Threading

May 28th, 2009

Poorly

Today I threaded my first screw. Kind of.

“Kind of” is a giant exaggeration. I learned how to make the longitudinal power feed and half-nuts work. In other words, the lathe was doing everything it was supposed to do in order to thread properly, but Gilligan was at the helm while the Skipper used the head.

I did, in fact, create threads of sort on a piece of steel. However they were extremely crude, and I managed to snap two corners off a three-cornered carbide insert. I snapped the first corner off by turning the drum shift the wrong way when I was trying to back the tool out of the threads. The tool continued on its way, ramming into the shoulder I had turned earlier. The second insert snapped when I reversed the direction of rotation. Something caught it from behind.

I decided to play with the quick change box, using one setting to erase the threads made using the previous setting. Probably not good practice, but fun.

It doesn’t matter. The point of all this was to learn how to make the lathe work properly. Making me work properly is a separate lesson.

The Clausing instructions aren’t very good. For example, there is a knob on the lathe mounted so the face is perpendicular to the floor, and the directions say it’s engaged when “vertical.” So I guess you snap it off the lathe and turn it on its side. I think “vertical” refers to the positions of the three letters on the front of the knob. If “A” is at the noon position, the “A” setting is engaged. And so on. Seems like it works that way.

I still don’t understand the 29.5-degree business for threading. I can’t say I’ve studied up on it. Since it didn’t matter today, I stuck a threading tool in the tool post, used a dead center in the tailstock to set it on the level of the center of the work, and fed it perpendicularly into the metal. I think it would have worked, had I shot for a coarser thread and avoided running the insert into things. It finally occurred to me to use the VFD potentiometer to control the movement.

The Moly-Dee seems totally wrong for turning. Not that I would know, of course. I was moving the shoulder farther down the work today, using the Moly-Dee as fluid, and it seemed like the AR tool refused to bite until the Moly-Dee burned away, indicating that it was lubricating really well but failing utterly as cutting fluid. WD-40 was even worse. People have recommended Ridgid pipe threading oil as a general cutting fluid, so I have some on the way.

Once the tool started to bite, it produced two types of chips. Tiny straw-colored chips or long blue ones. Correct me if I’m wrong, but I believe I’m supposed to be shooting for the least color change possible. The tool just sat there unless I applied considerable pressure, so I couldn’t make colorless chips.

I don’t know if I’m supposed to use the power feed when turning a piece to a smaller diameter. I figured it would be impossible to adjust. The manual cross feed seems jerky. I have seen the phrase “slip stick” used to describe machine tool motion, and I think that may be what I saw today. But I eventually overcame it.

I sat down and began an instruction manual for the lathe, in English a mere college graduate can understand. I wrote nine pages today, and I took it with me to the garage. Helpful.

It’s pretty sweet, sitting on my shop stool, making good use of the backrest, watching lathe videos and screwing with scrap in the newly air conditioned tool oasis.

I think I have the four-jaw chuck worked out. Once you start using it, you realize you have to do one axis at a time.

I have a list of things I need to do, to get to the point where I’m actually doing something. Might get there in a week.

6 Responses to “Threading”

  1. Bill P Says:

    I see you have now learned that carbide inserts don’t like shock loads.

    If you are going to do threading, you will need to get a fishtail gauge to insure that you are perpendicular to the stock, and also to check your tool bit when you grind your own. Threading to a shoulder is a challenge. Ideally you would like to have a short section of reduced diameter between the shoulder and the last thread, giving you time to throw out the feed. Lacking that, go slow, get fast, or hand crank the spindle near the shoulder.

    As far as 29.5ยบ goes, when you set the compound up at that angle, advancing the compound slide will cut on only one face of the V thread. Why do that, you ask? Well, plunge cutting straight into the work cuts both flanks of the thread. Since the two chips curl off and collide right in the thread, they can cause trouble. Cutting on one flank eliminates that problem. Generally, you make almost the full advance on the compound, leaving the cross slide set at zero, then as your last pass go in on the cross slide .001 and clean up both faces. Note that you should start out very small, .001 advance on the compound for a scratch pass, and gradually increase it, then decrease the DOC as you get closer to the full depth. The reason, the deeper into the work the tool protrudes, the bigger cut it is taking off the thread flank.

    Some materials are a pain to thread. Cold rolled steel is an example. Try aluminum to start. Use lots of coolant. Run about 1/4 of the SFM you would normally use.

    There is some arithmetic to be sorted out before you embark on this great crusade. You will need to calculate the amount of advance on the compound. Hint, it is surprisingly easy. You will also need a way to measure to see if you got what you were after, like thread wires. More on that as time goes on. Get familiar with major and minor diameters, and thread forms. When you are ready, we will move on to the steps that you need to do to produce a good thread.

  2. Steve H. Says:

    I wasn’t trying to thread to a shoulder. I was trying to thread to somewhere near a shoulder. I ended up threading INTO a shoulder.
    ,
    And a piece of carbide is still stuck in the work.

  3. jdunmyer Says:

    As Bill says, cut a groove in the part that’s fairly wide, and as deep as the minor diameter of the thread, this will give you plenty of time to disengage the half-nuts. Run the lathe at its slowest speed, and start with a fine thread, not coarse, the carriage is moving slower and things are easier to control. Do an even thread such as 20, 24, etc., so you can engage the half-nut on any graduation on your thread dial. Get a dial indicator that’s attached to the ways and use that to set your stopping point. Do a practice pass or 2 w/o actually cutting, to get the rythym of cranking back the cross-slide and THEN disengaging the half-nut. I always do that if I’ve not cut threads in a while.

    DOC on the compound will be on the order of .800/TPI, it varies with the exact form of your threading tool.

    I’m unsure from your post, but it appears as though you might have been using the FEED to cut threads instead of the leadscrew and half-nuts. That won’t work.

    Look at the HSM board for message threads on threading, there’s been several.

  4. og Says:

    Maybe I’m telling you something you’ve already figured out but you don’t cut the thread all at once, you cut a little bit, then go back and cut a little more, etc. Apologies if this is already known to you. That’s the porpose of the thread wheel, by the way. You start the cut on one mark, and then go back and restart on the same mark. Bloody difficult to cut blind threads by hand. CNC machines do it like falling off a log.

  5. Bradford M. Kleemann Says:

    Steve,
    Since you are apparently taking prayer requests, I have a 4 to 5 mm wide kidney stone. The urologist gave it 50% chance of passing on its own. I am seeing the Urologist at 12:15PM EDT (your time) today for an X-ray to see if it has moved at all. I sure hope (and pray) so! I don’t want laser surgery again.

  6. ErikZ Says:

    Don’t they have ultrasound machines for breaking up kidney stones now?