Rotary Table Jig Plate

Rotary Table Jig Plate

I bought a new 6" rotary table for my manual milling machine just before Christmas. It's your basic Chinese made table, but functions as expected with no real tight spots in the rotation. As a bonus, the seller also sent me a set of 3 dividing plates and the hardware to mount and use them with the table. I've had great luck dealing with this guy: https://www.accusizetools.com/ Because he's in Canada, shipping is reasonable, fast and no brokerage fees! I usually check his ebay store first, he usually lists everything for sale there as well as on his website.

I bought a 4" rotary table many years ago for the mini mill. I made both a tapped jig plate as well as a 3 jaw chuck adapter for this table, then also CNC'd it. For the most part, the 3 jaw chuck just stays on it. It's a very handy CNC device to have and I still use it often.

A basic rotary table has 3 or 4 t-slots that you use with clamps (often custom made) to fixture your part to be machined to the table. This can be very akward and difficult to use. Making a tapped fixture plate for the table greatly expands the part clamping options and simplifies the setup of the part.

The jig plate I make here was not my idea. I got the idea from a Youtuber named Clickspring. In one of his videos, he shows the jig plate he made and the features it has. After watching that video, I knew that I wanted the same design and features on mine.

Key features:
1. Jig plate is centered on the table using an MT2 plug inserted into the center hole of the RT.
2. Center plug is drawn tight into the taper from the bottom of the able using a sleeve and capscrew.
3. Center plug it tapped allowing clamping of the workpiece in the center of a hole.
4. Center plugs can be custom made to suit the job.
5. Center plug is used for locating the RT under the spindle of the milling machine.

 Here's the PDF to the 'As Built' plate

To get started, I scavenged a piece of 1/2" x 7" steel flat bar from work. I should have got one of our guys to torch out the basic shape using our CNC liberator machine, or a circle cutting guide, but I figured I could manage. I would have never guessed that I could cut this circle out using my Bosch jigsaw and some good quality metal cutting blades, but 1/2hr later it was done.

Cutting the plate out

The jig saw blade used

Next step was to drill and tap some holes through the plate so that I could mount it to the lathe faceplate.

I used the live center in my tailstock aligned with a centerpunch mark located on the center of the disk to locate it on the faceplate. This was not too critical, since it's just the first machining. Everything after this would be located using a dial indicator on the outside machined edge. I could have used about 2 more hands to hold the plate as I got it snugged to the faceplate.

I faced this side, and machined the OD to about 6 1/4". I bored the 1/2" locator hole in the center of the plate, then drilled and reamed the 1/4" through hole. After flipping the plate around to machine the top side, I used a chunk of 1/4" rod in the tailstock to suspend the plate while I got the clamping bolts started and snugged down. I then dialed it in to less than .001" before tightening it down fully.   

I experimented with different tools to try and get the best surface finish. Using automatic cross feeding, I found that if the crossfeed was set too slow, the tool would 'starve' for chips and cutting would be erratic. If set too fast, the machining marks would be quite coarse. I also tried a few different carbide inserts, some with a big tip radius and some with a small tip radius. I found that the small tip radius would make a more uniform cut. Finally I tried a hand ground HSS blank, sharpened to the nines. This is what I used for the final pass on the top side of the plate. The finish was still not as good as I wanted, but using some sandpapers and finishing with scotchbrite made the result acceptable. 

After facing the top side, I scribed the circular rings at 1" increments using a 60deg threading tool and set the depth stop to ensure that they were all the same depth. While I was at it, I thought it would be handy to scribe some fine lines perpendicular to each other that go through the center of the table. For this, I used my handy spring loaded graver tool and carefully ensured that it was on dead center. I rotated plate 90deg and used a square to ensure the the previously scribed line was at 90 deg to the top of the cross slide. 

Lathe operations done

Finally I bored the top 'plug' hole to exactly 0.750". I can just turn future plugs out of 3/4" drill rod and they will fit perfectly. 

The jig plate was now ready to turn to swiss cheese with the 1/4-20 UNC tapped holes. Of course, the easiest way to drill all the holes was to do it on the RT itself. In order to do this, I would first have to make the MT2 locator plug that is mounted into the taper in the center of the RT. I happened to have an MT2 dead center for the mini lathe that has never been used. I decided to machine this to fit the RT. Since this is the most important piece of the whole project, I need to ensure that the end that locates the jig plate was machined perfectly concentric with the taper. Since you can't hold a tapered part in a 3 jaw chuck, the only solution was to make a sleeve with an internal taper that matches the MT2 taper. I set the compound slide of the lathe to the same angle as the MT taper (by indicating along the length of the taper) and bored a small sleeve. I was then able to machine the part while being held securely in the sleeve. I tapped the top end of the locator plug 10-32. I also needed to thread the bottom of the shank, but quickly found out that this end of the taper was hardened. After annealing the entire piece and cooling, I was able to drill and tap the opposite end. Basically, this was all to have an exact 1/2" diameter x 1/8" cylinder sticking up above the table. The Jig plate has the mating 1/2" hole in it. 

The MT2 shank being held in the sleeve

Finally, I need to make a sleeve that fit into the bottom of the RT that could be used to pull the MT shank into the table. Nothing complicated there. 

Draw bolt, sleeve, MT2 shank and jig plate

With these parts done, i was able to assemble the jig plate onto the rotary table. I held the plate temporarily in place with a 10-32 button head cap screw through the center of the MT2 shank, exactly what it was designed for. I then proceeded to drill the hole patterns to about 80% through the plate. Before removing the plate, I went around again with a countersink and put a generous chamfer on each hole. When I was done, I removed the plate and drilled the rest of the way though using the drill press.

Although I would have liked to tap all those holes using the rotary table for indexing, it just wasn't going to work. The tap has to go about 1/2" through the bottom of the jig plate to form the full thread. Rather, I set this up in the milling vice, and X-Y'd to each hole and power tapped them. I used a neat idea that I picked up from Abom79 on Youtube about making a tap driver. I made one of these for the 1/4" tap which made the power tapping almost enjoyable. I will definitely make these for any tap that I need to drive with the mill. Thanks Abom!

Tap Driver Sleeve and Tap

Since I was going to use 1/4" hex socket cap screws to to mount the plate to the table, I need to also make the 4 t-nuts. 1/4" t-nuts are not that common, besides, I didn't have any on hand.  Some strait forward milling, drilling and tapping and they were done.

T-Nut and 1/4-20 Socket head cap screw

Finally, the alignment plug. I was going to make it with a cone point, but after thinking about it, it would be easier to line up a point in the spindle with a hole in the plug than a point to a point. (I think) So that's what I made. Just a simple plug that fits snugly into the center hole of the plate, and is accurately center drilled.

Aligning the center of the rotary table under the center of the spindle

So that's it for now. It took about 10 hrs of enjoyable shop time to complete this little project with a material cost of $0. I Just found a great deal on a 5" 3 jaw chuck and have ordered it. That will be the next device to mount to the RT. It will align to the table using the same method as the jig plate.
Till next time.