# 4th Axis Trunnion Table project



## spumco (Jan 6, 2018)

At the request of TomS, here are some details of a trunnion table I recently built to take advantage of my 4th axis.

Basic idea was to have a very heavy-duty trunnion table that was as big as my machine travels permitted, but that was easy to remove so I could stick a 6" vise or other stuff on the table.  Also wanted a fairly standard hole pattern for fixtures.

Biggest issue to overcome was the fact that I don't have a hoist in my garage, so I couldn't just make a big base plate to mount the drive and tail support in one piece for easy setups.  Way too heavy for me to lift.  That meant making the trunnion removable while still being repeatable.

Answer to that was:
1. Make the tail axle removable.  Loosen the axle clamp screw, and the axle slides out of the trunnion tail plate.  The axle doesn't rotate with the plate (the plate has a bronze bush pressed in) so no fussy bearings as this thing only rotates at about 8RPM max.  That, and I has some bronze stock on hand.

2. There is a locating pin in the 4th axis spindle with a 2MT end and a straight-side .875" end.  The 2MT end goes in the spindle and the other end sticks out about .625".  The drive  plate has a slot/pocket in the end with two angled shoulders at the bottom.  Those shoulders are tangent to the center of rotation, and bear against the locating pin.

3. Installation of the trunnion means turning it upside down, dropping the pocket/slot over the drive end locating pin until the shoulders hit the pin, and sliding the tail axle in the bush.  Run the drive screws in to the faceplate T-nuts and snug everything down.  Because the pin only touches the drive plate pocket at the intended contact surfaces, there's no 'nudging' anything around to get it back to square and parallel along the X-axis.  Nor do I have to worry about pushing a pin with clearance through the drive end in to the 4th spindle and then having it bind up if I'm off a bit.  There's zero clearance between the pin the the drive pocket contact surfaces.  All I have to do now is bolt it in and sweep the table along Y to zero the A-axis.

*Materials* - I went with hot rolled steel.  I considered aluminum, but steel is about 3x stiffer than aluminum and I didn't want any flex.  And hot rolled doesn't warp as much as cold-rolled or some of the alloys.  It's not heat-treated or hardened - what was the point?  If I ding the table I'll just stone it out.

Everything is a simple milled surface, except for the working table surface.  I used a buddy's surface grinder to flatten that down and it looks like pretty awesome. A tenth's indicator on the middle of the table barley twitches when I stand on the table - no flex.

*Size *-  The table is 1.25" (ish) finished thickness, and 5.5"x18" long.  Working surface is about 16"  long.  Height/offset from rotational axis was as far as I could get it with main plate thickness.  Had to chamfer the edges about 3/4" to get them to clear the table.  Working surface is 2.008" from the rotational centerline

*Features:*
1. Holes are tapped 3/8-16, and the top .5" is reamed for 3/8 dowl pins.  Totally stole that idea from Saunders Machine Works (NYC CNC - youtube) and it's an excellent idea.  Pattern is 1"x1" triangle pattern, so holes along the X & Y axis are 2" apart.  Center of hole pattern is aligned with the rotational axis.

2. The fixed axle and floating tail end plate mean that when the table grows in length due to temp changes it doesn't bind up.  There's about .025-.050" clearance between the tail support and the outboard end of the tail plate.

3. All connections are bolted, and there's enough slop that everything is adjustable.  I knew I couldn't machine it _that_ perfect, so during initial indication & squaring I just snugged it all up lightly and got the tail axis dead-nuts with the 4th drive axis (brass shims under tail support).  After that, I rotated the 4th a whole bunch of times to let it settle in without binding and then started slowly tightening all the connection joints.  Dowel holes were nudged parallel with the X-axis and the table was finally tightened to the drive/tail plates.

*Future Changes*:
1. If I did it again, I'd avoid screwing up the intended hole pattern.  Completely my fault, but I exported a bad version of the CAD file that had the inboard hole pattern off-set by about .1" and then went and machined it out on my buddy's VMC.  Now the 2nd and 4th row of holes is offset along X and none of my fixtures are reversible!  Still perfectly usable - I just have the 'final' version saved in CAD so when I model a fixture I set the fixture hole/dowel pattern to the as-built pattern.

2. Make it longer.  If it were about 2-3" longer I could get the mill head all the way down to the working surface at the extreme ends of the machine travels.  I'd still have room on the machine table for the 4th drive and tail support.  Not a huge deal, but now it's a bit limited in working length if the parts are really off-set from the rotational axis.

So, there you go.  Just ignore the naughty little part in the fixture...

-Spumco

Overview.  Note brass shims under tail support go get the axis lined up with the 4th drive.  Also note the total hack-job splash shields over the 4th stepper and top.
The big block to the right is a tail-stock with a retractable ram that supports the long overhanging part (currently retracted).



Closeup of tail end.  Dowel pin in center of axle is used to find Y/Z zero.  Fusion 360 CAM requires all 4th axis moves to use the center of rotation as the WCS and tool orientation references, so this was way easier than swinging the table around and indicating the table surface a bunch of times.



Here's a shot looking down the drive plate slot/pocket.  You can just see the locating pin at the bottom.  When it's upside down, the angle surfaces at the end of the pocket bear against the pin.  There's clearance between the end of the pin and the bottom (left in photo) of the pocket.


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## magicniner (Jan 6, 2018)

Inspirational project man! Now I need one!


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## TomS (Jan 6, 2018)

Thanks for posting this.  I've got a rotary table I can use for the drive end.  You've got me thinking.


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## spumco (Jan 7, 2018)

TomS said:


> You've got me thinking.



_"Rut Roh, Raggy"_


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## magicniner (Jan 7, 2018)

Scooby?


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## spumco (Jan 7, 2018)

magicniner said:


> Scooby?


Bingo.  Got it in one.


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## spumco (Jan 9, 2018)

So here's the mostly finished project from the first photos.  This is what a trunnion table is good for....

Racegun!  

Stainless slide and Gr5 Ti comp.  Surface finish came out pretty sweet.  4Fl 3/8" carbide, TiN coating, 200SFM (2Krpm), full DOC and 0.010" WOC for the finish pass.  Sounded great after the 'new end mill squeal' went away.

Slide racker and some mostly cosmetic lightening cuts will finish it off.


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## MontanaAardvark (Jan 14, 2018)

To quote Homer, "Ummm!!!  Race gun!"

Tell us more!  Or show us more.  

This type of carrier for the A axis is something I had thought of only for palettes of identical pieces that all need to be machined.  Something like this VMC carrier:




Are you using the rotation to cut those 45 degree sides on the slide?  That adds something that isn't apparent in the VMC tombstone fixture.


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## spumco (Jan 15, 2018)

They are 50 degree cuts (tri-top) because 45's wouldn't match the slab side 'body-line' properly and still leave enough meat on the top of the slide.

Top and rear swoop were done at 90.  All cuts done with side of tool so the tool marks could be stoned out easier (no end mill circles).

I'm using an off-set trunnion to get the barrel as close to the center of fixture rotation as possible to reduce side or rotational loads on the table.  If I used a flat plate along the A-axis rotation I could have two fixtures opposite each other and cut two in one program, but the slides would be sticking out quite a bit and I might get chatter - and drilling way off axis would be doom.

It also means my 4" vise fits on a fixture plate with the top of the jaws just above A-axis center.

If I had a brake rather than relying on the 80:1 worm gear I could probably hang things out farther from center without worrying about it.

I'l post more when I get the remaining cosmetic lightening cuts and slide racker dovetail finished (i.e. no longer my problem!).


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## spumco (Apr 11, 2019)

Year and change later - major update (understatement).

New home-built 4th axis based on a hollow-bore harmonic drive gearbox.  The old worm drive one had unacceptable backlash.  The new one is rigid, backlash-free, and accurate way beyond my hopes & dreams.  Once I replace the stepper with a servo I should get over 100RPM A-axis rapids.

New trunnion table made to fit the orignal ears, minus the buggered hole pattern in the original version.  Same pattern as the Saunders Machine Works fixture plates.

All new tailstock with a proper 5C spindle and bearings.  Mounted on linear rails so I can move it in X (shorter parts) without losing position in Y or Z. Since it's located in X with a ballscrew, I can also use it to tension slender parts by installing a 5C chuck in the spindle taper.

Because the 4th axis has a 3MT taper, I mounted a boring head and line-bored the tailstock bearing pockets in place (at 30RPM - slow).  Everything stays concentric and no shims anywhere!

I've even made provisions to mount a motor on the tailstock and turn it in to a lathe spindle - eventually.

More details available if anyone's interested...

-Ralph


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## JimDawson (Apr 11, 2019)

Very nice work


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## macardoso (Apr 11, 2019)

Dear god, how long did it take to line bore that housing. That looks miserable.


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## spumco (Apr 11, 2019)

macardoso said:


> Dear god, how long did it take to line bore that housing. That looks miserable.



Not that bad, honestly.  I used a high-polish positive rake insert (not in the photo), and I milled the bearing pocket out to within about 0.075" of final bore in a vise (with the normal spindle).  The line boring was just a way to ensure Z-axis concentricity.  Took about 4 passes per pocket until I got a thumb-force press fit.  And the pockets are only 0.750" deep(ish).

Did the front end, then unbolted the ball nut, slid it off the rails, and flipped it end for end.  The rails kept everything aligned for the outboard pocket.

Glue the bearings in with some retaining compound and done deal.

But yes, it took _some_ time at 30RPM.  First pass I wound up advancing the tailstock too quickly and got really nice threads instead of a smooth bore.

A better photo below of the setup right before I switched inserts.  Second one is final alignment before tightening the tail base plate down.  Got a 1" hardened chrome rod trued up in the 4-jaw and then clamped the other end in a 1" collet with the tailstock loose.  Tapped it in true while rotating the rod to average the errors and then snugged up the 4th axis and tail base.  

The rotational axis of the 4th and tail are concentric, parallel with the X, and perpendicular to Z within the limits of my ability to measure  - maybe a couple of tenths here or there.

-Ralph


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## macardoso (Apr 11, 2019)

Beautiful work!


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## Matt81 (Sep 29, 2019)

What reduction ratio did you go with on the harmonic drive?


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## spumco (Sep 30, 2019)

Matt81 said:


> What reduction ratio did you go with on the harmonic drive?


50:1 (or 51:1 to be exact).

I'd like to say that this was the precise ratio I needed, based on a careful engineering study.  Reality was that I was looking for anything 100:1 or less and got the 50:1 drive dirt cheap.  I wanted something fast - I was tired of 10RPM on my worm drive 4th - and lucked in to this one.


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