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07-30-2012, 05:57 AM
(This post was last modified: 07-30-2012, 06:01 AM by Mayhem.)
Up until now I have had no problems in single point threading. However, the coarsest thread I have done is probably 12tpi.
I am currently trying to cut a 6tpi thread and I am not winning 
I have a old change gear lathe with a 2tpi leadscrew. I set up the gears for 6tpi and verified it by taking a .001" cut and measuring the 'scratch' with a thread gauge. However, as I advance the compound (set to 29*) on successive passes, the thread just re-cuts itself. After several passes, the major diameter is reduced and there isn't much depth to the thread, despite decent chips coming off. Regardless, the thread still measures 6tpi.
Initially I didn't use the threading dial, as the thread I am cutting is divisible by the leadscrew pitch (I never use it on even number threads) but I tried using it and the problem remained - despite using the same mark on the dial (it takes ages for it to come around!). I engaged the half nuts about an inch before the cut to ensure any backlash had been taken up.
I ground a 60* bit out of a 1/2" blank and checked it against a gauge before honing the cutting edge. I ground 10* relief angles on both sides and left the top flat. I am wondering if I need more side relief, given the helix angle of a 6tpi thread? I don't really know/understand the whole geometry of the tool/helix angle but thought it may be something I should consider.
I didn't take pictures, as I was somewhat disgusted and thought it better to walk away, counting to 10...
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07-30-2012, 06:30 AM
(This post was last modified: 07-30-2012, 06:33 AM by Highpower.)
(07-30-2012, 05:57 AM)Mayhem Wrote: I ground a 60* bit out of a 1/2" blank and checked it against a gauge before honing the cutting edge. I ground 10* relief angles on both sides and left the top flat. I am wondering if I need more side relief, given the helix angle of a 6tpi thread? I don't really know/understand the whole geometry of the tool/helix angle but thought it may be something I should consider.
I don't understand it all either Darren, but I think you need a whole lot more relief than that for a 6 TPI thread. You may have a vertical shear bit there, instead of a threading bit. 
Have you tried putting some Dykem on your side relief cuts to check for rubbing? 
Willie
Willie
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Its funny, as at the time I didn't think of the relief angle at all. I was concentrating on 60* point, sharp, perpendicular to the work, compound set at 29*. It wasn't until I was thinking about it on the train this morning!
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Maybe you need to start riding the train first and then start your thinking. 
Willie
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I hesitate to ask but although you set the compound to 29°, is the 60° tool perpendicular to the workpiece?
I fear you'll have to do up a video of what you're doing (even if the WORLD will then see what you're doing wrong )
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Helix angle is a function of pitch and diameter. What is the major diameter? If say, a 6TPI was on a 12" OD part, the helix angle would be much lower than on a 1" diameter. Just a example. Think of the helix angle as the smaller of the two angles adjacent to the hypotenuse of the wedge geometry if you took a single thread path of one revolution and "unwound" it to a flat surface. You get a triangle, right? That's all a thread is. A long wedge wrapped around a stick. Tightening a bolt in a nut is like pushing two wedges together and getting a force along the axis of the two that draws them tight. Clear as mud, yes?
I'd expect to see a 6 pitch on something no less than 1 3/8". The helix angle is calculated along the pitch line. Formula is:
HA = arctan(L/((OD+MD)*3.141/2))
HA is Helix Angle
L is Lead
OD is Outside Diameter
MD is Minor Diameter
For example, on my 1.375, the HA is 2.2095°. You can use the calculator here to check your math, or do it for you.
http://www.americanmachinist.com/calcula...tting.aspx
If you have 10°, you have plenty of clearance.
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Yes I can see that we are going to have to have a video to sort out this problem 
I have found that cutting threads can do so easy once you understand the principles such as compound at 29 degrees, tool bit razor sharp, and at 60 Degrees. But then something will always come along to kick us in the teeth that says this isn't as easy as we thought.
I would definitely put some dykem of marker pen on the tool bit to check for rubbing on the sides of the bit, you may need to cut a steeper relief on the leading edge of the bit. I always grind a slight relief on the top also so that I can get a much sharper cutting edge on the leading side of the bit.
DA
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Thanks Tony. The major diameter is 4", so using the calculator you linked to the helix angle is 0.7598.
Thanks DA. I'll check to see if it is rubbing. From what I understood the top was best left flat but I'mm willing to put some relief on the top.
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Make sure that the compound feed is parallel to the trailing edge of the bit. 29 degrees isn't the same on all lathes (some start at 0, some at 90). Handle towards your belly, swing right to 45 and then back 15. It'll say 30 or 60. Whichever, but it's very important that it is parallel to the trailing edge, if you are feeding with the compound.
You can kinda see the needed clearance angle by eyeballing things - hunker down and look at the thread from the side, see if your toolbit has enough clearance. At that pitch it will need a lot.
This might be a good time to practice the art of picking up a thread, rather than scrapping the part. Disengage the back gear so the spindle turns by hand and with the halfnuts engaged, fiddle the bit in where it needs to go, minding the backlash, then back the compound out until it's clear of the work, and zero the crossfeed dial. Turn the spindle by hand, watching the tool as it enters the cut (cutting air). If all is well, then resume threading.
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I always believed the reason for 29° was so that the trailing edge of the 60° threading tool would be 1° shy of parallel with the compound. That way all the cutting will be done with the leading edge of the cutting tool.
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