photo 1: Finished post at work on author's Logan.
(click on photo for a closer look, 85 Kb)
Note, several years after I wrote it, I revised this article extensively. (04/03/00) Photos which were lost when Bill Gray's disc crashed have been replaced, and some new information and pictures added. -KF-
Hopefully this article will inspire others to build this useful accessory. There might well be a few ideas here that will prove useful even to old shop hands who take on this project, but it is aimed primarily at beginners. Old hands will have their own favorite way of performing an operation, and have little need of advice from an upstart like me.
I need to thank Fitch Williams, who started a toolpost slightly ahead of me, and finished long before I did. He found a couple of things that needed changing, and talked me out of one or two dumb ideas. I doubt this project would have turned out as nicely as it did without Fitch's encouragement and good advice.
I credit John Stevenson with a elegant and solid design, but I was prompted to write this article because I think his tooling and experience exceeds my own by quite a bit. I had to do some things my way, because I don't think _I_ could have done them John's way. I'm grateful for his encouragement to go ahead with presenting my experience in this project.
I'm going to confess to some of the mistakes I made along the way....early on I would get pretty discouraged when I'd scrap a part. I think much of the hobby literature contributed to this, for several of reasons:
In order to understand my comments, it is important to read Stevenson's article ...I won't make much sense otherwise. I'll try to stick to his terminology, even if it isn't what would come first to my american mind. Enough preliminaries, lets get to the meat of the matter:
John specified BDMS for the wedges, known in the States as as CRS. This is a rather difficult material to turn, as it wants to tear rather than cut. I scrapped a couple of parts while experimenting with speeds, feeds, tool geometry and coolant. Using 12L14 or some such would have made things a lot easier, I think. (Update: After three years, the threads on the wedges are gitting a little loose: I'd consider making them from O-1 and tempering to a dark straw next time) A thin mix of kerosene and 30W oil, small nose radius, edges honed to mirror finish, plenty of rake, >50SFM, and 5-6 thou per rev. gave a fair finish. I turned to about .001 over, then filed and emery papered to "dead nothing". Center the work carefully, as the 1" stock dimension is used without a truing cut.
I deviated from Stevenson's instructions in that I didn't face the wedges to length in the lathe. Instead I left the larger ends as-sawn and about 1/8" too long. Prior to cutting the dovetails, these were fly cut to length while installed in the block, insuring the length was exactly correct. This also means only one spacer, of non critical length need be made for the milling. If you look carefully at photo 2 you might be able to see that the milling marks from the fly cutter align exactly between the wedge face, and the tool block.
photo 2: Wedge face is a perfect match to block, as they were milled at one setup. (71 Kb)
I first translated 3/8" BSF thread to 3/8"-24. I scrapped a couple of wedges before I decided that I would never get a smooth thread at the low speed required by my reflexes. Finally, I settled on a technique of threading to half depth, then finishing with a die.
Then I decided that 16tpi would be more durable, so I started again. Murphy lept on this opening, and loosed the setscrews on my tailstock, resulting in a .005" taper. In the end, I turned 14 wedges to make 4 good ones. (I started out to make two posts) Bits of the mistakes became washers, adjuster nuts, etc....in a couple of weeks, I had managed to use up (hide!) almost all 10 of them.
My 3 jaw is a pretty sad affair, so I centered each wedge "dead nuts" in the four-jaw.
On Fitch's recommendation, I waited until the block was bored to mill the wedges. There will be some inevitable runout in the bores and wedges. They will likely fit best in just one configuration, and possibly not at all in others. By putting off the milling you can choose the optimal orientation. In an age of mass production, we sometimes forget that parts don't always need to be interchangeable, and that hand fitting and matching can often make the "one-off" job come out much nicer than "to print" production items.
I left a short 3/4" shoulder on the side wedge, thinking this would decrease any tendancy to bind in the main block. I would advize others NOT to do this, as it precludes an improved methode of boring the block, as I'll explain below.
My choice of material really hurt me here. Culled from the local scrap yard, the man said "It's either 4130, or 4140...be careful or it will harden up on you". Well, I can't say he was wrong, as it gave a much nicer finish than the 1018 I used for the wedges, but I found that I couldn't drill it without work hardening the walls of the hole...made for a lot of effort, and frustration, but determination saw me through it. Lesson to others is that there is always risk in bargains.
As the opening photo shows, this is one big block of steel, looking a little oversized on the 10" Logan.
For a better fit on my machine, I moved the mounting hole 1/8" away from each bore (toward the "arrowhead" corner) Beveling the rear and right upper corners disguises this offset, retaining a symmetrical look. Though the Logan has a greater swing than the Myford, the Myford must have a more expansive compound. Fitch said when he set the post on his 9" South Bend "It looks like a part fell off a D9 Cat (A large bull dozer) and landed on the lathe"..He's considering making a scaled down version for that machine.
I moved the 1/2"-20 hole off-center (towards the dial) in the T-nut, to bring the tools yet a little closer to the compound. After using it some, I can add to Fitch's testimony that it is at least a solid as it looks...looks that I'm getting quite used to, by the way.
Fitch pondered some over how high to place the wedges, since his Taiwan lathe had a different center height than the Myford John had in mind. In the end, it seemed that the wedge center-line should be at the same height as the spindle's. So if you use a between centre boring bar, you can just bolt the post to the compound via the mounting hole, and have at it....you don't even need a T-slotted cross slide. But then if you don't have a slotted cross slide (as I don't) then you probably don't have the between centre bars (as I also don't)...another way was needed.
The 6:1 ratio of length to diameter makes the 3/4" bores the tool block's most demanding operation. After a difficult time on the first bore, I figured out a much easier way to do the second. While I used a boring head on my mill-drill, the idea should work equally well on the lathe.
The trick is to bore the 1" diameter first, rather than last. Then install the side wedge (use the side wedge regardless of which hole you are boring) with the stepped washer upside-down. You can now indicate on the washer's 3/4" step, and bore the 3/4" diameter from that end, needing to go only 2" deep...an inch less which makes a big difference in difficulty. I hung the block out the side of the mill vise, and was able to remove the wedge due to the height of the vise rotary base. If doing this on a lathe with a spindle bore less than 1", remove the chuck to get the wedge out of there. This is a nice use of parts you have to make anyway to "fixture" the job. For best accuracy, the washer hole should be bored to a close fit on the wedge....I ground a short boring tool from a 3/16" toolbit.
This is why I said _not_ to put that 3/4" diameter on the side wedge as I did. To make the above work, I had to bore some of the 3/4"diameter from each end. Of course that left a step, but the first hole was a good fit on the front wedge, and the 3/8" shank of the side wedge cleared the step by a wide margin
Purists will point out that doing it this way will not produce ideal alignment, and they will be correct. However, because the 1" stock diameter will not be perfectly aligned with the rest of the wedge, a slightly skewed bore _can_ be a better fit than a perfect one!
Because only the front wedge really requires the 3/4" bore (except for the washer on the side wedge) you get two chances to get one "just so". Remember, until you cut the dovetails, you can still swap the wedges for best fit. If you are lucky enough to make the first bore a really good fit for the front wedge, you can save some trouble by only boring enough of the other 3/4" diameter to accept the stepped washer nicely.
If you follow my example, and do the boring on a mill-drill, you will find there is not room to get a caliper in under the boring tool when it is withdrawn, making it difficult to check your progress. Neither will there be room to remove the boring head from the spindle, and it is of course undesirable to unlock the table once boring has commenced...same goes double for raising the head. Removing the boring tool from the 'head loses it's zero...what to do?
I decided to sleep on the problem, as I felt I must be overlooking something. I was: unscrewing the boring head from it's shank gives lots of room, without disturbing any critical settings! A trick that may come in handy regardless of the project....would even save "real" mill owners some cranking up and down.
I chose to use soldered-in pins (71 Kb) acting on the side of the wedge, instead of a dog point screws to keep the wedges from rotating. If you place them in the sides, then the slot for this pin can be milled into the front wedge at the same setup as the 3/8" slot, saving you from the difficulty of milling the two slots at exactly 90 degrees. Also, putting the slots in the sides avoids cutting the dovetails through the slots, which seemed ugly to me. (Sorry John!) This photo (90 Kb) of the side wedge shows the slot for the pin, and also my 3/4" shoulder non-improvement. Note that the slot does not extend to the face. I think this is a good idea, even if you choose to stick with Stevenson's design otherwise...One less cranny for chips to collect in. Wait 'till after the dovetails are milled to solder in the pins, so you don't need to slot the spacer ring. Whatever method you choose, mill the anti-rotation slots into the wedges, and pin (or screw) them whilst milling the dovetails to guarantee alignment.
Before drilling the holes in the top for the depth stop screws, measure your screw heads. Fitch found that when placed as shown, the heads didn't quite clear the dovetail, and had to be milled off some. This may be due to american screws having slightly larger heads. I moved my screws back by 1/16" to be well clear. A taller toolblock would avoid the need for the screws altogether, but this is NOT a good idea, as the screw heads shed swarf, and thus avoid trapping debris under the adjuster nut, and sisturbing the tool-height adjustment.
The last step is milling the dovetails on the block. I was very nervous about this but it went well. Make some tool holder blocks first, to get used to using the dovetail cutter without risking so much work, and you'll need them to gauge progress on the block. I made a pair of KDK tool holders for a friend, which have female dovetails with near identical dimensions as the tool block. Actually, the first dovetail on the tool block went so well that I let my guard down, and the second has one point rather duller than I intended.. works quite well, but I was angry with myself for two days over that.
I made my parting blade holder differently than John showed.
John's parting blade holder drawing.
I had several of the "T" cross section blades, which don't clamp so nicely under countersunk screws. Since this is quite different from what John describes, I tell how to make one in some detail:
First you need to work out how much rake angle you want. Figure out how large a diameter you think you might be able to part with the blade you intend to use. I hope to part 3" stock with a 1/2" tall blade. (optimistic perhaps?) With the rear of a 4.5" long blade on the compound and the tip at center height, figure out how high the lower edge will be 1.5" back from the tip. ~.3" in my case. Thus I have .25" of steel under the blade, and .050 extra adjustment. 4.6 degrees was the most rake I could use and without having to hang the blade off the side of the compound. I've used this tool on brass and mild steel with good results. More rake might cause trouble on brass.
At first, it seemed impossible to do this job on my mill-drill with no tilting head, and no tilting vise. It took me a while to work out just how, but it was really easy. Start with the wedge, just 3" of 1/2" square CRS. Drill four .250 holes .050 off-center, and counterbore (I used a 4 flute EM) to 3/8. A 3/8 counterbore does not break through the wall, so you can hold the stock well down in the vise. Set this part aside, as the remaining operations on the wedge will use the main part as a fixture
I clamped the block in the mill vise by it's ends, at a 30 degree angle. If you have Angle blocks, use them...I just used the pendulum angle gauge in a "handyman special" tri-square; good enough for this job. The wedge slot is milled parallel to the long axis of the stock. A line was scribed 1/2" above the height where the top of the blade was to fall (scrapped one block coming up with that dimension), and the the table was positioned to place the corner of a 1/2" 2 flute on the line. The slot was cut to depth (.550 ) by feeding the Z axis only. Take very shallow cuts so that the mill leaves a nice finish on the walls of the slot. The four 1/4-20 tapped holes are placed .050 above center, using the same hole spacing as the wedge (which I leave for you to work out).
The stock I used for my wedge was zinc plated to slightly over dimension. A little work with file and emery paper soon gave a good fit in the slot. Install the wedge with a .060 shim under it...I used three layers of precision wood fiber shim stock in which bottles of a favorite beverage (Red Hook ESB) are supplied! Set the holder flat (on parallels) in the vise. I then milled the slot for the blade with a 3/8" e.m. using a 1/8" e.m. to create some clearance for the wider top part of the blade. It would probably be better to dovetail this slot a few degrees, but lacking a proper cutter to do that, I left it square, which seems to be working fine. If using the hollow ground type blades, dovetail the slot 5 deg. top and bottom.
When trial fits with a parting blade show a rather tight fit, call the slot done. Now remove the screw from one end hole, and mill the outer surface of the wedge until the mill is near the second screw. Stop the mill, re-install the end screw, and remove the middle two. Put them back after milling up to the last screw. Should be obvious how to finish the wedge. Cutting off this outside corner helps clear the chuck jaws when parting nearby.
Prior to milling the dovetail, I took ~1/2" off the back of the block, with the aim of reducing overhang...if you start with 1" x1.5" stock instead of 1.5" square, you'll save this step.
Next the dovetail was cut on the back at the rake angle. I don't have a sine- bar, (or jo-blocks for that matter) so I got the angle exactly right by indicating on the fixed jaw, and "tangent-ing" with the X & Y machine dials.
After the dovetail was cut, I clamped the post in the mill vise, and used it to hold the blade holder (upside down) while I milled off the lower rear corner. Then the holder was turned over, and the hole for the adjuster stud was drilled and tapped at the 4.6 degree angle. Doing the work in this way avoided the need to make any compound angled cuts. The resulting irregular rhombus shape of the holder looks a little odd, but I kinda like it! Photo 5 also shows that I under cut allowing just a little extra reach if the blade is hung off the compund. The meager support under the rear of the blade is no problem, as cutting forces push the rear of the blade upward, rather than downward.
I don't like scars from set screws on my boring tools, so I split my boring bar holder and clamp that way. (photo 69 Kb) The slot was made with a pneumatic cutoff tool, and doesn't look nearly as nice as it works. I've just bought a slitting saw for the mill, so will make another at some point. (Update: I used the saw to clean up the slit from the abrasive blade, rather than starting over..the slit is slightly off-center the bore, but no harm)
I was planning to make a clamp type knurling tool to custom fit the post, but found one available for US$30...(Update: they have gone up to nearly $50, but Enco still has the lowest price I've seen) if it turns out not to be a bargain after all, then I'll be back at the drawing board. You will note that the adjuster nuts in the cutoff tool photos are not knurled. (yet!) Update 4/30/00: The knurling tool worked great, except that when the 5/8" shank was in a normal tool block, it stuck so far forward that the I could barely retract the cross slide far enough to use it. So I made a custom tool block ( 99 Kb) that dispenses with the shank altogethor, saves a few inches, and holds the knurling tool more securely as well. Note that there is no adjuster on this 'holder, as the height is non critical with this "pliers" style of knurling tool. The fact the most of the middle of one side of the dovetail was removed by the large counterbore doesn't seem to matter either...it works great.
Your imagination and needs are the only limit to the handy accessories you can make to fit this toolpost. I plan to keep a few "blank" (dovetailed only) holders on hand, to be customized as the need arises. The dovetails can be cut four at at time by stacking parts in the mill vise. A C-clamp (UK G-clamp) to compress the stack is a good idea for safety though. One great decision I made was to buy a really good 1/4-20 tap. This thing all but turns itself, and has taken all pain out of the many tapped holes required when making up a brace of holders.
One very handy accessory is this special tool changing wrench (90 Kb) I made. The brass head prevents mushrooming the threaded end of the wedges when you give them the tap needed to free a tool holder. Also, you need only find, and bring to hand a single tool to exchange bits. The brass head is od 1" stock, and is milled out to suit the wrench. It is held on with JB weld. Cut off one end the wrench, and roughen it on the grinder to help the bond. The grooves in the head are not only decorative, they limit the size of chip that might split from the head after extended use. I liked this so much, I made a larger version with a 17mm wrench for the draw-bar on the mill-drill.
So far the toolpost has been all that I hoped for. I'm glad I pressed on in spite of the difficulties. Thanks again to John for the great design, and for permitting me to link in his drawings.... and to Fitch for the encouragement. Also to Bill Gray for providing a place to share this information. If you choose to build one, I think you will be pleased with the outcome. I welcome correspondence with those interested in making their own.
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Kevin's home page
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Photos by author using Kodak DC215 Zoom digital camera. (except for parting blade holder, which were done with a Casio QV-10)
Linked drawings by John Stevenson
Last revised: 4/04/00