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Myford puzzle?

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Cornish Jack27/12/2010 14:36:35
1228 forum posts
172 photos
Well, it is to me.
There must be a great many Myfords in use and the various accessories are pretty numerous also but I have not, so far, come across a reasonably priced source of Myford nose thread dies. Even taps are not much in evidence. The only die I have thus far discovered was priced at an eye-watering £GBP156.00!!!! and that was just an hexagonal job - not the adjustable sort. I managed to get a tap at a reasonable price but no dies. Anyone know anything different or does one just rely on getting a good finish from thread cutting?
Rgds
KWIL27/12/2010 14:52:01
3681 forum posts
70 photos
As you say Taps are OK (Tracy Tools).
 
In the case of the thread, a screwcut one is quite adequate, it does not have to be perfect, its job is merely to close the Register faces, the Register radial fit and the rear flat face are the critical surfaces which must be correct. Why would you need a die in this instance? If you are worried about thread form, do not be,  merely round the crest of the thread for safety purposes.
chris stephens27/12/2010 15:03:12
1049 forum posts
1 photos
Had you thought about using a hand thread chaser to clean up your machine cut thread? Hand chasers allow you to create a "correct" thread profile and also allow you to shave off whiskers till you get the right fit. I did this recently to salvage a Norton crankshaft that someone had repeatedly hit very hard with a hammer. Worked a treat!
chriStephens 

Edited By chris stephens on 27/12/2010 15:03:47

John Stevenson27/12/2010 15:33:30
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5068 forum posts
3 photos
Jack,
The problem with trying to put a thread on with a die at this size and presumably such a short length is you are liable to get a drunken thread of poor quality given the amount of material it has to move and the effort required to use it.
 
One way to get nice threads is to use one piece of a Coventry die set as a multi tooth screwcutting tool.
 

Any 12 pitch Whit form die will do, the diameter doesn't matter, these will put the correct root and crest radii on without any grinding or experience needed.
 
John S.
Ray Lyons27/12/2010 16:32:48
200 forum posts
1 photos
Try RDG Tools. I bought both the tap and die about 2 years ago. Not cheap but well under £20. 
Tony Pratt 127/12/2010 16:55:22
2319 forum posts
13 photos
Jack,
You really don't want to be die cutting a thread this size even if one is still obtainable. The torque needed to cut would be quite substantial and quite often tearing can be a problem. A screw cutting tool isn't that hard to make and can be used on other jobs in the future, the root and crest form really doesn't have to be perfect to do the job but of course chasers will give you the correct radii
Tony
Cornish Jack27/12/2010 17:19:36
1228 forum posts
172 photos
Thank you one and all.
I wouldn't consider using a die to produce a thread of this diameter. The query was just why such a ubiquitous machine would not have a REASONABLY priced tap and die set for thread FINISHING purposes. Thread chasing is obviously the way to go, as JS et al have said. 
Season's greetings and a warm workshop to all for 2011
Rgds

NJH27/12/2010 17:32:25
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2314 forum posts
139 photos
Hi Jack
The chasers, both conventional and Coventry inserts, are available quite cheaply from Tracy Tools.  ( Not too far from Cornwall either  - although if you choose to visit I doubt that a chaser is all that you will buy!)
Regards
 
Norman 
 
 
Chris Trice27/12/2010 18:55:53
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1376 forum posts
10 photos
The register does centralise the backplate on the spindle but there is a tiny clearance otherwise you couldn't screw it on. In fact the threads conical form does play a part in centralising the chuck/backplate/accessory too, which is why you want to form the thread well.  Obviously it depends what machining tolerances you work to but technically it is possible for the parallel part of the backplate to not touch the parallel part of the register once it's screwed on tight if the thread in both is true. Similarly, a wonky thread can throw the backplate off to the side so that the register contacts it in just one place on one side even if the rear face contacts uniformally. This is why it's good to machine a backplate in situ on your specific lathe to ensure the chuck being mounted to it runs true and why a small error may be seen if the chuck is transferred to another lathe.
 
None of this applies with a tapered mount obviously as they self centralise assuming again everything is machined true.

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