Diamond Scrollsaw Vibration

[davequa]

I have an old Diamond 19 scrollsaw and it has severe vibration at anything over half speed. The counterweight on the drive shaft seems to be in the correct position (ie vertical when the saw is at its lowest point). The saw is bolted down well to a rock solid bench
Does anybody have any ideas as to how I can cure the problem

 

[Gill]

I wish I could help, Dave, but I'm no engineer so let me just welcome you to the forum instead .

Gill

 

[jonbikebod]

Welcome to the forum Dave,
I am not familiar with the Diamond but do know a little about balance.
The counter weight as you describe moving in predominantly the opposite direction to the saws frame is only able to cancel some of the vibration. Initially you would think a counter weight of the same mass as the reciprocating parts of the saw would cancel all the vibration and in the vertical plane it will, the problem is the counterweight is rotating so between the top and the bottom of its motion it creates vibration horizontally. In practice the counterweight is around half the mass of the reciprocating part reducing the vertical vibration rather than cancelling it but avoiding introducing too much horizontal vibration. (An additional flywheel rotating the other way could be arranged to cancel nearly all the vibration but would also be rather complex). So, this is a compromise all scroll saws (as far as I am aware) use and it seems to usually be adequate.
Has the saw always done this? Could a weight have fallen off?
I would suggest unbolting the saw from your bench (which sounds like it should be enough to tame any scroll saw) and then try and determine if the saw is vibrating mostly up and down or mostly back to front (assuming that is the plane the flywheel rotates in). I hope you can now see that increasing the counter balance weight will improve vertical vibration at the expense of horizontal and vice versa. As you usually have your saw bolted down, you can probably afford to have more horizontal vibration than would be wise with a ‘loose’ saw. Experimentally adding more counterbalance weight may well improve things.
Do also make sure the counter balance is in the same vertical plane as the reciprocating parts (or as close as possible) or this can cause the saw to rock from side to side. This might happen if components have been assembled the wrong way round.
I hope this helps,
Jon.

 

[davequa]

Many thanks for the reply Jon. It has certainly given me a few things to think about. I will strip the saw down this week and have a bit of a fiddle

 

[jonbikebod]

Re-reading my mail it sounds very confusing but like all these things the central idea is quite simple.
Let me know how you get on Dave,
Jon.

 

[davequa]

The saga continues.

I've had a chance to try out a few things so, Jon, if you read this I'd very much appreciate your thoughts.

The balance weight is a solid rectangular piece of steel 3" x 1" x 1/2" and is fixed to the pulley shaft so that it is vertical when the arms are at their lowest point.

I reset the pulleys so that they were completely in line and set the balance weight as close to the plane of the pulleys as possible. All the bearings and pivot points seem to be in good condition.

I've tried the following (all with the saw NOT bolted to the bench.)
1. With initial setup - quite severe vibration above half speed
2. Moving the balance weight through 10 degrees clockwise or anticlockwise seems to have no effect
3. Moving the balance weight through 180 degrees also seems to have no effect
4. Moving the weight through 90 degrees seems to improve things considerably (short test as it was getting late and I didn't want to annoy the neighbours!) I'll try it again tomorrow. Doesn't seem logical!
5 Most of the vibration seems to be in the vertical plane as the saw acts like a 'hovercraft' and seems to float above the bench. A small push on the side moves it in that direction

Now for several questions:
1. If I remove the balance weight completely what is the likely effect? Will the saw shake itself to pieces? (can't do this at the moment as I need a puller to remove the pulley)
2 Is a rectangular shape for the balance weight the best or would a pear shape (of the same weight) with the widest part centred around the shaft be better.
3 The saw has hard rubber feet so when I bolt it to the bench I will put 1/2" neoprene pads under each foot - that should help reduce vibration??

Sorry for the long post - vibration is now at an acceptable level and is probably OK for 90% of my requirements. However I would like to understand what is going on as it offends me that it is not rock steady throughout it's speed range!!

UPDATE
I've just bolted the saw to the bench with the neoprene pads under the feet and the balance weight 90 degrees from it's original position. The result is a great improvement. Still some vibration at high speeds but it is manageable. I think I will try it without any balance weight sometime soon. Now I guess I should start cutting some wood

 

[jonbikebod]

Hi Dave,
I am suspecting this counter weight may have been miss-identified. I have done a quick web search for some photos of your machine but not managed to find any. If you could post some photos it may well make things clearer.
I get the impression the weight goes from the periphery at one side, past the centre and on to the periphery the opposite side, as you describe the weight as vertical rather that ‘at the top’. If this is the case, the bar is in balance with itself and so it will not be working as a counterweight. You also mention the weight is attached to a pulley and I get the impression this is on the motor shaft. To work as a counterweight it would need to be attached to the crank that moves the saw arms. This is because belts tend to slip (unless they are toothed belts) so the effect of the counterweight would keep changing.
To answer your questions, removing the weight is probably fine and may well improve things. My Hegna clone has just a heavy flywheel, no balance weight and negligible vibration. The part of the weight that will cause vibration (hopefully cancelling another source of vibration) is at its most effective on the periphery of the wheel. Around the centre it is in balance with itself and only adds mass to the wheel. The shape is only really dictated by the consideration of getting the mass at the periphery. With the bar you describe it is the placement that is important. I do wonder if its purpose might be to drive something else from the motor shaft, an accessory flexible drive for example.
Neoprene could dampen vibration but it could also allow enough flexibility for it to vibrate, negating the dampening effect of your bench top. Experimentation would be the way to go there.
The other thing to consider is dampening the vibration by fixing something heavy to the machine’s table. This is one of the reasons cast iron is the preferred table material. Easy to try experimentally and bolted under the table if it proves effective.
Jon.

 

[davequa]

Hi Jon,
I guess my word pictures are not as lucid as they could be
The balance weight is a 3" metal bar with a hole about 1/2" from one end. The shaft for the pulley passes through the hole and a grub screw secures the bar to the shaft. The bar extends from the centre of the pulley to the periphery (like the hand of a clock). This pulley/weight/shaft combination is driven by a belt from the motor and itself drives the the crank that moves the arms. Hope that is clearer. I'll take a couple of pics and post them here.

Your help is much appreciated

 

[jonbikebod]

Hi Dave,
Apologies for taking so long to get back to you, I sort of got taken over by events.
Nothing at all wrong with your descriptions but pictures both fill in the gaps of any description and make it easier for anyone else to follow the thread now or sometime in the future.
So, as we were then, certainly a balance weight but I would have expected there to be a significant difference between the weight being at the top when the reciprocating arms are also at the top (would expect it to vibrate like a foundation compactor) to the weight being at the bottom when the arms are at the top (cancelling some/most of the vertical vibration with perhaps more fore and aft vibration that bolting the machine to the bench should tame).
As that doesn’t seem to be the case (and it was presumably correctly set at the factory) something else must be causing the vibration like a worn bush or bearing – most probably of a reciprocating rather than rotating component?
Jon.

 

[davequa]

Hi Jon,
Assuming I have done everything correctly here are a couple of pics of the Diamond saw. The balance weight really does not seem to do much in terms of reducing vibration no matter which orientation it is set in. AS soon as I get hold of a puller I will remove it and see what happens.

pic1 - General view of pulley arrangement

Pic 2 - closeup of balance weight

One thought I've had is to by-pass the pulley that drives the accessories and see if that makes a difference

 

[jonbikebod]

Hi Dave,
Hadn’t forgotten about you but getting vey little time at the computer at the moment. The photos speak volumes, I must say yours looks like a very well engineered machine, could easily be a one-off from its appearance at least. Be aware the pulley that drive accessories is also a reduction pulley slowing the speed of the cranking pulley by several orders. If you run a belt from the motor directly to the big (cranking?) pulley on the left I am pretty sure it will either run far too fast or the motor will stall too easily. The pulleys appear to be made of aluminium the holes in the left hand one would normally be there to reduce weight. This isn’t much of an advantage used in this way so I suspect they are proprietary items or were originally made for something else. From the photos it appears the ‘balance weight’ is acting more as a connecting rod – converting the rotary motion of the pulley to a reciprocating motion needed for the saw arms. It also appears to be made from aluminium which is usually used for its light weight. The opposite criteria to that normally used as a balance weight but it would explain why its position doesn’t alter the vibration much.
What I still cannot work out from the photos is where the arm(s) attach one of the pulleys to generate that up and down movement. It looks to be the horizontal green bar with the nylock nut on it in pic 1 is being driven, is the drive for this from the other end of the shaft for the left hand pulley? In that case removing the balance weight isn’t going to make much difference. Replacing the balance weight with something heavier – iron or bronze – should improve things a lot though.
Your machine has a tubular steel frame (and table?). Cast iron is the more usual choice of material - for most static woodworking machines – because it naturally tends to absorb vibration.
Jon.

 

[Geoffrey]

Hi jon you are right the pitman arm is on the over side of the horizontal
shaft .I have one of these saws and it vibrates like mad at more than half
speed i took counter weight off my saw and it made no diffrence.
Its a heavy duty saw and will cut thick wood ect but is very good at fine work its notment to run at full speed

Regards Geoff

 

[jonbikebod]

Thanks Geoff,
The answer of sorts seems to be “they all do that”. If the maker didn’t manage to fix the problem (and the counter weight would seem to be an attempt at just that) then it seems the only ways forward are to re-engineer the machine (a mass similar to the reciprocating parts of the saw moved in a reciprocating motion equal and opposite direction to the saw components would work) or working around the problem by running at lower speed for fine work.
Thanks for your input; I suspect you have saved Dave a lot of going around in unproductive circles.
Jon.

 

[davequa]

Hi Jon/Geoff,
Many thanks for all the advice and help. I've decided to call it quits on the saw vibration for the moment. In truth it is unlikely that I will need to run at more than half speed anyway and at this speed the vibration level is acceptable

 

[tomrad]

Hi Dave
I have just stumbled onto this thread so I may be a bit late with this comment. I have a Diamond saw which vibrates badly at high speeds. I contacted the manufacturer some years ago. Their reply was that, until 1995 (I think), these saws all did this because they had a fairly simple balancing mechanism. Saws built after this date had a much better system. It could be fitted to older machines at a cost of about £50. In the end I found that I liked working at low speed so I didn't have the work done.
Mine is for sale if anyone is interested.
[email protected]
Tom
Rochdale
Lancs.