ML7 Novice Questions

mark wrote:


Simple answer, that has worked for well over a hundred years, is to not try cutting with the spindle running in reverse.
Avoiding going hard from forward to reverse (stop the spindle in between in between) is another way to keep the heart rate at a more reasonable pace.
Reverse is a bad thing at high enough speeds that you must worry about inertia removing a threaded chuck on startup.
IMO such modifications that would cause the chuck to stay in place, mostly serve to make life in front of my klathe just a little less enjoyable, as it complicates the removal and installation for little benefit. I have yet to see a good reason for running the lathe in reverse fast enough to spin a chuck off at startup.
I know it is standard on the new Myfords. I blame that on lawyers, not on common sense.
A better idea that I have seen was a spring catch that prevented the switch from being thrown straight through from the forward to the reverse position, so it had to be pushed out of the way to reverse.
Nothing as fun as a chuck going forward at 2000 rpm and the spindle going backwards! :-) Mostly only an issue with 3 phase power, as most single phase motor will not hard reverse, but must be brought down to a slow rpm before the start windings will kick the motor in the other direction.
Cheers Trevor Jones
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mark wrote:

You cannot usually drive the spindle backwards with the leadscrew handwheel. The geartrain does not allow it at the ratios used.
Cheers Trevor Jones
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DR_G wrote:

Get over the idea that grinding tools is any sort of black art.
By that logic, so is machining. Best left to those in the know, doncha know.
It is a skill, like machining, that one must learn by doing, rather than reading about.
Avoidance solves nothing and costs more.
The angles are, in general, open to interpretation, and capable of being varied (often by a huge amount) to see what works. Just as there are different angles given for HSS tools, there are even MORE variable in tips for insert tools. A Kennametal or Iscar catalog will set you right to work grinding HSS, esp. once you see the prices. In carbide tooling, you generally get what you pay for. You just have to know what you want, and why, to make them worthwhile.
A sharp edge with a little clearance in a couple direction will cut most materials well enough. Flat on top for brass and threading tools. A radiussed tip for a nicer finish, most of the time.
Grind a tool, try it, try different geometries to see what works. Some tools work quite well on many different materials, despite what the books say about how wrong the geometry is.
I use a lot of carbide tooling daily at work. I use a lot of HSS at work as well. More HSS than carbide. I turn steels from mild through some semi exotic stainlesses, Brass, Beryllium copper, titanium, graphite, plastics, and a few other materials. HSS works well for all of them.
I do not work in a production shop environment, rather in maintenance, where I have to make one or two of most of the jobs that come in.
Cheers Trevor Jones
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At the outset of the purchase of Dr G's ML7, I pointed out that the manual was available via the YahooGroups MyMyford site. I have just checked to confirm this as I have an original copy and have sufficient knowledge gleaned over the years to write about overhauling them.So I am not 'au fait'with scrabbling for information.
It would now appear that the reverse facility was omitted when Myford made his machine.
In the words of the great American humourist, O.Henry, an evasive answer is appropriate.
N.
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ravensworth2674 wrote:

I had an ML7 with a reversing motor on it. I now have a Super-7 with a simple on-off switch. It has not complicated my life at all to be without the reversing facilities.
I would rate it as something potentially handy to have, but by no means a requirement.
Cheers Trevor Jones
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Trevor Jones wrote:

I don't think that works ... but I haven't got a Myford. </???>
Much better to keep the gear train engaged during the entire thread cutting operation, and withdraw the tool and rewind by hand between cuts, to a point comfortably beyond the start of the cut (so any backlash is taken up when you go back in), then advance the tool and cut in. Don't touch the tumbler reverse.
Forget about the threading indicator, at least to start, if not forever. Learn the cutting action first, then worry about the indicator (if you particularly want to) once you can cut threads.
I don't use a threading indicator at all, and I also often cut threads while turning the lathe by hand, I don't use the motor. That loses the smoothness of the cutting speed a bit, but it avoids collisions (!) and it is much easier to end the cuts accurately.
You can make a mark on the chuck so that each cut ends at the same place, but you should really also have a "groove" (I don't know the correct name for it) which is the same diameter as the inner diameter of the thread at each end of the thread. That's for external threads of course - when cutting anything but the largest internal threads use a tap.
Inserts can be useful here, thread cutting inserts come in two main types, ones for a particular pitch and ones that cover a range of pitches less accurately. They are a bit hard to get hold of, and can be expensive.
To begin with it's probably better to plunge cut straight in, but after a while, and especially on larger threads, it's better to cut only one side of the thread form.
You do this either by advancing the top slide between cuts by a percentage of the value you advanced the depth of the tool on the cross slide - 50% for 60 degree threads, 46% for 55 degree threads if I got the math right.
Or you can set the top slide at half the thread angle (with the tool still at 90 degrees to the work) and advance only the top slide between cuts, the geometry then does the calculation for you, and you can lock the cross slide for extra rigidity.
Cutting threads is not easy at first, but persevere and you'll pick it up.
:)
Hope I didn't make any errors here ...
--
Peter Fairbrother



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Peter Fairbrother wrote:

On a metric thread, with an inch pitch leadscrew, the halfnuts and gear train MUST remain engaged (There are some ways around this, but somewhat complicated). When cutting an inch pitch with an inch leadscrew, safe is to use the same number. No worries, no having to remember which numbers are correct.
Now. If the pitch of the thread being cut, is evenly divisible by the pitch of the leadscrew (inch pitches , inch leadscrew again) there is no need at all for a thread indicator, as the thread will engage correctly at any point that the halfnuts will engage. Some pitches will require the use of only one number, others have several that will work.
Many lathes have the chart for the threads and the appropriate number to use, right on the machine near the thread indicator.
One lathe I use at work has no half nuts. Threading is done with a ballscrew, and the reverse lever by design. A bit of adaptation, to get used to.

Reversing the spindle, or even stopping it while the tool is in contact with the work, is deadly on insert tips. At 25 dollars/ 12 pounds or so per decent quality insert, I recommend staying completely away from the carbides, and sticking with the HSS. Learn to grind a tool, and you have the skill forever.
I make the apprentices at work hand grind their threading bits, to their satisfaction, then we look them over with a comparator. They have to grind a UNF threading tool with all the correct geometry, in order to pass. Most are quite capable of grinding a tool to dead nuts perfect once they practice a bit, whether they think so or not. Guys get all freaked out about grinding tools. It's dead silly simple, with a bit of practice. You don't get any practice by avoiding it, either.

Even most of the CNC lathes dont do this any more. It's very hard on tips, with some drastic loads applied.

Huh? That sounds like it makes life more complicated than it has to be. If it works for you, though...

That is the more common practice. Use slightly less than half the included angle of the thread, so as to cut primarilly on the one face, but apply a shaving cut to the other. If you use exactly the half angle, there is a possibility of leaving steps in the face that the tool runs parrallel to, causing a rough thread, with steps down one flank.

It is easy. To the point of being a bit boring. (ooh, lathe pun!)
The first thread I ever cut with a lathe was a acme thread on a mild steel 5/8 inch bar, to replace the leadscrew on my bench vise (unwisely used to press out too big a bearing). The threaded portion of the bar was 7 inches long, IIRC, and I ground the tool bit using a angle gauge and a micrometer to measure the tip width. Once I got that done, there was never any fear of cutting threads with a silly little V tip. :-) I did this before I had ever got any real training in the craft.
The recipe is thus:
Set zeros (or note dial readings) on cross and compound slides. Feed in compound (topslide) for depth of cut Engage halfnuts Cut Disengage halfnuts* Crank out cross slide Crank carriage back to start point* Crank in cross slide to zero Feed in compound for next pass Engage halfnuts* Repeat until final depth is reached.
The starred items will be skipped if you are hand cranking the spindle back to the start. You MUST pull the tool back from the work before reversing the rotation, though, or you get to learn how to pick up on a partially completed thread (less fun).
Threading, and grinding HSS tools, are dead easy. Only fear keeps guys from plunging right in and getting the job done. Practice is key. If you do not do it, you do not get any practice, now, do you?
Cheers Trevor Jones
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Peter Fairbrother wrote:

Got that wrong. It's 58% for 60 degree threads and and 52% for 55 degree threads. Sorry.

Forgot to mention, the "topslide travel" should be 15.5% more for 60 degree threads and 12.7% more for 55 degree threads than the required depth of cut for the thread (hope that makes sense) when using this method.

Ooops - sorry, too early on a Sunday.
--
Peter Fairbrother


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Peter Fairbrother wrote:

I can understand now why I have never seen this method in use. :-)
Too many things to remember. :-)
Cheers Trevor Jones
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On 20 Apr, 18:19, jontom snipped-for-privacy@hotmail.com wrote:

cant argue with all of that keith ...very good
just to add there is a full video here ...amongst others
http://www.eng.hmc.edu/E8/Videos.htm
all the best.mark
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Mark, Norm, Keith,
Thanks for all the comments. If you look at this link :
http://www.rdgtools.co.uk/acatalog/HIGH_SPEED_TOOL.html
The sets at the top of the page show eight tools. The tools in questio are the finishing tool (I think the 'pointed' one, third from the left) and, the threading tool (again, I think, the asymmetrically pointed one seventh from the left).
OK so the finishing tool is symetrical viewed in plan, and has som rake angle on the top of the tool, BUT I did not realise that thi finishing tool would need modding. Rounding the nose as Norm suggeste will certainly improve matters significantly, but I am at a loss t know why someone would sell this as a finishing tool when it will quit obviously give a poor finish. This is what confused me last night. I they had said 'this tool must be radiussed to your requirements' the fair enough, but there was no such instruction.
The threading tool is asymmetrical in plan, and has a flat top - n rake angle (for some reason unknown to me). I have tried setting th leadscrew to 1.2 mm pitch and running the thread tool along some M1 threaded bar just to check that I am doing the gearing correctly. Th result was fine - after taking up the initail backlash, the too followed the existing thread perfectly, and the thread angle looke spot on. Part of my initial question was how the hell do you do mor than one pass of a thread tool (assuming you need to do several cuts t get the correct depth), and how on earth do you use the thread pitc indicator device? I have read the Myford Manual, but this seems aime at people who have previous experience of thread cutting.
Thanks again everyone,
Garth
-- DR_ ----------------------------------------------------------------------- DR_G's Profile: http://www.rcgroups.com/forums/member.php?u 168 View this thread: http://www.rcgroups.com/forums/showthread.php?tg459
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You are practising on brass Garth ..you will almost always get a good finish on that no mater if you used a cold chisel in the tool post ..... have a go on mild steel black bar .this is the worst to get a good finish on ...and will show up the tools for what they are .

Forget about the thread dial indicator for now ....it's for advanced speedy thread cutting .....you get the same results going slow for now, stopping at the end of the cut.....taking tool out of cut at the end of the cut . ...noting dial reading .......then reversing ....and not taking it out of thread cutting mode .....stop........put back into cut .....advancing beyond last reading by a small amount. start. take as many passes as you like ......but don't undercut ............leave small core sticking out of the bar as lead- in ......this core is the same as the thread core ....should be prepared before you start..and can be taken off after .......
YOU ALSO should prepare a core at the end of the cut ....this is your manoeuvring space at the end of the cut. you also need to use the tailstock centre or live centre ...if your bar is sticking out of the chuck some way .........corresponding to it thickness....so there is no chance of bar flexing.

all the best.mark
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Garth, The delightful thing is that we are all right although we differ a bit here and there. It is called finding your own personal equation. Going back to George Thomas, you will get a better insight when his books arrive as he is one of the gurus. We lesser mortals are struggling to express our selves properly. In the meantime, I would concentrate on plain turning rather than screwcutting and if you have these 'pointy tools' they were probably ground by people who are far from clued up. Mark Jones, Keith and I have been at it- making mistakes and learning from them.
Moving to Mark Rand, I suggested to Garth that he armed himself with GHT, Sparey and Bradley and whilst GHT had a Union T&C, he did most of lathe tool grinding on a quite simple grinder but honed by hand and gave very pedantic instructions even to the type of 'oilstone'. OK, we can now buy diamond stuff with fascinating grits and ex production tool and cutter grinders for 'shirt buttons' but old GHT and the old mob were able to produce exhibition quality stuff on their ML7's. I have already confessed to owning a plethora of grinders but I now have shaky, limp all sorts of things including fingers.
We'll get you there- Garth
Norm
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