Drilling and tapping carriage for rear tool post

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kwoodhands

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I am making a rear mounted tool post. I will have to bore holes in the carriage for 1/4-20 cap screws. I am wondering if cobalt drills or even HSS drills will work. Also , will HSS endmills work for a counterbore. Only 4 holes are needed. I will start with #6 drill and try to tap the hole.
mike
 

BaronJ

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Mike, it might be a good idea to tell us what lathe you have and a picture or two of what you are trying to do.

With regard to your question, the saddle is most likely cast iron so HSS drills and end mills will do what you want. But I would be very careful drilling holes in the saddle, it far too easy to damage a hidden surface.
 

kwoodhands

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Grizzly 10x22 lathe. I have drilled and tapped the carriage, it is cast iron and very easy to drill and tap. I changed from 1/4-20 to 10/24 . This required smaller holes and still plenty strong. I am done and tried the tool post last night. Worked great for parting 3" 6061 aluminum.
I never had a problem parting off 1" rounds or less. The big stuff was a problem before. Had to hacksaw and then face to dimension. Now I can part off and only face to clean up.
Thanks for your post,mike
 

BaronJ

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Hello Mike,

Good to hear that you have achieved what you wanted without causing any damage. 👍

How about a picture of your work, I would be interested as I'm sure would others, particularly if they are contemplating doing something similar.
 

Shopgeezer

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I have never understood the advantage of a rear toolpost for parting. Do you use the parting tool upside down, or run the lathe in reverse?
 

packrat

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quote " Do you use the parting tool upside down" Yes that is right
 

BaronJ

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Hi Shopgeezer,

Yes you use the parting tool upside down running the lathe the right way.

You say you've never understood the advantage of a rear toolpost for parting.

The advantages are quite a lot, the swarf drops out of the cut, rather than staying on top of the tool with the risk of it going down the side and causing a jam.

The rigidity is vastly improved because more of the cross slide is above the saddle rather than at the end where there is more wear and less support.

The bulk of the tool holder will have less overhang, the holder body not over hanging the table edge.

Correctly set up the blade will always be at the right height and dead square to the work in both directions. Apart from being able to work closer to the chuck, which isn't always possible with a parting blade in a QCTP.

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Here are some pictures of my Norman patent rear tool post. The tool block is interchangeable with the front one. So if needed I can swap them around for a particular job.

The parting blade is 2 mm thick by 12 mm wide and 200 mm long. In this picture the blade is new and has not been ground yet.
 

Shopgeezer

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So the forces on the tool would be trying to lift the tool up in a reversed parting tool. In the front it would be pushed down. I would think that the upside down position would be less rigid since the whole tool post is being lifted from its mount. In the front it is being forced down.
 

BaronJ

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So the forces on the tool would be trying to lift the tool up in a reversed parting tool. In the front it would be pushed down. I would think that the upside down position would be less rigid since the whole tool post is being lifted from its mount. In the front it is being forced down.
Yes that is correct, but since there is normally less play and wear at that end the opposite is true, there is also more engagement of the slide. On a small lathe the spindle would be being pushed down rather than being lifted.
 

RM-MN

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When you have the parting too in the normal position any flex in the cross slide as the blade cuts causes the blade to dig in. With it in the rear the forces are to lift it and move the blade away from the cut. Even with less rigidity it will cut easier with the blade in the rear.
 

Cogsy

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When you have the parting too in the normal position any flex in the cross slide as the blade cuts causes the blade to dig in. With it in the rear the forces are to lift it and move the blade away from the cut. Even with less rigidity it will cut easier with the blade in the rear.
Surely this can't be true? If your tool is on centre then it is being deflected in the direction of rotation which has to be away from the material no matter if it is a front or rear post. Same thing if you're above centre and being deflected it's always in the direction of rotation and cut depth will increase (which I believe is why it is sometimes recommended to have parting blades set slightly high on less rigid machines although this would equate to slightly low on a rear parting tool).
 

BaronJ

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Surely this can't be true? If your tool is on centre then it is being deflected in the direction of rotation which has to be away from the material no matter if it is a front or rear post. Same thing if you're above centre and being deflected it's always in the direction of rotation and cut depth will increase (which I believe is why it is sometimes recommended to have parting blades set slightly high on less rigid machines although this would equate to slightly low on a rear parting tool).
If you look at the forces when parting from the front, the chuck is forced upwards and will cause the tool to dig in because it is pulling the tool post towards the chuck.

Parting from the back, the reverse is true. The rotation of the chuck pulls the chuck down against the cutting force of the tool, this is pushing the tool away from the chuck.

I would never advocate having the tool set to anywhere other than accurately on the center line.

I think that people have a lot of trouble with parting because they simply don't understand the dynamics of rigidity and positional requirements.

Anyway haven't we been this way before recently, when we discussed the parting blade getting trapped when parting aluminum. Which since I've actually found to be caused by the aluminum being smeared down the side of the blade. It seems that only the soft gummy types are the culprits.
 

Apprentice707

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I have used a rear-mounted toolpost on my Myford S7 for 25 years without any problems. I use HSS and a Ganze type tool holder with inserts depending on what material I am parting off, both work well. The most important things to watch out for are tool height and keeping tool overhang to a minimum, the cutting edge must be on the exact centre height of the lathe. I seldom machine aluminium, but when I do build up on the tool cutting edge is always a problem frequent cleaning during operation keeps this to a minimum.
Baronj's pillar toolpost has inspired me to add one to my Chinese made 8 x 12, just wish it had a Tee slotted cross slide like the Myford !!

Happy metal shaping y'all.

B
 

goldstar31

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I've written umpteen times on the topic and the the rear mounted inverted blade goes

In and out of cut
whereas as front mounted parting tool

Goes into cut and can

DIG IN---BANG
Sorry Cogsy but it is not getting through.

I have had a Geo Thomas one which is angled down wards at 7 degrees, has the top cutting edge ground with a 140 degree female 'top' and a male vee of 140 degrees and the normal cutting edge is 7 degrees in hss and will cut 2" mild steel.

But the nice thing is that there are 2 blades and when cutting, the swarf is 'narrowed' into nice little curly rolls and doesn't seize and the little curlies simply drop down.

Heminngwaykits sells castings but mine came from the scrap box.

If my memory is correct, Ian Bradley on of the Duplex tean withDr Nrman Hallows made the first one and Thomas made one- the horizontal variety and then did the present design which- if we think about it only requires the front of the tool refreshed. Usually only a touch on the grinder!

I hope that this helps

Norman
 

Rodney Brown

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Actually with a rear toolpost, the tool can be run upside-down and the chuck in the normal direction (or vice-versa -- tool normal, chuck reverse).

I have an AXA toolpost on the compound, and a BXA toolpost mounted directly to the saddle... easy to do with my Nardini since it has T-slots in the saddle for a rear-mounted attachments. I built some blocks so that I can index it to exactly the same position whenever it is mounted. This gives me fixed tool offsets for tool holders where I want them to be dead-on the centerline (BXA-mounted drill chucks, MT3 tool holders, etc.). A DRO for storing those offsets is my next big investment in this lathe! The primary advantage to me is that my rear tool post is mounted solid to the saddle, not on a compound. This makes it about 10x more rigid for parting, T/P grinding, etc.).

Already said, but something to watch out for... the saddle is designed for cutting forces into the lathe, not lifting the saddle from the lathe (regardless of the direction the chuck is running). Parting with an upside down tool is my preferred method, but if the tool catches it pulls the saddle up from the ways. A bad catch can tear a saddle off the machine.

20200504_101249.jpg
 
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Cogsy

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If you look at the forces when parting from the front, the chuck is forced upwards and will cause the tool to dig in because it is pulling the tool post towards the chuck.

Parting from the back, the reverse is true. The rotation of the chuck pulls the chuck down against the cutting force of the tool, this is pushing the tool away from the chuck.
Let's assume the tool is exactly on centreline of the work and is making a shallow cut into the workpiece. If we now deflect the toolpost in the direction of rotation (as we get from cutting forces), it will be towards the saddle from the front, or towards the ceiling in the rear. In both cases the distance from the tool tip to the workpiece will increase and depth of cut will reduce as in both cases the tooltip is rotating around a pivotpoint (the tool holder) and describing an arc. Also in both cases, the toolpost is now effectively a spring and will be trying to restore the toolpost back to straight - as we continue to feed, possibly as we reduce our feedrate, the spring in the toolpost brings the tool tip back down, depth of cut increases and we can have a dig in.

If we think the chuck itself is deflecting instead, it's essentially the same result as above but removes the toolpost deflection completely so there is zero difference between a front or rear post in terms of the deflection.

Rear parting holders undoubtedly work but it's my opinion that it's more about gravity assisting with chip clearance. If anything, for a similar rigidity front vs rear toolpost I would expect any looseness in the saddle gibs would mean an upside down parting tool would end up further off centreline than the front.

I would never advocate having the tool set to anywhere other than accurately on the center line.

I think that people have a lot of trouble with parting because they simply don't understand the dynamics of rigidity and positional requirements.
I have seen it in writing from reputable sources (once on the Sandvik website even) although I do set mine as bang-on as I can. Too much away from the centreline against the rotation direction is obviously bad for dig-in and too much in the direction of rotation can lead to the work 'climbing over' the tool. I believe the recommendations I've seen that advocate for 'slightly high' (for a front toolpost) such that the deflection in the toolpost and the tool itself result in the cutting being done exactly on centre. Probably easy to refine in batches of hundreds on a CNC machine but not easy to do at home.
 

Cogsy

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I've written umpteen times on the topic and the the rear mounted inverted blade goes

In and out of cut
whereas as front mounted parting tool

Goes into cut and can

DIG IN---BANG
Sorry Cogsy but it is not getting through.
Any parting tool can dig in - set your rear tool too low and try it out and you'll get yourself a dig in. If I have a dig-in then I'd much prefer the force to be being applied down through the saddle onto the ways than up which would try and rip the saddle off the bed.

You've written ad nauseam about having a rear parting tool but to my knowledge you have never been able to explain why it is "better" so I don't see what isn't "getting through".

I have had a Geo Thomas one which is angled down wards at 7 degrees, has the top cutting edge ground with a 140 degree female 'top' and a male vee of 140 degrees and the normal cutting edge is 7 degrees in hss and will cut 2" mild steel.
I can, and do, cut more than 2" mild steel also. I assume virtually everyone here does although maybe I've overestimated the ability or size of the Myfords? (I've never used one myself).

But the nice thing is that there are 2 blades and when cutting, the swarf is 'narrowed' into nice little curly rolls and doesn't seize and the little curlies simply drop down.
Gravity is certainly assisting with the 'curlies' dropping down but I don't like making 'curlies' with my parting tool as they can jam in the groove. I prefer actual chips which get flung out (annoyingly into my face sometimes). I also like nice thin parting blades so I don't waste much material for example when parting off piston rings, or when using a narrow, bifurcated cutter to shape crankshaft journals and the like. Actually this is the main reason I have never bothered with a rear parting setup - I use my parting tool for far more than just parting off so I want it to be at the front where I can see it and control it better.


If my memory is correct, Ian Bradley on of the Duplex tean withDr Nrman Hallows made the first one and Thomas made one- the horizontal variety and then did the present design which- if we think about it only requires the front of the tool refreshed. Usually only a touch on the grinder!
My toolholder (purchased, probably Chinese) takes standard blades, has built-in angles and only requires the front of the tool to be ground as well. Nothing new here.
 

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