Standard Myford tool clamp and 'quick-setting' tool and toolboat.
Standard Myford tool clamp and 'quick-setting' tool and toolboat.
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My next move, on my old Myford ML10, was to construct a 4-way turret which accepted 1/4" HSS bits ground to shape. This was a simple affair which did not stop at indexed positions but required a small square to be used off the top-slide against an edge of the turret. Accuracy was not great but speed of working increased considerably. I made several turrets some of which held boring tools, others threading and chamfering tools, still others bar turning tooling. One turret held a general set consisting of knife tool, 60 degree threading tool, small parting tool, and a LH cranked round nosed tool.
When I purchased my new Super 7 I also acquired the Myford indexing 4-way toolpost. This accepted larger tools (5/16" sq) which is actually of little advantage compared to 1/4" bits which cut just as well and are cheaper and, as it's name suggests, the turret indexed around it's 4 positions using a hardened ratchet as a stop. I have not measured the repeatability of it's setting but I have read that the system used is not of the utmost accuracy (about .003" resetting error can be expected). An improved design has been described by Geo. H. Thomas in ME magazine, and also in his book "The Model Engineers Workshop Manual". Plans and materials are also available from Hemingway. The improvements mainly concern the indexing mechanism and the use of a large conical seating in place of the flat seating of the Myford original. I have found with the Myford version that if the clamp is done up only reasonably tight (not over tight) distortion of the top-slide occurs and it is noticeably more difficult to move. The cause of this problem is elusive as the base of the turret is flat (I checked it) and so is the top face of the top-slide, so I can only assume that perhaps the holding stud is not square and it is forced so on tightening, thus distorting the slide. One modification I made was to make a new clamp washer with a ball thrust bearing fitted to the underside. This clamps up tight with less effort, and more importantly, releases easily without having to clout the handle with the palm of the hand.
4-tool turret thrust bearing.
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One problem with 4-way toolposts is that sharp tools are poking out from all four corners, just waiting to catch the unwary. I lost a few choice bits of flesh this way at first but have never taken the trouble to cover the tool bits, one learns to avoid them! Another problem is that the turret interferes with accessibility when working close to the tailstock, especially when a bulky rotating center is in use. Turning the turret to one side does not usually help much as the cutting angles of the tool are not presented to the work correctly. To correct this defect Dave Lammas (in ME magazine 1985, Vol 155, No.3758, pp136) designed a triangular 3-way turret. This design allows for much better access to the workpiece at the expense of the loss of one tool position.
The Lammas 3-way toolpost.
Another option is to use quick-change toolholders, but these are not cheap. Each tool requires it's own special holder which attaches to the base unit using a dovetail fitting and a simple quick-acting clamp. Jack screws on the holders allow the height to be set for each tool so that repositioning accuracy is good. The tool can generally be fixed to the base unit in one of two positions, either for normal turing or facing the headstock. Given the number of different tools that I use I would need at least a dozen holders, and the fact that the base unit would need to be removed to use any other tool not held in a holder, I can't say that this system is either economical nor efficient.
Mention should be made of the tool 'boat' design, common in American type toolposts and also the special Myford slide rest tools whose shank is machined to sit on a boat. Whilst this removes the need for shim packing the action of rotating the tool on its spherical seating to set tool height alters the rake angles. Not too serious for bar-turning tools, but more so for threading tools. Unlike shims, releasing the tool from the clamp to reposition it requires re-setting to center height (it does with the Myford version anyway). The system has less to offer to my mind than simple shims which, once set to height, can be slid around to any position. The Norton type of toolpost was also designed to dispense with shims, using as it does a central round pillar onto which clamps a split rectangular bar with a locking bolt one side, and a hole for mounting the tool on the other. This design has nothing to recommend it either, each tool swap requires the block to be re-set to center height so speed is again slow.
Photo shows Myford tool boat and tools.
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Toolholders for boring tools are best made to attach to a rear toolpost (described below). This will give the most rigid support. An exception is where a tapered bore is required and the tool must be mounted on the cross-slide. Commercial boring tools with a square shank cannot be adjusted for overhang, and a better option is to use home-made boring bars with inserted bits mounted in a purpose built holder. The holder can be a simple V-grooved rectangular bar under the normal tool clamp, or a bar which has been bored at center height, then split with a fine saw. More sophisticated versions use a split bush with an eccentric hole so that tool height can be set by simply rotating the bush in it's bore. Again, either the normal tool clamp can be used or screws passed through the split to clamp the tool.
One remarkable tool holder was described by Tubal Cain and called the 'Gibraltar' toolpost. This large casting bolts to the cross-slide directly and it's only function is to hold the tool as rigidly as possible. It's particularly useful for machining crank axles where tool overhang is unavoidable. The fact that this holder is so successful serves to demonstrate the importance of rigidity.
Other specialised variants include designs by Geo H. Thomas for retracting toolholders. These are specifically for use in threading operations where it's useful to be able to quickly withdraw the tool at the end of a cut so that the carriage can be traversed back to the start point. The holders are designed to reposition the tool point to precisely the same depth setting so measuring the depth of cut is made easier (the micrometer dial only needs to be turned inwards instead of both inwards and outwards to withdraw the toolpoint). The first example of this type is a stand-alone holder which can be held in the 4-way toolpost, a small lever is moved forwards/backwards to position the toolpoint. A second example is more sophisticated in that it is the entire topslide which is retracted carrying the tool with it. Both of these designs work on a camming principle and are detailed in his book 'The model engineers workshop'. The plans are also available from Hemingway.
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3.2 Rear Toolpost
I've given some information already on the advantages of a rear toolpost in the construction article for the version I made for my own lathe. A wide range of attachments can be mounted on the rear toolpost quite apart from lathe tools. If you are going to mount a parting tool in a small turret it is essential to make sure that the blade is held at exactly 90 degrees to the work or it will jam in the cut. All but one of my turrets don't index as I felt this facility was not essential here. Anyway, it's awkward to machine more than one to index correctly with the same base unit. However, the square sides to both the turret and base offer good datum surfaces, and for the parting tool I simply set the right-hand edge of the turret square with the base using a handy flat piece of steel, clamped it up then set the blade square to the lathe axis. So if I need to swap turrets it's very quick to reset - just hold the flat steel against turret and base and tighten the clamp.
I have many small turrets made up to hold specially shaped 1/4" HSS bits for those odd jobs. The turrets are very simple to make from 1-1/2" square ms section, just saw off a chunk, face both sides in the 4-jaw and drill through 5/16", then mill the slots in the lathe or vertical miller, finally drill and tap for the clamping screws.
My rear toolpost base is also used to hold tooling for boring, knurling and spherical turning, but these items will be described later.
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Again, in the home workshop environment we can get away with using shapes of tools which would be useless in industry, their tool life under stressed conditions would be counted in seconds, but at the slower speed, smaller cuts and finer feeds we have to use on a light lathe they have some definite advantages. The use of plenty of top and side rake, and side clearance, allows the tool to cut much more cleanly, but at the expense of a somewhat weakened cutting edge. The nomenclature of tool angles is given in the diagram below:
The illustration above uses a 'roughing' tool as an example, the positive approach angle is used for deep cuts and throws the chip away from the work, but at the same time the cutting forces will tend to push the tool away from the work (and vice-versa) so that some springing will be evident with long, unsupported workpieces. My favorite all-purpose tool is shown below, and is a simple knife tool shape. This will take 1/8" cuts off cleanly forming tight continuous curls (provided there is no negative approach angle), and will produce a reasonable finish too. I always use more top-rake than is thought correct but this gives a cleaner cut. It's not ideal for brass, which (in theory at least) should have very little top rake to avoid dragging the tool into the work, but to be honest, I find it works well enough and doesn't throw brass chips quite the 10 metres radius as does flat top tooling! The difference between this tool and the roughing tool is that there is no positive approach angle, in fact this is usually slightly negative (1 degree or so) so that the tool can be used for facing as well as turning at the same setting. If setup like this you cannot take deep turning cuts with it (no more than 50 thou - otherwise the tool will be pulled into the work and cut undersize, finish will be poor, and chip clearance is worse resulting in a tangled 'bird's nest' of swarf around the tool). With lighter cuts the finish is good, especially if a small radius is allowed on the leading corner and a small trailing flat. If the above is a little confusing I should point out that a trick I use is to give the tool a small negative approach angle, but when I want to turn a significant amount of metal off I turn the turret slightly anti-clockwise so that I can take deeper cuts. I turn the turret back when I want to face the workpiece. If I want to turn AND face at the same setting I leave the tool as it is and put up with taking finer turning cuts. Hope that makes sense.
Illustrated below is a home-made holder for triangular tungsten carbide throw-away inserts. I only made it because I was given several packets of the tips and had no other way of making use of them. The tip shape is similar to the all-pupose knife tool and will turn and face at one setting. The only problem is that (as with most tipped tools) back or top rake is not allowed for, so more pressure is required to make it cut metal. Not so bad on substantial chunks of steel (above 1/2" dia) but not so useful for fine work as it causes the work to spring more. Each tip has three cutting edges, and as one becomes dulled the tip is simply indexed around to bring a fresh one into play.
The threading tool shape will depend on the technique you use for putting on the cut. See cutting V-threads for more info on this. As I use a 'set-over' technique for cross-slide mounted threading tools I can use 7 degrees of side rake on the cutting edge. This means that left and right hand threads require separate tools of course, but this is a minor disadvantage compared to the improved cutting action. On ordinary mild steel flat-topped tools (for 'straight-in' feeding) work well enough and give a good finish, but try the same on tough stainless and you will appreciate being able to use a little side rake and cut only one side of the thread.
The finishing tool has only one purpose, and that is to take a fine scrape leaving as good a surface finish as possible. It is awfully tempting when using single tools (i.e., no indexing turret or QC holders) to use the finishing tool to turn to size AND add the finish cut. Better that you consider the finish tool to be the finely-honed precision instrument that it is and use it for nothing else. The characteristics of the tool are that it should have a large-ish area of cutting edge in contact with the work at any one time, and that there are no sharp angles (leading or trailing) to gouge the workpiece. These requirements can be met either by a wide flat-fronted tool with radiused corners, or a round-nosed tool of very large radius (or more correctly the side of an ellipse). Either will do the job. You should take a finishing cut fairly fast (hence the need for a large cutting edge), as trying to take a slow fine cut will result in intermittent cutting. Normal ms (not free-cutting) is a bu**er for doing this, you will be going fine then the tool stops cutting leaving a 'hill', then starts cutting again leaving a 'trough'. Good cutting fluid helps minimise this effect a little. The finish cut can be very fine if the tool is up to it, 0.0002" is not incredible (taking about half a thou off the diameter), but more usually 0.002" will be satisfactory. Hence the need for a very finely honed tool, and also explains it's delicate nature.
As far as tables of cutting angles are concerned I don't feel they are of much value to the home machinist. They are of very great value to the production shop for repetitive work, where tooling has to be optimised to the job for best economy. But I don't see your average model engineer grinding up a tool with the correct angles especially to cut that bit of Naval Brass or aluminium bronze. No, I think he's more likely to attack it with the old reliable knife tool. However, I think there are broad classes which can be dealt with as deserving their own tooling; Cast Iron and hard alloy steels are best dealt with using carbide tipped tools, Ordinary steels and phosphor bronze can be readily machined with HSS tooling (cutting speed and lube are more important for getting a good result), for brass your steel HSS tooling will work OK but tools with less rake and kept very sharp are better - don't try cutting brass with a tool that's been used to cut steel without giving it a rub with a slipstone, Aluminium can also use the same HSS tooling as used for steel for most jobs, but more rake and a higher speed (plus paraffin as a lube) will give a better finish.
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The main requirements for successful parting are that the lathe mandrel bearings and slides be in good condition and well adjusted, that the tool be as rigid as possible with minimum tool overhang, that parting should be done as near the chuck jaws as possible, that adequate lubrication should get to the blade and cutting edge, that the blade shape should encourage chip clearance and straight cutting, and that the tool should be at 90 degrees to the workpiece so it is not deflected to one side or the other as the cut progresses. Lathe speed should be about half that used for turning work of a similar diameter - don't go slower as you want the chip to come off as a thin continuous ribbon, and with very slow speed it's easy to end up cutting a thicker chip by mistake which might jam. Quite a list of requirements, but if you can satisfy them all you'll have no problems.
My first parting tools were laboriously ground from 1/4" square HSS bits with the blade portion about 1/16" by 3/8" long. This type of tool is fine on a small scale because it's easy to give it good clearance by making the tip wider than the base, thus alignment is far less critical. Unfortunately it also weakens the blade at a critical point so that parting large diameter work (larger than, say, 3/4") is unsafe. I also bought a commercial 5/16" shank carbide tool which worked well until the end snapped off (it was a cheap tool I guess), but it never had the capacity for work larger than 1" anyway. For larger diameter work I bought an Eclipse parting tool holder with a 5/16" x 1/16" section blade and had no end of trouble with it. It would *always* dig-in at critical moments and parting work larger than 3/4" was a nightmare. I eventually concluded this tool simply wasn't up to parting larger diameter work. The blade shape was a bit odd, having side rake for some reason I'll never understand. This was just asking for the blade to cut on one side more than the other leading to deflection and eventual dig-in. Grinding the top flat helped a little but I think another problem was a simple lack of mechanical strength of the narrow section blade making it very susceptible to deflection.
I replaced the 1/16" blade and holder with the larger version Eclipse tool with a 3/32" x 1/2" blade. Since then I've had no problems at all, not a single dig-in that wasn't directly attributable to an insecure workpiece. I make sure the blade tip is ground dead square, and in addition add a little back relief using the edge of the grind wheel. I know this means that as the blade is sharpened the center height will change a little, but this is easy to compensate for and it's worth it to get the performance. Now I can part 2" diameter mild steel, under fine power cross-feed, with no problem.
(c) Chris Heapy 1996.
3.3 Turning Tools
Most of my lathe work is done with ground HSS toolbits, with odd jobs catered for with carbide tipped tools (for castings) and carbon steel (special form tools). I find HSS good to work with, can be sharpened and honed easily, and in the home environment the cutting edge lasts long enough for it not to be chore to keep them sharp. I'll cover sharpening of lathe tools in a separate section.
TOOL ANGLES
THE KNIFE TOOL
Home-made tool for holding carbide inserts.
THE THREADING TOOL
THE FINISHING TOOL
Photo shows (left to right) finishing tool, knife tool, and threading tool.
3.4 The Parting Tool
Few things instill more apprehension in the inexperienced model engineer than parting large diameter workpieces, especially those who have had a bad experience with the job. To have a parting tool dig-in and break is not one of my all time best experiences. More irritating than anything was the unpredictability of the problem, sometimes the job would go fine, and then just when you were getting confidence the damn thing happens again. My problem was lack of understanding of the forces involved and the mechanical requirements for the operation to succeed. One could do much worse at this point than to read Geo. H. Thomas' article on parting and parting tools in his book "Model Engineers Workshop Manual", where you'll find a wealth of practical advice.
Photo shows selection of parting tools (left to right) 1/16" and 3/32" Eclipse replaceable blade holders, carbide tipped, and HSS tool ground from 5/16" sq bit.
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