A Shattering Experience………

[Anonymous]

The RH crankshaft bearing housing and cover for my 4″ scale traction engines come as one casting, and need separating before machining can commence. An easy job for a slitting saw. Here’s a picture of the final cut on the first part:

 

Thirty seconds after I took this picture the saw shattered. Here’s a picture of the bits:

 

Where’s the rest of it? I don’t know, I haven’t found all the bits yet! The furthest, and largest, bit was found by the opposite wall, a good 12 feet away.

 

Cutting parameters were 41rpm, depth of cut per pass ~10mm, and a feedrate of 80mm/min, giving a tooth chip loading of 0.05mm. So what went wrong? I’m not sure, but here are some possibilites:

 

1) The gullet of a tooth got jammed with swarf due to an over-ambitious feedrate – seems unlikely since the cutter shattered on the last cut, and I’ve subsequently split the second casting using the same cutting parameters with no problem

 

2) The cutter wandered and then jammed – possible, but I haven’t experienced this when using a slitting saw in the horizontal mill

 

3) The part being separated moved – this is my favourite; the cutter shattered when the final cut was almost finished. There is ‘gouging’ in the part (see picture below) that suggests that at least one tooth had broken before the cutter shattered. I suspect that the part moved, a tooth broke and that then precipitated the beak up of the saw.

 

When I split the second part I didn’t try and cut all the way through with the slitting saw. I left about 6mm and used a hacksaw to finally separate the parts.

 

However, the purpose of this post is not to prove that I was an idiot, I’m sure you know that already, but to highlight the fact that when the saw did shatter it spread bits all round the workshop, even though the rpm wasn’t high. So, when it says on the box ‘Caution, cutter may shatter, wear eye protection’, it means it!

 

Was I wearing eye protection? Actually I was, a proper pair of goggles from the local professional tool shop. They’re £1.45p + VAT a pair; an absolute bargain! I highly value my sight, especially so since I’ve come close to losing it in the past for medical reasons.

 

My advise is to get some goggles and wear them, if you don’t already do so.

 

Regards,

 

Andrew

Edited By Andrew Johnston on 04/10/2011 11:50:08

[Anonymous]

[John Stevenson 1Participant @johnstevenson1]

Were you using a key in the arbor ?

 

I never run slitting saws or thin cutters with a key as i want them to spin if they get hung up.

Sooner have a grove on the arbor than wearing a cutter.

 

John S.

[StovepipeParticipant @stovepipe]

Wear them ALWAYS when you’re machining anything, especially with the quality of some of the cutters being marketed today. I wear them even when using a moderate device such as a Dremel. 10 seconds to put on the goggles is much better than years of having your eyes attended to. Also goggles are much better than so-called safety spectacles, especially if you wear spectacles already.

 

Dennis

[NJHParticipant @njh]

Hi Andrew

 

Now if you had been using a hacksaw instead of the slitting saw then it wouldn’t have shattered – but then I suppose you would be!

 

Very good advice on the goggles – I recently broke a 6mm end mill and it went across the workshop like a bullet! I always use goggles now and I’ve also invested in one of those clear plastic shields mounted on a magnetic base – quick and easy to position it gives a bit of extra protection and keeps swarf/coolant on the machine instead of on the floor.

 

Regards

 

Norman

[KWILParticipant @kwil]

Andrew,

 

How flat was the surface in contact with the table? I have been known to interpose a piece of paper, the paper does have a use in stopping sliding about.

 

It is difficult to see but is the rear clamp OK. With you I would expect to be but for the education of others, it should lean down on the job.

 

K

[JasonBModerator @jasonb]

The second photo clearly shows where your problem lies. You have got one of those duel direction blades that need a rocking motion of the arbor so all the teeth are cutting

 

Whats the cut edge like if you lay a straight edge on it, would show if the blade deflected beyond its limit.

 

Any chance the blade got overheated, this usually results in the outer edge buckling and is why you often see slots in circular saw blades, the slot closes up rather than the blade turn into a Pringle.

 

J

Edited By JasonB on 04/10/2011 17:50:19

[Les Jones 1Participant @lesjones1]

Hi Andrew,

I two theories that may explain what happened. One stresses in the casting causing the cut to close up on the blade. Two If the bottom surface was slightly concave with the axis of the curvature parallel to the cut then when most of the metal was removed the clamps would pull it closer to the table again causing the cut to close up. I cannot think of a way to show if either of these theories could be correct.

Les.

[Ian S CParticipant @iansc]

I might be wrong, but I seem to remember some one telling me years ago that unless there no way of doing it, use three clamps, you’v got to have enough room to do it of course, not always possible. Also the paper under the casting can help distribute the load. Ian S C

[Richard ParsonsParticipant @richardparsons61721]

Andrew Hi

OMG what a comedy of errors. Your rear holdfast looks very wrong it is sloping ‘uphill’. If the casting moves to the left it will loosen. Secondly you have no stubs fixed to the table so that the bottom of the casting nothing to push against. Thirdly you have nothing to hold the part you are cutting off down. Fourthly I think you are running your milling machine in reverse so the spindle can come undone and your cutter is tending to lift the casting from the table. My old Astra ran anti clockwise you are running clockwise but your feed is in the right direction for the rotation.
If you look at your cut section you will see where the seizure occurred. It is almost opposite the 6.3” mark on your ruler.
Personally I would have used my band saw for the job. The blades are cheaper. The work would be clamped in the vice with an extra holdfast and jacks for good measure.
Hum ho you will not make that mistake again.

[JasonBModerator @jasonb]

Richard if the blade were set to push the work down onto the table surely that would result in climb cutting and be more likely to cause a jam.

 

As for the rear clamp I think its the camera angle making it look like its going uphill when infact its just like the other one set at an angle across the table.

 

J

[Anonymous]

Here are a few further comments on the machining side. Sorry, if I missed anybody out.

 

JohnS: Yes, I was using a key. When I put the slitting saw on the arbor I did hesitate, but decided that since this was a 6″ cutter, and I would be taking reasonable cuts, 10mm, it seemed better to use a key. May be I was wrong. In my limited experience when a slitting saw spins on a deep cut it breaks anyway, as I’m not quick enough to knock the power feed off before catastrophe.

 

Norman: I agree, but the casting is 1.5″ thick at worst; I’m not that fit!

 

KWIL: Mating with the table was pretty good, I did run a file over the bottom of the casting, but it wasn’t really needed. The third clamp played no part, as there wasn’t one. What I will say in my defence is that it was never my intention to completely separate the casting with the slitting saw.

 

JasonB: Well spotted, there’s my mistake, no wonder they’re called saws rather than cutters. At no time did the saw get hot, or even warm. I dropped the clutch after each pass to check, and when I was picking up the pieces they were all cold. The cut surface is pretty flat; I can just get a 1.5 thou feeler gauge between a steel rule and the surface if I push a bit.

 

Les: Sound theories but I don’t know which one might be correct either, although I lean to the second one.

 

RichardP: I can assure you that the clamps were correctly fixed. My mill will run in either direction; the control panel makes no claim as to which it considers forward or reverse. Even if I reversed the spindle direction, and the saw, surely it would still tend to lift the casting. The only way for the saw to push the casting down is to climb mill. I don’t understand the comment on the rule; the numbers are purely arbitrary, the ruler is just to give a feel for scale. It happens to be at about 6″ because that’s where the rule was in balance on the casting.

 

Let me re-iterate that it was not my intention to separate the two parts of the casting with the slitting saw, but simply to ease the effort required by the hacksaw. Here’s what I think happened. On the final cut I didn’t leave enough metal, and as the cutter reached the end of its cut the casting moved, possibly a la Les’s theory, and momentarily trapped the cutter, causing it to shatter, before the casting itself factured. On the second part I left about 6mm of metal and everything worked perfectly. No funny noises, hot blades or shattering experiences, just a nice clean cut and a couple of minutes work with the hacksaw to finish off.

 

Regards,

 

Andrew

Edited By Andrew Johnston on 05/10/2011 17:39:15

[Gary BrookeParticipant @garybrooke63895]

Hi there Andrew glad to see you did not get hurt.As a beginner ihave just bought a sliting saw to cut some grovesin screw heads and wondered if any one could give advice as to what speeds to run at. Are they like grinding disk with a maximum speeds.
Also JohnS suggestion off running the abour without a key whilst I can see what he means. Is it best to leave that sort off operation till I have more experience all advice as always greatly received .
As always stay safe
Regards Gary

[Martin WParticipant @martinw]

Gary

 

If you want data on cutting speeds then this site should load a pdf file. If you go to page 13, the penultimate page then there is a list of cutting speeds for slitting saws in a variety of materials. It also offers advice for tooth count, speed variations versus material types and thicknesses etc.

 

Hope this helps.

 

Martin

[Martin WParticipant @martinw]

Gary

 

Just checked the site I suggested and page 13 of the pdf document is NOT the penultimate page , I assumed it was as it appeared at the bottom of the list but there were more!!! The good news that some extra information is included on these pages.

 

What’s the old adage ‘Measure twice and cut once’

 

Cheers

 

 

Martin

 

 

[Clive HartlandParticipant @clivehartland94829]

Its interesting looking at the photo Andrew that you have a two way cutting saw?

I have had a 6″ saw split in two and it was when it was breaking through in aluminium. I think that the gullet between the teeth matched the width of the remaining metal and locked up on the one tooth!

I remember I did not use a key but it still split. Ground down it made a good parting tool.

The ones I use have alternate teeth cutting L and R with an intermediate tooth that cuts central.

 

Clive

[Gary BrookeParticipant @garybrooke63895]

Thanks Martin.
Lots off information there will study it and have a go.
Regards Gary

[Sub MandrelParticipant @submandrel]

Interesting stuff. Perhaps it is bidirectionalmetallic blade?

 

I’m amazed (and concerned) that it did that at just 41rpm. I normally wear goggles for many tasks, but not normally using a slitting saw. A useful cautionary tale.

 

Neil

[Author]

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