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Projects ... Crossfeed Modification (updated 10/06/05)
This modification changes the lead screw to a thick 7/16-20 thread, a larger mount, adds a deep grove ball bearing and a thrust bearing. Plus the hand wheel diameter has been increased from 2" to 3".
Both changes follows Steve Bedair's modification only with an added thrust bearing and a few other minor things, click here to see Steve's construction details. Thanks Steve, for your web site.
This modification adds a new dimension in the use of the crossfeed. Smooth, effortless and a real feel of control. Smaller backlash ... was .030" to .040", now .005" to .010" and even better after an adjustment. I can now actually see and feel a change of .0005" in either direction. Well worth the time and effort.



Here are the two versions of the lead screw and mount assembly...



The new assembly has a 7/16" lead screw, a larger nut (aluminum shown, see below for 936 Bronze), a larger (and longer) beefier mount with room for a deep grove ball bearing and a thrust bearing. An end cap that provides a preload for the thrust bearing assembly and keeps the LS/Bearing assembly together. Finally the dial assembly with a few of its own modifications.


The above picture shows the details of the thrust bearing and end cap. The ball bearing is press fitted inside the mount, then shown is the LS raised portion that presses against the ball bearing inner race and the thrust bearing. The thrust bearing has three main components ... left washer with ball groves, ball bearings in a cage, and the right washer that also has ball groves. The left washer is hand pressed on the LS, the ball bearing cage and the right washer float ... they do not touch the LS. When the end cap is mounted (using four 4-40 screws) it pushes the right washer into the ball bearings, this provides the alignment. Looking closely at the end cap there is a small lip (.015") that presses against  the washer.  The preload depends on those 4-40 screws that  need to be tighten a little at a time. The goal is the have a little preload but no gap between the end cap and the mount. If there is a gap just reduce the .015" a little at a time using 600 grit sandpaper.  It works .. so far!

The picture on the right shows the 4-40 screws and the mounting of the end cap. The lip should have been a little wider to prevent the screw heads from breaking out. The LS end was drilled and tapped for a screw so the dial assembly would not pull off. I didn't want to redo the keying for the dial, so instead I drilled and tapped for a set screw under the dial, then drilled an access hole for the Allen wrench ... it looks good (see picture top of page).
The indent on the end cap at about 3 o'clock is a locator, so that I will install it the same way each time (there is another one on the rim of the mount). Once the scribe line for the dial is in place, the indent is no longer needed.


End Cap Mounting
Drilling and tapping for those 4-40 screws was a bit tricky... didn't want the screw heads to breakout on the perimeter of the end cap but also did not want the threads to breakout inside the bearing area. This is where a DRO really came in handy. I was able to find the center of the mount then precisely move to the hole location to drill and tap. See right. (The tilt that you see in the picture is due to the camera!)
 

Tapping 4-40
Bearing selection:
I wanted the largest ball bearing available and still provide a thick wall for the press fit,  I chose the 6201 with an ID of 0.4724" and OD of 1.2598", the thrust bearing is 51201. 
 
Lead Screw details:
Material I used (I believe) was 4140 with a diameter of .630". I wanted the ball bearing contact area to provide a snug slip fit, same for the left washer of the thrust bearing, so I turned down those points to about 0.4725", then used 600 grit sand paper for a smooth finish. The rest of the LS was turned down for the 7/16" threads, the section inside the mount (no threads) and the dial assembly. All in all the 4140 was easy to turn, smooth with a nice finish.
I don't have a reverse tumbler on my lathe (next project?), but I have access to a 7/16"-20 LH tap and die. Threading the 7/16" rod took a lot of time and effort but finally came out good, maybe better then a lathe would (a least for me).

Mount details:
I used aluminum for the mount. Nothing really special, after I turned the bearing section to match the dial diameter (1.652"), the midsection was turned to 1.1" (instead of the 1" old style). Then the mounting lip was turned to 2.5". Next the bearing chamber was bored out so that the bearing would have a press fit (about .001" or so). The entrance to the bearing chamber was bored out to about .002" larger. Next a .5" center hole was drilled for the LS to pass through. Excess material length was cut then refaced. Finally the mounting lip and midsection was milled for fit (photo on right). Holes were drilled and tapped, the end cap made.


Milling the Mount
Crossfeed nut: (updated 10/06/05)

I finally found some 936 Bronze 1.25" round stock. Milled down the sides to make a rectangle (.625"W x .875"H x 1.1"L). Used a 4 jaw chuck to turn down one side for the holding screw, drilled the hole for the thread and tapped (1/4"x20). Turned the part around for the 7/16"(Left Hand) LS hole (photo on right), here I made one change ... I drilled the hole smaller by .0046". I should have used a 25/64" (.3906") drill, instead I used a W (.3860"). I wanted a tighter fit, more thread contact from about 65% to 72%. Maybe less backlash. I rounded the corners of the nut for final fit. This left about .14" plus all around the LS. Much more than the old style.

Tapping 7/16-20 Left Hand

See new nut below.


New 936 Bronze Nut
Hand wheel:
Used aluminum, increased the diameter from 2" to 3". That doesn't sound like much, but it makes a huge difference when using the wheel!! See Steve's site for details.
 
Final notes:
This was a fun project; it took a lot more time than I thought it would. But I probably saved a lot of time by using a CAD program. I was able to draw parts and assemblies then take measurements to see how things would fit before making any cuts.
 

If you have any questions or comments drop me an e-mail!!

Projects ... Crossfeed Modification
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