Weird lathe bed

[AjohnwParticipant @ajohnw51620]

I was a bit stupid in my last post. It's an attempt to get a narrow guide with a dovetail bed. Not a bad one really but to work the rear one would need to be a little clear.

I should have looked more before posting – wife was calling.

So looking at it again

Lets take away the inner gib strip and just use the outer dovetail. If there is a thou clearance the saddle can rock by that much and cutting forces will cause the saddle to pull down onto the bed at the back due to the angle. This is why circa 60 degrees is often used for this sort of thing as it has less mechanical advantage than say 45 degrees. This will increase friction. There has been all sorts of arguments about the ideal angle.

Put the gib strip back and unless the saddle is longer a thou of clearance will allow the same degree of rock but the effect of cutting force is acting on a vertical face. The gib strip also has a very substantial section of saddle behind it to help resist cutting forces far more than the usual rear dovetail has.

There is another effect that might influence Tubal Cain but doubtful. If anyone has adjusted a myford for how slides out to be they may find that it tightens along the bed. Often there is too much wear to get even close. If they then mic it up they may find that the difference needed to do this was minuscule. Even hard to measure with a tenth reading mic. Temperature can have an influence on that. Unlikely but one of the factors. It also has a different effect when the saddle is moving which is reckoned to be the main advantage of a narrow guide as the forces used should be close to the centre line of the guide – ideally they need to be exactly in line and can be on a prismatic bead. That aspect is a fact. This is probably why he would put things back as they used to be. Also because he would regain the stiffness running along the rear of the front rail. My memory of that is that it really was substantial.

Putting the gib strip inside taking cutting forces looks strange but I suspect it does have an advantage. It will be the part that wears most first so can be replaced. Tapered gib strips are wonderful things to use. They support the gib strip all along it's entire length and are extremely easy to adjust precisely. Some larger lathes have been known to use them and if some one messed about setting one up it wouldn't take long to realise just how superior they are to the usual screws. Pity they didn't add a wear strip to the front dovetail. In fact it's a pity they don't use wear strips in all sorts of places on lathes and other machines. At one point in time some slideway grinding people could add them.

The other aspect about narrow guides is where the cutting tool should be in relationship to them. Smack in the middle of the length is one argument but the real point is that they shouldn't be at the end nearest the chuck as that encourages wear as they can't spread the load along their length. True of any guide. Some bed designs have large flat areas that help to get round that problem. Modern prismatic beds are hardened but it doesn't help in respect to saddle wear.

As drawn there is a significant area taking the cutting forces via the gib strip. Much more than a 7's bed proportional to the size of the lathe.

It's not bad really but would need some changes to make max use of a narrow guide.

John

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Edited By Ajohnw on 14/09/2016 00:22:36

[RainbowsParticipant @rainbows]

I have never heard of the top flat of a lathe way getting worn out like the sides do thought that might just be my sheltered lifestyle.

That in mind it looked like it would be relatively easy to add a wear plate onto the part where the gib used to be.

The diagram says hardened and ground but I am not sure if a lapped bronze plate wouldn't be better. Better to have the wear plate wear rather than the bed after all. Depends if the bed was hardened I guess, maybe I can get J Stevenson to just chuck an entire lathe in his nitriding friends machine

[Paul LousickParticipant @paullousick59116]

My old Southbend lathe does not have gibs on the slide for the saddle, but uses 2 shear profiles and a 3rd for the tailstock. A retainer (non adjustable) is attached to the rear of the saddle to stop it lifting.

Saddle locking is by a screw on the top of the saddle and a clamp nut on the underside.

The lathe spindle base is also attached to the same 2 shear guides as the saddle.

A history of lathe bed designs can be found at: **LINK**

Paul.

[Alan JacksonParticipant @alanjackson47790]

On my Stepperhead lathe I used the front bed guide as a narrow guide with the gib strip on the outside dovetail and used the inside vertical face of the saddle as the guide face for the full width of the saddle. The rear be dovetail has a dovetail gib strip adjusted to prevent any lift of the saddle. The leadscrew is positioned centrally and as high as possible between the bed guides to minimise the twisting moment from the leadscrew pushing or pulling the saddle. both gib strips are dowelled in position.

Alan

Edited By Alan Jackson on 14/09/2016 10:14:37

[AjohnwParticipant @ajohnw51620]

The Southbend and it's clones Boxford etc are designed on the basis of the correct use of a narrow guide Paul. Many others too. One side effect is that the guides must be able to pass either side of the headstock. Another unlike some minilathes swarf doesn't tend to fall on the leadscrew. Chances are that on lots of lathes with a prismatic ^ will have the rack in the right place but for some reason the leadscrew further off centre to catch the swarf. The rack fits in it's natural place directly below the guide and very close to it and on centre. Ideal. The lead screw can also be on centre but has to be further away from the guide. In theory it to should be as close as possible.

The other advantage of the vertical V is that it can offer pretty large wear surfaces and also tends to self tighten under load so in theory at least there is no need to hold the saddle down firmly as there can be on other designs. All it leaves in an oil film under load. Wish people luck trying to adjust other types to that state. Even temperature will mess it up, I have heard that it's not possible to mill on some lathes because nothing holds the saddle down.

Gap beds mess things up and without hardening wouldn't be a good idea really on prismatic beds due to the position the tool finishes up at right at the end of the guides. My guess is that this why modern lathe beds are hardened.I fail to understand why the guides can't pass either side of the head and tailstock. I'd guess that the only lathe that does that now is Schaublin. Maybe it adds to the cost because the headstock casting turns out to be different and gearboxes if fitted need a cover. I don't see it though. Everyone knows that the cross slides wear as well so why not harden those ? Often that problem crops up before the bed has much in the lines of wear. Some use the narrow guide idea on these too – the width of the dovetail looks rather narrow in proportion to ithe cross slide width.

I've seen the lathe bed design book before. A lot of what is in there comes down to trying to use flat guides a la Myford on the basis that they are simple to produce accurately. The dovetail bed saddle can be a lot wider than it usually is effectively making it more narrow guide but it can't compete with a vertical ^. Strips can't either really due to strength factors but they could be bigger and butcher. Th ^ needs zero adjustment and is easy to make in practice. The saddle in put on the same machine as the bed and offset by some amounts so that one of it's sides is ground when one side of the guide on the bed is ground. Then the offset is changed and the other sides done. The saddle can be jigged so that the amount of dish the lathe will face turn is controlled. Another table can carry the tailstock. Same for the headstock. They could be milled in exactly the same way. I have always wondered about the rear ^ some lathes have. Seems to me that if made this way it ensures that the saddle sits level and the angles match. If it's just a flat it's height relationship to the ^ needs to be rather carefully controlled.

As just about everyone who has anything to do with machines will pull a face when they look at 7's I have always wondered why Myford didn't change earlier. They did on the last machine they introduced. There are repercussions though. The ^ goes into the saddle so it should be thicker. That reduces the swing over the cross slide so the spindle needs to be higher. The bed would be fine. The equipment needed would have to be paid for so prices would go up. Their dovetail bed lathes just about all have rather narrow saddles and a gap in some cases. I'm sure I saw a photo of a lathe found in the factory where they had reduced the width of the dovetail on the bed considerably leaving immense flats either side. The 3 morse lathes came late. Maybe down to worries about the surface speed and area of the front cone bearing when it's enlarged that much. Some S7 owners tell me that ML7's will drill much larger holes due to their reduced friction. I'd say some of that is probably down to the need for extremely precise adjustment. ML7 plane bearings have their own problems anyway, It isn't just a case of removing a bit of shim and doing them back up again. They have to be scraped back into alignment and to fit.

Typical lathe headstock bearings really annoy me.

John

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[Neil WyattModerator @neilwyatt]

Here's real heresy.

The 'ideal' model engineering lathe might combine features of both a mini-lathe and a Super 7.

The S7 has heavier construction and a gap bed, but aside from that the capacities and capabilities of the two machines are closer than most would dare to admit.

Does anyone dare suggest what would be the best combination?

Neil

[Roderick JenkinsParticipant @roderickjenkins93242]

One argument in favour of the design of the Myford 7 series was that they were intended to be a universal machine in the days before vertical mills were available at a similar price. The wide shears and positive attachment in the vertical direction lend themselves to a better set up for milling in the lathe than a machine designed for turning only.

JohnW may pull a long (winded?) face but the Myfords where not intended to be tool-room or production lathes, they were more a general purpose machine and, as that, they were very successful. The advent of the affordable milling machine changed the market, or so it seems to me.

Rod

[blowlampParticipant @blowlamp]

Posted by Neil Wyatt on 14/09/2016 11:25:29:

Here's real heresy.

The 'ideal' model engineering lathe might combine features of both a mini-lathe and a Super 7.

The S7 has heavier construction and a gap bed, but aside from that the capacities and capabilities of the two machines are closer than most would dare to admit.

Does anyone dare suggest what would be the best combination?

Neil

Yeah, maybe a Boxford.

Martin.

[Neil WyattModerator @neilwyatt]

Posted by blowlamp on 14/09/2016 11:50:49:

Posted by Neil Wyatt on 14/09/2016 11:25:29:

Here's real heresy.

The 'ideal' model engineering lathe might combine features of both a mini-lathe and a Super 7.

The S7 has heavier construction and a gap bed, but aside from that the capacities and capabilities of the two machines are closer than most would dare to admit.

Does anyone dare suggest what would be the best combination?

Neil

Yeah, maybe a Boxford.

For once, Martin, I am in total agreement.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/09/2016 12:36:29:

Posted by blowlamp on 14/09/2016 11:50:49:

Posted by Neil Wyatt on 14/09/2016 11:25:29:

…

The 'ideal' model engineering lathe might combine features of both a mini-lathe and a Super 7.

The S7 has heavier construction and a gap bed … < etc >

Yeah, maybe a Boxford.

For once, Martin, I am in total agreement.

Neil

.

Boxfords with a gap bed are not exactly common:

[quote] One unusual, possibly unique model, probably constructed during 1949/50, was a gap-bed model of which only one example is known and currently in South Africa. [/quote]

MichaelG.

[AjohnwParticipant @ajohnw51620]

I like that post Martin. Actually I sometimes think that one way of getting a pretty capable lathe is to buy one and have it reconn'd. Then comes the problem – what the recon people might do to them. Ok that's from my father that had lots of machines done but it suggests some care is needed.

I don't see Myford's that way Rod. I see them just as Chris Heapy did – a budget lathe. That is what they are. Once some one beats them on price they are going to have problems even though given that the Myford is in excellent condition they are much better machines. As I am a designer I can't help wondering about what they could have done differently. Not much in the line of a cheap change and most changes that can be made to lathes that make them say superb tend to be expensive. Plus of course Super 7's can be superb in some ways until various things wear and then they do have a bit of a problem – best taken care of by sending it back to Myford. Their costs were very reasonable and I would hope that the work would be done as well as it can be.

Long winded ? Simple really don't bother to read it. It's a lot shorter than a certain book that is about on narrow guides and why. Myford's bed design is widely seen as a bad idea and that why lathes stopped being made that way. English beds too.

Headstock bearings are a pain but that would take another post.

Myfords 7's have been used to for production work. I saw them around when I used to visit closure auctions. Lots of small ML's were used even pretty recently in some areas but not the 10's it seems or at least not the roller bearing ones. I walked into Reeve's one Sat morning near the time a new design had appeared in ME. Packed solid with people and a brand new Speed 10 on display. I looked and one bloke said you can change the headstock bearing on those easily, pity the dam thing isn't bigger. Many nods of agreement and a couple of oh it's not so simple as that. It isn't but they are easy to change. So personally I do wonder if they should have changed that at some point. They did on the ML10.

John

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Edited By Ajohnw on 14/09/2016 13:31:42

Edited By Ajohnw on 14/09/2016 13:32:23

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