Hw pistons how round ?

[jonny neate]

Just chucked up a hw95 piston in my lathe (4 jaw chuck) and it's the first time if had a serious attempt at setting up a 4 jaw.

I have it held chuck end by the seal locating flange and have made a mandrel that slides inside the piston and is supported by a live centre in the tail stock .

The best I can manage is 6 thou runout , Does this sound about right.

Jonny

[bigtoe01]

I have trued a few then added bearings, you have to be careful to not take to much off if relying on the cocking slot with a cocking shoe

[jonny neate]

But how much is too much .

Jonny

[MattyBoy]

Surely 6 thou is an acceptable tolerance?



Matty

[jonny neate]

I want to take the diameter down a bit as I intend to fit bearings

[bigtoe01]

last 26mm piston I did used the old style shoe, I took 0.2mm off the piston to true it and leave space for the bearings, the bearings were 5mm wide full circumference with milled flats to allow airflow so the bearings would not compress air...this was for a 95 done around 2 yrs ago.

You need to experiment a little, take a bit off, measure it all up and see how well the bearings and cocking shoe look to be, just go easy. I will say i started to prefer buttons over full bearings...i made these 8mm.

Diana pistons are the worst although the last 280 piston i chucked up was near perfect so I touched it up at the nose and polished it... it went in with no bearings as they were not needed..same with the HW57 i have.

[edbear2]

Just chucked up a hw95 piston in my lathe (4 jaw chuck) and it's the first time if had a serious attempt at setting up a 4 jaw.

I have it held chuck end by the seal locating flange and have made a mandrel that slides inside the piston and is supported by a live centre in the tail stock .

The best I can manage is 6 thou runout , Does this sound about right.

Jonny

How many places along it have you checked the run out? 0.006" is horrendous for a turned component generally

ATB, ED

[jonny neate]

Jonny

[wonky donky]

I don't turn the outside of the piston down to add bearings. 80, 99, 95 pistons have a slight flair at the skirt which is hardened, sometimes very hard. The flair itself will be out of round. I do polish it after machining the grooves.

The problem of "out of round" I have found is in the cocking slot area, the rod & the flair. The slots not going to create a problem, the flair can be machined true-ish & the rod centralised if need be.

Setting up in a four jaw chuck assuming I'm short stroking, I will machine the nose first. Setting up to less than a thou quite easily.

Turning it around & machining the skirt is a different matter, I will put the clock further down the piston, not on the flair. I will then take the least amount as possible off the flair just to try to get an even surface. I cant say it will be true but 3/4 thou is never going to be a problem.

With a 26mm piston I cut a groove 5mm wide with a diameter of 24mm, a 25mm piston 5mm wide 23mm diameter.

When making the bearings I cut the i/d 0.05mm larger than the piston groove but the width dead size. The o/d I now cut 0.10 below the cylinder dia. ie for 26mm cylinder 25.90mm o/d bearing.

The width I fit by hand by placing the bearing on top of some 1000 grade wet & dry on a flat surface removing a small amount untill it fits the groove perfectly.

It's important to "deburr" the i/d of the cylinder as there will be lots of sharp edges, for this I use a 3 leg honing tool in the lathe. It does the job quickly & easily without removing too much material.

THIS IS A BIT ROUGH BUT GIVES AN IDEA

Not answered the question really has it!

[jonny neate]

Some really good information .

Jonny