Building a 7" rotary table

[Firebird]

Hi

One of the best things I have done is to fit a 4 jaw independant chuck to my vertex 4" rotary table.



Setting jobs up in the mill is easy and very accurate using a DTI. It's so useful that I'm reluctant to remove it when other jobs come up. What I could do with is another rotary table.

Have you seen the price of these things now 

It was a big expense when I bought mine about 6 years ago but they have about doubled in price, beyond my pocket now.

Plan B.   Can I build one  I remembered seeing Arnold build one so I dug back and found his build and have just finished reading it, excellent Arnold 

I'm going to have a go at something around 7". Like Arnold I have started by throwing a few bits on the bench.



Some 50mm x 15mm steel bar, a gear, origin unknown but big enough to get a 7" table and thick enough to cut T slots if I wish. Some brass rings for the worm wheel, a bearing and a bit of 1/2 steel plate.

This will be one of those projects that lies on the bench and gets worked on every now and then so don't expect any rapid results 

Cheers

Rich

[tel]

I'll be looking in as you progress Rich! 

[arnoldb]

Cheers Rich 
It's a fun bit to build.  I use mine all the time and never regretted effort I spent on it.
I'll definitely be following along; good start 

Kind regards, Arnold

[Don1966]

You have my interest Rich, I be tagging along.

Don

[swilliams]

You have my interest Rich, I be tagging along.

Don

Mine too

Steve

[ths]

Me too! Hugh.

[Firebird]

Hi

Thanks gents.

A question If I may. The rotary table I have is 90:1, ie one turn of the handle = 4°. I think Arnold opted for 72:1, ie 5° per turn and I have seen 60:1, ie 6° per turn. I can see the advantage of 90:1, a finer feed and possibly more accurate? Anybody got any thoughts on this.

Cheers

Rich

[Maryak]

I can see the advantage of 90:1, a finer feed and possibly more accurate
Rich

Hi Rich,

IMHO the finer feed is the key.

90 : 1 gives the possibility of smaller increments of division using a vernier scale on the handwheel. With the correct dividing plates all worm drive ratios divide as accurately as the tolerances of construction permit.

Best Regards
Bob

[Jo]

Two of my rotary tables have 90:1 and that seems about right. The BCA is 180:1 and you spend for ever winding the handle but as she is for very fine work thats ok .

One feature you might like to consider is some sort of means to disengage the worm so that the table free wheels: it saves a lot of winding. And a lock to keep it in position.. and stops for either end of a cut....

Jo

[arnoldb]

I used the 72 tooth approach for a couple of reasons:
There was only limited space for the worm wheel, and I didn't want to make the teeth on it too fine, so instead of 90 teeth at a smaller module, the 72 let me use a bigger module, which made it easier to mesh things up reliably.
It's easier to work with 5^o increments on the dials; no need to keep track of where the last position was and count divisions; I can just read off the current position.
Anybody going through the build log will see that I made a bit of a stuff-up with the vernier scale - so instead of 0.001^o accuracy I was left with 0.01^o accuracy.  I've found this to be more than adequate in practical use.

As I mostly use it for small parts, when milling curves and so on the feed rate matches well with good control, though I have found the feed a bit course when going over a 50mm radius - at that point the finer feed afforded by a 90 tooth gear would be nice.

The zero-able handwheel is a pleasure to use, as is the eccentric adjustment to remove backlash; in fact, properly adjusted there is pretty much zero backlash on it.
In retrospect, one thing that I would like to add to the table is an adjustable degree ring instead of the fixed markings that I used.  This would be very handy when I use the screw-on chuck adapter to transfer workpieces already mounted in chucks between the lathe and RT.

Kind regards, Arnold

[Firebird]

Hi

Thanks for the info. I figured 90:1 would be best. I may have a practice at cutting the worm wheel on some scrap ally. I have some old largish whitworth taps that I will dig out and see what they cut like. If thats ok I can then work out what size worm blank I will need for 90 teeth.

Cheers

Rich

[mklotz]

The BCA is 180:1 and you spend for ever winding the handle but as she is for very fine work thats ok .

Mine is a 90:1 as well and, as Jo points out, winding that handle over any significant angular distance can be tiring and tedious.

In your design consider arranging for some sort of fitting at the outboard end of the handle shaft so you can mechanically "plug in" a common battery-operated power screwdriver. 

[Firebird]

Hi

Marv, I have been out playing this afternoon and the same idea occured to me. I'm looking into taking the shaft right through and out the back with the possibility of fitting a stepper motor.

I had a practice run with a couple of worm gears. The first one I used a 1/2" Whitworth tap which is 12 threads to the inch. First job was to turn a blank from some scrap ally.



This is the setup I cobbled together. I just messed about with various spacers until I got the blank on centre height.



The blank spins freely and is not held on. The downward cutting force keeps it on. Sorry there are no more photos of the first attempt. You can see I didn't quite get centre height but not bad though.



My second attempt I used a 5/8" Whitworth tap which is 11 threads to the inch, slightly coarser. Again I just messed about with various spacers until centre height was attained. alitlle more accurately this time.



Not too bad at all.



And compared to the first attempt.



A bit of video, not very good I'm afraid but you get the idea.

<a href="https://www.youtube.com/watch?v=f79QUrznS7k" target="_blank">http://www.youtube.com/watch?v=f79QUrznS7k</a>

To calculate the blank size I used this formulae.

Thread pitch x number of teeth divided by 3.142

For the 1/2" whitworth tap with 12 tpi

0.0833 X 90  ÷ 3.142  = 2.3870"

For the 5/8" whitworth tap which is 11 tpi

0.0909  X  90  ÷ 3.142  =  2.6040"

I have a 3/4" whitworth tap which is 10 tpi that I am going to try.

0.1  X  90  ÷  3.142  =  2.8644"

Cheers

Rich

[Firebird]

Hi

This morning I finished the experiment by cutting a 90 tooth worm wheel with a 3/4" whitworth 10 tpi tap.



Compared to the 11 tpi and the 12 tpi





This is me trying the worm wheel on the cross slide lead screw of my Myford which is 10 tpi. it rolls along nicely.



A bit of video of the cutting

<a href="https://www.youtube.com/watch?v=H-vL93ObVY0" target="_blank">http://www.youtube.com/watch?v=H-vL93ObVY0</a>

I think the 10 tpi is the one I will go for, it looks right. The tap I have used is not ideal. one tooth has a chip out of it and is a taper tap. I'll see if I can beg or borrow a better one.

Cheers

Rich

[Firebird]

Hi

another question if i may. Just read Arnolds build again and he gashed the blank with a slitting saw prior to cutting the teeth. He held the blank at an angle of 4.5°. Iv'e looked but can't find the angle of 3/4" 10tpi whitworth, anybody know what it is?

Cheers

Rich