Tuesday, 10 November 2020

Saddle ballscrew assembly - getting some bits together

Preparing to get on with the Bantam CNC conversion

Here's what I've got in the pipeline, heading its way from various emporia in Korea and the UK. Spending beer tokens is the easy bit, although getting them in the door past the Domestic Manager can be fraught. But that's no reason to hold back.

Ballscrews:

I've gone for parts from the DY Global scrapyard in Seoul, stripped from some bygone machine or other. Rather than take my chances with some shark on AliExpress flogging bent (literally) rolled ballscrews, I thought I'd go for second hand decent ground jobbies. In some cases, they offer new old stock parts from the likes of HSK, Hiwin, TBI etc. In this instance, there was nothing suitable for my needs. However, I finally settled on the following:

1205 ground ballscrew for the cross slide. I've convinced myself the ballnut will (just) fit in the cavity in the saddle under the cross slide. I'll clean up the rough cast cavity to give it a bit of margin but it The ballscrew itself is longer than I need but I'll machine it down when I have actually designed something for it to go in.


And a 2005 ground ballscrew for the saddle. It's longer than I need but as I'm not yet certain what that is, I'd rather cut down a longer ballscrew than curse a shorter one.



From DY Global's description:

Main Size (Metric) 
  • Shaft Diameter = 20mm
  • Lead  (Pitch - distance by 1 rotation) = 5mm
  • Nut Diameter = 44mm
  • Flange Size = 60mm X 44mm
  • Nut PCD = 55mm
  • NUT Length = 41mm
  • Overall Length : 1090mm
  • Helix parts Length : 980mm
  • Motor Side  (Fixture Part) diameter for coupling : 12mm
  • Motor Side  (Fixture Part) diameter : 15mm (applicable to AK15, EK15, BK15, FK15)
  • Support Side diameter :  15mm (applicable to AF15, EF15, BF15, FF15)
  • Ball Screw Support Unit : BK15+BF15
Ballscrew supports:

The big one seems to come with a set of supports. These are the generic BK15 and BF15 supports that you see on all the Chinesium sites. There's a reasonable drawing here ballscrew supports - Hiwin.

And for the ballscrew / ballnut themselves, the Issoku ballscrews home page is a good start. Issoku HTR2005C7 ballscrew (page 68) and Issoku HTR2005C7 ballnut (page 12) look about right.

As for the cross slide ballscrew, I simply modelled it myself, along with a ballnut created from the dimensions on the ebay page. I'll check and correct that when I have something to measure up. I suspect it's so old there's no data on the internet.

Linear encoders:

I've also received an "EHM" miniature magnetic linear encoder from M-DRO that should fit underneath the cross slide table itself. The scale is a magnetic self adhesive strip of about 1mm thickness and the head is just 10x12mm (40 long), so can be buried out of the way. Resolution is 5um - not much point going for any better. Wasn't cheap but hopefully it will at least work.

For the saddle scale, I have a full sized HXX glass scale that I bought in China, also 5um resolution. It was sized for this machine so is 700mm long, with 600mm measurement range. I can cut it down if required but ideally it will be OK as it is. I've modelled it up in Fusion.

Drive pulleys:

For the cross slide drive, I plan to use an HTD 3mm pitch, 15mm width timing belt. The largest pulley I can fit in under the cross slide in its extended position would be 32 teeth (32-3M-15) and for the motor I should be able to get something like the 10t (10-3M-15), assuming that's not too small for the motor shaft. With a 150mm long belt (150-3M-15) on these pulleys, the centre distance will be around 42mm. I want to keep it fairly compact.

Cross slide limit switches:

ZF seem to do some sealed miniature microswitches. These should just about be able to hide themselves under the cross slide, within cavities in the saddle - I don't mind machining bits off here and there. Got some Omron ones too, in case they don't work out. These are not sealed but appear to be half decent quality (hopefully) and appear to be intended for industrial use.

Oilers:

Finally (for now), I ordered some oilers. The Bantam is rather light on lube points, as John Stevenson pointed out. I will press a few of these into the cross slide etc to facilitate oil getting where it needs to be.

Thrust bearing:

I want something half decent here, so I've gone for a Nachi 6200-2NSE9C3 Sealed Ball Bearing (C3 Clearance) 10mm x 30mm x 9mm. I'm not planning on any fancy paired back-to-back assemblies here. This is a single row deep groove bearing and it should be up to the requirements without having to step up to a double row bearing. I can't see it collapsing under the load it will see somehow.

Rotex coupling:

14mm Rotex coupling and orange spider will join the stub of the original leadscrew with the new ballscrew, removing any axial load. This is only allowing the handwheel to operate the ballscrew, while the servo and encoder are all on the ballscrew side of the coupling.

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