screw cutting

[minerva]

Hi guys,
I've had my metric Mini Lathe for almost 5 years now and having made the decision from the out-set to "go metric"
single point screw cutting has presented few problems until now that is!
I am currently fabricating an Ornamental turning Engine (lathe) and I need to attach my wood-turning chuck to the main spindle. The problem the thread is 3/4 x 16tpi which sadly doesn't figure on any of the change wheel data attached to the machine.
I have done a search through the archives and found this quote from Bogs which suggest that my machine my well be capable of carrying out the task if I can only figure out the change wheels involved

[ftp]July 07, 2010,

post by Bogstandard

Most lathes usually have a means of cutting both types of thread, and it is usually a gear change and technique change that is required to be able to cut them.
Normally to cut Imperial on a metric machine, you leave the half nuts engaged permanently and reverse the lathe to get back to the start of the cut rather than disconnecting the half nuts and using the drop in dial.

I would suggest you have a good look at your threading chart that comes with your lathe.

any info would be appreciated
T.I.A

Terry T

 

[kvom]

Perhaps you can find a 3/4-16 bolt somewhere local; cut the head off and go from there.

Otherwise a 16tpi thread is a 1.5875 metric pitch vs. a standard 1.575 metric pitch, which is 99.2% accurate and may work/

 

[Omnimill]

Exactly what make and model Lathe have you got Terry?

Vic.

 

[Tin Falcon]

try this

Calculator
you will likely need a 21 tooth translator gear to get you close.
The other option is convert with a sae lead screw and half nut and thread dial..

A-55 B-20 C-80 D-20 will get you close 3.5% error put a 21 in instead of one of the 20s and cut the error in half.
Tin

 

[picclock]

Hi Jasonb

You don't state the pitch of the leadscrew, but if its metric, 1.5mm, then the following should work.

55,60,75,65 or 50,40,55,65. These will get you to 0.06% pitch tolerance.

I have a pitch calculator written by Paul Bussieres which I think I got from the yahoo 7x minilathe group. I would post a direct link but Yahoo seems to want to stop me (member of group around 4 years, now I can't even access the site due to there change of policy - sigh)

The program is nifty, in that it allows you to enter the gears you have and will find the best ratio with those available. I Think the gears are M1 so you can purchase individual (metal) ones from technobots if needed.

Best Regards

picclock

However the program is free and I can you a copy if you PM me. I don't think this forum has a section for files like this.

 

[minerva]

Well what service :bow:
posted the problem at eleven last night, went to bed and on waking all is revealed!!!

Kvom,
I must say I considered this option but decided that I would attempt to do it from a blank as the very nature of even the most basic OT operations requires fairly accurate concentricity of the chuck

Vic,
the lathe is a Real Bull CJ0618A (which is a basic 7X12) I have over the years carried out most of the mods advocated by the Mini lathe fraternity and I must say it has proved to be a great workhorse, it is certainly as accurate as it needs to be for my requirements although it must be said I tend to stretch the "envelope" so to speak. and for this reason only I intend to upgrade to a larger machine in the very near future.

Tin,
thanks for the link, the basic info given will, If I can purchase locally the extra change wheels will do the job I feel sure. This episode has highlighted a big gap in my knowledge of screw-cutting in general which I intend to pursue as soon as time permits.

picclock,
in common with most Metric Minis you are right in assuming a 1.5mm pitch lead screw, I have just phoned Arc Euro who have the extra gears I need so I can move on. I will send you a PM with regard to the software, many thanks.

 

[Omnimill]

Glad you're sorted Terry. Arceuro are very good aren't they! I had a quick look at the Real Bull website and it does say you can cut 12 - 52 TPI on your Lathe.

Vic.

 

[Tin Falcon]

Tin,
thanks for the link, the basic info given will, If I can purchase locally the extra change wheels will do the job I feel sure. This episode has highlighted a big gap in my knowledge of screw-cutting in general which I intend to pursue as soon as time permits.

glad to help. you are quite welcome. You may want to read this http://www.machsupport.com/docs/Mach3_Threading.pdf it is written for cnc lathe threading so you may want to skip over some info but lots of good info and imho not overly technical.
Should be a good read for you. A place to start your pursuet of knowledge for threading.
tin

 

[picclock]

Found a link to the gear selection program on Varmint Al's site

http://varmintal.net/alath.htm

Its about halfway down the page, along with a DOS version.

Best Regards

picclock

 

[minerva]

Thanks once again guy, advice much appreciated. should get the reqd. change wheels today and I'll let you know how I get on.

Tin,
the link looks interesting. I will look further when I have some spare time!

Kindest regards
Terry

 

[kustomkb]

Hi Terry,

If you are building a rose engine, be sure to see this fellows site. He has drawings available. I built his cutter head for my Father.

http://www.ornamentalroseengine.com/rre/index1.php

 

[Ken I]

Imperial / Metric conversion normally uses a 127 Tooth gear somewhare in the geartrain.

There is usually a 127T gear somewhere in the gear set.

ie 127 being the nearest to 254 (25.4mm = 1")

So if you change your drive by 127:120 then 1.5mm pich becomes 1/16"

ie 1.5 x 127 / 120 = 1.5875mm (1/16")

Regards,
Ken

 

[Tin Falcon]

I do not think one could fit a 127 in the mini at least not and have room for other things.
Tin

 

[Dan Rowe]

Tin,
I have the hand crank threading gears for a Sherline lathe and they used a much finner pitch for the metric conversion gears which include a 127 gear.

Dan

 

[mklotz]

47/37 = 1.27027027... (0.02 % error)

and takes up a lot less room.

If you're looking for gearing lash-ups to approximate a pre-determined ratio, you might want to take a look at my GEARFIND and GEARATIO programs. That's exactly what they were written to do.

 

[raggle]

My Exe 2-1/2" lathe has a 12tpi leadscrew and a 18 tooth gear permanently on the spindle. Simple gearing for a given TPI requires a leadscrew gear 3/2 times the desired thread

e.g. - required 16 tpi - 16x3/2 = 24t on leadscrew

The standard gears "rise by threes" - 18, 21, 24, 27, 30, 33 and 36 teeth in my set.

1mm pitch = 25.4 tpi, so (25.4x3/2) = 38.1 teeth

one day I WILL find such a gear but in the meantime a 38t one will have to do, although it gives a .0026 error.

Other metric pitches can be achieved by compounding the train.

I realise this is imperial to metric and not the other way around but I'm intrigued by the apparent complication I see with geartrains. The lathe, by the way, dates from 1924. Mine is M/c #104 and is a recent acquisition following an 8-year search. I will describe it more fully in the appropriate section when I get the chance. Meanwhile it is shown on

http://www.lathes.co.uk/exe/page2.html

Ray

 

[Maryak]

47/37 = 1.27027027... (0.02 % error)
and takes up a lot less room.

Marv that's probably the best compromise for smaller lathes. The banjo on most small lathes will not accommodate a 127 tooth gear and it's also likely that the guard door will not close if your are lucky enough to get the gear in place...................Ask me how I know :

Best Regards
Bob

 

[mklotz]

If you have the space for two pair of gears,

30:19 - 37:46

will get you there with 0.001% error.