Thread: Is there a prize for the worst thread ever cut HELP!

No, it’s not an Ozmestore lathe, it’s an Optimum Maschinen. Looks very similar though, doesn’t it? It has a few differences in spec and is slightly larger. I do get a few bits and pieces from Ozmestore though – excellent service. Unfortunately, the lathes he advertises on ebay aren’t viewable – big turn off for me.

I've always stripped my lathes before use and done a little work here and there to tidy up and make things work as they should. This one I didn't bother so much as the finish, at first look at least, was so much better than on other machinery I've bought before. So much for that idea, I'll have to tear down the gearbox one day and have a look in there I suppose.

I ground negative helix and a very conservative end relief as a datum for the last thread I cut. I find with too much end relief, the point breaks of if not careful, PITA halfway through a thread! Unfortunately, that ‘abysmal’ thread was done on a part for a small drill sharpening jig I was working on at the time, so I’ll have to start that project again once I’ve sorted the threading.

As you have said earlier you have not had problems in the past with cutting threads,so why start to rethink about your processes now.

Your sharpening of the tool bit and work method must of been correct or close to it,or was it just luck.

th62 you have mentioned grinding negative helix twice in this thread, what exactly do you mean by this? Can you post a clear picture of the tool looking "straight down the barrel" at the cutting end?

Originally Posted by Techo1

th62 you have mentioned grinding negative helix twice in this thread, what exactly do you mean by this? Can you post a clear picture of the tool looking directly at the cutting end?

Agreed, this has completely lost me too. A thread tool is actually one of the easiest to grind, with the caveat that the angles need to be accurate angles. However there are only two sides to grind! There is no back rake, and the two ground faces are ground to provide sufficient side and front/end relief. If either of these is insufficient the tool will rub/not cut correctly. You don't grind "helix" in to a tool, in some cases it may need to be ground to allow for the helix of the thread, however as noted above, no special allowance need be made in this regard in standard V threads other than simply sufficient side relief.

Originally Posted by Pete F

Ok, what do you think the end relief is there for?

It's for cutting clearance, 5 degrees would be plenty, this tool obviously has more than enough.

All the reliefs are connected, when you grind the side reliefs that affects the front relief. If you grind the two side reliefs at a different angle you then have a positive helix angle - either left or right. For this tool bit, both side reliefs were ground at the same angle, so effectively there is no helix angle - what I call negative helix. Perhaps not accepted nomenclature, but on the positive side, I know exactly what I mean.

Originally Posted by Techo1

It's for cutting clearance, 5 degrees would be plenty, this tool obviously has more than enough.

ALL reliefs are for "cutting clearance", and if they are not sufficient the tool will rub. The photo was obviously clearer to you than it was for me, hence why I asked if it was possible for another photo. It isn't normally an issue, but thought it would be worth checking. Likewise if it's above centre.

All the reliefs are connected, when you grind the side reliefs that affects the front relief. If you grind the two side reliefs at a different angle you then have a positive helix angle - either left or right. For this tool bit, both side reliefs were ground at the same angle, so effectively there is no helix angle - what I call negative helix. Perhaps not accepted nomenclature, but on the positive side, I know exactly what I mean.

While you may know what you mean, you're correct in saying that it's neither accepted nomenclature, nor, I'm afraid, accurate. If you plunge in, as you tried, the tool is a simple form tool, and the angles you've ground will form the flank angles of the thread. If on the other hand you were to feed in at precisely 30 degrees, you could (theoretically) have a single sided tool. However I'd suggest doing yourself a favour a it seems to me you're unnecessarily complicating things, so just grind the tool symmetrical at an included angle of 60 degrees, and feed at 28 degrees just as you've done.

Originally Posted by th62

All the reliefs are connected, when you grind the side reliefs that affects the front relief. If you grind the two side reliefs at a different angle you then have a positive helix angle - either left or right. For this tool bit, both side reliefs were ground at the same angle, so effectively there is no helix angle - what I call negative helix. Perhaps not accepted nomenclature, but on the positive side, I know exactly what I mean.

If I may suggest... I would now try to cut the same thread, with the same tool, in Plastic like PVC or Delrin. Then in Aluminum or Brass.
If you can cut the plastic well, then your tool is sharp. If Aluminium and Brass come out well, you know you can trust the angles of the tool are correctly ground.

Next try threading a leaded free-cutting mild steel. If that comes out good too, it would confirm that it is the particular steel you were using that does not lend itself too well for machining. If you have problems with the leaded steel, then you may have a problem with lathe rigidity. Maybe spindle preload too loose. Maybe a gib strip is bent or twisted, mimicking a properly adjusted slide where in reality it is loose.

Some more ideas to look out for: looking at your first pics, I see you use the standard 4-way toolpost. I have never seen your lathe make in flesh... but I have seen other Chinese lathe's 4-way toolposts. Often the toolpost surface where the toolbit rests is extremely roughly finished. One can almost imagine to see the toolbit rock on the rough surface. Something worth checking and if necessary correct. I have also seen 4-way toolposts wobble on the compound, because the mating surfaces are not flat, because the tightening bolt had been over-tightened and the top surface of the compound bulged upwards around the clamping bolt..... . I also know that the compound slide on some China lathes can be wobbly. If nothing else helps, you may try to clamp the threading tool directly onto the cross slide, using a massive plinth to suit center height. If now a plunge threading cut was to work well, you would know it is the compound that has to come apart and be improved.


EDIT: actually, looking at your previous pics, the surfaces that are turned do not look like a very good surface finish to me. This could be an indication of steel not suitable for machining. Many steels that are optimized for welding, are actually very poor to machine.

Well, now it starts, I should have known it would - sooner or later.

Pipeclay, re thinking your processes is a necessary evil when things go wrong, I would have thought that obvious. As it turned out my sharpens were correct, or at least reasonable. If that was meant to be an insult - you succeeded, happy?

Pete F, not exactly sure what your problem is, Techo1 asked me to explain what I meant by negative helix - so I did, as best as I could in my language. I'm extremely sorry if my explanation and nomenclature doesn't meet with your approval.

I'll stand by my statement: side,front and helix angles are all interconnected - you cannot change one without affecting the others.

Why you suddenly went off on a tangent about plunge cutting is best known to you; however, in all my explanations, I said I used the compound (except for the final very fine clean up cuts), which when set at 28 degrees is hardly a plunge is it?

Why you two decided to ramp it up, I have no idea, but as I said, I should have realised it would eventually.

To the remainder who offered assistance, without insult, thank you...

You've GOT to be kidding me mate. My "problem" it seems is I just wasted a bunch of my time trying to help you out. You clearly know better so you're on your own buddy!!!

For the record THIS is what you said

The 58 degrees I mentioned was a typo, the compound is set at 28 degrees. With the compound at that angle, if you rest the cutting edge of the tool against the side of the compound slide, the tool sits parallel with the top slide (one or two degrees off). But I'll try moving the compound to 28 today and also try plunging.

With regards to grinding positive rake on threading tools, I know it's a no no as it changes the effective angle of the tool.
However, I find that when threading gummy stock like black iron or stainless, a bit of negative rake results in a much cleaner cut.
I usually use the top-slide set over method when threading and have tools ground with a bit of positive back and side rake for use with gummy stock.
For run of the mill threads I have found the benefit of a smoother finish outweighs the variance in profile angle which is virtually negligible.
If you are particularly worried about the final angle you could grind your tool a bit wider (61º or 56º) before grinding the rake but I grind my tools freehand and have no illusions that I can hold to such accuracy.
Cheers,
Greg.

I just spent a few minutes trying to photograph a correctly ground tool for right hand V threads but the compact digital camera cannot capture the detail required so I will try again tomorrow. I will also try to show why allowance for the lead angle of the thread is very important except maybe for finer threads of larger diameter. The thread th62 is cutting has a lead angle of slightly less than 2 degrees at the O/D and over 2 degrees at the minor diameter, add another 3 degrees for clearance and tool should have about 5 degrees side clearance angle on the leading edge, the trailing edge requires virtually no side clearance thanks to the helix angle of the thread.

You seem to have a very selective memory Pete F, I did write that, but then I wrote this - what I actually did.:

"Ground another bit (no honing and no helix), clamped in holder with a 12mm overhang and (with fingers crossed) cut a thread using coolant and the compound, finishing of with a couple of very fine plunge cuts to smooth the trailing edge of thread. Huge improvement, the thread isn’t cut to correct size or cleaned up after - just a roughy to test straight off the tool. Still not as good as I’d like, but approaching useable. Presumably attention to tool sharpening should improve. I may send off for a diamond or Arkansas stone next and try that."

Selective memory isn't all it's made out to be!

Negative: characterized by the absence of distinguishing or marked qualities or features; lacking positive attributes (opposed to positive ):

Less than zero and opposite in sign to a positive number that when added to the given number yields zero

So I guess negative would be - zero nothing naught zip!
I don't think it can be explained any better than that.

Without a better picture of your cutting tool th62 it's hard to say that what you refer to as helix is actually clearance on the leading edge of the tool to allow for the helix angle. The first picture here shows a tool ground with about 6 degrees of clearance for lead angles up to around 3 degrees, a keen eye should be able to see the difference between this and the other end of the tool shown in the second picture which has virtually no clearance although the tool looks OK to the untrained eye.

DSC_0015.JPG

DSC_0009.JPG

This last one shows where the tool with virtually no side clearance has rubbed on the leading edge resulting in built up material welded on to it. This tool cannot possibly cut a nice clean thread.

DSC_0013.jpg