Minilathe C3 Chuck Balance

G'day.

I am happy with my C3 minlathe, but it has an annoying tendency to vibrate at certain RPM's when fitted with a 100mm 4 jaw chuck.

I have balanced the chuck as much as I can after I noticed that the holes in the back were assymetrical, but the problem remains.

3 jaw chuck, faceplate and (home made) ER 25 collet chuck are no problems but the 4 jaw means that I have to select the RPM's that I run it at.

The vibration is definitely harmonic as I can reproduce it at 360, 720, 1080 RPM.

Any ideas?

* Danny M *

Hi Danny,

I had exactly the same problem when I replaced the 80 mm standard chuck for a 100 mm chuck. The problem lies with the balance of the backplate to convert the spindle flange to accept the 100mm chuck. The holes in the plate are assymetric and counterbored. The counterboring makes the imbalance much greater than would be the case if the holes were not counterbored.

You could try making little steel plugs to fill all the unused holes and counterbores in the backplate. I actually replaced the spindle with the 80 mm flange for one with 100 mm flange (available from Amadeal) and this completely cured the problem, see:

http://mikesworkshop.weebly.com/headstock-upgrade.html

I hope this helps

Mike

A non self-centering 4 jaw will always be out-of -balance unless the job is perfectly symeterical. Or am I going mad?

Gordon, your spot on. Danny, you'v answered your own question, just don't run at the speeds that vibrate, this sort of thing happens with other machines, it's what caused the crashes of the Lockheed Electra(4 engine turbo prop), when the engine revs were adjusted the problem ceased, the aircraft flies on as the P-3 Orion.

Ian S C

Ian SC

That reply is not your usual sage advice and its not very helpful. It reminds of the Fawlty Towers guest that did not switch her hearing aid because it used up the batteries!

Danny's problem is that his chuck is out of balance.

As Michael says the best cure is to add or remove metal until balance is restored.

Ian P

Many 4-jaw setups will be out of balance anyway.

Nonetheless my advice would be to add plasticine or similar to the chuck until its in balance, then drill out a plug of steel of the same mass from the backplate at the diametrically opposite point. You can work out how much by weighing the plasticine and using the density of steel.

I'm sure this approach has been described in ME/MEW in the past.

Neil

Hi Danny,

I would first check that the chuck housing / body runs true then I would strip it bare so only the chuck body is left then mount it back on the lathe and see if the problem gets any better - if it does then the problem is with the parts left on the bench and if it doesn't it is time to take a real close look at the chuck body .

The casting could be thicker in one spot or it could have casting faults (voids ) that were not picked up on at the factory .

If it is the chuck body then I would then make a mandrel out of steel ( either turn one up or use silver steel ) then drill centres that run true in each end .

Then set this mandrel up in the four jaw chuck so it runs true and make sure there is enough sticking out of both ends of the chuck to allow mounting between centres

if you then mount up a dead centre in the spindle and one in tailstock you can use the lathe as a crude balancing jig by running this mandrel between centres loosely give it a spin and look for the heavy spot .

You can use small discs of steel held on with supa glue to play around adjusting the light spot to get it in balance and it is then a matter of removing the same weight of material from the heavy side , either way it will at least show you where the heavy and light spots are .

By using steel to add weight to the light Side that is round and of a know thickness ( a disc shape ) it would be easy to use an end mill of the same diameter to plunge cut to the same depth as the metal disc thickness to remove the equivalent amount of material from the heavy side

Ian

Thanks to all of you for the advice. Weather is going to be lousy this weekend so I shall get busy in the workshop and report back when I have solved the mystery.

* Danny M *

Think about it before you start mutilating With a four jaw chuck, unless all four jaws are equal distance from centre, it must be out of balance

Roy

Further on from XD's post why not perform a static balance test

Mount the Chuck on a round bar say silver steel then place the bar and chuck onto a set of parallels

Place a mark on the circumference of the chuck and rotate it approx 90 degree - does it continue to roll or does it come back on its self

repeat until you have gone 360 degree - this will show you if the chuck body is out of balance, you then add small weights to the body of the chuck until you can rotate the body to any position and its remains there

Obviously it requires you to have access to a flat/level surface and some parrallels - the thinner the better to reduce drag during the rotation

its not as accurate as a dynamic balance test but will point you in the right direction

cheers

This balancing method works quite well, but first make sure all 4 jaws are the same. The steel bar will have to be clocked centrally on the lathe also.

Unless it's a self centring chuck, your wasting your time, the point of balance on a independent four jaw chuck will often be different, depending on what is in the chuck. Ian S C

Edited By Chris Trice on 28/06/2014 14:02:08

G'day.

The chuck is an independent chuck. I mounted and dialed in a 0.5" steel rod to run true, then removed the chuck with the rod and placed it on a set of ball-raced rollers that I use to make fishing rods on. It rotated by itself.

The problem is the 'extra' holes in the back plate. When filled with lead shot and plasticene I can get the thing nicely balanced, so next move is to dig out the filling, weigh it and turn up some plugs to fit in the holes.

I shall take some photo's of the set-up in case they may be of use to anybody else with the same problem.

* Danny M *

Edited By Danny M2Z on 28/06/2014 16:21:16

I think you would only have problems with out of balance if you were running at full speed. If you are turning off centre in the four jaw, it sometimes pays to reduce speed a little, just in case a vibration is set up.

Is the rest of the lathe balanced ie., the motor armature, although you can still get the vibration caused by a single phase motor if that's what you have, maybe yours is DC, electronic control. Ian S C

For what my advice is worth:

Since the chuck is 4 Jaw independent, and bearing in mind the comments re the likelihood of the jaws being asymmetrically placed in use; before checking the balance, remove all the jaws, and then work to get the bare chuck and backplate into balance.

Howard