Thread: Rotary Table CNCing
So for Xmas I treated myself to a rotary table... I didnt want to spend a fortune, but I wanted one that was a bit above the usual nasty chinese and indian ones. So I went over to Chronos and had a look at what they had and decided on this new design Soba 4", a copy of the 4" Vertex. OK its not a Vertex, but is a cut above their usual quality. It has a MT2 central taper so can mount a chuck, or use a centre...
One reason I chose it is that its a good candidate (IMHO) for CNCing...
Removiing the handle, reveals a 10mm spindle
and removing the vernier collar reveals a usefull 15mm deep by 18mm dia place to clamp a stepper mount to.
The plan is simple. Machine a circular collar 14mm thick 34mm OD, 18mmID with two or three 4mm tapped holes for locking grubscrews, to fit over the bearing. Place a 10x18x5.5 thrust washer on the 10mm spindle, drill out a L050 coupling and lock to shaft while using some puller arrangement to pull shaft outwards and rotating table to push shaft out - this will preload the thrust bearing. A 67mm length of 40mm OD, 3mm wall tube forms the basis of the stepper mount. This will locate into a 40mm dia, 8mm deep bored hole in a 10mm piece of aluminium 56mm square drilled out for a NEMA23 motor and counterbored 38.1mm. the tube is retained in the plate by 2 or 4 grubscrews through the sides of the plate. The general idea is shown below:
Last edited by irving2008; 06-01-2010 at 07:27 AM.
Nice one Irving, problem I had was fitting a thrust bearing on the inside to take the trust in reverse so to speak. There is not a lot of room inside the unit but I managed to do it although I had to do some machining inside the block to make room for the thrust race.
Your diagram shows only one thrust race so OK in one direction only.
Use an Oldham coupling, Lovejoy couplings don't take misalignment into account.
I have done one of the 4" tilting units, the cheap and nasty ones just to see if a type17 motor will run it, yes it does but only for positioning, not enough power to overcome cutting forces if you want to machine say an arc.
.John S -
Yes I pulled the worm out and internally there is a cast iron to cast iron bearing. It would be possible to fit a thrust bearing in there but the problem is finding a small enough OD diameter with an ID of 10mm. Possible a needle thrust if i could find something small enough, but the smallest is 24mm OD, or 18mm as a ball thrust washer. Another option could be an oiliite washer maybe?
It really needs to be <15mm OD i.e. about the same dia as the worm gear, I think, because of the offset of the spindle. The alternative would be to bore out the base of the cavity just above the wormwheel to a diameter of about 20mm to allow the eccentric to rotate the thrust washer into when disengaging the drive. Not impossible to do, but tricky, and means stripping the rest of the table down.
Where is the misalignment? using my proposed tube mounting approach everything is concentric. And I have several LC couplings going spare.
I am using a type 23 1Nm unit which I have here, but I could get a 1.85Nm type 23 from Zapp if needed. I measured the torque needed to turn the table and the 1Nm motor should be able to give about 5rpm for positioning and 1/2 of that for cutting, assuming alloy only (20N cutting forces as per other discussions on torque). My main interest is not cutting arcs but engraving handwheels and cutting gears...
06-01-2010 #6Where is the misalignment? using my proposed tube mounting approach everything is concentric.
Bet you end up using a flexible coupler of some sort :naughty:
Irving 1Nm is more than enough,I used a type 17 at 0.36Nm just to see if it works and it does.
I use 1.8Nm on the ones I do commercially but that's only because I get the best deal on this size of motor from China.
.John S -
You reminded me, I had to fit a sintered bush inside the offset bush as the thrust I used inside was a needle and so did not provide for any side thrust as ball units do. It is not easy to assemble due to the offset arrangement but at least it works as long as I dont try to reduce backlash to much.
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