Modified Square Threads
Modified Square Threads
(OP)
I'm looking for information on 10 degree modified square threads. I need detailed design information - beyond what is in typical sources such as Machinery's Handbook, H28, etc. Such things as clearances at the major and root diameters, tolerances on the lead, etc. Also, applications where this thread is still used. Why is this thread more difficult to manufacture than an ACME? (I am not a machininst.) My application is rotary to linear motion - where the rotating screw drives a gear rack. Anyone know of a similar application, even if ACME threads are used? High strength, non-overhauling and minimized radial loads on the screw are the requirements. Efficiency is not important. Plenty of input torque available. Low speed - about 300 rpm.





RE: Modified Square Threads
RE: Modified Square Threads
RE: Modified Square Threads
I have inherited an existing design, and am not able to make significant changes (i.e. changing the thread form) at this point. During initial grooming of this equipment, this design has had considerable problems with the meshing of the threads (once per revolution clunks, etc.). I'm not sure if this thread was dimensioned properly, but reference material on this thread is extremely limited. I have found the information provided in Machinery's Handbook duplicated in other references, but nothing beyond that.
The screw directly drives the rack. The rack is basically a segment of a nut. Since the nut does not fully encircle the screw, the height of the rack relative to the screw must be established at assembly. At each revolution of the screw, the start and end of the thread are engaging/disengaging the discontinous thread on the rack. This is where the once per revolution clunk originates. Which brings up another question... what should the start of the thread (Higby cut?) look like?
RE: Modified Square Threads
There is a big difference between cutting a square thread and cutting a thread with angled flanks, especially if you are doing it on a lathe with a single point tool. When you cut a conventional thread, you normally arrange to remove nearly all the material on just one side of the thread - you get a much better finish that way. You do this by setting the tool up so that it can be progressively fed in at an angle (29.5 degrees if you are cutting a standard thread, 9.5 if cutting a 10 degree). But with a square thread, you can't do this. You have to finish each side of the thread separately, and use a narrower tool. But the difference between a 10 degree and an acme would not be so great.
As I said, can't help much with the dimensions. You need to reverse engineer them by the sound of it. You may need to try and figure out the intended profile using an optical projector - you can use castable metrology resin to investigate the nut threads. I've never heard of a "Higby" cut (along with many other things !). But I don't see why you should get a major "clunk" if everything is made and set up correctly. However, if you are remaking the external thread component, (which appears to be shorter than the rack), you could easily arrange for a progressive lead-in and lead-out at the ends of the thread simply by chamfering the blank before cutting the threads, (or chamfering the threads themselves if you prefer).
RE: Modified Square Threads
Higbee cuts should have a 45 degree lead-in although I have seen cuts beginning with a milled half radius on some equipment. To prevent your clunking problem use 2 half nuts that fully release at the end of your stroke/cycle much like a lathe lead screw. The single half nut idea your working with is pure nonsense, why reinvent the wheel?
RE: Modified Square Threads
RE: Modified Square Threads
RE: Modified Square Threads