# Themac Tool Post Grinder DIY Spindle



## Yorkus (Jun 10, 2020)

I have recently acquired a Themac J45 TPG. Unfortunately it didn't come with any of the accessories, such as spindle extension & different sized pulleys to vary the spindle speed. I am confident I can make the pulley's but I think the spindle extensions will be a little more involved. I guess at this point I am wondering if anyone else has done this & any advice you could offer. I have started by measuring the spindle bore taper to get a plan on how to proceed. 
It appears to be 7/8" per foot taper or very close to that. The Thread in the spindle appears to be 5/16"-24. If anyone has better information or a drawing that would be great. 
And before anyone mentions it I know Themac is still in business and selling parts & accessories. The problem is this is just a hobby for me and I can't begin to justify buying new at the prices I was quoted. These machines are priced at almost $3000 new and to buy a single spindle extension is over $200.

Thank You ahead of time for any help and/or advice!

York


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## petertha (Dec 4, 2020)

Hi Yorkus. Sorry I didn't see your post until just now. Did you already make your parts? I have fiddled with making custom Themac arbors (J35). Their taper is their own recipe (from the horses mouth). I can dig up my notes if still of interest.


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## Yorkus (Dec 6, 2020)

Hi Petertha

Thank you for the reply. I understand you not seeing my post for a while if at all since it is not the most common subject.
It's been a while since I did anything with the tool post grinder, it ended up going on the back burner for a while. 

Before putting it all aside I measured the Spindle Taper and came up with 0.0353" per inch of taper. As I understand it, this is about 2 degrees.
Shortly after that I made a short spindle and it seem to fit well. I am needing to make some more spindles for different size grinding wheels.
Now that winter temperatures have set in I'm not as quick to go out to the unheated shop. 

I am very interested in your knowledge on the subject if you don't mind sending your findings and any possible experience in making spindles.


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## petertha (Dec 8, 2020)

Sorry for the delay. I found my drawing of an ID grinder arbor that I made, but looks like I didn't take a picture of it yet. My drawing shows the taper angle I settled on after measuring a few Themac arbors I have & basically taking the average. I then made a few taper test slugs without the thread end & checked the fit by slight twist & Jiffy marker rub off. They seem to fit as snug as the stock ones. 

Hopefully this isn't too convoluted. At the time I wanted an known, documented angle reference that I could set up anytime I wanted to make another arbor. For the test tapers I set my compound angle by using a parallel bar extending from the spindle, mini sine bar & appropriate spacer. But for the arbor I made I held a Themac arbors in my collet chuck & adjusted the compound angle by running DTI down the surface (exactly centered of course). When I measured my resultant taper it seemed to correspond to the drawing & test slugs so I think its close.

The other thing I learned is the 5/16-24 thread that I cut according to the book (class 2A, OD & pitch diameter) fit too tight in spindle thread. I then measured the OD & PD of my Themac arbors & they are on what would be considered on the loose side. So I don't know what to say about that other than check it beforehand or cut a thread & check that independently. I'll try & round up some more info from my notes,


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## petertha (Dec 8, 2020)

I just re-read your post. You referenced 7/8" per foot, so that's 0.0729 in/in. My drawing shows .0391 in/in equivalent of the taper angle, times 2 for included angle = 0.0782 in/in equivalent. Very close. Themac never disclosed their value when I chatted them a while back, they said they have had the same turning/grinding setup for long, long time. Maybe you have the recipe.


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## petertha (Dec 8, 2020)

Small wheels are hard to find in my neck of the woods. I had these cut by a water jet guy from 1/4" commercial wheel. Working good so far. I wear a shield just in case.


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## petertha (Dec 8, 2020)

My self made wheel dressing tool


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## Yorkus (Dec 9, 2020)

When I measured the arbor taper I used the following method:
I removed the arbor from the THEMAC motor and removed the Pulley from the drive end.
I then put the drive end of the grinder spindle in a collet in the lathe and supported the other end with a steady rest.
Using 2 DTI's I arranged one as a travel dial to measure the 1" of travel. The second DTI was used with a Brown & sharp bore attachment to measure the taper. What I came up with after averaging repeated measurements Into & then out of the spindle, was 0.0354" taper per 1.00".
My original post stating the taper was 7/8" included was before I had a chance to measure accurately.

For the threaded end if I recall I single point threaded it most of the way and then ran a tap over it to finish the treads.
Seems to fit quite well.

I have attached photos of the one arbor I have made so far. On my second attempt the material I used was case hardened and I had a hell of a time with my lathe getting through the case to the softer material in the middle and ended up with a bad taper. When I get back into this I am going to ether use different material or anneal it before starting. As you can see the surface finish is not the best and the "rub" test shows an "OK" contact area.

Part of the reason this project went on the back burner was because I sold my Atlas Commercial 12x36 Lathe and Purchased a Taiwan Import 12 x 36 gearhead. Looking forward to getting some more time on this Lathe to compare to the Atlas.


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## Yorkus (Dec 9, 2020)

I really like all the parts you have made for yours.
I will need a way to hold the diamond nib to dress the wheel. I like what you came up with.
As for the wheels I stopped into a local grinding Company to see what they may have for sale. 
He gave me the wheel that I made the arbor for,  OD 1.750", ID .75", Width of 1.00"

Since then I found the package you see in the photos of the wheels from Ebay.
There are a number of option on ebay so maybe try that if you find you need more.


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## petertha (Dec 9, 2020)

Tour parts look good. Goes without saying hopefully to cover up your lathe bed when grinding as especially dressing the wheel insitu. I put a vaccum nozzle on when I do this.

Somewhat related, I think I posted this elsewhere but here is what I came up with to accurately measure in-feed when using TPG. Its my tenths reading mini dial gauge, held by a clamp via the stem, an aluminum stop block with some neodium magnets glued in so it can be quickly positioned & attached to the cross slide base. The clamp is bolted to the table moving surface via nuts in the T-slot. I find once the TPG is running, the dials become a bit lively, probably due to some micro vibration transferring through the compound. This is kind of a direct reading, finer resolution method. It also picks up the teeniest variations of locking the table.

I think TPG's definitely have their place. But my big issue with them is they are nearly, if not totally impossible to position to the work when the end of the part needs to be supported with a tailstock center. The spindle or extending motor assembly is always trying to occupy the same real estate as the tailstock casting/quill.


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## Yorkus (Dec 9, 2020)

I have found the Tail Stock getting in the way to be a common problem when cutting lite tapers with the compound. I made a #2 size pulley wheel for the drive side of the Tool Post Grinder and to give the pulley a crown in the middle I put the compound at 1.5 degrees off parallel with the lathe bed. I had to extend the tailstock quill almost the full 4" travel to get enough room to clear the compound crank just enough to advance the cutter. I was turning between centers with the work on a mandrel. One way it would seem to help when it comes to the TPG would be to use a larger grinding wheel so you can put more distance between the compound and the work. Just curious what Lathe you have? Size?
I was aware of the good practice of covering the lathe ways when doing anything in the grinding realm. 
So back to the THEMAC arbors, I guess the only surprise I have from your info is the difference in what you have as the Arbor Taper of 0.0391" per inch as opposed to my 0.0354" per inch. I didn't have the ability to put a factory arbor on the lathe to measure since I don't have one, but I really thought I did a good job of setting up to measure the bore of the spindle. Seems to me that on a very small taper like this that almost 0.004" is a large difference. 

Thank you for the PDF drawings with dimensions. You have made it very easy to understand compared to my hand written diagrams and notes that I didn't really remove all the bad measurements before putting this on the back burner.


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## petertha (Dec 10, 2020)

You've raised a good point about the taper. I have 2 ID arbors plus 1 collet chuck arbor all with same taper shank. The ID arbors I was able to (kinda) hold between centers using the center drill pip on the thread end and the screw hole on the wheel end between centers. The collet chuck I held a 0.250 ground pin in my collet chuck but I can almost guarantee that was iffy. 

Anyways I first rotated the ID arbors with DTI on the OD. It was pretty concentric runout but I did see variations, likely due to the thread hole not really being a center hole. Then I put my DTI on the compound, found vertical center of the arbor, advanced in until DTI registered zero on one end of taper, traversed the carriage to other end of taper, advanced in again to same DTI zero & read off my DRO X displaced value. I'll have to dig my notes because I did see some variation in arbors and different rotate positions, but like I mentioned I took an average of those & that's what I made my drawing & taper test slugs from. Could well be some measurement variations though. Also my compound turning is not as good as with a taper attachment so some hand finishing was required - more room for variation!

Then when I made my arbor I decided to set my compound in the more classic way - DTI-ing along the taper of the arbor itself. My plan was to make a few more arbors but I need a specific tool for a job & got busy with other things so I only have the one.

One thing occurs to me, when I blue check a Themac arbor, its always being engaged by the threaded section before the tapers come in contact with each other & that might have some influence over the rub off. When I made my test slugs it was only the taper section, no threads. I blued with felt pen & gave a half turn. 

I wish they would just publish the taper & we could just get on with life, LOL. So I wouldn't throw out your number quite yet. It sounds like it might be a logical nominal taper ratio but who really knows.


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## petertha (Dec 10, 2020)

Regarding tailstock 'in the way' I suspect that's why these machines were invented, probably 100 years ago. I would love to have one.


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