Small Scroll Saw Build

[MJM460]

Hi Chuck,

Been quietly following along on your great project.  Great to see you in the shop again.

My commercial unit, actually it's my daughters, but stored in my shed, vibrates a lot too.  The commercial approach is to put on thick vibration absorbing feet, not very sophisticated.

It is an interesting problem to balance as you have the not only the unbalance of the frame moving up and down but it has a small amount of rotation as well.

I suspect that if you attach a weight on the back of the frame (might need an opening in your back board so it is not ugly above the cutting table) to balance the frame around the pivot  (with blade and the little con rod in place), you could get a pretty good result.  If you really want smooth, the weight has to be on an arm geared to the frame so the weight moves up and anticlockwise as the frame moves up all the while in static balance.  But even Marvin might think that was over the top.  Keep is simple will probably still give you the best balanced saw in the known universe.

MJM460

[MJM460]

Sorry Chuck,  just noticed that I had missed a few posts.  If you are moving the pivot anyway, might as well shift it towards the blade to the balance point as well.  Always better not to add weight to a forced vibration.

Chris's four bar linkage  Ryobi is an interesting approach to both the blade advance and balance problems though it will still move a little towards the work on the up stroke.  We need to work out an orbital mechanism like the jigsaws.  That really helps clear the chips.  I must go out and have a closer look at mine.  Too cold now but I will report back.

MJM460

[cfellows]

Sorry Chuck,  just noticed that I had missed a few posts.  If you are moving the pivot anyway, might as well shift it towards the blade to the balance point as well.  Always better not to add weight to a forced vibration.

Chris's four bar linkage  Ryobi is an interesting approach to both the blade advance and balance problems though it will still move a little towards the work on the up stroke.  We need to work out an orbital mechanism like the jigsaws.  That really helps clear the chips.  I must go out and have a closer look at mine.  Too cold now but I will report back.

MJM460

Thanks for your comments and suggestions.  The parallel arm linkage like the Ryobi is, by far and away, the most common mechanism used in today's scroll saws.  However, the C-Frame design, like mine, is purported to enjoy a longer blade life and the overall design can be kept smaller.  And, since there is only one pivot point instead of 6, it's mechanically easier to control unwanted blade motions caused by play in the pivots.  The trade-off, of course, is vibration.

I have several options for mitigating the vibration and will continue to report on how I'm doing.

Chuck

[cfellows]

Rummaging through my stash, I found a 10 tooth timing pulley that will fit on the motor.  I decided to make a 30 tooth timing pulley for the crankshaft.  I'm making it out of brass to add weight and provide some flywheel effect.  I have a 3/32" thick slitting saw that I modified to cut XL timing pulley grooves. 



The outer diameter is 1.89", the hub is 1.25".  Overall width is about 7/8".  I've designed I new crank disk that will have counter balancing.  If that isn't enough I can also cut away part of the web on pulley to add more counter weight.

Tomorrow, I will use my CNC mill and 4th axis to cut the grooves.

Chuck

[cfellows]

So here's a multi-part video.  The first couple of segments show the process of cutting a 30 tooth, XL timing pulley.  The last segment shows a test of the scroll saw with new pulleys in place.

<a href="https://www.youtube.com/watch?v=TPgbOhfXK0g" target="_blank">http://www.youtube.com/watch?v=TPgbOhfXK0g</a>

Chuck

[cfellows]

Here's some pictures...

This is the 12" x 5" x 1/2" steel plate attached to the bottom of the saw:


And here is the drive unit with the new pulleys and the new brass, counter-weighted crankshaft:



And here it is all buttoned up, including the switch:



All the balancing measures and slowing down the drive has the saw behaving pretty well, not much vibration.  I had no idea how big of a counter-weight to put on the crank, but it did help.  I may have to put a bigger motor on it, which won't be a trivial matter.  It's kind of bogs down on 1/2" or thicker plywood.   I really made it to cut thin materials like plastic or plywood no more than 1/4" thick, usually even thinner.  I'll use it for a while and see how it works out.

Chuck

[Kim]

That is a really nice little scroll saw you made there, Chuck!
Now what are you going to make with it?
Kim

[MJM460]

Hi Chuck,

That is a great result.   Good to see that your solutions to the problems worked out well.  I like the creative use of the brown stuff, it probably even adds a bit of damping.

Complete balance is tricky to achieve because the frame motion has a component of rotation, and that needs a counter rotating couple for complete balance, and almost certainly not worth the complexity.  Anyway it is a forced vibration scenario and your heavy plate underneath is quite a valid way to reduce the response.

You could probably test if your static counterbalance is good by slipping off the belt and spinning the crank.  If it always stops in the same spot, the low side is still heavy.  If it tends to stop at many or even just a few places your balance is probably as good as you will get.

Earlier I said I would check my daughters Delta machine.  It has the extra pivots like the Ryobi.  I think the elegant simplicity of your design beats the complexity, particularly for such a handy small machine.  If you need to cut 1/2" stuff, perhaps the next project could be a band saw.

MJM460

[cfellows]

Good idea on removing the belt and spinning the assembly to see where it stops.  Hadn't thought of that, but will give it a try today.

Thx... Chuck

[cfellows]

Round 3... I checked the balance as MJM suggested and found that the counterweight I built into the crank wasn't anywhere near enough.  Because everything is mounted in ball bearings, it all moves very freely so I didn't even have to spin it.  I just turned the weight to the top and it rotates right back to the bottom.

So I'm going for round 3 on the Crank.  I'll get the new crank done today and include some more pictures.  I'm also not happy with the power of the little motor I started out with, so I'm replacing it with a larger, 90v DC motor.  The first motor was 1.75" tall and was just barely cleared by the bottom of the arm.  The new motor is 2.5" diameter and won't fit under the arm, so I'm mounting it out front where there is no clearance problem.  It's still tucked back under the case far enough that it's not too unsightly.

Chuck

[DICKEYBIRD]

I'm also not happy with the power of the little motor I started out with, so I'm replacing it with a larger, 90v DC motor.

What kind of motor did you come up with Chuck?  I'm always looking for 90vdc motors that are smaller than treadmill motors.  Did you find an inexpensive Chinese one? 

[cfellows]

I'm also not happy with the power of the little motor I started out with, so I'm replacing it with a larger, 90v DC motor.

What kind of motor did you come up with Chuck?  I'm always looking for 90vdc motors that are smaller than treadmill motors.  Did you find an inexpensive Chinese one?

Unfortunately, I acquired the motor many years ago, I think from Lincoln Surplus Center. 

http://www.surpluscenter.com/

I looked on their site and couldn't find it.  I had the voltage wrong, it's rated at 100 volts and  no-load speed is 3000 RPM.  Dimensions are 2.5" x 5" long, 3/8" shaft.  Guess that doesn't help you if you can't find one.  The part number is E51964 BPM1012.  It's been sitting in a drawer for 12 or more years, so I'm guessing your chance of finding one are pretty slim. 

[cfellows]

Got the counter-weighted crank done today.  Turned out nice, but it's still not heavy enough. 



Not doing any more with the crank.  Might hollow out one side of the brass drive pulley to remove more weight from the light side.  I'm going to put it together and see how it behaves. 

Chuck

[DICKEYBIRD]

Thanks for checking on it Chuck.  I'll bump into something one of these days.  I like the 90v range because I have a few extra KBIC-120 speed controls lying around & they work great.

I picked up a 24vdc scooter motor cheap on ebay & run it @ 38v on my homemade toolpost grinder.  I took it apart & added some cooling holes plus broke it in carefully.  Seems to do pretty well but I'd like more power eventually.

Could you drill a couple holes & fit some scrap carbide chunks in your counterweight?  I think carbide is at least twice as heavy as brass.  Dunno if it's worth the trouble but it might be an interesting experiment.

[cfellows]

So, I got everything put back together.  A couple of observations.   Supposedly, the smaller motor had a maximum speed of 2500 RPM and the larger motor I just installed was supposed to run at 3000 RPM.  Well, the new motor runs a lot faster than the smaller one, maybe close to twice as fast.  Fifty percent voltage on the variac is about as fast as I want to run this thing.  Vibration is less with the new crank, but at anything over half throttle, it starts vibrating quite a bit.  The good news, this larger motor has way more than enough torque and I can run the saw at a very slow speed without it lugging down.  With its 3/4" stroke, it cuts 3/4 inch plywood aggressively, even at slower speeds, and gives me a really smooth, controllable cut.  I'm actually very pleased with the results.  However, I think the addition of a hold-down foot is going to be a requirement.  More on that after a little design work.

Chuck