Clarke lathe help please

[daz]

I have a clarke cl500m lathe and am just about to start my first foray into making my own threads. I have turned loads of stuff on it in the past but always thought it was complicated to set it up for threading. After poking about in here for a while I realise it isn't.
My problem is that the instructions that came with the lathe to show spindle speeds does not tie up with what I have, ie the pulleys are in different positions. Does anyone out there have the same lathe that can help me out please.

TIA
daz

[andyf]

Hi Daz,

I'm not sure how the pulleys come into it, other than to get the spindle rotating at a suitable speed for threading. Many Chinese lathes won't really go slow enough; in that case, a handcrank in the spindle can take all the drama out of stopping in the right place.

It is the gears connecting the spindle to the leadscrew which set up how far the tool is hauled along during each spindle rotation, and thus generate the thread. For example, if you want a 16tpi thread and your leadscrew is 8tpi, the gears are set up to get the leasdscrew making half a turn for each full turn of the spindle, no matter how fast the spindle is turning.

Did you get a screwcutting chart with the lathe, showing what gears to use for different threads?

Andy

[John Rudd]

Hi Daz,

I'm not sure how the pulleys come into it, other than to get the spindle rotating at a suitable speed for threading. Many Chinese lathes won't really go slow enough; in that case, a handcrank in the spindle can take all the drama out of stopping in the right place.

It is the gears connecting the spindle to the leadscrew which set up how far the tool is hauled along during each spindle rotation.


Andy

Yup what he said....and, the torque required to cut say a 2mm pitch thread may be more than the belts can take without slipping.....

Definitely a case for a hand crank...with much better control over starting and stopping.....

[daz]

Cheers guys, I have all the gears and a chart for them. I have managed to figure that bit out, but I thought you were meant to cut screws on a slower speed than what you would use for turning? I think I might have figured out why it looks so strange, it looks like a belt has snapped in the past. Instead of the correct belt being installed someone has put one on that is far too large. The motor has been adjusted all the way out amd the drive pulley pulled out by 20mm to take up the slack on the middle position. I took all the belts off along with the pulleys, cleaned it up and refited with the pulleys in line, the belts don't come anywhere close to fitting. I have ordered the new belts in the right size, along with a new qctp, holders, tools and a 60 degree threading cutter. As far as the speed is concerned I'm sure the clarke can go down to 160rpm, is that slow enough or do I need to make a hand crank for it?
Daz

[John Rudd]

As far as the speed is concerned I'm sure the clarke can go down to 160rpm, is that slow enough or do I need to make a hand crank for it?
Daz

Experience will tell....

Best make a hand crank just in case...

[andyf]

Daz, I think (someone correct me, please, if this is wrong) the only reason for threading at a lower speed than when plain turning is the problem of stopping in the right place when threading up to a shoulder and the like, when overrunning could cause a tool crash. The coarser the thread, the faster the tool zips along, and quick reactions can be needed. Classier lathes than yours and mine have a back gear, which can be engaged to reduce the spindle speed by a factor of 5 or 6, so everything happens in slow motion. The primary purpose of a back gear is for turning large diameters, but it can also be useful when threading.

Best thing is to experiment on a bit of scrap bar, or even a bit of PVC pipe. With your threading tool, scribe a line round it not too close to the chuck jaws. Engage your threading gears and,  with the tool doing little more than scratching the surface, take repeated "cuts" and practise stopping on the line. The maximum overrun will indicate how wide a runout groove you should put round the work before threading for real.

Now I'm getting close to my pension, my reactions ain't what they were. I built a contraption to help me stop in the right place, so I don't have to resort to my handcrank - see http://andysmachines.weebly.com/semi-automatic-saddle-stop.html .

Andy

[Swarfing]

On the belts front you should have had three with it. Two small and a larger belt, the two small were for outside to middle setup or the large on it's own from motor straight to outside pulleys from memory to get all the gear speeds.

[doubleboost]

Hi
Daz
You need the slowest speed you can get for screw cutting (things seem to  speed up as you reach the end of the thread)
Turning the lathe manually would be good for your first try at cutting threads

Andy
That is a very clever set up you have there
Personally i never release the half nuts until the thread is finished
I have a inverter on my lathe which makes  speed controll and reversing easy
John

[andyf]

Hi John,

This is a bit  for Daz, but my setup works fine for screwcutting when I can disengage the half nuts and use the threading dial, but it's useless if I'm doing a thread which is foreign to the leadscrew, when they must be kept engaged to maintain sync between spindle and leadscrew.

I'm keeping an eye on this long-winded thread on the Model Engineer website http://www.model-engineer.co.uk/forums/postings.asp?th=49358 . The idea of combining a stop-operated dog clutch with an automatic reverse, so the motor can be left running forwards throughout, is very appealing.  If used in conjunction with a swing-up tool holder, all you would need to do is add on a bit more depth of cut as the tool traversed back.

Andy

[daz]

Thanks for the replies guys, very helpful.
I have ordered a QCTP, some new toolholders, parting off blade, 60 degree thread cutter and a set of belts. The ones that were fitted were wrong and as a result the main drive pulley had been pulled forward on its shaft to take up the slack (the motor tensioner was out at max!)
The tool post I have ordered (from Chester Tools) seemingly needs "some machining" to make it fit, or at least according to their website. Anybody have any ideas what I am going to have to modify???

Thanks
darren

[andyf]

Whatever it is, it should be pretty obvious when you get the QCTP and try to fit it, Darren. It may well be something simple like enlarging the through hole to suit the post which sticks up from your topslide.

Andy

[Pete.]

The toolposts come with a (barely) machineable nut that you modify to fit your top slide.

[John Rudd]

ditto what Andy said.....

Mebbe if you post up a link to the tp you bought will help....

[daz]

I doubt you will be able to see much from the picture in the link but fingers crossed,
http://www.chesteruk.net/Products/Product.aspx?productID=141
It's the third one down.

thanks

darren

[John Rudd]

Darren,

It could be what Pete said about machining the top nut.....

You wont know until you remove the existing tp and measure the height of the centre bolt and compare that with the overall height of the new tp.

You may get away with just replacing the lot and using a slightly thinner nut(dont omit the washer underneath) The other way would be to make a new handle/nut suitably threaded to match the centre bolt, wouldnt need a spanner then for tightening

[Stilldrillin]

I doubt you will be able to see much from the picture in the link but fingers crossed,
http://www.chesteruk.net/Products/Product.aspx?productID=141
It's the third one down.

thanks

darren

Thanks for the link Darren!  Two spare, mini toolholders arrived yesterday. 

I've been looking for a supplier, all summer...... 

David D

[daz]

Glad it helped someone else. They have a good selection of stuff for the smaller mills and lathes and are pretty cheap too, delivery can be a wee bit slow but that's made up for by their assisatance on the phonem

Daz