Author Topic: Building a model Drag Saw  (Read 5495 times)

Offline Brian Rupnow

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Building a model Drag Saw
« on: August 28, 2018, 06:09:17 PM »
I am going to build a model drag saw. The power for this saw will come from the second i.c. engine I ever built--The Kerzel hit and miss engine. There are so many things that I don't know about a drag saw that it makes my head vibrate, but I am a fast learner. I am going to use a driving mechanism similar to the one in the attached picture.  One of the first things I have to design is a "dog clutch" which enables me to shift a lever and put the engine "out of gear" so that I don't have to shut the engine down to stop the saw blade moving. I have at one time or other built everything in a drag saw (on other machinery) except for a dog clutch. Since I have the capability to cut my own gears, this saw will have a gear drive instead of a sprocket and chain. If you like this sort of thing, then follow along. Parts of this story may be boring, but parts of it will be something new.


Offline crueby

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Re: Building a model Drag Saw
« Reply #1 on: August 28, 2018, 06:27:22 PM »
Excellent!!
If you need any details about how they work, there are lots of patents about them that you can find on Google Patents, they usually have great diagrams of linkages.

 :popcorn: :popcorn: :popcorn:

Offline Alyn Foundry

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Re: Building a model Drag Saw
« Reply #2 on: August 28, 2018, 07:13:30 PM »
Hi Brian.

I too will be watching with interest.

The " dog clutch " was the el cheapo version apparently. More sophisticated saws had a friction clutch which massively reduced the impact on the engine whilst engaging the blade to cut.

Having actually driven a full sized Witte engine powered saw they are rather a handful. You hold the blade off the cut using an extension to the top linkage in your right hand whilst engaging the clutch with your left. Now you're right arm is going back and forth as you gently drop the blade onto the log. Not for the feint of heart, I can tell you!

Oh, don't forget that the connecting rod is made of wood. It's a safety device, designed to snap if something goes radically wrong.

Cheers Graham.

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #3 on: August 28, 2018, 08:22:44 PM »
This first bit is a head scratcher. Where do I start?--well, the green colored part is the available shaft end and flywheel hub on the side of the Kerzel opposite from the governor weights. It is 3/8" diameter x 1.125" long. The brown colored part is a steel shaft extender 3/8" i.d. x 7/16" o.d. that slides over the Kerzel shaft and is pinned there by a 3/32" diameter cross pin. (And maybe some Loctite). The dark blue gear is a 24 dp x 20 tooth gear which has an internal bore of 1/2" and has a bronze bushing (red color) pressed into it. It also has three "dogs" machined on one side. It actually "floats" on the shaft extender and is not keyed to it in any way. There is a shoulder on the end of the brown shaft extender which the gear rides up against. The  pink part is a steel sliding bushing with three "dogs" machined on it that will fit into the three slots between the dogs on the blue gear. It has a "groove" machined into the outer diameter, which a pair of pins on the shift lever fit into. It is capable of sliding along the shaft extender, and is also keyed to the shaft extender so it can not rotate freely. (I haven't quite worked that last bit out yet.)


Offline Jasonb

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Re: Building a model Drag Saw
« Reply #4 on: August 28, 2018, 08:37:34 PM »
You will find it slips into drive easier if one side is sloped.

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #5 on: August 28, 2018, 09:46:28 PM »
Okay---I got it!! Two pictures here, one with dog clutch engaged and one with dog clutch disengaged. The green cross-pin on the extreme right hand end is for my electric starter.


Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #6 on: August 28, 2018, 09:47:52 PM »
You will find it slips into drive easier if one side is sloped.
Thank you Jason. I've never made one of these before.

Offline Florian Eberhard

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Re: Building a model Drag Saw
« Reply #7 on: August 28, 2018, 10:05:43 PM »
Hey Brian

You had me thinking in the totally wrong direction first. What I had in mind by reading "drag saw" was some kind of powertool dragster saw  :Lol:

Something like these:

But I just learnt what a drag saw actually is  :embarassed:


With your clutch: don't you make it extra complicated that way?
I would make the dog and the gear one part. That way the coupler doesn't need to be able to slide sideways while the already has a bushing (wich of course can slide and rotate at the same time)

Florian
« Last Edit: August 28, 2018, 10:13:06 PM by Florian Eberhard »

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #8 on: August 28, 2018, 10:23:22 PM »
This is it for today. Since I have now modelled the dog clutch, I will decide whether to stop designing and make some real metal parts or do some further design tomorrow.

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #9 on: August 28, 2018, 11:23:16 PM »
I may very well put a ramp on the driving dogs. I just did a little test out in my main garage. I put a spruce 2 x 4 in my vice and got one of my handsaws. My son was here and had a stop watch built into his cell phone. I sawed at what seemed to me to be a reasonable speed, and he timed me for three different 10 second intervals. Seems like 14 strokes per 10 seconds is "reasonable". Six times 14 equals 84 strokes per minute. The ratio between my two gears is 108 divided by 20 equals a ratio of 5.4 to 1   So---5.4 x 84=453 rpm at the engine crankshaft. I don't remember what speed my Kerzel engine runs at, but I will probably start it tomorrow and check it with a laser tachometer. I know my single cylinder flathead engine runs at about 1000 rpm. I think the Kerzel runs slower than the flathead, but I will have to check it and see. If I have to have a second stage gear reduction built into this drag saw to make it work, I would much rather know now at the design stage than after it is all finished.

Offline Steamer5

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Re: Building a model Drag Saw
« Reply #10 on: August 29, 2018, 06:14:03 AM »
Oh goody.... :popcorn: :popcorn:

Hi Brian,
 Following along!

Cheers Kerrin
Get excited and make something!

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #11 on: August 29, 2018, 01:57:40 PM »
First thing to do this morning was machine ramps on the driving dogs. Others have told me this makes the dog clutch much easier to engage. Now I'm off to my garage to start the Kerzel engine and decide what rpm it runs at.

Offline crueby

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Re: Building a model Drag Saw
« Reply #12 on: August 29, 2018, 02:04:19 PM »
This first bit is a head scratcher. Where do I start?--well, the green colored part is the available shaft end and flywheel hub on the side of the Kerzel opposite from the governor weights. It is 3/8" diameter x 1.125" long. The brown colored part is a steel shaft extender 3/8" i.d. x 7/16" o.d. that slides over the Kerzel shaft and is pinned there by a 3/32" diameter cross pin. (And maybe some Loctite). The dark blue gear is a 24 dp x 20 tooth gear which has an internal bore of 1/2" and has a bronze bushing (red color) pressed into it. It also has three "dogs" machined on one side. It actually "floats" on the shaft extender and is not keyed to it in any way. There is a shoulder on the end of the brown shaft extender which the gear rides up against. The  pink part is a steel sliding bushing with three "dogs" machined on it that will fit into the three slots between the dogs on the blue gear. It has a "groove" machined into the outer diameter, which a pair of pins on the shift lever fit into. It is capable of sliding along the shaft extender, and is also keyed to the shaft extender so it can not rotate freely. (I haven't quite worked that last bit out yet.)


Brian, what keeps the blue gear from sliding sideways towards the moving dog when the dog is disengaged? Seems like it could slip sideways on the red bearing?

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #13 on: August 29, 2018, 03:31:20 PM »
I set up the Kerzel engine this morning and ran it. My laser tachometer tells me it is turning at 960 rpm. This is twice as fast as I require, in order to get the saw to make 12 to 14 strokes every 10 seconds. I can not make the engine run any slower, so I will have to put one more set of gears in the drag-saw gear train. I currently have a 5.4:1 ratio in the gears on the saw, so I will have to install a second set of gears at 2:1 ratio to give an engine speed of about 900 rpm.

Offline Brian Rupnow

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Re: Building a model Drag Saw
« Reply #14 on: August 29, 2018, 03:32:52 PM »
This first bit is a head scratcher. Where do I start?--well, the green colored part is the available shaft end and flywheel hub on the side of the Kerzel opposite from the governor weights. It is 3/8" diameter x 1.125" long. The brown colored part is a steel shaft extender 3/8" i.d. x 7/16" o.d. that slides over the Kerzel shaft and is pinned there by a 3/32" diameter cross pin. (And maybe some Loctite). The dark blue gear is a 24 dp x 20 tooth gear which has an internal bore of 1/2" and has a bronze bushing (red color) pressed into it. It also has three "dogs" machined on one side. It actually "floats" on the shaft extender and is not keyed to it in any way. There is a shoulder on the end of the brown shaft extender which the gear rides up against. The  pink part is a steel sliding bushing with three "dogs" machined on it that will fit into the three slots between the dogs on the blue gear. It has a "groove" machined into the outer diameter, which a pair of pins on the shift lever fit into. It is capable of sliding along the shaft extender, and is also keyed to the shaft extender so it can not rotate freely. (I haven't quite worked that last bit out yet.)


Brian, what keeps the blue gear from sliding sideways towards the moving dog when the dog is disengaged? Seems like it could slip sideways on the red bearing?
Nothing at the moment. Will have to figure out a retainer.---Brian