Question about the limitations of miniature lathes

[BtoVin83]

No matter what size of lathe you buy it will 1/2 the size you really need. On the other hand you can accomplish a lot by being patient and be 50 per cent smarter than the material

 

[benswollo]

A general rule of thumb regarding drill depth is to keep depth of holes to less than 10x the drill diameter to avoid significant wandering. Drilling holes in hardened steel requires significant force to make the cut. The tip of the drill doesn't cut well. For this reason, pilot holes are drilled first as the cutting forces are less for the smaller drill. This is especially important on a small lathe where there is a lack of available force and there is a small amount of power to do the cutting. Unfortunately, a first drilling a pilot hole means an even worse depth to diameter ratio.

Ideally, this is a job for a bigger lathe but given the lathes you are considering, here is how I would approach it. I would drill a pilot hole to about 10x the drill diameter probably using a 3mm drill. Then I would drill with a 5mm drill to slightly less depth of the pilot hole. This will give you a starting hole for the next operation, Repeat the process until you reach the full depth of the hole. Next, if you have a suitable boring bar, bore the hole to the final 5.5mm diameter. Then bore the hole to the diameter and 76mm depth using several passes. If you don't have a suitable boring bar, drill to final diameters.
Note that a boring bar capable of boring a 5.5mm hole to 100mm depth will be extremely flexible and prone to chatter. Carbide would be the best choice for this because of its superior stiffness. Even so, it is a daunting task.

Interesting. I will keep this in mind. It sounds like these machines are less than ideal for the task. I planned on learning on one of these machines as I don't have experience with mills or lathes.

 

[mcdanlj]

Interesting. How do people usually attain the level of skill necessary?

IMHO, A mini-lathe isn't a bad place to start gaining mastery. It's just that you are starting with a hard task, and it's easier to start with simpler as well as smaller projects and learn one thing at a time. I started with a mini-lathe and did simple projects in aluminum, brass, and plastic before upgrading to a larger lathe. Similarly, I started with a mini-mill and now upgraded to a larger bench mill. I learned a lot on both of them, on seemingly simple projects. By making mistakes.

I didn't understand the limitations of a mini-lathe when I started; I took the 7 and the 14 too seriously in the "7x14". I now have a 14x40 which seemed like a bit overkill when I got it but I keep being thankful that I hauled the 3/4 ton into my basement. I'm not turning large work, but the extra room sure comes in handy. Remember, for example, that the jaws stick out of the chuck when you have them open...

 

[ThinWoodsman]

Sorry, but must be blunt. IMHO, the Taig and similar are just toys.

Having started with a Taig mill and a 7x14 lathe, I have to agree. I'm sure someone who is very skilled can turn out good parts on these, but an amateur is in for a lot of frustration.

On the other hand, these machines are good to learn on. They are much less dangerous than the big iron, and you will learn the important stuff like how each metal sounds when being cut correctly, how to position the cutting tool for different operations, etc. You'll also learn a lot about improving the machines, i.e. making them more rigid, removing slop, adding missing features like a saddle lock.

 

[ttabbal]

Lots of good work is done on small machines. This particular project, a deep small diameter bore, is one that requires rigidity though, which is the weakest point on a mini-lathe. A lot depends on what you mean by "accurately". Before I started working with metalworking tools, "accurate" was about 1/8". Now it's about 0.001". And to others here, 0.0001" is more what they have in mind. I'm just not as skilled as them, yet, hopefully. I think of my skill level as "that apprentice that probably won't kill himself in the shop".

If you get one, treat it like a kit. You will need to upgrade and fix it up a little. Once tightened up and such, with a solid carbide boring bar, you might be able to get a good bore. You will have to take very light cuts and those little carbide bars are a bit easy to break, but I think it could be done. Patience is going to be a required quality. Particularly if you want to work steel.

If you think you will be doing this for any length of time, you would be better served by a larger more rigid machine. Even a 10x22 would be a big step up. I really like my PM1127, but that's another good bump in price. You should also look at the tool grinding thread. These smaller machines benefit from razor sharp HSS tooling. And it saves some cash over the insert tooling.

 

[mcdanlj]

On the other hand, these machines are ... much less dangerous than the big iron...

Just be aware that less dangerous means "maybe less likely to kill you" but don't underestimate what a mini lathe can do to you. Long sleeves tangled in the work or chuck could permanently maim you. Long hair tangled in the work or chuck could kill you if you aren't lucky enough for it to pull your hair out... There is a lot of information out there on how to operate a lathe safely. Take it seriously, even on a mini lathe. There's a whole section of this site on mistakes to learn from, and plenty more elsewhere.

 

[mikey]

The part would have a final overall length of 131mm. Of that length, 53mm would be turned down to an outer diameter of 13mm. The remaining length would have an outer diameter of 26mm. A 5.5mm drill bit would be used to create a hole 100mm deep in the center, starting from the larger side of the piece. Then, starting from the larger side, a 9mm drill bit would be used to re-bore the 5.5mm hole to a depth of 76mm.

I am wondering if all of these operations could possibly be performed accurately on either of the aforementioned machines. Any insight will be much appreciated.

You might get away with it with the 7x12" lathe, which has a 20mm spindle bore. The entire work piece is a little over 5" long, which leaves enough room to drill it, I think. You would need to turn the smaller diameter first so that it can pass that end into the spindle bore, then drill the large end sticking out.

You have to keep in mind that anything beyond a 10:1 depth:diameter ratio is considered deep drilling and accuracy will be affected. You have not told us what the part is for or how accurate the hole has to be. If the 9mm part has to be accurate then you're looking at boring a hole just under 3" deep with an ID of about 0.354". That would require a 1/4" carbide bar that has a depth capacity of 2.5" at best. And if the bore has to be accurate, with tight tolerances, then you really need to know what you're doing with that boring bar.

I think a 7X12 lathe can handle it, at least capacity-wise.

A Taig won't have the spindle bore capacity or the bed length to get this job done.

Your post suggests that you need a lathe for just this one job. You might want to put some thought into this. If you only need one part, it will be cheaper to have it made for you. If you need many parts then buying a lathe makes more sense. If you plan to get into this hobby then the lathe you buy needs to be able to accommodate the biggest stuff you intend to work with. More info will help us to help you.

 

[Aaron_W]

Are you looking for a mini-lathe simply based on price? Do you have the room for a larger lathe?

I don't know what the used machine market is like in your area, and it does vary, but you if you are mostly looking at price you can occasionally find larger , small lathes (9x20, 10x22) used for $900-1200 that might be better suited to your project. If you have the room and the interest sometimes you can find larger lathes for not a whole lot more than the small ones.

The small lathes seem to hold their value better as almost anybody with the interest can find space for a mini-lathe and can transport them without special equipment. You are dealing with a much smaller group of people who are interested in the larger lathes. A 7x14 weighs about 100lbs, a 9x20 about 225lbs, 10x22 around 300lbs, but a 12x36 jumps to 900lbs+.


Also I haven't seen anybody mention it yet, but don't think it stops with the purchase of the lathe. Most lathes come with just the bare essentials to get you started, it is not at all uncommon to almost immediately spend 50-100% more on additional tooling. Drill bits, boring bars, reamers, cutting tools, tool holders, chucks. You also have the measuring tools, calipers, micrometers, dial indicators etc etc.


Mini-lathes are a good learning tool, and can be quite capable if you stay within their size limits. A lathes nominal size (7x14, 9x20 etc) is theoretical maximum, 50-60% of the diameter and maybe 75% of the length is more realistic.

 

[john.k]

i would think a Mini with 1 gnat power would not drill or turn something like 4140PH,and even be a battle turning annealed 4140.......meaning needing a series of cuts sneaking up on the finished dimensions.,drilling being similar series of increasing sizes..........However I started with a Swift ,and it was too big,but still managed to make 3/16 gun screws.......and without any of the digging in troubles the small lathes have.

 

[wrmiller]

A long time ago when I was just getting started, I managed to make a titanium compensator for a competition pistol on a Sherline lathe and mill. Basically a 2" long part with a .360" bore through the three chambers of the comp.

I would never attempt that again, but I got a little ****** when this 'master machinist' said it couldn't be done. It took three attempts and a custom ground boring bar provided by a friend.

I would suggest a larger lathe if you can afford it, as the larger work envelope and extra rigidity does help.

Best of luck.