Gear set up on db10g for .9 thread

[Stephen Gray 2Participant @stephengray2]

[Stephen Gray 2Participant @stephengray2]

Hi I need to cut a 9.5x.9 mm internal thread on some brass I have a db10g lathe but the threading table only shows gearing for 8.8 any ideas

[JasonBModerator @jasonb]

It would help us if you say if the machine has a metric or imperial lead screw and what pitch that is.

J

 

Are you actually cutting M9.5x 0.9 or trying to cut 1/8" BSP which is 9.7ish x 0.91ish

Edited By JasonB on 29/02/2012 20:20:15

[Stephen Gray 2Participant @stephengray2]

Hi lathe is metric the pitch for the thread is .9 according to the thread gage

[JasonBModerator @jasonb]

But have you tried a BSP 28t thread gauge on it as I doubt you could see 0.007mm difference with the naked eye.

0.9 is a very unusual metric pitch and 9.5mm dia is also a bit of an oddity thats why I feel its 1/8" BSP which is 28tpi and 0.383" dia which allowing for tollerance on teh OD could just come in at your 9.5mm

[Stephen Gray 2Participant @stephengray2]

Don't have a bsp gage but I think your right just had a search,

the threads on a presher gage which come as bsp 1/4 or 1/8 will have a search for a tap .

Thanks

[Les Jones 1Participant @lesjones1]

Hi Steven,

I have a DB10 G but it does not show a setting for 8.8 mm ( Or 0.88 mm) so the gearing must be different. The first gearing is a fixed ratio of 0.75 (A 45 tooth driving a 60 tooth)

The ratios available in the gearbox are these

A B C

I 0.7 1.75 0.875

II 0.4 1.00 0.5

III 1.00 2.5 1.25

The leadscrew is 2mm pitch

So with 1:1 ratio in the change gears the pitch of the thread cut would be 1.5 times these values (0.75 * 2 mm leadscrew pitch)

So gearbox setting A II would cut a 0.6 mm pitch.

You want 0.9 mm pitch so you need to gear up by 1.5. If you have the same change wheels as I have you will have a 40 tooth and a 60 tooth. If you arrange to have the 60 toothe driving the 40 tooth it should cut 0.9 mm pitch

Les.

 

 

Edited By Les Jones 1 on 29/02/2012 21:34:52

[Les Jones 1Participant @lesjones1]

Hi Steven,

After looking at other replies I realise I made an assumption that I had no information to support. I assumed it was the metric version of the lathe like mine. If yours is imperial ignore all the above information.

Les.

[Stephen Gray 2Participant @stephengray2]

Hi les just checked the lathe for 0.88 the gearing is

H. 45

I

70 80

I

60 H speed setting C

[Les Jones 1Participant @lesjones1]

Hi Steven,

Your lathe must be a different version of the DB10 G than mine. I do not have a 70 tooth change wheel.

Les.

Edited By Les Jones 1 on 29/02/2012 22:30:26

[Ady1Participant @ady1]

What tpi are you after??

It all boils down to tpi, 25.4 for a 1mm thread etc

If you can give me the tpi and your gears, plus your leadscrew tpi, I can find your perfect or nearest combination.

(Nearest combinations are a problem because the right tooth moves around)

 

Edited By Ady1 on 29/02/2012 23:50:28

[Ady1Participant @ady1]

Hi lathe is metric the pitch for the thread is .9 according to the thread gage

Metric what? 2mm? You havent even told us what your leadscrew pitch is yet.

To help we need

Leadscrew pitch. 2mm?

Thread pitch 9mm? 9.5mm?

 

It doesn's matter whether the thread is cut on 9.5 inches or 9.5 metres, but we need the thread pitch and the leadscrew pitch.

Then I need your gears, and your gearing configuration.

Most lathes are 1-2-2-1 headstock-compound-compound-leadscrew

 

I can crank through 50 million combinations and permutations in about 3 minutes…but only if I know what you have to play with

 

Edited By Ady1 on 01/03/2012 00:19:05

[Ady1Participant @ady1]

Got a 28tpi match for 0.9mm

2mm leadscrew is 12.7 tpi

with 20.25.30 35.40.45.50.55.60.65.70.80 and 63 the nearest is

20 45/50 63/50 and 50

which gives an error of 0.00176336, about 0.2%

So you're going to need to have a few weird number gears to have a hope of an exact match.

[Ady1Participant @ady1]

Got nowhere with an exact match on a metric 12.7tpi, up to 85 million iterations.

A doddle with 8tpi though, loads of combinations

20.20/20.20/35.40 etc etc etc

[Ady1Participant @ady1]

25.4/0.9 is 28.222 tpi

Tried a "devo max" (scottish colloquialism)

12.7tpi

gears 19.20.21.22.25.30.35.40.43.45.47.49.50.55.60.63.65.70.73.80

closest is:

19 50/47 60/49 55 for an error of 0.01491%

85 million iterations

[JasonBModerator @jasonb]

ADY you need to bear in mind that the driver is fixed on these lathes so just punching the available rations into a computer will not work. You need to take account of teh fixed gear and then work with whats left.

Does it have to be screwcut, can you not just use a 1/8" Tap?

J

[Ian S CParticipant @iansc]

Proberbly best with a tap, because its proberbly BSP tapered thread which is a little difficult to do on the lathe, maybe not impossible, but I don't know how it's done. The only 1/8 BSP tap that I'v bought cost about $NZ40, lucky me I was'nt paying for it, a friend needed it. A few yrars later I got one of my own, much better price 000, very handy. Ian S C

[Les Jones 1Participant @lesjones1]

Hi Ady,

I can feel your frustration. It was this that made me start the thread "Forum niggles". This is a case where information of what the Item was to be used for would probably point towards a BSP thread. I cannot work out from Stevens description of the settings for 0.88 (Unknown units) how the geabox and change wheels are set up. My table of geabox ratios did not come out very clearly as its format was changed between typing and display of my reply. As you are probably not familiar with the DB10G gearbox it has two knobs one marked A, B , C the other I, II, III This give 9 ratios but two are the same ratio. I agree that a tap is the most sensible solution.

Les.

[Stephen Gray 2Participant @stephengray2]

Problem solved Managed to get reducer 1/4 to 1/8

just for interest I uploaded pics of front of the lathe why it has metric and imperial I don't know only had the lathe for 2 weeks and the manual lost in the post?

[JasonBModerator @jasonb]

That looks more like a DB10VS than a G

In which case it will cut metric and imperial pitches though the imperial may be a fraction off.

If you put on the changewheels as per the 28tpi on the imperial chart you will get your 1/8" BSP thread

J

[Les Jones 1Participant @lesjones1]

Hi Steven,

Your lathe is the more modern version with variable speed than mine but the screwcutting setup is also different. Mine is only designed for metric screwcutting where on yours the gearbox provides the conversion ratio between metric and imperial. On mine I need to try to work out a sequence of gears to give an approximation to the imperial pitch I require. This is why I had worked out the ratios of the gearbox on my lathe. From your picture I now understand what the "H" means. It means that a spacer is used in that position so the first 45 tooth gear does not influence the gear ratio. It is just an idler to make up space. The first ratio will be the number of teeth on the spindle gear to the 80 tooth gear.

Les.

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