Foru-Way Toolpost Question

[Nigel Graham 2Participant @nigelgraham2]

Having just fitted a boring-table to my Harrison L5 lathe I have started making adaptors for the tool-holders I have.

The 4-way toolpost of unknown make and provence, came as just the tool-holder itself, a rather non-OEM Tee-slot central bolt that has to be replaced, and locking handle, but nothing else.

The underside has a circular boss for radial location in…. ?? It also has 4 shallow indexing holes, with arcuate ramps into them to give the ratchet action.

The 3/8″ dia bolt is smaller than the hole through the block – I can sleeve it to a better fit. The thread looks 3/8 X 16 tpi, either BSW or UNC. This bolt seems site-made from a long bolt and slice of hexagon bar.

The handle and stud had no indexing function. They were simply a clamp.

..

Question: would the indexing holes, approx 3/8″ dia but only about 1/8″ deep, have engaged sprung pins, or sprung balls; in a sub-plate that was the part fixed to the lathe slide?

OR…

Would a spring under the boss, have pushed the tool-holder itself up clear of a static pin or pins when the clamp-handle is operated? The pin(s) would need be low enough to keep the location engaged, although it is barely 1/4″ deep. A properly-made central stud would help here.

OR….

No springs, just a static pin or pins and the ramps lift the block off them when rotated manually?

.

Incidentally, for occasions when I need turn short tapers, I am also investigating mounting the Harrison and Myford compound slides on this boring-table, without modifying that, albeit with considerable loss of tooling headroom.

Mounting the original Harrison top-slide in its designed form would need a huge hole through the table, compromising its intended purpose and a disservice to its maker. “Borrowing” the Myford top-slide might be feasible via a sub-plate held to the boring-table, like the rest of the fittings, by Tee-nuts and studs.

The table is beautifully made, with hand-scraped top surface, and I bought it from the maker via an advertisement on this forum. That was at least two years ago but I have only just fitted it! (I have a factory-made boring-table too, but found it is for the L5A, a significantly larger edition of the lathe, so not suitable.)

Separately also making raising-blocks to use the QCTPs and rear parting-tool sets.

 

[Howard LewisParticipant @howardlewis46836]

If the lathe is a Harrison, being British, I would expect the threads to be Whit form.

The ramps act as a ratchet to locate the four way post. On my lathe the pawl is spring loaded, with a flat face which bears against the end of the slot in the bottom of the toolpost.

If the slots have a rounded end, I would expect to see a cylindrical pawl,  possibly with the end face machined at an angle to match that of the ramp in the bottom of the toolpost.

In this way, as the toolpost is rotated, the pawl is pressed down against a spring, until the angle face contacts the ramp, which it then follows up, until it hits the circular end of the ramp, to provide a firm location for the toolpost, to resist cutting forces.

The external lever merely acts as a clamp, although the post need to be located by a close fitting post / sleeve, in the central bore.

If sufficient clamping force is applied, the toolpost can can be positioned between the fixed locations for some machining operations, or to increase the clearance angle on a tool, for a particular job.

Howard

[DC31kParticipant @dc31k]

If you find one of the many free online manuals for lathes of a similar class, one of them will have an exploded diagram of the typical four way toolpost.

At least two of the variations of a Colchester Student manual show details. The L5 manual itself has a early variation, using a conical boss.

The Patent for such a toolpost is also available from Espacenet (GB1127018A). Note name of patentee.

[DiogenesParticipant @diogenes]

What Howard says, I think – it’s not desirable for the base of the toolpost to lift off the deck because chips and debris will get underneath it.

The circular boss will originally have fitted in a counterbore, to allow a more accurate location/rotation of the toolpost & registration of the pawls, and offer them protection from side-loads.

 

[Nigel Graham 2Participant @nigelgraham2]

Thankyou –

The toolpost bears no maker’s name, serial-number or other identifier, and was “supplied” without the housing for the pawls.

It might not be Harrison-made, and the mis-fitting stud suggests life on a previous lathe with a Tee-slotted top-slide. (c.f. The photographs on the Raglan thread.)

It is now clear what I need do – as you advise. Needing an intermediate spacer nearly 3/4″ thick gives depth for the counterbore and sprung pin-type pawls, with a different assembly design from my original envisaging.

If the spacer is very slightly undersize the overhang will help protect against swarf ingress, too.

I can probably modify the stud by replacing the hexagonal head with a thread to fit the base, and a sleeve for concentricity, but it will be better, and with little more work, to make a new one.

.

Sorry _ I failed to find that patent. I do not know what Espacenet expects more than the reference-number, but my attempts all returned “not found”.

Examining lathes. co shows my TP resembles that made for Harrison L5, but close study of the photograph suggests that is as far as it goes. One 4-way post looks like any other externally, but the general principle seems to need a slide and post made for each other by the manufacturer. My lathe’s top-slide and this orphaned tool-post are insufficiently compatible.

Similarly, the Colchester lathe has a top-slide designed as part of a 4-way TP assembly, with a complicated mechanism to give easy, rapid indexing. I note it uses balls rather than pins, as pawls.

….

 

I am aiming for maximum flexibility: the 4-way TP; simple QCTP set risers, and possibly a mounting for at least the Myford top-slide borrowed as needed. It looks as if that could be arranged on this lathe to give very high-angle coning, as e.g. for a Vee-pulley. Trying that on its own lathe needs an adaptor plate or slide-outriggers for the clamping-screws, and the handwheels as made may foul each other anyway.

There are ways round the lost, fine length control but on most of the turning this Harrison does, diametrical accuracy is more important than length, within reason. So a saddle-stop set by rule or spacers would cope with that. The feed-shaft tail might accommodate a handwheel but seems not to give nice integer travels. I want to keep the 1/4″ -pitch lead-screw for screw-cutting only.

The lathe is designed for easy swapping between cross-slide and boring-table without removing the hand-wheel, but even with a recess cut in the wall-insulation I’d need move it sufficiently far from the wall to make the workshop even more cramped. So I’ll think about that one!

 

Why do I want the boring-table? For a start, I have a steam-wagon cylinder block to make….

[Earny49Participant @earny49]

Howard  I have a Harrison mill and all the threads on it are either UNC or UNF. I would expect that Harrison lathes of that era would also be UNC /UNF

 

David E

[Nigel Graham 2Participant @nigelgraham2]

Thankyou David. The era of mine is fairly old so it might be all BS threads, but I will verify what I have.

I will have to replace the stud and it needs only match the handle, but it makes sense to make both ends the same. 3/8″ UNC and 3/8″ BSW are both 16tpi so without much in the way of metrology equipment it won’t be easy to determine this exactly.

In any case, as I have no idea who made the thing the thread could be of either type anyway. I have thread-gauges but assessing the relative fits on clamp-set studs or known bolts that have seen little or no use, would be as good a way as any.

[Howard LewisParticipant @howardlewis46836]

To determine whether the threads are Whit or Unified, you need thread gauges. These will show whether the thread is a 55 degree (Whit) form or 60 degree Unified form. Metric are also 60 degree, but not interchangeable. (Metric quote actual pitch, say 8 x 1.25; whereas Unified are specified by diameter and tpi, , say 1/4 x 20).

You can screw a UNC thread into a BSW if the tpi are the same, but BSW into UNC will not go!

(Our Chief Engineer would not contemplate Unified threads, so often we had to tap out a UNC thread to take a BSW bolt!)

Howard

[Howard LewisParticipant @howardlewis46836]

Some toolposts are located by a tapered base and recess. This has the advantage that once clamped down, the taper resists any movement; in this way, the toolpost can be used in positions other than located by the pawls.

When I made my 4 way rear toolpost, rather than making a ratchet and pawl, I used a single dowel which located into one of the four holes in the base.

Howard

[Nigel Graham 2Participant @nigelgraham2]

I have thread gauges but still could not make sense of it until suspicion dawned….

The shank is 0.375″ dia, but the thread is somehow, M10!

Testing the stud with gauges, then both the handle and stud with various screws and nuts, soon showed a UNC or BSW thread does not mate with either, but a clean M10 nut moves smoothly along the stud, and a good-quality M10 bolt fits the handle properly.

I had not expected that at all, but as I have said the stud is probably not original to the tool-post, and the unit’s provenance is unknown. It just sort of came with the lathe, I think, but in a load of random bits and bobs.

….

The problem with a static pin as I had originally envisaged using, is as outlined above, that of having to lift the tool-post with the risk of swarf finding its way in there.

The post I have can be clamped off-pawl (it would just push them down into their burrows) but it might not be very rigidly held without unfair straining on the handle. There are times when it could be useful but in that case it’s better to select a tool in the first place, that will perform the task without artifice.

However you raise an important point with using a non-rotable mounting; that of holding tools whose alignment to the work is critical (screw-cutting, parting, form-cutting). I have partly made the basic mounting-plate but still need make an intervening raising-block to hold the tool post itself.  I will look at providing some angular adjustment somewhere.

[Howard LewisParticipant @howardlewis46836]

Sadly did not not make plain that it is the dowel in the rear toolpost that is raised, for rotation, not the toolpost which remains resting on the base.  One dowel in the toolpost body, four dowel holes in the base.

Any swarf lying about gets wiped off when the post is rotated, so does not get underneath.

And I make a point of brushing and wiping swarf away as often as possible.

Examination, after eight years of operation, confirms this.

The post is unclamped before the dowel is raised, the post rotated to the next position, and then the dowel reinserted and clamped again.

You only need a thou or two clearance, to allow rotation, so the chances of swarf finding its way between the post and base are like the clearance, pretty small.

Howard

[Nigel Graham 2Participant @nigelgraham2]

Ah, I see. Thankyou for that.

I noticed on of my QCTP blocks has a hole right through it, near one of the rear corners. I wonder if that is for a location pin in a similar way.

[DiogenesParticipant @diogenes]

I know it sounds like an extra job you don’t need, but if you are intending a ‘free’ 4W, it probably would be a good time to provide a chunkier stud, so that the toolpost has a better chance of staying where you put it, ‘rotation-wise’.

..let’s say that I have an ‘informed suspicion’ that M10 / 3/8W are too small, and too coarse, to provide reliable clamping of an ‘unrestrained’ toolpost.. 😉

..unless it’s only half the size I think it is..

 

 

 

 

[old martParticipant @oldmart]

A lot of top slides (compound) have tee slots for the toolpost without any particular means of locking or indexing. Unless you cut the tools very short, they won’t fit in a four way, we only use two tools in ours and I made a two way for one of the lathes.

[Nigel Graham 2Participant @nigelgraham2]

Diogenes –

I intend to make a properly-fitting stud, and to fit indexing stops.

I have already pointed out the “supplied” stud is not original but improvised by some previous owner – apparently by replacing the head of a bolt with a welded-on slice of hexagon bar.

The tool-post body is small – about two-and-half-inches square flanges – so the clamp-handle with it should be fine with the major diameter of the stud fitting the block properly.

 

OldMart –

I do have short, small-size HSS tools and tool-blanks suitable for this device.

None of my lathes have slotted top-slides.

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