Cutting Acme threads.

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[FONT=Verdana, Arial, sans-serif]Im cutting a 1/2 - 10 tpi acme thread that will elevate the torch on a cnc table Im building . When I ground the cutter I got the tip width a little wide, closer to the width required for 9 tpi. According to the thread gauge I have. As a result at .055 inch thread depth (rather than .060) the pitch dia came into tolerance by the 3 wire method. Will that be a problem when I try to make the nut. Not sure if I'll make a tap or try to single point the nut.[/FONT]

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If I drill the pilot hole for the female thread 10 thou over will that correct the shallower root on the male thread?

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There is already some clearance given by the difference in minor diameters. The external should run 0.3800/0.3594. The internal should be 0.4050/0.4000. If your minor is within these limits, then you should be fine, even if the root is a little wide. If your PD is in tolerance, and your minor is, then you are good. I would expect to see a problem only if your PD and MD were both on the very high. And even then, the clearance in the design will let you get away with some. I would get a tap only if this were a long nut, and tap it normally. This will give you a normal PD fit. Only if there was an interference with the minor would I then go back and bore it larger. I didn't do the math, but my feelings are that you are good.
 
Thanks told me what I'll probably mess up myself that's why I've held off making a new split nut for my lathe. Standard threads are a piece of cake but I still need to learn making acme threads.

Todd:roflmao:
 
Thanks Tony. At .500 od the .060 thread depth or depth of cut would have given me the ..380 minor dia. At my current .055 depth of cut (measured of the dial on the cross feed) my minor dia would be .390. The book calls for a .410 tap drill so I guess I'll have clearance?

Greg
 
Greg, I think you are getting close to an issue I have with the common practice of going heavily by the thread depth (or height, whatever you want to call it). That is more of a guideline, really. You have hard numbers to go by with both the Major and Minor diameters. You cannot go out of tolerance there and have a truly compliant thread, regardless of what you dialed in on thread depth, or height. If you major, minor, and PD are right, that means that your root width is right. Certainly if your tool has an error in the width, your minor and PD will not be exactly proper in relation to each other, but it doesn't mean you are making an unusable thread....within limits. But to pass a commercial evaluation....maybe not. For Acme/Stub Acme, I have used a finer pitch tool and offset it in Z to compensate for the narrow nose, and still had everything fall in tolerance. Not the same when you have too wide a nose, but if it isn't too bad you may still get away with it.


In other words, I pay little attention to the actual thread height and more to the major, minor and PD. If you major is on the high, you may need to cut deeper per side to get the minor in, and naturally that makes the height greater, and the converse is true. Only if you measure all the elements will you know for sure that the thread complies with standards completely.
 
Thanks again Tony. Decided to single point the nut so I should be able to make the two fit. As they say two out of three ain't bad, at least we got the OD and PD right.

Greg
 
Went with 1/2 inch thick brass plate,this screw sets the torch height, a bit of backlash I don't think will affect the height control. Didn't measure it but seams fairly low.

Greg
 
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