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Non-tapered gib adjustment, scraping and guidelines - help needed.

Miannini

Plastic
Joined
Apr 17, 2023
Location
Brazil
Before I get expelled from this forum, the machine below is from a friend, who got it at a black friday sale.
Ah, the sweet smell of a mini-lathe packaging grease! Now I know that I'll have lots of problem to solve.
So my friend bought this mini lathe, which is a Sieg C1, rebranded by brazilian seller Manrod, with 10" between centers. It's very similar to that FoxShop that does not have the compound slide. Power feed is only in the main carriage, not available in the cross. It sat there for many years (since 2017 or 18), unused, unplugged, untested.
Then, the trashy side of me picked it up and offered all my limited wisdom to adjust it to "working" condition.
Dismounted everything. Spindle bearing were tapered (good), but poor quality. Replaced. Machine ways were scraped, terrible at first. Handles have been made (it was missing). Cross-slide leadscrew was replaced by an trapezoidal one (did the nut yesterday - very awkward trapezoidal tapping by hand, but finished product is ok).

I started scraping a few years ago, using an german Hawera Handschaber. As Steffan Gotteswinter said about it, crazy rigid handle. It really beated me when using it.
So, in the last months, while I was without am milling machine, I decide to belt grind about 1/16" of it's handle thickness, to make it more flexible. Of course that it was a lousy approache to reduce some of it's original 5mm of handle (more than 3/16"). In the end, I finished with most of it (freehand on a belt grinder, imagine the mess I made!) around the 1/8" intended thickness. It became much more flexible, even tough now I have the world first concave handle scraper. In usage it felt easier to glide the blade and take cuts, reducing some of the chatter. I also adapted my reciprocating cordless saw with 13mm (1/2") stroke to hold a scraper blade: much faster than hand for the rough stage.

Then today I went to reassemble and adjust the carriages. Hmmm... non-tapered gibs. Let's see how they adjust. Of course, very rough. Checking them on the surface plate, they were (obviously) bowed. Tried some bending, got better. 0.05mm (2 thou) feeler gauge was just passing under the center of the back side, called it good enough for trying to bend without a proper alignment fixture. Went to the diamond stone and made some 8000 figure eights. Thing became pretty flat and beautifully hand lapped. Beautiful. Lapped front side flat also. However, since I haven't scraped the dovetails, that would be a lapped surface to milled surface, nowhere for oil to go. Probably I'll to some gib scraping for oil retention. Will have to make a wooden fixture to hold the slanted sided gib.

Gib back side was also very rough, got lapped. Screw indentation were very bad, just a mess that the screw made on the gib back side. Why do they use a flat tip screw on a slanted surface contact of the gib? Sanded the bolt tips to a very nice half sphere using sandpapers up to 5000 grit. Looks like a mirror finished steel ball bearing. Tomorrow I'll get the round diamond burr on the dremel and make polished dimples with correct angle on the back side of the gib, where the screws contacts. I also used a diamond file to deburr all edges on dovetails and ways. My lathe (Manrod 13x40") uses rounded tip bolts on rounded indentations. Other machines uses slanted cut pin spacer for contacting the gib flat on it's slanted surface. Any advantages of one or other? Guessing that both slanted and ball and socket creates the same anti-lift force as on milling vise jaws, right?

Then almost time to adjust. I'm having trouble finding information on adjusting these type of non-tapered gibs (are they called flat or parallel gibs?). They are usually not found on better machines. How to properly set them? One of the four screws is always hanging out of the base. In hand setting, I could get smooth movement, but there was some play detected by hard hand pressure. I was adjusting it with the first three screws in contact, leaving it smooth moving, then retracting the carriage to engage the last screw (and disengaging the first one, as the gib is larger than the mating part) and readjusting. No play means stiff movement, not possible with hand pressure, only by leadscrew (which can be inserted after the carriage-gib are adjusted).

If someone can help with tips for adjusting these gibs and comment on anything I did wrong or could have done simpler/better, I'll appreciate. Also, judge my scraping. I was trying to avoid scraping the dovetails, but I'm guessing that I'll need at least to check them using gage pins and a micrometer. I was hoping not to mess with them, but that may be the main cause of the binding.
 

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Typically 3 screws are used to adjust the gib, and the 4th (often located between the first and second) is used to lock it, and is usually not engaged. Adjustment is tightening them as tight as possible, that still allows free movement. There should not be play, it should not have tight spots. If it is not consistent across the usable travel, then the dovetails are not flat or not parallel, dealing with that sort of error is documented in many places. If this answers you question, maybe delete this thread before someone does it for you. As you seem to already know, they don't like wasting bandwidth on such items here.
 
Those cheap machines are a disaster and not precision. A couple of things. depending on travel the center 40% of gibs need to be lower then the ends .001". So as it wears it gets better. You can search Youtube - Richard King Scraping - Oslo Norway where I show the classhow to straighten a tapered gib, but it works the same as a straight gib. Have to be careful though as the Chinese cast iron can be brittle. Also search Adam Booth and Keith Rucker on adjusting a Gib. They are using a indicator to check the lost motion on tapered gibs but it is the same technique you do to a straight gib. When I adjust straight gibs with a set screw, I tighten the ends using the indicator push pull. then finger tighten the middle.

Also the set screws need to be long so you can lock them with a jam nut. When I see those gibs with no dimple in the gib, I put it together and drill a clearance hole through the tapped hole so the angle of the dimple pushes the gib straight in. leave the gib loose and the feed screw nut loose where it tightensto the crosslide and screw the nut as close to the casting / operators side and see if it lifts the cross-slide. check with feeler gage. If the nut thickness is aligned right. it should move easy and the ways should be tight. Then tighter the nut.

You will need .0005 to .001" oil gap on the gib. Or lost motion. That small machine can bend so be sure careful not to bend when push pulling it. Stefan G took 2 of my classes. Also on You Tube search Richard King Scraping Sweden where the students are scraping and testing cross-slides. Jan Sveere also rebuilds Myford lathes that have square ways. He has attended 5 of my classes. On the Sweden class the one student bump scrapes, you could try that as it is easier to scrape sometimes.
 
Those cheap machines are a disaster and not precision. A couple of things. depending on travel the center 40% of gibs need to be lower then the ends .001". So as it wears it gets better. You can search Youtube - Richard King Scraping - Oslo Norway where I show the classhow to straighten a tapered gib, but it works the same as a straight gib. Have to be careful though as the Chinese cast iron can be brittle. Also search Adam Booth and Keith Rucker on adjusting a Gib. They are using a indicator to check the lost motion on tapered gibs but it is the same technique you do to a straight gib. When I adjust straight gibs with a set screw, I tighten the ends using the indicator push pull. then finger tighten the middle.

Also the set screws need to be long so you can lock them with a jam nut. When I see those gibs with no dimple in the gib, I put it together and drill a clearance hole through the tapped hole so the angle of the dimple pushes the gib straight in. leave the gib loose and the feed screw nut loose where it tightensto the crosslide and screw the nut as close to the casting / operators side and see if it lifts the cross-slide. check with feeler gage. If the nut thickness is aligned right. it should move easy and the ways should be tight. Then tighter the nut.

You will need .0005 to .001" oil gap on the gib. Or lost motion. That small machine can bend so be sure careful not to bend when push pulling it. Stefan G took 2 of my classes. Also on You Tube search Richard King Scraping Sweden where the students are scraping and testing cross-slides. Jan Sveere also rebuilds Myford lathes that have square ways. He has attended 5 of my classes. On the Sweden class the one student bump scrapes, you could try that as it is easier to scrape sometimes.
Thanks Richard!
I have also found an old copy of “Machine tool reconditioning and applications of hand scraping”. They mention the same clearance you mentioned ! Also, it’s written that the parallel gib should fit snuggly on the slot, which is not the case on this machine. I suspect it’s too narrow and short, which is way adjustment tests leaves a straight mark on the sliding surface, probably from tilting and hitting heavily on the stationary surface top edge.
Will mill a new close fitting gib, maybe from brass or bronze (I have stocks in closer sizing in those material, my cast steel rod is much bigger), then flat, lap and scrape it. Will post pics here. I’m not expecting to achieve Schaublin result, but any improvement on such a crude machine is appreciated.
Will also use two gauge pins and check the parallelism of dovetails with the mike.
Will search forum for home made straight edges for scrapping the dovetails. Maybe using the 2” rod of cast steel I have.
That kind of work makes me wanting to have a surface grinder and a shaper. Will try to make my best at the mill.
 
Before measuring the dovetails with pins make sure the flats are coplaner (same height or parallel) to each other. if they aren't and say one is higher than the other the pin will ride higher on the dovetail and measure wider
 
Progress made: since the original mini-lathe gibs were so lousy, undersized, thin and bent, I deiced to machined a set of new brass non-tapered slanted gibs (about 1/8" x 5/8" x 60 degrees). Tight fitting on the assembly. Without any bolt adjustment, they are gliding smoothly up to a point, then it takes real resistance to move on, a sign that dovetails are not perfectly parallel or straight.

Also measured the dovetails of my 13x40" lathe cross slide using gauge pins cut from a cold-rolled stock: it spreads out of parallelism to a max of 0.035mm (1.5 thou) towards the machine back side, hence that's why it got tighter as the cut progressed and why I got a very slightly convex facing surface when grinding with the toolpost grinder: it's probably moving backward a little (need to check which dovetail side or if both are out of parallel). Noticed that the tapered gib is now flat enough (it was bent about 20 thou towards it's back), but it was already slightly shorter in height than the dovetail fitting (about 5 thou). Should I need to increase the height, am considering to loctite (# 480) a 0.10mm or 0.30mm brass shim to it's top surface, them re-machine bottom flat (sliding surface) and scrape.

Now I'm into the process of machining a triangular straight edge, then will scrape and use it to blue the dovetails on the tables and the lathe bed. I'll probably mount them between the divider center's, machine, rotate 60 degrees to machine second surface plane, then machine the last surface to 90 degrees. Am using a 1-3/4" round cast iron stock about 10" long.

Also ordered a large enough dovetail mill cutter, so maybe if I decide it's too bad/hard/impossible to fix the dovetails with my scrapping technique, I'd have the option to carefully try to re-mill the dovetails after indicating them on the milling machine table.
 








 
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