Bench grinder spindle thread

As DoN has mentioned, using a DI and turning the spindle with the carriage feed engaged will show if there are errors in the selected gear set.
I generally just rotate the chuck the number of turns equal to the TPI to make sure the travel is 1.00".
Before I assembled a variable speed drive, I was using a hand crank fitted into the back/left end of the spindle for short threaded sections such as lens adapters which may only use 3-4 turns of thread. When cutting up to a shoulder, the variable speed drive is a huge benefit.
For a fine thread, using a hand crank is fairly easy work, but there are several other required additional steps which make cutting the thread successful.
This is a basic outline: Unplug the power cord for hand crank method Setup the compound at about 29 to 29.5 degrees Cutting lubricant applied to work area Setting the cross feed dial to zero for the scratch cut Set the compound dial to zero for the scratch cut Make the first pass scratch cut Backing out the cross feed enough to clear the workpiece Reverse feed to locate the carriage for a second pass (by hand if using a crank) Set the cross feed dial back to zero for the second pass Advance the compound for a reasonable cut (depending upon hand or power cut) Make the second pass with cutting lube Repeat from Backing out the cross feed
It helps to have a (quality) nut of the desired thread nearby to use as a gage.. otherwise, there are cutting depth specs on on threading gages for setting up the cutting tool prior to threading (perpendicular to the workpiece axis). A quality nut would be one that doesn't rattle when run onto a quality tap.. which will insure that the resulting thread will fit any other standard nut of that size/pitch.
Adhering to the thread specs will generally prevent cutting too deep, although getting familiar with a machine generally includes seeing how well it responds to operator input, so when nearing the final recommended depth of the cut, trying the nut can prevent undercutting and the need to start over.
The edges of the thread crests can be sharp and possibly a little rough.. removing the hand crank and running the spindle at a reasonable speed will allow the operator to wire brush the freshly-cut thread (carbon steel brush bristles for a steel workpiece) and/or run some emery cloth over the area (apply a protective cover/shop rag over the ways to keep any shedded abrasive off them). A 3-sided file can also be used to very lightly chase along the thread to break the sharp edges of the thread.
--
WB
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I had to generate a spreadsheet threading gearbox chart since the original was missing. I added the compound infeed at 29 degrees, assuming the proper bit tip width, and the indicator line-matching rules. As I mentioned before I usually leave the bit more pointed and the calculated infeed cuts a somewhat shallow thread, so I can cut in to the number quickly before measuring with three wires.
The bit will cut the proper thread form if I then disconnect the tumbler and move the spindle a tooth backwards, to cut the thread wider but not deeper.
jsw
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On Mon, 19 Dec 2011 06:17:00 -0500, "Wild_Bill"

Thanks. Pretty much all that I did except for the indicator thing.
I could only find a scanty reference to the depth of each pass - they recommended 0.003" on the compound. That makes it about 0.0025" depth of cut per pass. For the 1/2"-20 the depth of thread is about 0.060" so that is about 25 passes. Soudns about right? Does this vary with how beefy one's lathe is?
Michael Koblic, Campbell River, BC
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A lot depends upon the overall characteristics of the machine.. rigidity, fit, adjustments of gibs and so on. Much of the methodolgy is trial and error, and/or making notes and sticking them on the wall.
25 passes for a fine thread seemed excessive (although slow and easy beats fast and oh-shit).. cutting depth will be about half of that mentioned. http://www.engineersedge.com/screw_threads_chart.htm Actual infeed (perpendicular to workpiece axis) is x2 for diameter measurements.
When the spindle is hand cranked, the operator gets feedback thru the cranking arm wrt the depth of cut during the pass.. going a little light is better than stopping during the pass, for sure. A very sharp cutting tool and a quality cutting lubricant will definitely enhance performance.
When threading under power a 9x20" lathe with the slow speed selected can produce more torque than choosing higher spindle speeds, so cuts can be relatively aggressive.. depending upon the variables mentioned above.
Some home shops are equipped with fairly heavy-duty lathes capable of producing lots of blue chips all day long.. yours and mine are jewelers lathes compared to those machines, but very useful none the less.
--
WB
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    [ ... ]

    Depends on how beefy the lathe is (including the toolpost and the length of threading tool extension, how long the section of workpiece is compared to the diameter (the skinnier, the shallower the pass, but if you can support the other end with a live center, you can get away with a bit more), the material being threaded (I like 12L14, but if you are planning to weld to it, forget that material. :-)
    If you have 3" or less of the 1/2" stock sticking out, you can likely do it as follows:
1)    Light scratch just to verify that the threading gears are     set right -- compare the scratches to a thread gauge).
2)    I would probably do four passes of 0.010" actual depth
3)    And then finish a couple of 0.005" passes. (Make sure you     know whether your lathe cross-slide and compound are calibrated     in terms of diameter off the workpiece, or radius. (This is why     the threading gauges have a "double depth" column as well as the     "single depth" one.
    The threading depths there are with free machining metals, 12L14 steel, 360L brass, 6061T6 brass. If you have some of the gummy "steel" from Home Depot -- experiment until you find values which work for you.
    And be sure to lubricate the threads each pass. Some of the Rigid high sulfur pipe threading oil for steel, perhaps some Molly-D, nothing for the brass, and just keep spritzing with WD-40 or kerosene for aluminum. I've also got some really high sulfur cutting oil which brushes on with an acid brush and stays put. It is called Sul-Flo, and is only sold in rather expensive quantities, so our metalworking club got together a group purcase, with the one who wanted it the most (from using it before) doing the sub-dividing into gallon plastic bottles. It looks just like flowers of sulfur (the fine yellow powder) in a thick oil. It is really nice when cutting tough steels.
    I tend to use pre-formed insert tooling specifically for threading most of the time, though I have ground my own Acme threading tools with the proper relief angles for the particular diameter and pitch I was threading.
    Good Luck,         DoN.
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I've done only a little threading on lathes bigger than my 10" South Bend. When I experimented with heroic depth of cut the point broke off, so now I don't take over 0.005" per pass on a one-off job on steel or stainless, especially one with previous time invested in it. For multiple parts I make extras and any with mistakes become the set-up and practice dummies. I think I've accidentally taken 0.025" or so when trying to catch the thread after removing the part from the chuck. No matter how beefy (sheepish, ratty) the lathe may be, if the work slips you have to reestablish the bit position in the thread.
You can tell if the work and machine have deflected from an overly aggressive cut by taking a second pass at the same setting to see how much of a chip is removed. If the work deflects the thread will become tapered.
A thread indicator as shown here speeds up the job considerably. http://www.micro-machine-shop.com/single_point_threading.htm Cut a groove with the parting tool at the left end of the thread and pop open the half nuts when the bit reaches it. On my lathe the gear train sound changes when the cutting pressure stops. Retract the bit, crank the carriage the carriage over, advance the bit and feed the compound in, wait for the proper line to match and close the half nuts. I make the retract-one-turn / move-right / advance-to-zero process uninterruptible so I don't lose my place when I stop the spindle to check the progress.
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On Tue, 20 Dec 2011 08:43:30 -0500, "Jim Wilkins"

At 2 AM last night I was hoping nobody would spot the error and call me on the depth of thread for 1/2-20. It is of course half of the quoted figure, so only 12 passes are necessary.
I am pleased to report that with the adjustment of the change gears I was able to cut two perfect 1/2-20 threads today! A nice Christmas present.
I re-did the cutter: It is amazing how many different instructions there are to grind one, some from reputable sources which seem plainly wrong (no side clearances). I added those and the cutting improved.
One of the joys of the 9x20 is the clutch at the lowest speed which effectively limits the available torque. Changing the cutter geometry stopped the clutch dis-engaging during the last couple of passes which was a problem before. There are those who disabled their clutch by epoxy. I have not addressed that because I tend to use higher speeds most of the time but it may be something I shall have to look at.
I use RapidTap, seems to work fine.
I do have a threading indicator but a cursory inspection would suggest that it is not working. In any case I felt I wanted to take that variable out of the equation for the moment. If I do more of this I shall re-visit it.
All in all I would like to thank everyone for their helpful advice. I feel almost like a proper machinist now.
Michael Koblic, Campbell River, BC
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wrote:
[...]

Not really. It disengages far too early rendering the low speed useless.

I do not think it turns at all. Pending detailed assessment.
[...]

That feature was not an option on the current project.
and disengage, crank

That I was not aware of. That will make things easier when I do this in future. I wondered how to hit the mark at 150 rpm.

How did one live without a lathe?
Michael Koblic, Campbell River, BC
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A quick look at the instructions etc on LittleMachineShop didn't show it, but on my old lathe the threading indicator pivots in and out of engagement with the leadscrew. Quick because then I noticed a puddle around my water heater...............
Before modern lathes (~1800) we lived like the Romans.
jsw
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    [ ... ]

    You snipped out the part where I mentioned that it could be mounted but disengaged, and was likely shipped set that way to reduce wear on the leadscrew until you are ready to do single-point threading.
    Is yours held to the side of the apron by a single screw? If you loosen the screw, can it be rotated so the gear on the bottom end engages the leadscrew? That should be all that is needed to make it work.
    [ ... ]

    I don't know -- though I must have at one point. :-) I remember the first access to a small lathe (a 6" South Bend, I think) at work back around 1960.
    Enjoy,         DoN.
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wrote:

It is both of those but will not turn.

Ah, the good old days.
Michael Koblic, Campbell River, BC
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    [ ... ]

    When backed away from the leadscrew, will the gear at the bottom turn by hand? If it is frozen, there is something too tight in the threading dial assembly, and that needs to be disassembled to find out why it is frozen.
    If it does turn, rotate it back into contact with the leadscrew (working the carriage back and forth a little until the gear lines up with the threads of the leadscrew and drops into engagement).
    Once that is done, with no half-nuts or longitudinal feed (if the latter is separate from the half-nuts) engagement, crank the carriage back and forth. The dial should turn. If it does not, look at the gear on the bottom and verify that it is turning (you should not be able to move the carriage otherwise given the conditions I have described.)
    If the gear turns and the dial does not -- check that the screw in the center of the dial is firmly tightened. (Assuming that it is held on by a screw through the center as in the photo on the website you posted a while back in this thread.
    It is possible that the dial is a permanent part of a shaft, and there is a setscrew or a pin connecting the dial to the gear. Mine is of the latter design. Make sure that those are in place and tight as well.
    Once all of this is verified, with the threading dial pickup gear enaged with the leadscrew, you should observe the following conditions:
1)    With the half nut (and possible feed lever) disengaged, the     spindle not turning, and the carriage cranked left to right, the     dial should turn as the carriage moves.
2)    With the half nut (and possible feed lever) disengaged so the     carriage is stationary, the spindle turning, and the     reverse/neutral/forward lever on the gear train to the leadscrew     in either left-hand (reverse) or right-hand (forward) (as when     you are threading) so the leadscrew is turning, the threading     dial should turn fairly slowly (unless you are set up for a     fairly coarse thread, which would cause it to turn faster).
3)    With the half nut engaged (as for threading) and the spindle     turning, the dial should appear stationary, but the carriage     should be moving. (Probably a good idea to set up for a very     fine thread while doing this test, to avoid running out of     carriage travel while you are looking at what is happening. :-)
    The threading dial shows the position of the carriage with relation to the leadscrew, to show you when to close the half nuts (with the spindle running) to track the same path as before. While waiting for the right number to come up, you can sometimes crank the carriage backwards to get closer to the number you want to close the half nuts at. This won't get you started cutting the threads any sooner, but you will be watching the carriage move towards the start of the workpiece, instead of sitting there anxious, waiting for the number to finally reach the right point to close the half-nuts. (This is also the reason that there are multiple numbers and index lines on the dial, so you can (when the threads are right) close without waiting for a nearly full rotation of the threading dial.
    You really *should* take the time to make sure that the threading dial works properly on your lathe -- it is a great time saver, so any time invested in making it work right now will be repaid many times over the life of the lathe.
    Enjoy,         DoN.
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wrote:
[...]

Exactly.
[...]
I did? My Alzheimer must be worse than I thought.

[...]
That and 76 other things. However, I have saved all your advice in a text file for future reference.
Merry Christmas.
Michael Koblic, Campbell River, BC
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    O.K. Remove it from the lathe and double check whether the gear at the bottom turns freely when it is unmounted. If not, loosen the screw which is likely in the center of the dial. If this frees it up somewhat, it is likely that there are chips or other debris either between the gear and the bearing, or between the dial and the upper bearing. Disassemble, clean, relubricate, and check again.

    You -- or someone in the thread, and I think that it was you, posted a URL to a page on how to single-point threads on a lathe. Somewhere down a few photos before you get to the threading dial photo.
    Better might be to take a photo of your own threading dial on your lathe, and put it on a web site temporarily -- then let us see the URL so we can examine what you have.
    [ ... ]

    O.K. Good luck with that.

    And to you (and all others here).
    Enjoy,         DoN.
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wrote:

More stuff for the file, thanks. BTW you now merit your own folder in the "Tools" section. Oh, wait, that came out wrong...:-)
I swear I have a "Tools" section for all the workshop stuff! Absolutely no disrespect!

Jim Wilkins just fessed up. Whew! I was worried there for a moment.

It is the same one as jsw linked.

More like "better weather". I really do not relish being in the unheated workshop right now. OTOH since the Christmas rush I was able to slow down and concentrate on strategic activities such as design and planning.
The thread cutting was just a light relief which I sort of stumbled into on account of a crappy die.
The worst thing is that I shall have to start the New year with woodworking - my router table needs a better support and, most importantly, wheels, as it rather gets in the way. That amongst other things means that all the metalworking machinery needs to go under cover.
Still, change is as good as a rest.
Michael Koblic, Campbell River, BC
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    [ ... ]

    I did not take it wrong. Besides, at MIT, "tool" is (or at least was) a term for someone who focused deeply on a problem -- usually schoolwork rather than hobby work. :-)

    Good!
    O.K. So that screw in the center of the dial is your key to disassembling it once it is off your lathe's carriage..
    [ ... ]

    O.K. I've got to go to my shop and do some sheet metal work -- cutting out, notching, and bending up a housing to mount a control panel (for a VFD) to the front of the drill press. I've finally replaced the single phase motor on the floor standing drill press with a three-phase one to allow easy speed control and easy reversing.
    I'm already using the VFD, but the control panel is a little out of convenient reach.
    The ability to do this is a result of a recent project when I got a small TIG welder at a yard sale this fall, and before the weather turned too cold, I welded up a stand for a corner notching shear which had been kicked (gently) off a workbench to make room for a 24" straight shear (DiAcro), so I can now use both. :-) (I also have a 24" DiAcro finger brake.)

    And learned that you really could cut threads on your lathe.

    Given that some woods are rather acidic, and the dust settling on the metal sliding surfaces of machine tools can lead to rust, yes, cover.

    Indeed so.
    Best of luck,         DoN.
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wrote:

In the department's model shop at MITRE I had a 24" shear and 24" finger brake on the bench and a 24" Triok 3-in-1 machine on a cart, likely from another closed shop. Although I greatly preferred the bench machines, the 3-in-1 would handle the same jobs adequately and occupied much less space.
I had to dispose of a 13" South Bend lathe (sigh!) to cram in a more useful small knee mill. The main machine shop did have a Hardinge HLV-H but they wouldn't let me run it. I had a Prazi clone in my lab which was good enough for the minimal turning electronics requires.
One of my flip-top tool stands mounts a corner notcher permanently on one side and a shear or compact bender on the other. The pivoting tabletop frame has holes to take a long pipe handle which is very useful to pull against for all three. I can bend the end of long stock by rotating the stand on its casters instead of needing a large open space for the stock to swing around, and it can be supported by a ladder etc.
jsw
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On Mon, 26 Dec 2011 20:17:23 -0500, "Jim Wilkins"

Interesting! I use all three lathes about 9 times as frequently as my mill.
That could change as I recently got a rotary table and for Christmas a collet block with a 5C collet set but I still do not see it beginning to equal the lathe use.
Michael Koblic, Campbell River, BC
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wrote:

This is the current inventory at a nearby second-hand dealer: http://www.brentwoodmachine.com/category_s/96.htm?searching=Y&sort &cat&show &page=1&search=%20lathe The $950 one is similar to mine, for less than I paid 20 years ago. This might be a good time to look around.
jsw
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