Download Plans for clamp Knurling attachment.
Most of the photos referred to in the text can be viewed by clicking on the thumbnails on the PROJECTS page.
Download Plans for base unit and turret.
Download Plans for clamp Knurling attachment.
The rear toolpost is a common addition to the amateur's lathe, yet probably the advantages it offers in terms of stability and cutting tool stiffness are not fully utilised.
The ability of the rear toolpost to better support a parting tool to resist deflection whilst cutting, and hence reduce the incidence of the dreaded 'dig in', is well known. It is able to do this partly because the top-slide is not used (removing one de-stabilising slideway from the equation), and partly because the tool is cutting on the reverse side where the forces tend to 'lift' the cross-slide. The effect of the latter is to push the surfaces of the slides into closer contact with each other. Both of these components exist when cutting tools other than a parting tool is being used (an exception being a boring tool set to cut on the near-side of the work).
Another advantage is that the rear toolpost makes a useful platform for holding special tooling. The range of tooling I commonly use with my rear toolpost is illustrated in photo 1. The advantage of the system is that all the attachments are rapidly interchangeable, and once set correctly centre height is automatically maintained. Of course I also use standard 4-tool turrets on the top-slide for other work.
This is a standard pattern base unit, to a design adapted from an illustration in the book 'The Amateur's Workshop' by Ian Bradley. Many other designs have been described in Model Engineer over the years but all serve a similar function. It is fabricated from standard mild steel sections and fixed to the boring table by two bolts placed immediately below the cutting tool. The action of tightening the clamping handle locks the attachment to the body using a long T-bolt which passes through the centre of the base column, and is thus also locked to the boring table. You can't get a much stiffer arrangement than this! The first turret I made held four 1/4" HSS toolbits, and indexing was provided for using a spring-loaded ball in the turret engaging four seats in the base unit. However, in practice, the fact that both turret and base column are square section means that it is easy enough to align tools without indexing, so none of the other tool turrets has mechanical indexing. The boring tools are held in axial alignment because the larger rectangular turret has a cut-out which only fits onto the base unit in two (cutting) positions thus ensuring alignment. I have added a 1/4" diameter rod 3/4 " long screwed into the rear face of the base unit, onto this can be attached my DTI which can be easily swung into position against work held in the chuck, and moved back again out of the way while machining.
All the standard turrets are simply cut from 1 1/2" square section steel and slots for the tools cut with a 5/16" end mill (in the lathe mounted on the rear toolpost with a 5/32" spacer between turret and base column), they are very quick and easy to make. Turret number 1 holds an Eclipse inserted blade parting tool (1/2" x 1/16" cutting blade) and a 45 degree chamfering tool. I have to say that I've never had much luck with this parting tool, either mounted on the top-slide or rear toolpost. Even when set exactly square, at centre height, freshly sharpened with just 3/4" blade outstanding, and using a new Super 7 with correctly adjusted slideways, it still has a tendency to dig-in when parting mild steel. I suspect that the narrow whippy blade (1/16" x 5/16") is partly to blame. I now only use it for brass and small steel sections (<3/8") or where I particularly need to get a narrow cut. A very recent addition is a 3/32" x 1/2" blade and holder - for which I made yet another turret. I have found this to be much stiffer in use and I can now slice through 2" bar without problems.
Turret 2 holds threading tools, 55 and 60 degree V thread form. Whilst you cannot 'set over' to 271/2 degrees with the tool mounted on the rear toolpost as you can with the top-slide, I've never found this a disadvantage. I still manage to cut clean threads feeding the tool straight into the work, and I think the extra tool stiffness - at least on larger threads - produces more accurate threads. I admit I take extra trouble to ensure the tools are honed razor sharp. Anyway, I have other threading tools on the top-slide if set-over is thought necessary (I would use this method for cutting worm threads so that some top-rake could be used on the tool), or where the use of a travelling steady is unavoidable. Another advantage of the rear mounted threading tool is that swarf tends to drop away from the tool tip rather than building up on the top of it, and the cutting progress is that bit more visible.
Turret 3 holds two shortened 5/16" shank carbide tipped tools. One is a parting tool (which never digs-in or chatters) and the other a LH cranked facing tool. Turret 4 holds internal threading tools (held upside-down to cut on the near side). All the turrets have a thick chamfered washer that the clamp handle seats against, but the real reason for this was to enable positioning of the handle out of the way (away from the chuck jaws) when clamped up tight. This was achieved by skimming the washers until the position of the handle was correct (with 5/16" x 22 tpi BSF thread 1 full turn = 46 thou so it's easy to estimate the cut required), the washers were then Loctited in place.
The boring tool attachment is the ideal medium for holding boring tools. Stiffness is the name of the game here, and the only loss compared to top-slide mounted tools is that you no longer have the index handle to gauge the depth of the tool. Not a problem of course if your lathe has a leadscrew handwheel, or you can make use of a carriage dead-stop to positively limit boring depth. You can't produce tapered bores this way, so making a Morse taper socket still needs the top slide mounted tool. The attachment shown is simply a rectangular section of steel the same cross section as the base unit column. A recess has been milled out so that it sits square onto the base unit. Drilling holes to mount the tool holders was carried out with the attachment clamped in-situ, thus guaranteeing they were at centre height. Note that the attachment can be turned round 180 degrees so that at least 2 boring tools can be mounted at any one time. Inserted-bit tool holders up to 1/2" diameter (using 1/8" HSS or solid carbide cutters - the latter the shanks of broken miniature drills used for PCB manufacture) can be held with the amount of overhang adjusted simply by pushing the holder in and out of the mounting hole. A 3/8" hole also holds some commercial one-piece boring tools that I have.
This was constructed because I wanted to make some of those attractive looking ball handles like the toolpost clamp handle shown in photo 1. I happened to have an old 60:1 worm speed reduction box from a geared electric motor, and thought I could use this to control a cutter attached to the worm gear shaft. From the photo you can see that the worm drive casing is bolted to a rectangular section of mild steel (similar to the boring tool holder), and the drive shaft extended through to the other side. A simple cutter holder is attached to the end, with a screw for adjusting cutting depth. In use, the cutter point is aligned with the centre of the work, and the knurled knob (on the end of the worm shaft) turned to rotate the cutter. The lathe needs to be run in reverse but at least it's easy to see the work in progress this way round. The 60:1 ratio makes it a little slow to use, but in compensation surprisingly heavy cuts can be taken with no chatter at all.
As with the boring tool attachment, machining was carried out with the body in-situ so that the bore of the spindle had no choice but to be exactly at lathe centre height. The plan was that by turning the attachment through 90 degrees a convenient method of accurate cross-drilling was also available (though this is available to me anyway through use of the Versatile Dividing Head to George Thomas' design). However, the D/M spindle would be significantly quicker to set up and the work remains in the lathe chuck rather than having to be transferred to the VDH. I did not propose to do much grinding with this particular attachment so elected to use phosphor bronze bearings rather than ball bearings as speed will be fairly low. I may well make a high speed ball bearing spindle later solely for grinding purposes. I have arranged things such that a small handle can be attached to the spindle so that the attachment can be used for manually 'spotting' hole centers using a small center drill on work held in the chuck/faceplate.
Picture shows the clamp knurling attachment which was constructed along similar lines to that illustrated in the Hemingway catalogue, the main modification of course being that the unit fits onto the base unit like all the other attachments. The plus points for this particular design are that the side cheeks (1/4" m.s. plates) hold the two arms carrying the knurling rolls rigidly in alignment resulting in more consistant patterning (picture shows the high quality of knurling produced), however the main advantage is that the clamp-type design relieves the lathe of the lateral pressure applied to the headstock bearing using standard knurling heads. This is particularly important with light lathes of the class normally used by model engineers.
In conclusion, it's clear that much can be done with the rear toolpost to aid speed, convenience and quality of working. Just having every tool set up and to hand while machining means that compromises about which tool to use are unnecessary.
(c) Chris Heapy 1996
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Most recent revision 06 July 1996