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Backplate debacle

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old mart24/08/2019 21:19:04
442 forum posts
42 photos

I am just in the process of fitting a new 160mm three jaw chuck to a backplate which had another 160mm on it. I had found that the backplate was undersize for the original chuck, which had a 142mm pcd for the 8mm bolts. I had drilled and tapped for six 6mm bolts at the same pcd, giving me a little more meat on the od of the plate.

The new chuck, by pure luck has six 10mm threaded holes at a pcd of 140mm. I have a supply of new 8/10mm titanium studs originally fitted to Westland Lynx helicopters to use for mounting the chuck. Using the rotary table on the mill, I plunged a solid carbide 8mm end cutting four flute end mill through the original 6mm holes in the backplate at the new pcd of 140mm. The register had to be reduced and now is 0.010" smaller than the chuck register. I will use 8mm nylock nuts, and a little low strength thread lock on the stud-chuck threads. I prefer six bolts to the more common three when mounting scroll chucks.

The second chuck was cheap on ebay, and has both sets of jaws, which the original did not. The jaws are not interchangeable between bodies, the new one has slightly larger jaws. It is either Chinese or Indian, the only marks on it are 3000 rpm, the serial number and 160.

Neil Wyatt24/08/2019 22:23:39
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16432 forum posts
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Two schools of thought.

Leave it undersize - people with less than perfect chucks often do this on purpose.

Just make the backplate thinner and try again - most have plenty of 'meat', yet only need enough thickness for teh fixings to be secure.

I've got one chuck on a backplate thick enough that I could have had four or five tries if I hadn't got it right first time, it's over 10mm thick!

Neil

Martin Hamilton 124/08/2019 23:30:36
130 forum posts

On my Sherline lathe i mounted an Emco Compact 8 er25 collet chuck, i mounted it onto the Sherline face plate with no register at all between the face plate & the ER25 chuck. I simply faced the face plate off & mounted the chuck so as i could tap the chuck into true, this worked very well for what i wanted. Depending on what collet i have in the chuck run out is .0002" - .0003", as you go up in collet sizes its around .0003" - .0005". I could spend more time tapping it truer but you end up chasing your own tail + i am happy with the run out i have got, if i was mounting a chuck on a larger lathe i would def use the same method where the register would not be a tight fit in the chuck recess so as to be able to tap the chuck true.

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