Donkey (Mechanical saw to everyone else)

So, as I really like Donkey's I found myself my own little mechanical one. Really nice chap by the name of Dave was advertising this for sale so I popped round this morning and picked it up. It is a set of Blackgates castings and the castings themselves have been well machined. I show it with the original quarter horse boat annchor in both pics but this motor has already been donated to my local scrap man. I have a nice new, quiet and efficient replacement in a fetching blue you can see in the second pic. I want to place this new motor here in the new configuration as the finished item will have an overall footprint of 7" x 18", I already have a sheet of ally plate ready for it and this is nice and compact to sit on a trolley under the bench.

The thing is, this is not really useable as it is. There is no cutoff switch for instance, there is no damper either and I think that is important.

So tomorrow I am going to reduce it to it's component parts and begin a rebuild - oil dashpots for instance on the shafts are something I will be adding. But if it was yours - what would you add and where could I adapt a damper from? I look forward to suggestions.

graham.

Looks good. I would suggest a car hydraulic damper( hols open boots and the like or it.. A NVR switch with a rod to activate it would be wise? Also a short roller at cuttu=ing height on the motor side to add support. MEW had a few good suggestions for bandsaws a while back.

Have fun! Ian

Hmmm not sure but have to agree that Dave really is a nice chap. Cant wait to see the final product. Nice to know its finally going to get some use. Good luck.

Cheers

Dave

Graham,

As its bedtime, without going in to detail I would suggest a small dashpot damper as is used on the Kennedy hacksaw. You can see some pictures on lathes.co.uk, or let me know if you want a bit more info.

If you'd looked around whilst at Dave's you could have looked at the Kennedy he had as well - unless it had gone by the time you got there.

Phil

Unlike Johns Axminster device the little Kennedy power hacksaws work just fine with a damper. The Kennedy takes a standard hacksaw blade and is probably around the same effective size as the Blackgates machine.

As I remember things correctly from a quarter of a century (ish) back the Kennedy in the departmental support workshop had a damper dashpot about 2" diameter with a fairly loose fitting "piston" effectively comprising a pair of thick washers with a couple or three large holes drilled in them. The amount of damping was adjustable via an L shaped rod sticking up through the middle which rotated one "washer" with respect to the other so altering the amount of free oil passage through the piston. About 1/3rd of a turn adjustment covering a range sufficient to let the beast cut adequately fast on solid bar yet be reasonably under control when dealing with reasonably thick wall tube. Waterpipe and the like. Very thin wall tube, such as Speedframe, was a bit too much for it. I imagine there was some sort of flap valve included so that the oil didn't get spalshed out when the unsympathetic hoiked the arm up rapidly. There was still noticeable drag when lifting. When the beast was running a certain nodding of the carrier arm could be seen. Probably not enough to indicate actual lift but certainly suggesting a reduction in blade down pressure on the back stroke. Some sort of geometrical magic going on.

Allegedly oil specification is important but ours seemed to work just fine getting a splash of whatever was handy when I noticed things getting low. No one else cared and the place wasn't exactly a regular haunt so for much of the time the oil level would ahve been wrong. Dashpot seems to have been a refinement rather than necessity as the saw still worked pretty well with it near empty. Decent write up on www.lathes.co.uk whith probably enough detail in the pictures for you to scale off.

Clive

Hi Graham, I'll follow this thread with interest as I bought one of these hacksaws a while back, no motor but everything else is there. Can you tell us what motor you bought and where from, please? I'm looking forward to getting mine working as I'm using a Rage chop saw at the moment for cutting slices of round and angle mild steel and it scares me witless, as well as pouring hot swarf on to my head!

Regards,

John.

Clive,

Well done, nearly remembered it right after 25 years!

The first "washer" you mention is in fact the piston, about 1/4" thick, with the holes in it. The second one is a thin disc (shim thickness) which can move up and down to either cover the holes or uncover them and let oil pass round it in the lifting position. Adjustment as you say, by a rod, threaded at bottom, which can vary the distance between piston and shim.

The only thing that stops the oil coming out of the top is a rubber boot, such as on car steering arms.

The makers specified the dashpot oil as "the special Wakefield Cresta V oil" (which was supplied). Though any reasonably thick oil will do.

There was a very strong pull down spring on the machine, the dashpot smoothed out the action.

Phil

Well then, looks like I will be doing a "how to".

John, I happened to have a motor under the bench which by happenstance is nearly new. I acquired it with another machine but I have since fitted a different motor. This one was in the "stores" as it were. It is a quarter horse 1300 rpm motor though from my reading almost anything will do, within reason. The Kennedy saws were fitted with 1/6th HP motors from Hoover. Obviously this one is a bit more modern, the one fitted was, as to be expected on a saw which was made some time back, of the massive cast iron casing variety. Probably nothing wrong with it at all but but it is decidedly heavy. And I have a newer one.

Phil, grateful for the description of the dashpot and that shouldn't be too hard for me to duplicate and again I should have everything in the "stores". The idea is to buy nothing and use what I have and looking at the job the only thing I am going to need to purchase is the small rubber or neoprene "boot" to cover the dashpot. I have seen them for sale somewhere. I just can't remember where, they were all pretty colours too. I would like a yellow one please!

The spring for downforce is also very useful and going from what John has said about the Axminster saw I was a bit puzzled as to why you would need to modify a brand new machine to make it actually work. Then I remembered, many people today can design stuff, but they often fail when it comes time to actually building a working example.

One design feature I will be stealing from the Kennedy machine though is the spring loaded release plunger sited along the top. Well today's job will be to dismantle it to component parts and remove the paint. Update later.

graham.

As I'm having a "shaky" episode, therefore fit for not much, I got out a Kennedy that has no oil in the pot and took a few pics of the dashpot to illustrate what Clive and I are talking about.

They are in an album "Kennedy Dashpot" here **LINK**

Note the gaiter is not the proper one, and doesn't work very well. The proper one is a concertina type.

Note also the curved sides of the piston. This allows the angle of the piston rod to change as the arm goes up and down without the piston binding in the pot.

The pot is 1¼" dia. and 3¼" deep.

Phil

PS  The creator of the album "Jo Robinson" is the Missus, not my alter ego.

Edited By Robbo on 26/04/2014 15:33:24

Graham,, Bogstandard is right, damper not needed but a weight on the arm is.. I build one of these a few years ago and it saves a lot of work. Two improvements i made were to extend the frame to take 12" blades and the crank arm to give a longer stroke, Otherwise it buzzes like a wasp in a jam jar.

Eddy.

Edited By Eddy Spriggs on 26/04/2014 17:41:30

Well I did have half a chance to do a little bit to the saw today (in between hanging a couple of new doors - most people buy new curtains or paint a bit. This one changes doors?) anyway first picture of the proposed "new layout":

Couple of issues (Dave was very clear about all the defects when I went to collect it, it was a project he never got around to. I have some of those myself.) and this is one of the most immediate problems. At some point during it's life due to the blade tensioning "system" (not very good by the looks) the blade has had a go at the vice. I was thinking of machining a bit off the side of the vice and put a piece of hardened gauge plate in there, it would certainly stop it happening in the future:

This is the actual hacksaw frame which holds the blade and rides along the top guide bar. I am not really impressed. Two bolts go through the frame behind the two large D shaped "Bearing Plates" and by adjusting the length of the bolts pressing on the two plates it takes up the slack on the bar so the hacksaw frame doesn't wobble side to side. This is "assisted" by the two sort of shim plates really held captive by two covers. All rather strange. I will definitely be changing the two D plates for something else. Phosphor Bronze, I have plenty of that. Steel against steel as a bearing surface doesn't really go. And what are the two bits of shim plate all about? Got to be able to do something a little more creative with that.

Here is another shot of one end of the frame:

Tomorrow I will get all the paint off because that can be doing while I fit the door handles the missus will no doubt be getting from that Big DIY store we all love so bloody much!

graham.

Hi to All

Nice find Graham rugged and straight forward piece of machinery and a useful addition to any home workshop.

A damper is not required as stated by other posters on a small machine just some means of adding extra weight for cutting thicker material.

Aim for around 85 to 105 strokes per minute and cutting on the "Pull Stroke" on a small machine.

A comment for John on the Axminster Hacksaw the "Air Spring" damper will be more effective if the top mounting pivot point is is moved to the right and fixed closer to the the saw frame pivot.

Anyone interested in "Power Hacksaws" do a search on "You Tube" and enjoy.

Eric

Although you can get away without a damper on these small hacksaw machines if you only cut solid material of a certain range of sizes, probably covering a couple of inches, around the sweet spot where downforce weight and blade tip co-operate to give best cutting you will run into trouble with tubes and material significantly outside that range.  With the aforementioned departmental Kennedy the usual sign that it needed a dashpot top up and, probably, a quick survice was a blade jam or breakage when cutting thin material. Usually the speedframe 1" square tube construction system which we used a lot of. Correct setting of the damper also made a big difference as to squareness of cut. For solid bar, especially at the larger end of the machines capacity, minimal damping was needed for a reasonable rate of cut. In thin materials and speedframe minimal damping resulted in a very unsquare cut but with the damping set appropriately a decently square end was produced.

Ordinary hacksaw blades aren't really stiff enough for power machines. Best blades we found were Keranous(?) Progressive Teeth type which could be got in a thicker (double?) than normal version as well as standard type. The extra stiffness gave noticably better results and the progressive teeth faster cutting. Gawd knows when the stash was got but when it was gone replacement proved impossible as the firm had disappeared. The dashpot also seems to reduce the pressure on the blade during the return stroke leading to longer life.

The Axminster style gas spring to control down force is just plain wrong and needing to add weights plain silly. Its arguable that a machine with a sliding weight on the arm with its position calibrated for different materials, thicknesses et al could be nicely engineered to both work well and be inexpensive to produce but for a firm of Axminsters size to supply a machine that doesn't work out of the box is plain daft.

Our Kennedy never had a cover over the dashpot in my time but I suspect the last few bits dropping into the cavity where what jammed up the dashpot giving me the opportunity to find out what was inside.

Clive

Edited By Clive Foster on 27/04/2014 11:53:17

So the parts spent last night in the Depainting Tank, yes I actually have one. The bits this morning:

I have been able to assess things much easier and the Saw Frame itself has a coggle on with only three out of the four bearing surfaces touching the surface plate:

Do you do all three and make the fourth come into the plane or just the one at the back? Have to make a few decisions here as this is not a simple casting, it is quite complex with several important factors to take into account; square, level, plane.
It doesn't matter what happens, the bearing surfaces are pretty much "fit where they touch" right now so anything is going to be an improvement.

This shows the blade actually installed. This is just unacceptable. The other blade holder is the same and a screw merely tightened up against the blade. I am not too keen on holding the blade by just pressing up against the other side of the casting. No wonder it took a skim off the vice!

This is my other Donkey, bit big for my workshop at home. Cutting a few slices off there for Mr Stevenson

graham.

Interesting observations on damping. My badsaw slows to a sudden and complete halt on cast iron if the cut is more than 2" long. 8" of 2" square MS balanced on the end got things moving again. It has no such problem with steel and blade is sharp. In contrast, cutting a 3/4" steel tube the other day as soon as it was past the top 'whoosh' to half way through and threw the blade. Would have been quicker to use the hacksaw :-/

So my thought at the moment is some combination of a damper and a weight.

Neil

Graham,

If it was me I wouldn't use hardened gauge plate as side packers for the vise because if it wanders it will still mark it but cost you a blade.

By all means fit a packer but make it sacrificial.

Neil

the issue you hah may be not enough teeth on the blade in contact with the work. On the top lots of contact then suddenly you're only cutting wall . You really need about 3 teeth cutting at any one time. I have issues cutting tube and you just stick a bung in it of wood and cut through the lot. There's lots of explanations why in various books but filling / bunging it works most of the time.

Regards Ian

The other way of controlling things when cutting tube with the bandsaw is to just take the weight with a finger under the end of the saw arm. It's much the same with the power hacksaw. Ian S C

Graham.

I've had one of these hacksaw's for a number of years & it's served me very well ( I get no fun from using a hand hacksaw!!)

A few improvements I made to mine are :-

1. I fitted a second bar parallel to the existing slide bar which helped in stopping the blade running off.

2. Fitted flip top oilers to the slide bar & shafts.

3. Secured the vice feed screw in the moving jaw so the jaw moves in or out when the screw is turned

4. Adding a weight to the handle improves the cutting.

Mine is mounted on a box with the motor enclosed underneath for the only reason that the motor is rather large ( It's the only one I had at the time) I must keep a look out for a smaller one. The whole lot is mounted on a shelf fitted with runners so it can be slid into the cabinet when not in use.

Ian

Well, as I had been good yesterday I was left to my own devices today

Made quite a bit of progress and a picture as they say, paints a thousand words. I just painted the bits instead (RAL 9001 for those of you interested, one of my favourite shades of white):

I had looked at the arm and the cranks and though not overly used the wear marks were apparent in places. This signalled a rethink of the bearing surfaces and the crank action. This was the original crankpin:

The hole in the crank arm has been over-bored already in the above picture and you can see the new configuration here:

As shown in the pic of the main arm I have made some "Top Hat" bushes for this main arm and the smaller crankpin on the sprocket, I made two Top Hats and put them back to back on the sprocket crankpin and the bearing surface is now 10mm diameter as opposed to the original 3/8". I also made new brass bearing covers for the hacksaw frame itself.

Not satisfied with that, I then bored the rear vice jaw, turned the thread back a bit on the bar and turned a groove in it, made a brass retaining plate and drilled and tapped it for the plate:

Oh, and if anyone thinks I have been polishing the bits up you are right. It's a friction thing:

graham.