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Wirecutter
09-07-2005, 11:39 AM
I thought a bit before posting, and I even tried a few things, but I'm stuck.

I've been practicing on my lathe a bit more now that I have the head of my Bridgeport taken apart. One of the things I've made is a little pill-jar sized container out of tool steel. I've picked up some good hands-on knowledge of cutting tools, lubricants/coolants, and getting a nice shiny finish. I even got to practice grinding my own stuff to get nice sharp corners in bored holes.
I threaded the tops of a couple of containers to 20 tpi. I had to use more primitive methods in some cases because I don't yet have a proper thread-chasing dial. All that is ok, though - I've managed to cut just one thread and not bung it up too badly. I just go slow and make small feed increments on the compound. (Yes, it's set to 29 degrees, just like the books say.)
I cut the top first, including threads. I make a rough cut of the opening taper of the thread with an indexed lathe tool. Then I cover all the ways and things, and then I use the Dremel with a grinder to clean up the taper. The only area I work in this step is just the very start of the thread, to help get the thread started. Part it off and put it aside, remove covers carefully, vacuum the whole area with the shop vac.
I then make the body of the container, and begin to cut threads. I measure everything a lot using the calipers, and when I get close, I start "test fitting" the top. Prior to a test fit, I make sure that the threads aren't "overcut" or otherwise rough.
Twice now, it's gone from "still just a little to tight to start" to "starts fine, but it's now a bit over, so it's sloppy." The last cuts are .001" at a time, but the slop I end up with feels like more than that.
This is all just an excercise for me, but I'd like to be able to get a nice precise fit. I'm looking for suggestions on how to improve the fit, so let it fly, fellas. Thanks.

Tony
09-07-2005, 11:52 AM
> Twice now, it's gone from "still just a >little to tight to start" to "starts fine, but >it's now a bit over, so it's sloppy."

wire,
its tough to get a good fitup with threads..
testing with the cap/top isn't really a good way to get 'precise' threads, but i know what you mean-- you're looking for a good solid feel when you screw that top on.

depeding on the condition of your tooltip, threadcutting almost always kicks up some burrs. those burrs will make the thread feel tight and you'll want to cut off some more.. but then you'll be cutting out the root too, and the root might be okay.

when it feels "almost right", instead of cutting the next thou off, try hitting it with some light sanding.. or a fine file. clean off the dust and then see how it goes.

-tony

Paul Alciatore
09-07-2005, 12:24 PM
What Tony said about raising a burr is dead on. Depending on the material being cut, threading often raises a burr on the top edges and that will make the thread fit tight or even not start. That's why precision threads are measured with the three wire method. The wires rest on the middle of the flanks, avoiding any burrs, and give a true reading. The burrs need to be rumoved. I generally make a light pass with 200 - 400 grit emery cloth before checking for fit.

Another tool that is absolutely necessary in my shop is a magnifier. It is almost impossible to see defects on a small thread with the naked or even properly corrected eye. I keep several sets of inexpensive magnifiers handy in the shop - 2X, 5X, and 10X. I also have two really good lenses in my pockets at all times, 10X and 20X Hastings Triplets. I have carried these for over 40 years now and couldn't begin to count the times I have used them. The original black finishes are completely worn off to shinny brass. Absolute necessities.

Just an observation/suggestion: I keep a couple of rolls of 1" wide emery cloth by the lathe (180 and 320 grit). It only takes an instant to tear off a small piece and use it to clean up the burrs and tool marks on the work. I apply a few drops of cutting oil to improve the finish and to help control the spread of any abrasive dust. They can be used on a finger tip or "shoe shine cloth" fashion on stationary or rotating work. This is much faster than setting up the Dremel or a tool post grinder and produces a very nice finish. But, of course, it is not as precise as a tool post grinder. Most parts don't require extreme precision. Wipe the oil/grit mixture off of the part immediately.

Paul A.

Harold_V
09-07-2005, 01:15 PM
You've received some excellent suggestions regards your threads. Polishing them with abrasive cloth before fitting is always a good idea, but be sure to clean them well when you do, otherwise the abrasive can be the source of erroneous fitting, or even the cause of galling. I generally wash the thread with an acid brush dipped in solvent, then blow it off with an air hose, though that may not work well for guys with their machines in their basements because of the smell and mess.

Your threading tool. Learn to grind it without any top rake. If you alter the top, it changes how the tool addresses the cut, altering the included angle of the thread. It also prevents a plunge pass for cleaning up the thread, a process that will often improve the surface finish drastically, especially in free machining materials such as 303 stainless.

Be aware, not all lathes are marked such that the compound should be set at 29°. Depending on the orientation of the 0° point, you may have to use the 61° mark instead. To insure you've set yours at the right mark, it's a good idea to set the compound parallel to the cross slide, then turn it 29° if there's any doubt.

Remember, the whole idea behind feeding at the angle is to create the vast majority of the cut on the side of the thread, yet still wiping the back side slightly, assuring good form and finish. This allows for better chip flow, and lowers, considerably, the pressure at the tip of the tool, especially after the first pass. Setting the compound at the wrong angle (greater than 30°)will truncate the thread, plus leave steps on the back side.

One other thing to remember when chasing threads. It's important that the compound feed your threading tool such that the cut loads the carriage against the propelling side of the lead screw. That prevents generating a drunken thread. When chasing an external thread towards the headstock, the handle of the compound should point towards the tailstock. When you switch to the internal thread, if you thread towards the headstock instead of towards the tailstock, the compound should be moved so the handle aims towards the headstock, assuming you keep it on the front side of the lathe. You can accomplish the same thing by placing it on the back side, with the handle then pointing towards the tailstock. It's very important to adhere to this process, especially on small machines, where the carriage has little mass and is easily influenced.

Good luck with your threads.

Harold

Lynn Standish
09-07-2005, 01:20 PM
You might try making some spring cuts also. The burr thing is very true, but if you make another pass at the same infeed setting, and the tool is cutting, it means you had enough deflection to keep the cutter from cutting to the intended depth.

Since there is more deflection further away from the chuck, by the time you get the far end to where it works, you have probably cut more than necessary at the part nearer the chuck.

tattoomike68
09-07-2005, 02:22 PM
I use a small 60 degree 3 sided jewelers file to knock the burrs off and shine up the threads, works nice for threads that are just a pinch too tight but you dont want to make 1 more pass and screw them up.

realy threads are better a pinch loose than too tight.
on tight threads one spec of dirt or a small ding will make them gaul up and be trashed.

Also when I file I like to put the spindle in reverse and use the file backwards and the file tracks away from the chuck,a much safer way to file on the lathe.

[This message has been edited by tattoomike68 (edited 09-07-2005).]

Rustybolt
09-07-2005, 02:25 PM
Sometimes if you up the spindle speed and hit the threads with a wire brush, that helps.

Mcgyver
09-07-2005, 03:45 PM
I'm not sure i followed the part about the dremel and cutting a taper. For a workman like job of it, undercut the ends to the major or minor dia (depending if its internal/external). This creates a start to the thread that won't get dinged if the end of the piece gets dinged. Is this what you were describing?

the picture below (for an angular contact bear leadscrew piece i'm making) shows this undercut on both pieces.

I don't know that I've every used an abrasive on threads i've cut. Its not going improve the accuracy and adds a bunch of cleanup work, both to lathe and work piece. Not that there's anything wrong with it, just don't think you need to.

remember the 29 1/2 degrees is simply to avoid cutting on both sides of the threading tool. The final cut should be at the same compound setting with a the cross feed a tiny bit past zero so you get the true 60 form, not 58 or 59. you can chase (go over it) one more time at this setting and it will sometimes take bit more off as it works any spring in the set up – even on one side it’s a relatively big cut the tool is taking.

Obviousl alignment of the 60 degree tool to the work is very important as is having tools ground to 60 - but you can easily get them very close with hand, eye and a thread guage and possibly a maginify glass/or loupe.

The last few passes are at a thou or so – it’s a decent size width of cut, and needs a corresponding small depth to keep the cutting forces reasonable. With the compound set at 29.5 to cut a 60 degree thread, advance the compound in a total amount equal to 0.750 divided by the threads per inch. I usually leave a little bit of clearance as well, ie the bore a is thou bigger than root dia and OD is a thou less than major dia. Always cut the male first, as you are doing, to use as a gauge on the female. .

with a bit of practice you can turn nice (not to a ground standard obviously) fitting threads without the bother of abrasives. It also helps to use a good material like a free cutting steel

http://i20.photobucket.com/albums/b201/michael0100/shotoflatheturnedthreads.jpg

[This message has been edited by Mcgyver (edited 09-07-2005).]

Wirecutter
09-07-2005, 04:42 PM
As always, you all come through in fine fashion. Thanks for all the useful info.

Mcgyver, that is almost exactly the mirror image of what I'm doing. I was cutting the female side first, though. That's the kind of end result I'm trying to get - nice work.

I think my problem is probably burrs. I may have a tiny bit of rake on the top of the cutting tool, but I was using a 60 degree gauge and magnification to grind the cutter. Paul, you are absolutely right about magnification. I've even been able to use the surgical telescopes to get a cutter back in the correct spot after disengaging the power feed. (See previous post - "First Lathe Project") I didn't have to do this on this particular project, though. Also, I don't use them all the time, but I probably need to use them more. I've been using optivisor type magnification.

Harold_V, I'll have to stand in front of the lathe to properly visualize your suggestions about direction of feed and such. I have been doing repeated cuts when finishing up without changing the crossfeed position (other than to stop, back up, and make another pass) It helps, but as I mentioned, it's not quite getting me there.

I'll have to try cutting the male first - which I haven't been doing. Getting a nice finish on the female side is a lot tougher, and that's why I'd been trying to do it first. I have had great results on the finish of tool steel using sucessively higher grits of wet-dry paper, 320-400-600-1200. (with the ways and everything covered up, of course) It may be time to make some kind of tool or finishing device to make this process easier on threads.

So, I'll head back to the lathe and cut some more. When I get good results, I'll report back with photos. Thanks again, all.

-Mark

Evan
09-07-2005, 04:49 PM
I keep handy wire brushes of the type that are about the size of tooth brushes. I have brass and SS ones. The brass ones are good for a quick swipe on aluminum to knock off any slight burrs and the SS are good for steel.

http://vts.bc.ca/pics/hold1.jpg

Wirecutter
09-07-2005, 06:29 PM
<font face="Verdana, Arial" size="2">Originally posted by Evan:

http://vts.bc.ca/pics/hold1.jpg </font>

Nice work there, Evan. Now you're just rubbing it in, aren't you? http://bbs.homeshopmachinist.net//biggrin.gif

IOWOLF
09-07-2005, 06:50 PM
I did this about 5 years ago....

http://img.photobucket.com/albums/v11/iowolf/can.jpg

------------------
The tame Wolf !

Evan
09-07-2005, 06:55 PM
I did this about 23 years ago:

http://vts.bc.ca/pics/work1.jpg

IOWOLF
09-07-2005, 06:58 PM
yea, But did the nut fit?

------------------
The tame Wolf !

Evan
09-07-2005, 07:05 PM
Yep, 6-32. First thing I ever made on my SB9, as a test.

crews1
09-07-2005, 07:37 PM
Regarding the taper, I always use the threading tool itself (no matter whether internal or external thread) to cut lead-in and cut-out angles.
I also use a threading tool to cut chamfers if it's close to the chuck, to de-burr grooves, I've even used an external threading tool to turn with.

Your Old Dog
09-07-2005, 07:42 PM
Interesting thread Wirecutter. Thanks for the post.

How many guys were sucked into dragging out the calculator on Evans post? http://bbs.homeshopmachinist.net//biggrin.gif And Evan, if that 6-32 is the first project you ever did on the SB9 then I should just push all my stuff over to one side of the shop and go back to working wood! I ain't getting very far very fast.

Evan
09-07-2005, 08:47 PM
I didn't say it was the first time I used a lathe, just my own lathe. http://bbs.homeshopmachinist.net//biggrin.gif

Harold_V
09-08-2005, 12:57 AM
<font face="Verdana, Arial" size="2">Harold_V, I'll have to stand in front of the lathe to properly visualize your suggestions about direction of feed and such. I have been doing repeated cuts when finishing up without changing the crossfeed position (other than to stop, back up, and make another pass) It helps, but as I mentioned, it's not quite getting me there.</font>

If by repeated cuts you're implying you go from external to internal threads, you're asking for trouble. It's not real obvious, and it doesn't happen every time, but take a look at it this way. When you feed a cutting tool towards the headstock, it cuts with resistance (the pressure of the cut), pushing the carriage back towards the tailstock. The pressure of the cut assures that the carriage can't move away from the screw.

When you chase in internal thread with your compound set with the handle towards the tailstock, aimed at the front of the lathe, not the rear, when you feed the tool for another pass, it advances towards the tailstock instead of the headstock. The cutting pressure then can move the carriage ahead of the screw because it's pushing away from the screw instead of into the screw. That is often the source of drunken threads, and also threads that aren't uniform in size, or so it appears. Because the tool can wander back and forth between the flanks of the thread, the thread won't be uniform in lead, or drunken. Although the pitch diameter may measure properly, the mating part often won't fit due to lead error.

I'd like to invite you to peruse this link:

http://www.chaski.com/ubb/showthreaded.php?at=&Board=gendiscussion&Number=22225&page=&view=&sb=5&o=

-----where this very subject was discussed at great length---after Jacin had generated drunken threads unknowingly. If you have time, and would like to be enlightened, try to read the entire thread. It's very beneficial for those that are in the learning phase of chasing threads.


<font face="Verdana, Arial" size="2">I'll have to try cutting the male first - which I haven't been doing. Getting a nice finish on the female side is a lot tougher, and that's why I'd been trying to do it first. I have had great results on the finish of tool steel using sucessively higher grits of wet-dry paper, 320-400-600-1200. (with the ways and everything covered up, of course) It may be time to make some kind of tool or finishing device to make this process easier on threads.</font>

It's generally a good idea to chase the external thread first, using wires to assure the proper pitch diameter. Once properly sized, you can use your male thread as a gauge for the female thread. That's not the best way to go, but who amongst us can afford proper plug gages for threads? Generating the female thread first leaves you with no way of measuring pitch diameter, and I don't recommend threads be cut by the formula. The error that can be expected by that method is typically greater than the tolerance of a pitch diameter.

Once you've chased a proper thread, it's not hard to improve the surface finish, assuming you have a sharp tool. By not altering the top of the tool, which should have 0 rake, you can take a few light passes (.0005" to .001") by plunging with the cross feed (not the compound). With the cut well lubricated, and the compound set properly, so the lead is correct, the tool will shave both faces of the thread evenly, improving the surface finish considerably. This doesn't work all that well with cold rolled or hot rolled steel, but works miracles on materials such as 303 stainless, brass and aluminum.

Wires are not just for inspection, they are the chief method of measuring threads while generating them on a lathe. A pitch mic or threading triangles can also be used. Once mastered, wires are very easy to use, and not expensive unless you purchase the calibrated individual wire sets. They're a very nice addition to any shop, but the inexpensive wire sets offered by supply houses are more than adequate for the home shop, and will elevate your work to a professional level. They can be purchased for less than $15, and will cover a range of threads generally beyond that needed by the home shop.

Threads are typically close tolerance in nature and shouldn't be cut randomly if you desire to have any semblance of interchangeability and reliability. Again, I highly recommend wires.

Harold

Norman Atkinson
09-08-2005, 05:13 AM
Again, this is a fascinating insight into screwcutting. It is the sort of thing that should be printed out- and refered to as important stuff. Not like my blether!!!

We limeys may have 55 1/2 threads as opposed to 60's but the problems are much the same. I have a soft spot for Martin Cleeve who published Screwcutting in the Lathe shortly before his death.
Cleeve- or Kenneth C Hart- lost his job and with only a 7" swing Myford make a substantial living producing special nuts and bolts on it.

For those who want a good read, may I commend it?
At this stage, I am trying to put the rest of his contributions into some one place- before I pop off as well.

Norman

Wirecutter
09-08-2005, 12:00 PM
Well, while I haven't fixed the problem, I've identified it.

Put my threads under a stereo microscope at work. The male side threads are fine, with perhaps just a hint of a burr at the peak of the thread. The female side is another story.

Under high magnification, it looks like cheddar cheese cut with a dull knife. It looks like the metal was "torn" rather than "cut". This makes sense when I think about it - I had a bitch of a time grinding the tool for the internal threads. I believe what happened is exactly what Harold_V warned about - I don't think I had 0 rake on the cutter.

I think I understand what Harold was saying about avoiding "drunken" threads. I want to make sure that play in the lathe carriage feed doesn't allow the "tail to wag the dog", or have the travel of the cutter in the threads to drag the carriage around. I don't think I've had this problem, and I'll try to explain how I think I avoided it.

I cut threads with the carriage feeding toward the headstock. In order to avoid crashing the cutter into an area I don't want threaded, I set the carriage stop. But wait, there's more. I run the machine very slowly, and I take up the slack in the carriage feed by manually "preloading" the carriage feed wheel toward the direction of feed. So the feed mechanism is in the mode of keeping the feed speed down, know what I mean? This allows me to stop the lathe, back out the cutter, and restart, and I always cut the same thread. The backlash in the carriage feed tells me when to turn off the motor - when the feedwheel stops (as the carriage contacts the stop), I can stop the motor before the feed mechanism binds into the stop. (IOWOLF, you should be laughing by now...) Yeah, I know it's an odd way to do it, and it might not work on a tighter machine. But it does work for me. (As I said, the problem is not with "cross threading", so to speak, it's with the quality of the cut)

So I think I've solved the mystery, and I'll go back and solve the orignal problem when I'm back in the shop. Thanks to all of you for the excellent information. I'm going to keep this for reference.

-Mark

Harold_V
09-08-2005, 01:39 PM
<font face="Verdana, Arial" size="2">Under high magnification, it looks like cheddar cheese cut with a dull knife. It looks like the metal was "torn" rather than "cut". This makes sense when I think about it - I had a bitch of a time grinding the tool for the internal threads. I believe what happened is exactly what Harold_V warned about - I don't think I had 0 rake on the cutter. </font>

From your description, I'd suggest your tool is improperly ground, and dragging on the thread. When chasing threads, one must account for the helix angle of the given thread, which necessitates extreme clearance on one side of the threading tool, depending on the direction of feed. Chasing internal threads further complicates the problem because of the tight radius of the bore. Often, the clearance ground is totally unreasonable at a glance, but very necessary if you expect your tool to cut. I'd suggest you look carefully at your threading tool for signs of tool drag on the relief.


<font face="Verdana, Arial" size="2">I cut threads with the carriage feeding toward the headstock. In order to avoid crashing the cutter into an area I don't want threaded, I set the carriage stop. But wait, there's more. I run the machine very slowly, and I take up the slack in the carriage feed by manually "preloading" the carriage feed wheel toward the direction of feed. So the feed mechanism is in the mode of keeping the feed speed down, know what I mean? </font>

That's a creative way to accomplish your mission, but I highly recommend against it. In a sense, you're climbing through the window to enter your residence when you have a perfectly good doorway that you aren't using. Further, If you've ever seen what happens to a lathe when you jam the lead screw, you'd quickly come to terms with my statement. Not only are you risking destruction of your lathe, you're also cheating yourself of learning proper threading procedures and enhancing your skills. . It's easy enough to pull out on a thread if you start out running slowly until your skill level improves. A simple pencil line on the threading tool is a good reference point at which you'd do so, or even a long travel dial indicator set on the ways.

There are other methods to ascertain a good thread without risking the machine. If you have the capability to run in reverse, you can thread on the back side, from the inside to the outside, where you can release the half nuts randomly. You can also run with your tool upside down and chase them on the front side. The options are almost endless-----if you have reverse.

Back to your threading method, you have avoided a drunken thread by loading the carriage, but the slightest irregularity in pressure would likely yield a drunken thread, especially if your tool is dragging and tearing instead of cutting cleanly. A couple simple changes will improve your results drastically.

Good luck!

Harold

Harold_V
09-08-2005, 01:46 PM
<font face="Verdana, Arial" size="2">We limeys may have 55 1/2 threads as opposed to 60's but the problems are much the same.</font>

Indeed they are. Thread wires for 55½° threads are readily available, and I highly recommend them. You'll be amazed at the improvement in your threads, and very grateful for that day when you must chase a thread without other means of knowing when it is to size.

Harold

lynnl
09-08-2005, 02:02 PM
Yeah, I think Harold hit the problem ...the tool tip doesn't have enough clearance to avoid dragging.

I always use a dial travel indicator to show me when to release the halfnuts. For both external and internal threads. And if it's real tight and I can't afford a landing groove, turn the lathe off and turn the last 2 or 3 revolutions by hand to avoid a crash.

Wirecutter
09-08-2005, 03:55 PM
<font face="Verdana, Arial" size="2">Originally posted by Harold_V:
That's a creative way to accomplish your mission, but I highly recommend against it. In a sense, you're climbing through the window to enter your residence when you have a perfectly good doorway that you aren't using. Further, If you've ever seen what happens to a lathe when you jam the lead screw, you'd quickly come to terms with my statement.

There are other methods to ascertain a good thread without risking the machine. If you have the capability to run in reverse, you can thread on the back side, from the inside to the outside, where you can release the half nuts randomly.

Harold</font>

The risk to the machine is the reason I go really slow. Also, this is the venerable South Bend 9, with a (somewhat loose) leather belt. (I have caused the belt to slip off a time or two) I only sheepishly admitted my technique for stopping the machine, because although it's ah, creative, I'm aware that there are better ways. I don't have a carriage micrometer or a threading dial. (Those are hen's teeth, they are.) I'm trying, though.

I tried using reverse when I made my surgical telescopes, but after having the collet chuck loosen on me (unscrewed a little) and ruin a part, I stopped doing that. I actually much prefer threading with the feed going away from the headstock (and a crash). It was just so disappointing to lose a part that way. (My first project, remember...)

Now the rake of the cutter to compensate for the helix of the thread - that's something I didn't account for, either. Ooops.

So anyway, the "to do" list now includes:
1. get some kind of carriage micrometer-stop (not so hard)
2. get a threading dial (moderate difficulty)
3. practice grinding tools a bit more, taking into account the tight bore of the workpiece and the helix of the thread.

Again, I thank you for the help with this. I want to make good stuff - I don't think I'll ever be as productive as ibewgypsie, but I can improve. Right now I take my time and ask a lot of what sometimes seem like silly questions. I hope others can benefit from the discussion - I know I will. Cheers.

-Mark

IOWOLF
09-08-2005, 05:38 PM
Like I was told once in high school shop class.
What did you cut those threads with a chainsaw?
Oh, and your expected chuckle is not warrented, I'll let you know when it is. http://bbs.homeshopmachinist.net//wink.gif
------------------
The tame Wolf !

[This message has been edited by IOWOLF (edited 09-08-2005).]

Wirecutter
09-08-2005, 10:08 PM
I'll certainly try harder next time.

BillH
09-08-2005, 10:23 PM
Evan, 6-32 nuts and bolts are pretty cheap you know... http://bbs.homeshopmachinist.net//smile.gif

[This message has been edited by BillH (edited 09-08-2005).]

Your Old Dog
09-10-2005, 07:31 AM
<font face="Verdana, Arial" size="2">Originally posted by BillH:
Evan, 6-32 nuts and bolts are pretty cheap you know... http://bbs.homeshopmachinist.net//smile.gif</font>

Bill, we're talking about a guy who needed to make his own road grader to get out of his driveway. I can only assume by that, that it's a long ride into town? http://bbs.homeshopmachinist.net//biggrin.gif

This is a threading related question. On the SB9A with gearbox and cross feeds, does it hurt to actually use the micrometer carriage stop while cutting threads until you can flip the halfnut? I thought the micrometer carriage stop was for working up to it by hand and not in autofeed engaged?

Wirecutter, I got both a micrometer carriage stop and a thread dial indicator of a small sb10 that happened to fit my sb9a. I answered a newspaper ad about old lathe for 200.00. I offered them $50 for the two and had the tools with me to remove them. This was an estate type sale and the equipment had virtually no value to them as it hadn't been used in 15 years. (that's how you'll all find my "stuff" someday !)



[This message has been edited by Your Old Dog (edited 09-10-2005).]

Your Old Dog
09-10-2005, 07:49 AM
[QUOTE]Originally posted by Your Old Dog:
[B] Bill, we're talking about a guy who needed to make his own road grader to get out of his driveway. I can only assume by that, that it's a long ride into town? http://bbs.homeshopmachinist.net//biggrin.gif

This is a threading related question. On the SB9A with gearbox and cross feeds, does it hurt to actually use the micrometer carriage stop while cutting threads until you can flip the halfnut? I thought the micrometer carriage stop was for working up to it by hand and not in autofeed engaged?

Wirecutter, I got both a micrometer carriage stop and a thread dial indicator off a small sb10 that happened to fit my sb9a. I answered a newspaper ad about old lathe for 200.00. I offered them $50 for the two and had the tools with me to remove them. It also had a taper attachment but they wouldn't let me remove it if it rendered the lather non-operational. By the time I got back (3 hours later) with my cross slide it was gone! This was an estate type sale and the equipment had virtually no value to them as it hadn't been used in 15 years. (that's how you'll all find my "stuff" someday !)

Harold_V
09-10-2005, 01:07 PM
<font face="Verdana, Arial" size="2"> On the SB9A with gearbox and cross feeds, does it hurt to actually use the micrometer carriage stop while cutting threads until you can flip the halfnut? I thought the micrometer carriage stop was for working up to it by hand and not in autofeed engaged?</font>

The risk of doing serious damage to the feed train makes that a very poor practice. You're right----rigid stops of any kind should never be used when engaging half nuts. Typically, the moment the half nuts are loaded, they won't release. If you're lucky, the gear train will be forced out of position and disengage. If you're not, the weakest link will break. What it does to the balance of the gear train and mechanical fastening points is hard to say, but the damage can be extreme.

He has been successful in using that method only because of the slow speed, and the fact that he has loaded the screw in the wrong direction, permitting him a narrow window of opportunity to stop the machine before the nut is loaded on the opposite face. Note that he is also not disengaging the half nuts because he lacks the threading dial.

The method employed creates other problems, especially if the thread being generated is expected to be useful right to the end, such as threading up to a shoulder, where no relief is permitted. Once the carriage makes contact with the stop, the lead changes, needless to say. One generally cuts a thread relief with a radius in the corners (that's not always an option if you're working to prints), or pulls out as the shoulder is approached.

Harold

Evan
09-10-2005, 01:11 PM
I put a small supermagnet on the front way as an indicator of when to stop for internal threading. If I bump it it simply slides. I then measure to the micrometer carriage stop an inch away to put it back where it belongs.