Mounting chuck directly to rotary table.

[old martParticipant @oldmart]

I saw an advert from RDG on ebay for a tilting 100mm rotary table with a three jaw scroll chuck. It was reduced in price because of backlash in the worm drive, and not having the mounting plate for the chuck. The table has three slots and the HBM chuck had three rear mounting threads. I drilled through the rear mounting holes after ascertaining that they were in the optimum position for converting to front mount. I drilled 6.2mm through the 8mm threads and then used a counterbore 6mm deep from the front for SHCS to fit just underflush. Three teenuts were made with 6mm threads in them and the chuck fits a treat. The original 8mm rear mounting threads were not touched by the modification. The tee slots in the table were not big enough for 8mm threads hence changing to 6mm. As for the backlash, it is only 3.2 divisions, I probably won’t bother to correct it. Mike and I will be discussing ways of stiffening the locking of the tilt mechanism, possibly with something at the other end of the hinge. The little 4 jaw independent Crown chuck will have to have an intermediate plate made up next week and use the inner mounting holesand with the tee nuts reversed in their slots.

[HuubParticipant @huub]

As for the backlash, it is only 3.2 divisions, I probably won’t bother to correct it.

Normally, these HBM rotary tables have a grub screw to adjust the worm backlash.

[David George 1Participant @davidgeorge1]

Have you thought about direct bolting the 4 jaw chuck directly to the rotary table. I believe you can drill a four hole pattern 6mm dia into the face of the rotary table missing the T slots and with plenty of depth for  a deep thread.

David

[Martin Johnson 1Participant @martinjohnson1]

Neat, but how do you register the chuck to the table for concentricity?

Martin

[bernard towersParticipant @bernardtowers37738]

Wouldnt you use a stepped peg for that

[old martParticipant @oldmart]

Right, some answers to questions. The Mounting holes were drilled to 6.2mm to suit the counterbore tool and the 6mm SHCS are under 6mm when measured. Also the tee nuts can slide slightly in their slots so I am confident in being able to get the chuck concentric with the table. If that is not enough movement, I will increase the mount holes to 1/4″ and turn the heads of the SHCS down by 0.010″ diameter. There is not very much room for drilling the table top for direct mounting of the four jaw, but I will look into the possibility, and will remove the top for close inspection. The 6″ Soba RT that I also have has four slots and tee nuts and SCHS for direct mounting of the 6″ lightweight Toolmex four jaw independent. I did drill and tap three 8mm holes in the top to hold a front mount Pratt Burnerd 5″ three jaw scroll chuck. Not without a hitch, though. One of the first three holes I made ran into a drilling for lubrication in the top and all three were plugged and the alternative set were successfully produced. The chuck has sufficient movement before the screws are tightened to allow an MT2 test bar in the centre to hold the chuck jaws central within 0.001″ tir which is good enough for most jobs. The Soba 6″ has adjustment for backlash, but I haven’t looked hard for that on the 100mm one yet. The adjusting screws may be hidden under the paint. I am not sure what the central hole size is yet, it seems to be tapered, but is probably straight, about 20mm and a custom alignment bar could be made easily.

[old martParticipant @oldmart]

A couple of problems which are being addressed. The first concerns the hole in the middle of the table. I removed the base fitting which can be adjusted for table end float and removed the top. When this was done, a MT2 dead center fits perfectly. The base fitting hole was slightly too small and obstructed fitting of any MT2 and only 0.020″, 0.5mm increase in the bore was needed to correct the problem. The second fault has been slightly more challenging. I made two 10mm countersunk SHCS fit the two low profile holes in the base and loosely fitted the RT to the Tom Senior with two shop made tee nuts. I opened the top to 90 degrees and attempted to run a clock across using the Y axis. I failed to get enough movement to square it up. The fixing holes were not drilled square to the hinge, the error measures 1.8mm, too much for the tee nuts to line up. I removed the hinge pin and put the top half aside and used the pin to set the base true and milled across the end next to the hinge to give a starting point for correcting the mounting holes. To minimise the machining of the holes, I will move both in opposite directions 0.9mm. The holes will be clocked and moved including the countersink and counterbore. Gary turned up an offcut of SG cast iron to mount the 4″ Crown 4 jaw, I won’t drill and tap the table top, and even with the adaptor,the Crown will be about an inch lower than the 100mm HBM 3 jaw.

[old martParticipant @oldmart]

I found the eccentric worm stop grub screw and have reduced the backlash to one division. The base mounting screwholes have been corrected as per last photo and the axis can be set inline with the Xaxis easily. I had a spare MT2 to MT1 socket with a cylindrical section to get the three jaw scroll chuck aligned when fitting it to the top. The end 25mm was cut off the MT2, even with the Morse taper fitting properly the base is not thick enough to accept any uncut one. Which brings me to the hole in the base, it is not in line with the hole in the top. The centre of the hole should be 5mm to the left and 23mm further away from the hinge. I probably won’t bother to correct it as the 6″ Soba can sit with its axis horizontal, I once used it with a tailstock support for holding the spindle of the Smart & Brown model A lathe.

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