Best Way to Make T-Slot Bolts for Lathe Compound / QC Toolpost

cinematechnic · May 10, 2018

I’m about to make my third toolpost hold-down bolt for my Hardinge HLV-H in less than a year. This new one is going to be for a Multifix E toolpost.

I haven’t had any problems with the bolts I’ve made but I wanted to tap into the vast knowledge base here. Just want to know if I’m doing things the right way and if anyone has any suggestions.

I tried advanced search and literally got nothing. Perhaps it a description problem with people using different terms to describe “hold down bolt”.

Please see the attached photo.

On the left is the T-nut I got with the KDK Series 100 toolpost, likely made by the shop that previously owned the HLV-H. It appears to be stainless, and hase a 7/16 - 14 thread. It is partially threaded so the bolt cannot go through the nut and contact the bottom of the T-slot.

In the middle is the bolt I made for my KDK Series 0 toolpost. It is made from C1215 steel and has a flat head (countersunk) 3/8-16 bolt that inserts from below. The pressure of the bolt being pulled upwards into the “T” part keeps it from rotating.

On the right is the bolt I made for my Tripan 111 toolpost, again made from a bar of C1215. Tripan are small and very precise toolposts commonly used on Schaublin lathes (my other lathe is a Schaublin 102N). The design is similar to the bolt for the KDK 0 but uses a M10 x 1.5 bolt and has a cylindrical projection to fit into the bore at the bottom of the Tripan and locates the 5/16” riser plate that compensates for the difference in center height between the HLV-H and the Schaublin so that I can swap toolholders between the two machines.

The bolt for the Tripan fits tighter and does not rotate at all even with no pressure being applied. I think it might be because of the longer unthreaded section fitting tightly against the bore I made in the “T bushing”.

What’s different this time: The Multifix E is about the biggest toolpost you’d want to put on a HLV-H. At it’s core is a hardened and ground splined cylinder 75mm in diameter. It can accept a bolt of up to 20mm and has holes for two 6mm pins that can be put into the T-nut to prevent the splined cylinder from rotating on the compound.

I don’t take heavy cuts and I’m usually operating on the “low” speed (0.5 HP), but I like the idea of having the Multifx setup being very rigid as this helps when you need very high accuracy (better than .001”) which I frequently do.

My questions are:

1. Is a bolt coming from below with a nut on top of the toolpost superior for any reason other than eliminating the possibility of the bolt driving through the T-nut to the compound? The stainless T-nut I'm using on the HLV-H (left side of photo) has an incomplete thread to prevent this

2. Is there any relationship between toolpost size and the size of the T-nut where it contacts the T-slot in the compound? In other words, how big should the contact are of the T-nut be to distribute the clamping force evenly and be sufficient for the size of the toolpost?

3. Any comments on materials? Is unhardened C1018 type steel OK? I used C1215 for the two previous ones but they were small enough that they could be made from 1.125” bar. Many more sizes and shapes availabel in C1018.

Thanks in advance!

dsergison · May 10, 2018

there is nothing besides clamping force and coefficient of friction between tool holder and top of compound going to effect rigidity..... until something over comes that it wont move. any slip fit pins or splines or such are just there for preventing EXCESS movement if you overcome the initial friction and break free.

If you don't push it or crash it you'll never need for any of that other detail.

Very nice stuff. It's obvious you care for it. You'r over thinking it in my opinion.

HuFlungDung · May 10, 2018

I would make up a T nut that fills the slot in the compound from one side to the other. Why not, it's just a few cents worth of iron. I'd start out with a piece of hot rolled flat bar or a piece of plate. I would tap as large a hole in the nut as I intended to make for the diameter of the stud on top. Also maximize that diameter so as to make it virtually unstrippable when using a long handle socket wrench to tighten the toolpost down. Loctite the thread when assembling the stud into the T nut.

The tapped hole in the nut should be carefully tapped or single point threaded so as to make the stud stand perpendicular to the compound. That is, don't be careless about hand tapping it and having it go cocked, because if it is crooked, then you unnecessarily 'bend it perpendicular' when you tighten the snot out of the hold down nut.

rons · May 10, 2018

My Hardinge tool post came with a solid bolt. The head is about 1.125" square. The thread is 3/8-24. The washer is .875" diameter. It had a repair weld on the bolt about 1" above the square head. So I made a copy just in case the original breaks. That was 15 years ago. The copy is still in a drawer.

Given the dimensions of my tool post, the middle bolt in the picture is the only one that would fit. I would tack weld the bolt to make sure it would never rotate in the bottom hole. Hate to imagine the situation where you try to loosen the top nut but the bolt spins in the T nut.

Your questions:
1. The Hardinge tool post and T nut dimensions do not allow that much dimension for a bottom thread. So the bolt thread is on top.
2. The rounded two sides on your nuts would reduce some contact area. I think totally square is a little better.
3. If the POS worked over bolt like mine can last this long then what you have already is just as good or maybe better. I think the thread and nut is more important. Eventually little pieces of the thread might chip off. They have on mine. You've got me thinking about making a new one from a grade 8 bolt and nut. My only hesitation would be not to use a set made from import crap.

cinematechnic · May 10, 2018

HuFlungDung said:
... I would tap as large a hole in the nut as I intended to make for the diameter of the stud on top. Also maximize that diameter so as to make it virtually unstrippable when using a long handle socket wrench to tighten the toolpost down...

I assume you mean to use a stud and make the female thread in the T-nut very tight so it binds?

rons said:
My Hardinge tool post came with a solid bolt. The head is about 1.125" square. The thread is 3/8-24. The washer is .875" diameter...Hate to imagine the situation where you try to loosen the top nut but the bolt spins in the T nut....

I was thinking that I forgot to bring up the issue of thread pitch for the bolt. I've heard that fine pitch applies more torque. The trouble is I can't find long fine pitch bolts, either UN or metric, larger than 3/8".

The original Swiss-Made (definitely not import crap :-) one-piece t-bolts from Tripan use M10 x 1.5 thread - slightly larger diameter and similar pitch to 3/8-16. They use an "Allen nut" internal hex nut to tighten. I copied that concept on the KDK 0 using a 3/8-16 Allen nut.

BTW, I made the nuts round so I could use a 5C emergency collet to hold them on both the lathe and the mill. But I was thinking of making this new T-nut for the Multifix from a bar of C1018.

Also, even though my flat head bolts are not really captured (the one on the KDK will turn by hand) I have NOT had any issue loosening them. I think the torque of tightening binds them enough to get the nut loose.

Thanks for the input!

Peter. · May 10, 2018

cinematechnic said:
. I've heard that fine pitch applies more torque. The trouble is I can't find long fine pitch bolts, either UN or metric, larger than 3/8".

If only you had a kind of machine that could take a suitable piece of metal and cut a fine pitch thread in it just for this purpose. Life would be so much easier if you had one of those!

rons · May 10, 2018

The one on the left is the original. Notice the damage to the threads. Ugly but still works.
The one on the right was made from a solid bar. Decided to make a slightly longer nut. The square nut is 1.125" on each side.

edwin dirnbeck · May 10, 2018

Here’s mine I have been using this for 12 years . Full width . It has

hardened half inch threaded rod tapped and welded in place .it is on a forbidden Lathe

Sent from my iPhone using Tapatalk

cinematechnic · May 10, 2018

Thanks for the replies and the photos.

I just realized to my embarrassment that the reason the bolt is fixed on the Tripan T-bolt and not fixed on the KDK T-bolt is that the diameter of the thread (M10x1.5) on the flat head bolt I used on the one for the Tripan was just slightly less than the diameter of the un-threaded section. With the 3/8-16 version the thread was slightly larger than the un-threaded section (about .002in larger).

The solution is so simple and I'm sort of kicking myself (mentally) for only having thought of it till now: Bore the hole for a press fit on the un-threaded section and then run a tap through it to create a little clearance for the threads...

Once the bolt is a tight fit where you can't turn it by hand in the "T-bushing", the pressure of tightening the nut will drive it into the T-bushing and there will be no problem with it coming loose.

BTW, I kinda think there might be some benefit to an aspect of the way my design works: The bolt is hardened but the T-bushing is not. The part that needs to be strong (threads) are rolled and hardened. The part that puts pressure agains the more vulnerable cast iron T-slot in the compound is unhardened low carbon steel.

What do you guys think?

edwin dirnbeck · May 10, 2018

Most small lathes,ESPECIALLY the hardinge lathes have thin and weak compound t slots. I have seen 2 beautiful hardinge lathes with cracked t slots. I would make the t slot nut as large and as tall as possible Makeit fit nice.Then tap as big as is allowable keeping in mind that if you make it too big the nut will be weakened.Harden the nut to around 53 rc. Then thread in a piece of high strength threaded rod from McMaster.Then weld the threaded rod to the t slot nut ONLY ON THE BOTTOM and grind flush. On top of your toolpost, you can use a cupling nut,they are taller and will have more thread engagement..You will NEVER WEAR THIS OUT.Edwin Dirnbeck

edwin dirnbeck · May 10, 2018

cinematechnic said:
I assume you mean to use a stud and make the female thread in the T-nut very tight so it binds?

I was thinking that I forgot to bring up the issue of thread pitch for the bolt. I've heard that fine pitch applies more torque. The trouble is I can't find long fine pitch bolts, either UN or metric, larger than 3/8".

The original Swiss-Made (definitely not import crap :-) one-piece t-bolts from Tripan use M10 x 1.5 thread - slightly larger diameter and similar pitch to 3/8-16. They use an "Allen nut" internal hex nut to tighten. I copied that concept on the KDK 0 using a 3/8-16 Allen nut.

BTW, I made the nuts round so I could use a 5C emergency collet to hold them on both the lathe and the mill. But I was thinking of making this new T-nut for the Multifix from a bar of C1018.

Also, even though my flat head bolts are not really captured (the one on the KDK will turn by hand) I have NOT had any issue loosening them. I think the torque of tightening binds them enough to get the nut loose.

Thanks for the input!

Mcmaster carr has high strength threaded rod in most sizes including metric.

rons · May 10, 2018

cinematechnic said:
BTW, I kinda think there might be some benefit to an aspect of the way my design works: The bolt is hardened but the T-bushing is not. The part that needs to be strong (threads) are rolled and hardened. The part that puts pressure agains the more vulnerable cast iron T-slot in the compound is unhardened low carbon steel.

What do you guys think?

This is where the Hardinge guys did something different, at least on mine. The T slot is not made from one solid piece. There is a removable plate on each side of the T slot, each attached with 3 screws. Each plate overhangs the tool post channel and that is the T slot. Any damage and the plates can be replaced. Probably something that anybody can add if they are brave and fearless.

EPAIII · May 10, 2018

You can see the Tee Nut and stud that I use with my tool post on the right here:

My thought when making it included:

1. I did not even consider a one piece design. Probably due to the massive waste of material and the amount of machining time needed to reduce a large starting diameter down to the size for the stud. It just isn't worth it. And if you start with a purchased bolt and just machine the head, you will wind up with a small Tee head: see my #2 below. Tee Nut and stud seemed to be the only way to go.

2. I have seen damage to Tee Slots (cracked) and to Tee Nuts made with steel that was not hardened. So I wanted a wide design that distributed the load over a wide area. I have some small (6mm) Tee Nuts that were made with the round type design and they became bent and even cracked over some time in use. I have replaced them with rectangular ones that are somewhat wider than standard Tee Nuts and have had no problems with them. It may seem odd, but the extra material in the corners of a rectangular design really does help. And I would recommend that it be made wider than the commercial Tee Nuts that are available, perhaps 50 to 100 percent wider.

3. Yes, do incorporate some means to prevent the stud from hitting the bottom of the Tee slot. I usually stop a plug tap when it reaches the bottom surface. If I go too far, I just use a punch and make a bunch of dimples around the bottom of the hole. That works too.

4. Material? I believe the one in the photo was made from 1018 steel and it has worked for years now. But if I was doing it again I would probably use 4140.

Your questions:

1. It is possible to drive a bolt or a stud too deep and into contact with the bottom of the Tee slot. Precautions should be taken in either case. A one piece Tee Bolt would positively prevent this, but it has other problems. I see no difference between using a bolt or a stud with a nut with a Tee nut in the Tee slot. You just have to take different cautions either way.

2. A relation between the sizes of the tool post and the slot and contact area? I guess a ME could produce some equations, but most Tee nuts are of a rectangular design and are sized about 50% longer than they are wide. IMHO, this is not always enough for a shop made, unhardened Tee nut and I tend to make them at least twice as long as they are wide. But this is seat-of-the-pants engineering. It does seem to work OK. There is no harm in going longer as some have suggested.

3. If it has a proper design, rectangular and extra wide, then material becomes a secondary consideration. I would not go to aluminum or brass: stick with steel. But 1018 can work fine. A better alloy may be better or even needed in some cases and I would start with 4140. If that is not good enough, I would try hardening it.

In summation: Tee nut and stud, rectangular design, extra length, and steel. That's about it.

cinematechnic said:
I’m about to make my third toolpost hold-down bolt for my Hardinge HLV-H in less than a year. This new one is going to be for a Multifix E toolpost.

I haven’t had any problems with the bolts I’ve made but I wanted to tap into the vast knowledge base here. Just want to know if I’m doing things the right way and if anyone has any suggestions.

I tried advanced search and literally got nothing. Perhaps it a description problem with people using different terms to describe “hold down bolt”.

Please see the attached photo.

On the left is the T-nut I got with the KDK Series 100 toolpost, likely made by the shop that previously owned the HLV-H. It appears to be stainless, and hase a 7/16 - 14 thread. It is partially threaded so the bolt cannot go through the nut and contact the bottom of the T-slot.

In the middle is the bolt I made for my KDK Series 0 toolpost. It is made from C1215 steel and has a flat head (countersunk) 3/8-16 bolt that inserts from below. The pressure of the bolt being pulled upwards into the “T” part keeps it from rotating.

On the right is the bolt I made for my Tripan 111 toolpost, again made from a bar of C1215. Tripan are small and very precise toolposts commonly used on Schaublin lathes (my other lathe is a Schaublin 102N). The design is similar to the bolt for the KDK 0 but uses a M10 x 1.5 bolt and has a cylindrical projection to fit into the bore at the bottom of the Tripan and locates the 5/16” riser plate that compensates for the difference in center height between the HLV-H and the Schaublin so that I can swap toolholders between the two machines.

The bolt for the Tripan fits tighter and does not rotate at all even with no pressure being applied. I think it might be because of the longer unthreaded section fitting tightly against the bore I made in the “T bushing”.

What’s different this time: The Multifix E is about the biggest toolpost you’d want to put on a HLV-H. At it’s core is a hardened and ground splined cylinder 75mm in diameter. It can accept a bolt of up to 20mm and has holes for two 6mm pins that can be put into the T-nut to prevent the splined cylinder from rotating on the compound.

I don’t take heavy cuts and I’m usually operating on the “low” speed (0.5 HP), but I like the idea of having the Multifx setup being very rigid as this helps when you need very high accuracy (better than .001”) which I frequently do.

My questions are:

1. Is a bolt coming from below with a nut on top of the toolpost superior for any reason other than eliminating the possibility of the bolt driving through the T-nut to the compound? The stainless T-nut I'm using on the HLV-H (left side of photo) has an incomplete thread to prevent this

2. Is there any relationship between toolpost size and the size of the T-nut where it contacts the T-slot in the compound? In other words, how big should the contact are of the T-nut be to distribute the clamping force evenly and be sufficient for the size of the toolpost?

3. Any comments on materials? Is unhardened C1018 type steel OK? I used C1215 for the two previous ones but they were small enough that they could be made from 1.125” bar. Many more sizes and shapes availabel in C1018.

Thanks in advance!
View attachment 228188

cinematechnic · May 11, 2018

Great, informative replies so far, thank you!

I realized yesterday that I already had an example of something similar to what I want to make. Made by the finest craftsmen and designed by (I assume at least) highly skilled engineers.

See photo below. That is the T-bolt that holds the carriage of the Schaublin 102N to the bed (the bed is split). It fits into a T-slot on the bottom of the carriage.

For those unfamiliar with the 70 and 102 series Schaublin, they have a setup similar to the plain turning Hardinge DV-59 but with a split cast iron bed instead of a dovetail steel bed.

The four points of contact seems like a good idea to me. That must have been done to help distribute the force applied by the bolt shaft preventing it from lifting in the center. The T-slot this fits into is cast iron so they wanted to protect it from damage (which would have affected the accuracy of the lathe since the carriage sits on that T-slot).

The bolt has a M10 x 1.0 thread (fine pitch metric) and close examination leads me to believe the part with the threads was brazed on. I can post a photo if anyone wants to take a look and opine.

cinematechnic · May 14, 2018

One more question: How important is it that the T-nut/T-bushing be hardened? I believe both 1018 and 1215 steels can be case hardened. Do I need to be concerned about dimensions shifting or parts warping if I have them case hardened?

I just changed the T-bolt (middle one in the picture in my OP) from 3/8-16 to a 3/8-24. The finer pitch bolt had a larger diameter unthreaded section.

Along with taking a light cut with a 3/8-24 tap resulted in a bolt that threads in tightly and then is a press fit when you get to the base of the unthreaded section.

So now I have a fixed bolt that will not turn at all on that "T-bushing". I also like the fact that the 3/8-24 is what Hardinge used and about the closest you can get to the M10 x 1.0 that Schaublin uses as the hold-down bolt for the carriage on the SV 102N.

Still plan to make the full length T-nut/T-bushing, but this gives me something I can use now. I plan to use the same bolts for the full length T-bushing.

McMaster has 3/8-24 x 3.5" flat head screws which is what I used. I like the fact that with this technique I get rolled and hardened alloy threads where I need the strength and can use unhardened steel for the "T-bushing". I may consider having my T-bushings hardened at some point.

rons · May 15, 2018

I didn't do that considering the surface area. As you can see from my old part, the thread is going to wear.

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