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ikdor
07-18-2016, 04:30 PM
I was single pointing this M12x0.5 thread, but it binds severerly at regular intervals.
http://i964.photobucket.com/albums/ae124/ikdor/IMG_2677.jpg
The pitch of the binding seems to correspond to the pitch of my lead screw.

I made another thread on a test piece and here you can see banding as well, every sixth thread is narrower.
http://i964.photobucket.com/albums/ae124/ikdor/IMG_0116.jpghttp://i964.photobucket.com/albums/ae124/ikdor/IMG_2675zoom.jpg

I have a cheap 9x20 chinese lathe, so I don't expect toolroom results, but botching a single point thread like this seems a bit too crude.

Has anyone come across this problem before? Is it perhaps an eccentric gear? Or a sloppy half nut?

Igor

Boostinjdm
07-18-2016, 04:48 PM
That would be about where the handle flops over from a sloppy rack and pinion. Try riding the hand wheel to keep it from flopping.

Yondering
07-18-2016, 05:04 PM
This doesn't address the root cause, but one thing you can do in some cases is cut the threads slightly tight, and lap the male/female together with fine valve lapping compound. This makes a nice smooth fit and will take care of the fit issue you're having.

You'd have to do the math with your gear ratios, but it's likely your thread pitch isn't exactly correct; it may have some small error that's just enough to bind up long threads.

Or what Boostinjdm said; that sounds pretty plausible if your carriage moves easily. If that's the case, along with riding the handle, you could tighten the gibs a little to take excess slop out of the carriage and give it just a little bit of drag.

Juergenwt
07-18-2016, 05:55 PM
I was single pointing this M12x0.5 thread, but it binds severerly at regular intervals.
http://i964.photobucket.com/albums/ae124/ikdor/IMG_2677.jpg
The pitch of the binding seems to correspond to the pitch of my lead screw.

I made another thread on a test piece and here you can see banding as well, every sixth thread is narrower.
http://i964.photobucket.com/albums/ae124/ikdor/IMG_0116.jpghttp://i964.photobucket.com/albums/ae124/ikdor/IMG_2675zoom.jpg

I have a cheap 9x20 chinese lathe, so I don't expect toolroom results, but botching a single point thread like this seems a bit too crude.

Has anyone come across this problem before? Is it perhaps an eccentric gear? Or a sloppy half nut?

Igor

Post a picture of your compound settings, incl. the tool holder and tool bit. This does not look like a thread. I don't think it is your lathe.

H380
07-18-2016, 06:07 PM
Try engaging the half nut on the SAME line on every cutting pass. The thread dial may be off. It looks like one or more passes are splitting your threads. Make sure you do not have chips in your half nut or on your leadscrew.

J Tiers
07-18-2016, 06:08 PM
If it is corresponding with the leadscrew threas, it is a "once per turn" problem. So, once pe turn, the leadscrew presumably binds or pulls on the carriage in such a way as to slightly move the tool away from the work.

Check the BACKSIDE gib, look to see if the leadscrew is bent. Your 3 mm (6x 0.5mm) is close to 1/8", so an 8 TPI leadscrew would be about right

ikdor
07-18-2016, 06:22 PM
The last image was not a full depth thread, so it looks worse.
I didn't disengage the half nut, but that probably didn't matter much as your can't miss a 0.5mm pitch with a 3mm leadscrew.
The leadscrew is protected by a springy thing, so no crap going in.

http://i964.photobucket.com/albums/ae124/ikdor/IMG_0123.jpg

http://i964.photobucket.com/albums/ae124/ikdor/IMG_0125.jpg

http://i964.photobucket.com/albums/ae124/ikdor/IMG_0124.jpg

It should be something with the leadscrew. What if the half nut was riding sloppily on the leadscrew, perhaps slanted? Then once the last nut thread let's go, somewhere else on the screw it has to pick up and perhaps with some slop.
I just tried to measure the progress with the dro, but I need a better chuck indexing method to do that properly.
The machine can not be worn, I'm the first owner and I don't use it enough.

Tony Ennis
07-18-2016, 07:18 PM
Turn the spindle by hand and see if you can feel when it binds up. If you can feel it, disengage the lead screw and turn it by hand again. Did it bind? Disengage the spindle from the leadscrew, engage the leadscrew, and turn the leadscrew by hand. Does it bind?

olcop
07-18-2016, 09:40 PM
I agree with the other poster that dosen't look like a thread----also that the tool you are using has a very unusual shape------also, have you insured that the compound is at a
29-1/2o setting in the dial indicator, and have you insured that the tool is presented to the work properly, and used a gauge to set it?
but, if this aint your first rodeo, please excuse the advice, you already know these things and have done them already.
olcop

J Tiers
07-18-2016, 10:52 PM
Tool is fine, if the angle is right. Ground to get up closer to a flange, etc.

danlb
07-18-2016, 11:38 PM
Several things came to mind. First was that a gear in the changegear section is binding, causing the drive belt to slip for a moment, but that would cause you to lose registration and the next threads would be off.

Is it possible that you are not using the thread dial (just putting it in reverse) AND are not retracting the tool bit at the end of the pass? That can cause funny flaws as the tool is drug through the thread in reverse while the slack in the leadscrew/halfnuts is taken up.

It's always hard to get a good picture of threads, but those don't look right and even look like you have double threads in there at some spots. If I did not know better, I'd say that you overlaid an 8 tpi thread with a 9 tpi.

Dan

CalM
07-18-2016, 11:57 PM
make a single pass , about .030, and show the result.

there is something fishy about that tool, and those threads.

PStechPaul
07-19-2016, 12:04 AM
It would help to se the side profile of the tool. It may be an issue of rake angle and relief.

http://firearmsdesigner.com/?p=52

http://firearmsdesigner.com/wp-content/uploads/2011/03/frontClearance.jpg

http://firearmsdesigner.com/wp-content/uploads/2011/03/08-squaring-cutter.jpg

J Tiers
07-19-2016, 12:46 AM
Looking at the pictures closely, I think the impression of double threads may be caused by a rather heavy burr thrown up on one side of the thread, as if the cutter was not really cutting well on that side.

The thread is rather fine, being 0.020 approx pitch (0.5mm) and corresponding to nearly 50 tpi, so the burr in question is actually only perhaps 0.15mm tall. The entire part, despite it's large appearance in the pictures is only a half inch diameter.

Since the OP is the first owner of the machine, it might not be worn, but it might easily be maladjusted, one or more gibs loose, spindle not properly preloaded, or anything along that line. There are actually a number of things that might cause such a problem. It is true that many of them are associated with a worn machine, but some are not. And even a new machine might have a slightly bent leadscrew, from some shipping or unloading mishap.

I think you should start by looking at the leadscrew, see if it appears to be bent. Then, do what Tony Ennis suggested in post 8.

Juergenwt
07-19-2016, 12:54 AM
Your setup looks ok. Tool looks ok. Do you have enough side clearance on each flank? Looks like something is pushing the material in the cut like a big burr. Tool at the center-line? Cutting oil?

J Tiers
07-19-2016, 01:16 AM
As for that burr.... It is a small thread, and may not be worth, even on a 9 x 20, fiddling about with the compound angle etc, etc. I'd suggest trying a straight on advance of the tool, if you have not already. That may help with the burr, might get half as much on each side.

Is the tool sharp?

How big a cut is being taken at a time? Meaning how much is compound or crosslide advanced for each pass?

dave_r
07-19-2016, 02:16 AM
Having recently learned to do threads on my lathe, that does not seem like a DFU error (like reversing and not taking out the backlash). I would look at the leadscrew (what drives the carriage forwards or backwards), and see if it rotates smoothly with the half-nut engaged. Maybe see that the threads are clean and maybe put your hand on it to feel for any odd vibrations or changes in speed (while making sure the carriage doesn't mash into something, including your hand).

Also look at the change gears (if they are accessible) to see if they are rotating smoothly with the half-nut engaged.

My lathe was semi-randomly wrecking the threads by having a sheared shear-pin on the leadscrew shaft. It still had enough friction, most of the time, to drive the lead screw properly, but every now and then, if I did something too fast, the leadscrew would either shift sideways or not rotate, and then the carriage is out of alignment and all the threads are wrecked.

But just shifting a bit every 3rd thread, regularly, really points to some part of the machine not rotating smoothly or even a change gear missing a tooth...maybe the gears aren't adjusted properly, so when one gear goes all the way around, there is just enough space for it to skip a tooth and then catch the next one?

ikdor
07-19-2016, 04:01 AM
Thanks for your thoughts guys. I think I have two problems; the irregular thread pitch and the tool ploughing too much. This closeup of my test piece is a bit better:

http://i964.photobucket.com/albums/ae124/ikdor/fcff5de5-9cce-4f11-88ea-5a82148ceede.jpg

There is more burr than I would like, so I think that if I want to get a nicer cut I need to add some top rake somehow and hone the edge. The pattern is also more clear here; the red ones are too narrow and the green ones too wide.

On earlier comments:
The thread on the final part looks double cut as I forced it through the mating part. I had to cut all the threads until only three remained on the mating part and still it binds severely, but a fat screwdriver to the top forced the double cut.
The tool could be sharper, this is straight from the shop. I took three cuts on the original piece: 0.15 0.25 and 0.35 from an OD of 11.9. I think the test piece was 0.15 and 0.25 only.
Tool was at the centre line, I used aluminium cutting oil, speed was dead slow. Perhaps too slow as it got irregular, but I wouldn't expect that could affect the gearing ratio. The part is a bit small and me too unskilled to run at high speed.
The tool has regular side rake, and with this small pitch it will never touch the thread beyond the tip.
I left the half nut engaged, but did retract the tool for the return journey. With all the backlash it would strip the thread right away otherwise.
Material is 28ST, SAE 2011.

Tonight I'll try the zero degree infeed and have a look at the gears and leadscrew. Gotta work now.

Richard P Wilson
07-19-2016, 04:03 AM
I think I'd check for end float in the leadscrew. If, as the OP says, its got those spring protectors on it, any play here might not be as obvious as on a lathe with a 'naked' screw. I also don't share his optimism that as its got protectors, no swarf or dirt can have got into the leadscrew/half nuts.
It might be to do with lack of flank clearance on the tool, but with such a fine thread this is unlikely, I think. It is a horrible looking thread, though, and definite signs of double threading .

boslab
07-19-2016, 04:40 AM
First off try a different peice of bar, they are often irregular as they get made from crap on average, I had to do hardness etc on rod samples, there was a spiral hard band round the rod corresponding to the pitch of the rod mills finish rollers, worth a try anyway before going medieval on your carridge and leadscrew
Mark

Tony Ennis
07-21-2016, 07:44 AM
Is there an update to this, ikdor?

gzig5
07-21-2016, 08:49 AM
First step is to try some free machining steel or 6000/7000 series aluminum and cut the same thread. Some material will not cut smoothly no matter how good the machine or setup. That pic shows the tool is more of a thread former than a thread cutter. If the tool is sharp you shouldn't need any positive rake, though it may help with poor turning material. To take the compound setting out of the picture, just plunge straight in with the cross slide. That shallow thread should cause no problems with that. Make sure you have enough front and side rake on the tool and hone it as well as you can. It looks pretty sharp in the pic but we can't see how much rake there is.

Also, it looks like you are cutting that material at full diameter as drawn. I bet there is a skin that is giving you problems. Turn the diameter down at least one millimeter and see how the thread forms on "virgin" material without that skin. It won't be the diameter you need but may tell you where the problem lies.

ikdor
07-21-2016, 10:37 AM
I tried again yesterday while pushing the carriage strongly to the right to take out any play, slop or similar kind of effects. Unfortunately the result is pretty much the same, I took a picture of the thread again and it was pretty similar to what I posted above.
I don't think there's something with the material, I need to take a truing cut on the material before starting as my three jaw chuck has so much runout I can't use round bar directly.... The 12mm thread came out of a 30mm bar anyway.
The effect is repeating with the lead screw pitch, so I'll check for bending and gear engagement next.

Sorry this is taking a while, but between my day job and the family it's hard to find the time.

Igor

Illinoyance
07-21-2016, 11:14 AM
Your close up of the thread makes me think you have inadequate clearance on your tool. If the threads were torn I would suggest adding back rake to the tool. As to the irregular pitch I would look at the leadscrew to see if it is bent. I would also look at the gear train to see if there might be a chip lodged in one of the gears.

It looks like two different problems. Solve one first, then tackle the other. I would start with the tool clearance angles. That is easiest to fix. Also check that your tool is at the proper height.

Peter.
07-21-2016, 05:38 PM
If your lathe is the type that has a leadscrew with a full-length groove to drive the feed gears, I would check that screw for burring of the slot in case it's causing the half-nuts to cam-out a bit each turn.

rohart
07-21-2016, 05:45 PM
The suggestion that you should try cutting the same thred in some free cutting leaded steel, or some aluminium, is quite right. That's what I would do.

Your thread is what I sometimes get when I'm threading some piece of nasty old pipe. Then, I'll get the file out and cut the burrs down way before I get to depth. Finally I'll run a triangular file through the threads, and try to get depth on a thin finishing pass with the tool. Awkward if I don't have a portable female thread to test it against.

Asks quietly ... [{(Now, don't take this the wrong way, but are you sure the lathe is spinning the right way ?)}]. Hmm... Did anyone speak ?

J Tiers
07-21-2016, 07:30 PM
Calling that sort of thing plowing is an insult to a decent moldboard plow. May be it is a snowplow you mean...... that would be closer to what most call "plowing". But, the usual threading tool IS flat-topped, so don;t get hung up on that. It's actually rather complex to make it any other way.

However, the problem may NOT be reliefs, or it MAY be. With that small a diameter, you could pretty much have a straight front, and it would not rub. A picture from the bottom of the tool, AND a picture of the tool from the end of the work, with the tip of the tool against the work, would be instructive. Close-ups, not the tool in an acre of background.

Far more likely, if rubbing is the issue, that the tool height relative to the work is the cause. Too high and it rubs. Just a hair too high and it partly cuts and partly rubs, "forming" the groove rather than just cutting.

So far, we have all sorts of info with the exception of what is needed. So let's see the pics.... they will make it clearer what is going on.

ikdor
07-22-2016, 01:46 PM
Aaaand back again. I tried it in brass with the same tool and a straight feed:

http://i964.photobucket.com/albums/ae124/ikdor/IMG_0159.jpg

And the brass likes the plowing of fine threads much better, the irregularity is perhaps even more clear now:

http://i964.photobucket.com/albums/ae124/ikdor/IMG_2682%20crop.jpg

Then measured the runout of the leadscrew:


http://vid964.photobucket.com/albums/ae124/ikdor/IMG_0161.mp4

Which came to 0.17mm runout. Combined with the flank angle of the trapezium thread this is not enough to mess things up.
Then I measured the possible axial motion in the leadscrew itself:

https://youtu.be/uT9X4sRYlu8

Holy moly!!!! It can move 0.5mm side to side! This needs some serious tightening up or some new bearings.
Bonus points for Richard P Wilson for suggesting the end float in the lead screw.

Thanks for your thoughts guys,
Igor

Illinoyance
07-22-2016, 02:31 PM
The threads still look like they were rolled rather than cut. Its like you were forming them with a tool with no clearance or the tool set too high. Look at the burrs raised on both leading and trailing flanks of the thread.

PStechPaul
07-22-2016, 02:44 PM
At some point I discovered that the nut holding the tail end of my leadscrew was loose, and things were much better after I took care of that.

Using the recommended technique of setting the compound to 29.5 degrees should help reduce or nearly eliminate poor thread form such as this, at least after the first or second cut. Since the cutting occurs mostly on one facet, it applies that pressure on the leadscrew and helps it maintain its longitudinal position and a preload on its thrust bearing.

It may be useful to try using the cutter upside down and running the lathe in reverse, threading from left to right. This will cause the leadscrew to put pressure on the thrust bearing at the headstock. It's possible that one of the thrust bearings is bad.

http://enginuitysystems.com/pix/tools/Threading_Clauson_1623.jpg

http://enginuitysystems.com/pix/tools/Threading_Clauson_1624.jpg

ikdor
07-22-2016, 02:47 PM
The threads still look like they were rolled rather than cut. Its like you were forming them with a tool with no clearance or the tool set too high. Look at the burrs raised on both leading and trailing flanks of the thread.

I realise there is still a microsopic burr, but this is roughly a 50TPI thread and the image is quite blown up. There is nothing to see with the naked eye. Here's a match head for scale:

http://i964.photobucket.com/albums/ae124/ikdor/IMG_2683crop.jpg

J Tiers
07-22-2016, 03:02 PM
The cutter has adequate clearance, as suspected. Top is on-center, as expected.

Interesting that with a straight-in feed, now you have a burr on BOTH sides. That is actually CORRECT. Some of the other photos showed it as being on ONE side, suggesting you were feeding in at the 29.5 deg angle with compound, which is textbook correct method, but is usually only important for larger threads.

Now, this machine is a 9 x 20, which means that one may not assume too much about how it is made.

Back and forth movement of the carriage with half nuts engaged is NOT an issue. I have perhaps 1mm movement, always have had, but I can cut threads well. The secret is to always have pressure on the carriage pushing it forward. That is the reason for advancing the cutter using the compound, it keeps cutting pressure on the front edge of the tool, and thus requires pressure from the leadscrew.

If for any reason the WORK "takes charge" and pushes the cutter, then you will have irregular threads. With such small threads, this SHOULD NOT be a problem, but you never know.

Questions:

Is the carriage well lubricated?

Does it move smoothly? Or is it stiff to move along the bed?

Perhaps a different test is in order. Put the carriage in the position where you have had the problem. place an indicator so that it is reading the carriage movement. This should be a plunger-type indicator, not one with an arm. The indicator should be the one that can read the longest movement distance Engage the half nuts as if you were going to do threading, but with no workpiece. With the indicator point against the carriage, turn the spindle by hand in the normal direction. Check to see that for every half turn (or convenient amount that you can measure accurately) that the carriage advances the SAME distance.

You might also check to be sure that the leadscrew remains steady in one position, only turning, and not moving back and forth in any way.

This can be done if you set up the indicator reading against the END of the leadscrew. Turn the spindle by hand as before, and make sure the leadscrew turns, but does NOT move end-wise.

Mark Rand
07-22-2016, 05:18 PM
Ignore the comments about burrs. Those are quite normal in size for the unfinished threads with a single point tool. Once you've finished cutting to depth, run a smooth file over the rotating part and any remaining burrs will magically disappear. The endplay on the leadscrew with the (minor) bend was probably the entire cause of the problem.

By the time we're too old to use them, we've generally got our machine tools sorted out to the point where they're worth using. :D

ironmonger
07-22-2016, 05:58 PM
Just a thought. .. Do you have any gears in the reduction path that have a 6:1 ratio? It might be something on the large gear that apears every revolution.

J Tiers
07-22-2016, 06:26 PM
Again.... END FLOAT DOES NO HARM. Neither does slop in the half nuts.

All that HAS to happen is the leadscrew etc should take up all the slop, and then stay steady. There must be a fairly "hard" stop to the end float, it cannot be "soft" so position depends on pressure, and it cannot have a "cam action" where it changes position as it rotates.

Your problem MAY be a "cam action", so that once per rev it moves slightly end-wise. This is not "end float", but rather an actual forced movement, due to some factor, a bent washer, a gear that is askew, etc.

CalM
07-22-2016, 10:36 PM
I realise there is still a microsopic burr, but this is roughly a 50TPI thread and the image is quite blown up. There is nothing to see with the naked eye. Here's a match head for scale:

http://i964.photobucket.com/albums/ae124/ikdor/IMG_2683crop.jpg

Burrs are best delt with by using sharp tools of correct geometry, and by deburring.
Nothing wrong with running a smooth file over the pushed up metal. Did you mic the diameter increase due to the burr?

PStechPaul
07-22-2016, 11:25 PM
A close look at the swarf might provide useful information.

Peter.
07-23-2016, 02:33 AM
I realise there is still a microsopic burr, but this is roughly a 50TPI thread and the image is quite blown up. There is nothing to see with the naked eye. Here's a match head for scale:

http://i964.photobucket.com/albums/ae124/ikdor/IMG_2683crop.jpg

There are a lot of chatter marks in that picture. I'd check the stability of the tool post and top slide and check your cutting edge with a loupe to make sure your clearance goes all the way to the top edge.

darryl
07-23-2016, 04:09 AM
You could well have a really poorly made set of half nuts, and or lead screw. It's new but it could easily have come with poorly made parts- some of what I've found on my machine have been poor, even though the basic machine performs well. I would not be surprised to find a drunken lead screw which periodically tightens in the half nuts- once for every revolution. It doesn't necessarily mean binding, but a change in the play between the screw and the half nuts would result in an inconsistent advance of the half nuts.

Here's a test you can do- set up a dial indicator to read carriage play, then watch the needle move as the lead screw pulls it along. Maybe you'll see a variation in the needle and will be able to correlate that with the position of the lead screw as it's rotating.

J Tiers
07-23-2016, 10:22 AM
....

Here's a test you can do- set up a dial indicator to read carriage play, then watch the needle move as the lead screw pulls it along. Maybe you'll see a variation in the needle and will be able to correlate that with the position of the lead screw as it's rotating.

Yes, this cannot be solved or rationally discussed unless there are measurements. Without data it is all guesses, and guessing is not useful. Most of the guesses so far have been shown to be wrong.

I had previously suggested both this and also reading the end of the leadscrew to check for "camming" as it turns. Maybe we will get the results of those tests. Unless he has an indicator with a decently long travel, it may be hard to do the carriage test. It will require at least 6mm or so of plunger movement, preferably more.

ikdor
07-31-2016, 06:39 AM
So.... I took the leadscrew bearings off, cleaned everything up and assembled it properly. Axial play in the leadscrew is now <0.01mm instead of the 0.5mm before.
Then cut a thread in brass:
http://i964.photobucket.com/albums/ae124/ikdor/IMG_2685crop.jpg

Regular as a clock.
So obviously something was causing a variable axial force (the gear on the leadscrew does have slight wobble) but this fix definitively made the system robust against it.
There is even more chatter visible in this picture, but we'll leave that to another time.

I appreciate all your efforts in helping me solve the problem.

Now back to making a lens holder.....
Igor

PStechPaul
07-31-2016, 05:56 PM
Looks good! Did you cut the threads in one pass? A second "spring" cut might smooth out the chatter marks, but they really aren't too bad.

GEP
07-31-2016, 08:29 PM
Looks good! Did you cut the threads in one pass? A second "spring" cut might smooth out the chatter marks, but they really aren't too bad.

Looks like the thread was cut in 1 pass. Look at the chatter marks

J Tiers
08-01-2016, 12:05 AM
It was a tiny thread, about 20 thou deep (half mm pitch). I'd still make 2 passes, but.... That depends on the machine making the cut. IIRC this was a "9 x 20" type, in which case that's not at all bad for one pass.

CalM
08-01-2016, 12:53 AM
Good job! It looks like you have changed the tool bit so as not to cast the offensive burr as shown in your first photos.

Or it is just the nature of brass to "chip" off and machine freely.

think your threading efforts will have more "learning"in store.

ikdor
08-01-2016, 02:53 AM
The tool was the same as before, it's just that the brass reacts much nicer than the aluminium in this setup.
I cut it with two passes at depths of 0.2 and 0.3mm using a straight feed and no lubricant. I wasn't really trying for a good thread form, I was more interested in the pitch variation.
I did learn though that I too really want a chamfer on the end of the blank to gradually start the thread. I knew lots of people do this but I was surprised by the ragged mess at the beginning of the thread you get if you don't.