Newb Lathe question - accuracy by hand?

[Rich05]

most of parts I have made has been with a CNC. I recently purchased a manual lathe. 1980s vintage no DRO or scales. 34" Chinese variety similar to Grizzly lathes.

Have been trying to cut some shafts to spec. Finding it quite hard to cut the part to the correct tollerance. Never go enough, then go too far..

Using the indicators on the handles seems very innacurate due to backlash and innacurate read from the marks.

I am wondering how do you guys get accurate cuts on manual machines, esp. lathes like mine. Do you just keep checking with a caliper? Seems like that could take a while. I have a bunch of shafts to cut.

Thanks Ri.

[Geof]

Originally Posted by Rich05 ...Using the indicators on the handles seems very innacurate due to backlash and innacurate read from the marks. I am wondering how do you guys get accurate cuts on manual machines, esp. lathes like mine. Do you just keep checking with a caliper? Seems like that could take a while. I have a bunch of shafts to cut. Thanks Ri.

Yes it can take a while.

Do some practice cuts to 'calibrate' your machine. Set the cross slide to one of the marks, take a cut, down and back without retracting the tool, and measure the size. Move the tool in one major division on the indicator, this normally corresponds to about 0.01", take a cut the same and measure. This gives you some idea of what the marks mean in terms of moving the tool position.

Always make sure you approach the size you want from the same direction to remove backlash errors.

If things seem loose try adjusting the clearance on the cross slide; if things are a bit tight it ensures the backlash is pushed 'back'.

Use a micrometer for measuring, calipers are really not good enough.

Like I said above it can take time; be systematic and makes notes to help remember things.

[LeeWay]

I'm assuming you haven't tuned it up. You take out all the slop you can in the ways. You may even want to scrape those, but that is involved. Adjust the backlash out as much as possible. Use a short a tool as possible.
Assuming again that you were taking light cuts, the tool may have been deflecting due to movement in other parts of the lathe, and then just a little more, it tightens up and is then too deep.
Very sharp tools and light cuts will help once you get close too.

I have to tune my mini lathe before any accurate turning every time. This is precisely why I am building another small bench lathe with some better parts.

[HuFlungDung]

Originally Posted by Rich05 most of parts I have made has been with a CNC. I recently purchased a manual lathe. 1980s vintage no DRO or scales. 34" Chinese variety similar to Grizzly lathes. Have been trying to cut some shafts to spec. Finding it quite hard to cut the part to the correct tollerance. Never go enough, then go too far.. Using the indicators on the handles seems very innacurate due to backlash and innacurate read from the marks. I am wondering how do you guys get accurate cuts on manual machines, esp. lathes like mine. Do you just keep checking with a caliper? Seems like that could take a while. I have a bunch of shafts to cut. Thanks Ri.

Yeah, don't manual lathes suck?

Backlash in the cross slide screw is the main bug in the works. You can work around this using trial cuts. By this, I mean to proportion your rough cuts so that you leave enough material for two 'identical' finish cuts. The point of this is to leave say .020" to finish, but take off the entire first .010 and measure it to see if you really got what you expected, and whether any taper is present. Then, take the final finish cut by advancing the tool exactly the proper amount remaining. By taking two almost identical cuts, you stand a chance of running the same experiment twice, and hopefully get the same results both times (except for the size).

If you are used to cnc, you might also be used to carbide insert performance which may be difficult to get on a manual machine. If you are slightly cautious, you'll always be leaving a little bit on the part at the last cut, because you may need to polish out a wee bit of taper or flatten the crests of the toolmarks to make a good fit that will endure more than one assembling. Anyways, what I'm leading to, is that you'll probably end up having to grind some razor sharp finishing tools that just wouldn't stand up for long in a cnc.

These finishing tools can be HSS or carbide. In my shop, we use mostly TNMG inserts on the manuals. The old worn inserts get turned into finishers by grinding a hook shaped gullet on the top corner. We use a natural diamond wheel, 1/8" thick with a 1/16" radius to do this. We grind right down from the top through the original chip-breaker and mostly behind the tip, although we do grind slightly through the front, in order to remove the worn area from the insert. I like to leave the factory insert tip radius alone, as its going to remain nice and square to the insert, just like original. In many cases, we'll end up with about a 40° rake angle on that hook tip, so its only good for scraping maybe .0002 to .005 off the part. Use cutting oil on the work for the final cut to enhance chip forming and reducing gummy built-up edge from forming. This built up edge forms readily in gummy C1018 material, C1045 is my preferred 'general purpose bar steel' because the extra carbon helps it turn a lot cleaner.

Go for a DRO on the cross slide. It may seem expensive but helps remove a lot of the unknowns from the 'results equation'.

Only time and many trials will teach you how good your machine is, and what it can be expected to do in a repeatable manner.

[blkhrt13]

go ahead with the slack removal.its a sure bet but if you still cant get repitition from your dials use the old school approach,a mighty mag base with an indicator.u didnt say if you tried to lock the axis you are not using.if you use the mag base method and you check for trueness by running a cut in air then a cut on metal it will tell you if u are experiencing back lash from wear or u are just not catching your numbers.If you havent read up on manual machining before try the starret student machinist book it will save you alot of time typing on your pc.i reread it occasionally for fun and entertainment.does that make me work obsessed?

[Crevice Reamer]

I'm sure that you know this--But just to cover all bases:

The cutting tool will remove twice as much material as the dial is set for.

Setting .001 on the DIAL = .002 material removed, because the dial is a RADIUS measurement, but material is removed from the DIAMETER of the work.

CR.

[Rich05]

Thank you for the great advise.

Should I be putting scales on the lathe? will this help. or do I have allot to work out and learn first with the lathe as it is?

I think this lathe is in decent shape but it may not be that accurate, can feel allot of backlash, and slop in the handles. The spindle and ways seem tight enough.

this is a picture of the lathe. I posted about if I should buy it for $800 a while back.

[mactec54]

Hi Rich05

That looks like a good buy for $800 even if you have to do some work on it try & see if you can get a ball screw for the cross slide at least a C5 grade ground ballscrew & then a read out as well as good tools you will be good to do any thing you want after some practice If you get a Ballscrew make sure it has the same pitch & same number of starts
as the screw in the machine & your dial will still match I have found that the Newall read outs are the best for manual lathes & mills no glass scales & readers to get contaminated
with coolant, oil, chips etc

[Rich05]

Originally Posted by mactec54 Hi Rich05 That looks like a good buy for $800 even if you have to do some work on it try & see if you can get a ball screw for the cross slide at least a C5 grade ground ballscrew & then a read out as well as good tools you will be good to do any thing you want after some practice If you get a Ballscrew make sure it has the same pitch & same number of starts as the screw in the machine & your dial will still match I have found that the Newall read outs are the best for manual lathes & mills no glass scales & readers to get contaminated with coolant, oil, chips etc

The Y axis has .001 increment on the dial. Right now the handle has 10 notches of play before it moves the tool in our out.

The X axis on way has .005 increment on the dial. Right now the handle has 5 notches of play before any movement.

Is this going to be a big problem having all this movement in the handles? Will replacing ball screws solve this? Where do I go about sourcing the right ball screws? What cost can I expect. Kind of tapped out cash wise right now.

[RICHARD ZASTROW]

I've mostly had DRO's on manual lathes. I agree with blkhrt13 (#5 above).

For final size cuts, attach an indicator to the saddle with a magnetic base. Place the plunger against the moveable cross slide. I usually use a .0001" indicator. Make sure you have "loaded" the indicator with enough travel to exceed the depth of cut.

This has been used, I think, since dial indicators were invented. (by Al Gore?) lol

Dick Z

[VWSatOz]

Make sure your cross slide gibs are not so tight as to cause stick-slip, they should be just close enough so that there is no sloppiness sideways but not too tight or you will not be able to get repeatability. When you back off and then wind back in the tool must obviously always come to that exact same same closeness to the work, and to see if this always happens use a test dial indicator with a very light touch-force finger that will measure "tenths".
The cross slide screw and nut must have very clean threads and be kept that way, no dirt or chips or grit as these get sloshed in and out by the oil and cause repeat errors, same goes for the thrust bearings behind the collar. If the screw gets exposed in use the fit a metal shield to prevent stuff falling on the screwthreads. Dont start the spindle after you have set the position you want, as any shock may make the slide jump forward from the "backlash-gone" position. Adjusting out the back lash if this feature is available will help a lot but it must not be made to bind at all, especially if the screw is not dead in line with the travel of the nut, it may be loose at the far end and get too tight at the near end.
The next trick is to get the lube oil between the nut and screw to be squeezed fully out as you advance the tool to the final set position each time, and if you pull back on the tool post by hand as you creep up to that final posi you will get rid of all the tiny gaps. I have even tried fitting a long weak spring to constantly pull the cross slide back all the time and this works fine for OD cuts but a bit tricky when boring internal as the spring should then be pulling the other way. The tool tip must still be razor sharp for the finish cuts and this may be hard to get if the same tool has been used for a lot of roughing down. Best to rough down all your shafts first as a batch and then rechuck for those last very accurate cuts so the tool stays honed very sharp.
The other thing that affects diameter accuracy is the gib-tightness of the compound upper slide, as any movement here causes a diameter error that even 3 axis DRO scales cant detect! Once again if you mess with a dial indicator and push-pull wiggle sideways etc you should be able to get this eliminated, gibs tighter is the best shot here, and will give you much better finishes when manual taper turning.
As said by others some materials are very gummy and cause build up on the tool and need very high top rake angle and smoothness and light cuts. If you can't get a good finish then you can't get accuracy either.
If/when you can take a tenth of a thou cut and measure a result of 2tenths off the job diameter, and know that when you can repeat the size off the same dial setting every time then you can call your self an expert!
A DRO will solve all these problems as you can SEE what goes wrong, even during a long cut the tool may jump in & out. Nitrided or Ball screw/nut sounds good too!
The spindle bearings must be good and all temperatures low and steady, and the job must not get hot.
Nuff Said!!!

[Geof]

I started my business a long time ago with a machine almost identical to that. The backlash you describe is more or less typical for that type of machine. Just clean everything up, adjust the gibs or setscrews to get the ways as snug as is comfortable for cranking the handles, and as several people have suggested if you want easy precision fit a DRO or just use a dial gauge. With practise it is easy to get plus/minus 0.001" just from the handwheel graduation.