Brevetti Prisma CE Mitre Saw: setting the blades

[retropic]

I'm currently working with a company that uses a Brevetti Prisma CE Mitre Saw. I've become increasingly unhappy about the cuts it's producing. On bigger 80mm oak moulding it seems to cut a good mitre, but put in a small black moulding (not sure of the catalogue number/supplier) the corners look like they've been cut by a blunt chisel. They're fine on the front face, but the corners are wide open with a gap of 1.0 - 1.5mm at worst.

So I turned to the manual for advice. This is helpfully written in Italian with a translation which appears to be by someone who failed their Italian O level 30 years previously and hasn't used the language since. I digress. The manual recommends making a frame and then checking it against a menu of options and gives 4 various permutations with which to compare your example:

1. Perfect joints - Do nothing
2. gap in joints at at back - adjust to make the angles (I think it said) to make bigger
3. gap in external corner - adjust to make the angles smaller (I may have these the wrong way round)
4. gap in the inner corner - rhumboidal. One blade is greater than 45 degrees, whilst the other is lesser.

So that seemed straightforward and I adjusted accordingly and cut about 8 frames, one frame after each adjustment, out of 2", 15mm MDF, (Which I believed to be perfectly straight). There was a vague improvement in the gap at the external corners, but I was far from happy with it, but it was an improvement.

Today I had to make 11 frames (1.2 metres by 0.9 metres) out of 80mm wide oak moulding. Decided I'd test the machine and cut each one, then pin it to test the set up. It became apparent that the blades were cutting rhumboidal (i.e. both blades were wrong). By frame 11, I think I'd cracked it. The front mitres looked fantastic, but even so there was still a very small gap in the outside corner. Fortunately the gap was easily repaired as it has a wax finish.

So I think I've got each blade now cutting at 45 degrees, so I'm wondering if it's possible that one or both of the blades are not running perfectly square in the vertical (a bit like how a car wheel runs when the tracking is out). Looking at the frames, I think it's the left hand blade that's the cause of the problem.

Does anyone have experience of setting one of these machines? Have I made any sense whatsoever? Do we need an engineer to set it accurately, or is it possible for an operative to do this successfully? Also how on earth do you work out which of the blades is making the inaccurate cut?

I've discounted bent/warped/twisted moulding as being the cause because surely not every moulding supplied can be out of true...?

Thanks in advance

[retropic]

I should have also mentioned that the 45 degree set triangle provided by Brevetti to check there's no gaps between it and the blade has helpfully been lost. I made a triangle out of mountboard on the Keencut, to get it as close as poss, then tweaked the nuts to get them accurate (ish).

[Steve Goodall]

Have you spoken to Garry White @ Wessex London- he is a bit of a guru on these saws.

It might help if you do business with Wessex - Garry is not a "charity" - but a very knowledgible bloke on these all the same...

[prospero]

Not all that familiar with saws but this seems to be the same problem that you can get with a Morso, which is cured by fine-tuning the angle of the left fence. The thing is, if one blade is off, the error is multiplied 4 times in a complete frame. The other blade may be off and depending on which way it is off it may partially cancel out the error of the other one or may add to it. All in all it's a juggling act. But very fine adjustments are called for. I'd tend to set one blade to as near 45 deg as possible and then only tweak the other one.

There is also the factor of different grain patterns in different woods. Although this is more relevant to choppers than saws.

[retropic]

Thanks. I've not spoken to Garry. The company is a customer of Wessex, so could legitimately discuss it with him, although I have a feeling the machine was purchased via Simons.

Thanks too Prospero. Good point about the problem being multiplied four times. I think I'm probably as close as I can get to a solution without an accurate 45 degree to set the blades. There's a really good engineering shop near here, I think I'll ask them to cut me an accurate set. I did mean to get one made when I was setting up my Morso, but never got round to it. Incidentally, talking of Morsos. I have set the spring tension as tight as I can, lubricated the runners on mine and still the blade either doesn't return to the top, or crawls up. New springs needed?

[prospero]

With the Morso, the blades usually need to be backed off a notch before letting them up or they will bind against the cut face.

It's known as the 'Morso Dance'.

If they creep up slowly without being in contact with the wood, then you either need new springs or a dab of grease on the block.

[stevebowen]

I had a similar problem when I purchased the same saw on ebay

After asking around, I was told the secret, one of these
http://www.axminster.co.uk/nobex-nobex- ... rod480222/

After cutting a piece, you use the 135 deg square to measure the angle of the cut, hold it up to the light and you can see if its slightly out, this gets one side aligned correctly and then do the same on the other side and do a test frame with a wide moulding, then make minute adjustments until it joins perfectly.

Hope this helps

[retropic]

Steve - thanks for that. I will get one of those and give it a go. Looks like an inexpensive solution to the problem.

Prospero - thanks too. Doh! Why didn't I think of backing off the blade so that it doesn't snatch the moulding as the blade goes back up! I have lubricated the whole of the block, so perhaps new springs would be a good idea. Will treat myself and see how it goes.

[avantime]

I had a Brevetti for many years and (not much help to the OP) never had to reset it. I did however regularly check that the blades were perpendicular to the bed - a small engineers square is best for this - also from Axminster.

We occassionally had a problem with gaps on the backs of the moulding - usually caused by the user of the machine trying to pull the blade through the wood rather than "letting the blades do the work"! Also we had a set of blades sharpened (ruined) once - they ground out the tripple chips!! This meant the wood was vibrated by the blades.

Most of the frames we made were oak / ash so thehardness shouldn't cause any problems.

[retropic]

Thanks very much everyone for your suggestions and advice.

After lots of headscratching and tweaking, I finally got there. It's now delivering spot on mitres.

Spoke to Garry at Wessex and it turns out the machine was purchased from them. We were prepared to buy another caliber to set the blades, but Garry's advice was "don't bother, they're not very accurate; buy a plastic school protractor from WH Smiths". The same day we found the caliber and I tried to set it with that, but he was right, it's not very accurate.

That plastic protractor cost 75p and made the job so easy. As Prospero said, those adjustments really do have to be very, very fine.

Avantine: thanks for your tips, as a result I've modified the way I use the machine, so that the blades do the work. And I've ordered a new pair of blades.

[Steve Goodall]

Well Done to Garry at Wessex

Sound advice & a 75p solution - now that is good customer service