Problem making metric threads on a Jet GHB1340A lathe

Yep. You cannot thread to a shoulder on a setup like this.

Short of running the spindle by hand, with a crank (some folks

*do* this) will have to have enough runout room to account for the spin-down time of the drive train.

My approach is to use a 3 phase motor, and plug-reverse it when the tool approaches the shoulder. A rotary converter supplies the 3 phase power.

Jim

Quoting from "Moltrecht":

"When cutting a thread with a split nut continuously engaged, the spindle speed must be slow enough to allow the operator to have complete control over all of the lathe movements at all times. Thus, the spindle speed used depends upon the judgment of the operator. He should consider his experience in cutting metric threads on inch lead-screw machines, his experience on a particular machine, the ease with which a particular machine can be manipulated, and the size and type of thread being cut. All other machine settings made in preparation for cutting threads are the same as before, except that the 127-tooth translating gear must be used in the gear train."

One good way to maintain necessary control is to rotate the spindle manually. I do this with a crank on the outboard end of the spindle. Another way would be to turn off power to the spindle with enough room left for it to coast to a stop. Then when almost stopped continue to turn the chuck by hand. The moral to this is - metric threads aren't as easy to cut on "English" machines as English threads are. Leave the gears disengaged from the motor when turning the spindle by hand. Cranking back by hand is a lot easier to do if you aren't dragging the motor along.

I made a hand crank for my EMCO lathe for the same reason. Slower, but can thread to a sholder.

Be sure to back out the tool before cranking back to the beginning.

Paul

Robert Sw> Quoting from "Moltrecht":

Simply make a dry run with the threading tool clear of the workpiece. Cut power when you think it is far enough along and see how far it coasts. Adjust the switch point accordingly. If it coasts more than 1/4", reduce the spindle rpm. You will always be safe because it will not coast as far with the tool engaged in a cut.

Randy

Thanks for the suggestions.

One issue I have with the coast method is that even using the slowest rpm (70), and dry running to figure amount of drift (1/2" in this case), it may not consistently coast that 1/2 an inch. With only a small relief area being allowed, this runs a good chance of damaging the part and tool. The hand crank method suggested by others seems like viable option given the short distance required to be threaded.

Will check tonight on the reliability of consistent drift or try and make a manual crank for the outboard section.

These responses are greatly appreciated.

Doug

(snip)

I'd suggest contriving a foot-actuated brake that can kill power and stop the spindle instantly. My lathe came with such a brake, and it is very handy when cutting threads close to a shoulder. At low speeds I can always stop within considerably less than 1/4 revolution. I usually make a relief groove about .050 wide next to the shoulder for the bit to stop in.

Thought about this also. Not sure where it would attach, but I believe it could be done as you suggest. Wish it had one already, simply for the fact that it makes using the lathe a lot easier (in my opinion).

I made a spindle brake and cutoff switch for my GHB1340 Jet. With the loowest speed being 70 its way to high and close in a lot of situations, and it always made me uncommfortable, expecially when working close to the jaws or a shoulder or in a blind hole when threading.

I used a small drum type brake and expand>===On 4 Oct 2005 09:13:52 -0700, snipped-for-privacy@sbcglobal.net wrote: >===

============================================== Put some color in your cheeks...garden naked! "The original frugal ponder" ~~~~ } ~~~~~~ } ~~~~~~~ }

Sounds like a very interesting modification. Do you have any pictures or maybe a rough drawing of how the brake assembly is connected to the gear train? If not, that's OK. Thanks for the detailed explanation of the setup.

Doug

Greetings DC, Now's the time to consider a 3 phase motor and a foot brake if the lathe doesn't have one. I'm not familiar with this lathe but if it's a bench lathe then maybe an electric brake. At the very least get a three phase motor. Using google to check previous threads on Rotary Phase Converters (RPC) will provide you with a wealth of information. Three phase motors are cheap and plentiful used. And you need two three phase motors. One for the lathe and the other for the RPC. I know this answer doesn't help you out much right now but others here will tell you how to solve you immediate problem. The three phase motor will allow you to instant reverse the lathe so if you are careful you can withdraw the threading tool at the same time as you reverse the spindle. Keeping the lathe running reverse until the tool is past the part will allow you to cut metric threads pretty fast. It's also real handy for power tapping. And some kind of brake can be a real finger, chuck, tool, etc saver. ERS

I don't know if you're talking about a mechanical brake, but you can brake ac motors electrically. I've not done it, but if I recall correctly, it's a matter of applying dc to the field winding. I'd think that the only problem would be with how much current to apply.

Bob

On my 13 - 40 lathe, from MSC, the motor shaft has a brake disk mounted on it. Automotive type master cylinder is hosed to a set of calipers and the whole affair is actuated by a long treadle, within easy reach of any area in front of the lathe. IMO, your time would be better spent learning to cut metric threads with a hand crank, etc., than fiddling around with trying to contrive a brake. Frustrations less; rewards more! You will feel better about your new machine if you perfect certain difficult procedures first rather than trying to modify it.

If he has three phase, then he doesn't really need the foot brake.

It's a snap to plug reverse the motor to a dead stop, or better yet throw it over into reverse as the tool is backed out, to bring it away from the shoulder.

Jim

If the thread is a short one consider setting up a hand-crank for the lathe. If this is not possible/practical you will have to anticipate how long the lathe will coast after power is turned off and 'pull the plug' in advance. As the machine is slowing down have your hand on the cross slide ready to rapidly back it off at the right moment. Then reverse the lathe back to your starting point. Be sure to have all the lash on the correct side of the tool before starting into the cut. Takes some practice and will supply a few moments of high drama but it gets easy.

Ken.

Ok here is my suggestion

Use the thread dial but only use the 1 position

Take the first cut and when at the end disconnect the half nuts and immediately stop the motor.

Notice how far the thread dial continued to turn.

Reverse the lathe and engage the half nut when the dial read 1

Stop the lathe and get set up for the next pass and leave the half nuts engaged and start the second cut.

Repeat as needed.

Hope this is clear and useful.

Good luck. Bill K7NOM

Yeah, that "snap" would be me breaking tools.

Bob Swinney>

You'd be suprised, it's easy to get into the swing of threading this way. I recently did a run of M8X1 fasteners for my motorbike restoration, granted it's a fine thread so the feed rate is slow, and if you make a mistake and over-run the thread end there's not much danger.

But some of them were pretty short, so I was threading right up to the collet, and nothing went wrong.

First few times doing this I would suggest using a setup part with large relief runout area, and getting used to the machine. But with the three phase power, the spindle comes to a stop and reverses, very predictably.

Jim

I think the hand crank is probably my best option. Still would like to install the brake as an option later (probably after warranty is up). Setting up 3 phase is also a pretty good idea, but I'm not ready to make the investment in rewiring and replacing motors.

Started making hand crank last night. Decided to design a model that will do a clamp fit over the spindle shaft so no machine modifications will be required (do not want to void warranty). Looks good so far. Should have finished today and ready for testing by tonight.

Thanks for the help.

Doug

Robert Sw> On my 13 - 40 lathe, from MSC, the motor shaft has a brake disk mounted on

I made a rear tool post to hold the threading bit. Then I run the lathe in reverse to cut the right hand thread. This way the direction is away from the head stock. The return stroke then can be by hand with no chance of hitting the chuck.

Jim