Thread: Threading...Aaarrghh!!!

Straight plunging cuts both flanks at the same time, as opposed to offsetting the compound, the closer you get to half the thread angle (in this case 30 degrees, the more it will tend to cut only on the leading flank). Incidentally the reason 29.5 degrees is often quoted, is that's about as close as you can get to 30 degrees and not risk over cutting the trailing flank. While it's true in theory, in practice, a few degrees less is going to make bugger all difference. Because the thread tool is a form tool, with the compound set around to almost half the thread angle, the tailing flank is just cleaned up by the tool as it goes deeper, and most of the serious cutting is done on the leading flank. It's one of the reasons it often tends to produce cleaner looking threads, especially on less rigid machines.

CNC lathes may look like they're plunging, but in fact they often slightly lead and slightly trail the cut to cause them to cut only on the leading or trailing edge as they do each pass. Because manual lathes are tied to the leadscrew with a strict ratio, we don't have that luxury unless the compound is slightly moved.

With all threading, and especially if plunging, it helps if the depth of cut is reduced as the thread gets deeper instead of just advancing the slide the same xxx amount with each pass.

Pete

Originally Posted by Pete F

With all threading, and especially if plunging, it helps if the depth of cut is reduced as the thread gets deeper instead of just advancing the slide the same xxx amount with each pass.

Agreed - one thing I did not mention. Depending on size, material etc typically I'll start with a DOC of 10 or 20 thou but as the threading progresses reduce that, eventually ending up with passes of 1 or 2 towards the end.

Michael

Originally Posted by Pete F

Straight plunging cuts both flanks at the same time, as opposed to offsetting the compound, the closer you get to half the thread angle (in this case 30 degrees, the more it will tend to cut only on the leading flank). Incidentally the reason 29.5 degrees is often quoted, is that's about as close as you can get to 30 degrees and not risk over cutting the trailing flank. While it's true in theory, in practice, a few degrees less is going to make bugger all difference. Because the thread tool is a form tool, with the compound set around to almost half the thread angle, the tailing flank is just cleaned up by the tool as it goes deeper, and most of the serious cutting is done on the leading flank. It's one of the reasons it often tends to produce cleaner looking threads, especially on less rigid machines.

CNC lathes may look like they're plunging, but in fact they often slightly lead and slightly trail the cut to cause them to cut only on the leading or trailing edge as they do each pass. Because manual lathes are tied to the leadscrew with a strict ratio, we don't have that luxury unless the compound is slightly moved.

With all threading, and especially if plunging, it helps if the depth of cut is reduced as the thread gets deeper instead of just advancing the slide the same xxx amount with each pass.

Pete

Well explained Pete. In one of your previous posts you said "at roughly 25 degrees clockwise, your compound is now set at the correct angle". This would make the other flank roughly 35 degrees when using the compound slide to advance the tool. I say this as I was always told to use the screwcutting gauge to set the tool square to the job as it is a form tool. When setting the compound over I use a vernier protractor or a set square for 30 degrees as this way the trailing edge of the tool just shaves the trailing flank.
If I use the 'plunge cut' method I will plunge the tool (remember there is no offset on the compound, it is parallel to the bed ways) until roughly about half the depth of thread then advance the compound slide (top slide) a bit and take a cut then back a bit and take a cut, repeat a few times then plunge a bit more cutting with the leading edge until the trailing edge is close to the trailing flank. Keep doing this until the cross slide has reached the desired depth of thread. The last cuts I do a light shave of both flanks to clean up any chatter marks and create a bit of clearance.
As with anything it takes a bit of practise. A good knowledge of the geometry is a must

Phil

Originally Posted by simonl

Not sure on that. I think I'm going to disagree but I'll have a think about it. Cheers, Simon

Originally Posted by cba_melbourne

shed, that is not the way it is described in the Hercus book. Unless I understand it wrong. They really mean cutting on both sides, without mentioning the word "plunge". Cutting like that is like turning with a form tool. The larger the tool contact area, the greater the cutting forces, and the greater deflection and chatter problems.

you both are quite correct, my bad.......
I was laying in bed last night after posting thinking about this and realized my stoopid
The worrying thing is that I would have cut my first thread when I was 13 or 14 and have cut so many since then.......

shed

It's all practice and finding out what your lathe can do. Chatter has never been an issue - the v belts slipping is my limitation.

For really big threads I sometimes use a 30 degree cutter to get to the depth required, and then finish off with a 60 degree.

Makes for a bit less strain on the lathe and a clean cut,

I ALWAYS use HSS, and lube on steel (which is what most jobs are).

Speed is as slow as she goes (72 RPM).

I've tried the 30 degree topslide method, but didn't get as good a result in uniformity.

Do whatever works best for you.

Cheers

Rob

Originally Posted by Steamwhisperer

Well explained Pete. In one of your previous posts you said "at roughly 25 degrees clockwise, your compound is now set at the correct angle". This would make the other flank roughly 35 degrees when using the compound slide to advance the tool. I say this as I was always told to use the screwcutting gauge to set the tool square to the job as it is a form tool. When setting the compound over I use a vernier protractor or a set square for 30 degrees as this way the trailing edge of the tool just shaves the trailing flank.
If I use the 'plunge cut' method I will plunge the tool (remember there is no offset on the compound, it is parallel to the bed ways) until roughly about half the depth of thread then advance the compound slide (top slide) a bit and take a cut then back a bit and take a cut, repeat a few times then plunge a bit more cutting with the leading edge until the trailing edge is close to the trailing flank. Keep doing this until the cross slide has reached the desired depth of thread. The last cuts I do a light shave of both flanks to clean up any chatter marks and create a bit of clearance.
As with anything it takes a bit of practise. A good knowledge of the geometry is a must

Phil

Phil, you're confusing the compound angle with the tool bit angle. The compound can be set to any angle, from +/- half the thread angle, as I mentioned before. To recap, we don't normally trail a cut, so don't use negative angles (well I've never seen it done anyway, though it's done in CNC), and the compound angle can be anywhere from zero degrees (ie it's plunging) to 30 degrees (ie the trailing edge of the tool is just skimming down the trailing edge of the flank). The tool itself however is ALWAYS set perpendicular to the work (assuming it's ground symmetrical, but let's not complicate things here). ALWAYS!

I think some guys get themselves in trouble because they try to overthink and overcomplicate all this. As Richard said, we all screwed up threads to begin with, and guess how I recognised the incorrect compound angle instantly when I saw it But try to put all the protractors and BS you've heard aside and just consider you are using a form tool to form a shape on the work, the shape just happens to be a V, or an ACME, or Buttress, or whatever, it doesn't matter. Set it up so the cross slide and top slide are parallel, now just plunge a thread to full depth. Turn it off and do another, but this time rotate your compound around a little. If you do that you'll need to loosen the tool post and square up your tool again. Cut another thread. Loosen it again, rotate it again, square up the tool again. Keep doing that until you get to around 30 degrees and you'll see how the tool behaves with each change of angle. Now keep going and swing it right around until it's at 45 or 60 degrees, again square up the tool and cut a thread like that just to see what a screw up looks like. You'll invest about 30 minutes and 6" of scrap steel in this educational time, but I guarantee it will be the best 30 minutes you've ever spent at your lathe in terms of learning how to cut threads.

Again, don't worry too much about geometry, helix angles or any rubbish like that, it's completely unnecessary and is clearly just confusing many people. With a HSS tool you've ground yourself, helix angle is completely immaterial for regular pitched threads, and the normal side clearance will take care of that. One gottcha is to NOT grind back rake however, leave the tool flat on top. If you change the back rake it will change the V profile (just trust me on that). Changing out a tool also shouldn't be necessary, one tool and the appropriate technique will cut threads to the size any of us here are likely to see.

I use HSS and carbide, and the results are identical. I thread as fast as I'm capable of reacting, just because there's no point in going slow and I have other things to do with my time. I need to get to work but found a photo of an unusual thread I cut a while back I thought I'd put up as it may be of interest. It's a Schaublin thread 19.70 x 1.6667 pitch 45/5 buttress thread, so it doesn't get weirder than that in my book. Trying to work out the gearing for the gearbox and grinding an accurate 45/5 degree tool was a little unusual, however the process to cut it is exactly the same as per a V thread. It was a while back I did this so can't recall the details, but looking at it I would have run down the 5 degree flank. That was just a test piece as I just wanted to test the setup, but it was spot on. That's 1020 steel IIRC, no cleanup, dies, or anything, just straight off the lathe, so the process does indeed work.

Schaublin Thread.jpg

Originally Posted by Pete F

I use HSS and carbide, and the results are identical. I thread as fast as I'm capable of reacting, just because there's no point in going slow and I have other things to do with my time. I need to get to work but found a photo of an unusual thread I cut a while back I thought I'd put up as it may be of interest. It's a Schaublin thread 19.70 x 1.6667 pitch 45/5 buttress thread, so it doesn't get weirder than that in my book. Trying to work out the gearing for the gearbox and grinding an accurate 45/5 degree tool was a little unusual, however the process to cut it is exactly the same as per a V thread. It was a while back I did this so can't recall the details, but looking at it I would have run down the 5 degree flank. That was just a test piece as I just wanted to test the setup, but it was spot on. That's 1020 steel IIRC, no cleanup, dies, or anything, just straight off the lathe, so the process does indeed work.

Schaublin Thread.jpg

You been making Schaublin drawtube adapters ?

Rob

Originally Posted by nearnexus

You been making Schaublin drawtube adapters ?

Rob

That was just a test piece Rob, but I need to make a 1" horizontal arbour for the Aciera and I have a couple of other things I may convert to native Schaublin/Aciera tooling. For example I have one Wohlhaupter UPA-3 with MT3 taper but it has a non-removable shank. Rather than dicking around with it I may sell it as it's a popular taper and instead get another UPA-3 with a removable arbour, and make up a new shank as a W20. It should be an interesting project as there's a few things in that including the differential thread on the boring head. It took me a while to get my head around the differential thread, but once I did I thought it very clever.

Pete

Hi,

Had some time today so i cut another M16x2.0 thread. Much better results, no nut slop and slightly larger lands (I think) and a nicer looking finish.

Did same technique as before, reversing and compound set at 30 or 60 degrees (yep confused there) and smaller cuts at the end. Also did some small plunge cutting at the end as well.

Gonna do a nut tree this week.

Cheers Ben.

Those threads look fine to me Ben, nice job. Yes the Hercus compound is confusing in that regard, it's why I say to turn the compound roughly 25 degrees anti-clockwise from being parallel to the cross slide. On our lathes I think it reads 65 degrees IIRC (my compound is generally just left there so I'll just check later).

If your lathe is metric there's no need to reverse the thing, it takes much longer if you do. Metric is slightly more time consuming to set up on the thread dial, as there are 3 possible positions, but once setup off the chart it's no more difficult than an imperial thread on an imperial lathe to pickup. Oh well, unless you're doing a weird thread like that Shaublin one above, or using transposing gears to cut imperial threads. Just be careful that the thread dial is locked in good and tight, I've had mine loosen and rotate off the leadcrew, with obviously disastrous results. IIRC I saw it do it at least once, and picked up the thread manually, but it still no fun.

Pete

Hi Pete,

I was looking at the thread on my W20 collets the other day, wondering about making up an adapter for the draw tube/spindle.

Coincidence, of the highest order

Not what one would call a common thread profile.

Instead I made up a screw on ER32 collet chuck for the Schaublin spindle, better option, gives feed through which my draw bar asian lathe hasn't got, and a few of my Schaublin collets (all Imperial) have seen better days. W20 is also fairly limited in compression range.

It's great having that old girl set up for ER as it saves swapping over on the Chinese machine. In fact having a smaller second lathe set up for collets is a really good idea IMHO, and a great excuse for more machinery in the shed.

Cheers

Rob

[QUOTE=Pete F;1794254]Again, don't worry too much about geometry, helix angles or any rubbish like that, it's completely unnecessary and is clearly just confusing many people. With a HSS tool you've ground yourself, helix angle is completely immaterial for regular pitched threads, and the normal side clearance will take care of that.the same as per a V thread.


That's a big statement to make Pete, even though you did qualify it by referring to "regular pitched threads". Leading edge geometry is very important and is the reason why insert manufacturers make anvils to go under threading inserts in 1 degree increments.

Having insufficient clearance under the leading edge is one reason that a lot of people have trouble cutting threads because the tool rubs on the flank instead of cutting cleanly at the cutting edge.

When I was an apprentice at AIS Port Kembla we were all( there were thousands of us over the years) taught to screwcut with the compound set in the normal position and to just advance the compound forward at every cut, the amount advanced is half the infeed on the cross slide so 10 thou in 5 thou along, and of course it is easy to take a skim on the back face as well just takes a bit of practice, you just have to watch out for the backlash in the compound, the easy way to do this is to slowly wind the compound back while the tool is in the thread on a long thread till you get a fine shaving and then just run it through on the same setting or on a short thread run the tool into the cut and stop the lathe leaving the lead screw engaged and then wind your compound back to nearly engage the back flank you may have to run it through a few times to take a shaving off. I have never cut a thread by moving the compound.
Will

[QUOTE=Techo1;1794382]

Originally Posted by Pete F

Again, don't worry too much about geometry, helix angles or any rubbish like that, it's completely unnecessary and is clearly just confusing many people. With a HSS tool you've ground yourself, helix angle is completely immaterial for regular pitched threads, and the normal side clearance will take care of that.the same as per a V thread.


That's a big statement to make Pete, even though you did qualify it by referring to "regular pitched threads". Leading edge geometry is very important and is the reason why insert manufacturers make anvils to go under threading inserts in 1 degree increments.

Having insufficient clearance under the leading edge is one reason that a lot of people have trouble cutting threads because the tool rubs on the flank instead of cutting cleanly at the cutting edge.

Sorry mate, I was very loath to even answer that comment, as the chances are the whole concept will just confuse some people who are just getting their heads around the whole thread cutting palava. My emphasis was to ignore immaterial complications like this and not worry about it, the OP did that and what do you know, he cut a perfect thread.

Nevertheless I guess I'm begrudging obliged to answer as answer as, with all due respect, what you wrote is just not true. On regular pitched right hand threads, on all manufacturers I'm aware of, standard RH inserts and holders will cut without any change in shims, they are specifically set up to do that, and it's normally 1 degree. As well as stating convention pitched threads, you apparently also missed the part where I very clearly stated "With a HSS tool you've ground yourself ...", so nobody was even talking about inserts. Nevertheless I have never heard of anyone having difficulty in cutting regular RH threads through having an incorrect shim in their holder, never mind "a lot of people". About the only time anyone here is likely to need a different shim to change the angle would be in cutting a very fast thread, say for example if they decided to make a Quorn T&C grinder, as at least one of the threads in that that I can think of is very fast (edit: no cancel that, now I think about that one I think even that thread isn't a V profile, so almost for sure you'll be grinding your own tool for it anyway). The other time that's reasonably likely is when cutting a LH thread with a RH holder, then you'll definitely need a shim to change the angle. Again to others who are just getting in to threading, don't worry about this, grind your own tool, ensure sufficient clearance angles, and you're good to go. Later on if you decide to get inserts, just be aware of the caveats to be aware of, especially the one about cutting LH threads.

Here are two documents that describe the whole shim/anvil business. I like the Sandvik publications as there's a wealth of knowledge available there in their knowledge base, they also have a free magazine available (I think 4 times per year??). I'm off to bed so couldn't find the one they do on threading, but it's quite good. Here's what they have to say about shims however. Insert clearance angles Another I'll just quickly link to is the Seco, very similar to the Sandvik threading guide I couldn't find http://www.secotools.com/CorpWeb/Dow..._Threading.pdf The part about anvils is on page 16, note the comment:

Originally fitted anvils. The table below shows the originally fitted anvils. These anvils are suitable for most operations when threading towards the chuck.

I'd suggest for those here who are just beginning with threading, forget all the above., and definitely ignore the diagrams regarding inserts if you grind your own tools. As I said above, always ensure your tool is at 90 degrees to the work and forget about anything else. Even if you get threading inserts and holder, the way they come as standard will cut 99.99% of the threads you'll ever need. Maybe with the exception of lefties.

Pete