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7x10, 7x12, 7x14 Mini-Lathe Information

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7x12 Mini-Lathe

                  A 14" bed extension kit was ordered from Chis Woods at Little Machine Shop, part # 1928.   Nearly all critical
    machined surfaces had some rather thick paint slopped about on them and this would have to go. The worst of it was in
    the area inside the webbing where the tail stock would register. Stripping required.

          First a critical dimensional check. The beds are ground so a point of reference for the grinder is needed and this would
    be the foot area. This makes it fairly straight forward to qualify the casting by placing it on a surface plate first checking for
    rock, indicating a twist and using a height gauge checking for bow or warp and flat/level relative to the surface plate and the
    "V" way relative to the flat way. The "V" requires multiple checks as angles are involved and the top of the "V" in truncated.
    This was done with the top half of a bed lock as shown in the photo below.


           Once the "V" is qualified then direct micrometer  measurement from the top of the "V" to the lower bearing area for the
    saddle gib area is assured. If the "V" is not qualified then the bed lock block and micrometer is required for thickness
    checks.  This bed checked out dead flat and all  relative positions in alignment except that the lower bearing block area for
    the saddle gibs, which are milled and not ground, displayed slight thickness variations which will be corrected latter when
    the saddle is fit to the bed.  

          After the checks were done the entire bed was stripped inside and out. Machined exterior surfaces were masked with
    Duct Tape to prevent nicks and dings while the rough handling was going on, there is no gentile way to do this. A spray
    type stripper for epoxies was used as gel types were quite ineffective. Small brass bristle brushes were required in the
    rough areas of the internal webbing and motor mount areas and repeated coats required even then. A good washing in
    a bucket of mineral spirits followed as well as allot of fine filing of the burrs and sharp edges.

    No serious voids  were uncovered but a good deal of casting sand was eliminated during this step. The part, sand cast,
    uses about a dozen cores, none of which line up well. Ergo there is a good deal of very soft filler used in the worst areas.
    As you can see from the photo just to the left of the height gauge most surfaces not ground or left natural, are fly milled
    and not gently.  None of the red you see in the photo was present after stripping. This is a thin coat of auto body glaze
    after sanding with a block sander. The interior of the webbing and motor mount areas are already painted with Glyptal
    electric motor insulating paint.  It's tough and quite resistant to oils.

    Painting the interior is a very tedious task and very time consuming. Masking the machined areas is nearly impossible so
    a steady hand, small art brushes, lots of angles of attack are the weapons to prevent re-coating the areas you just spent
    so much time stripping.

    The bed as delivered weighed in at 25 lbs. After stripping and knocking sand out of it and removing soft fillers, 24
    lbs. The 14" beds have several more internal webs and the right foot is fully cast unlike my short bed machine.  The
    motor mount area is wider to accommodate the larger motors normally fit to these machines as well.  Both feet of the
    casting are also longer.

    With the bed initial preparation completed and measurements qualified the next task is to fit the saddle assembly.
    Don't under estimate the importance of all the detail work. Everything references from the bed of the machine, if it
    isn't right, nothing else will be either. T'is the very foundation of your project.


          Using the height gauge a near side reference is marked to locate it in the same position each time.  Height is set from
    the base of the compound dove register as this is referenced not only to the bed but will be the reference for the compound
    later when fitted.  Note I used the gauge foot extension to do this to get as much distance between points as possible.
    Once zero is set the height gauge is rotated to the back . A dial indicator on magnetic base set and zeroed. Now pressure
    is placed upon the near side of the saddle (operator side) using the "V" as the fulcrum which will lift the back side until it
    contacts the height gauge and down hill distance can now be read directly from the dial indicator.  When this was first done
    the back of the saddle was .018" lower than the front!  

          The height gauge can be moved to the opposite side to detect difference in compound dove base height. In this case
    a few thou in difference.

           Next the measurement equipment is removed form the bed and finger pressure applied to opposite corners, such as
    front left and right rear to detect any tendency to rock and there was. Use of the dial indicator or a feeler gauge can
    quantify the amount. Roughly .005" in this case.

           Next one must identify the high spots and points of contact. Layout die applied to the saddle "V" and back flat and then
    setting it on the bed and sliding it back a forth a few inches will mark those places.

           As side to side was pretty close I assumed the "V" to be pretty close in that plane if but considerably high and the
    major problem area to be the back flat. As it turned out the "V" area was making contact on both sides but with little
    area in contact. The back flat however removed next to nothing of the bluing. Just a light pencil lead width concentrated
    on the high corner. As it was so far out I used a medium sized second cut file to knock it down. Easy, a few licks at a
    time and checking. This whole process is slow and painful. Once the rocking motion was close to nil a piece of wet/dry
    paper, 80 grit was placed grit side up on the bed under the flat. A piece of paper over the "V" to protect it and the saddle
    worked over this until all trace of rock was removed. (Reverse of the top right picture) Of course this makes the down hill
    condition even worse but full line contact made across the flat which also corrected the side to side difference for
    compound dove register.

           I had Kresser Machine run a quarter inch end mill down the roof of the "V". The reason this is needed is that the top flat
    of the "V" is about .200" wide (6 mm) and the relief in the roof of the "V" is but .157" wide (4 mm). As you attempt to remove
    material from this area to lower the saddle a ridge is formed that prevents progress. To be honest I didn't do this
    procedure straight away but it came as a consequence after working this thing silly for a week with little progress.  A bit
    more thought on this and the roof height would remain as is as it is the width I was after.

           Okay so now we have the rock out, side to side squared up and some room in the "V" to work with, now the hard part.
    Wet/Dry paper is cut to width to just cover the "V" and Duct Taped in place and WD-40 applied before the saddle is set
    down on it. On the flat several folds of paper are placed between the saddle and the flat.

          The paper not only protects the un-lubricated flat but adjust the height. If you don't shim it like this or make some other
    provision for height then as you sand the saddle "V" it will eventually take a new angle and not one you want. This
    takes awhile to get a feel for, allot of tape and allot of sand paper and allot of paper towels allot of solvent and a lot of
    patience. Work slow and measure often as was done in the beginning.  How many sheets of paper? Well a sheet is
    about .003: thick and were down about .021" at present plus the thickness of the sand paper so roughly eight ply will
    be close and close at this point is good enough. You want to be slightly high on the side your working and work it until
    your slightly low, then remove a single shim and do it again.

          Measure, measure, measure...this took about an hour a night for nearly two weeks. Measurements will help you get a
    feel for pressure, how many strokes to remove how much metal, what grade paper to use and so on. Nothing I can
    give implicit instructions too and each case will be different.

          When you get down to less than say .005" on the down hill trade the shim stock for a sheet of wet/dry on the back and
    do the same on the front, trade sand paper for a paper shim. You've reduced the angle enough to get much better
    contact and now you need it flat. Work both sides using a courser grade on the "V" than the flat. You'll get a feel for how
    much, where and when to quit long before your in trouble. (Remember, grit side up :)

          If you got all this, by now your within .0005" or so of down hill, no rocking motion, both sides within .0005" in height and
    using something in the neighborhood of 320 grit paper.

          At this point it just wiggles the dial indicator so I gave both sides a few strokes with a sheet of 1500 grit, then cleaned
    and again with 2000 grit, both with a liberal spray of mineral spirits and cleaned a final time.

          When finished I had full contact on all contact surfaces that would remove blue (actually a Sharpie at this point) on a
    single passing. When wet with WD-40 I placed the saddle at one end of the bed and gave it a tap. Don't do that, it slid
    like a puck on an air hockey table crashing off the opposite end onto the bench putting a ding in the saddle that
    fortunately for me was in a place of no harm.  Now WD-40 isn't the normal oil to use on the bed and heavier oil does
    induce more friction and the slide is less free. Acts like a joy block actually. Slides freely but near impossible to pull
    straight off the bed. When I was close the dial indicator was replaced with a test indicator with four decimal resolution
    and I ended with a wiggle in the indicator for down hill. Under .0002 side to side and zero rocking motion. Rechecking
    with the height gauge on all surfaces and re-qualifying the "V" with the bed lock block produced no measurable

           The finer and flatter the finish, the more precise the alignment (with the appropriate lubrication) the tighter
    things like gibs can be set limiting motion and precluding chatter and the lighter the oil that can be used. Joy block
    type finishes actually help dampen vibration using the elastic nature of the fluid as a shock absorber.  Such surfaces
    wear at a much lower rate due to low contact unit loading.

    Now, this is what I've done and I won't preach that as gospel to anyone. IF you disagree finish to what you like and use
    what lubricants you like, just get it flat and square and well aligned.  Your now caught up to the current state of the

         3/22/2007 Rereading some of this I see I miss explanation of a very important step. Cleaning up the dimensional
    differences of the bed underside gib contact area. This was done by making measurements along the way both front
    and back in half inch increments writing the dimensions with felt marker on the bed. Using a 4" second cut flat file the
    high areas were taken down as far as I dared. The the file wrapped in wet/dry 320 and worked until I was within
    .0002",  This took several days and allot of measurements. I was concerned about getting out of square but gentle
    pressure working short areas at a time seemed to work just fine, even with out a guide fixture. Now the ends of the
    bed on both ends taper to a smaller dimension that the area I worked but I reasoned that only the area covered by
    saddle travel need be worked over. Later during gib fitting I used yet another method to finish the underside to an
    immeasurable tolerance.


                                                Rear Gib                                                                                       Front Gib

           I made the gibs close to my own drawings  from A-2 house scraps except thicker .350", and omitting the center hole.
    That made it 3.15" between centers on the two remaining holes.

     The manufacture of the gibs was very straight forward and went off without a hitch. Installing them was another
    matter. I bought some brass shim stock and a set of feeler gauges to sacrifice for same. I also acquired  a few
    sheets of 320 grit orbital sander media with the sticky back like they use in body shops. This turned out the be a
    very good idea. Nothing is square on these machines and the gib registers were no different. Use of a single shim
    proved unworkable as the registers are .001" out of parallel with the bed end for end. This translates into one end
    contacting the bed and the other quite loose making the gib dig into the bed. I reasoned at this point the two shims
    moved outboard of the fasteners of different thicknesses might work quite nicely and it did. Once it got it pretty
    close I disassembled the unit again as I had yet a few tight spots here and there. Using the gib as a template I cut
    sections of the sand paper the full length and width of the gib and applied it sticky side to gib, grit to bed and
    reinstalled the shims. Running the saddle up and down the bed then hits only the high spots. If you remember I
    had it down to about .0002" to begin with but when you get things this close even that is too much. Both front and
    back are now with in .00005 to .0001"" flat and parallel to that same dimension from the top of the bed and that is
    as fine as I can measure.

      High spots gone the saddle fit a tad loose again so the shim packs were thinned a bit more. Thing is that
    sometimes you need a shim in some size other than standard .0005" increments. That's where the brass stock, a
    lapping plate and some 2000 grit wet/dry earns its keep. Now anyone who has ever done a bit of this close work
    would tell you that even bolt tension will move things a bit and the idea is to get a shim that allows enough torque
    to be applied to it to prevent loosening during use and that takes a bit of trial and error, I remade shims more than
    once. I might mention that clean is highly important as well as at this level of fit even dust on a shim makes or
    brakes it. I use brake cleaner and lint free rags. Oil has thickness so it need be cleaned and oiled each assembly
    cycle with the oil you are committed to. I'm committed to ISO 68 way oil and the supplier is unimportant but use the
    same one.

           I remade a rear gib and made it 6" long X .4555" thick and 1.030" wide. Only the length contributes to
    contact area however. Extra thickness for the extra length for stiffness and width because I was too rushed to mill it
    off. This time I also used all three holes and located two stand off set screws as per original except that they are
    centered perfectly and in line with the three hold downs. These were made 1/4-20. Upon installation I fit the center
    bolt, placed the shims as before on the outside corners for level and gap. Then fit the two outside caps and played
    the bolt tension until satisfied. NOW I ran the set screws in to make firm contact and tightened the lock nuts.
    Finally, as both sides of each retaining screw are now supported those can be tightened to a sufficient level to
    remain in place.

      The difference is night and day. Stick is gone and slip is even better. You would have to feel it to know what I
    mean and I don't think a video would make the point. A test indicator on the back shows ZERO displacement of
    saddle to bed and something under .0005" on the front.  Placing and inclinometer on the bed and elevating until
    the saddle moved gave a slip angle of 7 degrees for a frictional coefficient of .121 "stick" and maintained motion
    once underway to a 5 degrees slip angle for a friction coefficient of .089. This is about a 4/5 reduction from factory
    in dynamic friction.

      Note: the shims widely spaced and not full length but they are full width to prevent rocking. The rough
    measurement for shim thickness is above each but a bit of fine tuning on a sheet of 2000 grit with thinning in
    increments of .0001" per fitting was required to get it right. Of course that means I had to make each one twice and
    you over shoot the dimension one step the first pass. The different thickness is required as the saddle casting is
    not parallel to the bed at their mounting face.

        The six inch length was a vast improvement over the standard 3.9" stock and 4" X .352" thick version. The extra
    length will preclude the use of the motor guard but I don't plan on the stock location for the motor anyway.

        Note that the area for the cross slide screw is milled out in anticipation of a thrust bearing in the collar later.  The
    extra length is .250" and the slot width is .804" provided by a reground 7/8 end mill which was the closest thing to a
    20 mm in the shop. While I touched of the mill to go no deeper than .002" than the existing floor of the channel the
    extra length broke through the "V" roof which was raised .025" and widened .250" to drop the saddle earlier to get
    the cross slide dove rails level to bed. I could have gone back further as other have but I want to keep the tool inside
    the rails. I will epoxy a disc in this location to prevent chips from entering the "V" channel as this become exposed
    when the slide is extended with the bearing modifications.

    Note the even oil film on the back way? Full and flat contact! Fully adjusted this saddle will slide right off the end of
    the bed and go right back on without a hitch with even light drag all the way to the head stock area even though it's fit
    to near zero gap.

         Several notes in this view. Note how close I got the gib width to the bed casting? It's all about stiffness and
    surface area, especially on the rear gib. I will shorten the stand offs latter now that I have the shim pack sorted and
    add some thin washers under the jamb nuts as I did the button heads.

       Before you ask, yes the apron will bolt up flush BUT the rack pinion will not rotate. Instead of creating a stress
    riser by carving on it as the taper gib arrangement does, I will turn the excess width from the gear. It's about twice
    as wide as the rack and the shaft proper does clear this fit buy .002"

        Button heads again for a cleaner look and a bit more room. The stock caps are 6 mm X 12 mm at 1 mm pitch.
    These front caps are 16 mm long and the rear ones 20 mm long with 1.5 mm thick washers. That may be useful,
    right? The stand off on the rear gib are 1/4 X 1 at 20 TPI.  Don't worry about the coarseness of the thread. You don't
    use them in this set up for adjustment, only support and stiffness.

       When you get things this aligned and use a good lube it is just magic what happens. I wish there was some
    way to show or demonstrate this on a web page.


                                                                                          Top View, Rear Gib
Lathe Bed Preparation
Kahale & Martin AP Machine
Fitting the Saddle
Fitting the Saddle Gibs

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