PDA

View Full Version : How to Cut Thread with 1 tpi?



BigBoy1
09-28-2008, 09:58 PM
I want to make a spiral that will rotate and transport round balls from ball bearings from a lower level to a high level. The pitch of the spiral thread needs to be 1 tpi. The largest thread I can cut on my SB is 4 tpi. What do I need to do the be able to cut the 1 tpi threads and/or what modifications do I need to do the be able to cut threads with a pitch of 1 tpi? Thanks.

Bill

J Tiers
09-28-2008, 10:11 PM
You need the leadscrew to turn 4 times faster than for 4 tpi. Whatever gearing change will do that is what you need.

If you have a QC box, you need to change the drive gear setup to do that, the box will already be maxed out.

When that is done, the gearing ratio from spindle to leadscrew will be so high that you may want to drive the leadscrew and let the gear train drive the spindle. Otherwise you may break something.

garyphansen
09-28-2008, 10:22 PM
"Otherwise you may break something." That should read, You will break something! Your gears. The early South Bend single lever gear boxes has a setting for 2 threads per inch. They broke gears so South Bend dropped that setting. One thread per inch is too much to ask out of a lathe with an eight or six thread per inch lead screw. What you need, is a lathe with a lead screw with 2 threads per inch or maybe 4 threads per inch. Fine someone with a Lodge & Shippy Lathe. Gary P. Hansen

J Tiers
09-28-2008, 10:41 PM
If you reverse the drive and drive the leadscrew instead of the spindle, you should be able to do the job, IF you adjust the gearing for an added 4:1 ratio.

That will avoid the high ratio issue, and will drive the spindle slowly, instead of driving the leadscrew insanely fast. But take light cuts, because the torque is large and may (will) exceed the gear capability if you try to use deep cuts.

it's a workaround, and like any workaround, needs care.

Driving the leadscrew may be difficult.... finding a place to put the pulley.......??

Disclaimer..... I have not done this. But if I had to that is what I'd do.

Oldbrock
09-28-2008, 11:03 PM
You need a universal mill and set it up for helical milling. Assuming a 4 tpi feedscrew and direct drive to the dividing head not to the 40 to one drive, a 24 gear on the feedscrew and a 96 tooth gear on the dividing head spindle should do it. Use whatever cutter that will give you the required profile you need to make the balls fit. You will need to set the cutter to the correct helix angle. as I don't know the diameter of the transporter I can't calculate the helix angle for you.Peter

Oldbrock
09-28-2008, 11:45 PM
You need a universal mill and set it up for helical milling. Assuming a 4 tpi feedscrew and direct drive to the dividing head not to the 40 to one drive, a 24 gear on the feedscrew and a 96 tooth gear on the dividing head spindle should do it. Use whatever cutter that will give you the required profile you need to make the balls fit. Peter Sorry, somehow posted twice.

darryl
09-29-2008, 12:18 AM
I recently needed to mill a variation in thickness on the edge of a plastic disc. That variation turned out to be 1 turn per inch, because I had a leadscrew with - you guessed it- 1 turn per inch! This came off a garage door opener (and it was extremely messy, what with all that moly and lithium grease). Anyway, I made a makeshift nut by heat-molding some plastic around the groove and setting it up in the mill so the disc would travel as I rotated it by hand. By the way, this was definitely NOT a climb milling operation :)

If you were to be able to find such a leadscrew, you could possibly chuck both this and your workpiece in the lathe, supporting the outboard end of the leadscrew in the tailstock. A makeshift nut could be used to bring the carriage along. Of course this would be strictly a hand powered operation, and your workpiece would have to be hollow to fit over a section of the leadscrew. You could possibly turn down a section of the screw for the workpiece to fit on, and/or you could temporarily pin the two together.

For my use, I made up a nut to fit the leadscrew. I could have used the one that came in the garage door opener but I wanted a tighter fit. In your application it may not matter since the play during cutting the thread would be taken up at one end of the slop, and the position of the cutting tool would be proper whenever the tool was being brought in from the right.

Thanks for asking this question- now I see how I can go about doing a similar thing.

oldtiffie
09-29-2008, 12:40 AM
I note that the shape of the spiral groove is not specified.

A "V" or "U" or a "V" with a wide "flat" at the bottom will suffice for ball-contact and guidance. A "half-round" section is not needed.

With the 96:24 lead-screw:dividing-head spindle and a 1/4" pitch lead-screw the drive should be OK.

A horizontal or universal mill is not needed if a vertical mill and an end mill of the required shape is used in the milling-head spindle collets with the milling-head vertical and its axis directly above the axis of the work-piece.

It is important to realise that as it is likely that more than one cut or "pass" is needed that either the milling head/spindle will need to be raised or alternatively that if a "knee" mill is used that the "knee" (and table) will need to be lowered between "passes" so that the cutter can be re-set to the "start of cut" position without fouling the work-piece.

It is important that the gear-drive not be disengaged until the cut is finished.

Note that setting the dividing head axis for the helix angle is not required either.

Many people associate spiral milling with a universal horizontal milling machine with the cutter mounted on the horizontal arbor exclusively and exclusive to/of a vertical milling head.

small.planes
09-29-2008, 04:13 AM
Or you could mount a rotary cutting head with a burr (dremel would probably do) on the lathe cross slide (assuming you can get the 1tpi gearing) and use that instead of a lathe tool. Then spiral mill away :)
Youll probably still want to drive it from the leadscrew end of the gears, and actually doing the calcs for SFM etc would probably be a good idea. IIRC in chaddocks Quorn book there is a section about this sort of thing, as the head height adjustment is a similar course thread, and it has the sums in it.

Dave

Norman Atkinson
09-29-2008, 04:20 AM
Chaddock cut a 1" TPI in a 1 1/4" column on his Quorn and this is in his book.

He cut it using an ancient Drummond 7" swing mdel makers lathe.

NickH
09-29-2008, 05:09 AM
Chaddock cut a 1" TPI in a 1 1/4" column on his Quorn and this is in his book.

He cut it using an ancient Drummond 7" swing mdel makers lathe.

Doesn't the book state that he used the Quorn spindle attached to the late?

I have a micro mill head which mounts to the cross slide for this & other tricky stuff, works a treat,
Regards,
Nick

Errol Groff
09-29-2008, 08:35 PM
You could email someone with a CNC lathe (like me) a drawing or sketch of the part and they could let you know if they could do it for you. Our CNC lathe will make any pitch needed. The coarsest pitch I have made on it is .875 pitch.

lazlo
09-29-2008, 08:52 PM
Chaddock cut a 1" TPI in a 1 1/4" column on his Quorn and this is in his book.

He cut it using an ancient Drummond 7" swing mdel makers lathe.

Norm, if I remember correctly, Chaddock geared his Drummond to 1:1, turned the leadscrew by hand, and then used a thread milling attachment to cut the 1 TPI thread.

Similar to what Brockley and small.planes are describing.

darryl
09-30-2008, 01:41 AM
Does it have to be machined, or can it be fabricated? This idea's been bouncing around in my head the last day or so- Wind a spring, stretch it out so it describes the helix angle you want, then fit it over a shaft. Carry this idea further- say the balls you're dealing with are 1/4 inch diameter. Start with 3/4 inch wide flat bar and wind it into a spiral. Stretched so the gaps become 1/4 inch wide, that becomes 1 turn per inch.

That might be hard to bend- maybe start with 1/4 inch square bar, make that into two helical coils, then intertwine them and mount on a rod or tube. Space them evenly and you have a two-start thread at 1 tpi. You can keep going with this idea- if you can bend a flat strip in the flat plane, you could intertwine four of these and have a four-start thread with that same 1tpi (or nearly so).

Another idea- start with a flexible round 'rope' of some kind, maybe vinyl tubing, maybe some mic wire, extension cord wire, etc. Wrap that around a rod or tube at the right helix angle, then surround that with a thin sheet of something- waxed would be good. Pour a filler into the space between, and when cured pull the wire out. Mount this on the lathe, then with very slow speed going on, use a round file to even up the groove.

I must have a full tank tonite (not sure what's in it though)- here's another idea- chuck a piece of tubing in the lathe. Grind a cutting bit which will peel off a curly helix, then adjust speed and feed to get a consistent result. Let that define your spiral groovy.

oldtiffie
09-30-2008, 01:58 AM
Is there any need for a "groove" at all? Can't it be achieved by using two wires wound as helixes and spaced as required? The wires could be attached to or supported by pins that were drilled into a core shaft. Drilling those holes for those pins is a simple job for a vertical mill and a rotary table or dividing head.

Why is it assumed that a spiral of lathe lead-screw accuracy is required?

Is it?

What are the real requirements?

Norman Atkinson
09-30-2008, 02:29 AM
There is a lot of very varied opinions about making the Quorn thread. In fact Old Tiff and Professor Dennis Chaddock are in agreement about NOT needing a thread- or questioning it.

I recall cheating and getting mine in with the kit or correctly two kits( shame, shame and out with the sackcloth and ashes)
It does become a Chicken and Egg situation about 'the Spindle' ( here comes another bit of heretic burning because it is a bugger to make)

Someone was prattling about the Quorn and surface grinder bearings -and not getting very far. So could I jump into another attack from the Puritans/Purists/Purile and say
'Yer don't need to make a fancy, all singing Quorn spindle!'
A perfectly acceptable spindle can come from the other Model Engineering Services design which is called the Kennet. A far simpler but pretty good tool which I also built.
However, spindles are two a penny and one can go to the Stent which also has a simpler than the Quorn one.
Again, I have built one from castings but I have a mild steel fabricated one- which I didn't make.
So where does that leave us? As far as the Quorn is concerned, one could leave it out or settle for a thread which is easier to make or throw the baby out with the bath water and mount a home electric drill on the saddle and follow a 1" spiral as the old masters did.
Before even Old Tiffie and I were even thought of.

It all keeps the remaining gray matter exercised and somebody else will, I hope add his 'improvements'

A.K. Boomer
09-30-2008, 07:22 AM
And dont forget to install ball screws on your X axis...

Norman Atkinson
09-30-2008, 08:03 AM
AK,
Quorn Number 2 is under the bench somewhere. It's completed and I moved on to Stent2 which lies with it.
Somebody offered me a Clarkson Mark 1 with most of the goodies for a 100 washers with Big Ears Mum on them.
For something similar or two fills of UK gasoline, I got the fabricated Stent with the short wheel base.

I also have a set of Mozart's Balls.

Sort of the Inn thing

Cheers

Norm

Mcgyver
09-30-2008, 08:20 AM
For an exact helix, the spindle and screw have to be positively engaged. this means a gear arrange in the lathe or mill. Given that is a big cut in (possibly) a long spindly piece, I'd be thinking a cutter head with a ball end mill either mounted on the compound OR doing it in the mill with a hobbled together 4th axis connected to the table feedscrew.

All of these set ups imply a fair bit of work cobbling things together, which may be inevitable. However, given the application, it strikes me there may be a lot of latitude in how exact the helix needs to be - this may open up new ideas on engage screw to work axis is a less fixed manner.

For example, maybe instead of a gear chain from mill screw to 4th axis it could be a belt and 4th axis could be a simpler affair......or advance the carriage by a cable wrapped around something on the spindle. For example, for a 1" lead, have a section at the end of the work (at the tailstock say) turned down to .318 (1/3.14) and wind thin model control line around it. a couple of pulleys takes the cable around behind the tail stock and drags carriage along at a 1" lead. with a spindle and ball end mill mounted on the carriage, turn the lathe over by hand

there's a little bit to build, but less than gear trains

BigBoy1
10-02-2008, 05:36 PM
I've read all of the suggestions, comments, tips, etc. and think that I have several ideas on the way to go. Yesterday I was over at the local community college machine shop and discovered in the corner a 1946 LaBlond 16" by 78" that had a thread cutting setting of 1.5 tpi. Maybe if I oil the machine and get it backing running order, I can use it. It is not 1 tpi but I can change my ideas and design slightly to make it work. Appreciate all the help.

Bill

Fasttrack
10-02-2008, 05:44 PM
I've read all of the suggestions, comments, tips, etc. and think that I have several ideas on the way to go. Yesterday I was over at the local community college machine shop and discovered in the corner a 1946 LaBlond 16" by 78" that had a thread cutting setting of 1.5 tpi. Maybe if I oil the machine and get it backing running order, I can use it. It is not 1 tpi but I can change my ideas and design slightly to make it work. Appreciate all the help.

Bill


Mmm... sounds like a beauty! You ought to see if you can take it home with you! :)

aboard_epsilon
10-02-2008, 06:57 PM
HOW ABOUT YOU IF PUT A Mandrel handle in the spindle bore and turn it by hand taking mincing cuts with the appropriate gear in the train

all the best.mark

Norman Atkinson
10-02-2008, 06:57 PM
If I recall correctly, it was the thought that gearing a small lathe down with gears would strip the thing under power.

Again, Chaddock's 1" was a suggested thread and if I had access to a lathe with 1.5" thread 'in the gear box'. I would go for it.

Perhaps we are being too pedantic. Old Chaddock had all the qualifications and he still made suggestions- and left it up to the individual tastes.

BigBoy1
10-03-2008, 09:06 AM
Mmm... sounds like a beauty! You ought to see if you can take it home with you! :)

I'd love to take it home. I'm currently working on getting a 1944 South Bend 14.5" by 48" that has been under "restoration" for many years. It has been "buried" under loads of "stuff" and hasn't seen any work done to it in a long, long time. I think most of the parts are still there in boxes below it. Maybe if I succeed in getting the SB, maybe I can start working on the LaBlond!

Bill

derekm
10-03-2008, 11:53 AM
The stripping of the gears is because of the force to move the saddle is multiplied up (or not sufficiently reduced) and then finds the weak spot

One solution then is to apply an additional source of torque to the lead screw so that the lathe gear train only has to add the small remainder or even act as a brake to regulate the speed of rotation.

One crude way to achieve is to have a drum attached to the leadscrew with a cable wrapped many times around it running to a block/s in the ceiling and down to a weight.

How much weight/torque? - use a torque wrench to see how much it needs to rotate the lead screw.

S_J_H
10-04-2008, 02:34 PM
Out of curiosity I thought I would see if my little scratch built cnc lathe could cut a 1tpi thread/groove.
It handled it well. Spindle turning ~100rpm and carriage movement at 100IPM. Controlled by mach3.
This was the result of just a quick test.
http://i109.photobucket.com/albums/n48/S_J_H/cnc%20bench%20lathe/1tpitest004.jpg

Follow the link below to a short video of it cutting the 1tpi thread-

http://s109.photobucket.com/albums/n48/S_J_H/cnc%20bench%20lathe/?action=view&current=1tpitest.flv


Steve

darryl
10-04-2008, 02:58 PM
I don't think anyone had mentioned this, so here goes- is there any need to use metal for this part? If not, you could make it from plastic rod without a lot of hassle. Drill a small hole at a point near the chuck, insert the end of a piece of wire, then rotate by hand to wrap the wire to the helix wanted. Adjust the coils as you go to even out the spacing, etc. Tape the end down. Pencil in a line following this wire. Remove wire. Using a round file, make a series of short divets just to the side of this line. Continue by filing the high spots between these divots until you have a more or less continuous groove. At this point you can run the lathe slowly and continue to file, letting the groove lead the file.

Beware of rotating chucks and unexpected push rods, the file in this case. If you can mount the plastic rod on a bar to keep it well away from the chuck, so much the better. Maybe support the bar at the tailstock and have the plastic rod near that end.

I've done this with aluminim, and it takes awhile. I'd have no problem doing it in solid pvc or similar. I put the file (a round wood rasp I suppose it is) in a cordless drill in reverse, and it shears the groove to a pretty good finish. You need a well defined groove established to begin with though, or the file will just roll away on you. I did have to remove the tang from the file, and it just occured to me that the largest diameter you could use with any typical cordless would be 3/8 inch.

You're free to make a single start, double start, etc, and choose your helix direction. No gears, threading apparatus, etc required, just a slow spindle rpm.