Homebuilt Metal Cutting Lathe: The Bed

February 22, 2003

Once the bed was built up and scraped flat on the top surface, it was time to attach the ways plate.  Due to availability and pricing of cold-rolled steel stock, we chose to make the ways plate 1/2" x 5" x 24" -- the five inch wide plate was cheaper than 4 1/2" width in the half inch thickness, and the extra overhang (now a nice round inch on both front and back of the bed) won't hurt anything; in fact, it will provide better support for a long travel cross slide, which is required to turn work up to the full swing of the lathe.

Once the ways plates were acquired, the first step was to lay out the hole pattern for the bolts that will attach the ways to the bed.

Laying out the bolt pattern

Here, Mark is seen scribing one of the layout marks.  The blue material is layout dye, a strongly colored lacquer like material (similar to model airplane dope, at least by its smell) that brushes onto the steel and dries quickly, then takes a visible mark with a scriber that scratches away the dye.  The mark produced is fine, but has high contrast between the bright steel in the scratch and the dark dye around it; in addition, the scriber makes an actual scratch in the surface of the metal (at least as long as the scriber point is harder than the work) that can be felt with the point of a center punch to allow for the most accurate hole starting.  After laying out, we used an automatic center punch (aka prick punch) to make the drilling marks, deepened those marks with a hammered punch I made from a nail set (chucked in my drill press and the 60 degree point ground with my Dremel while the punch rotated), and then used the resulting marks to center the drill.  Since these were not location critical holes, we didn't bother to pilot drill a smaller hole; instead, we simply drilled straight through with a 17/64 drill, to provide clearance over the threads of a 1/4" x 20 bolt.

Actual layout marks

This is an image of the actual layout marks on one of the ways plates.  As you can see, even a cheap digital camera can record these from a few inches away (this shot was from about six inches); if you look closely you'll see a very fine mark crossing the obvious one; it's actually the same size scratch, but the angle was less favorable for reflecting the light, so it's not as bright.  The intersection of the marks is the point where we set the prick punch to make the starter mark, then deepened that mark with a center punch before starting the drill in the deeper mark.  We laid the lathes out to use two rows of four bolts to hold the ways on the beds; the separation of the bolts will make the join more rigid than the single row of bolts would do on the Gingery style bed; as well, we used a wider and much thicker plate, which will also add to the stiffness and overall strength.  These lathes should be capable of actually turning up to the eight inch swing we're building to, if we take care to give them adequate power and speed range.

Bolt holes, showing counterinking

In extreme closeup, two of the bolt holes that will accept the socket flathead screws to secure the ways to the bed.  These holes were drilled through with a 17/64" drill to provide thread clearance for a 1/4" x 20 bolt.  Once all the holes were drilled in the ways, we carefully centered and aligned, then clamped each ways plate to its bed and drilled two of the holes through, dropped bolts into those to act as locators (to prevent the ways from wandering as we worked) and then drilled the remaining six holes.  Better practice would have been to install the nuts on the bolts and tighten them before proceeding, which would also have allowed us to remove the clamps and thus have simplified the drilling operation, but we didn't yet have a method of installing the nuts on the bolts close to the side walls of the bed tube, and more than eight inches in; the three inch outside measure tube (with 3/16" walls) is too small to get a hand in, and in an effort to get maximum leverage for the holding power of the bolts, we put them too close to the tube walls to get a socket onto the nuts.  In the end, Mark molded a socket from Fimo® molding plastic, which after hardening in the oven and gluing to the end of a stick made a perfectly serviceable nut starter.  Once the nuts were started, we could wedge the blade of a screwdriver between the nut and the tube wall to hold the nut while the bolt was tightened from the top.

The countersinks, as you can see, are run a small amount below surface; the goal was to put the flat head of the bolt between 1/64" and 1/32" below the surface of the plate, so that if the carriage pushes chips ahead of it while making a cut, the chips won't wedge on the head of the screw and lead to a jam.

Flat socket head bolts, countersunk

As you can see from this image, the countersinking was a success; these bolts are nicely below the surface, but not enough so as to form a catch point to collect chips or oil.  Once all eight bolts are in place on each lathe, the half inch plate should conform closely to the bed surface under it, bringing the plate to reasonable flatness.  We won't need to scrape the top of the ways, since the flatness produced this way is more than adquate; with the tool point cutting at or close to center height, variations of a few thousandths in height as the carriage travels will be unmeasurable, almost undetectable unless the diameter being turned is tiny (in whch case, flex in the part will overwhelm inaccuracies in the lathe in any case).  The front to back position, conversely, translates directly into error in the work, every thousandth of misalignment in the back (reference) edge of the ways making two thousandths variation in the diameter of the work.  Because of this, we used the level and bluing to file the back edge of the ways true.  Unfortunately, there aren't any photos of that process, but it worked exactly like the scraping of the bed except that it went faster because there was so much less metal to work.  In fact, with the completion of that process, the ways are ready to proceed to the next step, which is to build the saddle; once the saddle (with its box slide and gib) is completed, we'll use the drag to locate wide spots and file the front edge of the bed accurately parallel with the back edge.

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