Rob G Posted July 26, 2009 Share Posted July 26, 2009 (edited) OK, i've got to turn a screw here. This thing has to be 2-1/4" OD. 4" Lead. Now how do i figure how many starts this screw will have. I know the diametral pitch is 12.5663 and the helix angle is 9.869572 if my calculations are correct. i want it to be 1/4 tpi per thread for the reason that my lathe can cut down to 1 tpi, and i have change gears for it to get it down to 1/4. now where do i go from here. if i'm not mistaken, this thing will be figured out more like a helical gear than a screw. Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
johnptc Posted July 26, 2009 Share Posted July 26, 2009 does it depend on the nut ??? or on its application ?? Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 26, 2009 Author Share Posted July 26, 2009 I am turning the nut as well. Quote Link to comment Share on other sites More sharing options...
johnptc Posted July 26, 2009 Share Posted July 26, 2009 in that case cant you choose the number ?? as long as the two parts fit together ??? Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 26, 2009 Author Share Posted July 26, 2009 (edited) it's got to be figured so that there is no extra lands in between each thread. for example if i were to do just 4 starts there would be large lands between each thread. there is some type of relationship for thread size, involving diametral pitch, pitch, lead, helix angle..... i don't know, tryin to figure it out....... Edited July 26, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
johnptc Posted July 26, 2009 Share Posted July 26, 2009 (edited) ok i get it now let me think about it now i understand the problem these guys http://www.roton.com/Mating_Components.aspx?family=7060295 make a 2 inch od 4 start threaded rod. 2"x1 which gives 4tpi taken together if you went to 2.25 od i think you could cut the threads deeper until the lands just disappear. Edited July 26, 2009 by johnptc Quote Link to comment Share on other sites More sharing options...
John B Posted July 26, 2009 Share Posted July 26, 2009 What thread form are you proposing to adopt? 1/4" pitch gives 4 threads per inch, This sounds more like a sinusoidal curve thing rather than what I understand to be a thread Have you a sketch of what you mean? The nomenclature you define has passed me by here I am afraid, but you have me intrigued. Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) the screw in that link is 1" lead. i need 4" lead. John B, i'm sorry, i meant 1/4 tpi, not 1/4" pitch. i've since edited the post. that is 1/4 tpi per thread. the actual pitch after all multiple starts is not known because the number of starts has not yet been figured. Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) **edited because my calcs were wrong. see latest post.** Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) **edited because my calcs were wrong. see latest post.** Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) another dim light bulb moment, and i think i got it this time. with a pitch of 1, this gives 4" lead with 4 starts as the lead is number of starts times pitch. lathe is set at 1/4 tpi as required this is right, and i'm trying to write an explanation, but it is so hard for me to articulate. for example the first thing i say in my idea above is "with a pitch of 1". i give no citation as to how i came up with this figure and it appears i snagged it out of thin air. however, now that the numbers are written down, they can be run through and proved correct so it really doesn't matter. i suppose as long as you have an understanding it doesn't matter whether or not you can articulate. what makes it so odd to explain is that it was figured using the relationship between the required lead and the required setting the lathe at 1/4 tpi. this is NOT the order that most screws are figured in, but it had to be done this way with this screw because it is so different. while i did figure it out i'm sure there is a better way to understand this......... Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
HWooldridge Posted July 27, 2009 Share Posted July 27, 2009 Four starts will have four separate threads on the rod. A single thread form is one continuous thread with a single start and finish, a two start will have two threads 180 degrees apart, a four start will be 90 degrees apart. Those are the only three configurations that I am aware of which are normally seen on worm gears and similar threads. On an engine lathe, you would cut one complete thread to depth and then index to the next position and cut that one. I work at a production machine shop and we cut multiple start threads all the time - although most of ours are formed with hydraulic rolls so both or all four are done simultaneously. You also might want to check Machinery's Handbook - they have some good info on multiple start threads. Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) your elementary explanations are frustrating as they reveal you don't understand what my problem here was. yes hw i'm aware of what starts are and machinery's handbook. the question is do you even understand what my problem is before you even give me advice. Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
John B Posted July 27, 2009 Share Posted July 27, 2009 Thanks for that, but I am still confused, talking through it may help both ways, unfortunately I am away for the next few days so I cannot get back to you quickly. Back to basics The pitch is determined at 4" (as I understand it ie. 1 revolution of workpiece gives the linear length of travel which is the pitch) The outer diameter is fixed at 2.250" The overal length of the finished item is not specified The depth of the thread may be determined by the type of thread form being used (ie. UNF is 60 degrees as Metric , Whitworth is 55 degrees, acme and square and buttress all having their own angles if memory serves me correctly, and the depth is also governed by the pitch which in turn will affect how many cuts/individual thread starts you will have to accommodate around the periphery They can also be coarse or fine pitch, the effectiveness for the application being in the depth of thread The appearance of this thread you propose to make is going to appear like a wire cable or a single cut that could be used as an oilway around a shaft rather than what I would term a mechanical thread What application is this going to be used for ? (as it is a very long linear movement for a small rotary movement which could mean any pressure on the thread will need a very large turning force required to move a load.) The nearest thing I can allude this to is rifling in a barrel of a firearm, maybe some gunsmiths out there could help. If the thread is going to be used to mechanically move something at the pace it would appear to be with a 4" pitch, I think I would seriously consider an alternative. I hope this helps and does not confuse you more, I look forward to seeing the finished item, (probably not as anxiously as yourself though) Good luck with it, I like a challenge too. Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 Pitch is the distance from a point on one thread to the same point on the next thread. Depth of thread is based on the pitch the solution here is 1" pitch with 4 starts. Quote Link to comment Share on other sites More sharing options...
johnptc Posted July 27, 2009 Share Posted July 27, 2009 the screw in that link is 1" lead. i need 4" lead. John B, i'm sorry, i meant 1/4 tpi, not 1/4" pitch. i've since edited the post. that is 1/4 tpi per thread. the actual pitch after all multiple starts is not known because the number of starts has not yet been figured. ok 4 inch lead very fast........... i have an experienced machinist i will ask ......... Quote Link to comment Share on other sites More sharing options...
Rob G Posted July 27, 2009 Author Share Posted July 27, 2009 (edited) ok i already posted the answer twice yet no one has picked up on it. the answer is the screw is going to be a 4 start w/ a 1" pitch. this answers all the information needed in order to cut the screw and meets the original requirements of having a 4" lead and setting the lathe at 1/4 tpi. Edited July 27, 2009 by Rob G Quote Link to comment Share on other sites More sharing options...
BIGGUNDOCTOR Posted July 29, 2009 Share Posted July 29, 2009 (edited) a 1" pitch will remain a 1' pitch no matter how many threads it has. One revolution will advance the nut 1". To advance the nut 4" per revolution you will need to be able to cut 1/4 TPI. There were some old lathe sub spindle attachments ( speed reduction head ) that allowed very slow chuck revs for stuff like this, but in all honesty in all of my years of going to machine shop auctions, I have never seen one. This may need to be done on a milling machine. OK --I reread your earlier post where you say that you can gear your lathe down to 1/4 TPI. 1/4TPI no matter how many starts will give you 4" of travel per revolution of the nut. All you need to do now is figure out what style thread will give you the best fit when cut to depth. It sounds like you are making a fly press perhaps? Edited July 29, 2009 by BIGGUNDOCTOR Quote Link to comment Share on other sites More sharing options...
HWooldridge Posted July 29, 2009 Share Posted July 29, 2009 your elementary explanations are frustrating as they reveal you don't understand what my problem here was. yes hw i'm aware of what starts are and machinery's handbook. the question is do you even understand what my problem is before you even give me advice. You are right. I don't understand your problem - good luck with your project. Quote Link to comment Share on other sites More sharing options...
jason0012 Posted February 8, 2010 Share Posted February 8, 2010 Are you trying to make a thread that is one turn in 4 x 4 lead? Like a gun barrel only inside out? A lead that extreme would probably be easier on a shaper or mill than a lathe . Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted February 8, 2010 Share Posted February 8, 2010 Pretty special lathe if it can do 1/4 tpi (4 inch lead). On multi start threads the pitch and lead are not the same. Lead is the advance per revolution (in this case 4") and pitch is the distance from one peak to the next. On a 4 start it would be 4" lead divided by 4 start or 1" pitch. A 1" pitch thread would have a 1/2" wide groove and a 1/2" wide land (less clearance) at the the pitch diameter regardless of the thread form. Quote Link to comment Share on other sites More sharing options...
Drq Posted November 20, 2010 Share Posted November 20, 2010 Much easier on a mill. I've tried to turn a couple super high leads on a lathe and its just not worth it. Started with indexable carbide, its broke. Moved to brazed on, it broke. Moved to HSS, it broke..... Because it ends up being such a deep thread the clearance you need on your tooling is just insane. One of the ideas I tossed around was to mount a milling fixture to the lathe with an appropriate end mill in it. This would at least let you cut square threads on the screw....the nut on the other hand ? Drq Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted November 20, 2010 Share Posted November 20, 2010 If I ever find myself in the position that I had to do it, I believe I would forge the nut. I would forge it in two pieces like a swage. Quote Link to comment Share on other sites More sharing options...
Drq Posted February 22, 2011 Share Posted February 22, 2011 Thats a really interesting idea Grant. Never thought of that, but with set of dies in the 160 ton press I have at work that would be easy as pie ! I'm thinking maybe turn the thread, forge the nut, and then throw the male back in the lathe and cut a little clearance on it. Hmmmmmmm........ ' Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted February 22, 2011 Share Posted February 22, 2011 FORGING THE NUT With full machine shop capacity I would do this: Machine a second male part maybe .010 -.015 thousandths big. Then I'd mill that down to half round, heat treat it and mount it to a heavy plate. This would be used to forge into a block of bronze. I would probably capture the bronze in a heavy steel frame or box to contain it. Forge two of these. Then clamp them on the screw and drill holes to bolt then together. Now the outside of the nut can be machined. Think I'd machine it square and slightly tapered and have a socket on the press frame for it to fit into (from the bottom). Now when the press operates it will drive the nut tighter into the socket. Quote Link to comment Share on other sites More sharing options...
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