Member postings for old mart

It is a good idea to take time to experiment with cut depths on different materials to find what combinations work best.

Most lathes should be stopped before changing any gears to prevent crashing, grinding and damaging the gearing. The exception is the leadscrew nut. The leadscrew nut is intended to be engaged and disengaged while the machine is running as it is impractical to keep stopping before using it. This does cause wear to the nut in particular, but it is unavoidable.

I keep a bookmark of the Motalia website, they have a thread chart.

For instance, 1/2 UNF female thread. Thread depth 0.0271" Multiply that by 2 because the thread is right round the circumference, and take it away from the nominal size.

0.500" - 0.0271" - 0.0271" = 0.4458" . The 0.4458"is the start size of the bore of the female thread. These figures can be rounded down to 3 places of decimals, ie, 0.446".

**LINK**

My take on exact diameter is when cutting with carbide inserts, a fairly deep cut is found to give a good finish. For instance, if a depth of cut of 0.020" is giving a good finish, and the penultimate cut gives a diameter of plus 0.040", then you will have the right size and a good finish with a final 0.020" cut. If, however, you take smaller cuts as you get near to size, the finish gets poor as the tool is not cutting at its best.

I remembered to check out the ACME threading bar when I was at the museum today, and have brought it home with me.

After a bit of a struggle, I managed to find your address and the tool will be in the post Monday.

If you are careful and take the cuts that I recommended in my post of the 11th, there is no danger of a crash. The hard thing I find is remembering to go back to the zero on the cross slide every time before withdrawing the tool. It helps to have the settings on a piece of paper and tick them off as you go. I would try out the leadscrew for fit during the last few cuts.

Edited By old mart on 12/06/2021 19:16:02

Edited By old mart on 12/06/2021 19:23:25

Edited By old mart on 12/06/2021 19:30:38

If you don't have the nerve to make the last cut a big one to get the better finish, but prefer to creep up the last bit, I suggest you try using a carbide insert intended for aluminium on steel. You can take a 0.0005" deep cut with one of these on steel and get away with it.

I would make the drift out of a bit of brass bar rather than steel.

Top slides are useful for most jobs and should always be ready for refitting. The lathe in the pictures has a choice of two different top slides, one is brand new even though it is 70 years old. Having the option makes for greater versatility. I also have a rear toolpost which bolts directly to the cross slide for when a particular parting off would be a challenge. That might be another mod you could consider.

Edited By old mart on 11/06/2021 19:07:20

Dead right, Tony, Germany in particular seems to charge high postage rates.

The lever on the apron is made for engaging and disengaging the leadscrew when the leadscrew is rotating. You should practice with the saddle right in the middle of its travel to give you plenty of time to see what is happening and stop without crashing.

I had a bit of a snigger when I read the comments about the tool I had made to hold the 22 size insert and still pass into a hole of 0.800". I have to confess two points regarding that tool. It is made of a bit of steel that was laying around, and I strongly suspect it is a bit of stainless steel. The photo is the only one taken of it and as soon as I fitted the insert, I realised my mistake. The insert housing is much too near the end of the tool and the insert overhangs the end dangerously. The entire end has now been remachined to give the best support that I could manage.

Regarding lending the tool, The chance of damaging it is remote if careful attention to the actual thread cutting is adhered to. With a 5 ACME, the actual depth of thread is 0.1" deep from the start bore of 0.8", resulting in a finished diameter of 1". This might have to be increased very slightly if the leadscrew is used as the go gauge.

To use the leadscrew as a gauge, you would have to remove the tailstock.

If the thread is produced in multiple passes normally, from first touch in the bore, depths of +0.015",(twice) + 0.014", +0.012", +0.010", +0.008", +0.006", +0.005",(twice) +0.004, +0.003", +0.002", +0.001", which make the total 0.1" deep cut. Any further passes to get the leadscrew just fitting to be made at 0.001" or less. The reduction in depth of cut as the threading gets deeper is because the ammount cut is getting bigger each time.

I can make another tool any time and your only requirement would be to buy me another insert if you broke it which is very unlikely, so the offer stands.

I'm back at the museum tommorrow and will be able to measure the working length of the tool, which will have to be longer than your nuts if the thread is right hand to give you time to disengage the leadscrew nuts after each pass.

Edited By old mart on 11/06/2021 18:49:30

I bought a Vertex electronic one years ago, the drawback with it is the 20mm body diameter, ok with R8 collets but a bit big for some machines. There are plenty of cheaper copies on the market at the moment. It has been used at least half a dozen times. I really got comfused when I saw my original post which I had totally forgotten about.

Edited By old mart on 10/06/2021 20:56:29

I cannot believe that it would have been stolen for its scrap metal value. It would be prudent when building a valuble model like that to mark a number of the components with the makers full name before assembling, to give the police a better chance of identifying stolen property in the very unlikely event of a loss.

I don't think the headstock is intended to be moved, it just sits clamped to the unusual bed vee ways. The saddle is very narrow and it looks like the apron can pass underneath the headstock slightly. The leadscrew is almost out of sight behind and beneath the rack.

Edited By old mart on 10/06/2021 20:31:31

I don't find that the spanners that you can get for tightening them are long enough. We use er25 and I have a pair of spanners with 10" long handles. As already mentioned, a ball bearing nut helps.

Who had that immortal catch phrase "the Germans bombed our chippie"?

I happened to get hold of a NOS 4 way toolpost which is bigger than the standard size for the lathe (Smart & Brown model A), and I decided to use it mounted on the cross slide at the same height as the other ones for the lathe. Because it is bigger, 25mm boring bars can be fitted and their increased rigidity plus the rigidity of leaving out the top slide would be an advantage. To partially make up for the loss of being able to make fine adjustments on the top slide, it helps to have an adjustable saddle stop and a box of slip gauges. The block of cast iron which the toolpost sits on helps somewhat with damping, but mild steel would work ok too. That boring bar in the picture is a 16mm with carbide shank which could enter a 20mm hole and bore 96mm deep.

Edited By old mart on 10/06/2021 20:08:25

Buy that lot at once will cost you a pretty penny, anyway welcome to the forum.

Not your GP, you want your BS, (belt supplier).

After grinding, the chuck holds all the sizes of ground stock that I have inside or out with a tir of 0.001" or better, I cannot expect better than that without fine adjusting (poor mans set tru) each time.