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Thread: balancing a lathe dog
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14th November 2013, 04:03 PM #1SENIOR MEMBER
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balancing a lathe dog
Hello,
I've graduated from chuck to turning between centres, am following a set of exercises in a Harold Hall book.
Have noticed that at 720rpm my lathe gets wobbly when turning between centres using a lathe dog on a 24mm diameter cylinder mounted between centres. At lower speeds all is fine. Hercus 9" speeds 720 530 410 310 100 135 75 and 60rpm
30+ years ago if my motor bike became wobbly I could change the speed and it would go away and I think some people mounted little dampers & pistons to dampen vibrations and prevent "tank slappers" (Honda F2 750 4 cylinder)
In a similar way, can people with variable speed lathes change the speed and the wobble goes away ? Or do you have to balance the weight of the lathe dog when this happens.
Is it a resonant frequency sort of thing that is stronger for particular weights and rpm or does it simply get worse as you spin the asymetric load faster ? AT 720 I cant go any faster.
Have tried googling and found a few references to balanced lathe dogs, and bolting a weight on the other side of the drive plate.
Any thumb rules ? ie I imagine the smaller the diameter of the workpiece the greater the influence of the asymetric dog/drive plate
Would bigger lathes "see" this problem less frequently than smaller lathes ?
Was wondering what the collective wisdom of the forum is on this issue - did a search but couldnt find any previous discussions
Any thumb rules ? Or does one simply get to know how the machine behaves and automatically start thinking about adding a balance weight under appropriate conditions ?
Relatively few google results - Maybe I've been using the wrong words in my searches - what are the right words to use to conduct searches ?
BillLast edited by steamingbill; 14th November 2013 at 04:17 PM. Reason: cut repeated sentence, added google search note
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14th November 2013 04:03 PM # ADSGoogle Adsense Advertisement
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14th November 2013, 04:13 PM #2.
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Ah! Tank slapping. Ever ride a 1970 Mach III Bill?
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14th November 2013, 04:19 PM #3SENIOR MEMBER
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No Bob I've never had the pleasure.
Was quite scary the first time it happened on the Honda.
Bill
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14th November 2013, 05:42 PM #4
Hi Bill,
It is not as bad on a heavier lathe that's for sure. Any out of balance part is going to affect a machine with less mass more than a machine with greater mass.
On the mars (hercus size) I use the face plate as my drive plate. If the dog throws the lathe out of balance I add a bolt through one of the faceplate slots, whichever one I feel is necessary. By moving the bolt in or out on the faceplate fairly fine adjustment can be made. A test run at top speed will soon tell you if your close.
Cheers
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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14th November 2013, 06:52 PM #5GOLD MEMBER
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Lathe Dogs
Bill
I am only guessing but perhaps the Lathe Dog is a bit large for the size of the workpiece.
Cant say I have had this balance problem, but a smaller lathe dog might fix it.otherwise just drop the revs back.
Often its just a matter of getting the feel of the machine & working within its limitations.
Lathe Dogs, come in all sizes, & they can be easily made in a lighter construction.
Some of the old forged ones were just too large & heavy for a small lathe.
regards
Bruce
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15th November 2013, 12:40 AM #6Cba
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Bill, on a lightweight lathe
- always use the smallest lathe dog, just large enough to hold the workpiece.
- avoid using steel or cast iron lathe dogs. These are made for heavy industrial lathes, and are invariably too heavy for a light hobbylathe (especially the ones with a bent drive arm). Use Aluminium lathe dogs instead. If you cannot find Aluminium lathe dogs, make your own from plate stock.
- Modify your catch plate to accept balance weights. If using the face plate, add balance weights as necessary. Changing the length of the bolt used to clamp the dog onto the workpiece, is also a way of improving balance.
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15th November 2013, 09:34 AM #7GOLD MEMBER
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any slight imbalance is accentuated the faster the rotating bit goes
rule of thumb ....slow down...best to keep to speeds where it doesnt shake and vibrate else things could go flying with disastrous results
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15th November 2013, 10:25 AM #8Cba
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Here a comparison of two totally different lathe dogs. I choose two that will clamp about the same workpiece size of around 20mm.
- The one on the right is (I believe) of Chinese origin, made of cast iron and is intended for a heavy duty industrial lathe. it clamps up to 22mm and weighs 196g. It came in a set of three that I bought mail order - I rarely ever use them, they are simply far too heavy for my Hercus 260.
- The one on the left (edit) is made of cast Aluminium and is intended to be used for light duty tasks (like with a light Hobbylathe). It clamps up to 20mm and weighs 22g (with the screw). I use it a lot, for example to turn over small motor collectors. It causes only little imbalance. I bought these back in Switzerland about 30 years ago.
1.jpg2.jpg
I cannot understand how someone can use the lathe dog on the right on a lathe of less than say 3-500kg weight. A lathe of less than 100kg weight would jump off the bench if not bolted down at speeds above 800rpm. On a 100kg lathe, the maximum speed of such heavy lathe dog without extra balancing weights would be about 200rpm. I do not know why it is hard to buy lightweight lathe dogs. Probably no dealer does between centers turning himself. Fortunately it is easy to cut such dogs from a sheet of Aluminium plate. Maybe someone doing home casting may consider running a batch, and offering them for sale? Chris
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15th November 2013, 02:03 PM #9Cba
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Here I found an ebay seller that has a set of these beautiful light lathe dogs:
Drehherz / Mitnehmer 6-Stück aus Alu, D= 6mm bis 55mm | eBay
I also found them in the Swiss Bruetsch&Ruegger catalogue (where I bought mine from). They make them in both Anticorodal order no 63600 (8 sizes for 6 to 70mm shafts), and in Brass order no 63620 (5 sizes for 6-40mm shafts). Chris
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15th November 2013, 03:03 PM #10SENIOR MEMBER
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ALuminium lathe dogs
Thanks for the tips.
I guess one trap for newbies is assuming that all the stuff they got with the lathe (2nd hand) is suitable for use with that lathe.
Will definitely follow up on the Aluminium lathe dog idea.
Looks like its not a resonance issue - with huge wobbles at certain frequencies - its just an unbalanced weight that wobbles more as the lathe goes faster.
Bill
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16th November 2013, 07:24 AM #11SENIOR MEMBER
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Hi Bill,
don't throw the heavy cast dogs just yet.
Try and get into the dark art of counterbalancing.
At some point in your machining life you will no doubt be required to do this.
All I do is roughly gauge how much weight needs to be bolted to the faceplate opposite the heavy side to negate this effect. You can get in the 'ballpark by putting the headstock in neutral and spinning the faceplate by hand and seeing if tends to stop with the heavy bit at the bottom. If it does, add a bit more weight (or move the weight you already have out further) until the chuck faceplate stops in random positions.
The weights can be old bearings, bits of plate, bits of shaft etc with holes in for the bolt to go through, and through the slot in the faceplate.
Counterbalancing is an important part in machine setup and it's great when you get it right as the lathe can nearly be run at full speed for the part you are machining.
Phil
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