Thread cutting - Can not get correct thread profile

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wannasteam

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Hello all

I'm having a go at cutting my first threads in preparation for making the worm gears etc for the PMR Drill press.

First thing was to chuck a piece of brass and practice and make sure I got the gist of the process.

I have used this as my reference http://www.metalartspress.com/60%20degree%20threads.pdf and have got all the preparation re thread pitch, 60 inc angle on the cutting tool, 30 deg offset of the compound etc.

My technique was not as good as thought as I am getting an asymmetrical (for a lack of a better description) thread profile. See attachment.

The other attachment illustrates the configuration of the cross slide and compound.

On a side note, I still don't fully understand the reason for the 30 deg offset of the compound. I just did it.

Any comments from the collective wisdom would be greatly appreciated.

Regards

Joe Pestana
Perth W Australia

DSC00465.JPG


DSC00471.JPG
 
Are you using the compond handwheel or the crossfeed to deepen the thread cut?
When you offset the compond to cut threads you advance the compond.
 
As Dave noted you want the compound to feed parallel to the thread face. The angle your compound is set is incorrect is all.
 
To restate what DP said, your compound should be 30° from the center line of the crossfeed screw. Yours looks to be set 30° from the bed. Feed from the compound, not the cross slide.
 
You need to use the fishtail gauge to both ensure that the tool is ground to 60 degrees and that it is perpendicular to the work.

From the photo is looks as if the side of the tool is perpendicular to the chuck, but it's very hard to make the 60 degree grind exactly centered.
 
Your cutting tool looks (roughly) OK - just align it better, so that it looks exactly/precisely like so:

ISO-metric_thread.jpg


BTW - imagine what would happen, if you were to rotate the tool 30 deg clockwise...
It's the topslide that needs to be rotated :)
As Dave says, get the angle right:
img135.jpg


It looks like your 30 deg is measured wrongly?

The reason for offsetting the compound is to do with feeding of the tool.
It is perfectly OK not to offset it and feed the tool straight in with your crosslide - the tool will cut equal amounts of metal from both leading and trailing edge.
A lot easier though is to set the compound at half of the included angle and feed the tool with the topslide. This way your tool is only cutting on the leading edge, but if you do not set the tool as in the sketch, it will still cutting both leading and trailing edge, causing drunken profile that you are seeing
There are variants of these two methods, but let's leave for another day ;)

Chris
 
Joe,

Looks like you have your compound set at 60° rather than 30° and that's why your threads are mis-shaped. Bring the compound around toward the headstock 30° more....In actuality you should infeed with the compound set at 29° so that the trailing edge of the thread is cut slightly with each infeed but the leading edge of the tool is doing most of the cutting. RH outside threads cut toward the headstock

Your first photo is the one that shows that the compound is too far around. In order to set the tip of the tool angles correctly, you need a thread gauge - looks like a flat dart with 60° outside angles on one end and 60° inside angles on the other and stamped with the full depth infeed on each side for various thread pitches.

Was delayed posting so will send this anyhow for another bit of info.....

Bill C.
 
A prime example of using that CURSED explanation of 30 deg. for the compound.
That lathe is marked just like out Clausing/ Metosa at school . I wish all the
so-called experts would STOP saying 30 degrees. and describe it. It seems some
people just cant seem to believe any lathe can have the compound angle marked
anyway but the way it is on their lathe. :-(
...lew...
 
Lew Hartswick said:
A prime example of using that CURSED explanation of 30 deg. for the compound.
That lathe is marked just like out Clausing/ Metosa at school . I wish all the
so-called experts would STOP saying 30 degrees. and describe it. It seems some
people just cant seem to believe any lathe can have the compound angle marked
anyway but the way it is on their lathe. :-(
...lew...
I am sorry but what are you saying,
 
I think he means that the compound is marked 0-360 counter-clockwise, so that turning it clockwise is the only way to get the dial to read 30.
 
kvom said:
I think he means that the compound is marked 0-360 counter-clockwise, so that turning it clockwise is the only way to get the dial to read 30.

The way my lathe's compound is marked is if the the compound is 90 deg from the work ( right angle) it reads zero, if you turn it counter-clockwise it goes from zero to 90 deg and the same if you go clockwise starting at zero, 90 deg from the work ( right angle).
 
Hello all

Thanks for all the replies.

To answer your questions:

Yes I use the compound to set the depth of cut. I back the tool away from the work using the cross slide then wind it back in to zero on the collar.

I've checked the angle of the cutting tool with a gauge and it is as close to 60 inc as dammit is to swearing.

Yes the tool is parallel to the chuck face.

Consensus seems that the compound should be set at 30 degs to the cross slide axis rather than 30 degs to the spindle axis. It appears that Angle A on Chris' second image is the key to this. I've got mine set to 60 degs and it should be 30. :Doh:

Have I got that right?

I'm at work at the moment but will adjust this as soon as I get home.

When I get this nutted out, I'll have some more questions about fixturing small pieces on the lathe... for another day.

Regards and thanks again for the info.

Joe



 
With the slide set at 30° (or 60° depending which way you measure it) - you are advancing the depth of cut parralell to the trailing edge - this way you cut with the leading edge only.

As pointed out earlier - you don't have to do this and in practice I seldom do.

However when you plunge straight in the tool is going to cut on both edges - this greatly increaces the force on the tool and the two chip streams "collide" with each other leading to tearing and poor surface finish. Chatter is also going to be a lot more problematic.

Typically I don't bother - but if is see signs of chatter or poor surface finish (depending on the material etc. etc.) then I swing the compound slide and do it "properly".

An earlier thread pointed out that you should set it to slightly less so that the trailing edge does some work and ensures that the opposite does not happen - ie if you are "overset" the trailing edge flank is going to become a series of steps.

Again this can also be overcome by using your cross slide for the final "tickling" finishing passes - but be carefull - chatter on the last pass can make a mess of everything.

Hope this helps.

Ken
 
Hi all

woohoo1 Success! Amazing how easy things are when you know how ;D


Ken I said:
With the slide set at 30° (or 60° depending which way you measure it) - you are advancing the depth of cut parralell to the trailing edge - this way you cut with the leading edge only.

<snip>

Hope this helps.

Ken

Ken This makes sense of it all. Thanks

I now have another question for you:

I need to make a worm gear (see attachments) with tooth profile as shown. The included angle is 40 degs. Based on what I've learned so far, I will need to offset the compound by 20 degs.

Is this correct?

Thanks to all :bow:

Cheers

Joe



DSC00475.JPG


DSC00476.JPG


DSC00477.JPG
 
Two ways to single point thread


Untitled-3.jpg


Hope this helps.

Best Regards
Bob
 
That's correct.

Another way of moving the slide is to make handweel adjustments for the 30° (whatever) so if you feed the tool in 0.2mm then you would offset it forwards by 0.1386 (from trig).

Why bother to do that when you can swing the compound slide ? - well when cutting a worm you can use a pointed tool with a very small flat tip and once you have advanced to depth you can offset the compound slide (in feed rate increments) to "widen" the thread to the correct worm profile - this gets you away from the problems of cutting on more than one edge that I mentioned earlier.

You can also cut to depth then back up slightly to improve the finish on the trailing edge with a light cut and then work your way forward with the leading edge until you get the right profile.

The flank finish on a worm needs to be really good if its not to "mill" away at the worm gear.

This process also allows you to use the same form tool on different pitches of worm and you are not reliant on how accurate you ground the tip flat.

Regards,
Ken
 
For the worm; you can purchase very accurately ground lathe tools for cutting that worm...if you don't have a tool grinder....off-hand grinding one is a project in itself!

BillC
 

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