Using a 33mm chuck on a 1 1/4" spindle

[sawdust maker]

Hi all

I've just got a second hand Nova DVR XP lathe. It has an 1 1/4" spindle and all my chucks are 33mm. I've sent away for some adaptors, but have noticed that the 33mm chucks will screw onto the 1 1/4" spindle. My question is:-

Is it safe to use the 33mm chucks on a 1 1/4" spindle as a short term measure, or am I in danger of damaging the threads?

 

[minilathe22]

There is a risk that the thread is only just catching, and could come off when used under power. I would not risk it unless you research the threads and are satisfied that you have enough overlap of the threaded parts (if that makes sense).

See this post for an example of another user with the same situation:

https://www.ukworkshop.co.uk/forums/newbie-mistake-t107977-15.html

 

[Retire2004]

Hi Sawdust Maker,
DON'T DO IT!!!
Never mind damaging the threads, you are more in danger of damaging your health!
Your Nova spindle nose is probably 1 1/4" x 8 TPI. ( approx. 31.75mm dia. x 3.12mm pitch) Your chucks will be M33 x 3.5mm Pitch. So there is nominally a difference of 0.25mm in register (and thread) dia. and 0.38mm in thread pitch. This means that there is only a very small area in actual contact and apart from that there will be substantial run out (wobble) when the lathe spindle is rotated.

 

[J-G]

DON'T DO IT!!!

Whilst I agree that is is bad practice, I have to correct certain assumptions.

Your Nova spindle nose is probably 1 1/4" x 8 TPI. ( approx. 31.75mm dia. x 3.12mm pitch)

While by no means a 'given', if Nova stick to the standards then 1¼" BSW or UNC is 7 tpi, (3.6285mm) and unless the thread engagement is less than 3 pitches (turns) then the chuck would bind if it were 8 tpi. At 7 tpi you could screw the chuck on 7 turns before binding and all lathe nose threads I've seen are about 5 or 6 pitches long. See the attached comparison drawing (I've assumed BSW but there would be little difference if it were UNC)

Your chucks will be M33 x 3.5mm Pitch. So there is nominally a difference of 0.25mm in register (and thread) dia. and 0.38mm in thread pitch. This means that there is only a very small area in actual contact and apart from that there will be substantial run out (wobble) when the lathe spindle is rotated.

The 'Register' will not necessarily be the same at the thread O/d - in fact it would be unlikely - so it is possible that the chuck and the lathe have the same 'register' even though it may well be a coincidence since one is Imperial and the other Metric! If they are not the same then Retire2004 is correct in the assumption that there may be 'wobble', only empirical tests would prove otherwise. As far as the 'area in contact' is concerned, 3.5mm Metric chuck on a 7tpi Whit spindle would have 81% of the contact that it would have on a 3.5mm spindle.

 

[sawdust maker]

Thanks to you all. I will just have to wait until the adaptors come until then I'll sit and look at my new pride and joy.

 

[minilathe22]

Not got a drive centre to do some spindle turning "while u wait"??

 

[Retire2004]

With regard to J-G's comments:-

I don't accept that I am the one making the assumptions! The Nova spindle is in fact 1 1/4" x 8TPI (UNC form and not Whitworth). I am aware that the standard is 7TPI, but lathe manufacturers often do this to try and prevent us from making/sourcing the mating parts ourselves.(faceplates etc.)
I think you will find that most spindle noses have the same nominal register dia. as the threads (with just a few exceptions). Most M33 chucks will have a 33mm register and most threads are in fact slightly smaller in dia. than their nominal size (when measured over the crest flat, or radius). Given that the thread is in fact 8TPI and the chuck screws all the way onto the spindle, somewhat nullifies your argument that the parts would bind if the spindle nose was 8TPI.(Maybe the thread tolerances are more sloppy than normal thus allowing this to happen!)
I am at a loss to understand your estimate of 81% contact area.
Regards
Tudor

 

[J-G]

First, it sounds as if Tudor considered my comments as a personal slight and nothing could be further from the truth so apologies.

Second, my 81% was a mis-calculation - I can see how I made the error - it should be 67% and I'll explain later.

I don't know Nova (and ought to have researched before posting) but I did say "While by no means a 'given' " to cover the eventuality. From my limited experience (and common sense thoughts) a register at the same diameter as the thread doesn't make sense. My Myford S7 has a 1-1/8 x 12 thread and a 1¼" register. Looking at http://www.lathe-chucks.com I see that all the threaded spindle sizes listed have a larger register - not by a great deal, but definitely larger.

I agree that most threads when measured over the crests are likely to be 'under-size' but that is not because the thread is under-size, but the crest has been truncated to avoid potential interference in case the 'nut' is at the lower limit of tolerance. the 'Effective' dia must always be correct and that is why threads should be measured using the 'three wire' method (or a 'Thread Micrometer'). The measured O/D is to all intents and purposes irrelevant - and actually why a register of the same diameter is possible - though, I would argue, bad practice.

I've done a new drawing - using the 8tpi Unified thread form - to better illustrate my point about binding. It also shows the thread engagement %, effective diameters and potential crest reduction and why that would not affect the binding.

Of course, I'm talking text-book sizes and not accounting for wear or poor manufacture which will certainly have a bearing on the matter but is 'intangible'. If Nova were to cut the thread with an effective diameter 20 thou undersize then you may well get a 33 x 3.5mm chuck on 4 turns. You'll see from the second illustration that there is no binding at 3 turns

 

[Retire2004]

I thank J-G for his comments. I stand by everything I have said but unfortunately disagree with much of what he has said.
"Sawdust Make"r seems to have accepted that he should wait until his adapter arrives, and not attempt to run his lathe with chuck in position.
Therefore I see no point in addressing D-J's latest comments in any further detail.
Kind regards
Tudor