Myford S7 Cutting Barrel Shaped Cylinders

Hi Gray,

The lathe is from around 1982. The saddle bears on the outside of the two shears (wide bed).

Mike

Hi Mike,

According to the above we are dealing with ONE Thou of wear generally across the bedways. This is the generating surface for parallel parts. With an additional maximum wear of FIVE and a bit Thou in the thickness of the bedways.

The FIVE Thou of wear extends the entire length of the front way, where you were turning your test piece, but at the same time the wear on the rear way is roughly FOUR Thou give or take Half a Thou. Thus the tool will drop only ONE and a bit Thou, which will increase the diameter by 0.000002". This is not the 0.0015" you mention at the beginning of the post.

Just for reference the tool will need to drop 0.027" in centre height to produce an increase in diameter of 0,0015". Plus this drop would need to be in the middle of the test piece, ("Barrel Shaped", with a gradual slope before and after to give the same diameter at each end.

The wear is as we have established fairly constant along the front edge, with no gradual 0.027" drop. Assuming that the Gib is adjusted correctly across the bedway then I am leaning towards the spindle adjustment. I do however think that the bed adjustments to get the two ends the same diameter originally may need revisiting, once the spindle has been checked out.

Regards

Gray,

I’ve just chucked up a bar that gives me around 10” from the bearing as leverage to check bearing play and found the following movement:

vertical: 0.0012” TIR

horizontal: 0.0016” TIR

I’ll search for previous bearing adjustment posts to see if this is ‘normal’

For interest, is it still possible to obtain Myford headstock spindles? I can’t see them on the Myford site, and I’d expect them to be £100s.
Mike

Edited By Mike Donnerstag on 30/05/2020 16:02:16

The FIVE Thou of wear extends the entire length of the front way, where you were turning your test piece, but at the same time the wear on the rear way is roughly FOUR Thou give or take Half a Thou. Thus the tool will drop only ONE and a bit Thou, which will increase the diameter by 0.000002". This is not the 0.0015" you mention at the beginning of the post.

How can it just affect the tool hight.?

If you move the front way down by 1 thou you not only lower the tool hight but move it away from the work. The saddle must rotate about the rear way. If the tool tip was over the rear way the only movement would be horizontal by 1 thou out of the work. If the tool tip was over the centre of the front way 3.5 inches in front of the rear way and 3.5 inches above it it forms a square the back corner of whaich is fixed and the front has dropped by 1 thou, The tool tip must then rotate down by near enough one thou and out by one thou. (Just by similar triangles). The consequence is an increase in diameter of 2 thou.

I woud be perfectly happy for someone to tell me the flaw in this argument. It's no different that the way we level the bed anyway. If you lower the front foot at the tailstock end you move the tool away from the work.

Incidentally I think that the max wear on the bed is 0.0005" variation not 5thou. I think the 5 thou refers to the maximum amount you can take off the top of the bed before more regrinds are not possible. Myford used to mount the bed o the bedway grinder and pull the centre down by half a thou producing a bed with a hump of 0.0005" from new, with wear this goes through 'flat and eventually to 0.0005" hollow when it's time for a regrind.

I'm sure there are many people more expirienced on here than me so perhaps someone could explain why I'm wrong.

regards Martin

Martin: The maximum wear on the bed, according to Myford, is given below, which is indeed 5thou, not 0.0005" (1/2 thou):

Mike

Edited By Mike Donnerstag on 30/05/2020 18:07:09

Having spent the last few hours searching to no avail, can anyone tell me:

1. What radial movement they would expect at the spindle nose, using a bar held in a chuck as a lever?
2. How to measure the spindle end float (axial movement)? Do I need to use some kind of lever against the bull wheel for example? I understand this should be 'measured' as zero.

Most of the ME threads I have been reading about setting up Myford bearings say how sensitive it all is, and several people have complained about end float resulting in the spindle locking up under an axial load, e.g. tailstock drilling. Mine runs nicely despite some fairly heavy tailstock drilling I've done recently, with no excess noise, no chatter and no locking up. I'm loathe to mess about again with the bearings unless I really need to.

Many thanks,
Mike

Mike

That is not how I read it. The above really talks about the maximum metal that can be removed before the bed is scrap. (0.0025" and goes on to say that whilst they keep the material removed to a minimum they may have to take between 0.005" and 0.010" off to get to a CLEAN surface. By this I take the meaning as 'ding free'. If you use your logic you could just as well assume that the allowance is 10 thou.

regards Martin

Martin: Apologies - I had only given you half the information! I've updated the photo in my last post with ALL the information that Myford gave on assessing lathe beds.

According to the document, the allowance in thickness is 5thou.

Mike

Edited By Mike Donnerstag on 30/05/2020 18:27:11

When I push very hard against the chuck with the dti set on the backplate I get less than 0.01mm movement. You can see hardly any movement of the indicator hand, it still has its factory setting from 1998. The other lathe from 1972 is much more used and gives a movement from around 0.01mm. Niko

Edited By speelwerk on 30/05/2020 18:17:59

Niko: I was using a bar held in a chuck, giving 10" of leverage from the front bearing, with all my might! Not that I'm all that mighty! I got between 0.03mm and 0.04mm on my metric gauge.

Here is my setup:

Anyone: Is this what you would expect?

Just to reiterate my questions stated earlier:

1. What radial movement they would expect at the spindle nose, using a bar held in a chuck as a lever?
2. How do I measure the spindle end float (axial movement)? Do I need to use some kind of lever against the bull wheel for example? I understand this should be 'measured' as zero.
3. Is it possible to obtain a 'new' spindle, if I deemed that mine needed replacement?

Edited By Mike Donnerstag on 30/05/2020 18:32:00

Using your method the newer one gives 0.02 mm and the older around 0.04 mm of indicator movement. Niko.

Niko: Interesting. So, only just under my own measurement. I think I’ll assume the bearings are adjusted okay and try something else, such as doing a test-cut between centres.

Mike

Is the lathe turning 'round'? Machine a short length of stock and use your micrometer to check its roundness - you should lock the micrometer once set to a nice slop-free fit and rotate the freshly turned portion to feel if it is notchy or tight then loose, which would suggest that there might be a problem with the spindle or bearings.

If that is OK, then I wouldn't worry about the spindle or its adjustment too much. You don't use the test bar as a lever at all - just light to firm finger pressure will do to establish wear, any more pressure is displayed as deflection and adds to the confusion.

Don't forget that the front bearing needs some clearance for an oil film to develop when it is rotating. The fact you stated earlier on that it was cutting cleanly makes me think there's not a lot to be concerned about with the headstock in general.

Martin.

OK. Point taken, I was working from memory, never the most reliable of sources which is why I asked where the info had come from. It still seems a large allowance to me but if that's what they say, that is what they say.

I still would be concerned about the variation front to rear. If it was even, as has been stated it would not matter too much. I'm fascinated to see the resolution of this.

regards Martin

I dont think the vertical wear on the top surface is as critical as you think, Martin. I can't quite visualise the amount of movement on a square/triangle as you describe. Besides, the tool point is not 3.5" from the rear shear. On a 1" diameter test piece it is only 1/2" from the spindle centre line, which on a Myford is offset toward the rear shear significantly for the specific purpose of giving good geometry under load on the front shear. So maybe more like 1.5" between rear shear pivot point and tool point. But also if the tool were moving outwards by the significant amount youi posit, the job would be turning larger toward the chuck, but as it is barrel shaped, it is getting smaller toward the chuck.

Hi Mike

The stand and the DTI are eBay cheapies. Seem to work just fine.

Re your bed measurements above it sounds like that scarred up area is the cause? It would pay to measure the width of the back shear itself and try to determine how much wear is on that very rear guiding surface without interference from that scarred up front shear.

Sounds like in the meantime you have the spindle movement in the ballpark. A little more than I would aim for but I think acceptable and almost certainly not enough to cause barrel shaped turning. No idea what the factory clearance spec is for the Super 7 oddball bearing set up. I aim for half a thou on plain bearings like the ML7 but I think your one thou or so should be passable.

Did you run a flat file over that scarred up front shear to make sure there is no raised ridge of metal around its edges? Might be worth a try.

Another thing you could try is setting up the lathe in the absence of expensive level by using the second method in the Myford owners manual that I described in a post above earlier, basically with a dial gauge on a long bar in the chuck to make sure no stress is put on the bed during bolting down.

Yet another thing I would look at is how you are holding your test piece for turning. Your OP said you only had 1" held in the collet. I would like to see double that if I were doing it. In fact I would fit the four jaw chuck and hold the bar in that. Just to eliminate the possibility of some movement of the job in the collet under load in a job sticking out so far. Don't use your three jaw chuck for this test as it is quite possibly bell mouthed if in similar condition to that bed!

Hopper: Instead of a flat file, I ran a small flat slipstone over the scarred-up front shear. So far at least, I don't think that area is to blame for the barrel shape, but I'll do some more testing today. I'm determined to get to the bottom of this and thank everyone who has posted so far for their help.

Regarding the stress on the bed, one thing I intend to try is to see how the bed reacts when the nut pressure on the right hand end is released. I'm still wondering if the problem is a combination of headstock misalignment and bed compensation.

Mike

One thing I forgot to mention is that the carriage gib strip should be fitted & adjusted correctly.

It should be removed and checked to ensure it has a slight bow, such that it contacts the bed first in the middle. This is to ensure full contact along its length as it is adjusted and only needs to be of a few thousandths of an inch, but it isn't critical just as long as the bow is there and can be flattened back out by adjustment without causing the carriage to bind.

If the centre of the gib strip isn't firmly touching, then when the saddle starts to become unsupported by moving over the gap in the bed, there might be a tendency for the cutting tool to deviate frome a straight line.

Martin.

Hi Martin,

Your analogy concerning the rear bedway pivot point would be correct if the surface of the bedway was indeed flat between the two points. If they were I would be forced to agree with you.

However having done several plots across the bedways given the bed wear shown in Mike's photograph the surface at each point approximates to a curve. The radius of this curve varies with the bed wear at each point of measurement. From my plots I get radii for the curvature ranging from 36 to 75 feet.

The centres of these radius are below the bedway and their positions vary laterally with regard to the Spindle centre. As the centreline of the lathe is very approximately at the top of these radii, any vertical, or lateral movement is going to have very little effect on the workpiece. Working with these radii and a bit of Pythagoras I got an average for the figure mentioned above.

Also as a point of interest. From the drawings on the S7 Screwcutting clutch I make the spindle centreline slightly less then 2" (50.27 mm) from the rear vertical face of the rear bedway.

Regards

Gray,

Hi Graham

I don't doubt that the actual situation is more complicated that my two points. You have clearly done more work on this than me. It's anybodies guess as to areas of contact underneath the saddle. I think my conclusion at this point would be that the bed has had it and needs a regrind.

I don't know idf the folowing test has been done but if a true test bar is put in the spindle or held between centres and the dial indicator is tracked down the bar noting readings and distance as you go if all else is correct apart from the bed and saddle that gives an absolute locus for the tool path.

best regards Martin