Revisiting the buttress thread.

[NeoTech]

So i my uncle pointed out to me that a 1.666 thread in mm equals to 1 and 2/3 mm. And converted to turns that would be 5/3 turns as a ratio.

When i threaded the buttress i made a hilarious amounts of calculations to arrive at the correct gearing.

How would you convert a turn ratio like 5/3 to a gearing setup. It would most likely be more precise than what i arrived at.

// Andreas

[chipenter]

Multiply both numbers by the same amount ie 5\3 X 10= 50 \30 until you have numbers that match your change gears .

[NeoTech]

In another thread i calculated the pitch of 1.666666 as a rounded fraction to 1.667 (i was shown). By setting my lathe gearing to a 3mm pitch i then did 3 / 1.667 = 1.8 and then went (1/1.8 * 10) * 3. Wich ended up as a 30/120/54 gearing on my lathe.. that worked.. nothing about that.. BUT.. if i do the math as ratios i end up with a 30/120/50 gearing instead.. will this be more or less accurate than my initial calculations?

Why im asking cause my ER32 collet i made threads into the drawbars but dont seat really well.. i think it has something todo with fraction "off" in the threading.

[vtsteam]

would it be 40/120/24?
or 35/105/21
or 30/90/18
or 25/75/15

[vtsteam]

45/135/27
50/150/30


5x/15x/3x?

[vtsteam]

Neotech -- apologies for the above if I don't understand the question -- are you essentially trying to get a step up ratio of 1.6666666.... with a train of 3 gears (or ratios)?

[NeoTech]

Vtsteam you are correct i try to get the accurate gearing for the 1.666 butress thread used on my aciera mill.

[RussellT]

Hi Andreas

I've read this a couple of times to get my head around what you're trying to do.

As I understand it your lathe has a gearbox but 1.66667 mm is not one of the pitches provided so you're doing it with changewheels as well.

You've set your gearbox to 3mm and your gears to 30/120/54.

If I understand this correctly - the middle gear can be any size you like as it is an idler - it only needs to be there because the other two gears won't touch each other.  30 and 54 give the correct ratio to convert 3 to 1.666667 (3*30/54 = 1.6666667).

This should work OK as long as the normal gear setup for using the gearbox is a 1:1 ratio (ignoring the middle gear).

If you used 30/120/50 you would get 3*30/50 = 1.8

Russell

[NeoTech]

Ah well i think i was fuzzy with my goals here.. I made the 30/120/54 gear change and threaded a ER32 collet that way.. Yes it works, but its a sloppy thread. Its accurate enought to take thread and putting force on it works..

But... (isnt there always a but).. when putting the same collet in the horizontal feed with different drawbar the slop becomes more apparent.. And i thought to myself.. well i could prob. redo this. But should i recalculate the thread to come closer to the ratio that was originally used. That 5/3 ratio.

But you kinda pointed out that my math was way off there.. =)
Converting the thread to a ratio by doing some excessive simplification doesnt really work out.. the 1.666mm thread, that can be read as a 1 and 2/3 of a milimeter, is a 5/3 ratio. But cant be calculated as i tried to.

And now when this is cleared up i need to revisit this damn buttress in a other fashion i guess to get the thread tighter and well better. =)
I do must say i get a better and better understanding of how to cut and calculate .. annoying thread pitches. ;)

[RussellT]

If you prefer to look at it another way you are trying to cut a thread with a pitch of 5/3mm which is your 5:3 ratio.

Your gearbox is set to 3mm so you need  to add in a 1:3 ratio as you are measuring in single millimetres.

That gives 5:3 *1:3 = 5:9.  Multiply by 6 and you have 30:54.

If you set your gearbox to 1mm the calculation would be simplified. You would need 5 turns of the gearbox gear for 3 turns of the spindle gear.  5:3 *10 =50:30.  Note that it's 50 on the spindle and 30 on the gearbox as if the gearbox is set to 1mm it has to turn faster than the spindle.

Russell

[NeoTech]

Aahhh there was the missing piece of information.. howto take the ratio in the box into account.. sweet.. ok. well then im spot on i guess..   =)

Need to make me a grinding rest for toolbits and get back to get the right somehow. =)

[Anzaniste]

Sloppiness in thread engagement is more to do with  clearance, isn't it?
Surely variation between thread pitches on male and female components  is bound to lead to thread tightening as more threads engage gradually taking up the clearance.

[NeoTech]

ah, well sure, my slop is not side to side though its "linear" in and out.  But well most often when threading i found that slop side to side is a clearance issue.

[steamer]

 Hey
I did a write up on doing exactly this a while back.  Check out modelenginemaker.com under quick machine mods if i recall.   I cut new threads for a W20 draw bar.

Davef

[steamer]

Nope
Look under metric transpostion gears

http://www.modelenginemaker.com/index.php/topic,649.0.html

Dave

[steamer]

With a quick change gear box and a 47/37 gear cluster,  ( 1.27027) , set the lathe to cut 12 threads/inch

12 x 1.27027 = 15.2432 threads/inch = .065602 pitchX 25.4 mm/inch= 1.6663mm pitch........we can talk about the missing parts of a micron....but..

Dave

[Pete.]

Andreas, silly question but - you know - it has to be asked.

You have got your two different angles on the correct side of the thread haven't you?

[NeoTech]

Steamer; thats alot closer than i manage to calc it. I do have a metric lathe bit somewhere i there i should be able to figure it.

Regarding angles. Yes they are correct i learned that after the first attempt. So made a small block with the angles for holding the bit and a fence to square against.

[NeoTech]

With a quick change gear box and a 47/37 gear cluster,  ( 1.27027) , set the lathe to cut 12 threads/inch

12 x 1.27027 = 15.2432 threads/inch = .065602 pitchX 25.4 mm/inch= 1.6663mm pitch........we can talk about the missing parts of a micron....but..

Dave

I gotta ask a stupid question.. i sat down and tried todo the math on using a gear cluster, getting closer to my 1.666 desired pitch.. Im on a metric lathe with gearbox though.
When setting the machine to a 3mm pitch i have a 60/120/60 gearset in the machine originally. I have a hard time to calculate the gearset that are in the machine already.. adding a cluster didnt really help me wrap my brain around this.. Sometime i really regret to be stupid. ;D

[steamer]

You're not stupid.

My lathe is english...and I'm sorry I missed that part earlier. 

However,

If your lathe is set to turn 3mm pitch, you need to add a cluster that will give you 1.6667...or nearly.

So...3 X = 1 2/3 or   3 X = 5/3.     We need to solve for X

Divide both sides by 3 we get    X = 5/ 3(3) = 5/9

So we need a cluster that has a ratio of 5/9   or  50 /90  or 25/45  or  20/36  or 15/27

In my case, I needed to make a wider spindle gear also ( same number of teeth...just wider) to put my cluster on.  You may be able.....depending on your lathe.....to space the spindle gear out to do the same thing.

Do you have any gears that are at least close to the ratio's above?

What other pitches can you cut?....If you can cut 5mm and put a 3/1 cluster or set of gears...you get there too.

Dave 

[Pete.]

I think your best bet is to list what change gears you have.

[NeoTech]

Well Pete, that would be one way.. But what would i learn by doing that? Its a little bit when i teach people using computers or programming. I provide the tools, the problem and a approach to solve the problem.. Not the solution.  =)

Steamer so let me see if i get this right..
In my initial post i talked about the 1 2/3 ratio that occurs with a 1.666 pitch. And how i could recalculate it to use a weird gear setup.
What i really missed out that by doing that i shouldnt touch the initial 1:1 gear that is in there (60/120/60) but add a 4th gear group to the equation instead and calculate that gear cluster instead. It make a bit more sense actually. Now i will just need to code a phone app that does the gear calculations for me.. cuz im lazy.. ;)

[steamer]

Well....that's the jist of it.   There are practical issues also.  Physically getting a cluster in might be one of them...and I don't know your lathe.

I'm assuming it has a 3mm pitch leadscrew.  What pitch is it?

It might be just that simple though....you should also look at compounding one of the gears in the train if you can,   If you have a change gear lathe, you may be limited on how many you can add to the "banjo".

My turn for the dumb question....do you have a manual for the lathe,,,and does it have a chart of threads it will cut?....you might be suprised to find some odd balls.....maybe this one is one of them?

Dave

[steamer]

Or to put it Petes way....what change gears do you have?

 

Dave

[NeoTech]

I have this lathe; http://www.optimum-maschinen.de/produkte/drehmaschinen/d-320/index.html  (yeap chinese crap clone slapped with german tag on it, holy crap this is a crappy machine).

And the large idler gear (120) is compounded with another gear. And the gear on the screw sits with a small bushing that can be fitted so it goes on the outside or the inside of the large compounded gear. I think this is mainly so i can switch between metric and imperial easier.

And i actually have no idea of what pitch my screw has. My lathe has a norton gearbox so i have been using that for calculations.