Hi,

I have this little lathe which hasn't really been used in the last couple of years, so I decided I'd better convert it to CNC which I could really use.



It is a Chinese conventional 9x20 lathe. They make them on several factories in China with different quality. This particular one was quite poorly made. The good thing in these lathes is usually their heavy cast bed and headstock and that's why it qualified as a good candidate for a conversion

Going to strip off everything and fit ball screws to both axes, with stepper motors to drive them and make up a new tray with appropriate guards. The plan is also to replace the AC induction motor driving the spindle with an AC servo motor. Quite some rework is needed to make all I want.

First, stripped off everything:


Headstock got new bearings installed and after mounting it back I checked for alignment to the Z axis:


It was found to be around 0.1mm out over 150mm. Simply filing the V-groove on the headstock in the right spots brought it to within 0.02mm over 150mm, which is a satisfactory result. It was a long process: filing a bit, mounting it back and testing and then back dismantling and filing more based on the test result. Took quite a few iterations, but it paid off

There was no enough room to put a substantial ball screw in place of the poor leadscrew, so I decided I to put the cross slide on linear guides to have a bit more clearance for the ball nut. On the other hand though, I didn't want to significantly reduce the clearance over the cross slide. I had very low profile Bosch Rexroth preloaded linear guides (10mm high) and I decided to you them, but also had to mill a bit into the cross slide as well as the carriage to achieve the required clearance.

Here is the carriage after milling the recess and drilling the holes for the linear blocks:


Going to mount the linear slides upside down as I figured it would be more convenient, so the carriage will have the linear blocks and guides will be mounted on the cross slide. The mounting holes for the linear blocks got counter-bored from the back:


Also, in the above photo, a lub channel has been milled into the flat sliding surface. The V-groove won't need that as it has a slightly smaller angle than 90° and there is always some clearance between the top of it and the V-guide.

On the left side, milled flat the surface for the Z ball nut assembly to be mounted.


This is the cross slide after milling the recess and drilling and tapping M3 holes for the guides:


Here's a top view, mounting holes for the X ball nut assembly have been drilled and counter-bored:


Will fill the unused holes with epoxy later.

Dan

The original carriage gibs weren't satisfactory for me. I wanted something more substantial and something that can be easily adjusted and locked. The original ones had the gibs mounted on screws which could be tightened for to adjust, but nothing to lock those screws themselves from unlocking.

I designed new gibs to replace the original ones. Here is my rear gib:


Couldn't find cast iron stock large enough to make it so made it two parts. The base is mild steel and the slide is cast iron. The slide bar sits on a step and is bolted to the base. It has the lub channel milled.

The 3 holes on the base match with the original bolt locations and will be used to tighten it against the slide for adjustment. The slots on the back flange will have bolts to secure it in place once adjustment has been completed.

Dan

Excellent write up.  I'm looking forward to watching your progress.  I have a 11x24 Rockwell that I want to convert.  You're making me really want to start now.
Mike

The front gib was more tricky in terms of mounting and size to prevent interference with the Z ball nut underneath. This is what I ended up with:


Again it's two pieces for the same reason, base being steel and the sliding bar cast iron. The two M6 holes at either end match up with the original holes on the carriage two bolts there will be used for adjustment. The 3 larger holes match with the 3 holes I bored in the carriage (bottom view photo in the previous post). These will have dowel pins.

It's hard to see on the picture, but there are M4 holes on the sides perpendicular to the M6 ones. They are for M4 locking set screws.

Dan

Looking real good so far Dan, nice work !
Have you decided on which ballscrews yet ? Buying new made to order ...eeeBay ? Chineee ?
Just curious as I am planning to do similar, again. Good screws don't come cheap, usually.
Thanks for the post,
Russ

Thanks Russ.

For the Z axis I have a 14mm diameter, ground NSK ball screw I got from eBay. It's used but it looks in a very good shape. For the X, I got a Hiwin 12mm diameter C5 rolled ball screw from 1aballscrew.co.uk who tend to offer good pricing and also do the end machining.

Dan

Are you planning on keeping it flat or using it as a slant-bed?

I turned mine into a slant and it helps keep the chips out of the z- screw.  Mine is far from done but it works and has made tons of parts.

Hi,

I am going to keep it flat. I have sheet metal guards designed to cover the screws so should be no problem there.

Thanks for the picture. I like your collet addition. Might consider it on my Emco C5 as I usually use bar stock there. Where did you get yours?

Dan

The collet closer is a Royal that was originally on a southbend 9" It uses 3c collets. I bought it on e-bay.

I added turcite to the bottom of my z axis carriage and had to rework my gibs as well. I have been thinking of adding linear rails as you are doing to yours.

I like yours so far, keep up the good work!