I have owned an ancient "Grayson" lathe for the last few years, and though rather tired and worn, it's still a very useful machine for
less critical jobs. However, it came only with a slightly oversized 3 jaw chuck which has been attached to the original backplate. Unfortunately the
backplate is way too small to register properly with the larger chuck, so the chuck is being "centered" (and I use the term loosely) using
only the mounting screws.
In a bid to fix this, I discovered from lathes.co.uk that this machine uses the standard
"Myford" spindle thread of 1 /18" x 12TPI, and some measurements with a vernier seemed to confirm this so I ordered a 4" Myford
backplate that I was planning to turn down so I could accurately mount the chuck.
Unfortunately it just doesn't fit, I can get about 3/4 turn on it before the thread locks up solid. I've measured the threads in the new
backplate and as best I can measure they are indeed 12TPI and 1 1/8", and I have re-measured the spindle which also seems correct. The only
thing I can think of is that the backplate has a Whitworth tooth form and the spindle has UNF - is this likely? If so, does anyone know where I can
get a suitable back plate?
[Edited on 13/9/11 by MikeRJ]
Mike, I'd say it's very likely that you have pinpointed the problem. Do you have 60 degree and 55 degree thread gauges?...that would confirm
it for sure.
The other possibility is the depth of the thread in one part or the other...do you have thread wires to measure the pitch diameter?
Alan
I bought one of those lathes back in the 70's
As it only had a 4 jaw, I bought a 3 jaw with a myford back plate, it fitted straight on no problems.
But a quick google found this article
LINK
Martin
cheers for that link mike, its the first time I've ever found anything about my lathe at all 
it seems to be a slightly different version of this: http://www.lathes.co.uk/hembrug/ same castings, different motor, controls etc...
(well I say mine its really my dads, but I use it more 
)
I'm also in need of a new chuck and maybe even new bearings though 
[Edited on 14/9/2011 by mcerd1]
I would just set it up true in a 4 jaw chuck and take a skim down the thread.It does not matter to much if the thread is slightly loose as the
location should be on the spigot.
Paul
quote:
Originally posted by Alan B
Mike, I'd say it's very likely that you have pinpointed the problem. Do you have 60 degree and 55 degree thread gauges?...that would confirm it for sure.
The other possibility is the depth of the thread in one part or the other...do you have thread wires to measure the pitch diameter?
Alan
quote:
Originally posted by paulf
I would just set it up true in a 4 jaw chuck and take a skim down the thread.It does not matter to much if the thread is slightly loose as the location should be on the spigot.
Paul
quote:
Originally posted by rf900rush
I bought one of those lathes back in the 70's
As it only had a 4 jaw, I bought a 3 jaw with a myford back plate, it fitted straight on no problems.
But a quick google found this article
LINK
Martin
You would really need to set up the lead screw to the correct pitch if the thread needs machining, it is also possible that the bore may be under size
so although correct thread form and pitch it wont fit.I would try some engineers blue on the spindle and see where it transfers to on the back
plate.
The other alternative is to enlarge the mounting screws in your existing back plate and then it may be possible to tap the chuck true with a set up
piece gripped in it before fully tightening the mounting screws it is then accurate for work around the same dia as set up with .
I have a few different chucks for my lathe and have one like this as it allows the chuck to be used for accurate light work that needs to be removed
and replaced or repetitive work with out having to set up dead true in a 4 jaw chuck each time.
quote:
Originally posted by MikeRJ
quote:
Originally posted by Alan B
Mike, I'd say it's very likely that you have pinpointed the problem. Do you have 60 degree and 55 degree thread gauges?...that would confirm it for sure.
The other possibility is the depth of the thread in one part or the other...do you have thread wires to measure the pitch diameter?
Alan
No thread gauges or thread wires unfortunately. Both are tools I've been meaning to buy for a while, so now seems a good opportunity!
quote:
Originally posted by paulf
I would just set it up true in a 4 jaw chuck and take a skim down the thread.It does not matter to much if the thread is slightly loose as the location should be on the spigot.
Paul
I'm a bit lacking in extra lathes, but I had similar thoughts! Assuming this is the problem I was thinking about clamping the new backplate to my faceplate, centering it up with a DTi and then attempting to modify the thread angle to 60 degrees. There isn't much to lose really, the backplates are pretty cheap.
If I did this, would I absolutely have to set up the lathe for screw cutting, or would it possible to just engage the tool in the thread, disengage the leadscrew and allow the thread to pull the tool/carriage in?