Twin start threads

Did you make sure that the backlash was properly taken out of the system when switching things round. Seems to me that with such a fine thread it might be possible to have enough backlash in the gears to let you effectively reconnect one tooth out which would offset the second thread enough to leave a flat between the two starts.

Some years since I did one so no longer fully up to speed without the machine in front of me. usually my hands remember better than my brain for this sort of thing.

Think I'd go back to the pen with a bigger simulated workpiece and set a coarser thread then try the effect of deliberate errors. Like what a happens if I set up one tooth wrong. Maybe wrap a piece of paper round the inner cardboard tube left when the kitchen roll is used up for a simulated workpiece.

Clive.

Depends on the gear train.

Although the overall ration is 1:1 for 8tpi thread with an 8tpi leadscrew. The first driver and driven gear are unlikely to be the same.

I have a sort of photographic memory for practical tasks, so once I have done something once I can recall it years later, but yes I normally do have to have the objects in-front of me.

I can see this being a long process this one

I really should have posted both links. This link takes you to video number 2. If anyone is interested in the theoretical basics demonstrated in video number 1, here is the link: http://www.youtube.com/watch?v=sbcH4Jg0HIs

Bill

Just to muddy the waters, if you want an 8 TPI 2 start thread, use either a 16 TPI hand thread chaser, or a Coventry Die Head chaser, grind the back teeth away until just the front two leading teeth remain, securely set the chaser square to the headstock, set the lathe to cut an 8 TPI thread and cut both starts at the same time.

Hi Allan,

I think that the problem is that the parent thread you chose is 8TPI - the same as your leadscrew and so the half-nuts will only engage in-phase with the work/leadscrew. The irony is that if you'd have picked any other even thread you'd have been fine with the threading dial method!

I would still advocate that the setting of the topslide in-line with the bed (facing the chuck) and doing things as I described above is easier - especially if you use the topslide starting position as 0 for first cut of the first thread and 0.063 as the first cut for the second thread - having taken up all of the backlash in the same direction towards the chuck. Remember to put on the cuts with two dial changes, an in-feed on the cross-slide and half that value on the topslide.

HTH

Jon

Edited By Jon Gibbs on 21/03/2017 09:26:10

Jon Gibbs has surely the simplest solution, but even if your top slide is set round at half the included angle of the thread it is simple trigonometry to calculate how far the top slide needs to advance to get a .0625" horizontal movement towards the chuck.

Tony

Edited By Tony Pratt 1 on 21/03/2017 10:30:30

Hi Allan,

Glad to see that you are having fun with your twin thread ! but I believe the REAL fun starts when you get to cut the internal thread and it gets even better when you try to get them to fit each other. This is a minefield into which I have not ventured yet. I too had a go at cutting two start threads and got some interesting results,not sure what they might be useful for,but they looked-------------different !

It seems like we both opened a can of worms,you with your thread,me with my table,but your problem should be easier to sort out.

Just reading the reply re my table from "Silly old duffer" [you are far from being silly or a duffer ! ] you are lucky indeed with your table. I would treat it with lots of TLC.

One other thing while I think of it,the mod I made to my cheap bandsaw by fitting it with a coolant system,could also be fitted to a lathe,it,s the same idea and would be a lot cheaper than buying a unit. Most of the bits came from my scrap box [which gets bigger every day ] and old washing machine pumps are everywhere.

thank you all for your replies, I have not had time to have another play today (parenting duties to be done) and I wont get chance to have another play now until Thursday, but I will update you all on my progress.

Larry, yes the next job is trying to make some form of internal thread to match this one which is proving such a issue in the first place lol

I did think about building a coolant pump for my machines, but then I found one at Axminster tools at about the right money, and as my milling machine and lathe are next to each other I just needed the one pump to feed both.

Allan

Hi Allen,

We do seem to stir things up,dont we ? Early on this afternoon,nothing better to do,I set up to cut an 8tpi [the fact is I was already set up for that anyway ] So,using a piece of 12mm bar,I cut one very light thread and then using Sparley,s method,I advanced the top slide by 1/16",1.578mm,if I remember correctly [old age is a curse ! ] and put on a second cut,just to check the spacing. Although I did not cut a full dept thread,the spacing looked fairly good,so I think this might be the way to go.Later,when I feel more brave,I will try the inside thread,that,s when things should get interesting. In the meantime,this system does appear to be fairly simple and is well worth a try.

I do have a coolant pump on my lathe which works very well,but what about the milling machine?. Does the stuff not go all over the place? what,s to stop it since there,s no tray?. I just apply it with a brush.

Hi again Allan,

Just like to say I love your workshop,lovely lathe !

Hi Allan,

A friend mine has a Bridgeport mill with a huge tray all around it,seems to take up half the shop,but it does catch all the spray. My mill is a Lux [ex Chester,but quite good for all that ]. I just mounted it on a box tubing stand,never thinking about the coolant problem,but now,years down the line,I,m thinking about making a tray for it. What puts me off is the thought of having to clear the junk from around it in order to lift it from its stand.Hindsight is a wonderful thing.

My workshop/garage/fuelstore/junkstore is 800 sq ft inside,9ft headroom,with an RSJ fitted across it [40 x20 ] It holds everything,sometimes the car even gets in,but not often.The junk area is slowly but surely taking over. Any ides how to deal with this? I mean,how can you throw out something which "might come in handy some time" . Does it ever? and if it does,can you ever find it? By the time you dig it out,you could have a new piece cut and the job finished. I,ve been there,done that,too many times,but still------it just might come in handy some day.

One other thing I forgot to check about the two start thread,do you cut the thread to full dept or only to half dept?

Seems like I came across something about that before. Must check it out again.

No it's still 3mm pitch or there won't be any room for the other thread but you do cut the 3mm pitch thread to the same depth as a 1.5

Clear ?

Forget the gearbox is set to 12 you are now working in mm and not imperial. Once it's set then forget it 12tpi and 6 tpi do not come into your calculations.

Edited By John Stevenson on 22/03/2017 13:12:17

I think we are both a little clearer now,I seem to remember about the thread dept from way back.

I just set up for an 8tpi,just to check the spacing and it looked pretty good but I can see why you would need to go only half the dept.

Looks like I am going to have try this,and I was just so nice and snug !

Does anyone know where I can buy gear cutters 1.25 mod?

I HAVE DONE IT

I took the majority advice and used the cross slide to advance across for the second thread, and just took smaller cuts and it seemed to come out ok.

19.5mm 1.5mm pitch

but after doing that, I needed a nut to go on it, so made a blank up out of some 25mm bar

and it works

thanks to John for the new gears so it is a true (ish) metric thread

and a big thank you to all of you for your help

Next job, 4 start threads, just for the hell of it.

Allan

You are just a show off I HATE YOU,I HATE YOU ,I HATE YOUUUUUUUU !

Good man,you got there before I did.YoU ARE NOW TALKING ABOUT A 4 START THREAD ? Is there no end to the man?

All I can say is,"Good for you ",I must now go and eat humble pie.