CNC bow forms?

[Robertfishes]

I was talking to a neighbor today, his family has a wood shop with a CNC machine I showed him my "Binghams" plans and asked him about helping me cut out the forms and making some permanant templates from something like formica. He told me his brother was the CNC guy and he would talk to him,I know they need numbers to input to the program and this may be a problem,But he seems very willing to help me out since their not too busy at this time.
Once the program is created and proven accurate he said they can save it on a disk for future use. If this works out I'm gonna ask him if we can make me a couple of extra forms for my friends.

[razorsharptokill]

That would be awesome. Even if you had the numbers you could provide to others so they could go to their local cnc machinist. That would be cool.

[Bob Sarrels]

My one piece Bobbcatt recurve press is made from alimunum on a CNC machine.  It is absolutly as true as you can get.

[Robertfishes]

I'm hoping his brother can program the CNC to do the forms,if they would have just spent another $100,000 on the CNC they could just scan it and it programs itself? I'm gonna meet with them monday afternoon. I could just do it with a band saw and drum sander

[bjansen]

I get the best results using a router and a template I make out of MDF.  I make the template to match one half of the form, srew it to my glued up plywood blank and use a pattern cutting bit to make the cut.  Then just flip the template to the other side and do it again.  You can achieve smooth, consistent results (and at a perfect 90 degree angle).

[Robertfishes]

what my problem is , well one of them is that I have no real wood working tools or experiance. 20 years ago I built wood decks, screen rooms, carports,etc. About all the tools I needed and used was a 10" mitersaw, skill saw, hammer and a drill.

[Andrew Wesley]

most CNC's can be run very accurately by a skilled hand. if they are using it for wood then i would just draw out your lines on the blank and then use the hand dials... or, most CNC's also come preprogramed with a ton of different settings to cut curves, blocks and angles. i'm sure one of those would fit or be very close to the forms...you just need to figure out your zero setting. but then, i dont know what kind of CNC they have. but there's an idea to ask them.

[Robertfishes]

I am going back tomorrow(monday) and will talk to the brother, he is the CNC man. we will likely cut the first one (half of the form) out of some scrap and see how it looks, if it is good then we can "mirror" the image to the other side to make the form complete. Brad that sounds like a great way. I don't have a router but I may sell another one of my Widows or Winchesters...I bought a 2 hp dust collector yesterday ,it was on sale...

[LPM]

In one of my former lives, I used to do all the CNC programing for Winnebago Industries Hardwoods plant

Modern software for CNC programing makes short work of duplication of curves or "arcs".

I assume that this CNC machine is a router machine.

 I would start by tracing out two perpendicular  lines from a carpender's square on paper big enough for the entire form.  Then trace the shape for the form within these lines.  Now you have to make a grid using the horizontal and vertical refrence lines that you drew.  

The original perpendicular lines are the referance lines that you will measure from as you plot your points along the shape of your form.

You can see that the finer the grid the more accurate the duplication will be.  I think that 1/2" grid lines would be plenty good.  

The CNC programer will start by CAD drawing a scaled grid to match the paper grid you used for the tracing.  He will then enter the points along the grid using the measurments you made on paper.

The CNC programing software should be able to use these plotted points as a means to creat an "arc".  Once the arc is created the CNC programer can enter tool paths and asign the correct tool for the job as well as cutter feed speed, RPM, lead in, lead out, depth, and direction.

A very important part of programming is the "spoil board".  This is the wood placed under the part you are making or in your case the form.

 It is usualy MDF partical board.  If the CNC you will use has a suction table then your spoilboard will have gasketing and holes drilled through it for suction to be aplied at the proper points to prevent the form from slpping as it is cut.  

The spoil board or creation of the spoil board is a good oportunity to check to see if the shape is going to match your hand traced sheet.

The program normaly cuts into the spoilboard just far enough to ensure that the part will be completly cut through.  Maybe .050" or so. Not much.  Once you have run the program on the spoilboard alone, you can see the shape lightly cut into the spoilboard.

If you have access to CNC by all means use it.  If your programer is good at spoilboards or Jig / Fixtures  there is no limit to the shapes for accent strips for risers or riser shape duplication to perfection.  Be sure to use the correct cutters and speeds to avoid burning or chipping the wood.  This is the voice of experience and hard Knocks!................

[rkmnt2]

Like LPM described....there's no magic to CNC machining...it's all about numbers. I build my forms that way because I have access to the equipment and I do the work myself but also because if I want to change something it's very easy to see on the computer how it will effect everything else and I have an electronic record of the patterns history.
  I think if I didn't have the knowledge and the cnc equipment available, hand tools would do just fine. Look at Hera's build-a-long! But then he makes everything look easy! lol