Lathe set up for machining large radii using cross slide profile follower

[GreensandsParticipant @greensands]

I am looking for photos/sketches for the general  set up showing how to machine a radius on the lathe using a fixed ball bearing arranged to follow a pre cut profile mounted on the cross slide. I seem to recall that it involves disconnecting the cross slide lead screen and replacing it with a suitably mounted lever.   I hope makes some sense.

[Dave HalfordParticipant @davehalford22513]

Coping or duplicator lathe.

[GreensandsParticipant @greensands]

Yes, specifically on ideas on how to modify a Myford lathe for copying purposes.

[DiogenesParticipant @diogenes]

..something like this?

https://www.denfordata.com/bb/viewtopic.php?t=3774

[Clive FosterParticipant @clivefoster55965]

Or maybe something like this.

BMW 6 cylinder crankshaft set up on my Pratt & Whitney Model B lathe for trimming down the webs to give it the weight watchers treatment to go in an engine for classic racing saloon car.

Template bolted across the bed with a follower fixed to the cross slide. Lathe tool is pretty much out of sight on the other side of the tool post. Double sided template with left and right handed insert tooling to do the pairs of webs for each crank throw.

As it’s a one way curve power cross feed works fine by pushing the saddle back as the probe runs down the template.  A fairly significant axial load hence plenty of bolts holding the template in place.

Bit bigger than what you intend to do but the principle scales. For accurate results you need to consider the effects of template follower radius and tool tip shape. I cheated and generated the template in place by scanning a works racing crankshaft using a dial gauge in the micrometer stop holder and replicating the shape by co-ordiante milling using a cutter of the same diameter as the probe. About 200 points per side I think.

Clive

[Ian PParticipant @ianp]

Greensands. You do not say what radius dimension you are trying to achieve but I used quite a simple method to turn the face of an 8mm thick, 100mm dia ali blank convex so that is was 2mm thick at the edge.

I used 2D CAD to work out the radius (I’m useless at maths) and made a ‘radius arm’ to that length, I found a convenient place on the headstock for one end of the arm to engage in and temporarily mounted a corresponding ‘divot’ on the carriage.

I kept the radius bar in place by maintaining steady pressure on the saddle handwheel whilst advancing the cut with the cross slide. Actually I found it best to do the final cut from the centre outwards.

The finished part was part of a pattern of a product to be cast in bronze resin and for a reason I now forget the designer insisted that the rear had to be completely flat with no spigot or fixing holes so I used superglue (and took light cuts!)

Radius bar (pointed at each end) visible below chuck

Turning in progress

After a polishing with Peek.

Ian P

———————————————————–

Footnote (off topic diversion)

The radius bar is the only bit I have left of a tyre-less, abandoned BSA 500 Twin motorbike that a school classmate and I pushed about a mile to my home (in mid 1950). We dismantled it for fun, learning a lot about engineering in the process. The radius arm is one of the OHV pushrods, ali tube with spherical male and female hardened steel ends.

[BazyleParticipant @bazyle]

Look up ‘curvilinear slide’ as used on ornamental turning lathes.

[Clive FosterParticipant @clivefoster55965]

Nice one IanP. Elegantly effective simplicity.

Somewhere on the internet there is a design for a more elaborate and “engineered” version of Ians simple and effective device. As I recall it the rod length was made adjustable, threaded rod in a tube with a nut welded on the end I’d guess. One end was to be fixed to pivot in line with the lathe axis on a solid bar clamped across the bed. The other pivot was to be on a drop bracket clamped to the cross slide. Theoretically this keeps the rod safely below the chuck, not completely convinced about this for a Vee & flat style lathe bed. Setting up is allegedly easier because the adjustable rod can be set to the desired radius and the fixed end clamped “anywhere” on the bed. It was said to work equally well at producing concave radii by clamping the fixed pivot on the tailstock side of the cross slide.

The “gotcha” when setting up is that the actual radius produced is defined by the distance between the fixed pivot and the tool tip. So some means of gauging that the tool tip is exactly above the centre of the pivot point fixed to the cross slide is desirable. If this isn’t possible mathematics and careful measurement are inevitable. If turning a concave radius there is no way of aligning tool tip and pivot centre so concave is harder. Figuring out what happens if the tool tip placement is in error is a job for those inclined to recreational mathematics. As is evaluating the positional tolerances relative to actual radius accuracy.

Many years ago I planned to make such a device using a pair of “obtained” rose joints for the pivots. In the event the rose joints went to a more urgent job and the idea of making the device got postponed until I needed it. Approaching 40 years later the need has yet to arise.

Clive

[GreensandsParticipant @greensands]

Ian – the required radius will be 4ins and so your method could well be applicable.

[JasonBModerator @jasonb]

Is this for a smokebox door? As you seem to be working on that end of the engine. If so working out co-ordinates and then blending the resulting steps will also work. No more work that the milling of the template shown for the crankshaft where it was worth doing due to the number required but for one offs co-ordinate cutting the actual part may be simpler.

Use of an online PCD calculator will give the co-ords instantly. Just use a round nosed tool and enter the pPCD as part dia plus tool diameter. Divide the circumference by the stepover and enter that as the number of holes on the PCD.

Or if you have a DRO on the lathe the ARC function will work out the co-ords for you.

[Julie AnnParticipant @julieann]

The ‘perfect’ way of forming a large radius on a smokebox door:

To maintain accuracy the follower needs to the same shape, and orientation, as the cutting tool.

Finding a hydraulic copy unit for a Myford would be difficult, but not impossible. I’ll bet somebody has parts from a Mini-Kop tucked under the bench.

Julie

[Andrew CrowParticipant @andrewcrow91475]

I have seen a simple way if you don’t need exact accuracy and that is to make the radius you require in a piece of say 1/4 steel plate and fix to the lathe bed. Then make a follower fixed to the saddle, by keeping the saddle follower pressed against the former use the cross slide auto feed from centre outwards and use the top slide to apply depth of cut.

Andy

[Andrew CrowParticipant @andrewcrow91475]

Sorry that should have said “fix the follower to the cross slide”

Andy

[Michael GilliganParticipant @michaelgilligan61133]

Andrew

May I refer you to this [historic] thread for inspiration:

Fusee Turning and Groove Cutting on my MYFORD

… unfortunately it has not migrated well from the old forum, but the gist is there.

MichaelG.

[JasonBModerator @jasonb]

It would help to know what axis the radius needs to be turned on. Are we talking a barrel shape where teh work is along the lathe axis or a dome/concave face where the work is across the axis? As some of the methods shown may only work in one situation.

[GreensandsParticipant @greensands]

Not sure if i know what a hydraulic copy unit is or how it works but I will opt for the cross slide  mounted follower method, keeping the top slide in contact with the template whilst making the cut under power.  Is there any advantage to be gained in using a tension spring to maintain contact with the template?

[Michael GilliganParticipant @michaelgilligan61133]

String, pulley, weight

might be simpler

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

For convenient reference:

https://www.lathes.co.uk/myfordminikop/

MichaelG.

[Clive FosterParticipant @clivefoster55965]

Concerning holding the follower against a template I’ve seen a reference to using an ex car or motorcycle valve spring pushing against the centre of the saddle to generate a decent force. It does need a definite push or pull to ensure contact under power feed. I have a small stash of die springs for this sort of job which are rather more powerful than the general run of “found in the useful bits box” springs.

If template following in your situation using a roller bearing would usefully reduce the friction on the template. Hand feeding whilst maintaining the pressure on the template with the saddle hand wheel would make the whole job more controllable and give you a feel for whats going on. Spring assistance would usefully reduce the effort on the saddle feed too. With small machines on this sort of job you have to consider the possibility of the saddle twisting slightly on the bed if relying solely on hand feed. It’s amazing how visible even a tiny error can be.

Light, consistent cuts from a sharp, carefully radiused tool are the order of the day.

Don’t rush it and you will be fine.

Frankly taking appropriate care whilst watching / feeling what’s going on at the cut is vastly more important than the exact method chosen.

Clive

[JasonBModerator @jasonb]

I wonder how much pressure is really needed, from hand turning and also mobing my ball turner without the handle you don’t really need much so just pushing with one hand while feeding with teh other may well do.

Another option would be a simple block with a tool mounted in it and simply slid over the cross slide running it to a template, much like this

[JohnFParticipant @johnf59703]

I have a recollection of seeing someone who attached a template to a Myford taper turning attachment to form a fusee for a weight driven clock — apparently it worked well and could easily be adapted to form a radius.

Where I saw it ??? But not too difficult to figure out

John

[JasonBModerator @jasonb]

John. Read Michael’s earlier post.

[Michael Gilligan]

On Michael Gilligan Said:

Andrew

May I refer you to this [historic] thread for inspiration:

Fusee Turning and Groove Cutting on my MYFORD

… unfortunately it has not migrated well from the old forum, but the gist is there.

MichaelG.

Thanks for that link Micheal, I had forgotten about producing fusee’s by a similar method, my experience was for making buffer heads for steam locos.

Andy

[JasonB]

On JasonB Said:

I wonder how much pressure is really needed………………………………..

Wonder no more. I did a quick lash up to get something similar to the Eccentric Engineering tool. Just set the insert of my ball turner flush with the edge of the body. Clamped a bit of 120mm thick wall tube to the cross slide to use as a template. Then just pushed the body of the ball turner along the template, feeding in the cross slide to put on the next cut. Similar could be done with a block of metal to hold a bit of HSS, or a Gibraltar tool post if you have one. may even be possible to just have the topslide loose on the cross slide so you can move it about by hand.

1″ steel with a 60mm radius to the end. Paper template cut against the pipe to check the profile.

[JasonB]

On JasonB Said:

I wonder how much pressure is really needed, from hand turning and also mobing my ball turner without the handle you don’t really need much so just pushing with one hand while feeding with teh other may well do.

Another option would be a simple block with a tool mounted in it and simply slid over the cross slide running it to a template, much like this

My experience is that the forces involved in mechanical template following can be much higher than one might expect. No doubt there is a strong link to the radius of the curve in that once below a certain radius you have to start taking things seriously. Use a roller bearing follower if you possibly can.

The main curve on the template for my crankshaft modification set up approximates to 2 inches radius, its an elliptical rather than circular section. The follower pin is 1/4″ diameter and 1″ long running in a full length lubricated plain bearing with a slot cut away for the template to run. Not enough room for roller bearings. Tailstock ended up right at the rear of the bed as was.

Following the template by hand using the cross-slide screw took about as much force as I cared to apply. So much that I contemplated using the saddle hand wheel to assist the template following by a light feed away from the template rather than against it. In the event the star-wheel clamped cross slide feed clutch was just up to the job, albeit using more tightening torque than I’d ever normally consider.

Obviously a 1943 built lathe will no longer be at full factory performance but Messrs P&W claimed the machine to be quite happy creating unbroken steel swarf ribbons 3/8″ wide by 1/10″ thick and 100 ft long so the feed clutches are appropriately substantial. Obviously the far larger saddle takes more moving along the bed than a Myford or similarly sized Model Engineers machine but pushing directly by hand is still eminently possible.

Trying template following on the SouthBend Heavy 10 I had until maybe 25 years ago didn’t go too well. The SouthBend has no hold down plate keeping the saddle down to the back side of the bed and tended to twist the saddle so it rode up the inside of the Vee guide. Something it had propensity to do when taper turning too. Late model camlock spindle version in good condition. Very accurate but annoyingly light duty.

Bottom line is mechanical template following works well but it’s important to recognise that considerable forces can be involved. Ians pivoting radius arm and similar more engineered versions take much less force. There is a reason why the professionals use hydraulics.

In practice if you are aware of the potential issues and take appropriate care the job will go just fine. Overthinking gets in the way of doing!

Clive

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