Turning between centres on Super 7

I plan to turn a part between centres but find the centre protrudes from the faceplate by about an inch which means the peg to push the drive dog round needs to be nearly two inches long and I would like this to be shorter.

To achieve this, I would like a stubby MT2 centre but did Myford once supply them?, otherwise, short of trying to make one does anyone know of a seller of stubby centres.

Not sure what the issue is with a 2" long peg or stud with nuts for the drive dog. You could also use a bent-leg drive dog with a 2" extension. Have used 2" and longer studs and bent leg dogs many many times with no issues whatsoever. No need to look for special short centres.

Of course any sort of drive dog turning needs extra care to keep operator and clothing well away from turning parts. Also wise to keep speeds down a bit due to the unbalanced mass. In the old days turners used a boot lace to tie the dog to the stud or a bent leg dog to the faceplate, but these days a nylon cable tie does a nice job. Just be sure to cut the tail off flush with the buckle, don't leave a sharp end.

Shorten it by turning? Ideally need a drawbar, if removing a lot of metal, but that should present little problem unless through-hardened.

cranked leg driving dog will mean you don't need such a long peg.

Or hold a bit of scrap in the 3-jaw, turn a 60degree point and then use the side of a jaw to drive the cranked dog no drive pin required.

Edited By JasonB on 30/09/2019 20:08:20

Thanks, a few ideas to pursue there.

I guess I just like the idea of the drive dog being close to the faceplate and away from where I will be cutting.

The centre for the headstock should be soft so it can be turned, usually done for perfect concentricity but if you feel it is too long then turn to your desired length. The lathe was supplied with a catch plate which is intended for driving work between centres. You may not have one as they are often separated when the lathe acquires a new owner. For a quick and dirty between centres job just keep a piece of bar with a 60° point and put it in the chuck and skim the point, it will now be dead true, use a cranked drive dog to drive off a chuck jaw.

mike

Jason beat me to it, must refresh before writing

Edited By Mike Poole on 30/09/2019 20:54:05

Mmm?, after a quick search on the net I see the genuine catch plate (that I haven't got) has a built in counterbalance, which is nice!

Just bolt something to the opposite side of your faceplate if you find vibration a problem

One of these could solve the drive dog problem, they are called face drivers.

Mike

Interesting again, I have seen something similar on wood turning lathes,

I wonder if you have to give it a tap to make the driving edges bite into the workpiece?

The chisels are spring loaded and do mark the end face where they bite into the job, sometimes you see the little marks left on the end of a component. They are often used for grinding but they are also used for turning. There are videos on YouTube.

Mike

Thanks, I’ve just this minute been watching a video about them

Face drivers would be ok for low torque work but a bit iffy for any sort of cut.

Tony

Is the centre also sprung so as to take care of the work expanding?

And is there a hobby market for these commercial devices?

I think these face drivers are intended for woodwork The only place I've seen them for sale deal with wood turners

Roy

**LINK**

I think this may be an hydraulic version but it shows a decent turning cut. They are quite expensive for a commercial item but a shop made version could be made.

Mike

Now that really IS impressive, Mike

MichaelG.

Pursuing my "sprung" query, it seems from the YouTube comments that this one is not spring-loaded. No doubt the significant heat generated is dissipated in the chips/sparks!

If the centre was sprung then the part could be ejected under cutting load. The driving teeth are the sprung parts.

C.J. - if you're worried about the length of the driving pin, make a extended driving pin with a fat base and just turn the end that engages the slot in the dog smaller.

Edited By Pete Rimmer on 01/10/2019 12:09:44

The driving teeth are the sprung parts.

Not on Rohm face drivers - see page 39 on Rohm face drivers

for a cross sectional diagram which shows the spring loaded centre. The centre has to move to allow the part to contact the drive dogs under tailstock pressure.

The Rohm driving disc can "wobble" to take up out-of-square bar ends, which makes for rather pricey items at larger sizes - I have designed a simpler, lighter duty version for turning graphite blanks at work with fixed driving dogs, where an out-of-square end is taken care of by the initially contacting dogs slipping & digging further in to the blank under hydraulic tailstock pressure under cutting conditions. When all 6 dogs are engaged, the blank doesn't slip any further and, as the initial heavy roughing cuts just produce a parallel cylinder, the axial displacement of the blank caused by the dogs digging in under load is of no consequence. The sliding centre is a parallel Morse taper 1 sleeve, with a die spring behind it - the spring rate was determined by trial & error (die springs are suprisingly cheap), with a Morse taper 1 centre providing an easily replaceable "point" if required (which it hasn't been for over 6 years that the home made unit has been in service).

Nigel B