Boxford 260 VMC rebuild

[mike-f]

Hello All,

This is a bit of an introduction and hopefully some of the information and pictures may help others faced with a Boxford CNC mill rebuild or update. First an introduction, I am a UK based design engineer based in the UK, I have owed a couple of other Boxford cnc's including a 250 CNC lathe and a a 190 VMC mill. At the time of purchasing the 260 I was using the 190 which is partially converted to Mach3 (manual spindle control). As a matter of course I have an permanent eBay search for any Boxford cnc parts or machines in the hope of picking up cheap tool holders, I was especially looking for an original machine vice. So it was found a 260VMC in very poor condition but complete with a vice and four tool holders. With biding very low it seemed a bargain for £225!

Of course collecting it was a challenge as it would only just fit in my car!











If you are considering purchasing one of these mills be warned they are pretty heavy at around 200 - 250Kg, I was lucky in that the machine was located on a farm and the seller had a forklift to get it into the boot of my car. even then it was pretty hard to get it in. Even harder to get it out at the other end.

My original plan was to strip it for parts like the vice and tool holders and sell the rest. But under the rust and muck it looked like it might make a nice machine.
First the bad bits:-
Rust everywhere, the cabinet was peelings and rusty so was everything else.
Electronics, 1980s drives and spindle. Spindle drive and electronics had obviously blown up at some point!
Other bits I did not know about until striped, Like spindle bearings shot, and very wonky Easy change chuck!

Good bits
Larger table and travel than my 190vmc
Larger ball screws 12mm vs 8mm in the 190
It has a happy ending!

The plan was to spend a few months striping and cleaning, followed by installing updated controls and spindle drive. As it happened it took a year and I ended up doing far more work than I planned!

More to follow.

[mike-f]

The first thing to do was strip down and see what needed doing. The bed ways and head come off pretty easy and most of the rust was on the surface only, The ball screws seemed OK but felt very gritty, so needed striping and cleaning at the least. The milling head is removed by removing the stepper motor and ball screw attachment bracket, the hole lot then lifts off. The actual milling head on the 260 can be rotated to turn the machine into a horizontal machine, with the head being pivoted. Once the slides had been removed the slideways looked pretty good with the original scraping still looking good. The base of the Y axis is bolted and glued to the cabinet and requires quite a bit of force to remove once the bolts are removed. The column is held to the Y axis with M8 screws from the underside, forming a very substantial and heavy assembly! Pictures below.

Attachment 32601
Attachment 32600
Above Z axis drive belt and pulleys
Attachment 32602
Above the Y axis removed showing the ball screw and nut

Attachment 32603
Above the spindle pivot for, which retains the head to the Z axis for rotation to horizontal position.


Above the spindle removed from the head, the bearings are are just deep grove ball races so these will be upgraded to angular contact bearing of the same size.


The vertical column and y axis base removed from the cabinet


Above the stripped out cabinet, waiting for a decision on what to do to it!

[mike-f]

So pretty much after checking all the parts I decided to rebuild the 260, I was originally going to scrap the cabinet as it was rusty inside and out, and also made machine pretty large for my cramped workshop. But in the end I kept it and do think it was the right decision. The plan was to replace all the electronics, replace the spindle motor with a 1Kw AC servo motor, make a new control panel with jog wheel, feed rate and spindle overrides. The controller chosen was a the very capable UCCNC UC400ETH. I had also decided to replace the spindle bearings with 7009 angular contact bearing instead of the deep groove ball races.


It took a few days to totally strip the machine, the main base is held in place with M8 cap screws one of which is hidden behind a cover that run up the edge of the column. With the bolts removed it is possible with large amount of persuasion to break the bond of the glue that seal and holds the base firmly attached to the cabinet.

With all the parts removed I made a start on the spindle upgrade this was to be straight forward and was to start off! after cleaning and removing the spindle first thing was to set up the spindle housing to drill and tap holes for a spindle nose cover, this is needed as the angular contact bearings do not have dust/ oil seals. so the plan was to use "NILOS" sealing rings and an spindle nose cover to protect the Nilos ring and act as an extra protection from swarf, I also decided to fit a Z axis wiper while the machine was apart and the Boxford is only fitted with bed wipers on the Y axis. Pictures below I hope will be useful to someone!

Attachment 32617
Above cross section of the spindle showing the Nilos ring (yellow) and spindle nose cover

Attachment 32618Attachment 32618
Above Z axis wiper, which was 3D printed in nylon.

Attachment 32619

Attachment 32620
Above test fit of a 3d printed spindle cover.

Attachment 32621
Above: completed spindle cover fitted

Attachment 32622
Above: I also added some oil grooves to the Z axis as it only had a 3mm hole at one end on each side.

Below are pictures of the Cabinet parts, clean up and painting.

Attachment 32623

Attachment 32624

Attachment 32625

Attachment 32626

Attachment 32624

Attachment 32627

Attachment 32628

Attachment 32629

[mike-f]

Right, if you’re thinking of picking up one of these mills, there are two key things to check before you part with your hard-earned cash:
Is the spindle running true? (Mine, er, wasn’t.)

How much backlash is lurking in the ball screws?

I’ll admit, I didn’t even think to check the spindle (not that it was important for the what machine cost). I’ve never come across a machine with such a wobbly one! The spindle itself was alright, maybe 0.005" out, but the Coventry Easy Change master holder fitted to it? A ridiculous 0.2mm off true. Pop a tool holder in, and it’s like watching a wonky spinning top. My first guess was a proper crash—someone giving it a right wallop. But I’m not convinced. The holder’s not shaped like it could bend easily, and the error was consistent along the taper. Feels like it was machined eccentric from the get-go. This being a late ‘80s model, ringing the manufacturer (still trading, mind you) for a refund was a bit optimistic.
By this point, I’d poured too much time into this project to chuck it in. So, my options? Shell out a few hundred pounds for a new master chuck body or try to salvage the dodgy one. Problem is, the body’s hardened to 55-58 RC, so turning it was out. Time for some workshop bodgery. I 3D-printed a mount for my Metabo die grinder, turned a mandrel on the lathe, and draped my trusty S&B Model A in as much protective cover as I could. Then, I set to cylindrical grinding the outer taper of the master chuck, this time true to the bore. The finish came out surprisingly decent, but I’m not keen on making a habit of it—my lathe didn’t sign up for this!
End result? Runout’s down to about 0.01mm, maybe a tad less. Not bad for a bit of shed engineering. Anyone else had to wrestle with a spindle horror show like this?




Attachment 32633

[mike-f]

Well, the spindle was a right faff, but the ball screws? No surprise there. These VMC 260s have a bit of a reputation for knackered ball screws. They’re 14mm with a 2.5mm pitch, and to top it off, they’re overhung with no end support. The single nut uses tiny 1.6mm balls, so the load rating’s a bit on the skimpy side. Finding replacements? Good luck. The nut’s a screw-mounted type, not a flange-mounted one, which are ten a penny.
Once I’d got the machine back together and wired up for a test, I braced myself to see how bad things were. The X-axis was the worst offender at nearly 0.18mm backlash. Y-axis wasn’t far behind at 0.09mm, and Z was the least awful at 0.05mm. I did track down a Chinese supplier offering rolled screws for £90-100 each, plus shipping. But knowing my luck, the replacements would probably be dodgier than the originals. So, I thought, why not have a crack at re-balling the ones I’ve got?
I started with the X-axis screw, dismantled it, measured the balls, and ordered replacements. When they arrived, the real fun kicked off. Those 1.6mm balls are so small they vanish the moment you drop one—like they’ve got a personal vendetta against you. For the first go, I meticulously counted out the exact number of balls from the old screw, then counted out the same number of new ones. Sounds simple? It’s not. After thoroughly cleaning the nut, I packed the grooves with ball screw grease and, with the patience of a saint, placed each ball into the grease. This took forever. Once all the balls were in, I slipped in a turned nylon filler rod to hold them in place temporarily. Fitting the screw back was a doddle… after a few practice runs, mind you. The result? Backlash down to less than 0.01mm—good as new, if not better.
The Y and Z axes were a breeze by comparison. I ditched the ball-counting nonsense. Instead, I slapped as many balls as I could into the grease, popped in the filler rod, held it just past the return path tube, and kept feeding in balls until the track was full. Worked like a charm first time and took about 20 minutes per screw. Piece of cake, really. Anyone else tackled ball screw woes on one of these? Or am I the only one daft enough to try?

[Lee Roberts]

Not had to do one of these but over the years I've rebuilt quite a few ball nuts for people on here, on some of the more expensive assemblies the balls themselves can be of different sizes, this is to do with preload etc

Not done any spindle grinding personally but have seen it done this way on the tube, think it was an Abom video...

Pleased you kept the enclosure and went for a restore, would have been a shame to lose it I think, look forward to seeing her alive again, good luck and keep going