How to draw a frustrum?

[gostraight]

how is this?

[gostraight]

there you go

[DARISC]

Just for kicks - two cylinders @ 45 deg.

[TimT]

neat idea but way overpriced

They are a specialized, semi-expensive tool. However the amount of (time) money they save in fab time easily makes up for the expense of the tool.

When making fishmouths and fitting tubing the pipemaster really cuts down on trial and error fit up, wasted pipe...waste time

I bought 3 of those tools....and they have saved me hours of time (money)...

[Zeke]

Originally posted by Walt Fricke
PS:

Tim - I had seen those nifty variations on the single plane body shape tool. And the price! I might buy the one for inch and a half if I ever get to making my own cage (plan - bend and cut and fishmouth everything, tack weld, and take to real welder to finish).

And the URL I gave doesn't work, but if you chop off some of the end you will get to the site that lists their various calculators.

Walt

I've built a roll cage. All you need is a chop saw with a metal blade. Two cuts at 90* to each other, the first one being the angle at which one tube meets the other. Instant fishmounth with a little shaping with an angle grinder.

For instance, a 90* weld will call for 2 equal 45* cuts. A 30* weld will call for a larger 30* cut and a smaller bite at 60* on the other side, more or less inversely proportionate to the angle of the weld. IOW, a deeper 30* cut of about 2/3rds more than the 60* cut. YMMV.

[randywebb]

you guys realize that the Greeks figured this out over 2,000 years ago...

[BertBeagle]

Originally posted by rsNINESOOPER
fabbers trick, cut the end of the length of tubing at the angle you would like it to pass through the sheet metal, then cut a slice at the same angle again so you wind up with a very short piece, now you have yourself a tracing jig.

That works good or, with a little less effort and if it is a one time deal, try this:

Wrap and tape a piece of thin pattern paper around the tube you want to pass thru the panel. Slide the paper "tube" down the real tube and trim the paper to get the angle you need. Trace around the paper onto the panel and cut your hole.

[Green 912]

My method is to cut a pipe slice or make a dummy from cardboard by wrapping it around the tube, taping with masking tape to firm it up, cutting to shape and tracing the hole from the inside of the little cardboard dummy
I have found that many times this is the best way for me as I can fit test and adjust as needed. For some reason calculations are ether right or way off. Simple systems work.

Dirty trick when welding a tube through sheet metal. drill the hole right where you want the center of the tube to be and stick a section of tube through the hole. Bend the sheet metal to conform to the tube angle using the tube as a former. Make little slits in the sheet metal if the angle is high and cut some if needed as well to keep from deforming the fire wall too much. After you have the tube in place you have nice little sheet metal flanges on each side to weld the the tube to.

I too have the expensive Tube Masters. They stay in the box until I have a hard or compound cut to make then they pay for themselves when you make the last fish on a compounud bend tube you just spent 2 hours tweaking to fit just right and did not just cut the tube too short. Anyone that has made up enough tubing has done that at some point and it is a royal b!tch as it only happens when you are short of time or that type tubing to make a replacement.

I used to hand cut tube but sprung for another expensive toy. It is nice to make compound cuts in one trip across the shop. They can get busy sometimes.

[Wavey]

Getting back to the original question -

This is really simple: a straight cut on an angle through round tubing creates an elipse that is specific to that angle. No complicated math or guesswork required. Any simple graphics or CAD program can draw the elipse.

I'm posting the patterns for some typical angles, for 2" tubing, as a jpg. Anyone should be able to copy this to their desktop and print it. Directions for enlarging or reducing to your tubing size are included. I can also e-mail as a pdf if anyone needs that, or provide any other file type. Hope this is helpful!

[Bill Verburg]

Originally posted by randy webb
you guys realize that the Greeks figured this out over 2,000 years ago...

Menaechmus, Euclid, and Apollonius were all Greeks that investigated the properties of the ellipse and other conic sections but not quantitatively.

You need trig(or software that knows trig) to quantitatively analyse the dimensions of the ellipse wanted here.

simplest formula for finding the major axis would be
(diameter of the tube)/(sin(angle between the plane and tube))

the minor axis dimension of course is always = (the diameter of the tube)

[klaucke]

Originally posted by Bill Verburg

simplest formula for finding the major axis would be
(diameter of the tube)/(sin(angle between the plane and tube))

However, it is worth noting that if you use this function, it is the angle b/w the plane itself and the centerline of the cylinder (as I believe Walt used), not the angle b/w the tube and a vector normal to the plane (as some people here have used for their angle measurement, in which case cos would be applicable).

[Bill Verburg]