The purpose of this attachment is to aid in the turning of accurate long tapers (i.e., greater than 2 inches or so). Typically this will include taper shanks for machine tooling, and any other shallow taper that needs a very high degree of accuracy. The advantages of this attachment are that turning can use fine self-act instead of using the top-slide handle thus producing a better finish, and that a much greater length of taper can be machined. The total cost should be no more than about £25 for the necessary steel sections, plus about 35 4BA cap head (or countersunk) screws, representing an enormous saving over the Myford factory product.
The attachment as described will fit all Myford Series 7 lathes, being bolted to the rear of the bed using the tapped holes which are standard fitments for mounting such accessories. The complete attachment, including the base, is fabricated entirely from mild steel sections, no castings were used at all. The 12" slide offers some 8" of useable travel, and the slide arm on my prototype can be set over +5 degrees to -12 degrees. I felt it unnecessary to provide for the more obtuse taper angle in both directions, and the range offered is of more use to me than, say, +8 to -8 degrees. In any case, the range is controlled by the placement of the locking screws and these can be located anywhere the constructor chooses (if you drill and tap multiple holes you can have any range you like, it's possible to set the bar to +/- 40 degrees on my prototype). Although not part of the original design, I will be looking to add a micrometer adjustment screw when the basic attachment is complete.
To some extent I used the Myford version as the basis of my design, mainly to ensure that the principle dimensions were workable and everything would fit. However, my version has a longer dovetail slide arm enabling longer tapers to be cut, a worthwhile improvement over the 6" usable range of the Myford design. The rest of the fabrication depended on the sizes of mild steel section I could obtain. I tried in vain to get hold of some 6" x 3/8" x 12" flat plate, but the widest I could find locally was 4". The wider section would have simplified the construction and saved a handful of screws and quite a few tapped holes. If you are able to obtain a suitable piece of the larger section then feel free to use it in place of the 2 pieces of 4" bolted together, but remember that the overall height will be 3/8" lower so you will need to compensate for this elsewhere (suggested later).
Constructing the base unit.
The base unit which carries the slide arm is constructed (see drawing) from 1 piece of bms 2" x 3/8" x 12" long (the back plate), and 2 pieces of 4" x 3/8" x 12" long screwed together with a 2" overlap (the top plate). Angled support braces are fitted at the ends also cut from 3/8" plate. The whole lot is screwed together with 4 BA cap head screws with their heads recessed into counterbores, though countersunk screws would be a cheaper and easier alternative - I just prefer to use hex socket screws when I can. Note that it's imperative that the mating bolting faces, and the corners of the support braces, are all quite square. The structure depends on this to present a flat and parallel surface for the slide arm to rest on, and also for the square alignment of slide arm to cross-slide. This is not too difficult to achieve, it just takes a little care. The support braces in particular need extra care, and these can be milled as a pair in the vertical mill or lathe and checks made with a try square. Don't forget to drill the 3 mounting holes in the back plate before final assembly, 6 mm diameter at 100 mm centers. When complete the structure is very stable and is an adequate substitute for a casting, you will also be very familiar with tapping 4 BA blind holes! Incidentally, if you were thinking that welding the components together would save a lot of trouble I should advise that this process is almost certainly going to cause the structure to warp a little. At least, it would if I did it (only having meager expertise with my MIG welder). If you think you can manage to do it without causing distortion then go for it - but don't say I didn't warn you.
The base unit, finished and painted.
Setup for machining dovetail.
As can be seen, the embryo dovetail is held down onto two parallels with 4 clamps, all situated on one side. A dovetail cutter is then passed down the clear side and it is machined to a finish. This job really needs a 1" dovetail cutter to get the depth of cut but I just managed it with my 3/4" diameter cutter. After machining the first side, and without disturbing the setup, 2 of the clamps are released and moved round to the opposite side and the nuts done up firmly - and only then are the remaining 2 clamps released and also moved to the other side. Using this procedure the bms bar will not have moved and you are free to machine the other side assured that the two edges will be parallel. To do this job on the lathe boring table will mean at least four machining setups, moving the bar between each. I guess it can be done, but you will have to use a DTI to carefully reset the work for each part of the operation. I should point out that this machining operation is likely to cause the bar to warp a little, as I discovered later. This can perhaps be avoided if you heat the whole thing up to dull red heat and allow to cool slowly, thus relieving any stresses in the bar caused by the rolling process. In any case, the six 2 BA bolts will hold it firmly to the flat part of the arm and this will likely straighten it out (it did on mine).
Another possibility for machining the dovetail in the lathe (which I've never tried) is to attach the 5/16" x 1" x 10-1/2" bar to a suitable length of hex material (perhaps 1-1/4" AF), which itself is bolted the faceplate. If you can swing this diameter, facing across the bar will give you both the 30 degree angles by using two flats of the hex bar. It may well turn out that the dovetail is turned slightly narrower in the middle than the ends (lathes tending to face *slightly* concave), but this could be quickly sorted with a fine file - only a thou or so would have to come off.
With the two parts of the dovetail arm screwed together you can drill and ream the 3/8" diameter hole for the pivot. This is marked out at the center point of the sliding arm and center punched. Use a pilot drill first (I used a No.31 drill because it was in the chuck), and follow this with a 23/64" drill and then the reamer. Make sure this hole is square to the bar or the arm won't sit flat on the base plate. To machine the ends, and to cut the curved slots for the clamp screws, I used a small rotary table (mine is 6" diameter, a 12" would have been preferable) as shown below.
Setup for machining ends of slide bar and curved slots.
To those used to loco building the cutting of curved slots will be a familiar task, and the setup you would normally use for machining expansion links can be utilised. The pivot point for the slots is the central 3/8" reamed hole - which (of course) is also the pivot of the slide arm. The curved slots are 1/4" wide and should extend to within 3/16" of the edge of the bar. A tip is to drill 1/4" holes at the beginning and end positions of the slot, this will make it easier starting the 1/4" mill and also aid chip clearance if you machine horizontally as I did. Use a slot drill rather than an endmill for this job. On one end (the right-hand end when the slide arm is in it's final position at the back of the lathe) you will want to mark the index. Whilst this is not absolutely essential it does help when setting angles initially. This is an easy job if you have a rotary table as you can use the index and handwheel to provide the degrees and fractions thereof. If you are using the lathe with the bar attached to the faceplate you will need to set up for dividing 360 divisions (at least 360 are needed - this will only give the full degree markings). The Geo. H. Thomas headstock dividing attachment is easily capable of providing sub-degree graduations. If you have neither then perhaps the only recourse is to wait until the attachment is finished, set a zero line, then set the slide to (say) 5 degrees with a combination square. This will provide you with two marks with the gap representing 5 degrees which you can measure directly and sub-divide, making markings manually with a square and sharp scriber. Another option (which just occurred to me) is that you could potentially make a separate linear scale attached to the base-plate and a pointer fixed to the slide arm. It won't be as accurate unless you want to take into account the fact that the degrees index markings will not be linear (the gaps will get narrower further from the zero point) but with a 6" radius it should be fairly close up to 5 degrees or so.
The engraved scale. Each small division = 10' of arc.
To find the position for the locking screws use a protractor or combination head to set the slide arm to whatever maximum angle you think you will need. In my case most of my tapers are narrower at the tailstock end. For example, say you wish +/- 10 degrees, you will need two holes each end as this is beyond the range of the length of the slot with a single hole (the total range with one hole being around 17 degrees). With the slide arm set at +10 degrees use a 1/4" drill to mark the end of the slots (at both ends of the bar) and drill and tap 1/4" BSF holes at this point. Swing the arm back around to -10 degrees and mark the oposite end of the slots and repeat the drilling and tapping.
Marking out the slide block with a height gauge.
Set the block up in the vertical mill (or lathe vertical slide) and machine a channel 0.325" deep between the two lines using a slot drill of about 1/2" diameter. Check the exact width of the channel with your calipers, then take a facing cut over the top to true it up (this is a sliding face so aim to get a good finish). The facing cut shouldn't be more than a couple of thou, if it requires more then take the equivalent out of the bottom of the channel or it won't be deep enough. Next, use your dovetail cutter to cut back the two sides leaving just a hint of a witness at the top edges. The last cut should shave both the side and the bottom of the channel - aim for the best finish possible here.
Machining gib strip edges to 30 degrees.
The gib strip is made from a piece of 3/32" bms section, I chamfered the edges to 30 degrees by holding the strip in a universal vice set at the required angle. You can always file or grind it to shape if needs be, the 30 degrees is not critical and there isn't too much work involved. Cut it to the exact same length as the slide block and check that there are a couple of thou clearance between the bottom of the block and the bottom edge of the gib strip, you want the bottom of the block to slide on the base - not the gib strip. Drill the 3 holes in the slide block tapping size (No.31) for the 4 BA adjusting screws at the positions indicated, then assemble the block with gib strip in place on the dovetail slide. Use a toolmaker's clamp to hold the ends of the gib strip to prevent it moving and pass the No.31 drill down the holes to make indents on the strip - don't drill right through it! Disassemble and tap the 3 holes 4 BA, and clean up the inside of the dovetail slot with a triangular needle file. Reassemble the block onto the slide again, this time with the 3 adjusting screws and locking nuts in place. Now, try adjusting the side play out of the block with the three screws and run it from one end of the slide to the other, squirt a little oil on the ways to prevent binding. With a bit of luck you should find it runs without getting stiff anywhere. If it does stick at a particular place you can unscrew the two parts of the dovetail base and a little judicial use of a fine flat file (file in one hand, bar in the other) will knock the bumps off. I had to do this a couple of times as the cutter had caused some chatter marks which I hadn't noticed. It would be best to rub the edge on a surface plate with a little marking blue to check which side needs filing, this will show up any high spots. It's important that the dovetail is truely parallel for obvious reasons.
The finished slide block.
The next job is to identify the final position for the pivot hole in the base unit. Place the slide arm in position so that it's back edge coincides with the rear of the base, and the right-hand curved edge is just flush with the right hand side (this will eventually have the index marked on it). Push a 3/8" drill into the reamed hole in the slide arm and twiddle it around to make a mark of the baseplate. Center punch the mark and then drill and ream 3/8". I have to admit that I needed to dismantle the two pieces of 4" x 3/8" x 12" plate to do this as I couldn't get a secure enough mounting with it in one piece to drill it, and it's very important that the 3/8" pivot hole is truly square. Ah well, it didn't take too long. The pivot itself is a length of 3/8" diameter silver steel. If the reamer was a hand reamer and not *quite* put all the way through the baseplate you will obtain a press fit, otherwise it just needs a drop of Loctite #601 to hold it in place. There is no torque applied to this pivot, it is for location only.
You can now reassemble the base unit and paint all but the mating surface for the slide arm and also the edge where the index mark will go. Use Myford green or grey enamel paint for a professional job that matches the original paintwork, you won't be taking it off once it's bolted in place - there's no need to unless you use the same holes for a milling head. The arm can be quickly pulled off and stored separately where it won't get covered in swarf though.
I make extensive use of a rear toolpost for various lathe operations, including parting off. So I didn't want to use the Myford arrangement of connecting the sliding block to the cross-slide. That method uses a standard pair of T-bolts in the same slot as occupied by the rear toolpost. Another option is to attach the link arm to the back edge of the cross-slide using a couple of screws, but this does mean removing the slide and drilling and tapping it to accept the screws.
The stub for attaching the link arm to the sliding block is made from a 1-3/16" length of 3/4" diameter silver steel. Turn one end down to 5/16" diameter for a length of 5/8" and thread 5/16" x 26. The plain part is 9/16" long which is the right height on my prototype (with a 1/16" washer beneath) to bring the top of the link arm to 1/32" below the surface of the boring table. If you had managed to acquire some 3/8" steel plate 6" x 12" for the fabricated base you will probably need to make this part about 3/8" longer to compensate. The other end of the stub is drilled with a letter H drill to depth of 7/16" and tapped 5/16" BSF for the clamping handle (or bolt). The mounting hole for the stub is just off-center of the top face of the sliding block (see drawing), which should be drilled and tapped 5/16" x 26 tpi to accept it. As the stub is not going to be removed again it's best to screw it in with a drop of Loctite on the threads.
The link arm is of simple tapered design made from a length of bms 3/8" x 2" x 4" long. A piece of 1/4" bms is screwed onto the end with three 4 BA countersunk screws, and this backplate is used to attach the link arm to the rear end of the cross-slide. This 1/4" thick end plate needs to be profiled to match the end of the slide, in particular clearance needs to be cut for the cross-slide dovetail (note that this cut-out is NOT central to the cross-slide). Machine the 3/8" slot in the link arm first whilst the piece is still square and can easily be held in the machine vice, then cut to the profile shown in the drawings. I found it easiest to use the bandsaw for this job, the part being clamped at an angle for each of the two cuts, finally finishing the rounded end on a grinder. The link arm is attached to the cross-slide with two 2 BA cap head screws, and the right hand one in particular requires careful positioning so it does not interfere with the gib strip adjustment screws.
Milling the 3/8" slot in the link arm.
I found removing the cross-slide un-necessary in the end, and managed to drill and tap the holes in the cross-slide for the backplate using a small re-chargeable cordless drill. Whichever method you prefer, unscrew the backplate from the link arm and mark out the positions of the two screws, then drill tapping size (No.22 drill). Place the backplate into position on the back of the cross-slide (make quite sure the slotted arm will be level in this position, clamping it down twisted will cause the cross-slide to bind) and hold it there with a toolmaker's clamp into the rear-most slot and across the plate. Use the two holes in the plate as a template to drill 1/2" deep for the securing screws, then remove and tap them full depth finishing with a plug tap. Open up the holes in the plate with a No.13 drill then reassemble the two parts of the link arm. Two 3/4" long 2 BA screws are finally used to secure the arm to the cross-slide.
Profiling the link arm with the bandsaw.
The link arm slot is 3/8" wide so as to accept a spacing bush. This bush is necessary because when you want to disconnect the taper attachment it's necessary to leave some clearance (1/16" in this case) between the top of the stub and the link arm. The link arm will normally stay in place attached to the cross-slide, there being no reason to remove it. The bush is turned from 3/4" brass and needs to be 3/8" overall length. One end is turned to 3/8" diameter x 5/16" long, and drilled right through 5/16" diameter. The bush is then parted off leaving a 3/4" diameter head 1/16" thick on the other end. It is placed on top of the stub and the clamp bolt passes through it and screws into the stub. To disconnect the cross-slide from the slide arm the bolt is simply removed and the bush recovered, this then leaves ample working clearance. If you really like making ball handles (and I do, well sort of - I like the look of them) then this would make a fine clamp handle for the link arm instead of a bolt, but if you can't be bothered then a bolt and washer will be just as effective.
The completed link arm.
Using a DTI to set the slide arm to zero position.
Having made the attachment you will be fully aware of how it works so I don't need to tell you. You will have the satisfaction of having saved considerable cash and the tool you have is at least as good as (and in some respects better than) the Myford version.
Using a bush to drill 2MT location hole.
One tip - it might be useful at some point to set the bar very accurately with a DTI for some commonly required taper (such as MT2 or 3, or Jacobs chuck tapers) and then drill and ream through the slide bar and base at some point near one end for a close fitting 5/32" dowel pin. Use a hand drill with guide bush for the drill bit (to keep it square) and so you don't have to dismantle the attachment from the lathe. There is plenty of room for several of these holes and they will do no harm. Stamp the setting next to each hole. Next time you want to make a 2MT shank it will take but a few seconds to set the arm with the pin and disconnect the cross-slide, the shank will be machined very accurately and will fit the socket first time with no question of filing or polishing it.
Locking bolt and dowel setting pin, placing the pin in the hole sets the slide arm angle to 2MT.
One last point. When you re-attach the cross-slide handle bracket make sure you screw the slide *all the way in* before you tighten up the screws. Otherwise it will be likely the feedscrew will not be properly aligned with it's nut and wear will occur quite rapidly.
The next project will use this attachment as the basis for a profile copying device, so you might get more use out of it than you thought!
(c) Chris Heapy 1996.
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