PDA

View Full Version : Thread cutting, tool holder hits tailstock center



taydin
07-19-2011, 07:12 PM
I am cutting a thread to a 16mm ST37 round stock using an insert type thread cutting tool. The thread is fairly long, about 100mm, so I'm using the tail stock live center to support the rod. Problem is, when starting from the far right end of the rod, the tool holder hits the cone of the live center.

Tried to work around this by starting the thread about 20mm away from the far right end, but this means several additional steps (cut an extra groove, cut off 20mm from the end, reface the end etc).

Then put the tool holder into the vise and used my grinder to remove material from the tip of the tool holder. This didn't help, it still hits the live center. BTW, here is the tool holder I'm talking about:

http://www.takimsas.com.tr/urunler/urun146_b.jpg

Then I inquired about a tool holder where the insert is aligned to the right side of the holder, as opposed to the left side. Nobody has such a tool holder here...

I then inquired about a live center whose tip is thinner so that it safely avoids the tool holder. There are such live centers, but they are categorized as "high precision CNC live centers" and are very expensive.

I can't grind my own HSS thread cutter. If I could, I wouldn't be dealing with insert type tool holders to begin with.

I read about a very unusual method. Basically, install the tool holder upside down and run the lathe in the opposite direction :) I don't know, seems too strange...

The other option I am considering is to take one of my 2 fixed centers, put it into the lathe spindle using the MK6 to MK4 reduction and then reduce its diameter to a suitable value. I would probably sacrifice a cutter in the process, but so be it ... Beats paying lots of money for a live center that I'm going to use very rarely.

Anybody can think about other options?

dockrat
07-19-2011, 07:34 PM
Taydin....niether elegant,extremely accurate, or in focus, but I have been known to use this in a tight spot

http://i253.photobucket.com/albums/hh67/Dockrat1/IMGP1575Medium.jpg

Gavin
07-19-2011, 07:40 PM
Use a half dead centre.

daryl bane
07-19-2011, 07:41 PM
If you don't mind using a dead center, there is what is called a "half dead center". I don't have a pic, but it looks just like a regular dead center, but 1/2 of the taper is cutoff just for this type of problem. It should be common enough to find one thru a good tool supplier. Gavin beat me to it.

gwilson
07-19-2011, 08:16 PM
Grinding a thread tool is not difficult. Do you have a thread gauge? I mean the flat,steel type with 60 notches milled into it. You just use those notches to compare your ground tool to,until you get it right.

That big,blocky tool holder will not fit into tight spots,as you have seen. It is better for larger diameter work.

Depending upon how critical maintaining the same diameter is on your threaded part,using the chuck option,unless it is a good,probably OLDER Jacobs chuck,or another good make, like Albrecht,can be .003" or more out,causing your thread to have a taper. Even a good old chuck will be .001" out.

I don't think a half center would clear that bulky holder either. Could you just grind more of the right hand corner off? You might not like the idea,but the holder would still be plenty rigid without the blocky corner. You could do it neatly,and put gun bluing on the ground away area. I know the holder cost money,but a neat job would make it better for small diameter work. Still,though,the insert is pretty large for a tight spot.

You should experiment with the idea of running the lathe backwards. Use some scrap till you figure out how it works. I have run the lathe backwards with the tool upside down when threading shallow lids. I start at the END of the cur,and thread my way OUT of the lid. It is a useful technique you need to learn.

A good idea to invest in a CNC type center when you can. The smallest type are the little Royal centers that have a rotating center INSIDE the taper,running on needle bearings. I use one when making knurls,which is a tight job.

taydin
07-19-2011, 08:35 PM
You guys are fantastic! Thanks for all the replies. I am searching for the half dead center in local websites now...

Carld
07-19-2011, 08:41 PM
Buy a live center for a CNC lathe. They have a long thin shaft with a small center on the end for close work on the end of a shaft.

taydin
07-19-2011, 08:41 PM
Grinding a thread tool is not difficult. Do you have a thread gauge? I mean the flat,steel type with 60 notches milled into it. You just use those notches to compare your ground tool to,until you get it right.

The tip of this insert is pointing a little down. Do I have to give this angle to a ground HSS cutter as well?



Depending upon how critical maintaining the same diameter is on your threaded part,using the chuck option,unless it is a good,probably OLDER Jacobs chuck,or another good make, like Albrecht,can be .003" or more out,causing your thread to have a taper. Even a good old chuck will be .001" out.

It isn't that critical really. It's a replacement threaded rod for levelling pads.


I don't think a half center would clear that bulky holder either. Could you just grind more of the right hand corner off?

Already tried that. I actually went to the extreme and now there is only 1mm support left for the insert. This holder is probably no good anymore.


A good idea to invest in a CNC type center when you can. The smallest type are the little Royal centers that have a rotating center INSIDE the taper,running on needle bearings. I use one when making knurls,which is a tight job.

I don't understand why a thinner tip live center is automatically in a separate category and so expensive. A cheap live center could as easily be made to have a thin tip. Is there anything else that's special about it?

Carld
07-19-2011, 08:47 PM
Probably expensive because it's for CNC and it's probably built stronger for the long nose.

aboard_epsilon
07-19-2011, 08:48 PM
Just have the rod that you are going to thread longer than it needs to be ....cut the extended bit to the depth of the thread.

about 3/4s of an inch will do

then you start cutting in fresh air ..your final thread cut should just lightly scratch the extended bit.

you wont hit the tailstock then..and you know you're at the right depth for your final cut .

all the best.markj

taydin
07-19-2011, 08:55 PM
Just have the rod that you are going to thread longer than it needs to be ....cut the extended bit to the depth of the thread.

about 3/4s of an inch will do

then you start cutting in fresh air ..your final thread cut should just lightly scratch the extended bit.

you wont hit the tailstock then..and you know you're at the right depth for your final cut .

all the best.markj

That is what I did with the first batch of 4 threaded rods. But because of a length mistake, that entire batch is now in the scrap bin. I'm trying to minimize the steps, because there is no guarantee that another oops won't happen...

taydin
07-19-2011, 08:58 PM
Taydin....niether elegant,extremely accurate, or in focus, but I have been known to use this in a tight spot

That is very clever indeed :cool:

So far all dead center I was able to find locally are fully dead. I will inquire in a few shops in person and in case of no luck, this option is a solid plan B.

mark61
07-19-2011, 09:24 PM
Why don't you get a tool that holds the insert verticly on the left side instead of laying flat on the top? They give you more room on the sides. Or as has been said-use a tool steel bit ground to shape.


mark61

QSIMDO
07-19-2011, 09:49 PM
Would it do to move more stock out of the chuck, set up the steady rest between the chuck and the tool and let the right end float?

Arcane
07-19-2011, 10:37 PM
It's a PITA but not impossible to do the threading in a couple of stages...you just have to "pick the thread up" again as you advance the shaft farther out of the chuck. It gets easier with practice...and it does help to have a good eyeball!

PixMan
07-19-2011, 11:29 PM
I had the same problem, so I bought a long-nose "CNC" live center for about $40 from Shars, and them made a slight modification to the tool holder I have that looks a lot like yours.

http://img.photobucket.com/albums/v466/kenm10759/Dads%20shop/IMG_1187-r.jpg

No problem at all with threading now, right down to some very small thread diameters.

GadgetBuilder
07-20-2011, 10:59 AM
A dead center slimmed down as you suggested should work. I have one in MT2 with a 9mm nose that may be shop made:
http://img339.imageshack.us/img339/4917/smallctr.th.jpg (http://imageshack.us/photo/my-images/339/smallctr.jpg/)

A tool post grinder might be preferable to turning if you're going to modify an existing center.

It's fairly easy to make a jig to quickly and accurately grind threading bits in HSS:
http://www.gadgetbuilder.com/ThreadingTools.html

Your carbide tool has negative rake but HSS threading tools are generally used with no rake.

Another possibility would be to make an extended nose to fit over your existing live center.

Or, make an extended live center from scratch, plans are available on the net (although I don't recall where).

John

gwilson
07-20-2011, 12:47 PM
The HSS threading tool you could grind should NOT point down hill. I am guessing that your insert points down hill because the carbide is brittle,and pointing it a little down might help it to not break in the threading application. Not all inserts point down hill. With that thin,tapered nose,the threading tool of carbide is probably more delicate.

Peter.
07-20-2011, 01:03 PM
I have a live center off of coronet wood lathe which has a MT1 taper. It has a thin point and narrow body and is very useful for this type of thing.

gwilson
07-20-2011, 02:13 PM
You could just take an ordinary DEAD center(not a ball bearing center) and just grind away 1 side of it to clear the tool holder. The tip must remain fully round: You need to stop the grinding when you have ground it down enough to leave the tip about 1/4" diameter of unground tip. Half centers aren't really completely HALF of a center.

Oldbrock
07-20-2011, 03:12 PM
I consider the day lost if I don't learn something new that day and I've been at machining for 65 years. Learn to grind a threading tool from HSS then hone it to a super finish, hone a flat on the end always maintaining clearance angles. for rh threads there need be no clearance angle on the trailing face but the leading edge must have helix angle + clearance, something else to learn. No such thing as can't unless it's machining a hypoid gear on a southbend 10K lathe. I did lots of carbide threading in an oilfield shop but not on my 10K HSS rules in my book for threading in small lathes. Get a piece of hss and go where others have gone before. Rough it with a large soft grade aluminum oxide wheel and finish with a standard fine wheel, keep it cool. Peter

taydin
07-20-2011, 05:51 PM
You could just take an ordinary DEAD center(not a ball bearing center) and just grind away 1 side of it to clear the tool holder. The tip must remain fully round: You need to stop the grinding when you have ground it down enough to leave the tip about 1/4" diameter of unground tip. Half centers aren't really completely HALF of a center.

I guess this would have to be a bench grinder and not an angle grinder? Most likely the high material removal rate of the angle grinder would cause too much heat deformation and render the dead center useless...

My two dead centers are chinese, most likely quite soft. So would it be a problem to turn them down on the lathe using a carbide insert cutter?

Scottike
07-20-2011, 06:32 PM
Get a ball bearing (steel ball) that's smaller than the minor diameter of the threads your cutting. Then take a piece of round stock smaller in diameter than your minor thread diameter and long enough to give you the clearance you need for your tool plus enough extra to mount in a drill chuck and center drill one end slightly shallower than you center drilled the end of the stock your threading. Place a dab of grease into the end of each piece of stock, chuck the piece into your tailstock, stick the ball bearing into the end (the grease will hold the bearing there and lubricate) and slide your tailstock up to your workpiece. Make sure the bearing is well seated into both pieces. The bearing will rotate against whichever piece has the smaller center hole. Make sure your drill holes are as large as possible to provide a substantial seat for the bearing, so that cutting pressures can't dislodge the bearing.

edit: you can do this with a cheap dead center if you want more accuracy than you can get with a drill chuck mounted piece, most but not all of the cheaper dead centers can be easily turned and drilled.

PixMan
07-20-2011, 06:36 PM
Just to clarify for those thinking a negative rake insert is "pointing down" and is exerting too much force....

Every negative rake lay-down threading insert I have in my small collection is actually cutting as at least a neutral top rake, if not positive rake. The holder is negative rake, the top of the inserts are either ground to a simple upsharp geometry, or like the one in my photo, molded with a chipbreaker design that is positive.

The main advantage to using lay-down threading inserts such as those isn't the carbide. In fact, even many CNC machines cannot achieve the cutting rates of the carbide in a threading cycle for the lack of processing speed for synchronization of the "electronic lead screw", so the carbides are tough grades and cut with lower force than traditional turning inserts would. But I digress. The real advantage to lay-down threading inserts is that of high-quality burr-free thread form that they can produce.

Because they can cut the complete form from crest to root, you often don't even need a thread gauge or thread wires to know when the thread is done. Turn the O.D. to nominal (.2362"/6mm for an M6 x 1.0 thread, for instance), and start threading. When the insert crests the thread and takes the O.D. down to just a bit smaller than .2362/6mm (don't have my Machinery's Handbook here), you're done.

Toolguy
07-20-2011, 11:13 PM
It's easy to make your own low profile live center. Get an old drill or dead center that fits your tailstock. Drill and bore and/or ream for a .001 oversize hole of 3/8" or 1/2". Get the appropriate size dowel pin or drill rod and turn a 60 degree point on one end. Face the other end square. Harden and temper to 50 to 55 Rc. Get a ball bearing the same size as the pin. Put some hi temp grease in the hole of the morse taper part. Drop the ball bearing in the hole, then push the pin in the hole, flat end first. The pin will seat against the ball bearing. That is the thrust bearing. The ball will only touch the very center of the pin for very low friction. The pin will have .001 clearance in the hole to turn freely. If you want to get fancy, you can put a small die spring in first to allow for heat expansion. This center works well at lower speeds, can over heat at high RPM. Ideally, all parts would be hardened and ground, but not essential.

taydin
07-22-2011, 03:33 PM
Ok, I took one of the rods that I had scrapped earlier, turned it down to like 14mm. Then I cut it to 100mm length and made the end of that rod conical, like the tip of a live center. I then chucked this dead center to the tailstock chuck, as suggested earlier in the post.

Then I successfully cut one of the levelling pad rods. Please evaluate the threads and let me know if you notice anything that needs improvement

http://www.taydin.org/web/thread_cut/scaled_img_2492.jpg

http://www.taydin.org/web/thread_cut/scaled_img_2493.jpg

http://www.taydin.org/web/thread_cut/scaled_img_2494.jpg

http://www.taydin.org/web/thread_cut/scaled_img_2495.jpg

taydin
07-22-2011, 03:36 PM
I used cutting oil and and 0.15mm per pass. When I made the last pass, I used 0.05mm depth. When finished, I didn't go over the threads using a file. I try to avoid using a file or sandpaper on the lathe. Instead, I ran the threads through a wire brush chucked to my drill press.

BadDog
07-22-2011, 04:10 PM
I've got a couple of "Concentric" (brand) live centers that work well for stuff like this. Much like those pictured earlier, the center spins in the middle of the taper shank, and has a small longish center hanging out without all that mass to get in the way. Also spring loaded so heat expansion doesn't cause trouble. Good for lighter jobs, I have a big Skoda and a Rohm for heavy jobs.

I've also got some carbide tipped "half" dead centers. Best for low rpm work like grinding, but can get you buy in a pinch on the lathe if you manage to keep lube in place and keep speed down. "White Lead" was the old time favorite when dead centers were used on lathes, but now hard to get, but there are various alternatives recommended.

And finally, something I've done when in similar circumstances is turn down a small extension stub to mount on the center, then start the thread where convenient (1/2" or so away). When done, saw or part of the support stub (if it matters). Couldn't really cut if off on a shaft where the centers may be necessary for later remount, but usually doesn't matter.

macona
07-22-2011, 06:10 PM
I used cutting oil and and 0.15mm per pass. When I made the last pass, I used 0.05mm depth. When finished, I didn't go over the threads using a file. I try to avoid using a file or sandpaper on the lathe. Instead, I ran the threads through a wire brush chucked to my drill press.


Threads look fine. You could knock the tips down.

Files are completely fine on the lathe as long as you follow a couple rules. One is how you hold it. There are diagrams on the web how to hold the file. Second rule, always use files with handles on them. Without a file the tang can get shove straight up into your wrist if something catches.

The reason for no sandpaper on a lathe is the grit. If you lay down paper before you use it then that is OK. Also emery, especially fine stuff, does not shed abrasive like paper back sandpaper. Also there is a technique to using sandpaper safely on a lathe.

Rustybolt
07-22-2011, 06:59 PM
looks fine. hit it with the wire wheel to knock off any roughness.