Chester DB10 thread cutting ???

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 23/07/2018 19:15:30:

Edit: This took some finding, but it seems to confirm that the G version was capable of cutting Metric and Imperial threads:

**LINK**

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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

[Les Jones 1Participant @lesjones1]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks, Les

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

MichaelG.

[JasonBModerator @jasonb]

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.

[Martin ConnellyParticipant @martinconnelly55370]

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

[Ian Skeldon 2Participant @ianskeldon2]

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

[Ian Skeldon 2Participant @ianskeldon2]

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

[Ian Skeldon 2Participant @ianskeldon2]

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

[Anonymous]

Posted by Ian Skeldon 2 on 23/07/2018 21:45:37:

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

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

[Les Jones 1Participant @lesjones1]

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.

[RedsetterParticipant @redsetter]

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.

[JasonBModerator @jasonb]

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.

[RedsetterParticipant @redsetter]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian Skeldon 2 on 23/07/2018 21:43:41:

… 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.

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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

[Brian WoodParticipant @brianwood45127]

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

[JasonBModerator @jasonb]

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.

[Brian WoodParticipant @brianwood45127]

Thank you Jason, but as I am completely unfamiliar with these Chinese lathes I have little idea how a spindle gear of 40 T [strongly suspected but not confirmed] interacts with the other gears in the change wheel section. I confess the layout confuses me. My Myford background doesn't sit comfortably with these layouts

I may well work on a leadscrew pitch of 2 mm as you suggest, but it will be speculative.

Regards

Brian

[Martin ConnellyParticipant @martinconnelly55370]

Brian, I used 12, 13, 14, 15, 16, 17, 18, 19, 20. The information in the various posts./ pictures for positions 1, 4, 7 and 9 give basic ratios of 12, 15, 18 and 20. Assuming (I know it is risky) the remaining gears go in an arithmetic series I just filled in the remaining positions because there would be no fractional gear counts. It could be 24, 30 36 and 40 and I could be wrong but the picture of the gear position holes looks pretty close to linear. The spreadsheet formula is simply (a tooth count)/(b tooth count)/(position tooth count)*12 for pitch in mm.

Martin C

I have just looked at Jason's post and the 12 in the formula and the 12 in position 1 match his 1:1 at position 1 view.

Edited By Martin Connelly on 24/07/2018 14:45:39

[Martin ConnellyParticipant @martinconnelly55370]

Ian the change gears you have do not have any odd values that could get a closer approximation than the 14.1TPI already suggested. The question already posted is can you fit a 127 tooth gear in position a. If so this would give accurate TPI values for future use and would be an option worth considering.

Martin C

[JasonBModerator @jasonb]

This shows the closed gearbox version for those that want to work out the ratios. Drive comes in bottom left from the banjo geartrain then up to the layshaft, then back down and out to the 2mm pictch screw bottom right.

Brian from the image on teh front of teh lathe the spindle is at the top with a fixed 40T gear. The group of three are some form of tumbler reverse for leadscrew direction and then down to teh 70T and 80T which seem to stay put. Finaly to A and B with B being on the input to the gearbox.

[Brian WoodParticipant @brianwood45127]

Jason,

The closed gearbox information is interesting, thank you for providing the link. Two things puzzle me though.

When one adds up the tooth counts on the interactions of the gears up to and down from the layshaft, for the wheel counts shown I get 43,45 and 45 on the left hand set—–on the right hand set I get 45,45 and 46 reading from left to right in each case.

I don't find that information reassuring at all, it suggests a two tooth slop in the left hand set and one tooth slop in the right hand set. Could the counts be wrong? One thing I know is that for common centres to be matched across gearbox ratios that can be slid one to another, tooth counts in the combinations usually add up to a common total

Thank you for the change wheel drawing, it now makes things clear that there is a 1:2 reduction right at the start with the 70 tooth wheel used as an idler. I dislike this hidden information, the initial 40T gear isn't identified at all and you seem to be expected to know the layout.

Kind regards

Brian

[Ian Skeldon 2Participant @ianskeldon2]

Hi,

and many thanks to you all for the advice offered, thanks again to Andrew for a very kind offer and confirming that the Harrison 300 will cut both (true imperial) and very close metric threads.

Redsetter, the thing I am making is a regulator tester, perfectly legal and legitimate as well as pretty common and can be bought as 'a recognised fitting for the job' if your prepared to pay silly money. I have made other devices which deal with the full cylinder pressure of 200bar (3000 psi) and had no problems (metric threads !).

A thread cut on a lathe is equally as good as one cut with a tap if it's done properly, both methods are of no use if not done properly, it is confirmed as 1/2" bsp as already mentioned. Engineers be they model engineers, rocket engineers often make the things they need or want, that does not mean that the product is inferior to a bought product which if you think about it for a second……has been made by an engineer.

[RedsetterParticipant @redsetter]

Ian,

The reason why you have to pay "silly money" for the recognised fitting is that it will have been produced under strictly controlled conditions from the correct materials, and will have been properly tested and inspected at every stage. It will also be covered by the manufacturer's guarantee and public liability insurance.

I think you are taking an unacceptable risk.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Redsetter on 24/07/2018 20:04:24:

I think you are taking an unacceptable risk.

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Now that you have clearly made your point … Could we please return to the interesting subject of screw-cutting?

Thanks

MichaelG.

[JasonBModerator @jasonb]

Posted by Ian Skeldon 2 on 24/07/2018 19:25:29:

and many thanks to you all for the advice offered, thanks again to Andrew for a very kind offer and confirming that the Harrison 300 will cut both (true imperial) and very close metric threads.

If you do go shopping for an M300 just be aware that Andrew has one with an imperial lead screw and that is why he can cut true imperial threads. If you buy one with a metric lead screw it will cut true metric threads and close imperial much the same as your DB10.

Also make sure whatever you buy that the correct change wheels are included otherwise you won't be cutting many threads. They do have a habit of getting lost when machines change hands.

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