Thread: Threading...Aaarrghh!!!

I'd suggest your compound is not set at the correct angle, most likely set at 60 degrees instead of 30 degrees. The compound can be set anywhere between +/- 30 degrees from perpendicular to the work, and the thread will cut ok. The closer it gets to 90 degrees, the more it's a plunge cut and will cut evenly off both flanks at the same time. Conversely, the closer to 30 degrees, the more it will cut off the leading flank. We don't normally set negative angles either, as things get on the way. Don't get too hung up on angles, nevertheless it is important that the angle is less than the angle of the thread.

Hi Ben, Looks to me like your compound angle is wrong? LH flank looks correct but the RH flank is not. To quote RC "Just plunge the bastard!" Ew Pete, here we go again.....

While you can cut threads with the compound set over, the tool must still be perpendicular to the work. Looks like you might have the tool in line with the compound instead.

Michael

I have had this exact issue. I had my compound set to 30 degrees and it came out looking like that. I will try Pete F's tip of 60 degrees. I used a fish tail to set up the tool post and single point insert at the appropriate angle. It seems that as one feeds with the compound it ends up going this way. If I feed the cross slide, I get a really nice thread!

Originally Posted by variant22

I have had this exact issue. I had my compound set to 30 degrees and it came out looking like that. I will try Pete F's tip of 60 degrees. I used a fish tail to set up the tool post and single point insert at the appropriate angle. It seems that as one feeds with the compound it ends up going this way. If I feed the cross slide, I get a really nice thread!

+1 Just plunge the bastard.

That's what I do and it turns out 100% every time.

I've been doing a heap of internal and external threading lately (up to 3 mm) and every one turned out fine.

Just forget about that 30 degree BS and plunge it.

Rob

Cut by using the cross slide if you wish to but dont forget to advance the compound slightly as well.

If off setting the compound, set it to just under the 60 degree mark on the cross slide.

Set the tool square to the job.

Adjust compound so as you have enough movement to cut the thread,zero it.

Touch the tip of the tool on your job using the cross slide, zero it.

Wind the tip clear of the job using the cross slide and return to the start of the thread.

Return the cross slide to zero.

Advance the compound your desired depth of cut for the pass.

After the pass wind the cross slide out return to start,zero cross slide advance compound and start again.

When you get down to your last couple of passes return the cross slide about .02 deeper than zero.

I would try an OD of 15.95 and total depth about .82mm.

Originally Posted by pipeclay

,zero cross slide advance compound and start again.

I would try an OD of 15.95 and total depth about .82mm.

There's something wrong with your depth calculation there, the actual single depth of thread is around 1.28mm, if using angular approach a little Trig. will give the depth at 30 degrees.

You are backing out the cross slide before reversing?

If you don't backlash will certainly screw up your screw profile.

Ray

Originally Posted by variant22

I have had this exact issue. I had my compound set to 30 degrees and it came out looking like that. I will try Pete F's tip of 60 degrees. I used a fish tail to set up the tool post and single point insert at the appropriate angle. It seems that as one feeds with the compound it ends up going this way. If I feed the cross slide, I get a really nice thread!

Whoa! No I didn't say to set the compound the 60 degrees. It may be marked that way on some particular lathes, but the angle is in fact 30 degrees (off perpendicular to work).

I'm not sure how some people are cutting threads, but some of the comments are with all due respect a little puzzling. Yes you can plunge threads, you can also drive tacks with a sledge hammer too. Most people find on lighter lathes the results are better by offsetting the compound, and I'd concur with that. I started by plunging, but once I got my head around having the compound offset, I find it gives a better finish on my lightweight lathe, is faster, and is no more difficult than plunging.

To set it up. As mentioned, some lathes mark their compound graduations so if the compound reads 90 degrees the top slide is parallel to the cross slide, others will say zero degree. It doesn't matter, for simplicity start with both slides going the same way. Now rotate the compound about 25 degrees anti-clockwise. The precise amount doesn't matter, just so long as it's less than the thread angle (ie either 27.5 for Whitworth or 30 degrees for most other common V threads). I won't complicate things with other thread profiles, but I've cut most, and the principle works exactly the same, just the angles change. Anyway, at roughly 25 degrees clockwise, your compound is now set at the correct angle.

Ensure your tool is at exactly right angles to the work. I grind my tools so they will be, but if using other than a QCTP that's set at right angles, you may need to confirm the tool is set correctly with a "Fish gauge". Assuming the work is already at the correct major diameter and the gearbox set, touch off on the work and set the cross feed dial to zero. Also set the compound slide dial to zero. Place a dial indicator on the lathes carriage but touching off the tool post so it's reading the depth of cut. Wind the top slide (ie compound slide in the thread depth by reading it off the dial indicator, and I like to then set that to zero at that point and then wind the compound back to it's original zero point.

To thread, make the first pass with the tool just touching and confirm the thread pitch. At the end of the cut, disengage the half nuts, back off the cross slide, but do NOT touch the top slide. Wind the carriage back and reset the cross slide to the zero point. Put on some more cut on the top slide by looking at your dial indicator. When your lucky number comes up on the thread indicator, engage the half-nuts and take another cut. At the end, disengage the half nuts, back off the cross slide, but do NOT touch the top slide. Wind the carriage back etc etc etc. Keep going until the dial indicator reads zero. Once it's at zero you're at full depth and the thread should be finished.

It sounds much more complicated than it is in reality. Having the dial indicator there makes the whole process almost idiot proof and means the only dial you need to read is to reset zero each time on the cross slide. An absolute no-brainer and makes the process very quick too.

Hopefully that explanation makes sense.

Pete

Originally Posted by Pete F

Whoa! No I didn't say to set the compound the 60 degrees. It may be marked that way on some particular lathes, but the angle is in fact 30 degrees (off perpendicular to work).

I'm not sure how some people are cutting threads, but some of the comments are with all due respect a little puzzling. Yes you can plunge threads, you can also drive tacks with a sledge hammer too. Most people find on lighter lathes the results are better by offsetting the compound, and I'd concur with that. I started by plunging, but once I got my head around having the compound offset, I find it gives a better finish on my lightweight lathe, is faster, and is no more difficult than plunging.

To set it up. As mentioned, some lathes mark their compound graduations so if the compound reads 90 degrees the top slide is parallel to the cross slide, others will say zero degree. It doesn't matter, for simplicity start with both slides going the same way. Now rotate the compound about 25 degrees anti-clockwise. The precise amount doesn't matter, just so long as it's less than the thread angle (ie either 27.5 for Whitworth or 30 degrees for most other common V threads). I won't complicate things with other thread profiles, but I've cut most, and the principle works exactly the same, just the angles change. Anyway, at roughly 25 degrees clockwise, your compound is now set at the correct angle.

Ensure your tool is at exactly right angles to the work. I grind my tools so they will be, but if using other than a QCTP that's set at right angles, you may need to confirm the tool is set correctly with a "Fish gauge". Assuming the work is already at the correct major diameter and the gearbox set, touch off on the work and set the cross feed dial to zero. Also set the compound slide dial to zero. Place a dial indicator on the lathes carriage but touching off the tool post so it's reading the depth of cut. Wind the top slide (ie compound slide in the thread depth by reading it off the dial indicator, and I like to then set that to zero at that point and then wind the compound back to it's original zero point.

To thread, make the first pass with the tool just touching and confirm the thread pitch. At the end of the cut, disengage the half nuts, back off the cross slide, but do NOT touch the top slide. Wind the carriage back and reset the cross slide to the zero point. Put on some more cut on the top slide by looking at your dial indicator. When your lucky number comes up on the thread indicator, engage the half-nuts and take another cut. At the end, disengage the half nuts, back off the cross slide, but do NOT touch the top slide. Wind the carriage back etc etc etc. Keep going until the dial indicator reads zero. Once it's at zero you're at full depth and the thread should be finished.

It sounds much more complicated than it is in reality. Having the dial indicator there makes the whole process almost idiot proof and means the only dial you need to read is to reset zero each time on the cross slide. An absolute no-brainer and makes the process very quick too.

Hopefully that explanation makes sense.

Pete

This is almost exactly how I do it, except I mostly reverse and don't disengage the half nuts, works spot on every time.

Hey I'm glad my explanation made sense, I must admit I was a bit worried it wouldn't!

Yes you can also reverse. Some lathes in fact do this by default (Hardinge for example), others by necessity if using metric or imperial transposing gears.

Nothing especially revolutionary about the way I described it, and from what I've seen it's pretty much standard. Old Skool is to not use a dial indicator, but I don't see any point in not using one. These days pretty much everyone has multiple dial indicators and mag bases, so why not use them.

There's a lot of misinformation about threading out there, and that's a shame, as it's not at all difficult. A few things I've seen is the emphasis that the compound needs to be set to exactly 29.5 degrees (for a 60 degree thread). That's complete rubbish. Obviously you can set it to that angle and it will cut almost exclusively on the leading edge, but it's not necessary, and I have no idea how people think they're setting half degrees on their lathe's compound. Obviously it works, and it's a wive's tale that has become popularised with the internet to become a "fact" merely by repetition.

Another thing I've seen is the emphasis that the compound needs to be quickly retracted at the end of the cut. In fact if terminating into a runout groove the carriage stops once the half nuts are disengaged and the tool will just sit there in the groove doing nothing. You can go make a cup of tea with the lathe running and come back and retract it if you want, it won't make any difference. A tip in this regard however is to adjust the cross and top slide handles when touching off so they cross slide handle is vertical, that way at the end of the cut it can just be flicked down to retract it enough on most smaller threads to clear them when running back to the start. It makes the process a lot quicker, as you basically just flick it between vertically up and down to end and start each cut.

It is a regional thing. The 29.5 degrees topslide method is taught and used in North America. Since much of the machining books and internet information of the English speaking world is of American origin, it may appear this 29.5 degree method is the only "correct" method. This is different in Europe, where the other threading method whereby the topslide stays set parallel to the ways but is offset for every pass is mostly taught and used. In practice, both methods do exactly the same thing, eg cutting the thread on one shoulder only. Each method has some minor advantage and disadvantage. If one has learned threading with either one method, there is certainly no compelling reason to change either way.

It is a similar thing with threading dials. These are most useful and used on lathes having imperial leadscrews and cutting imperial threads. Much less useful and less used on lathes with a metric leadscrew, regardless if cutting imperial or metric threads.

Originally Posted by cba_melbourne

This is different in Europe, where the other threading method whereby the topslide stays set parallel to the ways but is offset for every pass is mostly taught and used. In practice, both methods do exactly the same thing, eg cutting the thread on one shoulder only.

Sorry could you run that one past me again? I couldn't follow that sentence at all.

Originally Posted by Ueee

To quote RC "Just plunge the bastard!"

I do not recall that being an accurate quote

I was doing a John Weldon when I posted that on that forum...