So I bought a lathe....

Ever used an indexable carbide threading insert? At $25 or a bit better per three points, plus a tool holder, they are not inexpensive, but when a tip breaks, and you have the ability to install a new one, without losing your zeros... Sometimes they are a nice thing to have around.

Those abortions of brazed carbide so-called cutting tools, on the other hand, would be best used to plug any and all the body orifices of those that insist on foisting them onto the planet at large. Preferably after being heated to a temperature well beyond the brazing melt point.
They really need to be properly ground to be of much use, and, sadly, the folks most like to buy them, are those that do so to avoid having to grind tools in the first place.

I have picked up and resumed cutting enough threads (either from tool breakage, or from having removed the part from the lathe) to know that it should hold no fear, though threading alone seems to, for some.

Cheers
Trev
 
speaking of threading, I have cut several inch threads over the past wile, every one turning out great. I recently tried to cut some metric pitches, and the first pass is correct, but after that its just not being timed right, and its not following the previous pass. I am using a dial gage for my starting point, and engaging the half nut in the same place every time. I checked that my work hadn't slipped in the chuck, and that my tool was mounted solidly. I tried a few standard threads, which worked great again. Then swich back to metric... same problem.

What could I, or my lathe be doing wrong? I don't want to be stuck with only inch threading capability.
 
This one I can answer:dancingbanana:

You can only thread imperial threads with the indexer as long as you have a imperial lead screw,which in this case you do(you lucky bastard).If you want to cut metric threads ...NOW you have to keep the half nut engaged ALL the time...and reverse the lathe (with the half nut engaged) to a starting point and then take a cutting pass!!...I know this because I bought a lathe with a metric lead screw....I can cut metric to bet hell....imperial though,is alot slower..I dont realy care...I am in no mass production..just remember though..take your time..keep you cuts light...double ck you work!!!!


P.S......Spend the bucks.....Use carbide bits....you grind or inserts..much nicer cuts ,lasts WAY longer(for us beginners;)







Big JD-From the hills said:
speaking of threading, I have cut several inch threads over the past wile, every one turning out great. I recently tried to cut some metric pitches, and the first pass is correct, but after that its just not being timed right, and its not following the previous pass. I am using a dial gage for my starting point, and engaging the half nut in the same place every time. I checked that my work hadn't slipped in the chuck, and that my tool was mounted solidly. I tried a few standard threads, which worked great again. Then swich back to metric... same problem.

What could I, or my lathe be doing wrong? I don't want to be stuck with only inch threading capability.
Click to expand...
 
You obviously have the conversion gears that goes from the main shaft to the gear box. When cutting metric threads with an "inch" lead screw the half nuts must stay engaged all the time. You stop the lathe and the end of each pass, retract the threading tool and reverse the lathe to move the carriage back to the starting point.
You then advance the cutting tool the desired amount for the next pass and start the lathe in forward.
 
Ditto on keeping the half nuts engaged. Took me a while and much cursing to figure THAT one out.... :D

This means things can get very, very challenging if you're trying to cut your metric threads up to a shoulder that is big enough that you can't wheel the cross slide back quickly enough to avoid cutting into the shoulder. For those cases I like to use a tail stock die holder and just thread them by hand. For cases where the thread MUST be 100% axial but still run up to a shoulder you can cut the first few threads close to final depth using a single point and the lead screw but pull back on the cross slide early enough that you can clear the shoulder and shut down the motor...... 3 arms and hands come in handy for this little trick :D Actually it's not bad. You get the one hand on the cross slide wheel and the other on the power switch and hit 'em both together.

To make this little trick easier to recover from move your cross slide in close to the work and stop it on the zero or at least a major index point. That'll be your "home setting" to return to for the cross slide when doing the return trip in reverse. Do NOT let the tool stay at the cutting depth for the return or the slack in the screw and 1/2 nuts will cause a cut on the trip back. Then once set back to the "home setting" you can use the compound rest, set to 29 degrees of course, to advance the threading cut for the next pass.

For a thread up to a shoulder using the die holder you get it down to where it's close to the final size for the first few threads and then haul out your tailstock die holder. I've shown you one of mine in the picture below. In use the die holder slips onto the stub shown in the drill chuck of the tail stock. This stub fits into a reamed hole, for a nice slop free fit, of the die holder stub that is shown sticking out behind the die position. Then you screw it on and the existing threading ensures a good axial beginning for even the most critical applications. The die scrapes the last few thou off the first portion and performs the entire cut for the later portions. For threads done in the lathe that need to be decently axial but not critcally so just cutting the stub down to the nominal screw size and a few thou more is enough to let the die holder start and go on just fine. It's quick and does a decent job without the need for single point cutting. A simple project that is more than worth making for your own lathe. Keep the stub the same for all sizes and just make up new holders to suit each size range of dies from smallest to humungous.

dieholder1.jpg
 
Ok you have pointed out another error I have been doing, I have just been using my cross slide to advance the tool, and not my compound, so definitely not 29 deg. Yeah I have been meaning to make a die holding tool for the smaller stuff. I actualy was using a small lathe chuck, and hex dies on my craftsman 109. But I definitely need to cut metric barrel shank threads on the lathe. A question on your die holder, thats all one piece right(except the handles)?
 
Mooncoon, the tailstock holder I showed starts the die a bit more centered and with the thread aligned along the axis. As I said in the last post it's not as axial as cutting the threads with the single point but it's a darn sight better than a regular die holder started only by hand.

Big JD, I THINK (don't quote me on this) that the main body on this one may be two pieces pressed together due to the size of the die holder end. At least that's what I'm remembering. Sorry but I made this and the other one piece for the smaller 13/16th dies some 15 or more years ago. However on a postive note I CAN say that the handles on both screw in... I think.... :D

Two pieces or one makes very little difference. What you would do if you have a material shortage and need to do it in two is to turn the holder but leave the die recess undone or only rough turned and deliberately left small by quite a bit. Turn the stub to take the press fit and deal with pressing it first. A 1.5 thou per inch ensures a nice "welded" like fit that you can still push together with a big bench vise. Be sure to grease the stub and recess before pressing into place to avoid the risk of galling and jamming before it seats home. Nothing ruins your day worse than a press fit that didn't seat all the way. If you trash it you lost your time. If you live with it it'll eat at your soul every time you look at it. Waking up from nightmares in a pool of cold sweat over this concession is not unheard of either... :D

If you don't have a big bench vise for pressing things like this together add it to your list right effin' now. A good and big bench vise in any machine shop is worth it's weight in .45ACP and then some. You'll never regret buying big for this item but you sure will curse yourself when it comes up too light and small.

Once pressed together into one piece it's a simple matter of treating it as a single blob of metal and finishing the drilling and reaming of the guide pin hole and opening up the die recess for a nice light push fit. It shouldn't quite fall out but require a bit of a smack to knock it out. For the final sizing you'll want to use some very fine sandpaper to just polish down the crown of the cutting grooves that final bit for a .0005 to .001 over size fit.

The proper use of the compound slide for threading along with many other hobby machinist topics is covered in a most excellent way in a book by L. H. Sparey titled "The Amatuer's Lathe". A lot of the pictures are dated but the techniques are 100% sound. This book is worth the cost many times over. But if you can find it in your local public library then so much the better.
 
I like that jig on your tail stock. I just hold the die and holder in place with the drill chuck on the tail stock but I like your method better

cheers mooncoon
 
mooncoon said:
I like that jig on your tail stock. I just hold the die and holder in place with the drill chuck on the tail stock but I like your method better

cheers mooncoon
Click to expand...

Yes, that's how I used to do it before I made up that tool. Pull back the jaws so they are submerged and use the flat face of the chuck against the die holder with gentle pressure on the tailstock wheel to guide the first part of the cut. It's a simple trick but it works well. Especially with the smaller sizes where you can grab the holder equally on both sides and apply the starting torque evenly. The downside is that if it's a larger thread you need to put your hands out farther on only one side to get the torque needed and I was worried it would lead to an off axis start. The other hand being busy keeping the pressure on at the tailstock wheel. Of course if I was gifted with three hands I could have done it just fine....:D Ah, maybe I worry too much about this stuff but the holder/guide was a nice project and it sure has made doing this stuff easy as pie for me. It's well worth an evening's time to make it.
 
Big JD-From the hills said:
Ok you have pointed out another error I have been doing, I have just been using my cross slide to advance the tool, and not my compound, so definitely not 29 deg. Yeah I have been meaning to make a die holding tool for the smaller stuff. I actualy was using a small lathe chuck, and hex dies on my craftsman 109. But I definitely need to cut metric barrel shank threads on the lathe. A question on your die holder, thats all one piece right(except the handles)?
Click to expand...


WHAT?
You need to set your cross slide to 60 degree. or if you are cutting internal threads to -30 degree.

http://metalworking.majosoft.com/html/to_make_screw_thread.html

is a good over view on how to get started.
 
aninchlow the angle of the cross-slide is fixed, what you are talking about is the topside which can be set to different angles. The topside if 'set over' is aways set to half the inclusive angle of the thread, if its a "v" form. For a standard metre V-thread the inclusive angle is 60 deg, so the topside is set over to 30 deg.
 
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