Rotary Table Sub Table

[John PurdyParticipant @johnpurdy78347]

Many years ago I built GHT’s small 4″ rotary table and it has been invaluable over the years to profile the ends of things like valve gear parts, con rods, connecting rods and the OD of intermittent circles like eccentric sheaves. Since making it I have built up a selection of pins/disks that fit the central 1/4″ reamed hole to hold and centre various items, 28 so far from .0625 to 1.250 diameter. As useful as it is, I have found that on occasion, because there are only 4 slots, it is a bit of a challenge to arrange clamping of some items, particularly small bits, and still give access to the milling cuter and also be able to see what’s going on. I’ve often wished there were more places to put screws to hold things down.

Then last week the latest issue of “Engineering in Miniature” (and sadly the last!) arrived with an article by Rodger Vane on making a sub table covered with tapped hole for said rotary table. I thought “that’s just the thing I need” but then not having an automatic tapping head as he did, the thought of hand tapping all those holes,56 of them, sort of dampened my enthusiasm. But then I thought rather than use normal hand taps what if I were to use a spiral point or spiral flute tap. That way rather than 2 turns, back off 1/2 turn to break the chips, repeat a couple more times then back it all the way out the clear the chips from the flutes and repeat half a dozen more times till through the 3/8″ thickness, it would be just a case of tuning the tap a sufficient number of turns to go right through then backing it out.

So I decided to have a go. Having some 3/8″ 6061 plate I sawed off a slightly over 4″ square, cleaned up the 4 edges in the mill, and using the height gauge marked out the centre and a .400″ grid across the whole surface. Over to the mill, centred the spindle over the marked centre, zeroed the DRO, then using the DRO, spotted and drilled all 61 holes 10-32 tapping size. opened up and counter bored the 4 holes for the hold down screws and reamed the centre hole 1/4″ to match the one in the rotary table. Before tapping the 56 10-32 holes I used a 3/16″ end mill and the quill stop to counter bore each hole .025″ deep to eliminate the burr thrown up by the tap.

I found the tapping, using a spiral point tap, wasn’t as onerous as I had envisioned. Using the tap wrench guided by the quill and positioned by the DRO it was just a matter of positioning the tap over the hole, a squirt of cutting fluid, 16 turns down, 16 turns back up, move to the next hole, repeat ad nauseam! It took between 1 and 1 1/2 minute per hole and I broke it up into 3 sessions as it got boring after 30 minutes. After finishing all the holes, I noticed there seemed to be 8 gaps in his original hole pattern around the edges where there was room for another hole, so I drilled and tapped 8 more holes.

It was then just a matter of hack sawing off the corners and mounting it on a sacrificial piece of wood attached to the face plate, centreing it, and turning the OD and putting a small chamfer on the edge.

So here is the final result. Whether it will be any better than the original remains to be seen as I haven’t had any opportunity to use it since making it.

John

 

 

[Michael GilliganParticipant @michaelgilligan61133]

It certainly looks the part, John

MichaelG.

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Edit: __ This might provide inspiration for your next project:

 

https://www.shars.com/mitee-bite-10202-10-32-1-2-hex-brass-symmetrical-hex-clamp-10-pc-set

[JasonBModerator @jasonb]

I tend to use the spiral flute taps 95% of the time these days.

I suppose the plate could just have been left square which would have given a bit more room for clamps etc as there is nothing for it to hit.

[Chris MateParticipant @chrismate31303]

I just completed a 200mm plate for 160 RT.

I made lots of holes  and the tapping was an experience of patience. Theres still enough spaces in between if I really wanted to use pins, however I think I will indicate determine placement of each job every time and “pin” it down for repeatability by clamping as described in (1).

1-Made slotted plates from flat steel pieces, bolt down with purpose to lift subject off table 3mm , so I don’t mill in table, and at same time they position the subject on table(Subject rest on tips). The subject can be lifted off, and placed back again in same position.
2-Made similar slotted plates to clamp subject down on to the table, then it can be milled without coming loose.
3-Made outer row of 8mm holes/apped and 2x slotted stops to work between two stopping points.
These slots are curved and slide over the outer ring of 200mm disc  and made it with RT itself.
Theres a lot of flexability in clamping, and the make more different as needed..

4-All holes covered with 8mm short grubscrews to keep swarf out of threads. If I need the hole, I just screw the grubscrew in down, the other stud or bolts used just screw on top of that, the plate is thick enough, that way no grub screw loose and go missing.

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