Sources of precision machine parts for concave facetor

[FlashGP]

Hi all, particularly Makky

I have bitten the bullet and bought some linear bearings and motors from Banggood and am going to try my hand at making a prototype reciprocating concave faceting arbour.

I have yet to work out the dop stop, and am thinking of something like the arrangement used by polymetric.

Does anyone know of a local source of precision machine parts for instruments or CNC cutters?

Regards
Gordon.

[MakkyBrown]

Hi Flash,

I built my cnc machine from parts on ebay and aliexpress. It was a battle and I had numerous claims due to poor machining, faulty parts, wrong parts etc. A lot of the linear bearings are rubbish. I swapped out a heap of bearings from housings. There are some really good sellers on Aliexpress for bearings just need to find them among the others.
What sort of cnc cutters are you after?
I buy for Aliexpress not ebay for cnc parts/bearings etc etc. Try and ship with e-packet where possible. And what out for the sellers that never ship/pretend too . got to go.

[FlashGP]

Hi Makky

I am actually using the parts to prototype a cheap concave cutting attachment for my facetron.  But I am also thinking of making a surfacr grinder using a cup wheel to resurface laps, a bit like a fly cutter, but able to take small amounts off a diamond covered surface.  I doubt the components would be rigid enough to use propper milling parts to resurface a lap.

The trick with the surface grinder will be to get the rail for grinder perfectly parrallel to the lap as it sits on the plattern of the old Robuilt tub, hence it is a project for when I have the time to tinker.

WRT to the concave machine, the bearings won't be taking much weight, and I expect that by spacing them 300mm apart, I can minimise the effect of slop in the bearings.  E.g. a .3mm movement of the bearing would have 1/3rd of the yaw at a 300mm spacing than over a 100mm spacing between bearings. 

I had tried an attachment which used a slide running on bearings cut into an aluminium extrusion, similar to the VJ concave attachment.  The clearance for the bearings of the slide carrying the motor that drove the mandrel translated to at least 1mm movement off centre where the stone contacted the mandrel.  That and allignment error meant I couldn't be certain of the index the facets were being cut, which is no good if positioning of the facet is critical.

Regards
Gordon (AKA Flash)

[MakkyBrown]

Flash,

Have you drawn up what your building in cad or photos etc. I'm sure I'll have some ideas if you give me a clearer picture of what your building.

Concentrate on the concave machine first.
Your idea with the fly cutter, you may aswell just build a simple cnc machine and machine the lap on that. but hmm mine took at lot more work than expected and a bit of head scathing,  and I may have mumbled a few time I should just have purchased one  .  Your can get things pretty good with cheap linear bearings, I'm using 16mm bearing/supported rails on my machine and accuracy is really good. No error between cut layers, and probably under 0.02mm maximum play measured at the bit from the bearings in any direction. Given the bearings are ball bearing in plastic housing that's not bad imo. This play does not seam to be transferred into the job and every thing is cutting really nicely.

[FlashGP]

Thanks Makky.

My drawings are rudimentary, just scale block positioning in Word with 3 layers on a 1:2 scale, eg 15cm = 30 cm full size.

The bottom layer is 2 transverse 8mm bearings with an acme screw thread and brass anti backlash screw housing to provide mast allignment.  These sit on a 9mm ply plate (it's a prototype, if it works I can always use aluminium plate later). Another plate tops this with 2 linear bearings running in the direction of the travel of the carriage.  A further plate tops this as the mounting plate for the mandrel motor.  I have sourced an 11mm collet chuck that fits directly on the motor shaft.  A further motor with 30 rpm worm drive sits on themiddle plate and via an offset arm drives a linkage arm to a pin in the middle of the top plate..  the top plate is wide enough to rotate the mandrel arm through 90 degrees to allow me to cut grooves.

I am still thinking about a platten to rotate the assembly through 90 degrees around a point in the middle of the mandrel, this would allow me to cut facets at cross angles.  At present I have to cater for the platten bearings as the assembly fits over the top of these.

Regards
Flash

[Aussie Sapphire]

We have a few left over Darkside and BATT mandrels which may be of interest - left over from when we had the Polymetric dealership.


https://www.aussiesapphire.com.au/index.php?main_page=index&cPath=59_296&sort=20a&filter_id=135


cheers
Leah

[FlashGP]

Thanks Leah,

I'll see how I go with the machine.  The 13mm mandrels look good, Ove only ordered an 11mm collet so the 17mm mandrels woth 3/8th shaft won't fit.

I had been thinking of using 9mm mandrels, but the darkside is swaying me towards 13mm.

[MakkyBrown]

If you had a lathe you could make up your own cheap. Copper, brass or even just aluminum. Just use 1/4 brass as the shaft and loctite 680 the mandrels made from so round bar to it then machine perfectly to size.
Brass could be pretty nice for polishing.

[FlashGP]

Thanks Andrew,

I was thinking of that.  I know an ex TAFE teacher/engineer with a decent lathe with independent 4 jaw chuck.  I was also thinking of casting a couple of zinc mandrels around 1/4 inch brass rods. and machining them to size.

The cutting, pre-polishing and polishing mandrels all need to be machined to the same size.

Regards
Gordon.

[Rusted]

I looked into this a while back and found a site where someone was using 1/2" copper  tube for mandrels. maybe make them a press fit onto a steel master so you can change them when they wear.

[FlashGP]

Thanks Rusted, i hadn't thought about a press fit onto a steel mandrel.  It would straighten the copper so you would not need to machine as much off.  We have a 2t arbour press, it wouldnt take much to make a fitting to hold the steel part vertical for the press fit.

[Dihusky]

Clarie Trevena, on the Gold Coast makes concave facetors, has a small machine shop and might be able to help. AFG member and well known. If you want his phone number pm me.

[FlashGP]

Thanks Dihusky,

I was talking to Clarie on the weekend at the mineral and crystal show at Runaway Bay.

[FlashGP]

Hi Makky

Most if my parts arrived by Friday.  Just waiting on the last 3 or 4 shipments.  Purchased them form Banggood.  They ship from factories, as a result one order can be split across 5 shipments.

The linear bearings are a snug fit, far less play than I thought.  But they rumble a bit.  I am thinking of buying 4 nylon bearings to swap into the oscilator bearing housings.

The bearings holding the acme thread have tiny grub screws, I had to buy a set of tiny allen keys, not very dear from Bunnings.  The key needed is 1.27mm.

Assembled most of the bottom layer last night.  It is basically an allignment layer.  Havent taken a photo yet, am waiting on the anti backlash screw and housing for the adjustment screw.

Mrs Flash said if it works I should patent it.  I don't think I would sell enough to pay for the patent.  They cost about $10k for a lite patent or about $25k for a 20y patent.  Then you need to patent overseas.

Besides although it is a cunning contraption, it may not be novel enough to patent.  It is basically a reconfigured  2 axis cnc cutter.

Regards
Flash

[MakkyBrown]

Hi Flash, A lot of the linear bearing are crap. I changed most of mine for better ones. I think you mention you were using 8mm, my brother swapped to the plastic centres on his 3d printer and is very happy with them.
Cheers
MB