Screw cutting problem

Tony

Merely an attack of creativity I'm afraid.

My Pratt & Whitney has a single tooth clutch and my Smart & Brown 1024 will get one, or a functional equivalent thereof, if I ever manage to figure out an elegant method of fitting.

Jasons need for fine longitudinal feed is indeed the best justification for setting a topslide parallel to the bed.

I'm impressed that he generally doesn't have any issues with cross slide to tailstock interference. On a small lathe avoiding that sort of problem needs very careful detail design and dimensioning.

Hafta say I've never driven a lathe that didn't have at least potential issues there. Even the stubby topslide on my P&W with its gear driven screw and raised, angled, handle / dial assembly manages to have issues. Which really surprised me once mid job. Had to resort to an extended toolholder. I loathe the things due to constitutional distrust of anything forcing the tooltip to divine its support at considerable distance.

When it comes to short longitudinal feeds I prefer to rely on a bedstop to give positive end position rather than a dial. Must be well over 30 years since I last used a machine without a micrometer bed stop or at least a solid one set with the aid of a near freebie "gash" set of gauge blocks that no longer wring so can't say I recall what it was like to do without.

Lots to be said for the modern DRO scale on the bed if you don't have a longitudinal feed dial. My S&B has an excellent and accurate one so I'm in no rush to go electronic. I did hear of someone who had successfully fitted a small retracting tape measure to the tailstock side of his saddle and found it effective for approximate work. Presumably one of those tiny pound shop bargain things so clearyl attractive to the fiscally challenged (AKA normal) home shop person.

Clive

All brownie points fully deserved. Thanks for the link. Awesome work. And a beautiful little engine design.

When cutting threads the basic set up should go something like set the compound slide to half inclusive angle.Set dial on compound to zero.If possible turn a root diameter at either the start or exit of the thread using cross slide.When the root diameter is achieved set cross slide to zero.Clear tool from work by backing off just enough using compound slide.Select RPM and touch on work using only the compound slide.Wind down the bed till your of your work.Set your first cut on compound and and chose a number on your chaser dial and engage.At end of each pass(remember to disengage) clear work using cross slide and wind down bed past the work to the beginning.Wind in cross slide to zero , put cut on compound slide and engage on same number.Repeat until compound reads zero.I would set up like jasonB as it cleans up trailing face nice.

If you do turn a section to the root diameter do consider the form of the tool being used before you do. A full form insert, a partial form insert or a home ground HSS tool will not all have the correct radius on the end so may need to go in deeper than the root diameter.

Well, that's one way of doing it, out of many as discussed above. I think maybe there are as many ways as there are machinists!

Unfortunately the OP's lathe, an ML7, does not allow you to rotate the topslide to half the thread angle. It is limited at 45 degrees from the lathe axis so can't make it to the required 61 degrees or so. So he is limited to either straight plunge cutting or the common method used in industry of advancing the non-angled top slide by half the amount of infeed.

Yes I have almost always found screwcutting to be a "fit and try" effort the final five thou or so, for this reason. One advantage of the method keeping the toplside set parallel to the lathe axis is if/when you get to full thread depth (or what you calculate to be full thread depth) and your sample nut will still not fit on your thread being machined, or goes on but is a bit tight, you can take a skim along each flank of the thread to clean it up without adding thread depth. This will often then allow the nut to screw on beautifully. With the angled topslide, you don't have this option and have to keep on adding depth of cut until the side flanks clear the flanks in the nut. Probably more relevant on larger threads than smaller, which are easier to do with a die anyway.

Ramon, That is a lovely set of parts for your Atomatic 5cc Diesel. I am not familier with this design. Are drawings available or is it another of your re-design projects?

Keith

Hello Keith - how are you? Still beavering away I hope.

I was smitten by the Atomatic '4' as soon as I saw Les Stones version on Model Engine News. The drawings I did for mine were pre CAD (for me) and simple pencil on paper. Ron Chernich was going to re draw them in CAD for MEN and a set were sent to him. Sadly his illness got the better of him before he was able to do so.

There was an Atomatic '5' as well so drew it out without the need for scaling

I still have the original drawings I did so if you would like them I can either copy you a set or take some images of them - unfortunately I am now not able to scan them to send digitally.

Regards - Tug

I was picking up on the spelling rather than punctuation, but also thought the thread could do with a little levity.

Andrew

Oh, my apologies,I didn’t know the Myford had this limitation on its compound slide.Thanks for the Lesson 👍

Exactly, I use thread depth as a guide, but use a mating part as a gauge for final fit. I aim to achieve a shake free fit, and agree that a couple of spring passes can be the difference between go and no go. For brass and cast iron a brush to remove swarf dust also helps. I rarely use dies, many external threads are screwcut, or done using Coventry dieheads.

I screwcut a lot of 32 and 40 tpi ME threads, where thread depths are small, A few thou can be the difference between a shake free fit and a rattling poor fit. My ME taps and dies are old and carbon steel, so it's not clear that they are particularly accurate. Consequently trial fits are essential.

My original comment in response to Clive's comment was partly tongue in cheek. My lathe is ex-industrial and I have an Ainjest unit fitted. The unit was bought on a whim, just to experiment, but I've found It makes screwcutting, especially blind internal, much easier, and faster. So I don't see it as cheating. I use techniques that are appropriate to the machines that I have. I distrust people who are dogmatic about the right, or wrong, way to machine something. I recall a forum member telling me that CNC milling wasn't really true modelling, until they got a CNC mill that was.

Andrew