I'll also add that you should make sure your cutting edge is somewhere close
to the centreline of your tube. For ID cuts, it can be a touch higher and
for OD cuts, it can be a touch lower. You can quickly gauge the height of
your cutting edge by lining it up closely with tip of your live centre in
your tailstock or chuck or collet a truly round bar of known diameter, set
your verniers to its radius and use the depth gauge end to set the tool tip
to correct height. You just hold the verniers at the end of the bar with the
depth gauge pointing downwards across the end face to give centre height.
Or, of course, use a gauge if you have one:)
Make sure your tube's material grade is correct. If the cutter is sharp and
is close to centre, but your cutting is gummy, then you could have an
incorrect material grade and/or temper. Soft Al-alloys not only are far
*inferior* structurally, they are also much harder to machine and become
quite gummy and they can also blunt your cutting edges. You need to make
sure your material is structural grade ie. has a UTS of at least 250 MPa and
yield strength of at least 200 MPa and that its temper isn't lost.
O-rings are magic and it will surprise you how well they can seal even for
dodgy grooves but they need to be made with the correct widths and depths.
Ie. the width needs to be equal to or beyond a minimum and the depth (in
reference to the mating surface) equal to or less than a maximum (but not
too much less).
Math: this is real concern here. If you need a hand with the Maths, I (and
I'm sure others) are more than willing to assist. If you can provide some
numbers of what you're trying to achieve in terms of either/and desired
thrust, desired total impulse or what materials you have available or what
volume or flow constraints you have, I'm happy to go through it with you.
Troy
-----Original Message-----On
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
Behalf Of Keith and Mary Stormoo-ring
Sent: Saturday, 16 October 2021 11:06 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Worth it to continue?
Hi Joe,
In addition to the safety items that others have mentioned, a few tips for
grooves in thin wall aluminum tube from a hobby machinist whose machinedlots
of them on a number of different lathes.I want
1. I make a cutting tool out of a broken lathe tool by grinding the width
with a Dremel motor cutoff tool and give it about a 10 degree positiverake angle.
the lathe.
2. Keep the length of the tool very short so it is very solid.
3. Use a steady rest if you cannot cut within about 1 inch of the jaws of
Hopefully the lathe bore allows the tube to be set very near the jaws andnot
tightened very tight so it does not distort the tube shape.just be a
4. take very shallow passes (like 0.005") and then the final pass should
spring pass where you cut at the same depth.edges
5. I usually cut very close to 50% of the wall thickness at a maximum.
6. Practice on some scrap tubing.
7. Use some fine wet/dry 300-400 grit or Scotch-Brite to remove sharp
without really rounding the corners.
Hope this helps,
Keith
At 08:17 AM 10/15/2021, Jack Hanna wrote:
Hello!
I've been having problems, mainly with machining and in general when
trying to make a hybrid nitrous-paraffin ground only rocket engine it's
been quite rough, and the lack of a mentor and machining knowledge has
led to about three weeks worth of work just being a couple of tubes. I
have been trying to cut o-ring grooves for probably about 6 hours, and
have just completely failed. I technically have a couple of grooves in
the outer tube, but I worry that if I purchase lower profile o-rings
that it still won't be sufficient sealant and it will just be a
glorified bomb when I go to ground test it.
That's the other issue, the last chance I really have to test it is on
october 30th, and that deadline has me kind of cramming, and the
absolute last thing I want to do is to rush it, and make a bomb instead
of a rocket engine. I've also been bad and skimped a little on the math
and other parts for time, and my plan really just boils down to shoving
nitrous through a tube with paraffin, standing very far away, and
hoping for the best. Is it worth it to try and continue with this
subpar design, or should I scrap it and pick it up later when I have
more time / experience?
I really want to do this, but I also want to know if it's going to
actually be severely dangerous or just have no chance at success. How
much can you bodge a rocket engine and still have it work ok? Thanks.Â
- Jack