Putting on a fine cut..

Having always wanted to build a small single cylinder aero engine I thought it would be a good time to get some opinion on putting on fine cuts, say working to 5/10000" in steel or aliminium.

As my ml10 is graduated in 'thou' what are the remedies to get finer cuts?

Angle the compound slide to 6 degrees and put on the cut using that. 10 thou on the dial is 1 thou infeed.

regards Martin

Spareys the amateur lathe tells you all about this.

But you need to have a really sharp tool and well adjusted slides.

For the final sizing lap or use a cylinder hone. Good luck Noel.

working to those limits you will also need a tempture controlled workshop and some very high end machine's. as for measuring equipment i don't know. but i do know it will need calibrating professionally. i have never worked to those limits, however i do work in the aerospace industry where such equipment is the norm.

Hi Martin, as an apprentice I was given a Harrison 10 and a 1" mike and told to make 400 pins in silver steel, to a tolerance of 1/4 thou, the lathe was worn and there was no temp control, but I got the job done - it was for RR. Noel

Model aero is machine to within a thou and hone /lap the rest, just finished a 0.1 dragonflea but its a tad too small for me ,working on a glow humbug at the moment hope you have success with yours. somewhere around 1.5 to 2.5cc is a nice manageable size.

I've never bothered to do anything special when making aero engines even the ones with no rings just split the divisions by eye. Final fit is by feel not measurement. So long as both parts you are working on are at similar ambient temp you don't need to control the workshop temp

Here is the old chinaman taking a 1 though cut to get to 0.3937" (10mm in new money) Insert tool and cross slide set parallel to lathe axis which is a lot easier when working to a shoulder like here than having the slide set over as that also moves the tool sideways as well as a little way in.

Edited By JasonB on 25/06/2023 17:33:57

Precisely!

... and it's easier to make the piston to fit the bore, rather than t'other way round.

Below about a thou, various tricky factors start causing trouble, and they get progressively worse:

• Temperature - a rod warmed to 40°C is measurably bigger than the same rod at 20°C. If temperature isn't accounted for, you ain't measuring tenths!
• Calibration becomes vital. Second-hand gauge blocks and ancient micrometers aren't good enough.
• Technique is difficult and tiny flaws destroy accuracy.
• Although a large diameter dial helps, don't expect much from dials, and especially not small ones. Dial marks are usually indicative rather than accurate: the operator is expected to use a micrometer, calipers, or DTI
• The brand-new lead-screw of an ordinary lathe is unlikely to be tenths accurate and older lead-screws will be more-or-less worn too. The accuracy of fine-cuts is fundamentally limited.
• Tiny saddle, headstock and other alignment errors show up
• Work and tools both bend under cutting pressure, making it likely that cuts are not all of equal depth end-to-end. (Ever noticed that tools often remove more metal when reversed out of a cut?)
• Scale causes the relative surface finish to go to pot! As the size of cut is small compared with the lathe's automatic feed-rate, fine cuts leave spiral grooves rather than a smooth surface. Looked at with a microscope after a few passes, the surface resembles knurling. Is a tenths micrometer measuring between peaks, or has the anvil crushed them towards the valley bottoms? Dunno: unless the surface is good, the tenths are uncertain

What this boils down to is that lathes (and milling machines) struggle to take cuts that are both fine and accurate. Tool-room lathes and jig-borers do better, but they're elaborately engineered and maintained to hold accuracy. They have obvious limits too.

The answer to high accuracy isn't a lathe. Industry are fond of grinding machines but these are expensive. Fortunately small workshops can get good accuracy and surface finish with manual abrasives, notably emery papers,and lapping. Making pistons, the lathe is used to get close to size, but turning stops a little above target. After that a succession of ever finer abrasives are used remove unwanted metal, not cutters. Unlike cutters, correctly used abrasives improve the surface as well as removing metal. The main problem with abrasives are removing metal evenly, and the amount of time it takes.

Larger pistons don't go for a very tight fit: instead the seal is provided by piston rings which can expand slightly to fit. Although cylinders are bored very accurately in big engines, I've got the impression surface finish is even more important. There is no 'Running In, Please Pass'. Au contraire, I've built 5 model engines and they all needed a lot of running in! I spoiled several pistons by rushing the finishing stage. Patience in the workshop is not one of my virtues.

I rarely work to better than ±0.05mm and avoid high-accuracy if I can. I stay with innocent measuring because high-accuracy is dangerously additive. 'Chasing Tenths' could easily take over my life. As Chasing Microns is even more difficult, only real men dare go metric!

Dave

However when you know you need to take 3 and a half thou off the angled top slide gets you closer to setting 1.75’ than guessing the setting of a small dial. It assumes the your tool is sharp and the last cut you made took a similar amount off.

regards Martin

Most of my imperial micrometers read in tenths of a thou, but I have never had any difficulty in splitting the 0.001" divisions by eye, so the vernier scale is redundent for my purposes. You can try tests to find how accurately you can split the divisions on your cross slide using a DTI and a piece of ground stock in the chuck, a MT socket or new drill shank. Actually cutting this finely gets difficult, which accounts for the advice on lapping. I can cut 0.0005" with steel using carbide inserts intended for aluminium.

Under CNC control, taking a 0.005 mm cut in aluminium using a CCGT060204 insert (uncoated, ground) is not a problem (do this on a daily base) even on the mini lathe. In aluminium, I could cut as shallow as 0.002 mm (stepper resolution).
For steel I would use the same CCGT060204 insert insert but the 0.005 mm depth of cut would be the limit.

The biggest problem is measuring the diameter, not the turning part. For inside diameters my 2 point gauges are just not repeatable enough. That is not a gauge issue but more my ability to position the gauge the right way with the part in the chuck. Measuring a calibration ring with ground surfaces is not a problem.

When I trim pistons for model engines, I set the top slide at the same angle that the taper is for the crown, about 0.7 deg. It makes my Myford top slide , 1 notch is 1um on diameter or so. The steel liners with iron pistons, the liner wants to have at the bottom below the exhaust port , about Ø0.05 to Ø0.06mm bigger than the piston diameter. Then from the exhaust port to the top dead center mark, wants about Ø0.01mm then parallel to the very top of the liner. All the liner tapers can be lapped from a parallel bore.

The piston wants to be turned to fit ideally, with a slight crown taper, for about the thickness of the crown. On cast iron pistons, I recommend that the top slide be set to about 0.5 deg. Trim the piston until it just enters the bottom of the liner. Then only using the topslide to make the next cuts until it fits where you want it. To get the last little bit, you can use a light grey scotch brite on the piston.

I like the Kyocera ccgt type of insert in the PR1425 grade. I have a picture in my album of the insert.

As for cuts, clean the piston with break clean or any other degreaser and trim the piston dry. The amount that a lathe can cut is down to a lot of variables.