# Lathe compound slide vs surface plate



## Cadillac (Mar 17, 2019)

So I needed to remove my compound to get some measurements for a solid tool post block I'm building. Note I don't have any problems with the lathe cutting just a project and I like to verify stuff makes me sleep better. Surface plate was cleaned so I figured why not. A real eye opener on contact  area on matting surfaces. Background it's a 14x40 optimum lathe built in 2013. Here's some pics.






I pulled out the tenths indicator and did some mapping of the parts. Every surface was tweaked falling .002 the most on the bottom dovetail mount towards the operator. The gibb had a .005 smiley face in it and a slight twist.  The bottom base had a rock in it,the top slide was surprisingly the best shape but the top where the qctp mounts had a terrible matting surface.
  I first looked at how to straighten the gibb I found that pinging it with a convexed body hammer with a piece of copper in between worked a charm. I now can lay on the surface plate and barely see light touching at the ends. Figure scraping it in will take care of the rest. Then I started by flattening the mounting surface of the compound. Side note this is my first real scraping job that isn't playing around. I don't have a clue but seems straight forward I need 100% contact and stay level right. After about ten trips back and fourth I got these results.





I've got the bottom down to where I'm pretty happy about 95% there so I moved on to the top surface where the tool post mounts. This surface had maybe 30% contact and its was around the perimeter not where the tool post sits. It took acouple passes I'm not satisfied as it has a .0002 hole in it. Getting there though I think.


The bottom of this surface looks like it was either milled or a ruff Blanchard grind. It was not scrapped but was in good shape otherwise only .0004 variance in readings. I need to make myself a prismatic straight edge for the dovetails and work myself up to it. The lower male dovetail was the worst having a .002 difference from front to back good thing is it was even left to right so I need to tilt the base to level it out. It isn't scrapped just some flaking/makeup on it.  All in all I'm happy with the results so far alittle disappointed in the finding but figured it's only gonna get better from here. Like said I had no problem with surface finishes or sizing just some curiosity. Would love to see what a bad machine would look like?  If you have a surface plate and some time it may be a eye opener to go check out some parts. More results in time thanks for looking.


----------



## Ulma Doctor (Mar 17, 2019)

most folks would stop sleeping if they ever rubbed any part of their machine tools on a flat rock.

these are the same machines we take for granted, producing (imaginary) .0005" tolerances


----------



## Dabbler (Mar 17, 2019)

Nice to see someone check out their machine properly...  

Nice that you are correcting what you find!  You treat your machines right.  bravo!


----------



## Cadillac (Mar 17, 2019)

You mean the test report that came with the lathe wasn't enough proof? 




Can barely read the chicken scratch and always just below tolerance AWESOME! Guys arm must be numb from the stacks he's has to do in a day.
 Here's a pic of what the surface finishes are when disassembled just machined and some glitter. I did a quick blue not very surprised. The left bottom corner had alittle more on it I accidentally grabbed the surface and my fat thumb wipe it off. Right side ain't bad left side touches about a 1" on the ends, along the gib and a touch in the center. No buena.



Will get it dialed in just gonna take alittle love and a lot of effort!


----------



## Sblack (Mar 20, 2019)

please tell us what impact your work has on the operation of the lathe.


----------



## Cadillac (Mar 20, 2019)

Here’s a pic of the gibb. .005 bow shown on plate with light ,First blue and where I’m at now. Got it pretty flat good thing is the curve will be opposite the slide lock so it will actually straighten when locked. I was really surprised at how much it moved by pinging the backside. Can already see movement in gibb adjustment screws. 




 My goal is to get the handles to spin freely like I see Stephan G on YouTube. Watching him do his magic and showing how smooth the compound moves with NO play, tight,and right. Using a tenths indicator checking side play and having the needle not move dam he’s good. I’ll keep ya updated.


----------



## ericc (Mar 20, 2019)

This is actually a huge plus.  Having tight uniform travel just makes the experience so much more positive.  And, hitting a loose spot in the carriage feed while parting can be destructive.  Of course, one can just stop and adjust, stop and adjust, stop and adjust, ..., but if you forget, the rotating part can grab the parting tool and self feed against the backlash.  It is aesthetics, but also functionality.


----------



## astjp2 (Mar 23, 2019)

Did you have to cut a relief into the dovetail or did it already have one?


----------



## eeler1 (Mar 24, 2019)

Not sure I fully understand the pictures in the first post, but you don’t want too much coverage on any mating parts that move.  Maybe 60% bearing (or coverage, if I understand you correctly) is plenty.  The low spots (not showing blue) are there to hold some lubricants, too much contact or coverage or bearing leads to premature wear.  Or maybe I mis-understand what you are trying to do, but thought I’d mention it just in case.


----------



## Cadillac (Mar 30, 2019)

eeler1 said:


> Not sure I fully understand the pictures in the first post, but you don’t want too much coverage on any mating parts that move.  Maybe 60% bearing (or coverage, if I understand you correctly) is plenty.  The low spots (not showing blue) are there to hold some lubricants, too much contact or coverage or bearing leads to premature wear.  Or maybe I mis-understand what you are trying to do, but thought I’d mention it just in case.



The only parts that I want 100% contact is the bottom mounting surface of the compound and the top surface where the qctp mounts.

I have not relieved any of the dovetails, they have reliefs cut in already. I'm at a stand still with the dovetails anyways I do not have a dovetail straight edge yet. Gonna have to be a near future project.


----------



## tertiaryjim (Mar 31, 2019)

Cadillac
I was in the same place as yourself so I used some mild steel to make a straight edge for the dovetail. It was well worth the effort as rigidity shot way up.
If your machine isn't out very far, you can use the same gibb though you may have to put some shim behind it. I found that many factory gibbs are poorly fit. Getting the gibbs to fit right seems much harder than scraping dovetails to me.
The only down side is that steel is a pita to scrape.
Each step, each surface compleated can make a notable difference.


----------



## Cadillac (Apr 1, 2019)

The gibb had a slight bow in it show in one of the previous pictures. I was abl to get it straight by tapping it slightly with a piece of copper buffer and a hammer. I suspect it will move during scraping.  I just got a nice donor piece of cast from a part from work that should work good for a dovetail straight edge. I need to cut it out of the part but I'm hopeful it's a good donor the part had extensive machining on it and finishes were good. Will see?
I figured I was putting this time into the compound why not check the cross slide? It's also been bugging me that my lock hole for the cross slide is covered by the dro scale. I have a way to use the lock with some mods but needed to take the cross slide off so that settled that off it came. Kind of disappointing but expected it almost after seeing the compound. Two high spots on opposing corners about .0006 all four corners blued but i was pushing and rockin. I find its really hard to pick a surface to measure from? Nothing is flat every surface has some sort of hollow tap, rock, or bow. 
 The one thing I'm really not liking is the gib for the cross slide! Bent banana in both directions, slide side not scraped ground on both sides, top and bottom look to have been done on a shaper. Gib is smiling in the vertical sense which explains why when adjusting the gib it would want to raise out of the dovetail. Ive done alittle of the flat sliding surface of the  top slide. Idk about the gib having bows in both directions might be out of adjustment by the time I get it fitted and have read that shimming is not a long term fix more of a band aid. So I might take a stab at trying to make a new one I haven't tried getting replacement parts I suspect it being a hassle unless I can find a comparable part from a grizzly model or something idk haven't tried. I think I have the equipment to do the gib it just the talent I'm questioning. Well got some work ahead of me but I think it should pay for itself in the long run here's some pics. Last pic is where I've gotten with the bottom. Once the end got takin down the one side came right in the other side isn't far behind.


----------



## Cadillac (Apr 2, 2019)

I think I see a 12” dovetail straight edge in here somewhere? Still needs a lot of trimming before I start milling it. Hopefully a good candidate


----------



## astjp2 (Jun 7, 2019)

tertiaryjim said:


> Cadillac
> I was in the same place as yourself so I used some mild steel to make a straight edge for the dovetail. It was well worth the effort as rigidity shot way up.
> If your machine isn't out very far, you can use the same gibb though you may have to put some shim behind it. I found that many factory gibbs are poorly fit. Getting the gibbs to fit right seems much harder than scraping dovetails to me.
> The only down side is that steel is a pita to scrape.
> Each step, each surface compleated can make a notable difference.


Mild steel moves too much, you want cast iron and heat cycle it on a grill then ring it, ring it like a bell by hitting it with a hammer to get the grain to stabilize.


----------



## tertiaryjim (Jun 7, 2019)

Steel was all I had at that time and it was such a small straight edge for the compound that warp was not a problem. That was my first effort at scraping. 
The ways were so far out that that even a poor quality straight edge could make a huge difference in rigidity. I purchased a cast iron bar to make a longer straight edge for the cross slide and other projects.


----------

