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Tos metric threading dial

jrolands

Aluminum
Joined
Nov 6, 2009
Location
Norway
Hey guys

I'm looking for a manual for the threading dial found on TOS lathes (metric)
Can't seem to find much info at all regarding the use of metric threading dials.
 

WILLEO6709

Diamond
Joined
Nov 6, 2001
Location
WAPELLO, IA USA
you have to determine if you have a sae or metric leadscrew. If Omnitrade Industrial Imported it I think its darn likely to be sae. I know the sn45 I had had an sae screw.

If I recall correctly the stuff was in the change gears/ quick change box to thread metric pretty easy on the machine I had, but sold it off years ago. Replaced with an 8000 series Clausing Colchester where I just throw some levers for metric to inch.
 

jrolands

Aluminum
Joined
Nov 6, 2009
Location
Norway
The threading dial on this lathe, and all other TOS lathes I've seen for that matter, has an 4 or 5 division plates with different divisions on them.
 

jrolands

Aluminum
Joined
Nov 6, 2009
Location
Norway
Here is a scan from one of my books... It is for a 6mm leadscrew.

Is there more in that book regarding threading dials? I would very much like to read the rest as it is rather uncommon here in Norway. I haven't come across a single person that has ever used one, nor any reference to them in any literature here. All lathe threading here is done at low speed and reversing the bench.. :nutter:
 

Clive603

Titanium
Joined
Aug 2, 2008
Location
Sussex, England
So far as I'm aware there is no good description in the literature concerning the how's and whys of metric threading dial operation. Due to metric threads being rationalised by pitch, rather than turns per unit, and the associated lack of common factors the whole idea seems to be considered something of a mechanical malpractice. Most descriptions cover the simpler, inch, case and tack on the metric version as a confusing aftertaste. Lets take a run at it anyway and start at the beginning.

If the lead screw be stationary running the carriage back and forth lets it act as a rack driving the dial via its gear. For any given lead screw pitch and number of teeth on drive gear the carriage will move a specific distance. In principle the dial can be calibrated in terms of distance moved per turn or fraction there-of. Although not explicitly stated this is what is done in the imperial case. This idea is generally considered impractical in the usual metric set up which uses one dial and several gears to cover the various pitches.

If the carriage be stationary and the lead screw turning the screw acts as a worm driving the dial round via its gear so the dial graduations acts as lead screw (and spindle) turns counters. Its the mutual integration of carriage movement and screw/spindle rotation indicated by the dial that lets you select the right place to close the half nut so that successive threading cuts are in sync.

The normal metric set-up of interchangeable dial drive gears is a complete pain to explain but there is a system on some Harrison lathes which uses single dial and gear to cover most metric pitches. This set-up uses a 6 mm pitch screw with dial carrying 20 divisions of which four are marked with longer lines and numbered 30, 60, 90, 120. The gearing is such that the dial makes one revolution for every 20 turns of the lead screw when the carriage is stationary. Clearly the dial is actually graduated directly in mm of carriage movement viz. 120 mm per full turn and 6 mm, one lead screw pitch, per sub division.

Where the pitch of the thread being cut is an exact whole number sub multiple of the lead screw pitch the nut may be engaged on any line at random without upsetting thread synchronisation. This works for 0.5, 0.75, 1, 1.5, 2, 3 and 6 mm pitch.

For 1.25, 2.5, 5 and 10 mm pitch the half nuts can be engaged on any numbered line. In this case the pitches are exact whole number sub multiples of 5 lead screw pitches i.e. 30 mm.

For 4 mm pitch the half nuts can be engaged at either of a diametrically opposite pair of lines i.e. 60 - 120, 30 - 90 (works for unnumbered sub divisions to but .....). This works because 4 mm is an exact sub multiple of 10 lead screw pitches i.e. 20 mm.

I've never really understood why this system isn't more common as it seems to cover a useful range of pitches in a simple manner.

Hopefully that will help you figure out what's actually going on with your lathe. The essential point is that the engagement points need to be an exact factor of the lead screw pitch multiplied by any suitable whole number.

Clive
 

jrolands

Aluminum
Joined
Nov 6, 2009
Location
Norway
Wow Clive.. You really know your stuff.. After reading your post numerous times I think I got a grasp on it. Ill try to experiment a little with the dial when i get back to work and see if I can't post some pictures at the same time.
Thanks loads :D
 

daredo222

Cast Iron
Joined
Mar 8, 2007
Location
Norwich U.K. & Marvao, Portugal
1.75 pitch

Clive603,
That is a very eloquent description of the metric threading conondrum & a pleasure to read. Just one addition, to cut a 1.75mm pitch thread with a 6m pitch leadscrew requires a 42 tooth gear on the threading dial with one index line on the indicator. Such a gear would be about 160mm dia., completely impractical but it would also enable the cutting of .7mm pitch, the M4 standard. Easy to see from this why the reversing method is so frequently used.

Ray
 

Clive603

Titanium
Joined
Aug 2, 2008
Location
Sussex, England
As to "knowing stuff" (flattery will get you anywhere) I'm not gonna admit how long it took me to cotton onto things after it was first explained to me via the "sychronous distance" concept! Don't ask what sychronous distance is, just don't. The maths is nice but the practical is hell!

On further reflection I think a fairly simple way of drawing things out would make things very clear. Need separate drawings for each dial drive gear if you have more than one. Best done with a CAD program but perfectly practical, albeit tedious, by pencil and paper using a drawing board or graph paper.

Start by figuring out what the carriage movement distance against a stationary lead screw for one complete turn of the dial is (number of teeth on drive gear x screw pitch) and lay the divisions out as a distance scale. Horizontal or vertical to taste and best done on a 1 mm grid background. Extend the lines across the screen or paper, do the lines corresponding to the numbered divisions heavier or use dashed lines for the intermediate divisions. Draw the different thread pitches as a string of appropriately spaced Vees or heavy dashes running across the extended lines and parallel to the original scale. Put the centre of the first V or the first dash aligned with the first line on the scale you made. Obviously offset the strings of Vees or dashes enough so you can see which is which for different thread pitches.

A Vee or dash falling on a dial division line indicates a satisfactory half nut engagement position for the thread in question.

Don't forget that the last line on your scale is actually the same position as the first line, 'cos the dial is round. Its quite possible to have a thread pitch which needs more than one turn of the dial to come back into alignment. This sort of thing is quite easily missed when using the straight line development of circular systems.

There is another little exercise which can help if you don't quite follow how the dial integrates lead screw / spindle rotation and carriage movement to keep successive cuts in sync. Mount something with a nice coarse thread in the chuck, an old feed screw is good if its within the machine threading range, and set the lathe up to cut the same pitch thread. Put a suitable tool or indicator pointer in the tool post. Fiddle things around so the pointer or tool aligns on a thread groove with the half nuts engaged and the thread dial on its first line. Drop the half nuts and pull the spindle round by hand a half turn or so so the pointer or tool is clearly mis-aligned relative to the thread. Verify that the half nuts won't engage and that the first line on the dial is no longer aligned with the reference mark. Crank the carriage along until the pointer or tool once more aligns with the middle of the thread. The dial line will once more align with the reference mark and the half nuts can be engaged. The lathe has calculated how far you need to move the carriage to bring the tool back into alignment when the spindle was rotated independently. Obviously the calculation works t'other way when you are threading with the carriage stationary, the half nuts dropped and the spindle turning waiting for the dial to come up to the mark.

If you haven't got a suitable sample of thread wrap an adequate size bar with paper and use the lathe to mark a decently coarse "thread" on it with a pencil or pen as the tool. Obviously one "cut" only.

Clive
 








 
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