Brilliant or Boneheaded?

[ealanm]

I've recently started working with my Craftex B2229. This machine has separate lathe and mill motors, with a switch that toggles between them. One of the first things that occurred to me was that if it were possible to run both motors at the same time then the lathe feeds could be used as X & Y feeds on the mill. If the lathe chuck were removed for safety, and other than the issue of potentially drawing too much current with two motors running, is that a brilliant idea or a boneheaded idea?

[TomB]

My Shopmaster has seperate 1 Hp motors on the mill and lathe. They are plugged into the same 220 circuit and I use both routinely because I want the power feed while milling. I do not have CNC motors on my machine (yet) so I'm using the threading gears. I will warn you that this is a good way to take the teeth off a gear.

There is one possible electrical reason why a design might not allow running both motors. (Specifically if a machine shared a motor controller or VFD between motors then sharing would not work.) But I did a quick google search and looked at the schematic for your machine and it is only sharing simple switches. So you could put in some extra switches and have the same type capability as my Shopmaster.

Tom

[ealanm]

Thanks, Tom, especially for the tip about the gears. I'm not sure if I want to go this route, but it did occur to me that I could make some milling jobs a lot easier by doing it.

Tedd

[Torch]

I will warn you that this is a good way to take the teeth off a gear.

I have the same concern (and mine has 1-1/2hp motors). So I replaced the steel key on the A gear (the first one in the train powered by the chuck) with one I fashioned out of copper. Boat props have sacrificial shear pins, why not lathes?

[randyc]

I was hoping that one of the several millwrights on the forum would comment since they encounter shear pins routinely but maybe they don't check this section often. I hadn't looked here until a couple of weeks ago when I noted interesting (specialized?) problems encountered in the operation of multi-purpose machines. (My personal experience with 3-1 machinery is limited to older Emco Maximats, often found in the model shops of electronic companies.)

A shear pin, in the right location, is always wise (a key isn't a good choice except for high torque loads - pins provide more predictable failure). The correct location for best protection - as on a power boat propellor - is always the lowest speed, highest torque shaft. That would be the lead screw, in the case of a lathe (or the connection between lead screw and banjo gear).

Personally, I'm VERY sensitive to this problem. Although my little lathe has never been crashed, it is old and I would NOT like to have to track down expensive replacement parts, LOL. I've driven the lead-screw shear pin half-way through the shaft so that the torque required to shear is reduced by 50%. Here's a photo to illustrate the point:

Now that I think about it, I should probably replace the steel pin with a lower-strength alloy, like Torch suggested, driven completely through the shaft (to prevent distorting the pin hole on the driving side of the shaft). Perhaps a hollow brass pin ... I'll have to think about it and crunch a few numbers. That's of course presuming that I'll even remember this tomorrow morning (chemo-brain), LOL.

Cheers, good topic -
Randy C

[Torch]

The correct location for best protection - as on a power boat propellor - is always the lowest speed, highest torque shaft. That would be the lead screw, in the case of a lathe (or the connection between lead screw and banjo gear).

Actually, I had a reason for putting it where I did. The gears before the sacrificial key are happily bathed in oil inside the sealed headstock. The gears beyond are somewhat shielded, but still potentially subject to contact with chips. For example, I have found chips inside the change-gear box following boring operations on the lathe -- pushed through the spindle, I imagine. Putting a shear pin at the lead screw only protects the gears from a problem from that point on -- the gears are not protected if one gets a chip between its teeth.

That is not to say that a shear pin couldn't be installed at the lead screw too.

BTW: When I was looking into it, aluminium, brass and copper have increasing shear modulus in that order -- roughly. A lot depends on the specific alloy and the hardness/temper. I have no idea what alloy copper I used -- it's what I had on hand, it was just about the right thickness, and it has to be less than the original steel key (about 1/2).

Good luck with the chemo.

[randyc]

You might consider adding the pin to the lead screw as safety margin. Most lathes are quite tolerant of chips in the gear teeth, it's a common occurence since change gears are exposed to the environment. The typical banjo mechanism depends on friction to hold gears in place so the gears just slide apart when foreign material enters the train.

Sometimes when taking heavy cuts, the lead screw will stop turning because the gears have slipped out of engagement. I neglected to mention one further safety precaution that I added to my gear train that relates to this, here's a photo:

As previously noted, my lathe is old (about 40 years) and the source of spare parts uncertain so I went one further step to insure that the gears would be forced apart if overloaded. The photo shows the banjo arm and, as you can see, there is the typical amount of debris around and in the gearing. I removed the bolt that normally secures the banjo and replaced it with a "star" thumbscrew, under which are two belleville washers (spring washers), bearing against two flat washers.

The idea is to engage the leadscrew with the lathe in a fairly low spindle speed then hold the carriage with the right hand while slowly loosening the thumbscrew with the left until the gears disengage. Works great, although when taking heavy cuts on steel (about 1/4 inch for example), the screw does have to be tightened very slightly to prevent the leadscrew from disengaging under cutting load. As I noted earlier, the lathe has never crashed and I hope that it never will, LOL.

The reason that I suggested the pin as being superior to a keyway is the much smaller cross-section that is in shear and the ability to fine-tune the shearing point by drilling a suitable hole through the pin. The relative shear strength of common alloys, as given by "Machinery's Handbook" are approximately as follows:

high tensile steels (like 4140) 75 - 165 kPSI
low carbon steels (most common alloys) 45 - 75 kPSI
free cutting steels (like 12L14) 40 - 60 kPSI)
wrought brass 30 - 45 kPSI
aluminum (soft) 7 - 26 kPSI
aluminum (heat-treated) 14 - 48 kPSI

A typical small key might be on the order of 1/8 square x 1/2 long, made of low carbon steel. It won't shear until it is loaded to about 50 kPSI, which would represent a load normal to the center of the key of about 3,125 pounds. An 1/8 inch diameter pin, made of the same material, will shear at a load of about 1,227 pounds. By drilling a 3/32 hole through the same pin, the shear load will diminish to around 500 pounds. For a shaft around 5/8 diameter, the shear torque will be about 150 inch/pounds which is a relatively safe number for even small machine tools. As a comparison, most power feed mechanisms on vertical mills produce about that amount of torque.

Forgive the pedantry but many may find the above information useful, particularly for small machines that do not employ cut metal gearing.

Cheers,
Randy C

[Torch]

Forgive the pedantry but many may find the above information useful, particularly for small machines that do not employ cut metal gearing.

No problem, in fact, I'll add to the pedantry by mentioning that copper is apparently around 45 - 60 kpsi depending on the alloying elements. Another thing I found out was that hardened brass actually has a lower shear modulus than soft brass -- I guess it's more brittle.

Another potential advantage of the pin over the key would be the fit of the respective parts -- the keyed gears tend to fit their shafts snugly -- and of course are normally retained by nuts and regular washers rather than spring-loaded -- while a pinned joint generally relies more on the strength of the pin rather than friction. So I can see adding a shear pin to the lead-screw.

But I'm not giving up my sacrificial key -- in forward, certain configurations actually draw the gears together instead of kicking them apart. (I first noticed this when it kicked out in reverse -- while cutting a metric thread )

[jstinem]

I just loosen the lathe drive belts a little when I use the lathe for a power milling feed so they will slip if I do something really dumb. I have never had any sort of problem doing it. My Shoptask has separate switches for the lathe and mill motors but it only had one power cord for both. Running both motors sometimes popped the breaker so I added a separate cord for mill motor that went to another plug in another circuit. Taking the chuck off is not a bad idea either