Chester DB10 thread cutting ???

Hi,

I have machined an internal bore to the correct diameter and depth, cut an overrun groove at the blind face, all without drama. Then I came to the task of cutting 14 tpi.

None of the diagrams make any sense to me, certainly not what I used many years ago, I will upload a (poor quality) photo showing the lathe instructions for screw cutting to see if anyone out there can enlighten me.

Thanks,

Ian

Edited By Ian Skeldon 2 on 22/07/2018 21:39:56

A couple of slightly better photos;

Hmmm, looks to me like the lathe is designed to cut a selected range of metric pitches. If you're prepared to accept a sloppy thread 14tpi is 1.814mm pitch, so you might just get away with 1.75mm over a short length. Otherwise I'm afraid you might be up the creek watching the paddle float away downstream!

On a more positive note there may be undocumented gear combinations that are closer, or other gears that may work better. But that is beyond my experience. I expect those with better knowledge of the intricacies of same will be along soon.

Andrew

Top diagram gives feedrates for general turning nd facing under power feed (that's what the arrows indicate)

Bottom diagram is metric pitch along the bottom row.

A and B are the changewheels in the diagram, a selection of ratios at left in the top diagram.

Ratios are given in rows 2 and 3 of the bottom diagram, so its different to the top one - rather confusing!

Presumably 1,4,7 and 9 are some of the gearbox selector positions, with other positions giving further options.

Neil

Without knowing the 7 gearbox ratios, spindle driver and lead screw pitch it would be hard for any of us to give a gear combination to get to your 14tpi. If you want closer than Andrew's 1.75mm pitch suggestion then you need to sit down and work out what is going on with the gears, levers and gearbox ratios and come up with something that gives a closer match.

Quick look and if you could get a 58T gear then put that in position B, 105T in position A and lever in No1 that would give you 1.810mm. A 3D printed one would go or buy a cheap one from one of the bearing suppliers.

IF you had a 55T in the gear set then 100/55 = 1.818mm which is quite close, lever pos 1 again. It is actually quite an easy layour to work out whats needed just convert the tpi to a mm pitch and see what fits in lever pos 1

I see that Chester only list it as giving metric pitches so you won't ghave got a 63T in the set of gears supplied.

This is the lathe for those not familiar with it.

Edited By JasonB on 23/07/2018 07:33:36

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Pardon my innocence, but, what is Chester's distinction between a closed gearbox and an open one ... and which does Ian's lathe have ?

MichaelG.

The two images on the linked web page have different gearboxes. I would suggest image 1 is a closed gearbox and image 2 is an open gearbox. Closed gearboxes can have oil in them for lubrication. Open need regular lubrication or drip oiling.

Martin C

With a ratio of 1:1 for A to B these are the feed values for the various gearbox settings. (Worked out from the posted list of settings.)

1 - 1.00mm
4 - 0.80mm
5 - 0.75mm (*)
7 - 0.667mm (2/3 mm to be exact.)
9 - 0.60mm (*)

(* These are calculated from the fine feed values but the ratio is slightly different for different table entries.)

Assuming that only gears A and B can be changed and that the gears shown in the table are the only ones available the best setting I can come up with is gear A 120 teeth, gear B 40 teeth and gearbox position 9. IF the feed for position 9 is exactly 0.6 mm then this will give a pitch of 1.80mm (14.11 TPI) Before cutting the thread I suggest measuring the feed by turning the chuck exactly 10 revolutions and measuring how far the carriage modes with the DRO or digital calipers.
Thanks to Neil for explaining what the vertical and horizontal arrows mean. (I was thinking it was another gearbox setting.) and to Michael for posting a picture of the lathe.

Les.

Edited By Les Jones 1 on 23/07/2018 09:39:55

deleted- Martin already covered it.

Edited By Bazyle on 23/07/2018 09:02:00

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Oh ... is that all there is to it ?

Thanks

MichaelG.

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I was guessing that there might be some functional difference; like the provision for swapping individual gears in the 'open' one. ... Glad I asked! 

Edited By Michael Gilligan on 23/07/2018 09:12:06

I am surprised that your lathe will cut only Metric threads. I know that some machines are built this way,but for hobby use being able to cut the odd BSW thread is more than useful. My old Craftsman lathe cuts a fairly wide range of Metric and BSW threads and I find a need for both from time to time.

14 TPI is the old GB thread,and that comes along more often than you might think !

I managed to work out how to cut a few Metric threads which were not listed on my machine,by working out the ratios,using a calculator together with the gears supplied [too lazy to wake the old grey cells,what,s left of them ],so you might be able to do something similar. Even at this late stage BSW has not gone away !.

Sorry,forgot to add that I do have a changewheel to go from Metric to Whit,came with the lathe.

.

That's gracious of you, Les ... but I was quoting from Jason.

MichaelG.

Excuse my ignorance, as I know nothing about lathe gearboxes.

Looking at the two images, they look to be two entirely different gearboxes. One looks like a nine position Norton style box with just the one lever, whilst the other appears to have two rotary levers, each with three positions.. Not only that, but the instructions on the casing appear different as well.

Does this mean that they provide different ratios? Or are they functionally the same, and providing the same ratios? I assume that the change from from normal turning to facing is done by a lever on the apron which also changes the ratio.

Peter G. Shaw

I suspect you get the same number of pitches from either 1 lever x 9 positions = 9 ratios. 3x3 options on the knob version would also give 9 options.

Small lever to the left of the silver box on the apron selects turning or facing feeds, facing is usually haft the turning rate.

With the information available the available ratios without using the 127 tooth gear is as below. Nearest to 14 TPI is 1.8mm pitch with a=120 b=40 lever in position 9. I would advise do not disengage the half nuts when cutting this thread. Is the 127 tooth gear a duplex and if so what is the tooth count on the other half or does it go in place of the 100 in position a? If you put a=127 b=70 and lever in position 1 you should get 14 tpi. Once again do not disengage the half nuts. Test the thread by scratching a bar before cutting.

Martin C

Does it come with a 127T? did not see one mentioned

I saw Larry mention he had a metric to Whit change wheel and thought it was the OP. If one is available the tables are as shown.

Martin C

Martin,

Do you also happen to know the leadscrew pitch as well?

Regards

Brian

Sorry, no. This is all based on the pictures and information in this thread. That is why I suggested a scratch test, there could easily be errors.

Martin C