Slit sawing a shaft question

[graham-xrf]

I want to make a replacement shaft for a carburettor butterfly valve.
The original got damaged with a "twist" during the effort to to extract it from the carburettor body, because it was so seized up by some corrosion. The engine is from a real vintage aircraft - so much regrets. I have a scheme, perhaps not too clever, but that is why I am asking here on the right way to go about this.

The pictures tell it all. The need is to saw a slit from each side to create the slot for the round butterfly valve disc to slide into, secured by a countersunk screw across the shaft. The saw I found is 1.75 inch diameter, HSS hollow ground, 1/32" teeth x 90 of them, and it seems to just about jam into the slot, and seems to match the original slot just right. It has a 5/8" hole in the saw for the arbour.

The end of the tweezer in the picture is just to stop it rolling away onto the floor. Picture taking here was a comedy!

The butterfly shaft measures about 0.217", probably to fit through a 7/32" hole (0.2188").
I do not have a mill, nor access to one, but I do have a South Bend 9C lathe (as relative beginner!)

My thought was to try and mount the saw on a rod between the chuck and a tail-stock centre, or even between centres, kind of creating my own 5/8" shaft mill. Then to contrive some way of supporting the (new) shaft onto the cross-slide. OK - rubber elastic bands are not involved, and I already took some teasing, but the idea is there. Is this the way to do it?

How does one hold a shaft secure in place for a cut like this, even on a mill?
There are incidental questions - like how to make up an arbour shaft to turn the saw.
I would, of course, like to try and use my lathe. Is the plan unrealistic?
All advice is welcome, even if it is to find and pay a mill machinist!

I

 

[f350ca]

Its doable Graham. A milling attachment like this for your lathe can do a lot of work.

I used this a LOT before I bought a mill. It replaces the compound. The vice can be swung so you could hold the shaft vertical and bring the shaft sideways into the slitting saw mounted on an arbour held in a collet or three jaw chuck.
I've made new shafts but the ones I've done were simply half the shaft milled away and the butterfly bolted to the flat. On an aircraft your style would be a lot more secure.
Imagination is the only thing holding anyone back.

Greg

 

[royesses]

If you have steady hands you could use a dremel with a fiber reinforced cut-off disk to cut the slot. Support each end of the shaft clamped in its own v block and freehand the slot. You could make up a guide to keep the blade parallel to the shaft. The butterfly needs to be a tight fit in diameter and thickness so as to minimize air leaks at the ends. As Greg has stated a milling attachment for the lathe would be better. Just some thoughts on how it could be done.

Roy

 

[woodchucker]

If you have steady hands you could use a dremel with a fiber reinforced cut-off disk to cut the slot. Support each end of the shaft clamped in its own v block and freehand the slot. You could make up a guide to keep the blade parallel to the shaft. The butterfly needs to be a tight fit in diameter and thickness so as to minimize air leaks at the ends. As Greg has stated a milling attachment for the lathe would be better. Just some thoughts on how it could be done.

Roy

I don't think that would work. I don't think it would be straight enough.

 

[woodchucker]

I am new to milling has I have had a milling attachment for a lathe for quite a while. I don't think I would do this with a lathe milling attachment.
I would take an aluminum block, cut a V in it, shallow. The block will be sacrificial since this piece may buckle w/o support. Clamp your piece to the aluminum if you have Kant Twist clamps, or C clamps with a V that's how I would clamp the shaft to the block.

clamp the aluminum down to the table with the shaft on the side, and plunge the slitting saw into the shaft. Go easy, use plenty of oil. I would do the slit as the last operation. I don't think you could cut threads or the indentations after slitting it would be too delicate.

 

[Dave Smith]

I agree with Roy--use a reinforced 1 1/2" Dremmel type cutoff blade and carefully cut your slot by hand---they also make very thin 3" cutoff blades for a die grinder to get your slot started through the shaft---just take it slow and careful---you will then use a hacksaw blade to finish the ends of slot to correct length--I have made a new shaft with a slot years ago, but the newer ones just use the flat on the shaft---you could make yours with a flat and two screws to hold the butterfly--I think it would be just as strong as your old one, since it looks like a steel shaft instead of brass.---I always save all my used die grinder discs so I have them in all diameters for the needed small ones at times----lots of us have very steady hands for these small delicate projects---the more you do the better you get---I have also collected a large supply of new carb shafts through my 73 years-----Dave

 

[Chipper5783]

I think there are probably a few ways to approach that. No doubt that the cheapest and most straight forward would be to go see a small commercial shop. Even if they just did the slot and you did the rest - it would still be much faster.

However, on your own, with just a lathe - it should be possible. Most of the time & most of the effort will be in making tooling or set up components (which is pretty common - making the actual component is a minor part of the exercise). First of all, sort out your slitting saw. Make up a nice between centers arbor. Make it as short and fat as the rest of the project will allow (think this all through). Perhaps 1.25" bar, step down to 7/8" near the work, make a stepped sleeve for the outer end of the arbor - 7/8 near the work, then back up to 1.25" (you can use a regular 5/8 nut for the loading). The key is not required - just make sure your rotation is tightening the loading nut.

Make your self a sacrificial holder that you can clamp in the tool post, like woodchucker says, except not a V block. Just get a piece of bar that you can grip in the tool post and drill a cross hole near the end (say 7/16"). You will hold the piece vertically and plunge the slitting saw through the bar and into the project piece.

Now for the piece. Sort out the slit first. I would start with a piece of rod larger in diameter (say 1/2" dia) and a good bit longer. The extra material is to make it easier to hang onto and makes it stiffer. Once finished you cut to size and length. Turn
a shoulder for part of the length and thread the far end - sort of like making a bolt (mark the position in the chuck and center drill). Then pop it in that holder you made and tighten a nut so the rod won't move. Even add some adhesive, such as blue Loctite. Of course you referenced the hole so you can get the slit where you want it to be. You may have to finish out the last of the slit with a jewelry saw, perhaps a scroll saw blade.

As woodchucker pointed out, the piece is delicate. Fill the slit - I suggest brass shim stock and epoxy. Now, machine the shaft - pretty straight forward. Hold the large end (preferably in a collet, or at least a 4 jaw marked to the same orientation from the previous machining) and support the outer end using the center you drilled earlier.

Do the cross drill and tap. Cut the handles off, clear out the shim stock and epoxy (heat).

You described your self as a beginner. This project is entirely possible. All the steps are fairly easy. Working out all the steps and the sequence - that is called machining.

Take you time. Work safe. A common issue people have when they start out with machining is that they do not build solid set ups. Take your time, think every step through, keep everything as rigid as possible (seriously, make everything at least twice as strong as you think it needs to be).

Let us know how you make out. David

 

[graham-xrf]

Oh my! Thank you all so much!
My first reaction has to be "Yay - the plan was not completely fanciful unrealistic after all".
Next reaction was "Duh - yeah! That is how we don't spell "shaft" in the title, and don't know how to edit it".
Sorry about that! My thanks to Greg for the pictures of the milling attachment.

I get it that the piece cannot take much torque or force once the slit is cut, so use sacrificial support, and pack the slit for any operations afterward, like cross drilling the screw hole. Would one set it in a drilled aluminium square-section block?

Roy and Dave are clearly free-hand skill experts, and Dave, with a collection of carb shafts knows exactly what this takes.
Even so, I think I might go with the other David (Chipper5783). Having a nice solid machine with no "coffee-induced hand tremble", and be able to advance a slide with the shaft into a saw which is running on an accurate between centres arbour, sounds far safer for me.

I am guessing that this is might be slow(ish) cutting speed operation, with plenty of oil, and the sequence might be..
1. Make up the arbour (Er.. arbor - Please forgive the pre-Teddy Roosevelt spellings. I am from the UK!)

2. Turn and thread the shaft. I get it that part of it could be fatter for the slitting operation, but turning it down to final diameter, even if the slit was packed, seems awkward. If a finally turned shaft can be supported well enough, even in or on sacrificial aluminium, then going for it in one operation has some instinctive appeal.

I am not sure about this. David's description of slot filled with brass shim and epoxy sounds like something turn-able.
I might try making a slit in a finished turned shaft, and resort to a packed slot if we mess up and need to try again.

3. Get the supports and slide advance sorted out. A milling adaptor.

4. Make the cuts. Looking into the slot, the radius of a saw cutting first from one side, then from the other, creates a curved sides "V" at each end of the slot insides, and there is then enough clear slot length between the points of the "V(s)" for the disc of the butterfly to fit. One could perhaps advance the slit saw just enough to let in a hacksaw, then use it to square up the slot ends, but if slit sawing from each side works OK, we eliminate the hand skill operation.

I have nothing but admiration for Roy and Dave's freehand skills, but I think it needs years of practice.

There is lots to do here. What might be a suitable saw RPM?
I do have a (so far unused) electronic motor drive that allows slow speeds at high torque.
Other than taking a long time over it, is there such a thing as "too slow"?

 

[DaveInMi]

Material costs for this seems low. You could mess up a few and still not be out much but time. This is cheap education and so much more fun than having someone else do it.

 

[tweinke]

might be a stupid thought, but why couldn't you just mill a flat and mount the butterfly to that. Plenty of carbs are done that way.