Screwcutting on the lathe

Today I had another go at cutting screws on the lathe.

I need to do some lathe screwcutting to make the piston gland for my steam engine and I've never really got the hang of it. The gland will have an external thread of 18mm into an internal thread for the body of the gland. Not something for which I have taps and dies!

So today, I took some steel bar (sadly sourced from b&q and likely entirely inappropriate for model engineering) and attempted to put a 1.5mm thread on it. It is 11mm stuff and I turned it down to 10mm.

The approach was the 29 degree offset one with the cross slide advanced the pitch depth (0.95 more or less) and then the cross slide zeroed and retracted to to touch the bar and then advanced until it was zero again.

The challenge was getting the lathe to turn. At 60rpm (recommended in some books), it wouldn't cut at all (stalled). I could get a cut at around 200 rpm but only if the depth was no more than about 2 ./ 4 thou (0.05 / 0.08 mm). This wasn't so bad at the start when I could get twice that, but towards the end I needed to take very thin cuts at a speed which made me uncomfortable. I am practising to do an internal cut so have my newly-made saddle stop mounted and I don't want to crash into it at 200 rpm!

The tool is a small hss one which I sourced a while back and have never manged to use with any great success. I sharpened it on a stone before starting and it seems to remove metal OK at speed.

Oh - the lather is a SPG0618A - a Real Bull with a 500 watt motor running in back gear - so it's not the most powerful lathe, but it's not a total wimp either.

Am I doing something wrong, or is this about what you would expect?

Iain

Assuming the lathe took heavier cut swhen reducing the bar from 11mm down to 10mm then there is a pproblem as the threading cut is removing a lot less materialnals.

Can you post a picture of the setup 

Edited By JasonB on 17/11/2019 10:44:38

Iain, as Jason says there must be a problem somewhere, my mini lathe was 500 watts (brushed) and I had no problem cutting threads with it of various sizes, I used low range which had plenty of torque at low speeds.

Thanks, both.

I should be clearer!

I am making an external 1.5mm pitch thread on some 10mm steel bar.

Ultimately, I will be making an 18mm plug to go into a socket with an internal 18mm thread. I want to make sure i know what I'm doing in basic threadcutting (external) before I try and make an actual piece.

As a note, the maximum I can remove in steel is about 0.5mm anything above that struggles (hss or carbide). DOes that indicate a lack of power and if so, is it likely to be the bushes, the motor or the control board?

Iain

Has this lathe got back gear? One where you operate a lever to engage the back gear and then disengage normal drive by means of a pin or lever or something similar? Failing to do disengage normal drive when in back gear locks the drive on the lathe quite effectively.

If you only have speed control of the motor to change speeds then low rpm probably gives low power. If this is the case then consider a spindle crank, these have been discussed a number of times in past threads.

Martin C

Yes, Martin, it has a back gear and I'm using it.

I'm also starting a spindle crank.

Iain

Are you trying to cut the thread in one pass - ie a depth of 1.5mm? In that case there is little point in the 29 degree offset (if I understand 'offset' correctly), as this is to ensure that successive cuts remove metal almost entirely on one side of the thread. Try the effect of taking a much smaller cut, perhaps 0.2mm - this might get the job done even with a lack of power (etc) but taking rather longer.

Unless of course this is how you have been working, in which case I wonder where B&Q got their super-difficult steel?

Regards, Tim

I think you've already identified the chief suspect - B&Q Steel!

When I first owned a mini-lathe I wasted a good few months attempting to turn scrap and DIY Store metal. I foolishly assumed that a metalworking lathe would cope well with any metal. This is a serious mistake!

There are about 2500 different steels available and many of them do not machine well. Some don't machine at all. Similar with the many Brass and Aluminium alloys about - although many machine well, some are downright difficult.

My experience of DIY Store metal is that it is poor stuff intended for decorative and rough work not engineering. The aluminium is soft and sticky. The steel is hard and gritty. Brass is reasonable, but not good compared with others. Most of the metal I initially played with came from B&Q, but I can confirm other stores are equally unsatisfactory.

When I finally got round to visiting my local metal emporium and telling the nice man I wanted steel to machine it, he sold me some EN1A Pb. Enormously easier to work with than B&Q's unsuitable stuff. Same with Aluminium alloy and Brass. Now I avoid metal unless I know what it is and although I experiment with scrap, I will walk away rather than persist with an unlucky choice.

Although it's better than DIY Store metal, worth knowing that ordinary mild-steel doesn't machine particularly well either. It's OK rather than brilliant, which is why EN1A is popular!

When you're a self-taught beginner, trying something a bit tricky like threading for the first time will go certainly go badly if the metal is carp! My advice is to learn threading on some Aluminium or Brass bought deliberately because machinability is mentioned in the specification. Both metals are easier to thread than mild-steel and good to learn on. Aluminium is cheaper, but needs to be kept wet with WD40 or paraffin. With a bit of experience it gets easier to handle difficult materials in the workshop, but you need to develop a feel for getting the best out of your machine. This is particularly true of small tools - big powerful machines can often cope with sub-standard driving.

Dave

Another thought - the thread 10mm x 1.5 pitch is a standard metric thread, and you can buy threaded parts with M10 threads from anywhere that sells nuts and bolts. Would it be possible to start with a threaded component (such as a bolt or screw) and turn the rest of it to match your drawing? Yes, I know, not in the spirit of doing everything yourself, but it only means 'drawing the line' of what is acceptable in a different place.

Regards, Tim

Your depth of cut should start at about 0.1mm, 0.004" for about three passes, and then reduce gradually to 0.025mm, 0.001" for the finish cuts. A good guide to the depth of thread to cut are the tables that I normally use from Motalia. For your 10 X 1.5 mm, the depth is 0.92mm, 0.0362".

**LINK**

Thanks Old Mart.

That link is useful.

SOD - it's encouraging that you've found the same challenges and it might just be the metal. I've got some 12mm EN1A which I can try on.

Tim - I'm aiming at putting a 1.5mm thread on an 18mm piece - which is NOT standard, hence the struggle!

Iain

My tuppence worth: It could be the metal, B&Q bar stock is hot rolled steel for garden gates, it welds nicely but isn't too good for machining. Although it can be machined successfully.

Check the clearance angles on the sourced HSS tool, it could be a generic cutting tool, you may have to grind the relevant clearance angles yourself. The tool might be rubbing more than cutting.

Don't bother with the 29 degrees offset, it just complicates things. Unless I need the room to clear the tail stock I never offset my top slide when screw cutting, I just keep it on zero degrees in line with the bed, and feed in with the cross slide to the required thread depth, the top slide is advanced slightly by a thou or two every cut until the last few spring passes to clean up the thread. Keep it simple.

I have a Mini Lathe and it cuts threads nicely at around 100 revs or so.

Ian

The best way to get up and running with threading is a bit of brass stock, say 25mm diameter by 50mm long. You can do test cuts, and then turn down to fresh metal. I usually do threading by zeroing when the tool touches and then cutting down to the thread depth using the charts. If you have the nut which is to be used, or mating part, then stop cutting very slightly before the theoretical size and use the nut as a gauge for a perfect fit.

Iain, going by my experience with the mini lathe it does seem to me that something is amiss with your machine. I found some notes I made when practicing thread cutting and I had no problem with 60 rpm though I did most of it at 80 rpm but that was just me wanting to make sure I was a little more into the power band of the lathe. It would be interesting to see how you get on with the EN1A if you give it a try, hopefully it will be ok and the problem was just the B&Q material and not the machine.

Edited By Ron Laden on 17/11/2019 18:06:02

Another option is to screwcut away from the chuck and at least stop the worry of crashing stuff.

It's exceedingly common being the thread for spark plugs, glow plugs and lambda sensors on a wide range of vehicles.

I think I would still go with screwcutting on a large gland like that, won't need to be off by much and you will have a tight piston rod and easier to cut to the bottom of what is likely to be a fairly shallow gland recess.

You could easily get away with M18x1 which would make life easier, I use that quite often as I have taps so just screwcut the external.

Tracy tools sell M18x1.5 taps and dies if it helps although at £12 each, not an inexpensive option and not as satisfying as screwcutting your own.

Gray

Thanks, All.

I might go for 1mm thread on this. I was thinking of this initially, but thought it might not be strong enough - easier to cut though!

And I have to confess to being a fair-weather engineer. Coming back from work in the dark, the thought of going into my shed at 3 degrees is just too much! This weekend, perhaps.

Iain