovely messes if any one thing is wrong, and your level-winder doesn't reverse direction at the correct point. We had to lay 4000m of 22mm trawl wire on the bottom of a nice shallow bay once to get down to the bottom wrap and fix what the shoreside 'experts' had done with the gearing and first lay of the cable. Not a fun job on a 6500 tonne ship...
Does anyone have an old baitcasting reel to take apart and post pictures to study?
I don't have one of those, but I'm reminded of the inside of a yankee screwdriver. Maybe that could be a place to start.
Originally Posted by MilGunsmith
I'm pretty sure the Yankee screwdriver and its relatives use two nuts, one for each direction. The ratchet selects which nut propels the screw in which direction. The nut not chosen freewheels in both directions, and the chosen nut ratchets on the return. The screw does not reverse during operation. Since it is not a reversing screw so much as a reversible one, it retains fairly sharp points where the two opposing threads cross.
Note; "pretty sure" above because I have only disassembled a cheap Yankee clone, treasuring my real Yankees far too much to mess with them. I assume the ratchet design is siimilar but can't guarantee it.
Just a funny note; My wife just looked over my shoulder and said"self reversing screw why would someone want that, I just shoveled snow and our deck is full of them and they suck" I'm not sure whether to grab the drill or the snow shovel?
Yes, but it isn't really neccesary. They look the same as what has been posted earlier. They also use the same type of semiarcuate blade type of nut, called pawl in reels. These are available with ceramic coatings to lessen friction.
Originally Posted by MilGunsmith
Abu-Garcia made a lot of these of course, (and still do) but they used a more-or-less dedicated cam-controlled miller for this. I've seen it and have a pic someplace, but I don't think you can see how it works anyway.
Nowadays I imagine it would be done with CNC. Not sure though.
There have been threads about this here before, but I couldn't find them.
This isn't a new thing by the way... These infinite screws have been used in baitcasting reels since the 1800s.
There is an alternative though, that looks like a piece of twisted flat bar where the line rides over top of the flat bar. As the bar rotates, the edges will urge the line alternately to the left or to the right.
CNC is the way to go. But it could be done by other means. I doubt that this part was originally threaded on a lathe, but probably on some specialty equipment for cutting small dia course pitch square threads. This is not a reversing screw, but a square thread, 1.25-2, that I cut on my mill. I thought about doing it on the turning center, but it would have really sucked and took way too much time. I turned the blanks on the manual lathe, programmed, set up, and cut in less than two hours. A reversing screw cut on a CNC mill would not be any harder than this.