Am I doing this correctly?

A few years ago when I only had a very old Drummond lathe the plastic bushes in the rope cutter on the saildrive on my boat had worn so that the cutters had a large gap & I was concerned that instead of cutting rope it would jam between the blades.

I sent the unit to the manufacturer because the advertised plastic bushes (note the price £ 50-00 a pair) seemed too small. He advised me that the blades were worn & I needed a new cutter.

I could imagine a bloke sitting in his armchair making up the price as it went.

How much?- £ 525-00 inc VAT

Oh dear!!--just cannot afford that

Well order it now & I will do you a special deal at £ 500-00

Sorry I will have to go without as wife will not allow that

Tell you what as you are an existing user & will do it for £ 475-00

At that point I rang off & researched the plastic. For £25-00 I bought enough tufnol whale to make 12 rings. Each pair last a year but I have since found R1000 lasts 2 years. You can see them in my picture & now I have a decent lathe & a jig set up for the face plate they take 15 mins each.

However, I now have decided I really do need a new rope cutter. See 2 pictures I have researched the steel & had a Rockwell test done & it seems they are just made mostly from 6mm * 100 gauge plate. Not even hardened.

I have set up my rotary table with a piece of aluminium as a protector – see picture& have clamped the first cutter to a piece of 3mm steel to make a pattern & as a test. I intend to drill 3mm holes in the roots of all the teeth & log the position on my DRO. Then I will machine the outer circle & then the splay of the cutter edge along the outer edge of the 3mm holes.

After that it is a problem.

I hope to tip the table on edge, like cutting gears, & using a double angled slitting saw I should be able to cut the teeth, just following the holes. The teeth already having been eased by the 3mm holes. They do not have to match exactly – just cut rope etc.

Then mount it on a face plate on the lathe & cut the inner circle which is the important part of the cutter as it runs on the bush or the shaft. The thick central part is a simple turning task.

Is that the way to do it or is there a better way & should I be looking for anything special with the gauge plate to enable easy machining

Any forum advice appreciated

Sorry for the long post

The cutter in pieces

The cutter assembled

Part set on plate

The other blade

Holes drilled in pattern

Edited By Sam Longley 1 on 28/01/2018 19:19:46

Sam,

On the premise that if it looks like a rope cutter, is made like a rope cutter and then performs like a rope cutter, it must surely be a rope cutter

To my unpracticed eye you seem to be doing all the right logical things and going about it in a systematic fashion. There will no doubt be an expert on the forum that knows better than me but if it does the job in the end does it really matter a jot? Carry on the good work

Regards Brian

Sam,

Your proposed method of manufacture sounds fine if you don't run the cutter right through so you end up with a bevel edge.

However..... Are you really sure about your material? Gauge plate will have no resistance to corrosion in either fresh or salt water. Many years ago I was involved with development trials of 'Spurs' rope cutters for the RNLI and the metal parts of those were a cast stainless of a grade similar to 316. The original bearings were a fairly soft plastic consisting of 2 washers and a thin strip, the washers were put either side and the strip around the inner diameter, lifespan was a couple of months! They were soon changed to a moulded section in two pieces if I remember correctly of some Kevlar incorporated material which got us 12m. I can see your thinking re using a high carbon steel for hardness but I will be very surprised if the thin section of the sharp edge has not disappeared in a couple of weeks through corrosion. By the time you have made the blades and had them heat treated I imagine you will be looking at £100 they will definately need renewing or at the least resharpening in 12m time ad finitum, that doesn't make £475 sound too bad! As you mention a sail drive and that is probably not going to be in regular use you also run the risk of corrosion scale on the faces of the blades that need to pass each other jamming the shaft!

With your machinery what is stopping you machining back the faces of the existing blades a little (ok that will thin them and reduce strength but you probably won't need to take much off to true them up, the edge can be put back on with a die grinder) then making new 'bushes' accordingly to position the blades? At least they are a reasonable grade of stainless and are not rusting away!

Paul.

Paul

Thanks for the comment about corrosion.Had not given that much thought.

Will 316 stainless machine Ok though?

I only have a Warco WM16 mill & I am not impressed with its performance,It was hard work just cutting the 3mm steel plate I am doing for a pattern ( Want the pattern so I can put the cutter back on the boat)

When the metal hardness firm thought it was gauge plate I was quite happy as I was told that machined Ok.

But stainless 316????

Edited By Sam Longley 1 on 29/01/2018 14:31:07

Sam,

316 machines OK as long as you don't allow the cutter (either milling cutter, drill or turning tool) to rub without cutting as the surface will work harden and take the edge off whatever cutter you are using. If you use a carbide cutter and keep it cutting you should be ok. 316 in itself is relatively soft just very tough and work hardens.

Assuming what you have now is some years old it will definitely be made of something around the 316 grade - if it wasn't it would have rusted away by now! Who ever told you it was gauge plate is sadly a bit off track!

Paul.

As an alternative to Tufnol you could consider either acetal or Vesconite. I have made many cutless bearings and rudder bearings from Vesconite and it performs extremely well - a typical Vesconite cutless bearing for a 1”prop shaft will last for at least 5 years. **LINK**

Both acetal and Vesconite have very low moisture absorption and therefore little dimensional change, unlike just about every other engineering plastic.

Agree with what others have said re gauge plate in salt water - won’t last 5 minutes. 316 is ok to machine with low speeds and a decent feed rate, but will work harden if you let the cutter rub. I’m not sure the shape of the teeth are all that important - you could probably mill the overall shape out and then use an angle grinder to cut the teeth. If you make all the parts from 316 make sure that the blade which runs on the bush has a bit of clearance, as otherwise it will gall up and seize solid, which is not at all what you want to happen.

Not being a sailor I had no idea what this was so I had a quick Google and all is now clear! Apparently your original had unhardened 316 stainless blades *** LINK *** Now I am off to find out what a "saildrive" is

John

Can you just do a quick 'magnet test' on the original blade cutter material , to show us that it is a 'stainless steel' ?

It spent last season immersed in salt water - a magnet test is really not necessary

Interesting project this.

It looks like an Ambassador stripper from before they made them in two parts so you don't have to pull the propeller off to fit them.

The brochure says that they are made of the same stainless steel as the prop shaft so as to prevent electrolysis, which would lead us to assume 316 except some shafts are made of Duplex stainless. The YBW test said that the Ambassador was 316 and a rival product called Spurs was hardened 17-4PH.

With boat parts being ludicrously expensive it's always a temptation to make your own if the equipment is available, and I'm wondering about it myself now!

Edited By Brian Corrie on 29/01/2018 18:13:43

I don't see that the teeth need to be precision. Holding an angle grinder in a jig with a table and guard would allow you to cut and sharpen teeth.

Sam, Yachting Monthly “road tested” your unit and quoted the blade is made of unhardened 316 Stainless Steel, so that should resolve your material specification, in respect of gauge plate I would expect it to corrode if only in the vicinity of sea water let alone immersion.

Dave W

In my younger years I was very boat and that is what I would, have called a 'Z-drive'.

Built in rope cutters! Do they work on poly bags and fishing net?

John

By fluke I have a picture of myself pointing at the plug in the bottom of the saildrive i had to have the boat lifted in Cherbourg because I had water in the oil. It turned out that I had used a copper washer on the plug & should have used a simple O ring.

If you enlarge the picture you will see the Brunton self pitching propeller ( note the blades have flopped down)& just in front of it you might be able to make out the Ambasador rope cutter

Neil

The cutter is very effective. On my 2 solo round UK trips I hit nearly a dozen badly marked crab pots, 3 off Blakeney in one night. It cut them with ease. I have caught nets & that has been cut OK . Polythene is a problem & i did collect a very large sheet that stopped the engine completely between boulogne & dieppe. It did manage to cut some of it & i got prop turning to get 1.5 Kts but needed a diver to remove it. The cutter did chop a lot of it. It is also effective for cutting the long strands of sea plants that fishing vessels dislodge . One gets a lot of that around the Channel islands

Edited By Sam Longley 1 on 29/01/2018 22:33:59

Steve

I chose Tufnol at first because water is supposed to be a good lubricant for bearings made of it. Plus it , like R1000, is very cheap & it does not take me long to make the bearings. I have found that the metal has worn as much as the plastic & it has worn out of true as well so i have to keep taking up the tolerance. I am not sure about acetal but i will certainly research vesconite

Thanks for that

Steve

made contact with the supplier re Vesconite & stocks not available in UK . they promised to quote for the material ( supplies come from South Africa) but did not bother so i had to drop that one. thanks for suggestion any way. i will try acetal later on

On another thread it was suggested that some closure to threads would be a nice thing.

So to do that, here is a picture of the finished article.

A brand new professional made one is now £ 590-00 inc VAT. This cost me £ 44-00 for material, £ 18-00 for a milling cutter ( still Ok) a broken 3mm drill bit & some wear on a couple of cutters plus about £ 4-00 worth of tufnol & a £ 1-00 of bolts

I did find it odd that HSS cutters were the best for machining the stainless steel. Especially the boring bars which I ground from blanks. That was quite an enjoyable achievement.

I learned a lot & if I made another (or a couple for friends) I would have them laser cut & save a lot of time.I have been offered the use of a 6 inch rotary table & a good solid one would have helped a lot

So here is a picture to prove that I did it !!!!!!!

Edited By Sam Longley 1 on 20/02/2018 19:22:41