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

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

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**

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

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

M18 x 1.5mm is a fairly common thread – taps + dies should be available – easy to find such advertised

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.

HSS ground tool? Most probably you don't have the proper tip radius/width which means you are 'touching off' the start of the thread too far out, meaning you are stopping before the thread is fully cut.