Anyone good at trig? - need help please

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skiprat

skiprat

Passed Away Mar 22, 2022
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I don't have much hair, but I'm pulling the remainder out at the moment.[xx(]
Here's the problem....
I'm practicing thread cutting on my metal lathe.
It has a METRIC leadscrew ( So I can't release the half nuts )
I have done the first cut for a three start thread. (Using 8tpi)

How do I calculate how much to move the topslide and the cross slide to move the bit for the next thread?

I have googled the question for days with no luck. I don't want to join another forum just to ask that question.

I know there are some metal Gurus here, but don't want to interupt their Easter weekend by PM'ing them direct. But if you guys are watching, then..........

HELP PLEASE


200832117112_threads.jpg
 
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Are you cutting 8 threads per inch or 3 times that?

What angle did you set your compound at? Some people thread at
29-1/2° and some thread at 30°.
 
No trig should be needed. However you may need to set your compound parallel to the shaft. An 8 tpi has a .125 lead, so you would divide .125 by three (aprox. .0415 would be just half a thou shy, should hurt anything) and move the compound .0415 for each start.
 
Paul, I thought I needed the compound at 30deg ( 29.5 ) to ensure clean succesive cuts? If I have the compound parralel then won't I be cutting the rhs of the thread groove incorrectly? If it's perfectly ok to set it straight then that would solve my problem. I was contimplating rotating the stock 120deg in the chuck. But that seemed a bit primative. I'm certainly going to give it ( your method )a try now, thanks.
I ultimately want to do these threads in stainless and want to get them as clean as possible without chipping or scoring in the groove.

Joe, Hi again. I've got the machine gearing set for 8tpi. Coursest thread I can do ( I think )
 
Skiprat, I hope my math is right on this but you should move you compound out .048" and move your cross slide in .024"

I have my fingers crossed for you on this one.

Paul's method sounds easier!
 
Thanks guys. I'll give both methods a go.:D Once I converted the inches to mm. ;)

I was just eating my supper and the penny dropped on the 30 deg setting. In the pic I have it set 30deg to the base. I think it should be 30 deg to the perpendicular? Don't think it matters too much on these practice runs though ( esp in soft brass )

Joe, thanks for the technical answer, but if you don't mind, please would you let me know how to get it for future reference?

'Give a man a fish and feed him for a day - teach him to fish and you have fed him for a lifetime' Or something like that:D
 
Sure, let me get my machinist handbook out and make sure I'm right.
I cheated on these ones I had AutoCad open and drew it up real fast to get you the figures.
 
Originally posted by skiprat

Paul, I thought I needed the compound at 30deg ( 29.5 ) to ensure clean succesive cuts? If I have the compound parralel then won't I be cutting the rhs of the thread groove incorrectly? If it's perfectly ok to set it straight then that would solve my problem. I was contimplating rotating the stock 120deg in the chuck. But that seemed a bit primative. I'm certainly going to give it ( your method )a try now, thanks.
Click to expand...
To do three start threads you are not cutting full depth. Another thought would be to back your cross-slide up and crank in the compound until it lines the tool up with the next thread. Divide each of those amounts by three. Not to sure, but should get you the same result. May have to get into AutoCad myself and do a draw-up!
 
Okay Skiprat I found the easiest way for you to try!
Check this sight out.
You know that one angle is 30°(your compound is set to this) and the other angle will be 90° and the short side is your lead of your thread.0416(1 divided by 24, because you are cutting 8 TPI times 3).
Try these numbers out and see how this works.

http://ostermiller.org/calc/triangle.html
 
Originally posted by skiprat

Thanks guys. I'll give both methods a go.:D Once I converted the inches to mm. ;)

I was just eating my supper and the penny dropped on the 30 deg setting. In the pic I have it set 30deg to the base. I think it should be 30 deg to the perpendicular? Don't think it matters too much on these practice runs though ( esp in soft brass )
Click to expand...

Skiprat; I think you are correct. It should be 60 degrees to the base. The compound should be parallel to one of the edges of the threading tool. The reason behind the set up is to only cut on one of the thread edges. Use a fish-tail (threading gauge) to verify that the tool is perpendicular to the piece.

30 (or 29.5) degrees off perpendicular sounds correct. The key is that at 30 degrees, turning the compound dial by .010" moves the tool half that amount or .005" At 5.8 degrees, moving the compound by .010" moves the tool by .001" or one-tenth the compound dial measurement.
 
Thanks again for all the help guys. I'm going to play around with it again today. I am also thinking about getting an Imperial leadscrew so I can use a thread dial indicator. Hopefully that will make it a bit easier. I think I need to see if I can change the gearing to perhaps get 4tpi as the angle of the threads at 8tpi just doesn't look anywhere as steep as any 3 start pen kit coupler.

Randall, small world! I see you are from Harrisburg PA. I stayed at the Best Western in H'burg less than two weeks ago. I went about six times through the toll booth trying to get off the freeway at the correct turn off for the hotel. [}:)] Oh well, at least I managed to get rid of ALL my small change:D
 
M12.0x0.8Px2.4L triple start
Skiprat, this is the thread info for the multi-start tap that was ordered for the group buy.
I got this info from this sight.
I think you were on the right track.

My metric thread gauge doesn't have 2.4 but it has 2.5 and it is real close to 10 TPI.

If you do the math, according to the above thread size that the tap set was ordered at, it is very close to ten threads per inch.

My concern for you is, how are you going to cut the female threads?
You will need a small threading tool to get in there.(without the tap of course)

Keep trying you are very close! I'm cheering for you!
 
Joe, I have never been scared of cheating:D[}:)] The plan is to make an 'acceptable looking' metal threaded part for the pen. I don't really care if it is a completely made up non-standard thread. Once I have got the geometry looking good, I will make a batch of them with the machine all set up. At the same time, I'll make 2 solid rods with the same threads. About 2 inches long each. One will be ground into a tap and the other will become part of a mold for casting. Hopefully I can then try two methods for doing the cap threads, knowing that they 'should' match my male threads on the pen.

I don't think I'll have to worry about hardening the homemade tap
It will only be used for making the plastic cap inserts.

I'll keep you posted. :D
 
Actually, probably the easiest way to do this is to use math and geometry from a 30 60 90 triangle.

Picture a 30 60 90 triangle with the short side on the bottom. the 90 deg angle is on the left, and the 60 deg angle is on the right.

The 90 deg angle represents your crosslide, and the 60 deg angle represents your compound.

The bottom of the triange is your pitch.

The side on the left (made by the 90 deg angle) is Pitch times the square root of 3.

The side on the right (made be the 60 deg angle) is 2 times the pitch.

These equations will give you the distances to move the compound and crosslide.

So you will move the compound IN 2(pitch), and the crosslide OUT pitch(sq rt of 3)

OR

Move the compound OUT 2(pitch) and the crosslide IN pitch(sq rt of 3)
 
Originally posted by skiprat



I don't think I'll have to worry about hardening the homemade tap
It will only be used for making the plastic cap inserts.

I'll keep you posted. :D
Click to expand...
Lee and I have discussed this exact same idea. This is how he is going to approach this, by making his own tap! I guess great minds think alike.;)
 
Also...Skiprat...have you considered a quick-change tool post?

They make life MUCH MUCH easier on the metal lathe.
 
I'm working nights for this week so managed to get some time to play.
Thanks to ALL the really helpfull replies that I got to this post, I think I'm getting there.:D

I threaded this length of stainless steel using all the methods offered!!!!:DI did break SEVERAL of my new TCT tipped tools and even put a nasty gash in my live center:(
Two got broken by forgetting to change the motor direction and two by reversing the motor before retracting the bit. I'm going to stick to the good old HSS bits from now on.

As you can see, I made the tap too:D Pretty easy to make from the threaded rod. I don't know if it will work or not, but it feels pretty sharp and it 'looks' the part[:o)]

200832618628_threadsntap.jpg
 
Excellent job Skiprat!
I am glad it all worked out.
It looks as if you are now a pro at single point threading, and a triple lead thread to boot.
With the quality and craftsmanship in the pens that you have shared with all of us, I knew you could do it.

I can't hardly wait to see this worked into one of your designs.
Thanks for the update and please keep us posted!
 
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