Fwd: [OpenLathe] Re: Unimat & Metalmaster Small Bar Ways Lathes

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Sam Putman

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Mar 14, 2010, 1:47:21 PM3/14/10
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---------- Forwarded message ----------
From: charcad2006 <charc...@yahoo.com>
Date: Sun, Mar 14, 2010 at 9:06 AM
Subject: [OpenLathe] Re: Unimat & Metalmaster Small Bar Ways Lathes
To: Open...@yahoogroups.com


 

Wayne,

I'll be sure to look up Mason's book.

>Mason's lathe is done entirely with bar stock.<

Bruce and I are kinda fixated on stiff round bars for the "ways" of lathes. Big and small lathes. Unimat originally used bar ways, too. I'll point out that this design originated in Central Europe in the aftermath of WWII. A time when there was great disruption in industry and the economy. Similar to now, although arising from different causes.

At the present time round steel bars finished to -.001" tolerance are readily available by the inch. Speedy Metals and other internet retailers offer this. Alloy 4142, "Heat Treated, Turned, Ground & Polished". That's the "HT-TGP" cited. Yield strength of 90,000 psi.

fyi, this is better than standard drill rod, which has an allowance of -/+ .006" TIR per 12".

> 8020 Extruded aluminum is pricy. C-channel or square tubing
> is much cheaper. Also, the tubing does not have to be square,
> I have purchased some nifty 1"x4" square tubing.

This is worth some discussion.

What you say is generally true about pricing. I can locally buy A500 tube drops for 85 cents/lb and A36 hot roll remnants for 75 cents/lb. Meanwhile 8020 Garage Sale on eBay wants about $25 per 2' of 2" x 2" extrusion (2020 fractional series). Plus shipping. Which is still not much for a small lathe project. There are other considerations.

As supplied tolerances:

ASTM A36 hot roll mill tolerances (angle iron, C-channels, etc) are 1/8" per 12" vertically, horizontally and for "twist". I have a build of the 1964 Popular Science power hacksaw design well along in my shop. This uses 2" x 2" x 1/4" angle and other parts. Believe me, look not to A-36 for as supplied precision.

ASTM A500 (tube steel spec) allows 1/16" inch per vertical, horizontal and "twist" per 12". Think of a slowly twisting piece of stiff spaghetti.

Some tube mills claim to do better than this. But you have to buy direct in a semi-ruck load to get it. If you go through a steel service center their tube steel is a commodity and we're back to the general A500 specs.

"8020 t-slot aluminum extrusion" 8020's advertised tolerance is .01" per 6'. This is already much better than A36 or A500.

8020 is produced with 6061 aluminum alloy, T6 heat treatment. 6061-T6 has a minimum yield strength of 40,000 psi. According to my Materials Science & Engineering Text, anyway. We're already in the region of cast iron. Especially Sieg Machine Tool Company quality iron castings. I have found sand inclusions in the beds of a couple machines I examined closely. Just painted over and sent down the line.

Interestingly 8020 Inc. itself only claims "a minimum of 30,000 psi".

For a real Unimat clone we might go as small as 1515 series for the bed. That's 1.5" x 1.5" with a single t-slot. This would be for a 3" swing.

Workability.

It's well known that aluminum is much easier to work than steel. 8020's combination of greater as-supplied precision and workability offers a real possibility to hand-scrape it dead flat using simple tools. i.e. resharpened wood chisels and old files.

The Old Method of "the rule of three" is still valid. Three surfaces that are hand-scraped to fit interchangeably are dead flat. In the case of 8020 you have eight such "surfaces" to use with just two discrete pieces. ;-)

> I have discovered that places like skatebearings.com
> have a huge selection of bearings for low prices. It
> might be possible to adapt some for a spindle bearing.

eBay and others are great source of low cost bearings.

Mark

--- In Open...@yahoogroups.com, "waynegramlich" <Wayne@...> wrote:
>
> Mark:
>
> I've always been more interested in a small hobby sized CNC
> lathe than the big ones. If you can get your hands on the
> book "Building a Small Lathe" by L.C. Mason, it is worth
> reading. I got a copy via inter library loan at my local
> library and, well, um, I kind of made a photocopy. Mason's
> lathe is done entirely with bar stock. I personally think
> the round bar stock idea is a better idea (for a small lathe.)
> Anyhow, I just wanted to remind people that the Mason book was
> out there.
>
> A few other random comments:
>
> - 8020 Extruded aluminum is pricy. C-channel or square tubing
> is much cheaper. Also, the tubing does not have to be square,
> I have purchased some nifty 1"x4" square tubing.
>
> - I have discovered that places like skatebearings.com
> have a huge selection of bearings for low prices. It
> might be possible to adapt some for a spindle bearing.
>
> - I have had some very good success with HDPE (High Density
> PolyEthaline -- i.e. cutting board plastic) as a linear
> bearing surface. It drills like butter and runs very
> smoothly over round rods.
>
> I hope this project moves forward.
>
> -Wayne
>
> [much snippage]
>

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ben lipkowitz

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Mar 14, 2010, 6:50:26 PM3/14/10
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sorry to always be the party pooper but this guy has no idea what he's
talking about.

round bar ways are not "stiff" by any measure.

tensile strength has nothing to do with modulus of elasticity, even if
they use the same units.

cast iron is not used in lathes because of its tensile strength; it's used
because of its mass and high vibration damping factor.

you can't scrape together 3 pieces of 8020 and expect it to be perfectly
flat - they can form a saddle shape. the "3 plates" method is done with
round plates for a reason. there are many other better ways to get a
straight beam anyway.

TIR doesn't really matter when you're bolting it to a beam of 8020 or hot
roll.

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John Griessen

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Mar 14, 2010, 7:31:19 PM3/14/10
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ben lipkowitz wrote:

> tensile strength has nothing to do with modulus of elasticity, even if
> they use the same units.
>
> cast iron is not used in lathes because of its tensile strength; it's
> used because of its mass and high vibration damping factor.

Hmmm.... Aluminum tuning fork? Yep, that's what many low frequency ones are
made of.

The biggest stopper in cutting deeply with a lathe is chatter...
Sharp tools help and stiff mounting at the tool holder matters,
and I'm talking one of the old fashioned V shaped ways machines
made of cast iron.

JG

John Griessen

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Mar 14, 2010, 7:42:53 PM3/14/10
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John Griessen wrote:
> ben lipkowitz wrote:
>
>> tensile strength has nothing to do with modulus of elasticity, even if
>> they use the same units.
>>
>> cast iron is not used in lathes because of its tensile strength; it's
>> used because of its mass and high vibration damping factor.
>
> Hmmm.... Aluminum tuning fork? Yep, that's what many low frequency ones
> are
> made of.

This way thread makes me think a decent machining saddle shaped lathe cross slide
might be made from a good small rod adhered to a slab of cast iron
with the slab making one flat way and the rod the other way -- in place
of a V shaped way.

How do you make the hollow shape to mate with a rod accurately though?
This is looking way difficult!

Since it's fitting over a rod, it could be from a drilled hole.
Is there a way from the past to "true up" a drilled hole by lapping with
abrasive and rods of increasing size?

No way?

John

ben lipkowitz

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Mar 14, 2010, 8:11:06 PM3/14/10
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On Sun, 14 Mar 2010, John Griessen wrote:
> This way thread makes me think a decent machining saddle shaped lathe
> cross slide might be made from a good small rod adhered to a slab of
> cast iron with the slab making one flat way and the rod the other way --
> in place of a V shaped way.
>
> How do you make the hollow shape to mate with a rod accurately though?
> This is looking way difficult!
>
> Since it's fitting over a rod, it could be from a drilled hole.
> Is there a way from the past to "true up" a drilled hole by lapping with
> abrasive and rods of increasing size?

hope it's not too modern for you, but you can just bed the rod in epoxy
filler such as moglice or JB-weld. Use an angle grinder to cut the groove,
and then a DTI to make sure the rod and front face are parallel

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