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Help needed with screw cutting charts on lathe

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David Taylor10/07/2015 05:40:17
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128 forum posts
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Hi all,

I'm trying to figure out how to do screw cutting on my lathe. I've watched a bunch of videos about it and thought I understood but the tables on my lathe are puzzling me.

First off, how do I reconcile the metric thread pitches with the indicator table? I have been doing 25.4 / metric thread pitch to come up with a TPI but this rarely works out to an equivalent number on the table. Is just choosing the nearest TPI the best I can do?

I tried it with a M12 1.75 pitch for example and got a TPI of 14.5. So I figured both 14 and 15 TPI told me I could use 1 and 5 on the thread dial, but the thread came out a mess.

I'm also wondering about the imperial table. I'm guessing it's telling me the middle gears just need to be equal. Is that so? Not sure how to go about that as it has larger 120 and 127 tooth gears on there now and nothing else that came with it looks big enough.

And with the indicator table, how do I figure for TPI above 28, such as 40 TPI? Is it just like 4, 8, 12, etc - being divisible by 4 or 8 means I can choose any number on the thread dial?

I don't want to cut the screws to final size, but I want to cut them deep enough to make it easy to start a die straight and have it just finish the thread off rather than cutting the whole thing. This has not worked yet!

threading.jpg

russell10/07/2015 07:12:51
142 forum posts

what lathe is this for? I'm a long way from being an expert, but my understanding is that if you have an imperial leadscrew, its not possible to use the indicator dial to pick up the thread for a second pass. You need to stop, reverse, without disengaging, back to the start point.

not sure what you mean about the 'middle gears being equal' - but, if they are just idler gears (that is, the drive on one side, and the driven on the other side, with only a single gear on that pin) then the tooth count is irrelevant - it just needs to fit...

not sure about the higher pitches, but you might be right.

hope this helps - i've heard of using a sharpie or similar as a 'cutting tool' to practice.

-russell

Hopper10/07/2015 08:43:29
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3715 forum posts
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Bit more info on what brand/model of lathe would be helpful, as would a pic of the thread indicator dial and the levers for the M11, A3 etc mentioned on the charts above.

But, first thing you need to do to turn metric threads is forget about TPI and conversions and so forth.

From the look of the chart you posted above, your lathe will do all that for you by a simple change of the change gears between the headstock spindle and leadscrew.

You need to set up the gears as show on the metric chart above, that is: 24T on the main spindle, a 127 and a 120 on the same stud in the middle, and a 48T on the leadscrew. The 120/127 combination is the "translator" to make your imperial leadscrew act like a metric one, by the magic of mathematics that is to arcane to get into here.

Once you have those gears set up as above, my best guess is you select the positions of MII and E2 with whatever selector handles/knobs match those. This will give you 1.75mm pitch, according to your metric chart.

Then don't use the thread indicator dial on metric threads. Keep the half nuts engaged at all times and run the lathe backwards to move the tool back toward the tailstock between cuts. The chart above looks as if the chaser dial is set up for imperial only, and it looks like you have an 8tpi or 16 tpi leadscrew, which is why no indicator number is specified for those pitches -- you can use any number on TPI that are a multiple of the leadscrew TPI.


To turn imperial threads, your chart seems to show that you set up the gears with with either a 48 or 24T on the spindle and then change the gear on the leadscrew to value Z to obtain the desired TPI, as well as setting the relevant levers to A1, C3, M11and so forth as per the chart.  The idler gear in between can be any size that fits the space. Its size does not affect the overall ratio. That is determined by the first and last gears in a simple (non-compounded) train such as this one used for the imperial threads.

Happy screwcutting. And I can highly recommend the Workshop Practice series book on Screwcutting in the Lathe as the easiest way to get your head around all this stuff.

Edited By Hopper on 10/07/2015 08:47:16

Edited By Hopper on 10/07/2015 08:49:07

David Taylor10/07/2015 09:00:17
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128 forum posts
39 photos

Thanks guys. I tried leaving the half-nuts engaged and running backwards a few days ago but I have just realised what I did not do in that case was bring the tool out - I just left it in the work and I'm guessing the slop in the half-nuts wasn't doing the job any good. I'll try it again this time bringing the tool back each time I reverse.

I do have the gears set up as required, and had set it to cut 1.75mm pitch. I was only converting to TPI because I thought I should use the chaser dial to engage the half-nuts so was looking for the closest TPI to use those settings. FWIW the chaser dial has 1, 3, 5, 7 and lines between each number as well, so I assume that stands for 1 - 8.

The lathe is basically this one **LINK** - it has the same controls and gearbox, just a flat front rather than curved. You can see a picture of the chaser dial from that page. It seems to be similar to the Warco GH1236.

Regards, David.

Ady110/07/2015 11:51:35
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3463 forum posts
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You can actually disengage the leadscrew clutch for doing Metric stuff on a imperial lathe etc but you need to be able see the gears at the back when you engage and you need to return the carriage to exactly the same position for the start of the threading run (so a stop is required, I just use the tailstock)

I use the cog on the leadscrew because it tends to be the biggest and easiest to see

You view it like you would with a clock, with the carriage "already in position" you only engage your leadscrew as a mark on the cog at the back approaches a certain chosen point, 12 O'clock etc

This means you don't need to stop the machine and the leadscrew can disengage at the end of the cut

It's one of those wee tricks you will pick up with experience, some threads need a double mark between the last two cogs because of a double thread pickup issue(or even a triple), a splat of tippex helps to keep you right until you get your eye in

The leadscrew clutch I have on my machine is a single point engagement


You are basically using your last two cogs as a thread indicator when you use this system

Edited By Ady1 on 10/07/2015 12:07:27

Ian S C10/07/2015 12:37:50
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7447 forum posts
230 photos

Follow Hopper's instructions, main thing is to sort out the metric change over gear.

Ian S C

Steve Abbey10/07/2015 12:38:36
3 forum posts

I had a similar brain scratching issue on my lathe, being the first time I have tried to cut threads. I checked the pitch / TPI by setting up the gears, and then engaging the half nut. Using a vernier caliper between the saddle and the main housing, I then turned the chuck by hand - say 10 times, and watched the vernier to make sure I had the correct pitch. Hope this makes sense.

David Taylor10/07/2015 13:36:06
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128 forum posts
39 photos

Yes Steve, I get what you're doing. I'm sure my pitch is ok.

I think I have two problems - trying to use the thread chaser for metric threads, and I would not be surprised if I am not getting the cross-slide back to the same place each time I take it out and put it back at the end of a cut. I go back to zero but maybe I'm not accurate enough or something.

I'll try tomorrow without opening the half-nuts but still retracting the tool and see if that helps.

Lawrie Alush-Jaggs10/07/2015 13:55:40
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118 forum posts
32 photos

Hi David

I have previously owned a similar lathe from H&F. I also found screw cutting a little confounding when I first started.

Set the gears up in the head stock according to the chart for the thread pitch you want to cut.

Set the lead screw gears similarly. On the chaser you should see a list which tells you which number/s you can use for a particular thread. From memory when I was cutting a 14x1.25 thread the start numbers were 1 or 8.

Pick a number and engage the lock nuts when that number lines up with the indicator mark on the chaser.

When you reach the end of your cut, note the tool depth on and then retract. Wind the saddle back to your start position, wind the tool in to the next depth and then wait for the same mark to line up and once again engage the half nuts.

Do set the speed as low as possible as it is just easier to catch the end of the thread.

don't make the mistake I made which was to assume that the possible numbers for the chaser mean that you use each one in turn. You start on one number, say 8 and continue using that number until the thread is cut.

Good luck, it is very rewarding when you first cut a thread and the nut fits

Trevorh10/07/2015 14:43:12
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279 forum posts
79 photos

Hi David,

I Have the equivalent Warco Lathe and had a similar experience to you

have a look at this post thread

http://www.model-engineer.co.uk/forums/postings.asp?th=92459

I have to keep the half nut engaged all of the time and just use the forward/reverse lever otherwise I get the same results as you.

 

cheers

Edited By Trevorh on 10/07/2015 14:43:40

Ady111/07/2015 00:11:06
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3463 forum posts
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The other big problem with screwcutting is accurate depthing of the tool if it's withdrawn for the next cut

A DRO is invaluable and takes all the guesswork out of this part

Even a cheap vernier allows you to depth 2 hundredths of a mm with reasonable certainty

Hopper11/07/2015 00:33:31
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3715 forum posts
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Here's an old dodge/bodge for depthing the tool after winding it back.

Set the top (compound) slide to 30 degrees (or 27.5 for BSW etc) from the perpendicular to lathe axis.

Set the cross slide so the tool point just touches the work to be screwcut. Set cross slide collar to Zero (or mark it with a felt pen if not an indexable collar) And set the top slide collar to zero also.

Move the carriage toward the tailstock until tool is clear of the job. Wind the cross slide in by the amount of thread depth you want (as per calculation or chart). Set the cross slide collar to zero again, without moving the tool. You now have both dials reading zero and the tool in the position you want it for the final cut.

Wind the TOP slide back out until the tool now just touches the OD of the job. Wind the TOP slide in by the amount you want to take for the first cut. Take your cut. Stop lathe at end of cut. Wind out the CROSS slide until tool clears job. Reverse lathe until tool is back to starting point. Wind CROSS slide back to the Zero setting. Wind the TOP slide in by the amount you want to take for your next cut. Repeat this process until the TOP slide collar reaches the pre-set Zero mark.

By this method you are using the cross slide to retract the tool and reset it to the same point, and you are using the Top slide to set the depth of each cut. So it is easier to keep track of what is where.

David Taylor11/07/2015 11:59:49
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128 forum posts
39 photos

Well, good and bad. I have a clean looking thread now I am not opening the half nuts, but despite many checks of the gears and settings the pitch is off. It should be 1.75mm, but it stretches to somewhere between there and 2mm.

I'm not going to blame the lathe as there are many of these in the wild. But I have no idea what I've done wrong. Maybe I should count the teeth on the gears rather than relying on the stamped number frown

After watching some of mrpete222's videos I'm using Hopper's method of retracting with the cross slide and using the top slide for depth, only ever moving it in. Thats the way mrpete222 does it too.

I did once (a couple of years ago) manage to cut a 1/2" thread, so I can do it.

When I've calmed down a bit I'll have another go at some different size/pitch. Maybe M12 just isn't for me.

On the plus side I managed to turn a MT2 taper with a 17/64" spigot on the end to stick into my rotatary table which I used to mill round ends on the draw bar for my loco. Attempt #1 of doing those ends finished with the work getting grabbed out of my hands and winding itself up the end mill getting chunks taken out in the process. Much safer with it held centrally and clamped on the RT! Of course I slipped on the linisher and my painstakingly shiny round ends now have a slight flat on them but you won't be able to see it!

Regards,

David.

Bowber11/07/2015 12:17:19
169 forum posts
24 photos

When your just starting out screw cutting is relatively hard as you have to follow all the procedure in the right order, once you have done it a few times it starts to make sense and you can modify the procedure to suit how you want to do the job.

As has already been said you can disengage the nut for metric threads on an imperial leadscrew, but you need a stop and mark on the leadscrew, for now don't bother just have a practice with imperial threads (or metric if a metric lathe) and get a feel for how to screw cut. This is the bit were having an experienced friend really helps.

One quick note, you'll end up having to go deeper than the calculation or books will tell you, this is usually because you'll be using a sharp tip on your tool, you really need to stone off the sharp tip but you'll not get it exactly right. I can never be bothered to calculate it so I just do a few spring cuts and keep checking to see if the nut will fit.
Also chamfer the end first, it makes testing the final fit a lot easier.

Steve

Just seen you reply before mine, I was under the impression you were new to this so my apologies.

Edited By Bowber on 11/07/2015 12:21:39

Hopper11/07/2015 13:22:07
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3715 forum posts
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Posted by David Taylor on 11/07/2015 11:59:49:

Well, good and bad. I have a clean looking thread now I am not opening the half nuts, but despite many checks of the gears and settings the pitch is off. It should be 1.75mm, but it stretches to somewhere between there and 2mm.

 

Well that's progress in the right direction. "Stretches to somewhere between there and 2mm" Hmm

Are you using a good quality metric thread pitch gauge to measure it?

Is the error consistent all the way along the thread?

You could mount a dial indicator on the cross slide or carriage with the button bearing on the tailstock. Then, rotate the chuck exactly one revolution and read off the dial gauge exactly how much the carriage is moving. If it is not 1.75mm, something is amiss.

Did you buy the lathe new? Or is there the possibility that a previous owner fitted a metric lead screw to it? (I've seen this done on a used milling machine a mate bought. Drove him nuts until he went to make some new half nuts and discovered the leadscrew and nuts had been changed from imperial to metric at some time in its life.)

It might be worth checking the leadscrew pitch to make sure it is an even number of TPI. And, yes, it might be worth also checking the gears have the right numbers stamped on them - these things do happen in China.2

If all the gearing is correct, the problem may lie in the rather confusing array of lever positions. Make sure all levers are correctly located on their shafts and have not moved position. And double check you have the E2 and MII settings as per the chart you posted (for 1.75 pitch). If something is out of whack there, it will cause weirdness.

Edited By Hopper on 11/07/2015 13:26:59

Edited By Hopper on 11/07/2015 13:34:37

David Taylor11/07/2015 13:59:34
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128 forum posts
39 photos

If I am using the top slide to increase the depth of the thread, would having the wrong angle on it account for the incorrect pitch? Seems like it would more make the peaks and valleys run into each other than affect the pitch to me.

I have it as close to 30 or 29.5 deg as I can tell, but that doesn't mean it actually is one of those positions.

Imagine you lined up the top and cross slide so they're parallel, then swivelled the top slide 29.5 or 30 deg CCW. So feeding the top slide feeds the tool right to left on an angle. That's how I set it.

I'll have a chance to redo it tomorrow as the top slide is at present set for the MT2 taper.

I have owned the lathe from new. I've just decided it's time to get used to screw cutting to try and help preseve my dies.

Thanks for all the advice.

Steve Pavey11/07/2015 16:37:07
280 forum posts
32 photos

Top slide angle setting is not all that critical, as long as it's 30 or a bit less (for metric). It certainly won't affect the pitch. I don't understand from what you say whether the error in pitch that you have is consistent, or whether it varies along the length of the thread. If it's consistent, then the chosen ratio is wrong (or wrongly labelled on the lathe instructions). If it varies along it's length, then there might possibly be some sort of backlash problem, either in the leadscrew/halfnuts, or in the gears. Or possibly something slipping on a shaft maybe - do all the gears have keyways?

Ajohnw11/07/2015 17:47:21
3631 forum posts
160 photos

You might not be running the saddle along for a sufficient distance to remove play in the lead screw fit in the nut. I generally allow 1/2" for that but lathes vary and that would be for a coarser pitch. There is no where near that amount of play in the fit but that distance ensures the nut is hard up against the lead screw when the cut is actually taken. One thing for sure if the the saddle is run back to the start by reversing the lathe the lead screw will be hard up against the wrong sides of the flanks of the nut so more space than i have mentioned might be needed. If the saddle fit is adjustable it might pay to tighten it slightly so that there is a bit of resistance when pushed along by hand. Unlikely problem but ...............

The main reason for setting the compound slide at an angle is that the tool will only be cutting on one side which reduces the strain on both the work and the machine. Opinions vary, some set so that a very light cut is taken on one side and the heavier on on the other. Subject to the work being up to it I often take the last couple of thou off with the cross slide.

To be honest with a 1.75mm pitch I might just feed straight in but lighter cuts would be needed and slender work might bend. Best run the tool down more than once at the same setting to make sure that isn't happening. I can understand why you might want to leave your compound slide set for a morse taper. They are a pain to set precisely.

I've no idea of the situation with metric hobby lathes and screw cutting indicators as some images on the web show them with them. This could just be imperial models but they can work out on metric threads as well if the machine is fitted with one. In that case the manual should mention which pitches can be used with the various marks on them. With these screw cutting can be done in the "normal" way. Disengage at the end of the cut, wind back the saddle to the start by hand and re engage on the correct mark. This should also prevent the nut from riding on top of the leadscrew before it engages which isn't very good for it. Actually by using one of 2 gears to mesh with the leadscrew they can be used to cut most of the usual pitches or with more such as fitted to some Colchesters yet more pitches but it can take a very long time for the indicator marks to line up on those. The design for the twin gear Boxford one is in my album. The gears have to be cut at an angle to match the lead angle of the leadscrew - or maybe the entire indicator could be tilted. I understand that Boxfords use a 3mm pitch leadscrew. Might be best to check. Different gear tooth counts would be needed for other lead screw pitches. The gear teeth counts and markings on the dial are defined by repeat distances - length of leadscrew in terms of it's pitch that are exact multiples of the pitch to be cut. The imperial pitches that are actually used on anything have been set to values that allow a very simple single gear indicator to be used but even then some will allow more pitches to be cut than others - usually more obscure ones are added.

Last but not least if the pitch is definitely out - count the teeth on the gears. It has been known to be incorrect. Rare but very good at driving people up the wall.

John

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David Taylor18/07/2015 09:50:01
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128 forum posts
39 photos

I had time to look at this again today and figured it out.

While counting the teeth on the gears I noticed both of them were meshed with the 127 tooth middle gear. The bottom 48 tooth gear should have been meshed with the 120 tooth gear which sits on the same arbor as the 127 tooth one.

The way they were set up was okay for the imperial 1/2" thread I cut a couple of years ago and when I saw the correct gears were still installed I stopped looking so didn't pick up one of them had to move out to mesh with the other 'middle' gear.

So this time the thread was clean and to the correct pitch. I couldn't thread a nut on though, even when the thread looked pretty good. I had to run a die over it before the nut would go on. On the plus side I think this left a nice clean thread, which looked better than the turned one.

If I am turning a male M12 thread, do I turn the initial diameter to 12mm, or a bit under?

Regards,

David.

Ady118/07/2015 10:09:53
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3463 forum posts
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If I am turning a male M12 thread, do I turn the initial diameter to 12mm, or a bit under?

If you turn to 12mm the lathe thread cutting process can squish the peaks to 12.1mm kinda thing, but on a B+Q nut this can give you a fabby fitting thread because of error "tolerances" built into "standard" nuts and threads

More experienced folk in here can give you the details but not all threads and nuts are created equal

If my thread is a bit too big I occasionally skim it down on the lathe and try the nut again

It's one of those areas where you need to test for the best results

My 12mm tap creates a thread that 12mm B+Q bar fits into quite easily, but it's a bit of a loose fit IMO. It does fine for ordinary jobs, but it's not a very high standard of fit and you wouldn't use it for a critical job

Edited By Ady1 on 18/07/2015 10:17:18

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