Mini-lathe apron modifications

January 2008

Need to say that some solutions I had encountered don't work the same way neither with a Sieg apron or with different Real Bull mini-lathe aprons. It's up to you to adapt any of these ideas to your apron, if it is different from this one. The Real Bull apron has two ball bearings on handwheel shaft, so that part of the work was already done when I bought the lathe. This one is a simple machined piece of cast iron with straight geometric shapes.

Only a good fitted apron with correct mesh on gears, rack and good halfnuts engagement can make the carriage run smooth and precise. Many times forgotten, a bad gear-to-rack mesh produce irregular finishes on turning parts. Backlash on halfnuts dovetails or poor engagement on leadscrew produce the same when using auto-feed.

Several points had guided this project: 1- improve gear-to-gear mesh; 2- improve gear-to-rack mesh; 3- improve halfnuts-to-leadscrew engagement; 4- improve apron-to-saddle connection; 5- eliminate interference between apron handwheel and cross-slide handwheel; 6- eliminate interference between apron handwheel and motor controller box; 7- modifying apron to be swarf-proof without the classic gears cover, has I have other plans for a second round on apron and need space inside; 8- that on the end it still looks like a lathe apron.

Apron to saddle connection

Adjusting apron on mini-lathe saddle (or carriage)

Before any measurement, the project starts here. As the counterbored holes on saddle for the M8 cap screws to hold apron are about 2.5mm larger than the cap screws heads, the M8 threads on apron were re-tapped to M10 thread. The M10 cap screws head has the same 14mm diameter as the holes on saddle. Repeatable apron mounts on saddle are now possible for measurements and for definitive adjustments.

Gear-to-rack horizontal mesh

Adjusting mini-lathe rack

To improve horizontal mesh, a 2.5mm thick brass bar was mounted under the rack.

Adjusting mini-lathe rack

The rack was tight to bed ensuring the same rack-to bedways distance along the rack. When tighten the cap screws this distance was adjusted with a c-clamp and measuring with a micrometer from rack teeth under each cap screw to the top of bed "vee". Having done it the follow step can be done.

Gear-to-gear and gear-to-rack mesh

Mini-lathe apron gears shaft

Many have solved these issues by a off-set bushing to correct gear-to-gear mesh and then lowering the rack to improve gear-to-rack mesh. If the first can be done precisely, there is no way to do it when lowering the rack. A precisely bed drilling to relocate rack assuring optimal mesh isn't easily possible. You can improve it, but you can't take it to optimal mesh. For it to be precise with off-set bushings will be necessary to align first the rack (as described above) and then use two off-set bushings. First one to optimize gear-to-rack mesh and the second on handwheel shaft to optimize gear-to-gear mesh.

As the apron handwheel shaft already has two ball bearings, it stay as it is. I decided to do both gear-to-gear and gear-to-rack mesh adjustments on the gear shaft. The shaft was removed and the 8mm hole bored to 16mm. A new shaft was made to be mounted from outside trough the bored hole. Mounts on the front of apron with three M4 cap screws and the holes on shaft for screws are 5.2mm to be possible a small amount of final adjustment. On the end, the shaft has moved about 1.5mm up and 2mm left. The handwheel has before about 15 degrees backlash and now less than 1 degree (almost nothing). As the gear was running off-center, bored the gear on both ends and bronze sleeves inserted. A small washer and a M3 flat head screw on shaft end assure optimal gear position.

Halfnuts-to-leadscrew engagement

Mini-lathe leadscrew halfnuts lever

The problem found here was with detents on halfnuts shaft, as they were stopping the lever on a non-optimal position. Decided to make a new lever with a larger handle hub and a disc under the hub with detents to stop the lever, while the ball and spring is mounted on hub. Only with everything assembled the detents positions were marked and drilled. As the hub was made in steel, and the lever lock setscrew is M6 there is no need for a ding on shaft for the setscrew. It locks well without it and the adjustment is possible this way. Just need to move down the lever until the halfnuts engage properly on leadscrew and then release the setscrew on handle hub, rotate freely the handle until the ball meet the detent and tight the setscrew again. Original tapped hole for ball and spring adjustment setscrew was closed. The shaft hole was bored on back and front and two small bronze sleeves inserted.

Once more used silicone carbide glued to parts with double sided paper to grind apron and halfnuts dovetails. First, with silicon carbide paper on the base of one halfnut, had grinded the apron surface between dovetails. Then, with silicon carbide paper on gib, had grinded halfnuts dovetails. On the end, with silicon carbide paper on one halfnut dovetail, had grinded the apron dovetail opposite to gib side. Started with 180 grit, again with 600 grit and once more with 1000 grit. This is more important that many can think, as the gib must hold halfnuts without backlash. Any backlash will cause halfnuts wobble and irregular carriage movement, therefore irregular finish when using autofeed. For precision threading the same is needed. The gib was shimmed for full contact with halfnuts dovetails.

For the regular needed lubrication, and as halfnuts aren't accessible, one oil cup was added in the front of apron above halfnuts. The oil go inside through a drilled hole. I was thinking of making some lines with a scriber to guide oil to near the dovetails, but there is no need of it. The oil goes down to the top of upper halfnut and then to dovetails.

The lever works now smooth and firm, the engagement is good and the detents stop the lever with more precision, as they are now on a 22mm diameter circumference.

Mini-lathe leadscrew covers and halfnuts gib screws

Here I had made a personal choice. As I use to make both metric and imperial threading, and the lathe motor is reversible, I don't use the threading dial. So I decided to remove the threading dial as it was limiting carriage travel for about 20mm with the spring type leadscrew cover. Moreover, as it mounts on apron above halfnuts gibs setscrews, it makes impossible to use nuts on these setscrews. Without the threading dial and with the leadscrew covers nuts can be used on two setscrews. The middle one was adjusted using loctite to keep it in position. One 6mm setscrew is now on threading dial mounting hole to avoid swarf to get in.

Swarf-proof apron

Mini-lathe apron gears cover

On the entire work one guiding rule was that on the end it also look good. So lots of careful on decisions and finally decided for welding. This apron is a simple piece of machined cast iron with geometric shapes, making this mode particularly simple. Angles and cuts were calculated and 6mm cast iron bar cut to extend the lateral sides close to bed. After the welding job some polyester and paint. Two coats of hammerite (50% white and 50% black), both smoothed with 600 grit paper. For a cleaner finish a final coat of "marine gray" spray. Dark colors hide oils and the lathe remains looking cleaner easily. With this mod some space was gained inside apron to do additional mods on future. With it and the spring type leadscrew covers both gears, halfnuts and leadscrew are now swarf-free.

Eliminating handwheels interferences

Mini-lathe disengageable apron handwheel Mini-lathe disengageable apron handwheel

Everyone had experienced, when turning on a minilathe, the apron handwheel to cross-slide handwheel interference. The more common problem happens when the carriage is stopped in a position which the apron handle stays at 1 or 2 o'clock. Another common problem is when cutting close to spindle, especially when using faceplate, the apron handwheel hit easily the motor controller box. This project, to solve both issues, was limited by the two ball bearings on apron and designed for work with another mod already designed to do on future: the cross-slide extension. The two 9x24x7mm ball bearings forced me to made the extension base with 40mm diameter, necessary to drill and tap the holes for the mounting tap screws aside the ball bearings. Dimensions were decided for a comfortable work with the designed cross-slide extension and to eliminate handwheel interference with motor controller box. It's a mechanically simple project with a new longer shaft (gear was bored and key cuts done on gear and shaft). A coupling collar with female dog is mounted on shaft. A female dog on handwheel slides through the shaft to engage and disengage. A retaining ring on the end of the shaft prevents the handwheel to fall from shaft. A brass collar prevents the dogs engagement from swarf.

Every time the carriage is stopped, I can pull the handwheel back and the weight of the steel handle on the aluminum wheel is enough to turn the wheel until the handle stop in lower position. Cross slide handwheel is now much easier to use without interference with apron handwheel. Also handwheel pass now in front of the motor controller box and don't hit it anymore.

With the disengageable feature, handwheel backlash increased to 4 degrees. Still much better than the initial 15 degrees.

The aluminum wheel was painted with the same color from apron. Self etching primer (two parts) for aluminum was first applied and then "marine gray" spray.

Thanks to Mike Cox, and to many members of 7x12minilathe group who helped, with their answers, to make decisions.

Drawings

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