Indicator Base Plate for Lathe

[Susquatch]

Maybe a year ago or so @Dabbler and I were talking about various ways to mount an indicator on a lathe. I have used the normal gammut of methods from putting a mag base on the tool post, on the cross-slide, on the chuck itself, and on the gear box. The problem with the last three is that cast iron has low magnetic permeability. It doesn't hold a magnet as strongly as steel does. Nobody likes a mag base coming loose in the middle of using it.

@Dabbler suggested a piece of solid plate steel attached to the gear box behind the chuck. And so it came to pass that a small project was added to my long list.

A 1/4" thick steel plate provides plenty of magnetic attraction. So some time after that, I acquired a piece of 1/4" steel plate about 10" x 5". I don't even remember where I got it. My plan was to use the two existing M6-1.0 SHCS's holding the backsplash in place on the headstock behind my lathe to mount the plate. I located the screw spacing using transfer screws in the backsplash screw holes to transfer the locations to the back of the plate. Then I drilled the plate holes a wee bit bigger (1/4" vs 6mm) for some wiggle room.

Today I mounted the plate for the first time to test it. It worked GREAT!



This is my medium sized NOGA holder. At some point, I think I'll paint the base plate the same colour as my lathe. But for now it's fine as is.

More importantly, it works great. No need to use the relatively weak permeability of my cross-slide or headstock anymore. I'll still need to use my chuck on occasion and obviously also my toolpost. But this completely eliminates the need to use those marginal surfaces.

A small project, but very useful, and I'm very happy with it.

For anyone who is interested, the ground rod shows about 1.5 thou of runout in my 3 jaw. The end is about 5 thou so there is some concentricity error. But, it's a 3 jaw. I don't really expect better.

 

[David]

I had no idea that cast iron was less magnetic (if I’m understanding correctly) than steel.
Cool idea. I might steal that if I may.

 

[mickeyf]

Good idea - I like this!

 

[Susquatch]

I had no idea that cast iron was less magnetic (if I’m understanding correctly) than steel.
Cool idea. I might steal that if I may.

Absolutely! I'm quite stoked about it.

Same goes for stainless. In fact most kinds of stainless have very low magnetic permeability. But I'm guessing you did know that.

Another little tidbit is that thin steel (sheet metal) doesn't work well either. That's because it reaches field saturation earlier which limits the buildup of lines of force. 1/4" plate is fine. I don't know at what thickness it reaches its maximum. It probably depends on the kind of steel.

Don't forget about your tool post and tool holders. They are usually also made of steel and they make a great place to mount an indicator holder too.

 

[1018Machine]

Nice idea Susquatch!

 

[ChazzC]

Nice, Sus!

Before painting the plate, I'd try placing a piece of paper between the plate and your base to see how much pull you may lose (may be negligible, but worth checking).

With regard to plate thickness, K&J Magnetics has a handy (test-based) calculator to compare different steel thicknesses (you'll have to make some assumptions with regard to the magnet specs, but you can use the NOGA base dimensions & pull to approximate the specs).

 

[kstrauss]

Perhaps I'm misunderstanding but the data at https://nvlpubs.nist.gov/nistpubs/bulletin/12/nbsbulletinv12n1p1_A2b.pdf
seems to show that the magnetic permeability of cast iron and low carbon steel vary due to composition and heat treatment but are similar at perhaps 100 to 200 and vary somewhat with temperature. See graphs on pages 6-7.

Is it possible that the poor attraction to your headstock is due to the paint thickness or lack of flatness rather than the low permeability of cast iron? I've never had problems with the attraction of my indicator base to my Myford cross slide (unpainted).

 

[Downwindtracker2]

On my mill, there is so much bondo on the casting my mag base lamp hardly sticks. For serious mag drilling, we sometimes tack welded plates. You need some thickness to hold.

 

[Susquatch]

Before painting the plate, I'd try placing a piece of paper between the plate and your base to see how much pull you may lose (may be negligible, but worth checking).

Good idea. I'll do just that.

With regard to plate thickness, K&J Magnetics has a handy (test-based) calculator to compare different steel thicknesses (you'll have to make some assumptions with regard to the magnet specs, but you can use the NOGA base dimensions & pull to approximate the specs).

I looked at it but don't readily see how to use it effectively. I'm not familiar with N grading.

 

[Susquatch]

Perhaps I'm misunderstanding but the data at https://nvlpubs.nist.gov/nistpubs/bulletin/12/nbsbulletinv12n1p1_A2b.pdf
seems to show that the magnetic permeability of cast iron and low carbon steel vary due to composition and heat treatment but are similar at perhaps 100 to 200 and vary somewhat with temperature. See graphs on pages 6-7.

Hmmmm....... I guess that depends on your (their) definition of similar.

Is it possible that the poor attraction to your headstock is due to the paint thickness or lack of flatness rather than the low permeability of cast iron?

I very much doubt it. There is a very marked difference. I can rather easily dislodge the base when I sits on any of the cast iron surfaces including my cross-slide and 3 jaw chuck which are not painted.

I've never had problems with the attraction of my indicator base to my Myford cross slide (unpainted).

Well, again this depends on many factors. Your definition of good attraction among them. Another important factor is the individual character of your cast iron - they are not all the same. In fact, cast iron varies all over the map. Yours could be just better than mine.

To test the difference, put a solid bar/post on the top of your mag base, put the assembly on a test surface and then evaluate the force it takes to tip the base over by pulling on the top of the post. A fish weight scale is a perfect way to measure it.

 

[David]

When I was calibrating the mill, I noticed that my Noga base stuck like crazy to the steel extension I made but not so well to the cast iron column but I put it down to poor placement on the cast iron.

 

[ChazzC]

I looked at it but don't readily see how to use it effectively. I'm not familiar with N grading.

The N grading is specific to Neodymium magnets (more info than you want about magnets), but what you can do is enter the base dimensions with a thickness of say 1” and play with the N grade until you get the NOGA stated pull; that will give you a value that you can then use to see the impact of thinner plates. I think this is what I did when I decided on 3/8” plates for my camera ceiling & wall mounting plates:

I do use 1/4" plates for bench plates, but gravity is on my side there, and I don't use them to mount the heavy camera rig:

Ring flash not shown on the camera; additional cameras and other accessories can also mount to the rig at the same time.

 

[kstrauss]

Intrigued, I did a little testing.

I don't have a fish scale (I mostly trust the scales at the grocery) so I tried using a digital kitchen scale for some very crude tests.

I secured a cheapie mag base to the ground cast iron of my Myford. With a force of about 5 pounds attempting to tilt the magbase's arm, the arm moved due to its poor joint rather than the magbase releasing from the cast iron. I did the same test with the magbase on a 3/4-inch thick lump of O1 with a ground surface. Same results.

Certainly very far from definitive but it indicates that the attractive force to cast iron is probably not a limiting factor considering the typical forces on a DTI.

 

[Susquatch]

Certainly very far from definitive but it indicates that the attractive force to cast iron is probably not a limiting factor considering the typical forces on a DTI.

I agree with that. I have used my cast iron surfaces many times in the past and I probably will again in the future. I didn't mean to imply that there was zero attraction to cast iron or that cast iron limits the useability considering typical dti forces.

What I am saying is that steel is usually much better than cast iron. I've had mag bases fall off of my headstock or get knocked over on my cross slide at force levels that were a complete surprise. One little bump and off she goes. That should not happen. The attachment is there but it should be stronger than it is and a steel plate delivers the goods.

To put it differently, I would say that my cast iron is marginally ok, but my new steel plate and my tool post are both MUCH better and totally adequate. I just tried to push the magnet base off using the standard NOGA arms. The arms bend on the steel base, and the magnet falls off on the cast iron.

I'll try to objectively quantify the required forces after I dig my fish scale out of the boat.

 

[Susquatch]

I had a wee bit of shop time today, so I did the fish scale testing I wanted to do. I put a post on the top of my NOGA mag base and then using the fish scale, I recorded what it took to pull the base over as shown in the photos. I marked the location on the post for consistency. Here are the results:

Lbf Location
7.0 Cast iron gear box mount
14.5 New 1/4" Base Plate
13.0 New Plate with paper
16.5 Side of Tool Post

Note that paper under the mag base does affect the force needed, so I probably won't paint the plate. Perhaps, I'll blue it instead.

We cannot directly compare results with the Cast Iron Cross-Slide because it is a horizontal surface, but it was only slightly more than the gear box.

Clearly, the plate is better than the gear box even taking paint into account.

The tool post is much thicker steel than my plate and provided a stronger attraction. This suggests I should have used 1/2" plate instead of 1/4". Maybe if I ever get a nice piece of 1/2 plate, I replace the base.

As previously mentioned, cast iron varies considerably. Your results might be better or worse than mine.

 

[ChazzC]

Note that paper under the mag base does affect the force needed, so I probably won't paint the plate. Perhaps, I'll blue it instead.

Sounds like a plan

Clearly, the plate is better than the gear box even taking paint into account.

The gearbox cover may be thinner that the plate and the paint most likely much thicker (& uneven thickness) than the paper.

 

[Susquatch]

The gearbox cover may be thinner that the plate and the paint most likely much thicker (& uneven thickness) than the paper.

Yes, it's possible that the gear box casting is thinner. But I'd give that relatively low odds. It is painted, but the paint is thin.

On the other hand, my cross-slide is definitely thicker than the plate and is unpainted. It wasn't significantly stronger than the gear box despite its gravity advantage.

Regardless, the plate is clearly superior in my case so I'm glad I did it and the numbers support that.