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thetortoise
08-12-2016, 10:06 AM
Ok I should be able to get this to work but I am doing something wrong. I am cutting external metric threads on a Leinen lathe with a laydown insert. (16 ER AG60). I can cut the threads no problem, they look good and the controls on this machine are great. What I having trouble with is cutting threads that fit the way I want them without lots of fussing around with test nuts (gauges). I have a long travel dial indicator mounted to measure the perpendicular moment of the compound slide so I should be able to calculate the total depth of cut, do a number of passes, do a spring pass or two have have an accurate thread but this is apparently not the case.

the calculations are shown here http://mdmetric.com/tech/M-thead%20600.htm (they actually took the diagram from the German equivalent of the Machinery Handbook)

Anyway for M10 1,5 pitch that is say actual diameter of 9,8mm, with adjustments for a 0,2mm radius on the cutter (0,008 inch I think is standard for this insert as I really don't have a good way to measure it) I end up with a thread that should look like this https://goo.gl/photos/meC3waZMjVNGw2xb7

This means if I cut 0,999mm (ok 1mm from the radius I should have an ideal thread.)


Any ideas of where I am going wrong? The radius on the tool could be different, i could be doing something wrong with the indicator. I can calculate the amount of in-feed on the compound but as I don't know the exact angle I think the indicator is a better plan (I am using a pinned Mulitfix so have to set it to a bit below 29 and then adjust the compound until the tool is aligned so it isn't 29,5 degrees etc more like 26 but that isn't the problem.

thanks

Luke

wdtom44
08-12-2016, 10:21 AM
I often cut threads because I don't have a big selection of dies, and I want the threads true to the work. I find that after cutting to the specified depth for 29 or 30 degrees the thread is often not a good fit. I sneak up on a good fit, note the setting and now I can be very close or right on for any others I may be doing with that setup. With all the variables I don't think you can be right on the first time, at least it never works out that way for me. Not long ago I bought a set of thread wires for when I don't have a nut to fit to, haven't need to use them yet.

lakeside53
08-12-2016, 01:05 PM
Measure with thread wires or thread mic, cut a known depth (dial) then measure again; now you can easily adjust to final figure. Be sure to debur the crests before "fit" - the crests will often get slightly raised by the cutting process. With your AG60 (laydown insert) your pitch diameter is where you want to be measuring, then check that the crests are "decent". If you use fully formed version of the insert (but you need one for each pitch) you can just keep cutting until the od (to crests) is correct, and the rest follows.

thetortoise
08-12-2016, 01:50 PM
Thanks guys (lakeside53 I am from Seattle so I know your area well) Any good sources for full depth inserts? Although that does seem like an expensive proposition and if I am going to throw money at thread cutting tools I should probably get a pair of retractable Multifix holders. I will pick up a thread micrometer anyway, I drop stuff too much to deal with wires :)

It still seems like I should be able cut directly within 0,01mm or so of the target like any other diameter. I have many hours of lathe time, just mostly on my Schaublin 102. The screw cutting machine is a new addition so I have some things to figure out.


Luke

danlb
08-12-2016, 03:11 PM
There is no reason why you can not cut a working thread on the first try every time. I do it often, though I have to admit that I usually use sharp V tools without the radius. It is simple math.

External thread depth is P * .613 for a sharp V tool. Subtract the radius (.2mm) in your case. Quick calc says that if you start with a rod that is 9.96mm, and feed in .7195 mm, it should be good.

There are charts (and formulas) online that will tell you exactly how many passes you should take (and how deep for each pass) in order to minimize springback and excessive cutting loads.

Dan

Spin Doctor
08-12-2016, 03:33 PM
Personally I find carbide inserts for threading on manual lathes a poor use of technology. With one exception. That's if you can run the lathe at a high enough rpm to take advantage of the inserts capability. The only lathes I've ever run that can do this are Hardinge HLVs. And the only reason they can do this is because of their sngle tooth dog clutch. Now they arent the only lathes out there tha can do this. On lathes without that type of feature properly sharpened HSS is the only way to go in my opinion. As to holding a size or fit tolerance, on a manual machine it really comes down to having to check size with wires or a thread micrometer. Unless you have a really light touch or a way to slowly control your infeed I find setting a zero point and cutting to a depth on threads iffy at best. And my eyes aren't what they used to be.

Paul Alciatore
08-12-2016, 04:26 PM
The web pages that you refer to have a lot of data, but no where do either of them show the exact infeed for a thread when cutting on a 30 degree angle or 29 or 29.5 degrees. And you do not say how you are calculating that amount. So, the first thing I must suspect is that calculation.

You are in Norway so I will use metric.

I will start with the idealized situation. For a sharp Vee tool and a 30 degree infeed, you touch off on the perfect OD and then feed in by an amount equal to the metric pitch. That's simple.

But you have a tool with a radius on the tip. Assuming that radius is correct, you compensate for it by subtracting that radius from the perpendicular in feed. That may not be obvious and I had to do the math, but yes you do subtract that radius from the perpendicular in feed. But you are feeding in on that 30 degree angle. So a bit of trig is needed and you actually subtract r/cos(30) or 1.155 r from the metric pitch.

But again, you are using 29 or 29.5 degrees. So you adjust that factor to r/cos(29) or r/cos(29.5). But I doubt that this slight difference in angle is your trouble as it is going to be very small.

On top of all that, you are going to have to take into account the type of fit and tolerance of the thread.

Is this how you are calculating the in feed for the compound?

Of course, if there is an error in the radius of the tip, then that error will show up in the in feed needed. Do the manufacturers of inserts have an actual, published tolerance for the tip radius? Somehow I suspect that most of them don't.

wdtom44
08-12-2016, 10:57 PM
As you mention you need a different tool for each pitch. Do you know where to get a gage for grinding them? I have an older Brown and Sharp catalog that shows a No. 724 Unified and American Screw Thread Tool Gage, but I have never seen one in a tool catalog or at a swap meet. If I wanted to be precise I would set a dial caliper or mike to the correct setting for the flat on the end and compare it with my tool with a loupe. But a gage would be nice. The only place I have seen one was on display at a small museum shop at the Zagrey Farm in Colechester CT near where I live.

danlb
08-13-2016, 01:26 AM
Personally I find carbide inserts for threading on manual lathes a poor use of technology. With one exception. That's if you can run the lathe at a high enough rpm to take advantage of the inserts capability. The only lathes I've ever run that can do this are Hardinge HLVs. And the only reason they can do this is because of their sngle tooth dog clutch. Now they arent the only lathes out there tha can do this. On lathes without that type of feature properly sharpened HSS is the only way to go in my opinion.

I respect your opinion, but I disagree.

1) Single pointing with a hand sharpened HSS tool means that you have to be very precise to match the cheapest carbide insert for accuracy of the angles AND the tip radius. That said, you do not need to be that precise for most threads.

2) A full-form insert will cut not only the thread but it will trim the crest too. It's hard to do that with hand sharpened HSS.

3) A carbide insert does not have to spin at super fast speeds. It CAN be used at faster speeds, but that's not required.

You can use HSS. It works. It's just not the only way.

Dan

redgrouse
08-13-2016, 04:28 AM
Hi Luke, going back to your original question you are making the thread too deep. Look at the Maryland chart and the depth you need to go to is h3 which is pitch X 0.61343 = 0.920mm, you may have to go in another fraction but use this as a start point and you will,be very close but maybe a slightly tight thread.
If you want to feed in at 29deg then Paul is correct using trig to calculate the difference.

Personally I would never do this on small thread pitches and I have cut 1000's of threads, if it was a large coarse pitch thread then yes use the top slide set over for the feed but otherwise a straight feed in. is fine.
I use both carbide and HSS tools but these days for most I use full form tips unless it's a thread I don't have a tip for and not worth purchasing for a one off.
Hope this helps. Cheers John