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Thread: Lathe dogs - use of
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8th May 2014, 08:56 PM #1
Lathe dogs - use of
Hi
Are there any dos and donts re: the use of lathe dogs ? I have a few of the ubiquitous cast bent leg style but haven't used them so far. I have read there are balance issues with them. Is the rpm limited due to balance issues ?
And, if using a 3 jaw chuck as the driver . Is there any special thing to watch out for > Mike
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8th May 2014, 09:12 PM #2Pink 10EE owner
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I would say do not get caught up in them...
Interestingly, when using them on cylindrical grinders one publication says to always have the dogs be pulled by the driver..Light red, the colour of choice for the discerning man.
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8th May 2014, 09:13 PM #3
Hi Mike,
The only things I have noted is to make sure the leg of the dog is against the lathe jaw before you start the lathe. I have had dogs spin but only when I have used an ally packer so I did not mar the part.
I have not had balance problems with balance, even on the C10 at 2000rpm, but I only have dogs up to 1". The 2 machines I have used them on of late both weigh over 1t though. You may have problems on your little lathe, probably not on the chippie, but you should have no problems at all on the hendey, other than the problem with her not running yet..... you really should get her going, if she's as good as her English cousin then you are in for a treat.
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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8th May 2014, 09:57 PM #4
right
OK interesting . I will give the dogs a try , taking it slowly to begin with and see how it goes ... faster faster faster til things begin to vibrate
I am making a set of mandrels soon and the between centres turning method should be an ideal way to do it . Mike
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8th May 2014, 10:11 PM #5Cba
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Using a 3-jaw chuck with a lathe dog seems pretty odd to me. Lathe dogs are used to turn between centers. This means you need to put a center in the headstock. A 3-jaw chuck would only be in the way. Think about it, it would cover your workpiece where you want to cut, it would force you to use a very excessive workpiece overhang. The lathe dog itself is already in the way, no need to make this far worse with a chuck!!!
How to drive the lathe dog depends a bit on your lathe, and the type of lathe dogs at hand. Lathes with camlock spindle nose are the easiest... remove the chuck and put a pin in one camlock hole. Otherwise you need to mount a faceplate. Many lathes have a small diameter faceplate that is only used to drive bent lathe dogs. You can also use it to drive straight lathe dogs by adding a hole to mount a pin. Full size face plates have slots for bent lathe dogs. If using straight lathe dogs, it is customary to tie the lathe dog to the driving pin with a leather shoelace. It has some give and acts as a "fuse", and it also avoids the one turn "backlash" snap when starting/stopping the spindle.
Properly made lathe dogs have some inbuilt balancing, but this cannot be perfect for every workpiece diameter. If using a light benchtop lathe, unbalance is likely to cause unacceptable vibrations, certainly at higher rpm. You will have to optimize balancing. Sometimes it is enough to choose a longer or shorter clamping bolt to change lathe dog balance. Stay away from overweight massive cast iron lathe dogs, they may work well on a 1000kg+ lathe - but may turn a light bench lathe into a concrete vibrator.
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8th May 2014, 10:40 PM #6
Chris, this is what Mike means by using a 3 jaw https://www.woodworkforums.com/f65/mean-184364
I can't just use a pin in my camlock, my dead centers stick out too far.
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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9th May 2014, 12:19 AM #7Cba
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I see. You do it this way for convenience. Instead of using a proper center for your lathe.
But that makes me think, why in the first place do you want to turn between centers at all? Normally you only do it, if you need to achieve the highest precision attainable on a lathe. To achieve this goal, you ideally would want the tip of the center to be located at the geometric center of the front spindle bearing. Or as close as possible to this location. There the dynamic runout will be smallest. Dynamic runout is also known as bearing noise. It is the random variation of the rotational center as the bearing rotates, due to un-roundness of the bearing components in the case of taper roller bearings, or due to the lifting action as the spindle rides onto the oil wave in a sleeve bearing. The further out from the geometric bearing center you put the tip of the center (like when holding it in a 3-jaw chuck) the more you amplify this dynamic runout. And thinking you can nullify this by taking a skimming cut off the center surface will not work. Because this runout is dynamic and irregular, just like noise is. Hell, some people spend lots of money for high precision class lathe spindle bearings just to keep exactly this dynamic runout low, why ruin this by extending the tip of the center away from its ideal location?
That said, by using the 3-jaw chuck to clamp the center (and truing it with a skim cut) you would still maintain the second advantage of turning between centers, and that is accurate re-chucking of the workpiece.
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9th May 2014, 04:33 AM #8Distracted Member
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Chris I would think the errors from bearing noise would be tiny. Like, at a wild guess, a micron or two? For my purposes that's a theoretical issue, not a practical one. Spindle adaptors OTOH seem prone to larger problems. At least the one I made for the Graz, with all the care I could muster, runs out I think .02mm, even when registered to the spindle. I threw it in the drawer in disgust and now use a chucked centre. Even if your adaptor is perfect, or you have a centre that's native to the spindle and needs no sleeve, it still only takes a tiny ding or a speck of grit ...
On dogs, I gave up on the cast ones as they seem to twist under load. Maybe mine are in poor condition. I use a home made one consisting of ... wait, here's a pic. This was a quick fix that worked so well I never got around to finishing it. I'm ashamed to say it even still has the gutter bolts. The roll pin is a trick (bodge) I use if I get slippage. I grind a little groove in the shaft to engage it - only if that surface doesn't matter obviously. I can think of better solutions.
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9th May 2014, 08:53 AM #9Member
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The type of lathe dog in Bryan's picture can be made fully balaced by making the two sides exactly the same as the bottom side in the picture, including the drive leg on both sides.
I made a very similar one and it causes no vibrations at any speed on my lttle C6 lathe.
Andrew
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9th May 2014, 10:43 AM #10Cba
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I agree the error from bearing noise is tiny, but it gets amplified the farther the center tip is extended. Even then it will be well below 0.01mm even if cheap spindle bearings are installed.
I still do not understand why you use a chuck to hold the center, and then take a cut to true it. Real lathe centers are made of soft material, so they can be trued. The practice is to carefully!! clean the spindle taper when inserting the center. Then, only for jobs that require the very best accuracy that can be achieved on a lathe, take a tiny skim cut to true the center. But if instead you choose to held a center in a chuck, you will always and every time have to true it, because it will otherwise be considerably off.
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9th May 2014, 11:33 AM #11Banned
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What am I missing
I must be missing something here, whenever I turn between centres the chuck comes off and a face place goes on. The engaging tab on the dog sits in the slots on the faceplate. I then bolt a weight to the faceplate in the appropriate position to balance everything if necessary??? I make my own dogs to suit the lathe faceplate current at the time. Using a centre in the chuck surely would make it difficult to re-centre if the centre needed removing and re-inserting at any stage of the job???
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9th May 2014, 12:46 PM #12Distracted Member
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9th May 2014, 01:15 PM #13Cba
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> Yes. What's hard about that?
Not hard at all... just set the compoud to 60 degrees, skim the tip, then set it back to cut parallel. How long this takes depends on how fuzzy one is.
> No-one suggested removing and refitting the centre. But if you had to you could easily re-true it.[/QUOTE]
Too bad if you just had spent 10 minutes setting the compound with a DTI to cut a number of specific tapers that need to be as accurate as you can make 'em.....
There are more downsides of having the center in a chuck. Chucks are heavy and have lots of inertia, slowing startup and stopping. You also loose on available length between centers, depending on the workpiece this may matter on a short bed lathe. In case it is an old style chuck with threaded mount, it may limit you using it in reverse as it may un-thread itself ruining a smallish workpiece.
At the end of the day, it is a personal choice/preference thing.
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9th May 2014, 01:15 PM #14
U toob
hi All
this U toob video describes the method
http://www.youtube.com/watch?v=my9ixtJogCg
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9th May 2014, 09:35 PM #15Cba
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I did some googling on this issue and learned something. The method is indeed fairly common on large lathes. Lathes with a very heavy chuck, so heavy that that it is a pain to remove. Machinists prefer to chuck a center. It is much quicker than swap the chuck, clean the spindle taper, insert a center, install a catch plate, and when finished turning between centers do the same in reverse.
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