Cutting circles

[Peter907]

I'm learning to use my scroll saw and thoroughly enjoying the experience so far. I needed to cut a circle about 4" diameter from 7mm thick plywood. I thought I had a foolproof method for doing this, but soon realised I hadn't as the cut was spiralling inwards so that by the time I'd gone full circle, the cut had moved inwards by 2.5mm. I had it all clamped down solid, new blade fitted and took my time letting the blade do its work without forcing it. I'm hoping that someone can give me some advice on what I'm doing wrong and how I can cut a circle not a spiral. I will probably do this circle again freehand without any jig whilst I'm waiting for a reply, but I'm sure there must be a better method I can use in the future. All good practice. Thanks in advance.

 

[niall Y]

Hmm.................., puzzling. It's a method that works well with a disc sander ( with the wood roughly chopped to a circle) and on my overhead router. If you are sure bolt is a tight fit and central to the drawn circle, then it has to be the blade wandering.
Am not too familiar with scroll saws, and am sure others will respond who are a lot more clued up, but would something like a spiral blade not help?

 

[recipio]

I can only guess that the blade is bending a little. Try to increase the tension to the max. ?
If you have a disc sander it's easy enough to make a circle sanding jig for it. Just cut your circle freehand and sand back to the line. You get a nice sanded edge as well.

 

[Peter907]

Thanks, this was my second attempt. This time I made sure the hole and bolt were a tight fit and everything was clamped better and increased the blade tension. But got exactly the same result. A spiral blade might be worth a try, but I don't have one to hand and the blade I'm using is an Olson no.5 436D double tooth, so is very thin. I actually need to cut out a ring as per the two circles marked, so a jig would be ideal. I can do this freehand but would love to know why my jig method is not working as I expected.

 

[Inspector]

I haven't played with the scroll saw I bought last year so can't speak from experience.

I suspect the position of the jig pivot point needs to be adjusted fore and aft. The average cut position of the blade (it pivots back and forth slightly) is not in line with the rotation point of the jig causing it to cut in. If moved too far the other way it would spiral out and the circle would be bigger. Much the same principle as a bandsaw blade not cutting parallel to the fence requiring a fence adjustment or re-tracking the blade fore or aft until it does. Something to ponder and more seasoned scrollers can confirm.

Pete

 

[AES]

Personally I wouldn't touch a spiral cut blade with the proverbial barge pole, though I should say that some people swear by them (rather than AT them)! I think a lot depends on how much previous practice/experience you've had, and how (exactly) you learnt how to cut.

Personally I would only cut a circle like that, especially in thicker stuff, WITHOUT a jig, by free hand only. That requires some practice, though from what you said in your OP about "letting the blade do the work", suggests that you are indeed on the right track (sorry, no pun intended).

The problem with using a jig with some sort of centre "pin"/bolt/pivot>whatever is that your natural tendency while rotating the work is to have at least some pressure moving against/towards the centre pivot, i.e. there's at least some tendency to force the blade sideways (looking in the vertical plane). This WILL tend to cause a spiral cut, or, if the pressure is too great, will eventually snap the blade.

The ideal when cutting any line, straight or curved, is for the cutting force to be exactly ALONG the line to be cut, with no force at all tending to push the blade sideways in either direction (either in towards or outwards from the centre).

In other words, in the "perfect world", your blade will stay absolutely vertical with the job moving in a smooth arc into the blade exactly ALONG the line to be cut. That takes practice, and in my own case, even after a LOT of practice, I can't do it as consistently smoothly (with no vertical/sideways or backwards/forwards movements) as the experts you see on YouTube for example.

But I CAN do it OK sometimes, and like Arnold Palmer with his golfing displays of holes in one, "the more I practice the better I get"!

The same is basically true when it comes to spiral cut blades too (or so my own experience shows). One of the "essences" of the above-described cutting method is that the blade ONLY cuts when you move the job into the blade at 90 degrees to the cut line (give or take a little to allow for blade twist). And that fact of cutting at 90 degrees is in fact the "secret" to turning sharp right angle corners:- Stop moving the job forward into the blade, then pivot the job 90 degrees (or however many degrees you need, it can be more than 90) and the blade will NOT cut the job until you start moving the job into the blade again at the new angle. That's the essence of cutting a circle or any other regular curve - that continuous movement of the job into the blade because the blade will ONLY cut when the job's moving forward into the teeth.

But by it's very nature, the spiral cut blade is already twisted into a spiral by the manufacturer. That means that (looking downwards on to the job), there is NO angle that you can move the job into where the blade will not cut. On the contrary, whatever angle you move the job into, the blade will ALWAYS cut the job.

That's why I - and many others here previously have - semi-humorously - referred spiral cut blades as "the work of the devil"!

Having seen those posts I wanted to try for myself, so bought a packet of spiral blades, tried them, and found that for me anyway, those things are indeed the work of the devil.

BUT to repeat what I said earlier, there are other scrollers (many?) who DO use them and get great results, so if any doubt, a packet of a dozen spiral blades are relatively cheap, so why not give it a go?

But to go back to your OP. Personally, I would keep practising circles without a jig until you can do it.

Sorry the above is lengthy, but IMO anyway, there are actually quite a few different factors coming into ply when cutting circles, and again IMO only, one needs to fully understand what's actually happening to produce success - along with "practice, practice, practice".

HTH, good luck

Edit for a P.S. Assuming you're trying to produce a good result NOW (and practice does need TIME!) then I'd suggest that you try out the non-jig circle cutting as above, and if that circle needs to be accurately to size, and ROUND (!), then make sure you're always cutting OUTSIDE the line. Then take a scrap of MDF, ply, whatever is to hand, and make your circle jig but use it to SAND to final ROUND size. A belt or a disc sander does work well for this, especially if you haven't got HUGE amounts to sand off. Again, HTH.

 

[Peter907]

Thanks @Inspector and @AES . I've learned a lot from your posts.
It should have, but didn't occur to me that the pivot point on the jig should be at exactly 90 degrees to the cutting line of the blade. It was way out, so I think this was one of the main contributors making the curve into a spiral. Also my inexperience with the scrollsaws means I haven't quite got the technique of feeding the wood correctly yet. So @AES I will read your lengthy but very informative reply several more times

 

[Peter907]

.....and work on the subtleties.
The photo show my current (lack of) freehand skills, but it is nothing a bit of sandpaper won't cure.

 

[Peter907]

So far, I've only cut a few practice lines, made a couple of dolphins (compound scrolling) which I'm pleased with, and my third attempt at the ring.

 

[AES]

@Peter907: Well Sir, if that ring is one of your first freehand circle attempts I should say that at least in comparison to my own pathetic attempts, yours look pretty good. Keep practising!

Without wishing to add further confusion, the pivot point of your jig doesn't need to be at 90 degrees to the cutting line as you said above. It certainly helps a lot if it is ('cos once you've cut all the way round back to the start of the cut line it's a helluva lot easier to make the two cut lines join into one single cut - MUCH harder if you started off at some other angle and so end up trying to join the last bit of the circle cut onto an ever-narrowing triangle of waste wood). So coming in at 90 degrees is ideal (BUT cannot be done with a spiral cut blade I don't think - well not by me anyway)!

But what helps is to first draw a single straight line on the job to be cut (or on the jig base), measure it's length off at the desired finished circle diameter, then divide that diameter line EXACTLY into 2 halves. That "halfway point" is of course EXACTLY where you place the point of the compass, and especially for the possible sanding after cutting jig I mentioned above, that method works v well and is very easy and quick to do.

As you've already discovered, cutting nice swoopy curves for something like a dolphin is both a delight and is quite easy to do (nobody can tell if you've gone off line a bit on parts of the cut). But circles are a different kettle of fish and often even quite small off line errors can stand out like a bulldog's whatsits, hence the suggestion for the sanding jig.

One further small tip (again turning into a long post, sorry). When cutting free hand do NOT be tempted to go back a bit and re-cut if you see you've wandered off line a bit. It almost always ends up with a worse jiggly bit of the circle than you first produced! MUCH easier to just keep cutting forward, try to get back onto the correct line asap, and then sand (jig or by hand) the mistake/s off afterwards.

HTH

 

[Peter907]

@AES on the strength of your lengthy and very helpful replies, and a wee dram of Laphroaig , I have tonight purchased an annual subscription to this forum. Not even sure what I'll get out of the subscription, but very happy to say that in my short time on this forum I've already had far more than that value in guidance . Thank you for spending the time and effort to provide such useful and in-depth advice to a beginner like me, it is much appreciated.
Also thanks to you and others on this forum who gave me good advice in another post, steering me towards a secondhand Hegner in preference to my original ideas of a new entry level scrollsaw. The more I use the Hegner, the happier I am with this very capable machine. After much enjoyable trial and error, I feel that working with the scrollsaw is fast becoming the new learning curve I've been searching for to fill some of my spare time now I'm retired. Today I messed around for nearly eight hours doing various things with the Hegner. Having learned by my mistakes, I know I could do the same job again in just one hour, which has given me a real sense of satisfaction and the confidence to go on to do much more.
Cheers,
Pete

 

[AES]

Pete, when I first joined here I too got lots of help and advice, so I'm pleased to try and pay back a bit by helping others if I can. There are MANY other members here who do exactly the same. Thanks for posting

 

[scrimper]

That's why I - and many others here previously have - semi-humorously - referred spiral cut blades as "the work of the devil"!

, HTH.

Lol I think you have been watching my videos, I coined the phrase "the work of the devil"

 

[scrimper]

Peter you have received some superb advice from the experts here and there is not much I can add, cutting a perfect circle out on a scrollsaw is not the easiest of tasks and I now always do the job freehand so to speak. I have tried using jigs as you mention and experienced the same issue as you discovered. One of the problems is that due to the way fretsaw blades are manufactured they do suffer from a bias and tend to veer off line which is why you have to hold the work at an angle to the saw to cut a straight line.

As for spiral blades (just a normal blade twisted) don't go there, it will put you off the hobby for life unless that is you are one of those people with a special talent who do absolutely superb work with them.

(I talk about spiral blades in several of my fretwork videos)

 

[AES]

Lol I think you have been watching my videos, I coined the phrase "the work of the devil"

Yep! "Guilty as charged "mi'lud"

 

[Rene Mopper]

Using a jig i would sure use a spiral blade.

 

[Peter907]

You're all doing a great job selling these spiral blades to me now. I should have some tomorrow via Amazon Prime so I can have a go at the devil's work - just for the hell of it . (Pun intended)

It won't put me off as I have lots of ordinary blades to go back to. I will report back on the circle jig method using a spiral blade.

 

[AES]

Using a jig i would sure use a spiral blade.

Oh??? Can you expand on that advice please? especially with WHY you would use a spiral cut blade.

 

[AES]

You're all doing a great job selling these spiral blades to me now.

Peter: You' said, (above). Oh? Really? But a dozen blades are not a big investment, even if you do end up throwing 11 (perhaps 10) of 'em away!!!!

If the Post from Switzerland didn't cost so much, I could send you 10 (at least) FOC!

 

[Inspector]

FOC!!! Put me on that list should you ever find a big bag of money.

Pete