Feed speed

I have a Warco 250 MV. Changing the change wheels for different threads can be a hassle & whatever set of wheels one selects gives one the feed speed on the leadscrew & the cross slide. Too high & the cutter goes too fast giving a poor finish, Too slow & it is time consuming, especially facing cuts

As I do not do much thread cutting it would seem sensible to set the speed at a suitable speed for feeding the tools along the material when doing general machining. In addition I have a fairly ridgid set up for the cutters so my parting is rarely a problem. However winding the cutter in & out by hand is a pain & an even feed works better

I also find the constant meshing of the gears noisy & different ratios can be a little quieter, but because I have not found a nice "optimum" I tend to feed everything by hand

So I was wondering what set of wheels most users of the 250MV use if not thread ciutting but just want to travers or face cut. As for tools, i use tipped & HSS but I like Brazed carbide. However I am told father Xmas will be delivering a pair of diamond tool holders so that could be the tool of the future. The parting blade is an eclipse tapered 13mm deep HSS

Any comments please

Edited By Sam Longley 1 on 04/11/2021 17:16:57

Edited By Sam Longley 1 on 04/11/2021 17:18:28

Does the 250 not have a set of feed speeds on the chart? My 280 does

I just leave my gear train in the finest of the ones on the chart and just flick the front lever to get three different speeds as needed.

Yes , but which 3 speeds?The slowest is hardly moving, so you are saying set it as slow as possible. Is that it?

Edited By Sam Longley 1 on 04/11/2021 17:20:09

Edited By Sam Longley 1 on 04/11/2021 17:21:19

I use the change wheel set shown in the LH column of the top diagram in the manual and the headstock plate, ie. blank (prob. too fine for 2 dp), 0,03 and 0,06mm. These give me a good finish.

So far I've found that any of the standard-pitch screwcutting setups give too coarse a feed for normal turning when using the feedlever instead of the halfnut lever, and this makes me reluctant to change over to screwcutting if I don't have to. But it's a small issue to me, because the very fine feeds are great for surface flycutting and milling on the vertical slide as well as facing and turning in the chuck.

Edited By Mick B1 on 04/11/2021 17:32:19

If you use inserts look at the recommended feed per rev and start there.

When not screw cutting my lathe is usually set to 3 thou per rev IIRC - it was the feed rate of my most used insert tool.

Dave

Like Jason, I usually leave my WM250 set with the finest feed gears in place. To avoid too much noise from the gear train use a sheet of paper between the gears when setting up, as per the manual. I find that liberal application of grease helps to reduce rattle, also make sure that the right washers are used to prevent the sides of the gears rubbing.

John

So how do I equate that to TPI, which is what I need to know what change wheels to use? Metric lathe.

Edited By Sam Longley 1 on 04/11/2021 18:18:33

With the gears set for the slowest speed I find that the slow C and medium A lever positions give me two suitable speeds for finishing and roughing when turning along the lathe. The B and A positions give roughing and finishing for facing. I prefer to run a fast spindle speed with lighter cuts to a deeper cuts you get there at the same time either way.

So 0.0025" per rev finish turning, 0.005" per rev roughing turning and 0.0015" finish facing, 0.003 rough facing

this would be in the region of 200tpi rough turning and 333tpi rough facing

If you find that too slow then just use the next feed ratio up or the next one from that.

As well as the strip of paper method mentioned by John also make sure you have the washers between Banjo and gear pairs the right way round otherwise the sides of the gears can run and "ring". Also there is a bit of sideways adjustment of the banjo where it pivots that can also be used to make sure the gears don't rum

2 1/2 thou = 0.064mm

5 thou = 0.127mm

1 1/2 thou = 0.038mm

Edited By DiogenesII on 04/11/2021 18:41:28

Feeds are not usually measured in TPI. If yours is a metric machine it will be in mm/rev. The table from the manual should make it clear:

This for the WM250 but should be similar to your later machine.

John

Same here; I haven't changed any gears since I got the machine way back in 2012, I have never needed to with the small threads I need. I usually work within the small mm range plus I have sets of BA & ME taps & dies & only use the A,B,C lever position for feed ranges.

George.

To calculate inch-based thread on a metric lathe, work by pitch thus:

- Divide 25.4 by the TPI to give the pitch in mm. [As the full "sum" is 25.4 X (1 / TPI) ].

- Find the ratio between that pitch and the leadscrew;

- Derive the closest combination for that ratio for the change-wheel set you have

Unless you have an appropriate conversion wheel (ideally 127T but 63T is often sufficient) or can set a multiple compound train, you will be lucky to obtain highly-accurate inch-pitches.

Nevertheless you might find combinations acceptably close within say, 10 turns, often long enough for many applications such as pipe connections and studs. You may need cut a bit shallow and finish by die or die-nut.

I've created 'Excel' spreadsheets, using the standard arithmetical functions (you don't need delve into programming), to work out opposite combinations - mm pitches on Imperial lathes without a 127T or 63T wheel available. It took a lot of experimenting but I obtained quite a lot of close matches even with just a simple Driver-Idler-Driven set.

A further column showing the pitch-error for 1 and 10 turns; allowed me to tweak the combinations to close the larger errors.

So it should be possible to derive inch pitches for your metric lathe and the available change-wheels.

To give some ideas, by calculator and rounding to 3 decimal places:

32tpi = (25.4 / 32) = 0.794mm pitch

40tpi = 0.635mm p.

26tpi (Brass thread) = 0.977mm p.

1/8" BSP (28TPI) = 0.907mm pitch.

....etc.

The sum is 25.4/T mm; where T is the threads per inch.

I would not say the handwheel is hardly moving in this video at about 35 seconds in and that is only on the middle A lever position, finest of the gear trains

Edited By JasonB on 04/11/2021 19:09:17

Forget thread pitches or tpi when feeding,  You are not screwcutting. 

The only number that matters is the feed per rev.

Surely the purpose of the feed per rev is to provide a suitable rate for roughing which does not overload the machine or the tool, and then a reduced feed rate for finishing to give a good surface finish, with a shallow depth of cut.

If the machine ha sufficient power, and rigidity you might rough at 0.5 mm per rev, and finish turn with a smaller depth of cut at 0.1 or even 0.05 mm per rev.

If these figure are not listed in a chart on the machine, they can be arrived at by calculating the gear ratio required between the chuck and a Leadscrew of known pitch.

It just needs a little thought, and some simple arithmetic to make up a gear train to give the required ratio.

You are setting up to cut a helix, but shallower and of a different form from what you normally see as a screw thread.,but, for finishing, with a very fine pitch.

You are unlikely to get a good surface finish with a high feed per rev, so just have to be patient.

Rome was not built in a day..

For a 1.5 mm pitch Leadscrew, it needs to rotate at 1/15 of the speed of the chuck to provide a feed rate of 0.1 mm per rev. So you set up a gear train to do this,

Say 20T :driving 75T/20T driving 80T (3.75: x 4:1 = 15 ) The ratio is given by DRIVERS / DRIVEN

IF the gears are available. You might need to buy in some extra spares, especially the lowest and highest tooth counts.

If the lathe has a Norton gearbox, the same logic applies.

My lathe gearbox provides a finest feed of 0.119 mm / rev with a 40T driving, via an Idler, the 40T on the input to the gearbox. By making and fitting a 80T, the feed rate is halved to 0.0595 mm /rev.

In fact by using the 127/120 compound idler, this feed can be reduced even further., to 0.0562 mm / rev

Some gear trains will involve two compound intermediates. In this case, the tumbler reverse will need to set to, apparently, reverse the direction of the Leadscrew. But because of the two "Idlers" it will rotate in the normal direction.

Howard

Edited By Howard Lewis on 04/11/2021 22:17:40

Sam maybe it would be better for you to say what feed rates you want with your tooling, material and likely diameter of work. Then we can give a suitable gear train. Is yours a metric or imperial leadscrew?

In the opening post you say you are getting some Diamond holders but have also said things are too slow for you. The HSS bits in the diamond holders will have to run at approx 1/3rd the speed of carbide so straight away a cut will take 3 times as long assuming similar DOC and feed for both types of tool.

Throw in the fact that the variable speed machines lack grunt as the diameter of the work goes up and the spindle speed goes down and you are not going to be able to use a heavy feed at the slower speeds that HSS needs on larger work. You will just stall the spindle and risk board and motor damage.

These machines don't have the power or rigidity to take 0.5mm/rev feed rates on anything but the smallest work so you will have to learn that they won't remove metal at the rate of a similar sized industrial machine.

My comment would be that for most of us it is a hobby, so time is not often much of an issue. If you want/need commercial cutting speeds, buy a (much) more expensive machine that will accommodate your needs. A couple of grand for a new machine does not get you a great deal these days - expect either good user operation or good accuracy, but not necessarily both. Hobby machines are a compromise, which most buyers accept.

My previous machine had about 16:1 change of feed rate by simply selecting with 2 control knobs, but the facing speed was pathetically slow, due to other gears in the drive train, compared to the long-travel. My current one (the improved, later, version of the same basic machine) has the same ratios available with the two knobs, but the facing feed rate is much improved.

A change-wheel machine, which was my first lathe, was not for me. As soon as I changed it for one with a QCGB I realised I would never settle for anything less.

I only used the power facing on the previous lathe for a steady (very slow) finishing cut. My present model is far better in this respect, so the power cross feed actually gets used nearly all the time, including parting off

.

Some useful comments, Howard … but I must point out the contradiction evident in the two statements quoted.

I actually think it might be better to emphasise that [like it or not] when you use a lathe the process is generally akin to screwcutting … and the tools and techniques used to achieve a good plain finish are essentially ways of camouflaging that fact.

MichaelG.

I find it very amusing to see how many facing cuts some "machinists" make when starting a job in the lathe. Why wear out your tools doing something that is totally unnesassary ? Unless the end of the bar is just faced and not drilled/bored etc I take a little as possible from the face. I only face the end of the work piece before drilling if it is really rough and then only the centre of the bar. I spot drill/drill/bore etc first then just face/chamfer off the required flat surfaces. Saves time energy & tooling.

regards

Edited By Oldiron on 05/11/2021 12:33:41

Jason.

I would not dispute what you say. However, i am confused. This video at 22.45 suggests something different. Further in the operator takes some ridiculously large cuts later on & a silly 1500 RPM as a test. . Obviously one would not do this, but it does disprove one's comment somewhat.regarding the tool itself.  I am aware that some you tube videos can mislead. But it is a big cut at 1000 RPM. At one point the cut is almost half as wide as the piece of steel

So the cutter will take whatever  I throw at it. It stands to reason that if the lathe can cut it with a brazed tip then it must be able to cut it with a diamond tool holder & i should  not notice a slow down in speed.  Or is that different?

Diamond tool holder #1

Edited By Sam Longley 1 on 05/11/2021 13:03:30

Revs is only half of the equation, Its around the top end of what I would expect for a bit of 1/2" bar, I'd probably be at 100sfm compared to his 130sfm. But at the same diameter you could be at 100meters per min surface speed

Its when you come to turning larger diameter work that things have to change. take a piece of 100mm or 4" dia he would need to be turning that at 130rpm or 6" at 85rpm which is where the vari speed machines start to loose out so a shallower DOC or finer feed has to be used .

He is turning that bar with a feed of 0.006"/rev, In my reply yesterday I said I use 0.005" yet you say mine is "hardly moving" and his is fast? yet I'm only feeding at 20% less than him. Just because the spindle is turning fast it does not mean a high feed rate is being used.

Edited By JasonB on 05/11/2021 13:22:59