chuck mount for 4 in. rotary table

[jcs0001]

I went through quite a few threads on madmodder and came up with a system that should work with my new 4 in. Soba RT.

First - the RT:

I was torn between buying a 4 in. and a 6 in. but the price was good on this one and I figured the 6 in. would be a bit big for my small taig mill.  I just got it the other day and found a few design problems.  Set screws hold the graduated collar and the handle in place.  Both are hardened and of course gall the shaft.  Small brass slugs were put between the set screw and shaft (ala John Bogs).  I tried to centre drill one on the lathe first but being hardened it just dulled my centre drill - should have tested it first of course.



The setscrew for the graduated collar can be seen in this photo.  The setscrew for back lash adjustment is just below the 310 mark on the edge of the RT.

The set screw which I assume adjusts backlash did not seem to work.  It presses against the worm gear shaft housing and if that is free to move pushes the gears together, thus reducing backlash.  The only way I could figure to allow this to properly adjust was to slightly increase the hole diameters for the 4 bolts holding the shaft in place.  I snugged the three bolts (one won't fit if it is adjusted properly) up gently then adjusted the backlash set screw inwards till backlash was minimized and then tightened the three bolts.  That seemed to do the trick and there is very little backlash now.



Three of the four bolts holding the worm gear shaft in place can be seen in the photo.  The knurled brass screw on the left side is the locking knob - it would seen that the designer/builders know that brass against steel will prevent galling - why not do something similar for the set screws??

The inner face of the graduated collar and the outer face of the adjoining holder were also skimmed a slight amount on the lathe and this seemed to smooth things out.  I may eventually lap these surfaces a bit as there is still a slight amount of wobble between the two.

The RT centre does not have a taper but has a cylindrical hole below the bottom of the T slots. 

I have a small 4 jaw taig chuck and it seemed logical to install it on the RT.  It accepts a male 3/4 in. x 16 tpi  shaft from the rear.



Taig 4 jaw chuck with RT and finished aluminium chuck holding plate.

I had several pieces of round aluminium about 4.5 in. x 7/8 in. thick in my treasure box and decided to dedicate two of them to the RT.  One as a dedicated chuck mount and one as a tooling plate.



Cleaning up the edges and surface of the aluminium round.  I did a slight cut at about the 4.1 in. mark as a guide to ensure that I didn't get too carried away.  Other than facing each side I didn't reduce the thickness much.  That can always be done later if need be.

Once the aluminium plate was cleaned up the centre was drilled and tapped (in the lathe) with a 3/4 x 16 tap.  I used the tailstock live centre to keep the tap in line and it gave a good result.



Shows the brass threaded portion at the bottom of the aluminium plate.  The portion below the plate just fits into the beginnings of the T slots of the RT.  The steel portion fits down into the centre of the RT.

The next part of the project was a brass 3/4 in. x 16 tpi post that screws into the aluminium plate and extends above the plate by about 5/8 in.  This extension will allow the chuck to be screwed on the the top of the plate.  It is drilled through with a 3/8 in. hole.



3/8 in. steel post turned down to fit into centre of RT.

The final part of the project is a steel post turned down to fit into the hole in the rotary table and left at 3/8 in. diameter to fit into the brass threaded post.  This post allows the chuck and aluminium plate to be easily centred in the RT.  I may locktite this in place but have not done so yet.

The RT was set up in the mill and four mounting holes spotted near the edge.  These will be used to bolt the aluminium plate down onto the RT using T slot nuts.

Another 3/8 diameter post may be placed into the top of the brass threaded sleeve to allow centring the RT in the mill. 

As you can see in the first photo I tried to use the side of an end mill to do a final clean up of the edges of the plate (turning the RT to do so).  Lots of chatter ensued.  Giving this some thought I believe it is due to not locking the X and Y axis of the mill before milling the edge.  I will try this next to see if it cures the problem.

Hope this is of some help to someone else with one of these tables.  Thanks to others for posting their solutions for chuck mounting an a RT.

John.

[jcs0001]

In order to make this a bit more clear I have done up a quick drawing of the parts.  Most dimensions aren't critical - the small diameter of the steel rod (fits inside the RT) could perhaps be a couple of thou bigger to fit a bit tighter.  I suggest slowly bringing this diameter down and testing regularly with the RT.  The bottom end of the brass piece (shown as 0.555) could also be several thou bigger.



Perhaps this will make things a bit clearer.  I used brass because I had some in the right diameter and also drill rod for the steel rod - it was handy at the time.  Other materials could be used.

Just a note about threading the outside threads of the brass centre piece.  I haven't done much threading using change gears and none on my current lathe.  Decided this was a good project to get a start on it.  It took a bit of time to figure out the change gear set up but it was fairly straightforward.  I turned down the end of the 7/8 brass stock to 0.75 (turned out to be a bit more though - must have been asleep).

Got the lathe all set up for threading and proceeded to thread using my spindle handle - my lathe will only go as slow as about 150 rpm and that's too fast for me right now.  Not knowing how the threading dial works I left the the lathe in gear the whole time, bringing the cross slide out for each return and back in to cut.  It is slow but works - will figure out the dial very soon however.

Anyway as I was getting good threads I found that the chuck would not thread onto the threads.  Eventually measured the outside diameter of the threaded portion and found it oversize - no wonder it wouldn't work.  Gave up on trying to get it down with the threading tool (some good threads were formed by now) and put another lathe tool in the post and turned the OD down to just under 0.750.  The plan was to then chase the well formed threads with my threading die in 3/4 x 16 tpi.

However I found I don't have a die handle for that die and of course my lathe was taken up with the project at hand.  Bye dint of some really crude methods I managed to get the die to cut the threads to the point where the chuck and aluminium plate thread on nice and snug. 

The long and the short of it is that making a die handle for that die is at the top of my list.  Also being more careful in measuring will be at the front of my mind from here on in.

John.

[Jonny]

Are you locking the chuck down when threaded on?
You wouldnt want it to unscrew!

Did my Vertex RT at first with an MT3 collet and stub into recessed back chuck, run out a mile and was never rigid enough. Bolt direct to table, and centre up.
99% of my work needs to be centred with no hassle trueing up job every day, therefore chuck remains on permanent.
Also had same problem 4 T slots in RT, only one T nut could be used.

[jcs0001]

Jonny:

I figured I'd likely need a lock of some kind - maybe a cam or something similar that would fit against the side of the chuck or against a jaw - haven't got that far yet however.  Thanks for the reminder,

John.

[mechman48]

Hi jcs,
Have been looking through the threads & happened on your write up, I too have a 4" soba RT that was purchased a number of years ago, & along with other stuff was put away due to work commitments. The difference with mine is I bought the tilting table version with the view that if I needed to machine something at 45 degrees then I had the facility to do so.I retrieved it from the dark depths of my garage storage yesterday & opened it up,yep it was still in its grease proof wrapping & in mint condition so thats a bonus, I gave it the eyeball inspection & all looked fine until, like your comment,there was'nt a taper in the center only the recessed hole,pity that as a 1 MT taper would have been nice to have, any way on closer inspection I noticed a thread in the lower portion of the hole so I thought Hmmm! at least it has the probability to fit/make  a threaded boss to fix a chuck to it.

I decided to see if I could determine what thread it was so I guestimated what size it to be; I thought 6mm, so out came a 6mm tap, it promptly dropped through to the bottom of the hole, ok not 6mm, so next out was 8 mm tap, this did not fit either, it seemed to pick up the first thread then would go no further, strange! not wanting to do any damage out came a 5/16th UNC/F..sloppy fit... now this leaves me with a conundrum! could it be a 7 mm? not unknown but a strange size to boot, not having a 7mm tap I couldn't confirm this possibility.
Are there any modders out there who have found this anomaly too, & what have been your solutions to rectify this as obviously I would like to fit a chuck at some later stage. Now all this is based on the presumption that there is a thread in there, if not then my eyes need a serious 'looking at'(pun intended) but I'm sure I could see the start of a thread & the 6mm tap felt like it was engaging/catching on some type of thread!
Any help would be appreciated, thanks in advance guys.
George
Teeside

[andyf]

George,
M7 is unusual, but not unknown. The pitch is 1mm for coarse and 0.75mm for fine. Tap drill sizes are 6mm and 6.2mm respectively, so you could see if either of those will pass down the threaded part. A better test would be to screwcut 1mm and 0.75mm threads on some 7mm stock, and try them for fit.

A cruder but quicker approach would be to whittle down a bit of wood to get a square or triangular section which you can twist down the thread and thus get an idea of the pitch. If you impress 10 "threads" into the wood to measure across, that will be more accurate than three or four. An inch of threads would be great, so you could count the tpi if it turns out to be imperial, but half an inch would be nearly as good.

Andy

[quantumeer]

Along the lines of the wooden screw gauge, what worked for me on a difficult job once: lubricate hole, inject hot melt glue, embed screwdriver and unscrew when set. It made a nice rubbery screw to measure.

Mark

[mechman48]

Hi there
Conundrum sorted; decided to take another look at my RT today,unlocked the table & lifted it to 90 degrees ...again, this time took a look at the underneath..what a muppet I am    there's only a bearing retaining plate held in by a countersunk screw staring me in the face isn't there, nevertheless I sorted through my collection of allen keys to find to find one to suit..voila..unscrewed & removed the csk screw, the internal thread was bright crisp & clean looking, the csk screw looked like what it was ..a piece of c**p,  so out came the digi' vernier...7.51mm OD... switched to imp... 0.2955" strange size methinks..? referenced my latest Zeus book... nothing matches, the nearest I could place it to was 5/16ths.. 7.50mm or 0.3125" I retrieved my collection of thread gauges..18 TPI, obviously not metric.. not UNC, not UNF nor AF  then one of those silly thoughts came up... nah! couldn't be, could it? .. Zeus tells me its '5/16 WHITWORTH' rooted around in my 'old tap box' & found a solitary tap marked '5/16 x18 TPI Whit', where I got that from heaven knows! .. offered it to the thread...well, need I say more!! 
It brought to mind the old adage' measure twice, cut once' but in my case 'inspect carefully before asking the question' ... numb nuts comes to mind . At least I can make a fixture to attach a chuck at a later date.
Any how modders out there thanks to those who offered remedies.