Author Topic: Simple trunnion table design?  (Read 7492 times)

Offline grzgrz

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Simple trunnion table design?
« on: February 18, 2016, 22:36:31 pm »
Hi guys,

After experimenting with my Chinese 4-axis CNC4030Z-S, I'm planning on adding a simple trunnion table to the current setup. As I don't have any experience in machine building and I couldn't find too many blueprints on the Internet, so I'll appreciate any advice!

My rotary unit has a three-jaw self-centering chuck and a tailstock (that I want to replace). The height of the rotary axis centre is 55 mm. My first concept is a 200x100x10 mm (steel/aluminum) plate with two shafts, one fixed in the chuck and the other in a (self-centering?) ball bearing.

My questions are:

- Would a single-piece trunnion (i.e. a plate with protruding shafts) be any good, or should the plate be milled separately, drilled from both sides, and then the shafts would be inserted, drilled and fixed with screws?

- Is one ball bearing (support) enough or do I need another one (close to the chuck; I suppose I'd need it anyway if I wanted the upper surface of the plate at the rotary axis centre, unlike in my draft, where the centre is in the middle)?

- Is there any way to calculate the right size of the components in relation to stepper motor/spindle output and/or structural rigidity? (E.g. minimum plate thickness, shaft and bearing size etc.)

- Similarly, how can I choose the maximum length of the trunnion table without the need for additional support from beneath (i.e. rollers, assuming that the lower part could be semi-cylindrical)?

- What about slots and/or holes in the plate for quick fixture/workpiece changes? Any recommended patterns for a versatile trunnion?

- Finally, is there any way to facilitate the homing of the Z axis and the rotary axis? Has anyone ever tried e.g. mounting a tool probe on a trunnion table?

I hope I'm not making the design process more complicated than it should be...

Regards,

Grzegorz

Offline lloydsp

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Re: Simple trunnion table design?
« Reply #1 on: February 18, 2016, 22:46:08 pm »
GrzGrz,

There's nothing wrong with your basic design, so long as it's made of a strong-enough material.

I would have concerns about how to turn accurately coaxial shafts on the ends.  Other than that, I think it would be more accurate if you made it as coaxial as possible, then milled it to finished flatness on the machine on which it was intended to be installed.

Lloyd
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Offline EddyCurrent

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Re: Simple trunnion table design?
« Reply #2 on: February 19, 2016, 08:29:22 am »
If Z=0 is at the centre of rotation it implies that the table top should also be along the centre line or even below so that the workpiece could be packed up.
Similar to these; http://www.trunniontables.com/table-trunnions-c-1/stallion-quick-change.html
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Offline grzgrz

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Re: Simple trunnion table design?
« Reply #3 on: February 19, 2016, 17:15:40 pm »
Lloyd,

Would you consider aluminum a strong-enough material? I should be able to flatten the plate's surface if it's a non-ferrous metal. However, I don't yet have any coolant/mist system installed. Maybe applying WD40 on the fly would help?

EddyCurrent,

I've seen the Stallion trunnion before. Is its 'cradle' one piece (i.e. apart from the ball bearing support) or an assembly? A design where the plate is below the centre line seems more difficult to build. Also, is there a substantial difference in rigidity in a single-piece 'cradle' (again, without the support(s)) vs an assembly?

Do you think a slotted table would be a better solution over a hole pattern?

Thanks for your input!

Grzegorz

Offline lloydsp

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Re: Simple trunnion table design?
« Reply #4 on: February 19, 2016, 19:06:50 pm »
"Would you consider aluminum a strong-enough material?"

That depends entirely on its dimensions and the thrust it will be expected to withstand.

I can't say... would have to run all the dimensions/thrust/flexure figures for given alloys to even guess.

LLoyd
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Offline Bubba

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Re: Simple trunnion table design?
« Reply #5 on: February 19, 2016, 19:30:43 pm »
Would you consider aluminum a strong-enough material?
*****************
Yes.
As Lloyd stated, it all depends on loads expected, considering deflections,flexing, twisting, etc..
My first choice would be 4130 or 4340 steel, but I've build tools out of 7075-T7 or 6061-T6 aluminum that holdup well even in production environment. Keep in mind all materials will deflect so proper bracing in relation to loads expected must observed.. ;)
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Offline Bob La Londe

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Re: Simple trunnion table design?
« Reply #6 on: February 19, 2016, 20:10:32 pm »
7075 is stronger than many steel alloys, but with enough mass even 6061 is ok for some things. 

7075 machines very nicely. 

I cut 6061 all the time for molds and it cuts nicely as well.

How about a precision rod that is clamped into a precision groove cut in the trunion table?  Then you KNOW the rod is as good as its likely to get.  Then you can machine a flat surface on the table when you are done. 
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Offline grzgrz

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Re: Simple trunnion table design?
« Reply #7 on: February 19, 2016, 22:10:54 pm »
OK, let's assume I'm going to use the 7075 aluminum alloy. The frame of my gantry-type mill is built from the 6061 and 6063 alloys and has 3A (and 1.5A, for the rotary axis) stepper motors. The X and Y axes' max theoretical feed is 2500mm/min., Z's 700 and A's 500, respectively, and the spindle is a 0.8KW, 1,000-24,000 RPM. Can any of this information be used to calculate bending/stress/deflection, or do I have to provide the exact dimensions and weight of the machine's modules?

Bob, I like the idea of clamping the rods into the grooves/holes. What would be your recommended method for minimising the eccentricity/misalignment errors?

Grzegorz

Offline lloydsp

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Re: Simple trunnion table design?
« Reply #8 on: February 19, 2016, 22:50:30 pm »
I like Bob's solution.  A good, hardened, precision-ground steel shaft running the length of the whole apparatus, and embedded in a snug-fitting groove in the table portion would solve the problem without having to worry much about the alloy strength.

Lloyd
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Offline Garyhlucas

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Re: Simple trunnion table design?
« Reply #9 on: February 19, 2016, 22:57:37 pm »
I see a couple of issues.  If the axis of rotation is below the part being machined the thrust from the tool is likely to overload the A axis.  I think I'd use this extrusion http://www.paletti-usa.com/itemview.aspx/?id=709&cat=1 which would be stiffer in all directions including torsion than the flat plate and already has the slots.  Then I'd make two endplates to raise the axis up about 40mm or so.  I'd just bolt them to the extrusion at either end using the slots.  If you stack them and bore the holes for stub shafts that press fit you can get them very accurate without going crazy.
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Offline grzgrz

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Re: Simple trunnion table design?
« Reply #10 on: February 21, 2016, 18:25:07 pm »
Reflecting upon your comments, I still don't know (1) the pros and cons of using a block of metal vs a profile, (2) of using a long precision-ground rod vs two supports (see attached pictures of rotary tables) and (3) mounting the plate at the axis centre vs below.

For the long rod design, there are some extruded aluminum profiles with round holes varying from 5 to 12.5mm. Again, I don't know which diameter would offer enough support for, say, a 200mm (or slightly shorter) plate.

With limited Z-axis travel, having a trunnion plate below the A-axis centre would be more logical. My spindle bracket holds the spindle exactly in the middle, but I don't know if/how moving the former up a third would affect the overall performance of the machine. Any thoughts?

Regards,

Grzegorz