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Single point threading problem
- Thread starter No-ladder
- Start date
I've actually had really good luck threading drill rod (W-1) on my 7x14. I have only used HSS cutters. I also had much better luck using a hand crank rather than doing it under power. I usually take the first pass at about 5thou, then do a couple at 3, then 2, finish up at 1 thou, and with a couple spring passes when I get close. The thing that flashes the red light to me is that you have alternate passes where you are not cutting. That should never ever happen, something is moving. Do your dials slip? Maybe your toolpost is shifting? If you gibs are loose, I would definitely snug those up, with form tools, once you bind, it's all over.
I also recommend some cutting oil, especially on the last couple passes.
I also recommend some cutting oil, especially on the last couple passes.
I agree with all the above, so to consolidate the thoughts-
1 make sure the work can not flex away from the tool - use a centre in the tailstock end of the job
2 get the tool sharp you will do better with hss than carbide
3 get tool on centre height- too high and it wont cut and too low it will 'self feed' into the work
4 use coolant / lubricant, I prefer straight oil for 'drill rod'
5 get rid of any play by keeping top and cross slide jibs tight.
This should do it.
1 make sure the work can not flex away from the tool - use a centre in the tailstock end of the job
2 get the tool sharp you will do better with hss than carbide
3 get tool on centre height- too high and it wont cut and too low it will 'self feed' into the work
4 use coolant / lubricant, I prefer straight oil for 'drill rod'
5 get rid of any play by keeping top and cross slide jibs tight.
This should do it.
For threads that are too small to allow a center-drill, turn a 60º point on the end of the part, allowing in length for it to machined off later Then take a tap or end mill that has a center hole in the end of it (I use a cut off 1/4-20 tap, butt end) and put in a drill chuck mounted in the tailstock. Remember though, this is a dead center and needs lubrication and light pressure, but you can get a lot of support for small threads and turnings that way.
Dear No-ladder,
With you having an import machine and knowing that the import machines are not always made to the highest standards, I have a few more things for you to check on your machine, that may be causing threading problems.
When you turn threads and you come to the thread shoulder, you will turn out the cross slide quite quickly and at the same time you will be disengaging the leadscrew nut. This quick turning of the cross slide thread in the outwards direction may cause the relationship of the cross slide thread nut and the compound slide to change. Check that the fixing bolt/s between the compound slide and the compound slide screw nut have not come loose.
Also, on most modern lathes there is a shear pin situated at the driving gear of the gear train that drives the leadscrew. The shear pin is fitted just in case you run a tool into a chuck or worse. The shearpin is designed to shear, if something goes wrong. This shear pin may have sheared or is not fitting properly which will cause the relationship between the chuck spindle and leadscrew to change. If the shear pin shears, the leadscrew will normally stop turning altogether, but the nut securing the gear that has the shear pin in it may be tight enough to keep the leadscrew turning until it comes under load and slips slightly. There is normally also a key on the leadscrew drive gear, which may have been removed or was not replaced when gears were changed. So check the shear pin and or key that drive the leadscrew.
I've trashed many a thread before and have had to start again, so don't think that it only happens to you. Enjoy your threading. Geoff.
With you having an import machine and knowing that the import machines are not always made to the highest standards, I have a few more things for you to check on your machine, that may be causing threading problems.
When you turn threads and you come to the thread shoulder, you will turn out the cross slide quite quickly and at the same time you will be disengaging the leadscrew nut. This quick turning of the cross slide thread in the outwards direction may cause the relationship of the cross slide thread nut and the compound slide to change. Check that the fixing bolt/s between the compound slide and the compound slide screw nut have not come loose.
Also, on most modern lathes there is a shear pin situated at the driving gear of the gear train that drives the leadscrew. The shear pin is fitted just in case you run a tool into a chuck or worse. The shearpin is designed to shear, if something goes wrong. This shear pin may have sheared or is not fitting properly which will cause the relationship between the chuck spindle and leadscrew to change. If the shear pin shears, the leadscrew will normally stop turning altogether, but the nut securing the gear that has the shear pin in it may be tight enough to keep the leadscrew turning until it comes under load and slips slightly. There is normally also a key on the leadscrew drive gear, which may have been removed or was not replaced when gears were changed. So check the shear pin and or key that drive the leadscrew.
I've trashed many a thread before and have had to start again, so don't think that it only happens to you. Enjoy your threading. Geoff.
I am also having similar problems with threading.
I have checked the points as mentioned above and things improved but I am still having problems (maybe?)
I'm cutting 8mm metric threads so have set the change gears as 30-120-127-30 with the gear selector in 7.
According to the charts this should be giving me an 8mm x 1mm thread.
Doing all the usual things, not disengaging the lead screw, backing out and running in reverse, advancing the compound etc.
The result was an appalling thread. ie. looking at the V's it was like this \|\|\|
I use the fish tail gauge to set the bit at 90' to the shaft so decided to forgo advancing the compound and just use the cross slide backing out-reversing-advancing in a little more so the bit cuts on both sides at the same time.
This produced what appeared to be a reasonable result but was a bit too tight so I cut another pass that unfortunately cut a little too much so the thread is a little loose but still usable. (just)
(this is only practice before the real thing)
What I would like to know is how come advancing the 29.5 degree set compound produced such an awful thread but advancing straight in with the cross slide appeared (to my beginners eyes) produce a better thread.
Thanks
I have checked the points as mentioned above and things improved but I am still having problems (maybe?)
I'm cutting 8mm metric threads so have set the change gears as 30-120-127-30 with the gear selector in 7.
According to the charts this should be giving me an 8mm x 1mm thread.
Doing all the usual things, not disengaging the lead screw, backing out and running in reverse, advancing the compound etc.
The result was an appalling thread. ie. looking at the V's it was like this \|\|\|
I use the fish tail gauge to set the bit at 90' to the shaft so decided to forgo advancing the compound and just use the cross slide backing out-reversing-advancing in a little more so the bit cuts on both sides at the same time.
This produced what appeared to be a reasonable result but was a bit too tight so I cut another pass that unfortunately cut a little too much so the thread is a little loose but still usable. (just)
(this is only practice before the real thing)
What I would like to know is how come advancing the 29.5 degree set compound produced such an awful thread but advancing straight in with the cross slide appeared (to my beginners eyes) produce a better thread.
Thanks
To continue on with my previous thread, another area that can cause thread cutting on a lathe to trash, is on lathes with chucks that are threaded onto the spindle. If the spindle and chuck threads are not clean and tightened properly and snugly up onto the shoulder, this could cause mis-alignment between the spindle and chuck, and will trash your thread. Especially if there is dirt between the shoulder landings, which will cause the chuck to tighten on the spindle, as the cut loading increases.
The old man that taught me threading on the lathe showed me two methods. None of them use any form of slide stops. The first method was by setting your compound slide over by half the angle of the thread. (i.e.: 27.5 degrees for Imperial threads and 30 degrees for Metric threads or half the included angle of the thread being cut.) The compound slide is then used to advance the feed into the work, between each cut. The cross slide is only used to pull out and is set back to the zero mark, each time that a pass is made. The idea behind this method is to cause the cut to only be made on one side of the tool, for each pass. You don't want the tool to cut on both sides, as it will increase the load on the tool and could even cause the tool to fail, when taking heavy cuts.
The second method is by keeping the compound slide square or paralell with the work. Each time that a cut is made, the compound slide is advanced in the direction of cutting by one thousandths or by 0.01 mm The depth of feed into the work is controlled by the cross slide. This also causes the cut to only be made on one side of the tool for each pass.
My preferred method is the second method mentioned above, but try both ways and see what works best for you. There are many ways of skinning a cat! Enjoy, Geoff.
The old man that taught me threading on the lathe showed me two methods. None of them use any form of slide stops. The first method was by setting your compound slide over by half the angle of the thread. (i.e.: 27.5 degrees for Imperial threads and 30 degrees for Metric threads or half the included angle of the thread being cut.) The compound slide is then used to advance the feed into the work, between each cut. The cross slide is only used to pull out and is set back to the zero mark, each time that a pass is made. The idea behind this method is to cause the cut to only be made on one side of the tool, for each pass. You don't want the tool to cut on both sides, as it will increase the load on the tool and could even cause the tool to fail, when taking heavy cuts.
The second method is by keeping the compound slide square or paralell with the work. Each time that a cut is made, the compound slide is advanced in the direction of cutting by one thousandths or by 0.01 mm The depth of feed into the work is controlled by the cross slide. This also causes the cut to only be made on one side of the tool for each pass.
My preferred method is the second method mentioned above, but try both ways and see what works best for you. There are many ways of skinning a cat! Enjoy, Geoff.
I find not all lathes have the degree markings the same, so the easiest way to acomplish the correct half angle is to proceed as follows.
first set the compound so it is paraell with the cross feed ie. both cross feed and compound feed handles are on top of each other.
Next turn your compound 29.5 deg for metric and std threads or 27.5 deg for whitworth to the RIGHT.
Now set your toolpost & tool with a fishtail gauge so the tool is at the correct angle ( square to the item to be threaded).
Set your crossfeed & compund dials at zero after the scratch pass.
Use the compund to feed after each pass and use the cross to withdraw the tool at the end of the pass.
Do not reverse without withrawing tool to clear thread as the backlash will wipe out you thread.
Reverse lathe to the begining of thread allowing for backlash.
Return your crossfeed to zero position and increse the compound to the desired cut.
Continue till thread is complete.
first set the compound so it is paraell with the cross feed ie. both cross feed and compound feed handles are on top of each other.
Next turn your compound 29.5 deg for metric and std threads or 27.5 deg for whitworth to the RIGHT.
Now set your toolpost & tool with a fishtail gauge so the tool is at the correct angle ( square to the item to be threaded).
Set your crossfeed & compund dials at zero after the scratch pass.
Use the compund to feed after each pass and use the cross to withdraw the tool at the end of the pass.
Do not reverse without withrawing tool to clear thread as the backlash will wipe out you thread.
Reverse lathe to the begining of thread allowing for backlash.
Return your crossfeed to zero position and increse the compound to the desired cut.
Continue till thread is complete.
A couple of things may be going on with your project. 1) Your threading tool (which I hope is HSS) may not have enough releif below the cutting point. 2) Your tool is not at center of your work. Both of these could necessitate increased pressure to cut. If you are getting rubbing it may be 'work hardening' your work, causing more cutting pressure to be needed. Then suddenly it will start to cut BIG TIME. Best of luck.
We have a cheap metric lathe at work. I believe its called a select lathe. Anyway, when i was an apprentice(20 years ago) it was the only lathe available in our shop. I had to thread some 5/8 nc bolts further back. The old guys told me you couldnt thread on it, that the chase dial wouldnt repeat. Sure enough, the were correct. I also had to change the gears to enable it to cut 11 tpi. So in desperation i just started threading like it was a metric thread. Never disengaging half nut. I just backed out at end of cut and reversed lathe past beginning, fed in and took another pass. Problem solved. I never disengage on half nut when threading metric. We have a couple of more modern lathes that you probably could, but i learned on monarchs, clausing-colchesters, and leblondes. Lol still prefer the old monarch CW for threading.