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Form tool to cut big end / main crank bearings.

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David Kenyon 201/01/2019 19:17:28
185 forum posts
144 photos

I have roughed out an inline 4 cylinder crankshaft which now needs the bearings machining .

I am struggling to get a nice finish due to 'chatter' at the tool tip. I have adjusted the jibs on the cross slide so they are a little tight and set the tool to centre height.

What would you suggest the best way to grind the tip of my Hss tool to be able to take light cuts , left and right , and keep the bearing parallel.

Nick Hughes01/01/2019 19:33:24
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191 forum posts
128 photos

Relieve the centre of the tool, as shown in the first 3 min of this video:- **LINK**

Edited By Nick Hughes on 01/01/2019 19:36:07

JasonB01/01/2019 19:59:25
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Moderator
15355 forum posts
1580 photos

I tend to use a knife tool to go down each side of the webs and take as much out the middle as possible with that then change to an indexable parting tool for which I keep an insert that has had the middle ground away with a Dremel cutting disc to a similar profile to that in Nicks video. HSS would look like this

AJW01/01/2019 20:04:50
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268 forum posts
117 photos
Amazing how all our 'roughing' cuts come finished well then when we get to the ones that matter it goes downhill!

Alan
David Kenyon 201/01/2019 20:14:08
185 forum posts
144 photos

What is the best way to grind the rounded edges , I have some diamond files so I could do it by hand / eye with the files.

The bearing is only .437 inch wide so the tool is quite narrow .

duncan webster01/01/2019 20:16:02
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2074 forum posts
32 photos

I've used a diamond file to shape tool to Jason's profile. It's slower than a Dremel, but more controllable

JasonB01/01/2019 20:17:41
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Moderator
15355 forum posts
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Yes, just stone the corners round so you leave a small fillet in the corners.

FMES01/01/2019 20:25:57
580 forum posts
2 photos
Posted by JasonB on 01/01/2019 19:59:25:

I tend to use a knife tool to go down each side of the webs and take as much out the middle as possible with that then change to an indexable parting tool for which I keep an insert that has had the middle ground away with a Dremel cutting disc to a similar profile to that in Nicks video. HSS would look like this

Agree with Jason, but I finish to size with a shaped grinding wheel.

rb gas engine 237.jpg

Edited By FMES on 01/01/2019 20:27:34

David Kenyon 211/01/2019 13:18:55
185 forum posts
144 photos

Well the crank has not gone well. I ground a form tool as described and cut the four big ends first.

I achieved a nice finish with the lathe and then just ran some wet/dry around to polish the bearing.

Then I set up for the Mains , no matter how I tried I suffered vibration and a poor finish.

I tried adjusting tool height, different speeds , re grinding tool bits, ect but could not get the finish I achieved with the Big ends. I am now at the stage were I do not have enough material left to risk another bad cut so I am going to make up grinding wheel and see if I can save it by grinding down to size.

I am thinking the problem may be from cutting the Webs and Big ends first , weakening the crank and causing it to flex [vibrate]

Edited By David Kenyon 2 on 11/01/2019 13:20:25

JasonB11/01/2019 13:31:02
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15355 forum posts
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Did you put packers between the webs so it would not flex when tightening the tailstock?

David Kenyon 211/01/2019 14:21:08
185 forum posts
144 photos

Yes , I made some aluminium packers , I also tried replacing the revolving steady with a fixed and various amounts of pressure from the tailstock. Its been very frustrating , especially as the big ends came out so nice.

Roger B11/01/2019 14:48:40
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60 forum posts
15 photos

Is the crankshaft cut from solid or a casting?

Are you doing all the turning between centers?

Is it possible to support one end in a 4 jaw chuck to improve ridigity?

David Kenyon 211/01/2019 21:11:18
185 forum posts
144 photos

The crank was cut from EN8 round bar and turned between centres.

I could have used a 4 jaw but that option is gone now really as I don't want to risk it.

On the plus side I have wanted a tool grinder for a while so now I will get on with making one.

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