Chester DB10 thread cutting ???

Hi all and thank you so much for the help.

My lathe is the one with the single gear selector knob and nine holes selecting different ( spur ?) gears.

When I bought the lathe it was advertised as being able to cut both metric and imperial threads.

Sadly it looks as though Andrew and his description of my situation may be right. The thread needs to be 14tpi as the internal chamber will be subject to around 100bar air pressure and there is not enough material on the male component to turn it down and cut a metric thread on it, even if there was I would have to start this piece all over again with a smaller internal bore, oh bugger.

I called Chester to ask for advice, in short, I wasted the cost of the phone call, they didn't have a clue. So although my lathe cuts very true it will probably have to go at some point in the near future, to make way for something more capable.

Once again my sincere thanks to eveyone that tried to help me out with this.

.

Did your keep a copy of that advert, Ian ?

... or was it just what they were displaying on the web ?

I have a nagging suspicion that the previous model was more versatile in the screw-cutting department.

Such is "progress"

MichaelG.

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Edit: This took some finding, but it seems to confirm that the G version was capable of cutting Metric and Imperial threads: http://www.tphmachinetools.co.uk/products/548-new-chester-db10g/specs-pdf

Edited By Michael Gilligan on 23/07/2018 19:45:27

Yep I can see a bit of a trend here, it was indeed as advertised on the website, sadly I never thought that saving the page might one day be useful.

Over the next few days I am going to cut a metric thread on a male bar and see how that fits into a female test piece, any slop at all means it's a none runner.

PM sent - Andrew

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Update: Here's the manual for the G version [which eventually downloaded from Chester's site]

**LINK**

https://d28lcup14p4e72.cloudfront.net/171684/2071501/DB10G_Lathe_Manual.doc

Edited By Michael Gilligan on 23/07/2018 20:08:36

Hi Michael,
I have the DB10G lathe but the screwcutting gearing seems quite different to Ian's DB10. Soon after I bought mine I worked out all the gearing to see what extra useful threads could be cut. This is a link to these notes. (On Dropbox.) This may be of interest to you.

Les.

Thanks, Les

Purely academic interest for me [Myford ML7R, and a couple of much older machines] but much appreciated.

MichaelG.

Also worth noting that the imperial threads listed on these machines with metric lead screws will all be approximate, and same goes for metric threads on the ones with imperial leadscrews as you don't get a 127gear supplied, at best it will be a 63T

So if you really do have a need for both to exact pitches look closely at what the lathe will actually cut.

Ian can I ask what is the full selection of change gears you have. I can quickly put them into my spreadsheet and see what the closest match to 14tpi is.

Martin C

Hi,

Martin, I have on the lathe at the moment 70, 40, 60, and also supplied with a further 60, 70, 80, 90, 100, 105 and 120 teeth gears.

Thanks,

Ian

Does anyone know if something like a harrison 300 would cut both metric and imperial threads?

Hi Les Jones, I have downloaded your chart, thanks for that, I will look through it and see if there is anything there that I can try.

Thanks, Ian

I've got an imperial Harrison M300, and the answer is yes. Naturally the imperial threads are exact. The metric pitches are an approximation, using gears in the ratio 33/26 which gives 1.2692 instead of 1.27 so pretty close. These are the headstock controls, threading and feeds are set by the lower three knobs and 8 position rotary control bottom left:

And this is the threading/feed chart in more detail:

Not shown, but in the manual, substitution of a 56 tooth gear in the gear train allows diametric and module pitches to be cut. The gearbox uses gears in the ratio of 22/28 as an approximation to pi/4.

I imagine that many other comparable industrial lathes will have similar capabilities.

Andrew

Hi Ian,
I've just been looking at some notes I made about cutting a 14 TPI thread on my DB10 G. It finished with a 14.11 TPI thread which is the same as the method I suggested for your lathe. (This is about 0.8% error) I used a 7/16" whitworth tap as a chaser to finish off the thread. The thread (1/2" BSP) fitted a standard BSP fitting. I think it would be good enough for your purpose.

Les.

May we ask what you are making? With due respect, 100 bar air pressure does not sound much like model engineering. You may know exactly what you are doing, but I would have safety concerns about untested one-off components at such a high pressure.

I suppose you could opt for one of the gear/lever combination suggested in this thread that will give you less than 1% error and cut to 90% depth. Then finish with a tap, this would also give the correct profile to the valleys and crests which would not be a bad thing at 100bar.

If it is 1/2 BSP, as it seems to be, it would seem easier to use a tap, but at 100bar I think you should not be trying to cut corners. If the fitting is for a legitimate application in whatever the field is, then there should be a recognised fitting for the job and that should be used.

i

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That's quite encouraging ... perhaps not for the immediate job, but for the future:

If the lathe has room for a 120 tooth gear, you might be able to squeeze-in a 127

[at worst you could do it with a 'reduced pitch' pair, as demonstrated by Martin Cleeve]

If the lathe is to your satisfaction in other respects, I think this would be 'good value'

MichaelG.

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Edited By Michael Gilligan on 24/07/2018 08:19:31

I have become quite interested in this thread and if someone can supply the information I will attempt to work out the gearbox ratios

Thank you Martin Connelly for your reply but if someone can measure the saddle travel for say 10 turns of the leadscrew that will be every bit as good as getting a visual and measuring the pitch with calipers; then using Martin's excellent spreadsheet I will be able to work out the ratios for the nine steps in the gearbox.

Kind regards

Brian

Brian it is almost certainly going to have a 40T integral spindle gear so working out a few of the metric pitches from that suggests a 2.0mm pitch leadscrew.

Position 1 seems to be straight through the box. For example using the 1.5mm pitch from the chart on the machine

40 driver - 70 idler -80 = 0.5

90 -60 = 1.5

0.5 x 1.5 = 0.75 rev

0.75 x 2mm pitch = 1.5

Pity there is no 50T changewheel as that would work out well with a 90T driving it for 1.8mm pitch.