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Cutting Threads for the first time...SB 10k

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AJ

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#1
Hi all,
I'm about to cut my first set of threads for a project I'm working on. 1/2-20 both right and left hand. I'll be going by the Machinerys Handbook to learn. I know the basics, compound at 29 1/2 deg. 60 deg tool, major and minor diameters, cutting fluid, machine feed rates(20tpi), not to sure about spindle speed (I think it's just cold rolled 5/8 rod so 4 x 100 / dia. ?) but that's about it. I will be doing a couple test pieces to make sure I get this right. Also, I think I'll be grinding my own tool for the first time instead of using one my dad gave me... Any advice ? The pic is of a couple test passes just to get use to the lathes controls... No ryme, reason or size to the threads in the pic.....Any advice ? 20180908_224717.jpg
 

ericc

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#2
Use a fish tool to grind and set up your tool. Buy one or make one; it doesn't matter. Just set it up and get the angles correct. Watch a quick video to make sure that you are measuring the 29,.5 degrees from the right place. If you get this wrong, and lots of people do, it is better to just go straight in. Starrett makes a nice fish, but you can just download a template and make one out of some junk sheet metal.
 

Ed ke6bnl

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#3
You're fishtail has to be 90 degrees to the cutter while it's flat is against the part, just a thought.
 

pacifica

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#4
I like full profile inserts(only do one thread size per insert) which requires less depth, gives a stronger thread and needs less machining passes.Downside is you need one for each type of thread you want to cut.
I would also recommend machery's handbook(buy an earlier edition,27 or 28 on ebay to save money,dont get the guide) and a screw thread micrometer(chinese is ok).

Try to cut class 3 threads to learn quality threading. Then cut double start and triple start threads for a challenge!
faster speed 200 rpm or more can give a better finish on the material.
I would start out with free machining steel, delrin and brass to learn the ups and downs.
 

T Bredehoft

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Threading can be challenging, but you don't seem to frightened of it. forget minor diameter, there's no way in God's green Earth to measure it, short of an optical comparitor.
For setting you angle of the compound, (since numbers can start at any quadrant) visualize a 4 pitch thread, visualize the right side of the cut, set the right side of your tool parallel the right side of the 4 pitch visualization. clamp the compound there.
Make the first cut a fine scratch, compare it with a known thread or a thread pitch gauge. Make sure it's the pitch you want.
Make sure there's some lubricant on the thread and take a lot of passes to get to where a nut will screw on easily.

Don't sweat the small stuff, it'll come eventually, that's why we suggest first practicing on plasltic or Ledloy (12L14) (Free Machining Steel)
 

mikey

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Hey AJ, your threads look pretty good for a first timer - congrats!

I have a few suggestions:
  • Grind an accurate 60 degree threading tool. I would recommend 15 degree relief angles. This will give you cleaner threads. Don't forget to stone a small flat at the very tip or you'll crack the tip off. Be fussy about your angles and also when aligning the tool to the work.
  • Unless the work is really short, less than 2 X the diameter, use a live center if you can. This has a significant impact on fits.
  • Make your thread relief about 2-3 thread widths wide and about 0.003-0.005" deeper than the minor diameter.
  • Don't forget to put a 30 to 45 degree chamfer on the starting end of the work. Make this about 1-2 threads wide.
  • Stage your cuts as you progress. Try 0.005 to 0.009" for the first pass, then 0.005" for the next two, then 0.001" less for every pass until you are taking only 0.001" deep passes. Check for fit early and often. Use thread wires or a thread mic if you need a precision fit; otherwise, a nut will do.
  • Use cutting oil. Any threading oil or cutting oil works.
Go slow, take your time and it will go well.
 
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GL

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#7
I'll second Mickey and what you said about getting used to the controls. There is a rhythm to threading, feed levers, backing off the cross slide, adding to the top slide, return to zero on the cross slide, engage, disengage. One of those all senses involved and concentration relaxing things about machining. See the process, do the process. Scary at first, not so much later - until threading to a shoulder on an internal bore with a screw on chuck where you can't run the lathe backwards (see Joe Pie's video).
 

BtoVin83

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#8
I'll give you my take on the speed, generally the faster you go the better finish you will achieve. I don't end in a relief most of the time as it keeps me in practice and the rpm is governed by my reaction time to pull out at the same point rather than the correct tool speed. Even with a relief and using carbide your reaction time to pull the half nut tends to limit your speed rather than the correct tool speed. Coarse lead screws and fine threads help. Try and run as fast as you are comfortable with and it will probably be OK.
 

P. Waller

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Have at it, this part does not hold the wings on a commercial aircraft I suspect.
Chuck stock, set tool as square as possible, touch it off to determine a tool position then thread away until the part measures either over wires or to a ring gauge set.

You likely do not have the equipment to measure a thread in every possible physical aspect of its form.
 

AJ

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Well I really screwed the pooch on that one. Messed it all up but at least I know what I did wrong and one of you knew I might. Ericc, the first post thought I might set the compound from the wrong point and I did. I was about 60deg off but tomorrow is another day. I was out in my shop to late and was to tired and did`t pay enough attention . More to come..I`ll post pics of the threads I cut tomorrow...The threads I cut tonight, well let`s just say you won`t be seeing them..WAY to embarrassing ! Thanks for all the input..I need to read up on cutting threads a little more...
 

pacifica

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#12
I use a swing-up toolholder (http://mikesworkshop.weebly.com/swing-up-tool-holder.html),Full profile inserts, cut straight in(perpendicular to workpiece) ,lock the compound, I don't back out the crosslide after each cut, run at 150 rpm,check depth with a thread micrometer and can usually manage class 3 threads . Until I did these things plus what mikey does I had trouble consistently threading. A tight shoulder makes it more difficult since there is little margin for stopping.
 

mikey

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A tight shoulder makes it more difficult since there is little margin for stopping.
Made me smile ... a shoulder without a thread relief makes for tight sphincters! :eek:
 

AJ

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Here is my second attempt at threads to size(1/2-20)...Mind you this was with aluminum. I was running a test before I do it to my project pieces..I have some really close ups of the threads.. 20180914_204935.jpg 20180914_210736.jpg Lightly went over them with a file... 20180914_211041.jpg 20180914_210657.jpg 20180914_211300.jpg 20180914_211540.jpg
 

AJ

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Have at it, this part does not hold the wings on a commercial aircraft I suspect.
Chuck stock, set tool as square as possible, touch it off to determine a tool position then thread away until the part measures either over wires or to a ring gauge set.

You likely do not have the equipment to measure a thread in every possible physical aspect of its form.

What all would be needed to check all physical aspects of a thread ? I have and will now use the PeeDee wire thread gauges..and by the way I`m an A&P.....lol
 

P. Waller

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What all would be needed to check all physical aspects of a thread ? I have and will now use the PeeDee wire thread gauges..and by the way I`m an A&P.....lol
An Optical Comparator is an excellent tool for this https://www.msi-viking.com/Mitutoyo...MIiM-9h9283QIVA1YNCh0GXg2aEAYYBiABEgIUYfD_BwE
Measurement over wires will give you a Pitch Diameter but assumes that the form is correct to begin with as it can not measure this feature directly without a great deal of work.
Ring gauges will assure that a part is within the limits but also do not measure the form directly, if you were inclined to do so you may produce a thread that measures but has an incorrect form (-:
 

mikey

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#17
AJ, see that rough finish on the back side of the thread? As you get deeper, the right side of the tool is not cutting much; it sort of drags. To get a better finish, do not use the compound for the last few cuts. Instead, feed straight in with the cross slide but be careful because the depth of cut is deeper than when using the compound feed. Leave enough room to clean up the threads. This won't make the fit a lot better but your threads will look better.

The other thing that will do that is an inaccurate shape. Check the right side of the tool to make sure it is flat; any rounding or unevenness in the tool face will leave marks.

An accurately ground tool with 15 degree relief angle on the tool will pretty much eliminate this altogether.
 

AJ

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AJ, see that rough finish on the back side of the thread? As you get deeper, the right side of the tool is not cutting much; it sort of drags. To get a better finish, do not use the compound for the last few cuts. Instead, feed straight in with the cross slide but be careful because the depth of cut is deeper than when using the compound feed. Leave enough room to clean up the threads. This won't make the fit a lot better but your threads will look better.

The other thing that will do that is an inaccurate shape. Check the right side of the tool to make sure it is flat; any rounding or unevenness in the tool face will leave marks.

An accurately ground tool with 15 degree relief angle on the tool will pretty much eliminate this altogether.

Thanks for the advice but ya know what's funny. Earlier i had sharpened that bit and it cut amazing, the threads were like glass... i didn't resharpen it this time and you can tell. My cuts were to heavy and by the time it fit the hyme joint i didn't have enough material left for a couple finish passes.. All learning experience
 

mikey

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#19
Here is a screen shot of your tool:

Screen Shot 09-15-18 at 03.12 PM.PNG

Threading tools are form tools. Any deformity in the tool shape is transferred to your work. The accuracy of the 60 degree angle, the relief angles and the nose radius/flat all matter. The included angle of your tool is less than 60 degrees and the right side is scalloped near the tip. It will cut a thread and you may not notice any issues on a class 2 thread but on a precision class 3, yeah, you very well might.

Just so you know, I'm not being critical here. I'm trying to help you troubleshoot your thread form.
 

P. Waller

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Thanks for the advice but ya know what's funny. Earlier i had sharpened that bit and it cut amazing, the threads were like glass... i didn't resharpen it this time and you can tell. My cuts were to heavy and by the time it fit the hyme joint i didn't have enough material left for a couple finish passes.. All learning experience
Materials are not created equally from heat to heat, you can buy a dozen bars one month and they turn just fine, a year later another batch may prove more difficult doing the same operation. Buying random material that is of unknown manufacture is always a gamble when you already have a fixed working process running, often the change to a lower cost material or supplier will cost you in the long run.
 

AJ

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Here is a screen shot of your tool:

View attachment 275552

Threading tools are form tools. Any deformity in the tool shape is transferred to your work. The accuracy of the 60 degree angle, the relief angles and the nose radius/flat all matter. The included angle of your tool is less than 60 degrees and the right side is scalloped near the tip. It will cut a thread and you may not notice any issues on a class 2 thread but on a precision class 3, yeah, you very well might.

Just so you know, I'm not being critical here. I'm trying to help you troubleshoot your thread form.
Thanks, I don`t take your comment other then what it is, advise..I asked for this. How else and I going to learn if everyone does`t say anything ?? I take all these comments as constructive criticism and will try to do things better. Once again thanks..
 
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