# External Threads on a Lathe: Where did I go wrong?



## Codiak (Apr 22, 2011)

I've only had a mini lathe since XMas and still have to think through every step of threading carefully. Last night I made an error that I still don't understand.

Task: 

thread 10mm of round of alumimum 6061 externally
Lesser diameter = 44.5mm
Greater Diameter >= 45.5mm
Tools: 

Mini-Lathe Imperial
HSS 60degree tool
Digital Caliper
Threading fluid
1mm is the max height I desire or 0.03937 inches
22 TPI should give me a height of .03936 (1/22 * .866)

At this point I expected things to go smoothly 
I turned the stock to 45.5mm
set the compound slide to just under 30 degrees
cut a stop groove at 10mm
cut a test patter of .002 depth
the made 6 passes at increasing depths on the compound slide @


.010
.020
.030
.035
.037
.039

While the threads look really rough I was sure a little filing would do the job.

*What I can't figureout is how my Greater diameter is 44.5 and not 45.5??? *
(I don't have the tools to check the lesser diameter)

I've verified the stock rest of the stock is 45.5.


Any idea where I went wrong?

Thanks in advance 


posted to http://www.practicalmachinist.com/vb/general/external-threads-lathe-where-did-i-go-wrong-224253/ as well


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## precisionworks (Apr 22, 2011)

A photo of your setup would really help.


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## Codiak (Apr 22, 2011)

I'll reset and get a photo up this weekend.

In the meantime I'm going to change up my methods.
I've got more than enough power to do straight-on threading... not only can I do it cleanly in fewer passes, I can control the depth better.
Since I'm creating the male and female ends I should be fine.


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## Anglepoise (Apr 22, 2011)

You say that your compound is set to just under 30°. Some import lathes have the incorrect angle settings. Make sure, by simply looking,
that the side of your 60° threading tool is exactly parallel with the compound. I have seen this error many times before.
If its wrong, you will see immediately. It will stand out easily.


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## precisionworks (Apr 22, 2011)

Those are called "fishtail gages" or "center gages". Use the very small side V to make sure your 60° threading tool is exactly 90° to the work. Starrett makes a nice one for under $20 (C391)


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## PeterH (Apr 22, 2011)

If I'm following correctly, between an outer diameter of 45.5mm and a diameter of 44.5mm at the bottom of the threads, that gives threads 0.5mm deep radius, not 1mm.


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## wquiles (Apr 22, 2011)

Codiak said:


> I'll reset and get a photo up this weekend.
> 
> In the meantime I'm going to change up my methods.
> I've got more than enough power to do straight-on threading... not only can I do it cleanly in fewer passes, I can control the depth better.
> Since I'm creating the male and female ends I should be fine.


 
If you need help with the pictures, you can send them to me and I can host them for you 


I just did some external 10mm-1.5 metric threads today, straight-in (I am stubborn regarding the 29.5deg!), and it was this group here in this subforum that helped me do the threads properly:


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## Codiak (Apr 24, 2011)

So I figured out my issue I think and I'm going to chalk it up to newbie-ism rather than dumb frack!

I decided to do some practice threads today and to make things simpler on myself I decided to just cut straight-in using a tip of 16 so I didn't have to worry about the dial count. I set the gears and started cutting deep single pass threads... No issue here....

I then tried a few multiple passes...and ack they looked like crap... In fact the second pass cut through the peaks of the first pass. A third pass mass it worse.

So I parted the section and tried again.. This time using the thread dial and engaging at 1 only. Same thing happened. 3 passes and total crap threads

I went back to the manual to check the gearing.. 40. Any Any 40. Hmm that was different than the side of to machine which said 40 65 - 40. But I had 40 65 60 40 so that wasn't it.... Back to the manual. If you use 3 gears... And my head about exploded with the ahha moment. I wasn't running 1 to 1, any doesnt real mean ANY and the math fell into place.

End result I got perfect 6 pass 16tpi test threads, straight in cuts... I'll reset and do the 29.5 method, but am confident the results will be great.

Thanks for all the feedback!


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## gadget_lover (Apr 24, 2011)

Your insight was right on. The biggest stumbling block to using many lathe features is interpreting the manual. The threading charts are even worse than most areas since the chart layout and nomenclature changes from machine to machine, and from country to country.

The 1.5 puzzled me until i realized it was 1 DOT 5 and the dot was actually used as we would use a comma , which makes it a list of 1 and 5.

Daniel


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## 350xfire (Apr 26, 2011)

Will
Those are some ridiculously NICE threads man!!!!


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## metalbutcher (Apr 26, 2011)

Those are some of the nicest threads I've seen. What's the material and what threading tool are you using?

Thanks,
Ed


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## wquiles (Apr 26, 2011)

Thank you guys 

Real thanks to Daniel and Mac (and a few others in this sub-forum) who taught me to do threads back when I had my "small" 7x lathe when I started threading "D" and "C" Mags 

The material is 12L14 "leaded" steel. The threads came out very nice, and that was without any polishing whatsoever, cutting straight-in at zero degrees - here is a close-up (still dirty):






I just go really slow, and take light cuts as I am approaching the target. In this case I had the part to match in hand, so I just keep taking 1-2 mills until I got the fit I wanted (not tight, nor loose).

My threading tools all use sharp carbide inserts. This one in particular is a Kennametal tool:































Will


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## precisionworks (Apr 26, 2011)

> Those are some of the nicest threads I've seen.


+1

Will has everything working in his favor ...
Careful setup, with tool point at center
Correct choice of threading tool
Taking multiple light cuts instead of a few heavy cuts
Using a "thread friendly" material

Some metals thread horribly, leaving a torn surface no matter what is done. Free machining steels (those with lead added) make a thread look like it's chrome plated. 12L14 is such a steel, as is 1144 Stressproof - the addition of lead makes both steels easy to work. 

If more strength or toughness is needed, 4140HT is a good choice. It has a lower machinability rating than 12L14 or 1144, meaning lower speeds & feeds, but it also produces beautiful, shiny threads.


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## darkzero (Apr 26, 2011)

Very nice threads Will! That's a might purdy threading insert you got there too. 

I need to try me some of that 12L14, too bad my local IMS doesn't carry it. Or maybe they do. You guys order it online? How easily does it rust?




precisionworks said:


> +1
> 
> Will has everything working in his favor ...
> Careful setup, with tool point at center
> ...


 

So that's why! I almost always get "tearing" when threading larger coarser threads using cold rolled. I always though it was something that I was doing wrong but I never get that kind of result machining other metals. With the finer smaller diameter threads using CR1018 I don't get it so much but if I look at it up close I can see a bit. Threads still feel smooth though on the finer threads.


Major tearing CR1018






Better






Small diameter CR1018, minor tearing






Ti (grade 5) & stainless steel thread nicely which I never expected. Well my brother told me to get 303 which is better for machining.


Not the best threads I've done on Ti but I don't have the other examples anymore.






And I always like machining brass. This was done on my mini lathe.








Here's a brass version of the "seamless" Ti stash cans I make that I made yesterday.
















Ti version


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## precisionworks (Apr 27, 2011)

> I almost always get "tearing" when threading larger coarser threads using cold rolled.


I started to mention CRS by name, as it consistently produces rough threads, but thought that not many members were using CRS (aka 1018 steel). Unless a customer insists on CRS, I don't use any and don't keep any on the rack.

HRS (aka A36) is much nicer, although nothing compares to 12L14 or 1144 for ease of machining. Some people dislike HRS because it comes with a coating of mill scale. The mill scale peels right off as long as the first pass on the lathe is deep enough, at least .020" depth of cut. Then is isn't bad to work, costs little, and is the most common structural steel in the USA.



> How easily does it rust?


In a heartbeat:nana:

If rust resistance is needed, chromium & nickel are mixed in & the result is "stainless" steel. It isn't rustproof, but it does take longer for rust to form. In a non-stainless steel, some alloys like 4140 and 4140HT do a surprising job unless the humidity is very high.

Very nice work on the stash cans


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## metalbutcher (Apr 27, 2011)

Will,

Thanks for the info on the threading tool. It's not too bad price-wise so I just may have to pick one up. We have the same QCTP, Phase II BXA wedge style so that same size threading tool will fit.

Thanks,:thumbsup:
Ed


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## wquiles (Apr 27, 2011)

precisionworks said:


> +1
> 
> Will has everything working in his favor ...
> Careful setup, with tool point at center
> ...



Thank you Barry 

I would only add good lubrication to that list - I "never" thread parts dry. I use the LB2000 lubricant either manually (droping a few drops onto the area being threaded), or with the Accu-Lube applicator (like in the pictures above) as the air stream that carries the lubricant also cools off the part/insert, plus removes the chips from the cutting surface.





metalbutcher said:


> Will,
> 
> Thanks for the info on the threading tool. It's not too bad price-wise so I just may have to pick one up. We have the same QCTP, Phase II BXA wedge style so that same size threading tool will fit.
> 
> ...



Here is the actual part number for the insert I am using:











Will


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## metalbutcher (Apr 27, 2011)

Will,

I've been doing a little research and was wondering if those inserts are the type that are good for a limited range of TPI threads? I was looking at some inserts on the Kennametal web site and some of them are good for only one TPI.

Ed


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## wquiles (Apr 27, 2011)

metalbutcher said:


> Will,
> 
> I've been doing a little research and was wondering if those inserts are the type that are good for a limited range of TPI threads? I was looking at some inserts on the Kennametal web site and some of them are good for only one TPI.
> 
> Ed


 
When it comes to threading inserts, you basically have two options:
- full profile inserts - they work on only ONE TPI (say, just 20 TPI), but they do cut the full profile of the threads (both the tops and the valleys)
- partial profile inserts - they work with a RANGE of TPI's, and you typically have two: one for the "smaller", finer TPI's, and one for the larger TPI's. The one I pictured above and that I am using is a partial profile thread insert that works in the TPI range of 48-8, which is great for the work I typically do.
LT16ERAG60CB

Will


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## wquiles (Apr 27, 2011)

Here is a good link that visually shows the partial and full profile inserts for threading, as well as the advantages of the 29.5 deg vs the straight-in (zero degrees) that I use:
http://www.mmsonline.com/articles/threading-on-a-lathe

Will


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## metalbutcher (Apr 27, 2011)

Will,

Does that partial profile insert require an anvil for certain TPIs? I'm not real clear on when an anvil is required and the Kennametal web site does not explain it very well for my feeble brain.:huh:

Ed


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## wquiles (Apr 27, 2011)

metalbutcher said:


> Will,
> 
> Does that partial profile insert require an anvil for certain TPIs? I'm not real clear on when an anvil is required and the Kennametal web site does not explain it very well for my feeble brain.:huh:
> 
> Ed


 
I will defer to others like Barry that have more experience threading, but my limited understanding is that the anvil does allow for slight adjustments to allow for better clearance. These old posts has some good insight into the anvil and its use:
http://www.practicalmachinist.com/vb/general-archive/threading-helix-angle-86201/

http://www.chaski.com/homemachinist/viewtopic.php?f=42&t=83627

In my particular case, I left the anvil in the position it came from the factory, and have been using it that way since day one - I have never bothered moving/changing the anvil.

Will


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## metalbutcher (Apr 28, 2011)

Thanks for the advise guys. I just ordered the same threading tool that Will has for $71 from MSC. They have a promotion going on now for most cutting tools. I got one of the threading inserts at $15. Pricey but hopefully worthwhile. MSC has a "Get it the next day" special going on as a free shipping upgrade so I should have the new toys, I mean tools tomorrow. :thumbsup:

I also ordered this tool from Wholesale Tools for $3.75. Don't know if it will be useful but for that price I didn't have much to lose.

http://www.wttool.com/index/page/pr...RC+Left+Hand+Side+Cutting+Carbide+Tool+Holder

Ed


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## metalbutcher (May 1, 2011)

I found this note in the Kennametal catalog for the threading tool:

*NOTE: Toolholders with primary shank sizes larger than 1/2-inch or 12mm
are supplied with clamp and insert screw. Secure the insert with either
the clamp or insert screw. Do not use both.

Anyone know if there is an advantage of using one method to secure the insert over the other method? I'm assuming there must be otherwise I see no reason for supplying both methods when only one method at a time is recommended, but I could certainly be wrong. oo:

Thanks,
Ed


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## wquiles (May 1, 2011)

metalbutcher said:


> I found this note in the Kennametal catalog for the threading tool:
> 
> *NOTE: Toolholders with primary shank sizes larger than 1/2-inch or 12mm
> are supplied with clamp and insert screw. Secure the insert with either
> ...



Maybe wrong, but when given the choice, I always use both.


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## metalbutcher (May 1, 2011)

I sent Kennametal an email requesting information on using the screw versus the clamp and asked what the repercussions would be if one used both at the same time. I'll post their answer when I get it.

By the way Will, what camera and lens do you have that you can get such clear closeups?

Thanks,
Ed


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## wquiles (May 1, 2011)

I use a Fujifilm FinePix S100FS, normally with the lens set at 50mm, and in MACRO mode for the basic pictures, and lots of illumination. The close-ups are cropped to 800x600 (max. size allowed in the forum) from the full size image (3840 x 2880), using IrfanView.

Will


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## metalbutcher (May 1, 2011)

wquiles said:


> I use a Fujifilm FinePix S100FS, normally with the lens set at 50mm, and in MACRO mode for the basic pictures, and lots of illumination. The close-ups are cropped to 800x600 (max. size allowed in the forum) from the full size image (3840 x 2880), using IrfanView.
> 
> Will


I suspected a macro lens might be the key. I have a Canon T2i and am looking at a 60mm macro so it's good to see what I can expect from that setup.

Thanks,
Ed


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## onetrickpony (May 1, 2011)

precisionworks said:


> Those are called "fishtail gages" or "center gages". Use the very small side V to make sure your 60° threading tool is exactly 90° to the work. Starrett makes a nice one for under $20 (C391)


 
I grew up in the next town over from Fitchburg Ma. I wonder if Sawyer is still around? Time for Google.


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## onetrickpony (May 1, 2011)

onetrickpony said:


> I grew up in the next town over from Fitchburg Ma. I wonder if Sawyer is still around? Time for Google.


 
Guess not, 1915 was their end, according to the Davidson Museum. Cool old tools are cool, you should take good care of that little gage.


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## metalbutcher (May 2, 2011)

metalbutcher said:


> I sent Kennametal an email requesting information on using the screw versus the clamp and asked what the repercussions would be if one used both at the same time. I'll post their answer when I get it.
> Ed


I got a reply back from Kennametal.

"*It is preference only. some think the top clamp assures a better clamp than just the screw. Others like change out of srew only. The use of both can not be done due to the screw will be in the way of the top clamp. the use of one over the other has no documented difference in our testing. if you use both you may push the insert forward out of the pocket seat.

Best Regards,* *

Daniel L. Dunmire, CMP* *
Customer Applications Support Analyst*"

Ed


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## wquiles (May 2, 2011)

metalbutcher said:


> I got a reply back from Kennametal.
> 
> "*It is preference only. some think the top clamp assures a better clamp than just the screw. Others like change out of srew only. The use of both can not be done due to the screw will be in the way of the top clamp. the use of one over the other has no documented difference in our testing. if you use both you may push the insert forward out of the pocket seat.
> 
> ...


 
Good to know what they think about it, but I will of course continue to use both since it works well for me


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## metalbutcher (May 6, 2011)

Well, either the tool has a defect or the insert is defective because when I put the insert on the tool the hole in the insert doesn't come close to centering over the threaded hole in the tool. I only bought one insert so it's hard to say what's at fault here. I guess I'll have to call MSC Monday and see what they have to say. I'm rather disappointed. I went with Kennametal to avoid such problems.:shrug:

Ed


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## wquiles (May 6, 2011)

metalbutcher said:


> Well, either the tool has a defect or the insert is defective because when I put the insert on the tool the hole in the insert doesn't come close to centering over the threaded hole in the tool. I only bought one insert so it's hard to say what's at fault here. I guess I'll have to call MSC Monday and see what they have to say. I'm rather disappointed. I went with Kennametal to avoid such problems.:shrug:
> 
> Ed


 
Maybe the sent you the wrong insert. What were the exact part numbers of the tool and threaded inserts that you got?

Will


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## metalbutcher (May 6, 2011)

wquiles said:


> Maybe the sent you the wrong insert. What were the exact part numbers of the tool and threaded inserts that you got?
> 
> Will


Will,

That's a good point. I got the exact same tool that you showed in a previous post. LSASR-103 LT threading toolholder.
The insert I ordered was LT16ERAG60CB KC5025. Since I ordered only one insert the box it came in is not marked but it looks exactly like the one you showed in your previous post and it fits in the insert pocket of the tool, it just doesn't line up with the threaded hole very well. The anvil seems to be off a little also but when I put the screw in the anvil seems to tighten up into the pocket OK. Not so the insert.

Ed


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## darkzero (May 6, 2011)

metalbutcher said:


> Well, either the tool has a defect or the insert is defective because when I put the insert on the tool the hole in the insert doesn't come close to centering over the threaded hole in the tool. I only bought one insert so it's hard to say what's at fault here. I guess I'll have to call MSC Monday and see what they have to say. I'm rather disappointed. I went with Kennametal to avoid such problems.:shrug:
> 
> Ed


 
I could be wrong but I doubt it's the insert. Does your tool holder use an anvil? Since it's 3/8 IC I'm assuming it does. Are you able to thread in the screw with the insert in place at all? 

Not that it matters but I use Carmex holders. Depending on which anvil you are using, the rake of different anvils will tilt the insert making it seem like the insert screw might be crooked. This is just what I speculate since I've only used one type of anvil so I'm not sure how off the screw may appear with different degrees.


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## metalbutcher (May 7, 2011)

darkzero said:


> Are you able to thread in the screw with the insert in place at all?



Per my previous post, yes, I can screw the anvil in even though it appears to be just slightly off center. The screw does snug the anvil up against the pocket allowing the screw to tighten all the way. Not so with the insert in place. I'll try to take some pictures of the problem and post them. I'm not sure my lens will allow me to get close enough for any detail but I'll give it a shot.

Ed


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## metalbutcher (May 7, 2011)

I haven't figured out how to post pictures so I'm not able to show you the problem with the screw not tightening down on the insert.

Ed


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## wquiles (May 7, 2011)

metalbutcher said:


> I haven't figured out how to post pictures so I'm not able to show you the problem with the screw not tightening down on the insert.
> 
> Ed


 
Send me the pictures to my email (in my signature), and I will host/post them for you 

Will


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## metalbutcher (May 7, 2011)

Will,

Thanks! I'll send them to you right now.

Ed


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## wquiles (May 7, 2011)

metalbutcher said:


> Will,
> 
> Thanks! I'll send them to you right now.
> 
> Ed



Here you go;


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## wquiles (May 7, 2011)

I am still trying to figure out what might be wrong, but while looking for additional pictures on threading stuff/tools, I got a few pictures that show the full profile insert vs. the universal 60 deg insert (partial profile). The insert in the tool is the full profile 20 TPI insert. The insert on the tool is the partial profile insert:
















Will


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## Codiak (May 12, 2011)

Not as nice as Wills... but they work... the max height is 47.5mm and the bottom out at 46.0 @ 20 TPI.
Material is 6061 T6....


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## wquiles (May 12, 2011)

Definitely a notable improvement - you will get better and better as you do more and more threads 

Threading is about having the right tools, the right setup, about going slowly (far too easy to mess up since you have to make multiple pases), and of course lots of practice.

Will


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## Codiak (May 12, 2011)

Firstly, thanks for telling me my first threads looked like crap when you did. It made me really look at what I was doing.

Now can you define "slowly".... are you referring the FPS rate, the depth of cut, or "measuring twice" and checklist validation ;-)


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## wquiles (May 12, 2011)

Codiak said:


> Firstly, thanks for telling me my first threads looked like crap when you did. It made me really look at what I was doing.
> 
> Now can you define "slowly".... are you referring the FPS rate, the depth of cut, or "measuring twice" and checklist validation ;-)


 
I think I was a little bit more "gentler" with my feedback, but you are welcome 

Slowly means mostly how much you advance for each cut. For my Mag re-threads (and remember that I cut straight at zero degrees!) I usually do like 0.010" on the first pass, then 0.007", then 0.005", then 0.002", another pass at 0.0015", and then a few more passes at 0.001" until I get the fit I want (each tailcap is slightly different - just natural with mass produced parts), and I typically do a pass at 0.000" which does the final cleaning/polishing job. I then use a scotch pad to lightly polish the top of the threads a little, and you end up with this:


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## Codiak (May 12, 2011)

Just as a test I did my female threads at 90 degrees and adjusted the depth accordingly.
I then bored the lip to match the top my O-ring groove at 47.5mm... 
Glad I tested this, while the threads match nicely, my O-Rings need work... IE groove needs to be twice as deep! and I'll need smaller ones


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## precisionworks (May 13, 2011)

Are you using lots of good lubricant, like TapMagic? You cannot use too much.

Is the tool tip at dead center, plus of minus a couple of thou?

Is the threading bar set for minimum projection from the block ... if not, move it back as far as possible.

Is the insert new/sharp or is the very tip broken or dulled?

FWIW, straight infeed (radial feed) puts the most pressure on the insert and is more likely to cause tearing than feeding the compound at 29.5°.






The most used method is modified flank infeed:


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## precisionworks (May 14, 2011)

Here's a HSS thread form tool that was popular during the 1940's and 1950's (pre carbide days for most shops). Capable of cutting a big thread, like 1"x8tpi, as well as pretty small ones. Sharpening is fast since only the top face is ground. As it wears & is resharpened, it is rotated clockwise to keep the top face level.


























Remember, lots of lard oil :devil:


One advantage to threading inserts is the molded in (or ground in) top rake, which reduces the force needed to shear the material.


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## wquiles (May 14, 2011)

I just check my external threading tool and sure enough, it is like yours in terms of the top rake:


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## wquiles (May 14, 2011)

Another tip that I use for threading that I learned here in the forum is to use a "plain" dial gauge to help me determine how far I have to go while doing "blind" internal threading. Even though I have a DRO, I find the analog needle easier to read when moving (relatively) fast:


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## precisionworks (May 14, 2011)

> Does that partial profile insert require an anvil for certain TPIs? I'm not real clear on when an anvil is required and the Kennametal web site does not explain it very wel


Like Will, I run the standard -1.5° anvil ... but I found this on PM while looking at the threading tool I use:



> get yourself a chart or electronic version of an anvil-to-lead angle calculator. the holder's standard -1.5º anvil works for a great number of common threads, but you'll eventually want to get anvils of -2.5º, -3.5º and -4.5º for threads with higher lead angles. This cures any rubbing and/or chatter problems.



Here's one from Kenna: http://www.kennametal.com/calculator/thread_cutting_lead_angle.jhtml


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## precisionworks (May 14, 2011)

While looking for another shim calculator, I found a great (short) summary on threading (source doc at http://www.alpenazonta.org/general/modern-threading-technology )

 [FONT=&quot]Threading operations are much more demanding than straight forward turning operations. Cutting forces are in general higher in threading and the cutting nose radius of the insert smaller and thus weaker.

Comparing the feed rate for turning and threading, we see that in threading, the feed rate must correspond exactly to the pitch of the thread. In the case of an 8 TPI thread, the tool must travel at a feed rate of 0.125 inch/revolution. The nose radius of the threading insert is typically 0.015" . In the case of turning, the normal feed rate is 0.012 inch/ revolution with a standard radius of 0.032" . From this example we see that threading feed rates are typically 10 times greater than turning. Correspondingly, the cutting forces at the tip of the threading insert can be anywhere from 100 to 1000 times greater than those for straight turning operations. Thus the nose radius of a threading insert plays a vital role in threading and its dimension is strictly limited by the allowable radius at the root of the thread form as defined in the relevant standard. Unlike turning where the material can be sheared, if, in the case of threading, material is “pushed” then thread distortion will be occur.

Further, since the thread is formed by carrying out a number of passes over the length of the thread, the lead screw of the cross slide is working excessively hard, stopping and starting, moving forwards and backwards and this factor alone results in a limitation in optimization potential.

Partial Profile versus Full Profile Inserts

Partial profile inserts, sometimes referred to as “non topping” inserts cut the thread groove without topping or cresting the thread. These inserts allow production of a wide range of threads, however the nose radius of the insert ( the most vulnerable part of the insert) must be small enough to produce the smallest pitch. The depth of thread is also affected by the small nose radius. For example for a 8 TPI thread, a partial profile insert requires a thread depth of 0.108″ while the same thread with a full profile insert will be no deeper than the specified 0.81″. Thus a stronger thread is produced with a full profile insert and further, up to four less passes in producing the thread.

Multi Tooth Inserts

Multi tooth inserts are designed with a number of teeth so that each one cuts deeper into the thread groove than the previous tooth. Thus the number of passes required to produce a thread can be reduced by up to 80%. The tool life of these inserts is considerably longer than single point inserts since the final tooth is only machining a half or a third of the metal removal of a given thread.

These inserts obviously can give a big push to improve productivity, however, due to the higher cutting forces they are not recommended for thin walled parts as chatter can result. The design of the workpiece should have a sufficient amount of thread relief or run out to allow all the teeth to exit the cut.

Infeed Per Pass

The depth of cut or infeed per pass is critical in threading because each successive pass engages a larger portion of the cutting edge than the preceding pass. If a constant infeed per pass is defined, forces and metal removal rates increase dramatically on each pass.

Producing a 60° thread form using a constant 0.010″ infeed per pass will result in the second pass removing three times the amount of metal as the first pass. For each succeeding pass the amount of metal removed grows exponentially. Thus the pressure on the nose radius increases accordingly. The depth of cut should reduced on each pass in order to achieve more realistic cutting forces.

Infeed Methods

a) Radial – not recommended for general use

While, controversially, this method is probably the most common method of producing threads, it is the least recommended. Since the tool is fed radially (perpendicularly to the workpiece centerline) metal is removed from both sides of the thread flanks, giving a V shaped chip. This form of chip is difficult to break this chip flow can be a problem. Further, since both sides of the insert nose are subject to high heat and pressure, tool life will generally be shorter than other infeed methods.

b) Flank Infeed – generally not recommended

In this method of infeed, the chip formed is similar to that produced in conventional turning and is easier to form and guide away from the cutting edge, providing better heat dissipation. With this infeed however, the direction of infeed is parallel to one of the thread flanks (30° ) and the trailing edge of the insert does not cut only rubs along the flank causing burnishing of the thread resulting in poor surface finish and, maybe, chatter.

c) Modified Flank Infeed – highly recommended

This method is similar to the flank infeed except that the infeed angle is somewhat less than the 30°. This gives the advantages of the flank infeed method while eliminating the problems of the training edge of the insert. A 29.5° infeed angle will normally produce the best results but in practice an infeed angle of between 25° and 29.5° are generally acceptable.

d) Alternating Flank Infeed – not recommended

This method utilizes both flanks of the insert to form the thread and gives longer tool life since both sides of the insert nose are used. In reality, this method can result in chip flow problems, which can affect surface finish and tool life. This method is usually used for very large pitches and such forms as ACME, TRAPEZE etc.

Clearance Angle Compensation

The ability to precisely tilt the insert in the direction of cut by changing the helix angle is probably one of the most powerful features of the laydown system.

This feature gives a higher quality thread because the insert will not rub against the flank of then thread form and also give a longer tool life since the cutting forces are evenly distributed over the full length of the cutting edge.

In the diagram above, the cutting edge of the insert on the left is parallel to the centerline of the workpiece. Note that the clearance angles under the leading edge and the trailing edge of the insert are not equal. In the case of many thread forms, particularly coarser pitches, this inequality can cause the flank to rub against the side of then insert.

With the laydown anvil system, (a helix angle chart appears in most catalogs allowing ease of choice of the correct shim to be used for the application under consideration) the proper anvil will tilt the cutting edge of the insert (in the direction of feed) in a plane perpendicular to the helix angle of the thread. The clearance angles beneath the insert’s leading and trailing edges will be equivalent. This ensures that the insert will rub on the thread flanks and edge wear will develop uniformly.



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## precisionworks (May 15, 2011)

I'm working on a wood turning chisel that uses round RCMT inserts. The shaft is 24" long, made of 4140HT that's 1/2" diameter. One end is threaded as the shaft will go all the way through the wooden handle ... here's a close up of the 1/2x13 thread, cut with the Iscar ER16 coated insert that Curt sells.






Spindle speed was 250 rpm, or 32 sfpm. Finish shows some roughness due to the very low sfpm (and the 10x magnification). As much as I'd like to run this at 500 rpm, that's too fast for me (threading to a shoulder).


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## precisionworks (May 17, 2011)

The photo in the last post made me take a look at the machine & my setup, as 4140HT normally comes out better than it did. Snugged up the half nut gibs, tightened the compound gibs, checked out everything possible ... and increased the spindle speed to 500 rpm :thinking:

500 doesn't sound like a lot, but it demands fast reflexes when trying to land the tool tip in a narrow runout groove.

IMO, much nicer looking (please disregard the shiny tops, as I filed & polished the crests because they were rough). At least the working part of the thread, the shoulders, are acceptably smooth.


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## Codiak (May 17, 2011)

Those look nice!


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## precisionworks (May 17, 2011)

TYVM 

I sent those photos to my boss at the tool & die shop, who asked what the rpm was. When I said 500, he replied "Are you ******** nuts?!" I wouldn't recommend using that speed unless you've had a good nights sleep, a few cups of coffee, and no distractions. _It really is an invitation to disaster_, all kidding aside.

I believe the correct shim would give a better surface finish at lower rpm. Maybe Curt will speak up, but the tables I've seen indicate something around 4°, where the shim I used is 1.5°. I'd really rather thread at 250 rpm or less.


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## gadget_lover (May 17, 2011)

Those are great photos Barry. I looked at the first set and thought that the flanks looked like they were rubbed by the sides of the tool. The concentric lines are very regular. They reminded me of the stereotypical 'tool marks' you see on CSI all the time. I think the shim IS the answer.


Daniel


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## Name goes here (May 18, 2011)

I don't think it's the shim. The first batch of inserts I stocked were Iscar 16ERM AG60 and they were molded, which is the problem. The edges aren't as sharp as a precision ground insert and as they say "therein lies the rub". Literally. 

Too much tool pressure with the molded inserts so now the only thing I handle is the TiALN coated ground version. A couple bucks more but well worth it.


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## precisionworks (May 18, 2011)

I'd sure be happy to test one of those in 4140HT ... if you can cut nice threads in that (or even worse, 304ss) the rest is easy.


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## precisionworks (May 18, 2011)

> I don't think it's the shim.


I know nearly nothing about the shim, but the online calculators show 2.8° for a 1/2x13 thread form ... would that not give better clearance with less rubbing?


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