Fehlmann Picomax 50 CNC conversion/refurb

[Robin Hewitt]

It was a bit of a bodge springing mine, I added a spacer and 2 Belleville washers. Well 6 spacers and 12 washers to do all the bearings and nuts. But the reward was out of all proportion to the effort.

[Agathon]

It was a bit of a bodge springing mine, I added a spacer and 2 Belleville washers. Well 6 spacers and 12 washers to do all the bearings and nuts. But the reward was out of all proportion to the effort.

So, if I'm interpreting that correctly, you have a double ball-nut set-up like my machine?

I could do something like this although it would mean making a new retaining flange. I have to say that apart from grinding spindles, I'm not a fan of spring-loaded solutions. I'm also not convinced that backlash of 0.01 is much of an issue. My conventional mill has about 0.25 backlash and it's only a problem with heavy climb-milling or slot milling and then it can be overcome by reducing the feed rate and the depth of cut. Mind you the table and slide alone must way 200kg - mass helps, as do well adjusted gibs.

I meant to have asked what the rest of you mill consists of Robin - got any pics?

[Robin Hewitt]

I don't have a recent pic, here's an old one...

[Agathon]

I don't have a recent pic, here's an old one...

Very neat, especially the Z axis. Wouldn't mind seeing some details of this as I'm sure I'm going to have to go down this route eventually.

What sort of backlash have you achieved on the x and y?

[Robin Hewitt]

There's a build log from many years ago, I will try and link it...
http://www.mycncuk.com/threads/651-W...ht=Warco+Major
I fitted zero backlash double nuts, big disappointment, so I sprung them and it was like magic.
The springs mean that I have no backlash up to 1/4 ton but if I ever pass that loading presumably everything will go horribly wrong.
I do get some sideways slop in the quill if I forget to pack it out with thick, icky grease. Open to suggestions.

[Agathon]

There's a build log from many years ago, I will try and link it...
http://www.mycncuk.com/threads/651-W...ht=Warco+Major
I fitted zero backlash double nuts, big disappointment, so I sprung them and it was like magic.
The springs mean that I have no backlash up to 1/4 ton but if I ever pass that loading presumably everything will go horribly wrong.
I do get some sideways slop in the quill if I forget to pack it out with thick, icky grease. Open to suggestions.

I might give the belleville washer thing a go. The Fehlmann's vernier adjustment is clever, but I think a grub screw lock would be better as it would allow finer adjustment. Quality manufacturers have a nice trick of fitting a pressed-in brass plug in the adjustment nut before threading it so that you have a formed locking piece that doesn't damage the thread and needs little pressure to lock.

Regarding your quill slop, there isn't really a satisfactory solution. The fit of a cylinder into a bore is something that requires a degree of precision and careful fitting at the point of manufacture - surface finish is critical. Any fool with a cylindrical grinder and enough patience can make a cylinder to size and parallel to a micron or two, but getting the bore honed for a perfect transition fit is another issue altogether. Most of the Chinese machines fall at this point, although I have to say that Myford's VMC (Taiwanese) had a pretty decent fitting quill - though not to Swiss, German or the best US and GB standards (has to be said though that most of the machine tools produced in the UK were not of a particularly outstanding fit in this respect).

You could split the casting and put a clamp in place, but this is a pretty crappy solution as you're going to get lots of point contact at the clamp position (the bore will be an oval) and you'll still have the original clearance further up. Alternatively you could get the quill hard-chromed and ground to a nominal size slightly larger than the clearance, but then you have the difficult honing issue mentioned above. Not worth doing either of these, better to start again with a higher-grade machine and transfer your clever CNC additions to it.

If you've got space there's been some fantastic 1980s CNC machines on eBay recently including a couple of Deckel FP3/4s for about £1500! Fab things with superb ball-screws and servos (probably old brush types though).

[Agathon]

Very neat, especially the Z axis. Wouldn't mind seeing some details of this as I'm sure I'm going to have to go down this route eventually.

What sort of backlash have you achieved on the x and y?

I meant to have complimented you on the really super job you've done on the casings for the drives - very professional.

[Robin Hewitt]

Most kind. This is the magic of CNC, it frees you from straight lines and you enter a wonder land of flowing curves. If I only had 2 more axes I could do blobs.

[C1Geoff2020]

Hi Hanermo2 and thanks for your very comprehensive and convincing post on your experience of servos. I'm coming to the same conclusion. Steppers are fine for hobby machines, but for anything a bit more industrial a servo is the way to go. Fehlmann certainly dropped steppers within a couple of years and went to servos.

The Fehlmann is a slightly odd machine in that it's a drilling machine with milling capability - so despite it's industrial specification in terms of mass, the design is a compromise, with milling, and then CNC, as an afterthought. With that said they sold well in their home market. The latest generation of Fehlmann machining centres are built round a conventional square slide-way for the Z axis, but they still produce a manual and semi-cnc/manual version of the Picomax: http://www.fehlmann.com/en/products/...picomaxr-21-m/

I was interested in what you mentioned about direct drive. When Fehlmann switched to servos (see pic) they mounted the motors remotely - not sure if it's a belt or gear drive, but suspect belt. The latest semi-cnc/manual of the drill mill that I have also has remote servos - I'm not sure about their machining centres.

Attachment 22222

Having reassembled the ball-screw, replacing it proved interesting since the nuts, or rather the keys in the nuts, didn't want to go into the key-way. The key-way or the keys were obviously minutely out of line with each other. A rub over a stone got them to slide in, but the results from the screw were disappointing - still not sure why. It could be an alignment issue - which seems strange given the precision nature of the table and the screw, but it's the sort of thing I've regularly encountered with precision machine tools - a lot of fettling takes place when these things are built. Anyway, I decide to but the nuts on the in the original orientation which seems better, but I can't get the backlash better than 0.01 without sacrificing the smoothness of the screw. I will tinker some more today.

I've had 20 years experience rebuilding machine tools and generally I've found that even if a bearing's tracks look good through a loupe they may well be slightly rough when under load. This is sometimes due to poor fitting (hammering the inner) and tiny imperfections are left on the bearing tracks which will only show up under a microscope. I suspect that the ball-screw and nut assembly have similar issues. I'm not about to replace them so will live with the 10 microns of backlash.

Thanks for your offer of the Kluber Isoflex. This is a product I'm familiar which since it's specified by some of the manufacturers I deal with. I actually use an SKF LGMT2 which has a similar specification and was more readily available at the time I needed it. In fact the Fehlmann's central lubrication system covers everything from the spindle bearings through to the ball-screws and everything in-between. The Vaseline used was just to get the balls to stick to the nut during assembly - it will soon wash out.

If you're ever in my "neck of the woods" you'd be more than welcome to visit.

I have some parts from an older machine looks like PICOMAX100 that was being disposed of by a machinery dealer .
It used sevo motors and timing belts to drive the ball screws .
when I arrived the controls had been chucked in a skip , the sevos removed and the intention was to strip the rest to get rid of the 3000 K that was in the way .
I still have the sevos , the SF 32 spindle , some SF30 tooling , the roller slides , massive leadscrews ( maybe 30mm ) with ball nuts and bearings .
So true sevos when others used steppers ! - well as you say , it IS Swiss.

[Agathon]

I have some parts from an older machine looks like PICOMAX100 that was being disposed of by a machinery dealer .
It used sevo motors and timing belts to drive the ball screws .
when I arrived the controls had been chucked in a skip , the sevos removed and the intention was to strip the rest to get rid of the 3000 K that was in the way .
I still have the sevos , the SF 32 spindle , some SF30 tooling , the roller slides , massive leadscrews ( maybe 30mm ) with ball nuts and bearings .
So true sevos when others used steppers ! - well as you say , it IS Swiss.

What a shame it was scrapped - I would have considered buying it.

I've only just started working on the old Fehlmann again after a couple of years of inactivity. My machine is a pain as it was originally only two axis. The Picomax 100 was a three axis machine that used the quill rack and pinion driven from a worm and wheel belt driven from a servo as the Z axis. It sounds like a terrible idea when one thinks about the average drilling machine rack and pinion, but being Swiss there's virually no backlash. As I mentioned earlier in this thread, the extremely clever engineer I bought the machine from had made a worm and wheel drive (the aluminium housing seen on the right hand side) to drive the pinion. However, I have found that because he used a fully throated worm wheel it has been almost impossible to remove the backlash.


I will probably end up using a ball-screw to move the quill, but I have recently reinstated the machine's original worm and wheel drive (which employs a gashed bronze worm wheel) and have backlash of about 0.01mm - not bad. I shall see how it works in practice once I have mounted the stepper motor to the original 90º cross-helix gear drive to the worm shaft. There is significant backlash in this gear, but it will be unimportant in terms of the ability of the quill to remain in position because of the worm and wheel - the backlash is constant so it can be accounted for in the software. As I say, this is almost identical to how it was originally done on the three axis machines, however, it has proved difficult to employ a belt drive to the worm shaft due to differences in the head castings - see drawings.