08-27-2017, 09:15 PM
WARNING: This is a L-O-N-G post for a S-I-M-P-L-E tool !!!
For years, I used an old technique for chasing threads on an engine lathe. The method consists of running up the tailstock near the work to be threaded (with any tailstock tooling removed, of course).
A standard die in a die-stock was then positioned so that the back of the die was seated squarely against the tailstock ram and the die-stock handle was supported by the lathe carriage to prevent rotation.
There were a couple of problems associated with the method. The obvious one is that the chasing die cannot thread short workpieces. This is because the carriage blocks the tailstock from nearing the chuck/collet at the headstock of the lathe, even when the tailstock ram is fully extended as in this photo.
Although on small lathes, it is possible to disassemble the tailstock and reassemble it in front of the carriage, avoiding the above problem, it’s not something than many would like to do on a regular basis.
A second disadvantage that exists, no matter where the tailstock is positioned, is that the tailstock taper ends up containing most of the chips from the chasing operation. The incredibly loud whistle when blowing out the taper bore with compressed air gets everyone's attention !
Surely tens of thousands of variations on the following tool have been made, this is just the simplest way that I envisioned it. The die-holding tool shown below passes over the lathe carriage with adequate length to work all the way up to the headstock.
Short pieces of 1 inch and 2 inch rod salvaged from under the bandsaw, were welded to the ends of a length of 3/4 pipe (1.05 OD).
I performed a primitive stress relief in the kitchen oven after welding and before finish machining the tool (incidentally, all lathe work except the front bore was done between centers). Three cycles from room temperature to 400 degrees F, allowing time to stabilize at temperature and time to cool took the better part of a day.
One end of the composite assembly was turned to a MT-2 taper which suits the tailstocks on most small lathes. The other end was bored to accommodate inexpensive hex dies, available from any hardware store, as well as standard one inch diameter adjustable dies.
Incidentally, to date, I have never needed to chase threads larger than 5/8 inch (after which, my blistered left hand was sore for days) single-pointing would always be my choice for threads greater than 3/8 or ½ inch. This is not from lack of sturdiness, the Emco is stouter than the majority of 12 inch lathes, but the drive train leaves much to be desired – no back gears and the belts slip under high torque !
In fact, the limitations of the 8 x 18 lathe shown in the photos suggests that threading any diameter over about 3/8 or so should be done on a larger lathe, although before I obtained my Sheldon, everything had to be done on the little guy.
The headstock end of the die-holder was bored in the same set-up that the taper was turned so that the dies would be well-aligned with the lathe spindle (if the tailstock is properly aligned). The three adjustment screws that serve to prevent dies from spinning also allow slight centering adjustment to accommodate any dimensional variation. I am long remiss in replacing the socket head cap screws with some large head diameter, home-made knurled screws.
Opposing slots are milled through the pipe just behind the die head for clearing chips. They are visible in the above photo.
A crossbar near the tailstock end prevents the tool from spinning and damaging the tailstock taper. When the die-holding tool is inserted in the tailstock, the bar is oriented downward, between the ways. In either direction the lathe rotates, the bar is confined between the ways, preventing rotation of the die-holder.
As noted previously, adjustment screws were included for centering the hex dies but I mostly prefer to leave them loose for small size threads. This allows a little radial movement (like a floating reamer). But the die is still held perpendicular to the axis of the workpiece.
This is a simple tool for a simple task; I usually thread at 120 RPM for dies of 3/8 and under, using sulphur-based pipe threading oil and pushing the tailstock by hand, which is not a problem on small lathes like this one. Normally this would be a smooth trouble-free operation.
BUT this particular small lathe is single-phase powered so there is no instant reversing. As I near the end of the threads, I power down, let the spindle coast to a stop and then complete the few remaining threads by hand-rotating a large pulley attached directly to the spindle, first removing the belt for safety and to reduce the amount of torque required to rotate the spindle. This leads to the following ...
OK, prepare for diversions from the topic -
After the painful experience of chasing a dozen 5/8-11 studs (I didn't have a larger lathe at the time, nor the correct change gears to single-point 11 pitch) I had to come up with a better way - even if it took longer. There was no convenient way to turn the spindle manually by gripping some exterior feature. The large spindle pulley could be modified to accommodate some form of spanner but I didn’t care for that idea.
Since the gripping option (at least to my limited reasoning) was to use the spindle bore, this is what I came up with. Clearly this would be an impediment if one wanted to run long lengths of stock through the spindle. Fortunately that is a requirement that I've never encountered.
The internal expanding wrench is made from one inch O-1, torch heat-treated. Parts of the tool display the original finish from the torch treatment. The smaller parts are also heat-treated drill rod.
I have modified the “Compact Eight” by removing the large steel end cover, and replacing it with plexiglass shields at the front and top. This can be seen in a photo below.
Previously, checking the belt configuration to know the spindle RPM, required opening the unreasonably big end cover. That was inconvenient because a large compressor is located near the end of the lathe; the end cover could be only partially opened. Changing belts was also a PITA for that reason.
In the current configuration I can easily see what pulleys are engaged and therefore know the spindle speed. The top panel can be pivoted backward to provide access to the spindle pulley. The rear area of the lathe is left open for lubrication, fast belt changes and for application of the spindle wrench. (The length of the spindle wrench was established so that it cleared the plexiglass covers.)
When using the spindle wrench shown, I slip off one of the belts for safety, as mentioned previously, and to lessen rotational resistance. If the lathe somehow was powered up in reverse while the ½ inch drive socket wrench was set for forward rotation ….. well need I say more ?
(If the lathe was powered up in the same rotation as the socket wrench was set, the wrench would just rachet.)
The spindle wrench slips easily into the lathe spindle hole but attempting to rotate the wrench in either direction locks it to the spindle so that a socket wrench with 7/8 socket can rotate the spindle as required.
Getting back to the chasing tool:
When retracting the die, if the retaining screws have been left slightly loose (as I prefer), withdrawing the tailstock will leave the die on the work, to be unthreaded by hand and this may be desirable. It's very simple to grasp the die and power up the lathe in reverse, die ends up in your hands in a second or two. If any resistance is encountered by the die, well my fingers are a pretty good clutch.
If it's desirable to retain the die in the holder, a finger lightly placed over the front of the die will prevent it from slipping out of the holder. The lathe can then be run in reverse and the tailstock pulled by hand to unthread the die and die holder from the work. I don't see any particular advantage in doing it this way.
I would note that it’s much easier to chase with a die by first turning the OD to a smaller and more convenient diameter than the nominal diameter. There’s no need to expend the horsepower and the wear on dies by forcing them to cut off more material than is required to meet unified thread standards.
My personal rule of thumb is to turn the major diameter about 3% below the nominal diameter. In other words, multiply the nominal diameter by 0.97 to obtain the diameter to which the rod must first be turned.
For example using this simple practice, 1/2 - 20 UNF threads suggests that the nominal diameter be turned to .50 x .97 = .485 diameter before chasing the threads. For critical applications one should examine the unified standards for proper dimensions (refer to “Machinery’s Handbook”).
I made this die-holding tool about ten years ago and each time that I use it, I wonder why it took so long to think of it
For years, I used an old technique for chasing threads on an engine lathe. The method consists of running up the tailstock near the work to be threaded (with any tailstock tooling removed, of course).
A standard die in a die-stock was then positioned so that the back of the die was seated squarely against the tailstock ram and the die-stock handle was supported by the lathe carriage to prevent rotation.
There were a couple of problems associated with the method. The obvious one is that the chasing die cannot thread short workpieces. This is because the carriage blocks the tailstock from nearing the chuck/collet at the headstock of the lathe, even when the tailstock ram is fully extended as in this photo.
Although on small lathes, it is possible to disassemble the tailstock and reassemble it in front of the carriage, avoiding the above problem, it’s not something than many would like to do on a regular basis.
A second disadvantage that exists, no matter where the tailstock is positioned, is that the tailstock taper ends up containing most of the chips from the chasing operation. The incredibly loud whistle when blowing out the taper bore with compressed air gets everyone's attention !
Surely tens of thousands of variations on the following tool have been made, this is just the simplest way that I envisioned it. The die-holding tool shown below passes over the lathe carriage with adequate length to work all the way up to the headstock.
Short pieces of 1 inch and 2 inch rod salvaged from under the bandsaw, were welded to the ends of a length of 3/4 pipe (1.05 OD).
I performed a primitive stress relief in the kitchen oven after welding and before finish machining the tool (incidentally, all lathe work except the front bore was done between centers). Three cycles from room temperature to 400 degrees F, allowing time to stabilize at temperature and time to cool took the better part of a day.
One end of the composite assembly was turned to a MT-2 taper which suits the tailstocks on most small lathes. The other end was bored to accommodate inexpensive hex dies, available from any hardware store, as well as standard one inch diameter adjustable dies.
Incidentally, to date, I have never needed to chase threads larger than 5/8 inch (after which, my blistered left hand was sore for days) single-pointing would always be my choice for threads greater than 3/8 or ½ inch. This is not from lack of sturdiness, the Emco is stouter than the majority of 12 inch lathes, but the drive train leaves much to be desired – no back gears and the belts slip under high torque !
In fact, the limitations of the 8 x 18 lathe shown in the photos suggests that threading any diameter over about 3/8 or so should be done on a larger lathe, although before I obtained my Sheldon, everything had to be done on the little guy.
The headstock end of the die-holder was bored in the same set-up that the taper was turned so that the dies would be well-aligned with the lathe spindle (if the tailstock is properly aligned). The three adjustment screws that serve to prevent dies from spinning also allow slight centering adjustment to accommodate any dimensional variation. I am long remiss in replacing the socket head cap screws with some large head diameter, home-made knurled screws.
Opposing slots are milled through the pipe just behind the die head for clearing chips. They are visible in the above photo.
A crossbar near the tailstock end prevents the tool from spinning and damaging the tailstock taper. When the die-holding tool is inserted in the tailstock, the bar is oriented downward, between the ways. In either direction the lathe rotates, the bar is confined between the ways, preventing rotation of the die-holder.
As noted previously, adjustment screws were included for centering the hex dies but I mostly prefer to leave them loose for small size threads. This allows a little radial movement (like a floating reamer). But the die is still held perpendicular to the axis of the workpiece.
This is a simple tool for a simple task; I usually thread at 120 RPM for dies of 3/8 and under, using sulphur-based pipe threading oil and pushing the tailstock by hand, which is not a problem on small lathes like this one. Normally this would be a smooth trouble-free operation.
BUT this particular small lathe is single-phase powered so there is no instant reversing. As I near the end of the threads, I power down, let the spindle coast to a stop and then complete the few remaining threads by hand-rotating a large pulley attached directly to the spindle, first removing the belt for safety and to reduce the amount of torque required to rotate the spindle. This leads to the following ...
OK, prepare for diversions from the topic -
After the painful experience of chasing a dozen 5/8-11 studs (I didn't have a larger lathe at the time, nor the correct change gears to single-point 11 pitch) I had to come up with a better way - even if it took longer. There was no convenient way to turn the spindle manually by gripping some exterior feature. The large spindle pulley could be modified to accommodate some form of spanner but I didn’t care for that idea.
Since the gripping option (at least to my limited reasoning) was to use the spindle bore, this is what I came up with. Clearly this would be an impediment if one wanted to run long lengths of stock through the spindle. Fortunately that is a requirement that I've never encountered.
The internal expanding wrench is made from one inch O-1, torch heat-treated. Parts of the tool display the original finish from the torch treatment. The smaller parts are also heat-treated drill rod.
I have modified the “Compact Eight” by removing the large steel end cover, and replacing it with plexiglass shields at the front and top. This can be seen in a photo below.
Previously, checking the belt configuration to know the spindle RPM, required opening the unreasonably big end cover. That was inconvenient because a large compressor is located near the end of the lathe; the end cover could be only partially opened. Changing belts was also a PITA for that reason.
In the current configuration I can easily see what pulleys are engaged and therefore know the spindle speed. The top panel can be pivoted backward to provide access to the spindle pulley. The rear area of the lathe is left open for lubrication, fast belt changes and for application of the spindle wrench. (The length of the spindle wrench was established so that it cleared the plexiglass covers.)
When using the spindle wrench shown, I slip off one of the belts for safety, as mentioned previously, and to lessen rotational resistance. If the lathe somehow was powered up in reverse while the ½ inch drive socket wrench was set for forward rotation ….. well need I say more ?
(If the lathe was powered up in the same rotation as the socket wrench was set, the wrench would just rachet.)
The spindle wrench slips easily into the lathe spindle hole but attempting to rotate the wrench in either direction locks it to the spindle so that a socket wrench with 7/8 socket can rotate the spindle as required.
Getting back to the chasing tool:
When retracting the die, if the retaining screws have been left slightly loose (as I prefer), withdrawing the tailstock will leave the die on the work, to be unthreaded by hand and this may be desirable. It's very simple to grasp the die and power up the lathe in reverse, die ends up in your hands in a second or two. If any resistance is encountered by the die, well my fingers are a pretty good clutch.
If it's desirable to retain the die in the holder, a finger lightly placed over the front of the die will prevent it from slipping out of the holder. The lathe can then be run in reverse and the tailstock pulled by hand to unthread the die and die holder from the work. I don't see any particular advantage in doing it this way.
I would note that it’s much easier to chase with a die by first turning the OD to a smaller and more convenient diameter than the nominal diameter. There’s no need to expend the horsepower and the wear on dies by forcing them to cut off more material than is required to meet unified thread standards.
My personal rule of thumb is to turn the major diameter about 3% below the nominal diameter. In other words, multiply the nominal diameter by 0.97 to obtain the diameter to which the rod must first be turned.
For example using this simple practice, 1/2 - 20 UNF threads suggests that the nominal diameter be turned to .50 x .97 = .485 diameter before chasing the threads. For critical applications one should examine the unified standards for proper dimensions (refer to “Machinery’s Handbook”).
I made this die-holding tool about ten years ago and each time that I use it, I wonder why it took so long to think of it