Beginners Lathe

[Evan]

It's a hobby these days Bob. I used to do it for a living. I take as much time as I please.

Incidentally, if you make a bimetal strip as I suggested it is amazing how a tiny bit of differential expansion creates a HUGE bend. A iron lathe on an aluminum plate would be a sort of bimetal strip. A differential change of .005 could bend things by maybe .1" over three feet. The force would be in the tons.

Specifically, the yield strength for 6061 T6 is around 40,000lbs per sq". If I put my SB on a plate 1/2 by 8 by 3' that would give a cross sectional area of 4 sq". The force generated by expansion or contraction would be in the area of 160,000lbs or 80 tons. Should be enough to bend any lathe. The amount of bend can also be calculated but I don't have my scientific calculator handy. It would be the tangent of the angle created by the difference in length over the distance from the plate to the bed / plate length.



[This message has been edited by Evan (edited 03-06-2004).]

 

[sch]

Other resources for small lathes, perhaps
referred to on the minilathe site:
www.groups.yahoo.com Search for groups
7x10minilathe and 9x20Lathe
I know the 9x20 group has extensively discussed adding a back gear for L hand
threading. Photo sections of the group
may have this documented or a query to
the group will no doubt referr to a web site.
Bummer about having such a nice machine
shop just out of reach. Probably liability
concerns..... Steve

 

[bob]

Evan: Why would the lathe bend rather than the al plate?

On fits, I assume you agree that you couldn't make a living producing those fits on your SB

Bob

 

[J Tiers]

Evan, I submit the bolts would slip long before you got to umpty-bump tons force. Probably that would be the limiter on forces.

Howabout two greased shims on each bolt?

I'd use a chunk of "80 lb" steel plate, if I had it and could get it under the lathe.

 

[Evan]

J, yep, I expect the bolts would slip, but maybe not before enough stress was put on the lathe to warp it.

Bob, the lathe would bend because the modulus of elasticity of steel is far less than the yield strength of aluminum. In other words, it is way easier to bend steel than it is to crush aluminum. These are two very different properties. The flexibility of a material (modulus of elasticity) and the yield strength are not closely related. Think about pulling a piece of 1/16 piano wire longer. Then think about bending it.

Oh yeah, my main point here is just use steel, potential problem solved and cheaper too.

[This message has been edited by Evan (edited 03-06-2004).]

 

[Evan]

Bob, I'm not so sure about not making a living producing those fits. There is a deep pocket market for specialized astronomical gear that I know how to make. Time can be well repaid. I have had offers that I am not willing to accept. These devices I have made so far are for my grandchildren. My plan is to make very specialized instruments for well heeled buyers.

[This message has been edited by Evan (edited 03-06-2004).]

 

[JTToner]

Evan,

If its not too expensive I'd like to mount my SB9 on a metal plate. Since I'm neither machinist nor engineer I don't know much of the technical stuff. Which would be better, steel or cast iron? I'm thinking that iron since the lathe bed is iron, but I don't really know.

Also, if I went with steel, would 1/2" plate be less rigid than rectangular steel channel, say of 3/16" steel and 8"x2"x36"? I'm only guessing on the 36" length since I'm not in the shop at the moment. My bed is 42".

Since you have an SB9,maybe you can answer another question. If I can make the machine more rigid by mounting it on metal, I'd like to use a 1/2" instead of the 1/4" I'm now using so hopefully I can take more aggressive cuts. Either size will fit my QC tool post.

My concern is whether I'd be over stressing the gear train or the spindle bearings. What do you think?

Thanks,
John
Thanks,
John

 

[ztarum]

On the aluminum plate thing, both would bend. If they were fused along their entire length (like bi-metal strips) they would bend the exact same amount. The aluminum would be pushing and the steel bed would be pulling.
In reality, if the plate is only bolted at each end you will see most of the bending in the plate because the plate is much less rigid and there will be no force to keep the center of the plate from pulling away from the bed and bowing downwards.
If the lathe bed were a simple shape with a uniform cross section (like and I-beam), you could empirically calculate the bend that would occure. However, the lathe does not have a unifrom cross section which mean that the polar moment of inertia is not constant. Because of this a simple solution is not possible. Finite element analysis is probably the only way to get a true answer to this question. That or an expiriment. Anybody out there have a spare bed and some aluminum laying around

Zach

 

[bob]

I have an old SB and some al plate but somebody would have to lend me the measuring equipment as nothing I have would measure the effects here
Bob

 

[Harold Hunt]

Bob posted 03-05-2004 10:27 PM That the lathe is 4" thick and the al plate is 3/4.

True the lathe bed is 4" thick but it is mounted on stands 4 to 5 inches tall, these stands give the al a lever to work against.

While the al is not bonded to the lathe, the expansion will effect the flatness of the bed. Anyone want to unbolt the tail of the bed and insert a .10 shim and see if it affects the lathe?

Does anyone level their lathe after install? For close work at the chuck the error would be small, longer work at the tail stock would be affected.

If we put grease under the lathe and leave the bolts loose, that would allow the lathe to float on the al. My goal is to produce a more ridged machine not let it float.

The comment was made that the al would bend before the lathe. If that was the case, I would not need to install the al plate in the first place. :+(
Harold

 

[Harold Hunt]

For a test, measuring equipment might not be needed. From what I recall turning a long piece of stock full length then turning it end for end then running the tool full length is a old timers test for lathe bed straightness.

Harold

 

[Evan]

JTToner,

Steel or iron will work fine. They have the same CLE. As you saw in the South Bend forum I used a piece of channel iron (it's called that but is really mild steel). It works fine. I'm not sure if it will allow you to take heavier cuts but I am sure it reduces chattering. The SB9 is limited by its flat belt more than anything else as to how heavy it can cut.

BTW, I recently acquired a Unimat DB200. It has an aluminum body so last night I mounted it on a 1/2" aluminum plate. It really helped reduce chatter.

Also JT, a flat plate is nowhere near as stiff as the channel iron. Use the channel iron.

[This message has been edited by Evan (edited 03-06-2004).]

 

[j.carlson]

Buy American !!

 

[Evan]

I'm in Canada!!

 

[Evan]

Ztarum,

I am really tempted to try an approximation of the case but I'm not going to put an Al plate under my SB.

 

[ztarum]

I did a crude approximation and here's what I came up with.
A 50F temp swing will produce a differential expantion of 12 thousandths (for a 3 foot bed). If we assume a 1/2" x 8" x 36" aluminum plate and constrain it so that it cannot expand, a force of 12,900 lbs will be exerted at its attachment point at each end of the bed. If we further assume that the feet of the bed created a 6" lever, this force becomes a moment (or torque) of 6,500 ft*lbs. If we assume the bed of the lathe is a 6" high x 3" wide x 36" long bar of cast iron and apply the beam bending equations we get the results.
The center of the bed would have a dip of 25 thousandths. Sounds bad, but if we assume we are turning a 1" bar this will only result in a diameter error of 0.0003. Yep, three tenths.

A crude approximation for sure, but good enough to show that the impact will be small.

Zach

 

[J Tiers]

If we put grease under the lathe and leave the bolts loose, that would allow the lathe to float on the al. My goal is to produce a more ridged machine not let it float.
posted by Harold Hunt

Ummmmmmm........

The idea would be to use Belleville (cone) spring washers under the boltheads. They can be had with any selected pressure rating when rightly torqued, but would have the compliance to allow slipping with temp variance. This would be true with any mounting substrate.

And, yes, shimming will no doubt be required.

I do think that 1/2 inch stock is kinda thin. 40 or 80 lb plate would be better, adding mass and rigidity.

If you could find an old table-saw wing, that might be a good candidate if large enough. Cast iron (if older), and fairly thick and rigid.

 

[Evan]

Ztarum,

Nice work. However, in the real world the lathe may twist instead of bending in an ideal manner since the bed is composed of two beams that are unequal in strength front and back. The error could well be much greater than .0003

Channel iron is cheap.

1/4" channel iron with a two inch flange will be way more rigid than a 1/2" plate. Maybe even more rigid than a 1" plate.



[This message has been edited by Evan (edited 03-06-2004).]

 

[jeffeosso]

in canada?


try these guys

http://www.mts.net/~gerryt/prod02.htm

I have the CQ6230(FI-900AG) Light-Duty Lathe with 17-Inch Max. Swing Over Gap

jeffe

 

[Evan]

Seriously blurry picture of the mill/drill. Just who are these guys? Bad grammar too.