Mini Lathe Issues and Concept Execution

[WhiskeyHammer]

Hello all,

I recently picked up my first metal lathe (at a steal of a price) and, after some google searches, ended up reading a few of the threads here on mini lathe modifications and other tricks/tips for accuracy in your machining. I especially liked the thread about using tire depth gauges for precision adjustments. Brilliant!

Edit: I'm running Harbors Freight's 7x10 or x12 (Im away from the lathe right now)

Anyway, I had a couple of concerns about how my mini lathe operates that google couldnt resolve for me, and I was hoping to pick the brains of more experienced users:

• When I throw the autofeed lever on my machine, it sometime hesitates in the last little bit of the throw before making a loud (almot thud) noise and goi9ng the rest of the way to engage the lead screw threads. Is this normal?
• I chucked up a pencil to do some low cost familiarization and noticed that the longer my bit sat at one point the deeper it cut. All the videos I've seen in my research dont seem to show this kind of behavior.

Its possible that the guy sold me a bum machine, but I doubt it. He was a proffesional lathe guy for the local electrical utility company and seemed a decent sort, so its more likely that anything wrong with the machine is from my inexperience or mishandling.

Taking a step away from operational concerns, I have a concept I want to double check. I origninally bought the lathe just for threading purposes, but the more I learn about it the more I see how it can be utilized for other projects. One of which is very unsual. I need to cut a very very low pitch twisting square shape. Something like 0.025 tpi (1 complete turn every 40 inches). Obviously this is a little outside conventional lathe operations, but heres my idea on how to tackle it anyway.

First I need to regear the lathe for the low pitch. I plan on creating a very long banjo to create room for not two or three gear combos, but for five. According to my math (which I would appreciate a double check of) I can acheive the 0.025 with five gear combos when restricting myself to the commonly availbale gears for my machine. That should allow me to advance the carriage at the appropriate rate, relative to the rotation of the stock, to create the desired twist. I got my math from here.

0.025 TPI = 16 ÷ ((80 ÷ 20) × (80 ÷ 20) × (80 ÷ 20) × (80 ÷ 20) × (50 ÷ 20))

Secondly to cut the flat face of the square, I need a router bit mounted on the tool post vertically relative to the stock. I can do this by taking the kind of bit meant for mounting dremels and laying it on its side. Then just throwing the approaite router/milling bit into the dremel. Kinda like this but oriented so that the dremel head is pointing down...

Finally I need a way of reliablly indexing the peice for each of the four cutting passes. The idea here is to just throw a carraige stop on the ways to make sure it starts from the same point every time, and then using this guys indexing method to inepxensively start the peice at the appropriate angle for every cut.

[ame="https://www.youtube.com/watch?v=lqreWsmXG4k"]Indexing Method[/ame]

Skip to 4:08 for a good look at what I mean.

I'm concerned about the amount of force acting on the gears and about how fast the carriage has to feed to meet that pitch (it such a high feed rate that it might snap the dremel bit), so rather than run it on auto feed I'll probably advance it by hand-cranking the chuck.

Thougts, pointers, critques?

 

[TimTaylor]

Do you know anyone with a CNC mill? Seems to me the part you describe would be easier to do on a CNC mill using the 4th(rotary) axis.

 

[Tin Falcon]

When I throw the autofeed lever on my machine, it sometime hesitates in the last little bit of the throw before making a loud (almot thud) noise and goi9ng the rest of the way to engage the lead screw threads. Is this normal?
I chucked up a pencil to do some low cost familiarization and noticed that the longer my bit sat at one point the deeper it cut. All the videos I've seen in my research dont seem to show this kind of behavior.

None of the 7X machines I have ever seen have an auto feed lever. they do however have a half nut for threading. the half nut should be used for threading at slow speed that said the half nut may need some adjustment or may be worn.

Before you jump in to complicated setups I suggest you learn basic lathe operations.
I recommend downloading and reading army TC 9-524 Machine tool operations.
There are also books written specific to the mini lathe.
Tin

 

[MachineTom]

So you are going to cut a Buttress (straight sided) thread with a pitch of .025 tpi 4 start thread. With a HF mini lathe and a Dremel. While a thread, more like a groove could be done that would work, I seriously doubt your choice tools and lack of experience will provide that result.

A gear train has backlash between each gear, there is clearance in each bushing, and these are cumlative, How do you propose to address that as you make each pass. That milling cutter in a dremel, is held by a tiny spindle, with a sleeve bearing in a plastic housing. Nothing precise in any of that. And then you will advance exactly 90° and do it all again.

There are many really cool things that are made with the tools you have by the folks in this forum, cruise around the site, read look enjoy, then learn to use the tools you have. If you need that part made, a shop with 4 axis CNC mill can do that in less time than it took me to write this.

 

[jack.39]

My neighbor, usually full of B.S. to assure his own position and rank remained #1, told me once of a guy who, back in the 1930s, built dozens of working live steam locomotive models, using NO machine tools, only hand tools.

I told the neighbor he was full of sh!t. It took me 8000 hours to build my 1-1/2" Northern using lathe and Bridgeport, among other things. Has anyone heard of this rumored genius? jack

 

[russ57]

That sounds a bit like a Chinese whisper version of "LBSC" the UK author /machinist who apparently started by upgrading and making toy steam locos as a kid with very limited resources, eventually revolutionising model loco construction by building 'real' engines, not toys.
-Russ

 

[geoff p]

Whilst agreeing with the other folks about learning to use the tools you have, I was once faced with machining a screw of 100mm pitch on a 4" bar by 1-metre long. It was easy enough to set the leadscrew-gearing but ....

With a spindle-speed of say 50rpm, the saddle would be moved at 50x100 mm/minute, and would be almost uncontrollable. (for your case it would be 50 x 1000 = 50Metres/minute)

What I did was to drive the LEADSCREW from a separate geared-motor, then let the change-wheels transmit rotation back up to the main spindle. The surface speed of the work was far too slow so I made a milling-spindle to fit in place of the normal tool-post.

But that was on a toolroom lathe, and a long time ago.

Geoff

 

[WhiskeyHammer]

And that ladies and gents is why we put our ideas out there. When you test them against the battlefield of open source analysis and critques, you'll always come out ahead. Thoughts and responses below:

A 4-axis CNC would be easier but I personally dont have one. I'll check around with the local fab labs and see if anything ends up being feasible (cost and time wise). For the immediete future well assume that I'm gonna run with the lathe idea. Im not super onfident about being able to find affordable access to a the mill I need.

THe 7x10 harbor freight has both an auto feed lever and a threading dial. Ill have to try tonight, but it certainly sounds like I could be trying to engage the feed laver while the machine is running too fast.

In so far as gears, it sounds like what your saying is that the cumulative slop of 5 gears will mess with my ability to get consistency out of each pass. How about instead of gears, I lathe a few large and small wheels and run them as a belt drive? That should make that part of the setup super consistent.

Instead of a dremel, I reckon I could just as easily use a power drill to get a thicker more serious use cutting action. I feel pretty confident about the indexing methodology I listed, but the worse case scenario is that the gears are just tad off in thier angular consistency (between valleys) and I end up with a bad cut. If it is bad, itll be pretty obvious, and I can upgrade to a higher quality gear, or reasses my indexing methodology. Any thoughts on alternatives?

I love the idea of driving the leadscrew! I suspected that the rate of advancement on the carriage would be too insane for a steady proper cut, thats why I planned on handcranking the chuck. Nice, slow, and smooth. But I could do it just as easily with the leadscrew and that way I wouldnt have to remove my indexing setup between passes.

 

[Tin Falcon]

THe 7x10 harbor freight has both an auto feed lever and a threading dial

I see said the blind man on a larger lathe the the half nut and the auto feed levers are separate. so since it can function as both and chinese manuals are not always the best English and technical details are sometime not totally accurate. Either one would work as a label.
The harbor freight manual is a lot better than what I got with my grizzly . but that was ten years ago so they have had time to write a decent manual.
I do suggest you download this one as well.

http://littlemachineshop.com/Info/MiniLatheUsersGuide.pdf

Tin

 

[SmithDoor]

The South Bend 9C and 8 used the half nut back to 1920s for 8 and 1930s for 9C.

Just remember to use waylub every few hours on the lead screw.

The my manual said use NLGI2 grease or 20W machine oil. I use waylub harder find.
I may put drip on my lead screw so nut last a long time.

I just purchased my first mini lathe in November 2023.
I had look them for over 20 years . The upside to mini lathes is size and weight. My last lathe was 700 pounds the new lathe shipping is 130 pounds.
I may build in a wooden cabinet with wheels for more in home or garage use.

Dave

 

[SmithDoor]

I see said the blind man on a larger lathe the the half nut and the auto feed levers are separate. so since it can function as both and chinese manuals are not always the best English and technical details are sometime not totally accurate. Either one would work as a label.
The harbor freight manual is a lot better than what I got with my grizzly . but that was ten years ago so they have had time to write a decent manual.
I do suggest you download this one as well.

http://littlemachineshop.com/Info/MiniLatheUsersGuide.pdf

Tin

I like manual *****

I just purchased first mini lathe in November.
I just put my favorite tool post on this week. Last week was 5" 4 jaw backing plate.
Next is a carriage lock.

Dave

 

[kf2qd]

1 turn in 40 inches is not doable on a 7x10 mini-lathe. It is not doable on any lathe. It is something that would be done with a milling machine and an indexing head driven by the long axis lead screw, or with a CNC with a 4th axis.

As far as the other behavior of your machine - leaving the cutter sitting in place while the spindle turns will cause a mark or undercut. There is some deflection caused by the pressure of the tool against the workpiece.

 

[Jonathan Coates]

Have you considered buying square stock and twisting it? People do it all the time for making fancy gits for gates etc. A local fabrication shop would have a powered machine to do this, probably with stops to get exactly the right degree of twist (no pun intended). Or you can get a manual one from EBay etc. unless the part is very thick.

 

[SmithDoor]

1 turn in 40 inches is not doable on a 7x10 mini-lathe. It is not doable on any lathe. It is something that would be done with a milling machine and an indexing head driven by the long axis lead screw, or with a CNC with a 4th axis.

As far as the other behavior of your machine - leaving the cutter sitting in place while the spindle turns will cause a mark or undercut. There is some deflection caused by the pressure of the tool against the workpiece.

I can see your setup.
But is a problem and you may need a follower rest.
In CNC work they try fix this by taking a lot of fine cuts.
In production work it best to use a roller box {for engine lathes follower rest} .

It is also used to make long tapper like Canon barrows with slight change to setup.

Dave

 

[SmithDoor]

Note you turn very long parts on short lathe if the bar is smaller than the head stock hole.

If does fit the head stock hole you can only turn double the length of lathe by using a steady rest in place of the tail stock. Still may need a follower rest for small diameter bars.

Dave

 

[SmithDoor]

Here a setup for turning a square shaft in steady rest.
May not help but shows what can do.

Dave

 

[kwoodhands]

I once built a locomotive from various woods. Took me 6 months. Does not run on steam or compressed air, does a nice job of a mailbox. Someone liked too much , stole it from my yard.
Your neighbor can give Biden a run for his money on lies.
mike

My neighbor, usually full of B.S. to assure his own position and rank remained #1, told me once of a guy who, back in the 1930s, built dozens of working live steam locomotive models, using NO machine tools, only hand tools.

I told the neighbor he was full of sh!t. It took me 8000 hours to build my 1-1/2" Northern using lathe and Bridgeport, among other things. Has anyone heard of this rumored genius? jack

from

 

[Bazzer]

The OP has long fled these shores but nevertheless, I think with care the OP's concept might have worked. Absolutely no need to power up the lathes spindle as the cutting will be done by the Dremel type tool. Just hand turn the spindle with the calculated gear train to advance the carriage.

With regard to the 4 start thread I did not like the OP's idea there at all, I would do the 90 degree offsets down in the gear train, ideally drop off the gear that is driven from the spindle and rotate the spindle by 90 degrees and re-engage the gear train and so on. Of course this will only work if the number of teeth on the spindle are evenly devisable by 4.

If doing dodgy threading (this falls into the dodgy threading category) then I never disengage the half nuts or other engagement devices, you cannot go wrong if you leave everything engaged.

Using a Dremel brand is a crap idea as they are way too flexible and horrible for a job like this as stated by someone else, get a Proxxon as they have metal heads with ball races mounted in that head and have a nice 20mm nose to pick up in a decent machined tool mount.

 

[pete]

While it would be difficult, very long lead pitches are completely doable on a lathe. Maybe not easily or possible with one that has a gear box, but with a plain change gear lathe it's certainly within what could be done. You'd have to add a whole lot more gearing than any lathe usually comes with though. What you can't do is drive the lathe conventionally with it's own motor. Try that and your going to instantly break parts. Instead you have to slowly back drive the lathes head stock spindle by turning the lead screw that one turn in 40 inches of carriage travel. How to do it is no different than that helical milling on a milling machine with a universal dividing head geared to the X axis feed screw. So some kind of independent powered milling head just like the OP mentioned isn't optional. But a Dremel doesn't have anything close to the correct bearings to ever do this. Long lead helical pitches were done way before the first milling machine was ever invented never mind cnc. Very large caliber artillery barrels have extremely slow rifling twist rates, and those rifling grooves were cut since the mid 1860's with equipment not much different than a basic lathe. Even prior to that time period, weaving mills to name just one also required long lead grooves to be cut. A few types of hand operated printing presses also used them.

 

[timo_gross]

Deleted once again! The OP was asking his questions 8 (almost 9) years ago! I hope he solved his problem in the mean time, but it was never revealed if, or how.