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chuck mount for 4 in. rotary table

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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  :scratch: 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!!  :clap:
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. 

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