HELP: Single Point Thread Cutting



    Yep. At least probably better than from Home Depot. :-)

    O.K. The easiest to get is the pipe tapping high-sulfur oil from Home Depot or just about any other hardware store. Molly-Dee I order from MSC (one day delivery from them, and I seldom get off an order smaller than $100.00 once I have decided that I'm going to call them. :-)

    For a 40 TPI with a 3/8" OD, I come up with only 1.22 degrees, so your side relief should be sufficient here. With the 1/2", it is even less.
    [ ... ]

    O.K. One thing to consider, however, is that too small an infeed results in the tip burnishing and sometimes work-hardening the workpiece on one pass, and then digging under after one or more extra passes.
    Unless the depth of the thread is truly critical, I would check depth of subsequent passes, and try to adjust the feed so you can take a full depth pass (whatever works for you) for the final pass. Following it with another pass at the same setting will burnish the finish a little, if there was no spring to catch up to. (I'm used to my 12x24" Clausing being rigid enough for pretty much any cut that I am willing to risk.

    That "formed" assumes that your threading tool has a tip rounded or squared off just right to meet the spec for that thread. Mine (except for the final column) assumes a sharp point on the threading tool.

    O.K.
    Those both help -- though there are the various interfaces between the carriage and the tool (cross-slide ways, pivot for the compound, and compound ways) all contributing to give. Have you pulled the compound out at the pivot and made sure to clean out any chips before re-installing it? IIRC (and based on my old 6x18 Atlas/Craftsman), the Atlas design has a boss projecting from the cross-slide, with a dovetail turned on it, and the compound has two setscrews pushing in bronze or brass angled pushers to contact the dovetail. The Clausing, by contrast, has a hole in the cross-slide, a cylindrical plug on the bottom of the compound, and two holes 180 degrees apart going through the flange on the compound and T-bolts in a circular T-slot surrounding the hole. A much more rigid mount. Since Clausing and Atlas combined at some point (I hear stories saying that A bought B, and others saying that B bought A), hopefully the 10" has the more rigid design.
    Also -- what size of tool stock did you use for the threading tool? I generally go for at least 1/2" on my Clausing (where the quick-change toolpost and holders will accept up to 5/8" shanks.)

    O.K. That sounds pretty good.

    And remember -- if the diameter is too small, a live center can get in the way of the the tool and toolpost, so you may want to pick up a half-center (actually a bit more than half) to give you more elbow room. *That*, you will have to use with a lubricant on the center, as there are no live half centers. :-)
    The ideal lubricant for that is a lead compound, which is of course very difficult to get these days. Older machines often had a cavity in the tailstock holding some, with a dauber to transfer it to the center. Lacking that, I would suggest an extra pressure lubricant for the center -- perhaps a lanolin based case sizing lube from a store which deals in reloading equipment.

    Good. Keep your eyes open.

    [ ... ]

    I presume that I don't need to mention being careful to keep the abrasive grit from reaching the bed of the lathe. Cover with a cloth or newspaper or aluminum foil. At a suggestion -- the newspaper wet with oil will help to keep the grit on the paper.
    [ ... ]

    O.K. The trick for this is:
1)    Remove the chuck.
2)    set up a dial indicator mounted on the headstock and measuring the     register of the spindle.
3)    Take some wood dowel (broomstick is the common suggestion, if     it will fit into the spindle), and push down and pull up, noting     the change in reading of the indicator. Someone who owns a     plain bearing lathe should chip in here -- but I think that the     reading should be on the order of 0.001" deflection when dry     (e.g. after sitting overnight), and much lower when the spindle     was just being run, and thus has a good film of oil.
    Lacking a wood dowel, I would consider aluminum or brass rod, so     it is unlikely to damage the spindle.
    You can find web based information on pouring a new Babbitt bearing in place, and whatever scraping may be needed.
    [ ... ]

    Or -- use one of the on-line metals places. They cost more than a local place -- but you don't have to get lots more than you happen to need. Hmm ... I wonder whether MSC has any? Let's check the web site, since that catalog is too big to use while I've got a keyboard in my lap.
    Nope -- The only 12L14 listed on their page is machinable expanding collets, so I guess that the online places are your best bet. Not too many industrial places stock the 12L14.

    Which is why I listed all the possibilities that I could think of.

    [ ... ]

    O.K. Good luck,         DoN.
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snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote in

I'll try to pick up something today.

I feel your pain :-) For me it's BusyBee machinery and KBC tools.

It's a rather easy change to make, so I may do this anyway. Then I'll be sure that it's not contributing to the problems.

You may be on to something, see my comments at the end.

While my experience is limited, it is mostly on much heavier equipment than the Atlas. In school we used 12" swing Standard Modern-Lathes which would probably be similar to your Clausing in rigidity, and from there I went to running a 24" swing Monarch for a while before returning to school. At the same time I am enjoying the challenge of learning to work within the limitations of the 10" Atlas.

I am not familiar with the 6" Atlas, but it sounds similar to the setup on the 10" Atlas. It is a known weakness of the lathe.

1/4" clamped as short as possible in the toolpost. It's what I had on hand. I agree that larger would be better I among other things it would allow me to let it project a little more.

I have the case sizing lubricant on hand. I'll add a half center to the shopping list.

OK, wetting the paper with oil sounds like a good idea.

I guess my concern here is how hard to push. I want to find any play, but I don't want to damage anything. A little leverage goes a long way :-)

While it would be really interesting, it is probably more of a project than I can undertake.

I am in Toronto frequently, so it's not a really big deal I'll call around the metal suppliers. I see now that I was spoiled in my youth. My dad had a screw machine shop, and always had LOTS of 12L14 on hand in many sizes, as well as an assortment of brass and stainless rod.

I ran a couple of trials last night:
The first attempt was the same setup as before, but at around 600 RPM. My reactions are still adequate, but I wouldn't want try it with 16TPI :-) There was no noticeable improvement.
Next I tried with a live center supporting the right end at 40 RPM. Due to interference between toolpost and tailstock I had to go back to the lantern toolpost. This improved the thread at the right end, without making the rest any worse. Now the roughness and tearing are evenly distributed.
I think your comment about work hardening may be the key. One would expect to get progressively larger chips if the infeed is the same for each pass. Instead, it was noticeably variable, as if it occasionally "dug in" and took a larger amount on some passes. I am going to try cutting it more agressively to see if that helps.
Rob
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Where does this "4 times" rule of thumb come from? I am querying this because deflection is much more sensitive to diameter than length. e.g. comparing 1" diameter bar of 4" length to a .5" bar of 2" length, both loaded with 100 lbs at the free end, the deflection is _twice_ as great for the smaller bar. This is even worse if the deflection in proportion to the diameter is considered.
BeamOut''(30E6,I_tube 1 .5)Beam 0 (4 100) pos'n pt. couples shear bend. slope def'l force stress stress 0 -100 -400 -169.8 4074 0 0 4 100 0 0 0 0.0005432 0.001449
BeamOut''(30E6,I_tube .5 .25)Beam 0 (2 100) pos'n pt. couples shear bend. slope def'l force stress stress 0 -100 -200 -679.1 16300 0 0 2 100 0 0 0 0.002173 0.002897
Ted
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    You're assuming a constant loading (100 lbs in this case). I think that with smaller diameter workpieces, it is normal to take lighter cuts, so the loading should scale, too.
    Enjoy,         DoN.
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DoN. Nichols wrote:

Yes. This wasn't mentioned and is one of those things that anyone who has done some machining would know but, for a newbie, it might not be obvious.
Ted
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Happy Father's Day to all the RCM and HSM folks out there to whom it applies.
Brian Lawson, Bothwell, Ontario.
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Same to you Brian! Marty
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Right back at 'cha, Brian. Nice of you to mention it. Ken.
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Brian Lawson wrote:

I'll send a round of thanks out there as well and to Brian if it applies. Martin
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Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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Hey guys,
A better than average looking auction coming up Saturday July 17/04 (tomorrow!!) just east of Kitchener, Ontario. Wish my budget this month would let me go!! This place is just a few minutes north of the 401, just south-east (mostly east) of Kitchener.
<http://mrjutzi.ca/2004-07-17-cat.html
Take care.
Brian Lawson, Bothwell, Ontario
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After reading some of the answers to your post (and your other responses):
Point of honor: If you arbitrarily choose 80 sfm for the cutting speed of your 1018, it yields a RPM of approx. 853 for 3/8 diameter. The stated 40 RPM is extremely slow. Go as fast as your reflexes/skill will allow. Practice until you can approach 300 rpm. Your current 40 RPM translates into SFM of about 4??
Get some cutting oil, and brush on liberally for each pass. The old fashioned sulphur based cutting oil is quite good for single pointing.
Study the machinery handbook a bit more on thread form. A sharp vee is not considered standard. With that in mind, and after stoning a slight radius on the end of your tool, it should be clear that you will achieve proper thread form before a sharp vee occurs on the OD. Once you have done a few threads you will learn to gage the flat on the OD as a clue that you are almost there.
Burrs happen when machining. Suggest removing with file or emery cloth before reaching final thread depth. (Which you did!)
As to tool angles: When threading and using the compound rest at 30 degrees a positive rake at right angle to the left side of the thread profile will give best results with the tool mounted horizontally. (for 1018) The back rake only serves to put everything except the tip of the tool below the center line of the work.
As to the final pass producing roughness, it is VERY common. So before you get to that point, you start babying it. Extremely small infeeds when you approach full thread depth. Perhaps even a couple passes without any infeed at all. (Known as spring passes)

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snip
snip
It is difficult to hand stone a threading tool of this size without slightly rounding the edge on the last few thou, just where it matters most. You will finish up with a keener edge by using the tool as ground or, at most, topface only stoned. A really keen edge is vital for successfull fine finishing cuts.
Feeding in at 30 deg with 10 deg side and back rake will generate a slightly incorrect thread form although the error is pretty small. Top rake should be at 90 deg to the LH cutting edge so that this edge remains horizontal. Up to 10 deg rake can help particularly when cutting coarse threads. At 40 TPI zero rake should be OK.
Unless you have overriding strength or wear resistance requirements your problems would pretty much instantly disappear if you changed to brass or a free cutting leaded mild steel.
Jim
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First off, lose the rake. The complex geometry of the tool cutting into a curved surface, makes it almost impossible to get a tool with rake to have the proper thread profile. Back rake is only good for roughing out coarse threads and will need to be followed by a finishing tool. Side rake is only needed for course threads or very tough materials.
If you are in a situation that needs side rake, like production cutting coarse threads, where a tool change would be non productive, or cutting threads in copper or inconel, then do this...
Use only side rake. It should be perpendicular to the leading edge. This way the leading edge remains horizontal and cuts an accurate angle on that flank of the thread. Then set your compound very precisely to 30 deg. This way the accuracy of the trailing edge of the thread is formed by the travel of the compound.
Either way, for 40 tpi you do not need any. Your tool should be flat.
Next, be sure that your tool has the proper clearance. The leading edge of the tool needs more clearance than the trailing edge, Look at the end of the tool with the cutting surface up. The junction of the facets should appear to lean clockwise on a right hand external tool. For left hand, it should lean counter clockwise.
I suspect this clearance is your problem. The lack of clearance causes the tool to heel or rub against the thread before it can cut it. It causes you to use excessive pressure because the edge of the tool cannot cut and is instead forging the thread into the steel. Eventually you get enough pressure, that the work rides over the tool snags it, and forces it to cut where there is enough clearance, Usually, in the middle of the previous thread.
Paul K. Dickman
Rob McDonald wrote in message ...

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Thanks for the advice Paul
Rob
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<SNIPS>
Most of the advice you have received is well founded, although there is one point that I have not seen mentioned........
Cutting this small a thread makes center height *extremely* critical. Dead center is best, but If anything, you should be a few thousands low. Otherwise, you are "forming" the shape rather than cutting it.
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