Determined to Single-Point Thread

[Mrfix71]

Gidday!

I've had my "Big Dog" 7x lathe for several years, and have gotten to know the things that needed to be upgraded or replaced as I broke them. One thing I have barely touched on is threading. Dozens of time I have tried setting up to try but have run into various problems with the very loose tolerances in the machine being discovered or those of operator error. More recently, I have made "made a point" to learn how to thread using the lathe and complete an important project that a die set cannot.

First problem was understanding how to swap the plastic gear configurations and all the related quirks. Second has been apparent slop in the gears when setting up for threading, which has been fixed here and there where possible. Then, after jumping out of gear with any force, I have significantly improved the FWD/REV selector engagement. Other gears are either wobbling or moving in ways they shouldn't be and am looking at replacing the related parts.

Currently setting the lathe up for threading 20tpi for a project according to the chart on the machine. During testing, the torque required of the motor seems excessive, and used my hand-wheel attachment. Much worse, the scribed lines on the shaft look much more like 24tpi. The gear tooth count was verified according to the chart on the cover. Since there has to be gears between A and D somewhere, I used the 50/60 gears from the 19tpi line. My assumption is that I must be misunderstanding the chart, or the leadscrew is not the correct one.

I would appreciate experienced input on this common mini lathe process!

 

[jaek]

You need one gear between A and D, not a combination of two gears. Any gear that will fit works. You’ll need to swap the spacer on the leadscrew so that gears line up in the same plane.

 

[markba633csi]

The thing about simple gear trains is that only the first and last in the string determines the ratio. You could have twenty different gears between A and D and the ratio would still be set by A and D.
Now, with compound (stacked) gears this is not the case and each compound gear contributes to the ratio.
-Mark
Sometimes the charts don't match what's in the manual on some machines. Hopefully not the case for yours

 

[brino]

I believe @jaek is right.

use this configuration:

With gear "B" being any size the fills the gap.

NOT this one:

Brian

 

[Bi11Hudson]

Many years back, I had acquired a Horrible Fright 6X12 to keep in my locker at work. The whys and wherefores of that were involved with paperwork and don't fit here. Overall, I found the machine loose, very loose, but usable out of the box. The fact that the gears are plastic should indicate that there isn't much torque transmitted with them. The lead screw was 16TPI, not real ACME but (Asian?) a trapezoidal thread of some sort. It could have been metric .625mm or so.

Might I suggest using a piece of PVC pipe to learn to cut threads. The accepted depth of cut for a small lathe is around .005, give or take. I use less but mostly do very fine threads. By using PVC to get a "feel" for the machine, stress on the gears is reduced. The gear placement is important.

I believe @jaek is right.

use this configuration:
View attachment 424854

With gear "B" being any size the fills the gap.

NOT this one:
View attachment 424855


Brian

Spacers are provided (should be) to move the gears in or out on the shaft as appropriate. For the finer pitches, there must be a "stacked" gear set. Trying with the tool pulled back will give a "feel" for the gear train. There should be a "wobble" in the gears but no slippage. A piece of paper rolled in as the banjo is set will give a fair spacing.

There is a possibility that when things feel too stiff, something is dragging on the lead screw. Make sure the half nuts and the feed feather (if used) are not coming on at the same time. Try rotating the feed screw by hand, with the gears and half nuts disconnected. There should be minimal drag, just the bearings. There are many causes for dragging, especially since the machine has been in use for a while. Those cannot be adaquitely covered without hands-on experience. Normal troubleshooting is to seperate half of the machine and see which half is dragging. Then half again and again until the problem is down to one part or function.

.

 

[mmcmdl]

Last night I had the oppurtunity to mess around with a 7 lead threaded leadscrew . Of course it was hardened . The bearing journals were worn from .591 down to less than .250 and snapped the shaft . The timing belt pulley wore thru the bearing housing all the way thru the snap ring groove . No extras , had to machine a longer shaft to get the line going . First night back from vacation is always a blast .

 

[Mrfix71]

Thanks so much for clarifying the gear setup. I feel rather dumb for making it more complicated than it needed to be!
Good idea on the pvc pipe to practice on.

Previously gave up playing with that stamped banjo and machined one out of flat aluminum, copying one I saw online somewhere. It seems so much easier to get the setup installed, and used it once with a flip-up threading tool that I made a year or so before. Things seemed to be going great and I was excited......before realizing I destroyed the part by threading backwards. Instead of lashing out at everything, I displayed great restraint and quietly turned the lights off and departed from the crime scene for the evening. lol

 

[homebrewed]

Thanks so much for clarifying the gear setup. I feel rather dumb for making it more complicated than it needed to be!
Good idea on the pvc pipe to practice on.

Previously gave up playing with that stamped banjo and machined one out of flat aluminum, copying one I saw online somewhere. It seems so much easier to get the setup installed, and used it once with a flip-up threading tool that I made a year or so before. Things seemed to be going great and I was excited......before realizing I destroyed the part by threading backwards. Instead of lashing out at everything, I displayed great restraint and quietly turned the lights off and departed from the crime scene for the evening. lol

Making mistakes can be a part of learning a new procedure. Now you know one more thing NOT to do!

It's all good.