Author Topic: New Lathe  (Read 17616 times)

Offline micktoon

  • Hero Member
  • *****
  • Posts: 796
  • Country: gb
Re: New Lathe
« Reply #50 on: March 02, 2015, 03:53:05 AM »
Hi Oz , when I was at John's we were looking at the lead screw as mine sounds like yours with plain castllated nut, John's looks like there might be some sort of safety slip clutch in the nut ? But the rear of thevnut is still casttlated and can be disengauged.
  So it looks like its the same overall  idea but has a safety lutch built into the nut ?


  Cheers Mick

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #51 on: March 02, 2015, 12:11:25 PM »
Thanks Mick, it sounds like the idea there would be to set the bed stop at the point you want to stop the threading tool from advancing any further along the job. When the saddle hits the carriage stop the clutch detent slips and leadscrew feed is prevented from overunning the disengagement point .

 I guess its then a matter of disengaging the half nuts and returning the carriage to the pick up point of the next cut.

It looks an interesting and useful mechanism,hope John can give some greater detail of it's operation......OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline DavidA

  • Hero Member
  • *****
  • Posts: 1167
  • Country: gb
Re: New Lathe
« Reply #52 on: March 02, 2015, 12:17:55 PM »
   When the saddle hits the carriage stop the clutch slips  

 Wouldn't this throw your gearing out of sync and prevent the next cut from starting at the correct place ?

Dave.

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #53 on: March 02, 2015, 01:42:05 PM »
Dave, the clutch would need to be of the single engagement point detent type then it can only engage at the same point in any 360 degrees of rotation. Visualise the castellated type but only having one male and female tooth to understand the concept I'm thinking off.....OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline DavidA

  • Hero Member
  • *****
  • Posts: 1167
  • Country: gb
Re: New Lathe
« Reply #54 on: March 02, 2015, 01:51:45 PM »
Ah yes,  a dog clutch.  that makes sense.

I was thrown when you referred to the clutch slipping.

Dave.

Offline DavidA

  • Hero Member
  • *****
  • Posts: 1167
  • Country: gb
Re: New Lathe
« Reply #55 on: March 02, 2015, 07:00:44 PM »
Thinking more about this.  There may be a snag.

I suspect that this dog clutch will only work when cutting threads that have a direct relationship with the number of threads on the lead screw. I.e. if you have an eight tpi lead screw it will work with 8, 16, 32, 64 tpi threads.
This is related to being able to 'drop in' the half nuts anywhere with 8 tpi lead  screw and the above mentioned threads, but the need to use a thread dial indicator for others.

I suspect, but not sure,  that it wont work if you were to, say, cut a 5 tpi thread with a 8 tpi leadscrew and then try to re engage the clutch of the chuck is not in the same position it was when the clutch dis engaged.

Readers views welcome.

Dave.

Offline doubleboost

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 1620
  • Country: gb
  • Newcastle Upon Tyne
Re: New Lathe
« Reply #56 on: March 02, 2015, 07:16:57 PM »
I think the "clutch" on the lead screw is there in case the lathe is "crashed"
Tool post crashes in to chuck lead screw slips to minimise damage
John

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #57 on: March 02, 2015, 09:15:48 PM »
Thinking more about this.  There may be a snag.

I suspect that this dog clutch will only work when cutting threads that have a direct relationship with the number of threads on the lead screw. I.e. if you have an eight tpi lead screw it will work with 8, 16, 32, 64 tpi threads.
This is related to being able to 'drop in' the half nuts anywhere with 8 tpi lead  screw and the above mentioned threads, but the need to use a thread dial indicator for others.

I suspect, but not sure,  that it wont work if you were to, say, cut a 5 tpi thread with a 8 tpi leadscrew and then try to re engage the clutch of the chuck is not in the same position it was when the clutch dis engaged.

Readers views welcome.

Dave.

Dave, a single point dog clutch will always re-engage the leadscrew at exactly the same relevant index as it was disengaged from and there will be no variation in the relationship of gear engagement because they haven't been disengaged or had their relative timing disrupted.

It doesn't make any difference to the repeatability of re-engagement if the dog clutch is located directly on the drive end of the leadscew(like the harrison lathes) or if it is located on the input end of the drive shaft for the screwcutting gearbox.

The reason it cant ever be engaged wrongly being that it has only 1 engagement/drive location in 360 degrees of rotation.

Similar single dog setups do exist,one example is shown and described fitted to a Myford lathe in Martin Cleave's book on screw cutting.

Helixes aren't always downward spirals,sometimes they're screwed up

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #58 on: March 02, 2015, 09:29:04 PM »
Agree John, it being a safety device would make perfect sense considering the cock ups that some spotty Herberts in training college can make when inexperience gets the better of them.....OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline DavidA

  • Hero Member
  • *****
  • Posts: 1167
  • Country: gb
Re: New Lathe
« Reply #59 on: March 03, 2015, 11:15:08 AM »
...It doesn't make any difference to the repeatability of re-engagement if the dog clutch is located directly on the drive end of the leadscew(like the harrison lathes) or if it is located on the input end of the drive shaft for the screwcutting gearbox. ,

You are right ,  of course.
It would only make a difference if the gear train was disengaged between the leadscrew gear and the headstock gear.

Took me a while to clearly see the point; pun intended.

Dave. :hammer:

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 179
Re: New Lathe
« Reply #60 on: March 06, 2015, 04:31:52 PM »
I'm going to assume the half-nuts stay closed?

Quote:
...It doesn't make any difference to the repeatability of re-engagement if the dog clutch is located directly on the drive end of the leadscew(like the harrison lathes) or if it is located on the input end of the drive shaft for the screwcutting gearbox. ,

I'm going to disagree, lathes with single-tooth clutches (Holbrook, Hendey, Hardiinge, Pratt & Whitney - toolroom lathes) run them at spindle speed before driving the QCGB, not leadscrew speed after - at spindle speed it preserves the relationship between spindle and leadscrew (it can only engage once per spindle rotation so once per threading pitch as the half-nuts stay closed), a single-tooth clutch on the leadscrew can engage at any *leadscrew* pitch, with the spindle in an arbitrary position depending on the QCGB ratio, not the same thing! It MIGHT work for "native" threads, as suggested, as you'd effectively be opening the halfnuts and closing them on a random point on the leadscrew, definitely doesn't for anything else, e.g. metric on an Imperial lathe and vice-versa, which the spindle-speed dog clutch will do.

Another quote:
a single point dog clutch will always re-engage the leadscrew at exactly the same relevant index as it was disengaged from and there will be no variation in the relationship of gear engagement because they haven't been disengaged or had their relative timing disrupted.

Except that the spindle can rotate though an angle dictated by the QCGB setting and align the dog clutch without coming to the same angular relationship between spindle and leadscrew - e.g. for the 8tpi leadscrew, 11 tpi thread to cut, the spindle will have rotated 11/8ths of a turn (or multiples of) before it realigns the dogs. Assume the carriage is exactly where the dogs disengaged, the spindle will have rotated an extra 3/8ths of a turn and you'll be cutting a multi-start thread by accident...

I've looked into this a bit, as my lathe would very much like a single-tooth clutch, leadscrew/feedscrew reverse, threading stop and extra control rod, for cutting metric threads among others - added to the complication on my lathe, although the forward/reverse (R/H Vs L/H) selection runs at "spindle gear" speed is that the back-gear drives the spindle at 1/8th the speed of the "spindle gear" (on a concentric sleeve with its own bearings) that operates the QCGB etc. to get the "coarse" threads that really could use a threading stop disengaging a single-tooth clutch... Perfect if you need to cut 8-start 2tpi or 16mm pitch threads.  :palm:

Plans are being made, slowly, fitting it all in is challenging!  :bang:

There are a couple of single-tooth setups published, one is in Martin Cleeve's "screwcutting in the Lathe" (Workshop Practice series No.3), another in one of the model engineering magazines a couple of years ago, and all run the dog clutch at spindle speed to maintain synchronisation, there are versions for a good selection of hobby lathes.

Probably not what you want to hear, a leadscrew dog clutch would be MUCH simpler!

Online mattinker

  • Hero Member
  • *****
  • Posts: 939
  • Country: fr
Re: New Lathe
« Reply #61 on: March 06, 2015, 05:31:07 PM »
If I remember correctly it's a metric lathe. Unlike the imperial lathe, the lead screw must be kept engaged as the dial thread indicator doesn't work for metric threads. I seem to remember reading somewhere that three different interchangeable D.T.I's would be needed for metric lead screws. I'd be interested in a dog clutch for my metric Emco compact 8! I'm jealous of your lathe John!

Regards, Matthew

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #62 on: March 06, 2015, 06:18:01 PM »
Dave,yes I forgot about the gears in the QCGB relative to the back gears etc if the clutch is located before the QCGB input shaft.

However,with my original idea it won't disrupt the synchronicity if the dog clutch is on the output side of the QCGB because it is simply disengaging the final drive line to the leadscrew in a manner that it can only re-engage at exactly the same single relative index to the final drive shaft output. With this arrangement no gears or half nut indexes have been altered at all....OZ.

Helixes aren't always downward spirals,sometimes they're screwed up

Offline Manxmodder

  • Hero Member
  • *****
  • Posts: 743
  • Country: gb
Re: New Lathe
« Reply #63 on: March 06, 2015, 06:26:04 PM »
Matt, just for interest here is a metric thread indicator for a Harrison lathe. This would only be workable on a dedicated metric leadscrew and thread cutting gearbox...OZ.

LINK: http://www.ebay.co.uk/itm/Spare-Metric-Thread-Indicator-Dial-for-Harrison-Lathe-etc-Part-No-L16-4-14-/121264384998
« Last Edit: March 06, 2015, 07:56:41 PM by Manxmodder »
Helixes aren't always downward spirals,sometimes they're screwed up

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 179
Re: New Lathe
« Reply #64 on: March 06, 2015, 06:55:26 PM »
Hi Oz,

I have to disagree - only if the thread you're cutting is a multiple of your leadscrew pitch will it sync' up correctly by having a dog clutch on the leadscrew itself - believe me, over the last couple of years I've spent several  weeks in free time working on this! I've even spreadsheeted it and knocked up a lash-up physical model as my maths isn't so practiced nowadays, and it gives multi-start threads if you're not really, *incredibly*, EXTREMELY lucky (once in several tens for most threads, far more rarely for the oddballs like B.A., D.P. and module which have irrational numbers like Pi as factors) - the only way it works for ALL thread pitches is if the clutch runs at spindle speed! Honest!

This is probably why proper toolroom lathes run the dog clutch at spindle speed...

My solution (not yet built, CADded, simulated and appears to be likely to work, some of the commercial-off-the-shelf parts like gears and bearings already purchased) is using the existing forward/reverse selector but adding a single-tooth clutch with a sliding dog in place of a sliding gear pair (the gears with their dogs will be fixed and free on needle-rollers, the sliding dogs keyed to the shaft), and as I have to deal with the 8xspindle-speed drive to the screwcutting / feed geartrain, I've had to come up with a synchroniser very similar to those for the firing pin on machine guns back in WW1 which fired through the propellor without trimming its blades...

There's a VERY long thread on single-tooth clutches at
 http://www.model-engineer.co.uk/forums/postings.asp?th=49358&p=1
Where a lot of quite bright, practical engineers describe the mods necessary on a number of different lathes. It's worth reading, but everything in there is also in Martin Cleeve's book, if you dig a bit!

Matt, take a look at that thread - the principles apply to a lot of lathes (the Emco shares a lot of features with them) and could put you at square 3 or 4 rather than square 1?

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 179
Re: New Lathe
« Reply #65 on: March 06, 2015, 07:04:50 PM »
Oh, I almost forgot - Nice Harrisson, John! Didn't mean to hijack your thread!

Offline doubleboost

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 1620
  • Country: gb
  • Newcastle Upon Tyne
Re: New Lathe
« Reply #66 on: March 06, 2015, 07:06:32 PM »
No
Problem
carry on lol
John

Offline hopefuldave

  • Full Member
  • ***
  • Posts: 179
Re: New Lathe
« Reply #67 on: March 06, 2015, 07:33:08 PM »
Some Proper Drawings of Graham Meek's very tidy design:

http://modelengineeringwebsite.com/Meek_screwcutting_clutch.html

And a description:

http://modelengineeringwebsite.com/Screw_cutting_clutch.html

Worth a read, if only to marvel at the ingenuity :)

Online mattinker

  • Hero Member
  • *****
  • Posts: 939
  • Country: fr
Re: New Lathe
« Reply #68 on: March 06, 2015, 08:03:18 PM »

 http://www.model-engineer.co.uk/forums/postings.asp?th=49358&p=1
Where a lot of quite bright, practical engineers describe the mods necessary on a number of different lathes. It's worth reading, but everything in there is also in Martin Cleeve's book, if you dig a bit!

Matt, take a look at that thread - the principles apply to a lot of lathes (the Emco shares a lot of features with them) and could put you at square 3 or 4 rather than square 1?

Hopeful Dave,

Thanks for the link to the thread and the links to the drawing and description, stored away for later!

Regards, Matthew

Offline RussellT

  • Sr. Member
  • ****
  • Posts: 376
  • Country: gb
Re: New Lathe
« Reply #69 on: March 07, 2015, 03:11:04 AM »
I'm not sure whether this will help, but if anyone is struggling to visualise why a single point leadscrew clutch won't synchronise for screw cutting then this example helped me.

Imagine a lathe with an 8tpi leadscrew.  The half nuts can be engaged every 1/8th of an inch along it's length and the single point leadscrew clutch can be engaged every 1/8 inch of carriage travel - even if the half nuts are left closed.

 If you're trying to cut a 1.5mm thread then it will only be right when 1.5mm is an exact multiple of the 8tpi thread.  In that example every that's every 7.5 inches along the lead screw.  (60 leadscrew threads and 127 1.5 mm threads).

Russell

Offline Ginger Nut

  • Full Member
  • ***
  • Posts: 218
Re: New Lathe
« Reply #70 on: March 07, 2015, 10:11:12 PM »
Just thought will Mick be spending time teaching John how to use his new lathe? Will SNNC become J&MHC  :lol:

Offline doubleboost

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 1620
  • Country: gb
  • Newcastle Upon Tyne
Re: New Lathe
« Reply #71 on: April 04, 2015, 03:34:27 PM »
Harrison in its final resting place
DRO half installed

John

Offline John Rudd

  • Hero Member
  • *****
  • Posts: 2113
  • Country: gb
Re: New Lathe
« Reply #72 on: April 04, 2015, 03:39:05 PM »
That really looks the part..... :drool:

I have lathe envy......especially after being round Rob's this morning, but this just tops it..

When we gonna see swarf?  :dremel:
eccentric millionaire financed by 'er indoors
Location:  near Hull

Skype: chippiejnr

Offline Stilldrillin

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 4828
  • Country: gb
  • Staveley, Derbyshire. England.
Re: New Lathe
« Reply #73 on: April 04, 2015, 05:14:47 PM »
Looking good, John!  :thumbup:

David D
Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline Ginger Nut

  • Full Member
  • ***
  • Posts: 218
Re: New Lathe
« Reply #74 on: April 04, 2015, 07:26:23 PM »
Just watched video bet you were on your knees more than you are when you go to church John  :lol:

Looking good, the rear of the lathe looks like new.