View Full Version here: : Thread cutting adaptors on a lathe
28-04-2012, 10:46 AM
In Robin's (Tandum) QHY11 thread there is some interesting discussion (to me) about making adaptors on a lathe.
So that this info that maybe of interest to others is not lost I have created this thread. Maybe some one that is more thread savvy than me can copy some of the original posts here as well.
Of the many ways of cutting threads on a lathe, the most common technique is to use a tool that is shaped to represent a cross section of the thread to be cut.
As most thread forms that we come across are roughly triangular with rounded peaks and troughs, the cutting tool also has a triangular shape with a rounded tip and is normally just advanced (plunged) with the cross slide deeper at right angles into the material
to be cut over a number of passes, getting deeper each time until the correct depth and shape is achieved.
Because of this triangular shape, the deeper we go (and closer to our finished thread) the more work this tool has to do on each face and because of the pressures on (and geometry of) the cutting faces to the material there is a tendency for the edges to tear resulting in a rough thread of poor form.
A modified method is to turn the tool and tool slide approx half the angle of the thread to be cut and advance the tool using the tool slide on this angle.
This makes the tool advance into the material almost parallel to one face of the thread taking a small amount off the trailing edge and doing most of the cutting on the leading edge.
In the QHY11 example the thread angle was 60 degrees and Mark (Marki) used a set over of 29 degrees.
This halves the work and material removed by the tool, improves the cutting geometry and results in a cleaner cut.
Sorry for no pictures or diagrams, but if you Goggle - set over screw cutting
On another note, a good workshop resource was Chris Heapy's page where he describes this and other techniques including mods to an LX200.
I think this is an archive of the original site http://www.astronomiainumbria.org/advanced_internet_files/meccanica/easyweb.easynet.co.uk/_chrish/homepge2.htm
Let the discussion continue, Neil
28-04-2012, 11:43 AM
Almost all cutting on the lathe is a tearing action !
The trick is to have the right cutting geometry on the turning tool to match the material being used and the speeds and feeds to suit.
Every material and tool combination will have its own best range of surface speeds for cutting and different tools will have different feed rates that work better than others.
The use of lubrication (or not) will make a difference.
So rake angle, clearance angle, cutting edge sharpness, type of chip breaker, type of tooling (HSS, carbide) and significantly the coating type etc etc all make a huge difference.
The difference can border from impossible to easy, super smooth to full of chatter and rough torn finish.
You would not use the same geometry and tooling by choice on aluminium to stainless or plastic to cast iron !
Some materials are 'sticky' and some are not, so the tool you are using totally makes all the difference.
Ensuring that any swarf does not get caught back up into the cut will make a big improvement.
It should be possible to cut a high quality thread using the right machines tools and technique.
Taking off too much in a cut can result in a rough surface finish, but likewise, taking off too little may result in a poor cut.
Most sintered carbide insert tooling likes to take a heavy cut whereas a sharper tool will work fine in a fine cut.
The rigidity of your machine and the tool setup as well as backlash in your slideways and thread cutting lead screw can make all the difference.
If your machine has lots of backlash and isnt stable, then the heavier cut may cause chatter and result in a poor finish.
Try to ensure that the tool is as short as possible.
So I am wondering if the technique you describe is more about trying to overcome some machine deficiencies than a method to get a good thread cut.
Another technique is thread rolling, this deforms the material under pressure rather than cutting.
You can also mill a thread with a rotating milling cutter - but you'll need a CNC mill to do that effectively.
Very high quality threads are actually ground.
Hope that helps rather than confuses, but sometimes its a non trivial affair, that requires a bit of experimenting to get the best finish.
28-04-2012, 12:04 PM
I think most commercial adapters would be made in aluminium on a CNC lathe, a bit of an advantage to have. If you use aluminium it probably helps to know something of the big range of alloys, not all of them amenable to thread cutting.
You need to use a messy cutting oil as well for the best finish.
I think I'd use brass instead where possible, looks great, and the machining grades cut like a dream. It's 3-4x more expensive though.
As to the tools, you can grind them up yourself, or look at commercially available stuff like
28-04-2012, 12:31 PM
Thanks Rally,Geoff, I did not want to do all the work and I did leave a lot unsaid.:thumbsup:
Furthering the discussion and an inquiry from the other thread about set over, given everything else being equal, this is an accepted technique for cleaner threads;especially on small machines or difficult materials. Just takes a bit more to setup.
Unless it is for something very special, I will just grab an off the shelf cutting tool and holder and accept there will always be a compromise with tool geometry especially with Al., and then practice technique to help overcome the compromises. More swarf - yippee.:D
I love making swarf and always challenge myself to do the best I can regardless of what I am making. Practicing precision on a non precision project helps me learn technique with less tears, but no less frustration and scrap !
Just drew this up to help describe the technique, Neil
28-04-2012, 12:35 PM
Thanks for taking the time to clarify the technique Neil.
28-04-2012, 01:06 PM
Bugger, I just saw this after posting a long PM to Peter :screwy::P:D. I always use water soluble oil when cutting ali and alloy steels (I have a high volume pump coolant setup). Usually just use WD40 for hot rolled mild steel and nothing for brass or free cutting mild steel and bronze. I have found kero or thin machine oil is good for cold rolled mild steel both on the lathe and mill. I have also stopped using indexable carbide tooling for a number of materials and job on the lathe as I am getting better results from cobalt tool steel. I have found that the carbide tooling is good for brass, hard alloy steel and cutting and facing aliuminium but is useless for cold rolled mild steel leaving a rough finish whilst the HSS cobalt steel does the lot and you have a choice of profiles via the grinder. My lathe will only run at 2000rpm max and I think the carbide inserts are actually wasted on it. Different story on the mill which can run at much higher speeds producing a good finish on all materials. Here's a few pics of my latest project carved out from 2" 6061T6 round bar. I have placed a 50 cent coin next to the block to give you an idea of scale. The small bolts holding the front and back covers on are 10BA ~ 1.5mm, try drilling and threading those with a tap, nearly drove me insane :P.
A very good book on the subject is Screwcutting in The Lathe (http://www.fishpond.com.au/product_info.php?id=9780852428382) by Martin Cleeve, part of the Workshop Practice Series.
Your local public library is likely to have it :thumbsup:
The book is worth it just for the photo of the authors extensively modified Myford lathe!
Another person interested in miniature engines ;)
I am still slowly buying tooling and the back issues of SIC magazine, eventually I'll get MEB too.
I have also bought the plans for Steve Hucks' V8 (http://www.homemodelenginemachinist.com/index.php?topic=15608.0) and the blower, but it will be a looong time before I get anywhere near a completed engine :(
28-04-2012, 03:46 PM
Simon I had to walk away from it for a bit, it was driving me crazy. I have been working on methods to make the crank and think I have just about worked it out:). It will be made from EN16 (chrome moly steel) which is quite hard to machine and I will need a number of fixtures to support it as there will be a lot of pressure on the blank and cutting tool. The engine was designed by Eric Whittle and has 550 parts that need to be made (very small parts) I must be mad:screwy:. Here is a vid of the prototype running at 7500 RPM with a 12 inch prop.
28-04-2012, 03:48 PM
That's really good work Mark
Ah yeah I thought it looked like a Whittle case. Have seen some very fine examples over the years in ME mag, I'm sure yours will be top notch too judging by the photo's.
I am not looking forward to machining a crank, have read so many horror stories.. from memory(dodgy) Steve Huck made about a dozen of them before he got a usable one!
Strangely, some people seem to be able to spit them out without drama :shrug:
Anyway.. sorry to Neil for going OT :P
28-04-2012, 09:22 PM
Not sure if your thread profile sketch is just illustrative or intended to be more accurate, but I can see how you would potentially have a lot of problems with those threads, tearing during the turning process would be assured and problems with galling and interference in use !
The internal and external thread profiles actually need to have different clearances, and hence the thread profiles (in a perfect world !) would be different, but as a minimum the crest and root of the thread profile are flat or curved but never pointed.
Check out the drawing here for better detail.
They have shaded the male and female sections showing the profiles and how they mate together against the line drawings of the thread profile.
So the outside diameter of the male threaded part(eg bolt) would be smaller than the notional metric size (eg M12 bolt diameter is less than 12mm)
Likewise the inside diameter of the female threaded part (eg nut) would be larger than the notional metric size (eg M12 nut internal diameter is greater than 12mm)
There are simple tables in all the machining handbooks and elsewhere that you can use to get all the correct dimensions for the starting diameters and depths of cut.
Thread cutting can be a bit of an art form and to accurately cut them you ideally need specialised metrology equipment - thread micrometers, shadow graphs if you are grinding your own tools !
28-04-2012, 10:07 PM
Rally, yes it was just a quick and dirty sketch to indicate the general techniques.
If I am in a hurry, I will often start a thread (both internal or external) in the lathe for a few passes, the run a tap or die to get the more correct thread form.
This also helps me keep the tap or die cutting straight and true.
I have also made a few form tools for acme (square) or for very fine threads to get a better root radius.
The crown I finish to dimension, then wet and dry to finish the edges and relieve the fit.
Mark, love your work, envious of your dedication.
There are some projects I would love to tackle;
A tool and cutter grinder, so I do not have grind off hand.
A miniature Aero engine:
The inverted 4 cylinder Gypsy Major as in our Auster and eventually a radial 7 or 9 cylinder. As I said, I envy your dedication as I can see the work involved.
Lots of swarf though :D.
Have you seen this Aussie site about miniature IC engines:
29-04-2012, 12:11 AM
Neil I cheated on the tool grinder :D. I thought about buying the warden kit fron the UK but bought the commercial version from Hare and Forbes (below) instead. Its a little ripper when it comes to sharpening drills, slot drills, end mills both 2 and 4 flute and shaping cutting tools in cobalt steel for the lathe. Will even do carbide tips. I have that website in my favourites but you have reminded me of it. You are right about swarf tonnes of the stuff when machining from solid, still the worst thing is getting the cam and crank journals to align to specs so the cam gears have the correct clearances. At least a good quality DRO is needed to even get close, oh and a very precise tool makers vice doesn't go astray either. Just couldn't do it without them and the tolerences on this motor are tight. Biggest problem is the miniscule size of most the components, I have to work with a watchmakers magnifying mask just to see what is going on :P. Next one will be a lot bigger.
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