Thread Geometry

[Antman]

I was going to call this thread “Screw Geometry”
Question is, given the diagram of an external or internal thread is that looking at the screw if it was cut down the middle or is that a view along the helix angle? In other words should your 60deg tool ideally be inclined at the helix angle, or flat, or are those 2 different questions? I haven’t progressed to Acme yet but the same applies.
Thanx
Ant

 

[Maryak]

Ant,

For larger pitch threads it is essential to incline the sides of the tool to the helix angle, ( to prevent them binding/cutting as the depth increases); but the top of the tool should be at 90 deg to the work and in the case of a square thread of the correct width.

I use a round tool bit and holder to achieve this.

Hope this helps

Best Regards
Bob

 

[Deanofid]

Maybe I'm not sure what you're asking, but if you put a base on that 60° tool and consider that the base of a triangle that forms the shape of the tool, you want that base parallel to the axis that the helix revolves about.

In other words, get your tool square to the work. Use a fishtail.

Dean

 

[b.lindsey]

Ant, every single point set-up I have ever seen the tool is flat. I assume the relief angle takes the helox into account at least in some small way. Otherwise every different screw pitch would require a different angle of incline. Interesting question though and makes me wonder if the same theory applies to taps and dies too. ??? I am interested in other's replies too.

Bill

 

[Blogwitch]

As Bill has said, the tool is invariably flat, with the relief angle taking care of the clearance when cutting a normal screw thread.
Two cutting tools each for both internal and external will most probably suffice. 60 degs for US threads and metric, and 55 degs for imperial, BSF & Whitworth, you can forget about BA, as most lathes won't cut the pitches required, but just for your information, 47.5 degrees is the tool angle.

In the UK we tend not to use the 'FISHTAIL' , a 60 DEGREE CENTRE GAUGE, that is perfect for US machinists, but as we are more used to using the 55 degs as well, we tend to prefer what we call a SCREWCUTTING GAUGE, which should have 55, 60 and 47.5 degs, plus usually an acme profile as well. Go down the page a little, and you will come across both types.

http://rdgtools.co.uk/acatalog/EVERYTHING_ELSE_MEASURING.html

When it comes to Acme, trapezoid, square or buttress section type threads, the tool has to be ground specifically to give clearance to the helix of the thread, otherwise you can hit all sorts of problems when trying to cut them.

Bogs

 

[Richard1]

Top of my tool is flat, side relief is ground to about 2 degrees less than the helix angle. When looked at from the front there is a decided slope on the tool which matches the helix angle of the thread. I don't know if you need to do this or if normal side clearance would be OK I just do because I have the tools to do it. I will get some photos later if I can steal the time.

Richard.

 

[Antman]

Hi,
Thanks for the replies, but I must admit I am still confused. Most of my 60deg thread cutting has been fairly successful in that I can cut a screw that fits a commercial nut or a tapped hole. I have been using a fishtail to set the tool. Dean, your reply makes the most sense to me, thats what I wanted to know, thank you. Do I have it right, the form is shown looking along the helix axis? Does that not obviate the need to incline the tool to the helix angle as Maryak shows, or alternatively, incline the tool but then the form should change, being a projection at the helix angle? It will only make a very small difference to the shape of the tool or thread cut.
On another tack I have not had much success feeding the tool in at 30deg with the top slide and rake at the top. The cut seems to be all on one side leaving a V with a much shallower angle.

 

[PaulG]

Hi Ant,

Could you describe your threading process step by step for everyone. Have you been using the cross slide to feed in the cut?

Paul

 

[Deanofid]

On another tack I have not had much success feeding the tool in at 30deg with the top slide and rake at the top. The cut seems to be all on one side leaving a V with a much shallower angle.

Ant, this sounds like you might be going 30° off the wrong orientation of the top slide. At the risk of making myself look silly, have a look at the pic.
You want to measure the 30° starting with the top slide perpendicular to the spindle C/L.
Then set your tool square to the work, and your threads should come out nice, as long as you have the proper 60° tool shape, (or for whatever type thread you're cutting).

Dean

 

[Blogwitch]

Ant,

The feeding at 30 degrees with the topslide, as Dean has just shown, should only cut on the forwards face (in theory), it takes pressure off the cutting tool by doing it that way. The thread is finally cleaned up by a minute cut using the cross slide. That is why it is so important to set your tool exactly square to the job, AFTER YOU HAVE OFFSET THE TOPSLIDE, and in fact, the topslide should be set at 29.5 degs (half the cutting tool angle, minus 1/2 degree) not 30 degs for a 60 degree thread, so for an Imperial 55 deg thread, the offset figure would be 27 degs, that should allow a tiny amount of backcut to take place, keeping both sides of the cut fairly smooth, with no stepped ridges in it.

Maybe you need to call around to my shop again, and I can show you the threading process first hand. A few minutes actually seeing it done can save you hours of reading about it, and still not getting the grasp of the technique.

It looks like you are getting very confused with too much information from all directions, when in fact, it should be a very simple operation.


John

 

[Antman]

Yeah, like that, I've only tried twice, to be honest, once was ok, and a couple days later duff, duh ...topslide the wrong way.

THANK

you Dean.


On the more theoretical shape of the thread Maths is fun... for some people.
Ant

 

[Antman]

Gee thanks also Bogs, how to make an Acme tool. I want to do it in round HSS.
Ant

 

[Blogwitch]

Ant,

Methinks I can detect a little running before walking here. Get the very basics under your belt first, then progress slowly and surely onto the next stage.

The world won't collapse around you if it takes an extra day or two to perfect the technique.

If you don't do that, you will find that you will be using many half learned techniques on the same job, and you will end up with only a half OK result.

Practice, practice, then some more practice. Trying it twice, then only getting one good result, in my eyes is a complete and total failure. Doing it 100 times and then getting one failure is almost good enough. Failure, on a large scale, especially in your case, with the materials being so expensive, is not an option.

Bogs

 

[Antman]

Gee Bogs, I'm doin' better than you think, some of what I have made has come out quite ok. As for expense I have quite a load of decent round BMS from 20mm to 40mm, that I got from an agricultural co-op, sold to me as EN3B, for much less than I expected to pay. I wasn't able to get 12mm or 15mm, so I am not roughing 20mm stock to make M8 screws, but I was given some quite adequate 15mm bar that I’ve been using for bolts. The more I am into this hobby, the more it seems like having the right tool and knowing how to use it is what it is all about, and how long will it take. Tooling is where it hurts the budget. I paid a small fortune for 2 sizes of metric fine taps direct from Somta Tools but busted 2 of 3 Jap M4 taps, that I bought as a kid to make a slot racer and found in my Dad’s toolbox after all these years. I have decided that to make progress, to learn techniques by working on stuff that is not tiny and to have for future use I must make my own tooling. I made a versatile toolholder for the shaper that works really nice, and a parting-off toolholder that I am scared of using, both from Mercedes truck crank counterweights. For me, if I am to ever make model power, the *BLING* factor is important and I have almost finished a knurling tool. Next up must be a ball turner, also for bling and then a vertical slide, why I want to make an accurate screw and nut. If I can’t get Acme right, I will make the feedscrews with 60deg threads. The only internal work I have done yet is through boring, and I am pleased with my results. I should post some pics of my progress but I am on a really slow dialup connection with limited cap.

I like to assimilate the information, let it rattle around my little pip, then commit.
Ant

 

[Groomengineering]

Hi Ant. ACME isn't all that different from 'normal' threading, just that the tool has a 29deg inc angle and a flat on the end which varies w/tpi. (see attached little handy chart) Basically means a different tool for each tpi. A thread gauge makes them much easier to grind. Compound set at 14deg (1/2 inc - .5deg). ACME threads are all very coarse so plenty of relief all the way round. ACME is much less tolerant of 'a few thou off' than standard v thread forms, it's either perfect or it won't fly.

Not trying to put you off, but 'the' part is not the one to try it out on. Get some scrap and practice. As for making tools from round, it's a real PITA unless you have a block to mount the stock in.

Almost forgot, it's a hobby, so HAVE FUN! ;D

Jeff

BTW - Ever wonder why gears have that goofy 14.5deg pressure angle?



View attachment Acme form.pdf

 

[Antman]

Hey Jeff,
Why indeed? For maybe 200 years the better gears have been involute. I was disappointed to find that my lathe has gears with straight sided gear teeth.
For that matter why stick to a standard? I could make a screw and nut with a more tolerant angle, but still trapezoidal form. It’s only a tool.
I’d have more fun if I could do things faster, but I am making the things I want for now. Before I had ever laid hands on a lathe, I didn’t think it could take me days to turn a scew.
Ant

 

[Blogwitch]

Ant,

I didn’t think it could take me days to turn a scew.

That is why we do it mainly for fun and enjoyment, nothing too serious.

If you are pushed for time, you buy a tap and die to do the job, but then you learn very little.


John

 

[winklmj]

Yep, I wouldn't get too discouraged at this point (still learning right?). I've only turned a couple of threads just for fun but read a lot about it. Needed to do one for real on a new compound lead screw and would be threading under power (vs. using a spindle handle).

Turned the major diamater and a groove for the minor dia. Setup the compound at 29.5. Squared up the threading bit using my fishtale. Zeroed out my dials. Took a light test cut, etc... Then promptly started turning my thread and forgot you're supposed to advance the bit with the compound and not the crossfeed. DOH! :wall:

By the time I realized it, it was too late so I ended up cutting the whole thing at essentially 90 degrees. And it still came out OK. I'd say for small threads advancing at 90 is OK but can see where it'd be an issue for heavy cuts.