Screwcutting Crashes -

To answer JA's question, I'm doing advanced junior training rather than a real job at the moment. I'm experienced enough to cut threads the right size (mostly, ahem) but not experienced enough to get a good finish without failures, and the odd crash. I started threading 20m diameter aluminium rod, which is rather easier to learn on, and now I'm trying 6mm brass. To keep things easy I'm cutting 1.0mm threads because all the thread dial increments work at that pitch. Reducing diameter caused the latest outbreak of mistooks and made me question what I'm doing.

I initially plunged in at right angles and found that the cut is often poor. Then I tried the top-slide half-angle method; this improved the quality of the cut enormously but, especially with small diameters, brings the saddle, top-slide. tool-post combination dangerously close to crash point.

This morning I tried cutting the thread from left to right. To do this the lathe is run in reverse (no problem on a Chinese Lathe with a flange mounted chuck) with the tool upside down.

The main problem with this is the limited space for the tool in the 4-way. With the point at centre height, a 6mm tool only just fits. I couldn't fit a larger threading tool upside down in this tool-post.

Being unfamiliar with working in reverse meant I had to think hard at every step. With practice it would become routine. Results were good and there's no chance whatever of a crash. I'm still not too happy about the amount of overhang needed, and it's all too easy to leave the lathe in reverse!

A disadvantage of owning an affordable lathe has become very apparent. Turning down the 1/4" brass rod to 6mm really needs the lathe set up with the fast belt and fine-feed gears. (See how crudely the rod in the picture was turned down!) Cutting the thread needs the lathe to be set-up with the slow belt plus gears to produce the desired pitch. Although not difficult to change over, it takes at least 10 minutes. It must be wonderful to own a lathe with a full gearbox!

Agreed about writing the numbers down, and marking the dial with a Sharpie would have helped too. After lunch I'm going to try doing that with Hopper's method.

Thanks again,

Dave

+1 takes me a few seconds to change spindle speeds or select most feeds or threads. The longest time is taken getting the old eyes focused on the thread/feed chart.

Where's the smiley for a smug barsteward?

Andrew

The disadvantages of owning an affordable lathe are far outweighed by the disadvantages of not being in possession of an unaffordable lathe. Basically, not having a lathe.

You just have to cut according to your cloth and get on with it.

regards Martin

The worse the equipment, usually the better the operator has to be to get the same results. Think of it as a 'skills enhancement' machine. An amazing machinist will get good results off the worst form of treadle lathe... Wish I could.

Regards,

Richard

Dave,

Just wondering = why not just cut the thread with a die? Was it just for fun that you decided to test you and your machine or simply no die at the required size?

Phil H

Having a coffee break and pleased to report that the Hopper method is the best so far. You don't have to swivel the top-slide and therefore can get close to the chuck without worrying about sticky out bits hitting things. Also, cutting the thread from left to right with the lathe in reverse, the dreaded head crash can't happen. I'd assumed hopper-style that remembering to move both top-slide and cross-slide in proportion would be error prone. It's not!

I'm well pleased with the shower of tips that came in answer to this question, not to mention what I picked up from watching Joe Pieczynski's videos.

Thanks,

Dave

PS. Martin, of course, is completely right about any lathe being better than no lathe. I started with a mini-lathe and found that incredibly interesting and useful. Trouble is tools are highly addictive and I am so weak. I could easily end up spending my days polishing a workshop full of expensive kit...

Good result! Happy latheing.

Yes, time involved in change gear fiddling about is one reason many choose to use dies held in a tailstock holder for smaller threads and save the screwcutting for special jobs.

Now you are using the top slide parallel, slide it further back on the cross slide. This will allow you to mount your tool the right way up but behind the work. That way you will not have the height issue associated with mounting the tool upside down, and just run in reverse. This is putting the tool in a similar position to that I showed earlier except you will be using a standard tool rather than an internal one.

You will find that if you use either and extended too bit in a larger block, or use an insert threading tool, you will be further away from the chuck diameter wise.

To not crash into the chuck, when I installed the VFD, I also added a micro switch. This is just set up to trip when the carriage gets to the desired position.The lathe motor stops and so does the carriage. If there is a run out (thread relief area) then retract the tool. Then simple turn the switch to reverse. It goes back and ready for another cut when in the forward position again.

Previously I used to do the marker pen thing for internal threading and just looking at the thread relief when external threading. I like to use the angled approach when threading, except when I have some special situations where an angled retraction in internal thread can cause damage to the end face. In these cases, I leave the compound on zero.

Neil

Hi Phil,

Missed your question yesterday. The answer is a bit of both, mainly educational. Usually for real work I cut small threads (<10mm) with a die, and try to keep things simple.

However, I'm keen to improve my metalworking skills because I'm self-taught. In this case, I'm making a trial brass union joint. It happens that the scrap of brass hexagon I have is 3/8" so I'm putting an M9x1 thread on it. It's an unusual one-off and I don't have a die. So it's an opportunity to push what I know about the lathe. If it goes well I shall have proved I can make a non-standard part. If it goes badly I can get help off the forum and have another go. It's a good job I don't have to make a living like this.

I've already bodged a M9x1 tap out of silver steel to do the union's female. Hardening it correctly is yet another self test because my blowlamp skills are wobbly too. Will it cut? Who knows. Practice, more practice.

I enjoy setting myself this kind of exercise. It's a meaningful challenge without serious hassle if I get it wrong.

Cheers,

Dave

Thanks Jason, that's another improvement. I shall try that this afternoon. It's 'quite interesting' how many different ways there are to do screw-cutting and the obvious one isn't necessarily the best. I'm glad I asked.

Cheers,

Dave

Just an update. Cutting from the rear as described by JasonB with dials twirled as per Hopper works extremely well. I shall stick with this method from now on.

Neil Lickfold suggested using a micro-switch to cut the power on the way in. I think that's a practical proposition. I experimented with how quickly my lathe stops when the power is cut. At 480 rpm the saddle carries on up the bed for 25mm! However, at a more reasonable 200 rpm (which is as fast as I think I would ever screw-cut), the saddle stops within 2mm of losing power. That's fast enough to avoid a crash. The much greater distance travelled at 480rpm shows how much energy is stored in a quickly turning chuck. I wouldn't want to tangle with one at 2500 rpm!

Thanks again for all the good advice

Cheers,

Dave.

Edited By SillyOldDuffer on 12/05/2017 19:11:21

Dave, If you have a VFD Variable Frequency Drive, they will stop with in 1 second from 400 rpm aprox. I screw cut at about 200 rpm to 400 rpm.

Neil

I have a micro switch mounted on an aluminium block that clamps on to the lathe bed and can easily be repositioned to suit collets, 4 jaw or 3 jaw chuck as appropriate. It is a home switch for CNC and a limit switch at the same time. It has saved me from crashing bits together a number of times when using manual data input to move the carriage around.

Martin C