CNC lathe conversion - part 1

The kit

One thing I still remember vividly from the days of the CNC mill conversion is how I wished I had spent some money on a CNC kit that someone had already vetted, rather than trying to save a few bucks and doing it all myself. It was an interesting quest for sure, but it claimed 6 month of my life, and while it was good for me, I wouldn’t want to do it again.

“Good judgment comes from bad experiences, which come from bad judgement.” 

Feeling like I had completed this circle, I vowed I’d play it smarter the next time around so, when I ran into a good deal on eBay for a quality CNC kit for my lathe (20% off after some haggling), I just had to go for it. The kit I purchased was made by Billy from BD Tools. 

BD Tools has a very good reputation in the CNC conversion community, so I didn’t waste a lot of time researching it as I probably might have otherwise done, and in this case the quality of the product I received was even better than I had expected.

Everything included in the kit

Z motor mount

¼" (6.35 mm) flex motor coupling

Apron replacement, and X motor mount.

X motor mount, longer lead-screw for tool ganging, and brass nut with backlash compensation

The guy who sold it to me forgot to include the instructions in the package, but I decided to install it anyway, while I waited for the instructions to be shipped to me.

The first shocker was that it only took me 1 day to install this kit on my lathe, versus the 6 months ordeal with my mill.

The second surprise turned out to be that this kit wasn’t as plug & play as I had imagined, but it needed a little creative machining on the mill. Not a big deal since I do have a mill, but others might require the services of a local machine shop, adding to the cost and time required for the installation.

When the instructions eventually came, they basically followed step by step everything I will show here, but operating in the blind at the time was a little disconcerting.

The first step was to strip the lathe of course, and give it a bit of a cleanup at the same time. The nakedness of the lathe made the oversize 5” (12.7 cm) chuck look even more out of proportion. 

All drive gears removed

Electronics, lead-screw, apron, compound, rack, and tailstock removed. I love that chuck!

Most of the stuff I took off will never need to go back on.

Amazing how much stuff had to go

The left ball-screw mount is a direct bolt on replacement, while the right one, at the far end of the bed, needs to be fitted in place. I mounted the replacement apron and ball-screw loosely, and spent the majority of the conversion time making sure the ball-screw had no binding throughout its entire length of travel, before identifying, marking, and drilling the bolt hole positions. 

Screw binding is a bit of a sore subject for me, as you might remember from my previous post on “curing the lathe”, so I wanted to be absolutely sure this would never become an issue again.

I cannot stress enough the importance of taking your time at this stage. Proceed carefully, run the saddle back and forth often, periodically loosen then tighten the apron’s top bolts to check for screw binding, readjust the right ball-screw mount accordingly, and repeat until no more gains are being made.

Narrowing down the final Z motor mount position

Same thing from a different angle

First hole located...

... drilled, and tapped.

First bolt installed, apron and ball-screw binding rechecked.

Next 3 holes located with a transfer punch

More drilling and tapping action

The end plate position is now final

Saddle travels along the bed smoothly

Another surprise I wouldn’t have encountered, had I had the instructions, was that the cross-slide lead-screw didn’t fit the original hole. A quick comparison between the new screw and the old revealed the new to be much beefier. So drilling it was.

Enlarging the X axis clearance hole to ½" (1.27 cm)

Another view

With the screw installed, the brass nut would catch on the right corner of the slot and the bottom of it as well, and was unable to slide freely. My initial response was to widen and deepen the channel…

Aligning the lathe's X axis to the mill's Y axis

Shimming the setup until leveled

Problem

Neanderthal solution to the problem

All better now!

… in reality, I should have made the hole a bit larger (as per the M.I.A. instructions), and a later binding issue did force me to go back and drill it again, correctly this time.

Enlarging again to 17/32" or 0.532" (1.35 cm)

Perhaps all this milling could have been avoided in the first place, I will never know, but you should be aware of the possibility of it, should you choose the same kit. The instructions did not mention this step.

One other thing you might want to be aware of, should milling of the bottom of the slot be necessary, is that there is not a lot of steel over the V way. I might have milled 0.050” (1.3 mm) vertically perhaps, and ended up opening a porthole from the cross-slide to the V way of the bed.

V way visible inside the cross-slide!

Luckily for me, that’s not a problem, but again, there might be no reason for needing to do that, if you drill the hole to the proper size first.

The mill was however still needed for the last step, which was to drill and tap two holes in the cross-slide. It turned out that the instructions did specify the locations of the holes based on some measurements, but since I didn’t have them at the time, I used a transfer punch instead, which worked perfectly.

It did take a little imagination

Drilling to about ½" (1.3 cm) 

Tapping for ¼-20 studs

After that, it was just a matter of bolting the rest of the kit on, and checking for binding one last time.

Studs installed

Bearing block fits nicely

X motor mount in place

One characteristic of the original manual lathe that had always bothered me was the unprotected lead-screw, which pretty much guaranteed that all the metal shavings would fall and collect on its lubed grooves. Not that I used it much, but keeping it free of metal chip was always a real pain. 

The ball-screw is just as vulnerable to debris falling from above as the lead-screw once was, so I decided to make a cover for it. This wouldn’t have been possible before, but with all the apron’s hardware now removed, there was just enough clearance for a hand fit cover to slide in.

I looked in my scrap bin and found a small aluminum L channel, and I ripped it on my table saw.

Ripping a scrap aluminum L channel 

I match drilled holes from the rack I had removed. 

Reproducing the rack's hole pattern

Next, I welded the ripped slice back on the other side of the L channel, at a downward angle of 15˚. 

Rigging up a cover for the ball-screw

Later, I found another flat piece, and welded that on as well, horizontally. 

Adding a little more overhang

The end result looked terrible, but with a little grinding and sanding… nope, it still looked nasty… but it fit, and I didn’t care, the screw was partially covered, and my scrap pile got smaller.

While not sealing the ball-screw entirely, it is much better protected than ever before.

From the chuck's perspective the ball-screw no longer even exists

Completed conversion, minus motors, and RPM pickup (for threading)

I might end up removing the compound to increase stiffness, but I'll leave it be for now.

The ball-screw shield was cut short to be able to remove it without dismantling the apron

A larger L channel would have probably been a quicker fit, but then I would have missed out on all the welding fun.