Bench grinder spindle thread

Now I'm thoroughly confused. TMI, but thanks! ;)

-- If you're trying to take a roomful of people by surprise, it's a lot easier to hit your targets if you don't yell going through the door. -- Lois McMaster Bujold

I have great faith in you.

I *thought* it cut freely...but I will have another look at the profile. The side clearances may not be all that they can be.

As to the TPI issue, another 9x20 owner, no doubt as a result of the same experience, pointed out that I may have missed a step in the change gear set up.

In my defence I have to say that the diagram on the machine is not particularly easy to interpret *and* it is different from the machine manual. I will make the necessary adjustments and report the result.

Of course the pragmatic Kalashnikov solution would be to cut the first four threads as is and finish the whole thing with a die. But that would not be sporting!

Michael Koblic, Campbell River, BC

Yes.

It's a bunch of loose gears. But I think I may have been given the answer. See my reply to JSW.

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The reverse was done backed out. I shall review the cutter shape, though.

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The second sentence in OP.

Which is what I was trying to avoid. But now I am committed :-)

Michael Koblic, Campbell River, BC

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. 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.

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.

Arbor nuts for securing grinding wheels aren't torqued like some fasteners in other applications, but being able to attain a specific torque value is easy with a fine thread, and mating contact areas are of benefit to holding a position. All bets are off when considering low-grade hardware.

A factor someone mentioned is the deeper the thread, the weaker the arbor (or bolt/stud) for a given size.

With blotters on each side of a grinding wheel and the appropriate cupped plates, (typical bench grinder) arbor nuts generally only need to be snug, and a fine thread is beneficial for achieving snug.. not even close to a grunt.

The blotters become slightly compressed, making good/excellent overall contact between the abrasive and the plates.

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.

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.

jsw

Now memorize this:

jsw

BTDT (senior HS year in Org Chem) Remember six or seven of 'em, at least in part.

Have you seen the Ig-Nobel prize winning table?

-- Tomorrow is the most important thing in life. Comes into us at midnight very clean. It's perfect when it arrives and it puts itself in our hands. It hopes we've learned something from yesterday. -- John Wayne

OK. That makes sense. SOunds like it comes under the category of "beneficial" but not "essential".

Michael Koblic, Campbell River, BC

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

I believe there would be reasons for using fine threads from an engineering standpoint.. but if I did know, it's been forgotten.

In pondering my own experiences with a lot of mechanical assemblies, I'm certain that fine threads are most often chosen for fasteners associated with moving parts in power transmission components.

In automotive applications, items bolted to the engine block are often coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts, etc.. have generally always had fine threads IME.

In many applications the thread choice is related to the metal/material types which have threaded holes.

My understanding is that coarse threads were intended for steel screws in cast iron assemblies, and the fine threads came about for steel into steel.

Joe Gwinn

Yes, and the reason for the move to finer threads in steel is twofold: Fine threaded *screws and bolts* are stronger in both tension and shear (counterintuitive, perhaps, but check it out) and they are better at self-locking: they're less likely to loosen from vibration.

In favor of coarse threads, aside from some installation issues, such as less likelihood of cross-threading, they are less likely to strip out of a soft or fragile material. And it is easier to make a strong threaded hole in brittle material (cast iron) with a coarser pitch.

As for total strength, then, it's a matter of the relationship between bolt strength and thread strength in the hole. In hard, strong materials, fine is better. In softer, weak, or brittle materials, coarse is better. In the latter cases, bolt strength is much greater than the hole strength either way.

I can see how a coarse thread may be less likely to strip in a very brittle material, or perhaps in a coarse grained material where the size of the grains is large relative to the threads. But a properly cut (or formed) fine thread should be somewhat more resistant to stripping than a coarse thread in a reasonably ductile material, regardless of the strength of the material. I'm using "stripping" here to mean shearing of the threads as a result of tension on the screw.

Apparently this is generally true. The older theories that I learned are based on the fact that the stress on the first couple of threads results in failure of those threads before there is significant stress on subsequent threads, and shear strength was dependent on some product of thread contact area and the pyramidal strength of the thread form -- coarser threads producing stronger pyramids.

I haven't looked at any of this for 30 years, except in passing. From what I've seen, the current thinking is that shear strength is almost exclusively a product of shear area (divided by a factor for helix angle) and that this says finer threads are always stronger in shear, except, perhaps, in brittle materials.

Somewhere there must be some good engineering test results to clear this up, but I haven't looked for them.

The explanation I remember from college is that the first few threads are overloaded and fail sequentially because the bolt elongates more than the female threaded element, if it is a solid block. If the female threaded element has the same cross-sectional area and elongation as the male thread, as in a turnbuckle, the bolt is more likely to snap at a thread root as long as at least [3 to 5??] threads engage.

When a turnbuckle made personally by the great yacht designer L. F. Herreshoff was tested to destruction every part of it deformed, showing he had balanced the strengths all over without wasting material.

jsw

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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

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