Turning screw threads down

[T Bredehoft]

Cast iron has a completely different grain structure than steel. It doesn't transmit vibration so it damps chatter, etc. It works well for backing plates for that reason.
It also lends itself to being cast to shape then machined to finish size, shape, thread, and so on. That's for the high production outfits.

 

[mariner3302]

https://www.ebay.com/itm/183170266117?ViewItem=&item=183170266117 Just received the 'Students Shop Reference Handbook'. Going to read through it. I need to get the Machinery's Handbook but can't decide whether to get the latest one or a less expensive version that may not have info on 3d printing, new materials, etc...

 

[Charles Spencer]

When I make a new mounting plate I thread it first, then screw it onto the spindle to finish all other operations and that way I know everything will be perfectly aligned.

That's what I did when I made a back plate from a barbell weight.

https://www.hobby-machinist.com/thr...in-your-shop-today.14637/page-265#post-416421

 

[Technical Ted]

It would probably be better to make one from scratch than modifying one. I read somewhere that cast iron is better than steel for things like backing plates and the like because cast iron absorbs vibration better. Is that true and if so, should I forgo steel in place of cast iron for spindle caps, backing plates, etc? I thought things were cast because of cost mostly.

I agree it's best to make one from scratch. Or, sometimes you can buy a cast iron (CI) blank and just finish the machining on it.

As you and others have said, CI is best. A decent grade of CI is what I would call "dead" material meaning that it doesn't have the stresses in it that some carbon and alloy steels can have. Another plus is that it is a dissimilar metal from your spindle and isn't as likely to gall.

That said, every one I have made has been out of a carbon steel weldment, because that's what I had on hand and it works OK for my use. Haven't had a problem yet, although I do agree that CI is better/best. The ones I made are plenty good enough for my hobby shop. The last one I made was for my SB 15" 8" 3-jaw chuck. I welded it up and put it in for the day in my wood stove and cranked up the fire. I checked it a couple of times and it was beyond cherry red into an orange. I left it in there until the next day so it cooled down as the fire went out. When I took it out I had to remove the scale that had formed on it, which is normal during normalizing, but it machined very nicely and the burnt edges were nice and soft.

I don't know what your experience level is, but if you haven't done any single point threading in your lathe yet you might want to consider making a gauge the same size as your spindle first. It's easier to start on an external thread where you can see things rather than start with an internal one.

Good luck and have fun learning!
Ted

 

[mariner3302]

Thanks!! I'm actually really looking forward to this project once I have her back together!

 

[12bolts]

I find large threads easier to make than smaller ones because mistakes are easier to spot.

Ive also found that its a lot easier to repair mistakes on larger ones and machine them out. Smaller threads tend to melt and require more filler rod...........