screwcutting guide

Hi Jonathon,

Isn't there a table on front of the hedstock explaining the change gear setup? I don't have a Hobbymat lathe but this picture shows such a table, although it seems to not show the 32 TPI you want. So you may have to calculate the gear setup yourself. Martin Cleeve explains how in his book: Screwcutting in the Lathe (Workshop Practoce Series #3), have you read it?

Thor

Jonathan,. This is a metric lathe, ie metric leadscrew, but that doesn't matter. The threads will be approximate but near enough for all practical purposes where it is only a few turns for fixing something. Don't worry about accurate conversions using 127 tooth gears which are not appropriate in this context and on this lathe.

Also you will not be screwcutting under power. Initially you will have to turn the chuck by hand but you should make a mandrel handle (look that up seperately) fairly soon.

You first need to look at the table on the front of the headstock or page 18 - top right section where it says "n/1" " which is number fo turns per inch. Now there isn't an entry for 32 so lets do 16 first of all.
The columns are W, z1, z2, L. W is the mandrel position, L is the leadscrew position.
The numbers in the table are the number of teeth on the gears you will use. The row for 16 colum for W shows 55 so you take off the pulley and the gear from the back of the mandrel and put on the 55 here, making sure not to loose the small 'key' that fits the slot in the gear and mandrel to lock them together. Both pulley and gear may be tight but there are no grub screws to secure them just friction, rust, and old oil goo. The gears in the middle may get in the way so at this stage so loosen the bolts holding the 'banjo' arm they are mounted on and move them out of the way.

Row 16 column L is 40 so you need to put a 40 tooth gear on the end of the leadscrew. I think you can see what needs to be moved for that to happen, starting with the dog clutch assembly. Observe the movement of the dog clutch engagement lever. It moves up to unlock it. You need to set its position so it can move the dog in and out of mesh.

I will continue later but must go out now.

Do you know the pitch of your leadscrew, and what change wheels do you have?

Neil

Edited By Neil Wyatt on 01/08/2014 18:09:49

Hi Jonathon,
If your lathe uses modulus 1 change wheels then it would be worth using Neil's suggestion of using a 63 tooth gear for cutting imperial threads using a metric leadscrew. Have a look at his article which you can read here and his table of gear combinations here. I bought one of these 63 tooth gears and modified it to fit my Chester DB10G lathe. It has greatly increased the number of imperial thread I can cut.

Les.

Part 2
The two gears z1 & z2 are joined together on a stub arbor which is already in use on the banjo mentioned above. z1 mates with the gear on the main spindle.
To remove the stub arbor undo the small countersunk screw holding a big washer on the end of it. The previous owner probably dropped this and replaced it with a longer less droppable screw and nut. Put making a nice knurled knob on your to-do list and put it in a time capsule for your grandson to open as you will never get round to it. Note that the small screw tends to come loose in its own so keep an eye on it. Note which way round the stub arbor is as you take it off. The geas pull off and you put the two new ones on remembering the key to lock them together.
After refitting the arbor and wheel pair you have to mesh all the gears and lock the banjo in place with the bottom nut and washer. You want them not too tight and not too loose so trap a piece of newspaper beteen teh gears and tighten the nut which sets them about right. Don't grease or slosh oil over the gears as it traps swarf. Just a touch of thin oil and clean them periodically.

Part 3
Note that the leadscrew is permanently engaged so no half nuts to disengage. You have to wind it back as laboriusly as you wound it in, but also must retract the tool first owing to the backlash. You will not be able to set the topslide over at half the thread angle as the slots don't go far enough but actual cutting is covered on other threads. As they are only plastic gears take it gently and sometimes you need to help things along with the leadscrew handle.

Now we have set up for 16tpi using the train 55/65 75/40 on the 1mm leadscrew but you wanted 32 tpi. That means getting the leadscrew to turn half as fast per turn othe chuck. Simple - put an 80 tooth in instead of the 40, except there isn't one in the set. While there are programs to help if you set up the equations in excel and fiddle around you can soon find something suitable. eg 55/60 65/75

Another set model engineers need is 40tpi - 55/65 75/100

You don't need the 63 tooth gear in practice although it is a nice idea. With the invention of computers it is easy to work out combinations that are near enought for all fastener applications and you are unlikely to need ot make a precision thread in the opposite measurement base to your lathe

. The steps of 5 in the gear set is used by a number of other lathe sets and is able to produce all imperial and BA screws - it was a fun excercise way back with my BBC computer but rarely needed. I sent the tables to both ME and the importer of Hobbymats back in 1983 but neither were interested

I think Bazyle has it nailed for you Jonathan.

Note for Bazyle;

When playing with manually selecting change gears, I noticed that when approaching a 'good' selection for an odd pitch it was helpful to have large gears with small differences in tooth number.

The 63 is handy for metric/imperial conversion but if you can select a train that has the option of one of 55-57-60-63-65 in it, then you have three pitches with a maximum 'error' of about 2 1/2%, which is an acceptable error for small fixings (e.g. BA screws). (2 1/2% is half the relative difference between 57 and 60, the biggest proportional gap). That run of five gears in the middle of the size range is very handy.

Neil

Have a look at the program NthreadsP available for free here. You can put in your leadscrew pitch and a list of the changewheels ytou have and it will come up with the best combinations.

Russell.