Where's Harry Potter when you need him ..... the Dark Arts ...... Grrrrr.. SPT

[CrewCab]

or ................. Single Point Threading ??? ......... and how I broke all my cutting tools in one afternoon :-X

OK ..... this is my first attempt at said "Dark Art" ... :wall: .......... I've chosen an M12 x 1.75 thread .......... cos' that's what I need ..... I've abandoned practising on mild steel bar and am now using Delrin ... much easier on the tooling ;D ......... but .......


why is my 1.75 pitch thread ending up at about 1.75 plus a bit ......... or around 1.8 ish ???

Confused.com CC

 

[Brian Rupnow]

You may have the wrong change gears installed, or it may be a case of having a British Imperial lead screw that will not cut a metric thread.

 

[CrewCab]

it may be a case of having a British Imperial lead screw that will not cut a metric thread.

.0254/.025 x 1.75 = 1.778 ??? ............. Dammit Brian .......... you may well be right :-\

:wall: CC ???

 

[Bogstandard]

Dave,

As you have found out the hard way, you are now in the world of the black arts.

There has been a lot of discussion on this over the last month or so about this weird phenomononomonon.

Marv is the one who is having bad indigestion over this one, and already I have put my head thru a brick wall because of it.

It all seems to stem, as Brian said, from the changeover from imperial to metric leadscrews, or the non changeover in some cases.
With an imperial leadscrew, it is normally very easy to cut imperial threads, and only (but sometimes very close) approximations of metric threads.

With a metric leadscrew, up to now, you are on your own.
Both Marv and myself are still working on that one.

On the lathe I have just bought, that has a new fangled metric leadscrew, even with a lot of gear swapping, a majority of the metric and imperial pitches are only approximations.
So it looks like progress has sent us back to the start of the industrial revolution.

I might have a bit more helpful info when I get my new machine delivered. As I am going to give it the full range checkout, just to see what it can and can't do, with reference to cutting threads. I have loads of delrin in stock to try it out. That might then shed some light on the metric thread syndrome. The machine might end up where the salesman doesn't want it to be (if you have seen the Hancock film, you will know where), depending on the results.

John

 

[CrewCab]

you are now in the world of the black arts.

Not Kiddin' Dumbledore

I bow to others who have gone before ........... or ...... who are at least on their travels ........... good luck guys

Anyway ............. I will, when time allows, try and cut a 1.778 female thread ...........just so something can be salvaged ;D

Good luck gang ....... seems like I need to re-visit my tailstock threading do -dad ........ but can anyone help, large threads using steel are ........... to say the least........... not easy ................ in fact 12mm is a (female dog)

CC

 

[Bogstandard]

Dave,

What you can do, is cut the thread say to half depth using single point. Then finish off using a die. The die should be able to put the slight off pitch right, it won't be so hard to cut because of some of the material is already gone, and the thread will be guaranteed straight because it was started as a single point.

John

 

[tel]

That's how I'd go about it as well John. AND make a tap the same way while you're set up for the pitch (just in case).

 

[CrewCab]

The die should be able to put the slight off pitch right,

Tried that guys, but It looks like the pitch is a little too far off for the die to put it right

CC

 

[Bogstandard]

You are definitely in need of a few new spells.

John

 

[CrewCab]

You are definitely in need of a few new spells

:big: ;D :big:

That cheered me up after a crap day ................. Thanks John ...... ;D .............. and ........ ....... I think your right

CC

 

[Bogstandard]

I had a thought after I had posted about straightening it up. That would only really work for a short run of threads, anything over say 12mm (1/2") the overpitch would be running into where the next thread should be.

Don't worry, you are not alone, and I think in the short term the manufacturers will get the better of us, but somewhere along the line, someone, somewhere, will come up with a final solution.

Using my old Atlas imperial machine, I managed to find some gearchange charts in the fantastic manual, that gave me useable metric threads up to whatever length I wanted. Almost perfect pitches with the change gear set I already had. Weird setups, but they worked.

Maybe some sort of web search might come up with a few answers.
I did find that downloading the 'Grizzly' PDF files for lathes similar to mine showed me the way to an understanding of how threads were cut. Maybe they have similar ones for your lathe, to give you a workaround to the problem.

John

 

[CrewCab]

It's not an insurmountable problem as I'm sure imperial threads will be no problem, however I will check that in the near future ...... it's just I needed an M12 thread at the time, so M12 die to the rescue on this occasion ........ dam hard work mind ??? ........ I'm happy working in either imperial or metric, or even both so I can cope with using the nearest imperial thread without any real problem ............ tis' a bit of a beggar for the younger guys though where imperial measurement is a bit of a mystery.

CC

 

[John S]

CC,
What lathe do you have ?

.

 

[Bernd]

I did find that downloading the 'Grizzly' PDF files for lathes similar to mine showed me the way to an understanding of how threads were cut. Maybe they have similar ones for your lathe, to give you a workaround to the problem.

John

You know John when I first got my Grizzly (model G4000) I tried cutting threads on it. The standard 20 threads per inch and they didn't come out right, come to think of it now. I never did try cutting threads on that "darn" machine again. I also didn't dive into why the threads didn't turn out right since I didn't buy it to cut threads. I'm thinking I should try to go through and set it up again to see if I can cut a simple 1/2-20.

The Grizzly's mini mill's lead screw are some odd pitch. IIRC if you turned the dial 1" it actually traveled a few thou further if you put and indicator on it. I finally changed those over to true standard thread by buying the change over kit from Micro-Mark.

Bernd

Regards,
Bernd

 

[mklotz]

Using my old Atlas imperial machine, I managed to find some gearchange charts in the fantastic manual, that gave me useable metric threads up to whatever length I wanted. Almost perfect pitches with the change gear set I already had. Weird setups, but they worked.

For the benefit of others who may wish to do what John suggests, take a look at the CHANGE program on my webpage. It was written expressly to address this problem.

Basically, you feed it (via a data file) all the change gears you have as well as the pitch of the leadscrew. When it runs, you input the thread you want to cut (Imperial or metric) and the maximum pitch error you will accept for cutting that thread.

It then proceeds to write a file which contains all of your change gear permutations that will cut that thread with an error less than or equal to the maximum you specified. Print the file, take it to the shop and get to work mounting gears.

Some of the old, grizzled pros may be saying, "Feh! We don't need no steenkin' computer program to run a lathe." Try figuring out what gear combination you need to cut 11-1/2 tpi with 0.01% pitch error by hand and you may revise your feelings about this program.

 

[Bogstandard]

Marv,

I had forgotten all about your little elegant prog, and I am positive that anyone who has a change gear machine would benefit from its use.

My problem, with this new machine, I only have the basics that changes over from metric to imperial, everything else is done in the gearboxes, which I have no control over whatsoever. When I want to 'play' and make something special, as in the columns on my mine engine, I think now I have taken a major step backwards on that one.
Make it easier to do, but lose the independence of being able to make whatever you want.

Change gears rule!

John

 

[DickDastardly40]

Crew Cab,

Your post raises 2 points, the first being that you've broken all your cutting tools.

I would be interested in how they have failed as you shouldn't be getting close to putting too much pressure on a vee thread tool on only a M12 thread to break one.

How deep was your cut on each pass and what speed were you turning at, did you use coolant and were you using the direct approach with the compound parallel to the lathe bed or angular approach with the compound at 30 deg?

Were your thread tools hand ground, did you take the side clearances required into account and did the tool tip have sharp, small flat or rounded profile.

Brass is a good medium for practising thread cutting; no coolant is required and you can see how the chips come off. Practising on delrin may not show how the chip forms as it shaves and you get big swathes of candyfloss.

As to the pitch of the thread being off by a small but apreciable margin I can offer no suggestion if you wish to use a proprietry fastener to a thread you have cut, however an alternative way could be to cut both the male and female threads the same (but non-standard) pitch. I appreciate cutting a 12mm internal thread is a pain especially for a novice.

I rarely cut a thread if a tap and die is available and the fit is not critical.

 

[John S]

Once you can get by the basics of screw cutting it becomes a pleasure. No more rough looking threads and more to the point they are parallel to the rest of your work.

It's not that hard and with just one or two cheap reground tools you have every die ever made [ and unmade ]

I screwcut even simple everyday threads to but one dodge I use is to use a single piece out of a Coventry die head.

This shot is of a part completed 14 x 2mm pitch thread being cut on a piece of silver steel [ drill rod ] which as we know isn't the best material to thread. The advantages of this method are that it puts the correct root and crest radii on in the same pass.

One die piece will cut any thread of that pitch and for some reason ? it doesn't seem to affect it just plunge cutting as opposed to going in at an angle with the top slide. This suits me as I have removed the top slides from by two big machines and replaced then with solid steel blocks as rigidity is worth more to me than the angular feature used about every 3 months.

.

 

[steamer]

Marv,

The program is great as long as you know the pitch of the leadscrew.
Based on the above it would appear to have something not quite english or metric? What are you using for leadscrew pitch as part of your program?

Dave

 

[CrewCab]

CC,
What lathe do you have ?

John,

My lathe is a Chester 9 x 20 (Imperial I believe) This One

CC