Mini-Lathe dovetails

[redvan22]

Hi,
I'm a perfectionist when it come to some things and my mini lathe is one of them. I want it as close to perfect as I can afford.

Out of the crate, there was a lot of slop in the lathe cross slide, compound slide and carriage and I adjusted the slide gibs as best as I could but could never get them perfect because perfect at one end of travel, was tight at the other end of travel. This led me to think that the slide dovetails were not parallel to each other.

As for the carriage; well, broken bits of cast iron found in the chip tray one day after a disastrous parting off experience, sent me on a journey to find a solution to replace the flat plates with their set screws that hold the carriage down (or used to hold the carriage down) which I found and I'm currently in the middle of that project but, other factors require immediate attention before I can produce the needed components to complete that project.

I read that using a triangular file in the dovetails was a quick and efficient way to remove the high spots from shoddy machining and then, along with sanding the gibs on w/d paper to get a flat mating surface, was a good fix. Good enough for most but not me. Yes, there was a great improvement but still not enough. I did the same treatment to my mini mill and there was tremendous improvement allowing me to take a .020 cut in mild steel with minimal chatter, but still not perfect.

Well, the filing method left things far from perfect on my lathe and I was told by a member this was not the proper way. They mentioned scraping which is unknown to me so here I am...

So, what is the proper way to get the dovetails parallel and flat so I can adjust the gibs to get as close to zero slop as possible?

Mike.

 

[homebrewed]

Before doing much more you need to know what part(s) of the system are out of whack (and you are the arbiter on what constitutes intolerable out-of-whackness). Without the right measuring tools you are never going to get the results you want.

You will need a way to measure the distance between the dovetails at several points to get an idea of the variation. The actual distance is not very important since the gib will make up the difference. This will require two identical precision dowels that are small enough to fit in the dovetail and a micrometer that is big enough to make the measurement. To do it, put the dowels in the dovetails and place the anvils of the micrometer on the outer diameter of the dowels. Do this for both the cross slide and saddle. From there, scraping is used to even everything out. You basically want as little variation in the distance as possible (given how much time you want to spend on it, and what your ambition is).

To do it even better, you also will need to check to see how perpendicular the dovetails are to the spindle axis. This will determine the flatness of workpieces you are facing. I'm not familiar with this process so can't offer much help, other than guessing it probably involves facing a test piece to see if you get a concave or convex surface. On a well-made lathe the radius of curvature should be very large (although the actual shape of the surface is a shallow cone). I bet someone in this forum can provide some insight, though....and I would be interested to know how to do this myself.

 

[middle.road]

Hi,
I'm a perfectionist when it come to some things and my mini lathe is one of them. I want it as close to perfect as I can afford.

Out of the crate, there was a lot of slop in the lathe cross slide, compound slide and carriage and I adjusted the slide gibs as best as I could but could never get them perfect because perfect at one end of travel, was tight at the other end of travel. This led me to think that the slide dovetails were not parallel to each other.

As for the carriage; well, broken bits of cast iron found in the chip tray one day after a disastrous parting off experience, sent me on a journey to find a solution to replace the flat plates with their set screws that hold the carriage down (or used to hold the carriage down) which I found and I'm currently in the middle of that project but, other factors require immediate attention before I can produce the needed components to complete that project.

I read that using a triangular file in the dovetails was a quick and efficient way to remove the high spots from shoddy machining and then, along with sanding the gibs on w/d paper to get a flat mating surface, was a good fix. Good enough for most but not me. Yes, there was a great improvement but still not enough. I did the same treatment to my mini mill and there was tremendous improvement allowing me to take a .020 cut in mild steel with minimal chatter, but still not perfect.

Well, the filing method left things far from perfect on my lathe and I was told by a member this was not the proper way. They mentioned scraping which is unknown to me so here I am...

So, what is the proper way to get the dovetails parallel and flat so I can adjust the gibs to get as close to zero slop as possible?

Mike.

 

[martik777]

There are some youtube vids on this:

Search for "scraping mini lathe"

 

[homebrewed]

To scrape your dovetails you will need a reference surface that is at least as flat as what you want to end up with in your dovetails. Ideally this would be a length of aged cast iron that has been machined with an angle slightly more acute than the dovetail (so you can get it in there to mark the dovetails with pigment), then scraped flat. Expecting to get the angle to exactly match your dovetails is gonna be another challenge and really not necessary, you just want to get a blued-up flat surface in the dovetail to highlight the high spots.

Scraping the reference angle, in turn, will need a granite surface plate. The plate needs to be at least as long as the object you are scraping. You blue it up, rub the angle piece on it, see where it's high (where the blue pigment is transferred), scrape, de-bur, clean thoroughly. Repeat. Ad infinitum (or so it seems). I've done some scraping and it is definitely an exercise in patience. An opportunity for meditation, perhaps. There is a lot of info on the web about scraping so be prepared to spend some time doing your homework.

You also will need some way to sharpen the scrapers because they will dull during use. Steel can be sharpened using stones or diamond. Carbide scrapers will require fine-grit diamond. The green grinding wheels don't produce a good enough finish on the carbide to make a good scraper.

So you could be looking at a bootstrap approach, making tools to refine your lathe. And you probably won't be able to buy manual scrapers that are small enough to get into the dovetail --- something else to make....

 

[redvan22]

Dan,
I watch TOT all the time and since I already have the machine (MM 7*16), I don't understand your post. Yes, hindsight is 20/20 they say.

The post I made was about tweaking the dovetails which others have responded to with numerous methods and worthwhile advise but thanks anyway.

Mike.

 

[redvan22]

Now, this is a useful video...

I should have just went to YT to begin with... it seems there's nothing that can't be viewed on YT.... I wonder If they have videos about child-proofing my house. I've done all I can think of but they still get in.....

 

[Ken from ontario]

Yes, watching you tube can be quite helpful.

 

[middle.road]

Dan,
I watch TOT all the time and since I already have the machine (MM 7*16), I don't understand your post. Yes, hindsight is 20/20 they say.

The post I made was about tweaking the dovetails which others have responded to with numerous methods and worthwhile advise but thanks anyway.

Mike.

Sorry, I thought somewhere in ToT's series on his mini-lathe he touched down on tweaking the dovetails.
Must have just been bearings and gears.

 

[Sylvester Jenkins]

Hello mate! When you speak about your mini lathe with such adoration it gives me joy to read! I absolutely love my mini-lathe as well. They are affordable, portable and powerful for small to medium projects. A mini lathe is perhaps the best thing an aspiring woodworker should invest in.